Interfacial Interactions in Polymer-Layered Silicate Nanocomposites

Ryo Kato,*,† Christopher M. Liauw,† Norman S. Allen,† Ainhoa Irure,†Arthur N. Wilkinson,‡ John L. Stanford,‡ and Nurul H. Fithriyah‡

Centre for Materials Science Research, Dalton Research Institute, Manchester Metropolitan UniVersity,Chester Street, Manchester, M1 5GD, U.K., and School of Materials, The UniVersity of Manchester,

GrosVenor Street, Manchester, M1 7HS, U.K.

ReceiVed August 24, 2007. In Final Form: NoVember 1, 2007

Interactions between sodium montmorillonite (Na-MMT) and a variety of probes, some of which are intended tomodel components of a polyurethane system, have been studied. Particular attention was given to the effect ofpreadsorbed water on the adsorption behavior of the probes. Flow microcalorimetry (FMC), diffuse reflectance Fouriertransform infrared spectroscopy (DRIFTS), and wide-angle X-ray scattering (WAXS) were used to monitor theadsorption process. The probe set included alcohols, amines, ethers, poly(propylene glycol) monobutyl ethers (PPG),and 4-ethylphenyl isocyanate (4-EPI). FMC revealed that the preadsorbed water molecules on undried Na-MMThindered the adsorption of alcohol and ether probes, but had little effect on the adsorption of amines. Drying ofNa-MMT to less than 0.3% w/w H2O led to an increase in heat of adsorption and generally greater retention of theprobes. PPG showed strong interaction with Na-MMT due to multipoint adsorption. With dried Na-MMT, WAXSrevealed that PPG of molecular weight (MW) 1000 was partly intercalated into the gallery while lower molecularweight PPG (MW 340) did not intercalate the Na-MMT. DRIFTS spectra of 4-EPI adsorbed on undried Na-MMTrevealed urea linkages, indicating formation ofN,N′-bis(4-ethylphenyl) urea. In contrast, with dried Na-MMT the4-EPI formed a urethane linkage with hydroxyl groups present at the edges of the silicate platelets.

IntroductionExfoliation of a layered silicate in a polymer matrix results

in a very high interfacial area at low silicate volume fraction andhence a unique set of composite properties such as low density,high mechanical strength, and much reduced gas permeability.1

Achievement of this ideal exfoliated state is not easy, particularlyvia melt blending of layered silicates with polyolefins. However,in the case of polar polymers such as polyamides and polyure-thanes, in situ polymerization in the presence of the layeredsilicate is often more practical. Because of the high polarity andsmall size of monomer molecules and polymerization additives,together with the potential for polymerization between theplatelets, exfoliation occurs more readily, with even unmodifiedlayered silicates showing some potential for exfoliation.2 In thevast majority of cases that involve melt blending of layeredsilicates with polymers, the metal ions on basal surfaces of thelayered silicate must be exchanged for organic cations that reducethe overall polarity of the gallery environment and decrease theattractive forces between platelets. Such modification generallyincreases the compatibility with polymers that are usually intendedfor nanocomposite applications such as polyamides andpolyolefins.3-5 Achievement of exfoliation and good compositemechanical properties depends on much more than simplymatching the surface free energy of the platelets to that of thepolymer matrix. If the match is too close, the strength of adhesion

between the silicate platelets and the matrix can reduce and canresult in a decrease in the strength of the composite; this isparticularly true of polyolefins.5 Successful formation of anexfoliated polymer-layered silicate nanocomposite (PLSN) istherefore a complex balance of competing interfacial interactionsthat require both understanding and a means of control, ifsignificant advances in the state of the art are to be realized.

Formation of a PLSN is dependent on penetration of monomeror polymer into the gallery of the layered silicate duringpolymerization or melt compounding.6,7 The penetration ofmonomer (or polymer) into the gallery space may be con-sidered an adsorption process that can be studied by flowmicrocalorimetry (FMC). FMC has proved to be a useful toolfor characterizing filler surfaces in terms of their surfacechemistry and their interaction with surface treatments in thatboth heat of adsorption/desorption and the amount of probesadsorped/desorped can be determined.8,9Our research group hasconducted several FMC studies on the adsorption of fatty acidsonto metal hydroxides and on the interactions between pinenesand silica.10,11

Adsorption into the galleries of MMT is complicated by theattractive forces between platelets and the surface chemistry ofthe platelet edges. Jaynes and Boyd investigated the nature ofthe siloxane surface in smectites by measuring the adsorption ofaromatic hydrocarbons from water onto clays of different cation-

* To whom correspondence should be addressed. Tel.: 44-161-247-3325. Fax: 44-161-247-6357. E-mail: r.kato@mmu.ac.uk.

† Manchester Metropolitan University.‡ The University of Manchester.(1) Ray, S. S.; Okamoto, M.Prog. Polym. Sci.2003, 28, 1539-1641.(2) Lees, G. C.; Liauw, C. M.; Wilkinson, A. N.; McIntrye, A.; Burrows, D.

(Kay-Metzeler, Ltd.). Method for the Production of Polymeric Material. U.K.Patent GB2400107B, March 4, 2004.

(3) Usuki, A.; Kawasumi, M.; Kojima, Y.; Okada, A.; Kurauchi, T.; Kamigaito,O. J. Mater. Res.1993, 8, 1174-1178.

(4) Artzi, N.; Narkis, M.; Siegmann, A.J. Polym. Sci., Part A: Polym. Phys.2005, 43, 1931-1943.

(5) Kadar, F.; Szazdi, L.; Fekete, E.; Puka´nszky, B.Langmuir2006,22, 7848-7854.

(6) Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi,T.; Kamigaito, O.J. Mater. Res.1993, 8, 1179-1184.

(7) Vaia, R. A.; Ishii, H.; Giannelis, E. P.Chem. Mater.1993, 5, 1694-1696.(8) Ashton, D. P.; Rothon, R. N. InControlled Interphases in Polymeric

Materials, Proceedings of the Third International Conference on CompositeInterfaces (ICCI-III), Cleveland, OH, May 21-24, 1990; Ishida, H., Ed.;Elsevier: New York, 1990; pp 295-305.

(9) Fowkes, F. M. InAcid-Base Interactions: ReleVance to Adhesion Scienceand Technology;Mittal, K. L., Anderson, H. R., Jr., Eds.; Brill: Leiden, TheNetherlands, 1991; pp 93-115.

(10) Liauw, C. M.; Rothon, R. N.; Lees, G. C.; Iqbal, Z.J. Adhes. Sci. Technol.2001, 15, 889-912.

(11) Diaz, L.; Liauw, C. M.; Edge, M.; Allen, N. S.; McMahon, A.; Rhodes,N. J. Colloid Interface Sci.2005, 287, 379-387.

1943Langmuir2008,24, 1943-1951

10.1021/la702620c CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 01/19/2008

exchange capacity (CEC).12 The organoclay derived from thelow CEC clay adsorbed more benzene than the high CEC clay,therefore indicating that benzene prefers to interact directly withthe siloxane basal surface. Ray et al. studied the effect of theplatelet edge hydroxyl groups (mainly silanol (Si-OH) andaluminol (Al-OH)) of MMT on the dispersion of layered silicatein poly(butylene succinate).13 The urethane linkages that werepresent in the chain extender component formed hydrogen bondswith the hydroxyl groups on the platelet edges; this led to verystrong interaction between the matrix and layered silicate platelets.Nam et al. prepared the flocculated poly(L-lactide) (PLLA)/claynanocomposites using a surfactant carrying alkyl chains end-capped with hydroxyl groups.14 IR spectra revealed that theflocculation structure of silicate platelets resulted from hydrogenbonding between hydroxyl groups of the surfactant and thehydroxyl groups at the platelet edges, and those at the PLLAchain ends. Thus, both the basal surface and the hydroxyl groupsat the platelet edges will interact with monomer or polymer, andan understanding of these adsorption processes is key to thesuccessful production of PLSNs.

Layered silicates are characterized by the presence of watermolecules that interact with exchangeable cations, platelet edgehydroxyl groups, surface oxygen atoms, and with each other.Therefore, the adsorption of monomer or polymer on the claywill be affected by adsorbed water. The character of interlayerwater is greatly influenced by the moisture level and the interlayercations.15,16The interlayer water adsorbed by clay can be foundin two different states: the readily removable water that can bedriven off by heating to about 100-150°C, and the water thatis held more firmly. These firmly bound water molecules arethought to form the inner hydration sphere of the interlayer cations(water bound directly to cations). On the other hand, the readilyremovable water consists of outer sphere water molecules thatare involved in water-to-water hydrogen bonding. Furthermore,the adsorbed water will form hydrogen bonds with oxygen atomsat the clay surface at sites where Si4+ is replaced by Al3+. Becauseof the positive charge deficiency, due to octahedral substitutionsin the case of montmorillonites, the negative charge on the surfaceoxygen atoms is delocalized and the adsorbed water moleculesform weak hydrogen bonds with the surface oxygen.17 Themolecular vibrations of H2O in montmorillonites were character-ized using infrared spectroscopy by Bishop et al.18 Absorptionsnear 3620 and 3550 cm-1 were assigned to water bound directlyto cations, and absorptions near 3450 and 3350 cm-1 wereassigned to additional adsorbed water molecules.

In this work, the strength of interactions between undried anddried sodium montmorillonite (Na-MMT) and a range of probesintended to model components of a simple polyurethane (PU)system will be studied by FMC. Generally, the structure of alinear-segmented PU is in the form of a multiblock (A-B)n

copolymer. The soft segment B-block is normally composed ofa polyester or polyether, and the hard segment A-block is thereaction product of a diol or diamine reacted with diisocyanate.Therefore, the probe set for the FMC measurements, designedto model a reactive PU system, included simple alcohols, amines,ethers, and an isocyanate. The nature of the interactions between

the Na-MMT and the probe and the chemical environment inwhich the adsorbed species reside will be investigated usingdiffuse reflectance Fourier transform infrared spectroscopy(DRIFTS) on samples isolated from the FMC cell. Wide-angleX-ray scattering (WAXS) will be used to examine the effect ofprobe adsorption on the basal spacing, which, together withknowledge of the dimensions of the molecule and infraredadsorption peak shifts, can indicate the mode of adsorption.

Experimental Section

The Na-MMT used was supplied by Nanocor as a polymer-grademontmorillonite, Nanomer PGW. Na-MMT conditioned in ambienthumidity (undried) and dried (140°C for 3 h) Na-MMT with<0.3%w/w H2O were used. Goldstein and Beer reported that the Na-MMT(Nanocor PGW) consists of rounded edge, variable shape, porousagglomerates with particle size of 5-45 µm range, and specificsurface area of∼4 m2g-1.19The probes investigated included ethanol(EtOH), 1-butanol (nBuOH), 2-propanol (iPrOH), and diethyl ether(EtOEt). Decahydronaphthalene was used as the nonadsorbing probe.These probes were intended to model the OH functional end groupsand ether linkages of polyether polyol chains. Poly(propylene glycol)monobutyl ether (hereafter referred to as PPG) of molar mass 340and 1000 g mol-1 (i.e., PPG 340 and PPG 1000, respectively) wereselected to represent “one” arm of a polypropylene oxide diol ortriol type polyol. 1-Butylamine (nBuNH2), octylamine (nOctNH2),4-ethylphenylisocyanate (4-EPI), and 4-ethylaniline (4-EAN) wereselected to represent an aromatic isocyanate and various reactionproducts. All solvents and probes were of HPLC grade or 99%+purity where appropriate. The heptane, alcohols, diethyl ether, anddecahydronaphthalene were dried over freshly activated (350°C, 3h) 4A molecular sieves. The amines were dried over sodium hydroxidepellets.

The swelling volume of Na-MMT in the various solvents wasmeasured using a 10 cm3 measuring cylinder. The sample (0.2 g)was sprinkled onto the solvent surface (10 cm3) and allowed to fallslowly to the bottom of the cylinder. After 24 h, a swelling equilibriumwas reached, and the swelling volume was recorded. After theswelling volume in the solvents and pure probes was determined,the samples were isolated and air-dried before DRIFTS.

The FMC used was a Microscal 3V that was upgraded to theall-PTFE fluid path “I” (inert) specification. The instrument waslinked to a Microscal thermometer bridge/control unit whose outputwas fed to a Perkin-Elmer 900 series interface. The cell outlet wasconnected to a Waters 410 differential refractometer, whose datawas fed into the second channel of the 900 series interface. The 900series interfaces were linked to a PC for data manipulation usingPerkin-Elmer chromatography software (TotalChrom version 6.3.1).Energy calibration of the FMC was achieved via passage of a knownamount of electric power for a fixed time period through a filamentintegrated into the cell outlet connector. Calibration of the differentialrefractometer was achieved using a 20-µL calibration loop. Heat ofwetting measurements were conducted using a cell temperature of30 °C ((1 °C) and a probe flow rate of 4.0 cm3 h-1. A knownamount (98 mg ((1 mg)) of Na-MMT sample was placed in theFMC cell, and the sample was left to equilibrate with air for 10 min.The probe was then introduced to the FMC cell, and the wettingexotherm was recorded. After thermal equilibrium was attained(usually 15-30 min), thermal calibration was carried out and twoto three peaks were generated. Adsorption and desorption of probesfrom heptane solution (0.2% w/v) were also conducted using FMC.The cell temperature of the differential refractometer was 40°C, andsensitivity was set to 4. Samples were left to equilibrate overnightat a carrier fluid flow rate of 0.3 cm3 h-1. The flow rate was thenincreased to 4.0 cm3 h-1, and the system was left to settle for ca.1 h. Data collection was then started, and after 5-10 min the inletwas switched from heptane to the solution of probe in heptane. Afterthe refractive index data reached a stable limiting value (typically120-150 min), the inlet was switched back to pure carrier fluid and

(12) Jaynes, W. F.; Boyd, S. A.Clays Clay Miner.1991, 39, 428-436.(13) Ray, S. S.; Okamoto, K.; Okamoto, M.Macromolecules2003, 36, 2355-

2367.(14) Nam, P. H.; Fujimori, A.; Masuko, T.e-Polym.2003, no. 005.(15) Hendricks, S. B.; Nelson, R. A.; Alexander, L. T.J. Am. Chem. Soc.1940,

62, 1457-1464.(16) Farmer, V. C.; Russell, J. D.Trans. Faraday Soc.1971, 67, 2737-2747.(17) Sposito, G.; Prost, R.Chem. ReV. 1982, 82, 553-573.(18) Bishop, J. L.; Pieters, C. M.; Edwards, J. O.Clays Clay Miner.1994, 42,

702-716. (19) Goldstein, A.; Beer, M.J. Eur. Ceram. Soc.2004, 24, 3187-3194.

1944 Langmuir, Vol. 24, No. 5, 2008 Kato et al.

the desorption process was recorded. After desorption, thermalcalibration was carried out and two to three peaks were generated.The differential refractometer was then calibrated using the 20-µLcalibration loop, and seven to eight calibration peaks were obtained.The amount adsorbed was determined by normalization of therefractometry data deflection for the nonadsorbing probe to that forthe adsorbing probe, once the deflection of the latter reached itslimiting value. After completion of the adsorption/desorption cycle,the sample was removed from the cell and dried at 70°C for 20 h.At least three replicates of each measurement were carried out, andthe spread of data was reported as one-half of an error bar. Thespread of data is half the difference between maximum and minimumvalues calculated from the raw data.

Infrared spectroscopy of the Na-MMT samples recovered fromthe FMC cell and dried, as outlined above, was carried out usingDRIFTS. A Spectra-Tech DRIFTS cell (with microsampling cup)was fitted to a Thermo-Nicolet Nexus FTIR bench purged withwater- and CO2-free air at 20 l min-1. The MMT samples werediluted to 5% w/w in finely ground KBr (ensuring that the samebatch was used for the entire range of samples), and the sample wasgently folded into the KBr with a microspatula. Absorbance spectrawere made up of 164 scans with resolution set to 4 cm-1. The peakareas of the C-H and Si-O stretching vibration bands (A(C-H) andA(Si-O), respectively) were determined using a two-point baselinethat also corresponded to the integration limits. For the C-Hstretching bands, the baseline/integration limits were 3124-2740cm-1, and those for the Si-O stretching bands of the MMT were1323-766 cm-1. These peak areas were used to calculate the peakarea ratio (A(C-H)/A(Si-O)) that can be used as a relative measure ofthe amount of probe retained by the Na-MMT. To more effectivelyhighlight IR bands associated with adsorbed species, spectralsubtraction was carried out using heptane-washed Na-MMT as areference (i.e., MMT that had been washed with heptane in the FMCcell and dried in the same manner as the samples). Peak areameasurements were carried out on unsubtracted spectra only.

WAXS patterns were obtained using a Philips PW 1729 X-raygenerator fitted with a Philips PW3050 goniometer. The Cu KR tube(nickel filter, wavelength 0.1541 nm) was used at an anode voltageof 50 kV and anode current of 40 mA. The divergence slit was setto automatic mode, and the receiving slit was 0.2 mm. The systemwas run using Philips X’Pert software over a 2θ scanning range of1-10° at a scan rate of 0.3° min-1 and step size of 0.01°. Samplepreparation was as follows: silicone paste was thinly and evenlyspread onto a coupon cut from a silicon single crystal, and a layerof clay was then smoothly spread on the paste. The sample was thenplaced in the diffractometer for analysis.

Results and Discussion

Dehydration and Rehydration of Na-MMT. The level ofambient moisture adsorbed onto Na-MMT decreased by around10% w/w on drying at 140°C for 3 h. Readsorption of wateroccurred relatively quickly; within 7 h under ambient temperatureand humidity ca. 10% w/w water had readsorbed. On loadingthe DRIFTS cell or FMC cell, the dried samples were exposedto ambient humidity for less than 2 min. In this way, the adsorptionof loosely bound water was kept below ca. 0.3% w/w. TheDRIFTS spectrum for dried Na-MMT showed a decrease inintensity of bands between 3500 and 3100 cm-1 relative to thatfor undried Na-MMT. From the peak assignments of wateradsorbed on MMT given by Bishop et al.,18 the latter bands canbe assigned to loosely bound water. Water molecules bounddirectly to cations (i.e., those forming the inner hydrationsphere) absorb at 3620 and 3550 cm-1. The intensity of thesebands did not decrease under the drying conditions used, indicatingthat in this MMT water was removed only from the followinglocations:

1. Water associated with edge hydroxyl groups (mainly Si-OH and Al-OH).

2. Water from the outer hydration spheres around the metalcations (associated with the inner hydration sphere via water towater hydrogen bonding).

3. Water associated with surface oxygen atoms.The WAXS patterns of undried Na-MMT and dried Na-MMT

show basal reflections of 2θ ) 7.1° and 7.2°, respectively,indicating that the drying of Na-MMT does not greatly influencethe basal spacing of the Na-MMT used in this study. The interlayerspacings corresponding to these reflections are 0.29-0.31 nm,which is consistent with the thickness of a water molecule (0.30nm). Therefore, in this case, water forming the inner hydrationsphere effectively sets the interlayer spacing.

Swelling Volume.The swelling volumes for undried and driedNa-MMT in heptane and the neat probes, together with theirdielectric constants, are shown in Table 1. Swelling volumeswere determined as a prelude to the heat of wetting determination.Medout-Marere et al. showed that when the contact angle iszero, the wetting enthalpy is the product of surface enthalpy ofthe liquid and the specific surface area of the MMT.20Therefore,swelling of Na-MMT will lead to an increase in the surface area,which affects the heat of wetting. The heat of immersion ofMMT in water has been studied by Me´dout-Marere et al.,20 whoshowed that the MMT was expected to give heat of immersionenthalpies in water in the order of 0.3 J m-2 and the BET specificsurface area of MMT was between 43 and 50 m2 g-1; the MMTwas therefore expected to give an immersion heat in water ofabout 13 J g-1 but 48 J g-1 was obtained experimentally. It wasconcluded that this observation resulted from the increase ofspecific surface area of Na-MMT in water, and obtained specificsurface areas of 110 and 277 m2 g-1 in low- and high-pressurewater vapor, respectively.20 Moreover, from the experimentalviewpoint excessive swelling of Na-MMT in the FMC cell couldlead to incorrect data being obtained and/or blockage of the filterin the cell outlet connector. It is immediately apparent that theswelling volumes of Na-MMT in amines are higher than that inthe other compounds. Water adsorbed in undried Na-MMTfacilitated swelling due to an increase in gallery polarity. Withboth dried and undried Na-MMT, the three alcohols, diethylether, and heptane gave similarly low swelling volumes. Thisdata clearly indicate that polarity is not the sole determiningfactor because dielectric constants of amines are smaller thanthose of alcohols. De´kany showed that the swelling volume isinfluenced by two major factors: the relative ease of penetrationof solvent molecules between stacks of platelets (tactoids) thatare assembled into aggregates, and penetration of solventmolecules between platelets within tactoids (i.e., intercalation).21

The former effect will influence the degree to which the tactoidsseparate from the aggregates (disaggregation), which is studied

(20) Medout-Marere, V.; Belarbi, H.; Thomas, P.; Morato, F.; Giuntini, J. C.;Douillard, J. M.J. Colloid Interface Sci.1998, 202, 139-148.

(21) Dekany, I. Pure Appl. Chem.1992, 64, 1499-1509.

Table 1. Swelling Volumes of Na-MMT in Various Probes andSolvents for Undried Na-MMT and Dried Na-MMT, and

Dielectric Constants of These Probes

swelling volume (cm3)

probesdielectricconstant

undriedNa-MMT

driedNa-MMT

ethanol 24.3 0.48 0.421-butanol 17.8 0.48 0.422-propanol 18.3 0.52 0.421-butylamine 4.5 0.87 0.65octylamine 1.06 0.68diethyl ether 4.3 0.52 0.42heptane 1.9 0.52 0.42

Interfacial Interactions in PLSN Langmuir, Vol. 24, No. 5, 20081945

by Medout-Marere et al.20 as mentioned earlier. The latter effectwill influence the amount of intercalation by solvent that maylead to an increase in the layer spacing (interlayer swelling). Thebasal spacing of Na-MMT in the liquids was investigated byDekany et al.,22 who showed that the basal spacing of Na-MMTin methanol (1.59 nm) and in benzene (1.42 nm) is higher thanthat in the dried state (1.23 nm), which indicated that interlayerswelling took place via intercalation of the MMT by the molecules.Both these effects led to higher swelling of Na-MMT in amines.Aragon et al.’s study, comparing the basal spacings of neutralamine complexes with MMT, showed that the basal spacing ofn-amine/MMT complexes rose linearly with the chain length ofan amine containing between four and 16 carbons.23 DRIFTSspectra provide some insight into the interactions between theamines and Na-MMT; spectra of the Na-MMT sample recoveredafter measuring its swelling volume in 1-butylamine, showed ashift in the N-H stretching bands from 3367 and 3290 cm-1 (inunbound 1-butylamine) to 3248 and 3181 cm-1, when adsorbedin Na-MMT. The reduction in energy of the vibrations indicatesinteraction with water molecules and/or the Na+ ions themselves,thus indicating that the amine is intercalated into the Na-MMT.Formation of such intercalated structures led to the increase inswelling volume. Arago´n et al. reported that the basal spacingof an octylamine/MMT complex was 2.85 nm, whereas that ofa 1-butylamine/MMT complex was 2.30 nm.23 This differencein basal spacing explains the higher swelling volume of the former(Table 1).

Heat of Wetting. In FMC, adsorption of probes from dilutesolution is usually studied; therefore, the heat of interactionbetween the probe and the substrate is actually composite valueof the heat of desorption of the carrier solvent, the heat ofadsorption of the probe, and the heat of demixing of the probeand carrier solvent. If the first and last items are small, then theheat of interaction becomes closer to a true heat of adsorption.It is therefore important to have an appreciation for the heat ofinteraction between the carrier solvent and the Na-MMT andindeed the heat of interaction between the neat probes and theMMT. Under these experimental conditions, the only speciesbeing displaced are the components of air and perhaps someadsorbed water. Whether or not there is sufficient vapor to adsorbonto the substrate in the vapor front that probably precedes themoving liquid front, before initial wetting of the substrate in theFMC cell, is a moot point. With the latter in mind, we havesteered away from describing the exotherm as a heat of immersionand have used the term heat of wetting instead. The heats ofwetting of dried Na-MMT in the neat probes and heptane FMCcarrier solvent are shown in Figure 1. For visual convenience,the sign of the exothermic energy has been reversed. Unfortu-

nately, attempts to measure heats of wetting for undried Na-MMT were unsuccessful because of the consistent appearanceof a strong endothermic peak that may be associated withdesorption of water. In Figure 1, it is evident that 1-butylaminegave the highest heat of wetting, followed by octylamine andthen ethanol, with heptane being the smallest. Me´dout-Marereet al.20 showed that the heat of wetting of Na-MMT in water is-33 J g-1, and thus the heat of wetting obtained for Na-MMTin 1-butylamine was almost 2.5 times that in water. Moreover,DRIFTS spectra revealed that N-H stretching bands of Na-MMT, recovered after measuring the heat of wetting in1-butylamine, were shifted to lower energy (110-120 cm-1)relative to those of the pure amine. Therefore, the higher heatof wetting in amines is due to both an increase in the specificsurface area of Na-MMT in amine and the exothermic ion-dipole interaction between the amine and sodium ions. Althoughthe heats of wetting of alcohols are considerably higher thanthose of heptane and diethyl ether, they are somewhat lower thanthose of the amines. This may be due to the alcohols interactingmainly with the hydroxyl groups at the platelet edges, with littleor no penetration into the galleries. This argument is certainlysupported by the swelling volume data. Barshad showed that thebasal spacings ofn-alcohol/MMT complexes did not changebetween one and eight carbon atoms.24 Moreover, the DRIFTSspectra of Na-MMT samples, taken after measuring their swellingvolumes in alcohols, do not show adsorption of alcohols. Thisresult indicates that alcohols adsorbed on Na-MMT evaporatedupon air-drying and, consequently, that interactions between thealcohols and the Na-MMT are weak. However, intercalation ofalcohols should perhaps not be completely ruled out as theadsorption density may be so low that the alkyl tails lie parallelto the platelet surface; the thickness of an alkyl tail is not vastlydifferent from that of a monolayer of water (Table 2); therefore,no swelling will be observed (see later). It is also apparent thatthe interaction between diethyl ether and Na-MMT is weakbecause the swelling volume and heat of wetting of diethyl etherare almost the same as those of heptane.

Adsorption Behavior of Alcohols and Amines from Hep-tane. FMC can determine not only the heat of adsorption anddesorption but also the amount of probes adsorbed and desorbed.Unfortunately, the amount of 1-butylamine adsorbed/desorbedcould not be determined because the refractive index of1-butylamine is too close to that of heptane. For visualconvenience, the sign of the exothermic energy has been reversed.We may not need to take into account the swelling (interlayerswelling and disaggregation) of Na-MMT in the probe solutionbecause the concentration of probes is very low. On the otherhand, although we need to keep in mind that the swelling volumeof undried Na-MMT in heptane is higher than that of dried Na-MMT, dried Na-MMT shows stronger adsorption activity relativeto undried Na-MMT; therefore, the observations are not likely(22) Dekany, I.; Szanto, F.; Nagy, L. G.J. Colloid Interface Sci.1986, 109,

376-384.(23) Aragon, F.; Ruiz, J. C.; MacEwan, D. M. C.Nature (London)1959, 183,

740-741. (24) Barshad, I.Soil Sci. Soc. Am. Proc.1952, 16, 176-182.

Figure 1. Heats of wetting for dried Na-MMT.

Table 2. Sizes of Probes Determined by the Space FillingModels of MOPAC Computation

probesheight(nm)

width(nm)

length(nm)

ethanol 0.42 0.46 0.641-butanol 0.42 0.46 0.892-propanol 0.48 0.52 0.671-butylamine 0.42 0.46 0.91octylamine 0.42 0.46 1.41diethyl ether 0.42 0.47 0.904-EPI 0.42 0.67 1.11water 0.30 0.33 0.39

1946 Langmuir, Vol. 24, No. 5, 2008 Kato et al.

to stem entirely from differences in available surface area arisingfrom different degrees of swelling. The FMC data for undriedNa-MMT and dried Na-MMT are shown in Figure 2. It is evidentthat the preadsorbed water molecules on undried Na-MMT mostnoticeably affected the adsorption activity of the alcohols. Theadsorption energy of ethanol on dried Na-MMT is about threetimes higher than that on undried Na-MMT, and the amount ofethanol adsorbed on dried Na-MMT is around twice that onundried Na-MMT. Moreover, the drying of Na-MMT resultedin greater retention of the probes. About 60% of the initial amountof ethanol remained on dried Na-MMT after the desorption; incontrast, all ethanol was removed from undried Na-MMT duringdesorption (i.e., flushing through with heptane). Water moleculesthat are adsorbed on the basal surface, platelet edge hydroxylgroups, and as the looser-bound outer hydration sphere aroundthe sodium ions will hinder the adsorption of alcohols and leadto weak interaction with undried Na-MMT. Alcohols thereforemainly interact with undried Na-MMT via the adsorbed water;such an indirect interaction (via a water bridge) will inevitablybe weak. After (140°C for 3 h) the Na-MMT is dried, the innerhydration sphere around the sodium ions still exists and interactionwith the sodium ion will still be via water bridges, though innersphere water will be more polarized than outer sphere water.Adsorption activity on dried Na-MMT is therefore mainlyenhanced because of exposure of edge hydroxyl groups and the

highly polarized inner sphere water. The energies of the specificinteractions are best compared by examining the molar heats ofadsorption (Figure 3). Although, for reasons given previously,the molar heats of adsorption were predictably greater onto driedNa-MMT, the heats of interaction were similar regardless ofalcohol structure. These results indicate that steric access, ratherthan differences in the energy of the interaction, is the mainfactor influencing the adsorption activities of alcohols.

The interaction of amine probes with MMT did not appear tobe significantly affected by the presence of preadsorbed water;both the energies and levels of adsorption were similar for bothdried and undried Na-MMT, with similar levels of amine retentionbeing evident in both cases. On DRIFTS spectra of undried Na-MMT recovered from the FMC cell after treatment with amines,the N-H stretching band is shifted to lower energy relative tothe unbound (pure) amine. This indicates that the amines maystrongly interact with undried Na-MMT via preadsorbed watermolecules. The latter water molecules may be acidic as a resultof polarization arising, for example, from interaction with anisolated silanol group. The amount of probe retained on Na-MMT can be compared using the infrared absorbance peak arearatio A(C-H)/A(Si-O). The area ratio of dried Na-MMT aftertreatment with octylamine (0.018) is the same as that of undriedNa-MMT (0.019), which is entirely consistent with the FMCdata. The strength of interactions between amines and Na-MMTwas compared by monitoring the area ratio after heating at150°C for 20 h.A(C-H)/A(Si-O) for undried Na-MMT after FMCtreatment with octylamine decreases by 30% after heating becauseof the vaporization of octylamine, whereas theA(C-H)/A(Si-O) fordried Na-MMT remained unchanged. These results indicate thatpreadsorbed water molecules do not affect the amount of amineadsorbed on Na-MMT but do affect the strength of interactionswith the MMT surfaces. It is interesting that the adsorption energyof amines from heptane solution is smaller than that of ethanol,whereas the heats of wetting of amines are higher than that ofethanol. This data may be due to the disaggregation andintercalation of Na-MMT in the neat amines. Amines are expectedto penetrate the Na-MMT tactoids and split up the platelets,hence increasing the effective specific surface area and givingrise to a large heat of wetting. However, in FMC the amineconcentration in the heptane carrier fluid is too low to give riseto disaggregation and intercalation, and the effective surfacearea is therefore low and is reflected in the relatively smalladsorption energy. The heats of wetting of Na-MMT in alcoholsare approximately the same as the heats of adsorption of alcoholsfrom heptane solution, suggesting that the effective surface areasof Na-MMT in the neat alcohols are the same as those in dilutealcohol/heptane solutions used in the FMC study. Drying of theNa-MMT resulted in a reduction in the molar heat of adsorptionof octylamine from heptane, which supports the idea that thenumber of acidic adsorption sites (probably polarized water

Figure 2. FMC showing the effect of preadsorbed water: (a) undriedNa-MMT and (b) dried Na-MMT. NM) not measurable becauseof similarity of the refractive indices of 1-butylamine and heptane.Note that the sign of energy change for adsorption is reversed to aiddata comparison.

Figure 3. Molar heats of adsorption for undried and dried Na-MMT.

Interfacial Interactions in PLSN Langmuir, Vol. 24, No. 5, 20081947

molecules associated with edge hydroxyl groups) reduced. Afterthe Na-MMT was dried, the reduced molar heat of adsorptionof octylamine approached the increased molar heats of adsorptionof alcohols; this is likely to be due to a predominance of electrondonor/acceptor interactions (interactions of the lone pairs of thehydroxyl oxygen atoms and amine nitrogen atoms with the gallerysurface and edge adsorption sites). The similarity of the molarheat of adsorption of diethyl ether on dried Na-MMT to that ofoctylamine and the alcohols lends further support to thepredominance of electron donor/acceptor interactions.

Values of interlayer spacing (determined by WAXS) of Na-MMT recovered from the FMC cell, after adsorption/desorptionof the alcohol and amine probes, did not show any change relativeto pristine Na-MMT. This result suggests that these probes werecertainly not adsorbed in the galleries in a closely packed,vertically orientated manner. This is particularly true of undriedNa-MMT because of the steric hindrance caused by watermolecules forming the outer hydration spheres around the Na+

ions. Therefore, these probes are mainly expected to interactwith the top and bottom surfaces of tactoids and with plateletedge hydroxyl groups. However, the wide end of the interlayerspacing distribution (discussed later) is sufficient to accommodatethe probes if they lie parallel to the silicate platelets. The basalreflection for Na-MMT appears as a broad peak with a maximumat 2θ ) 7.1-7.2°, and by Bragg’s rule this corresponds to a basalspacing of 1.23-1.25 nm. Taking 0.94 nm as the platelet thicknessof Na-MMT,25 we find that the interlayer spacing is 0.29-0.31nm. However, the basal reflection is quite broad, reflecting adistribution of basal spacings. The low and high tails of the peak(i.e., 2θ ) 6.0° and 8.0°) correspond to a distribution of interlayerspacing, varying between 0.17 and 0.53 nm. High levels ofdisorder close to the edges of tactoids contribute significantlyto the wide end of the interlayer spacing distribution. Vali andKoster showed that the aggregates of MMT are composed oftactoids that contain a core of coherent silicate platelets surroundedby disordered and bent silicate platelets with frayed edges.26Themost stable conformations of the probes were determined byMOPAC computation using parametric method 3 (PM3) theory,and the sizes of probes were estimated from the space-fillingmodel (Table 2). The minimum thickness of a water molecule(0.30 nm) coincides with the average interlayer spacing of Na-MMT (0.29-0.31 nm). Moreover, all the probes can beaccommodated in the gallery if they lie parallel to the platelets,particularly if the wider interlayer spacings close to tactoid edgesare considered.26

Adsorption Behavior of Diethyl Ether and PPG. Diethylether was chosen to assess the contribution of ether groups inpolyether polyols, though it is appreciated that the ether groupsof PPG are likely to be more sterically hindered than those ofdiethyl ether. Both the energy and amount of adsorption of diethylether on undried Na-MMT were very low, and the adsorptionwas fully reversible. Drying of the substrate increased the energyof adsorption 5-fold to 2.0 J g-1 and doubled the amount adsorbedto 2.8 mg g-1. Roughly half of the diethyl ether was retainedafter desorption from dried Na-MMT. The heat of adsorption (inJ g-1) of diethyl ether onto dried Na-MMT was about four timeslower than those of 1-butanol and 1-butylamine (which are ofsimilar molecular size to diethyl ether). This is due to ethergroups having only hydrogen bond acceptor characteristics byvirtue of the lone electron pairs on the ether oxygen. The alcoholsand amines, however, are both H-bonding acceptors (O and Nlone pairs) and donors (the alcohol and amine hydrogen atoms)

and can therefore interact with a wider range of surface sites.Consideration of heat of adsorption per mole of probe, however,yields similarity between adsorption of diethyl ether, alcohols,and octylamine onto dried Na-MMT and may mean that thecontribution of the terminal OH group of the PPG to adsorptionon dried Na-MMT is relatively insignificant. On dried Na-MMTthe stronger retention of ether groups could result in reducedaccess of the terminal OH of PPG to the Na-MMT surface.However, with undried Na-MMT the contribution of the terminalOH group to the overall adsorption activity is likely to be greateras the adsorption of alcohols was less affected by preadsorbedwater than adsorption of ethers.

From the thermal data, both PPGs appeared to be stronglyretained on Na-MMT as the heat of desorption was zero or closeto zero. This high level of retention is due to multipoint adsorption,where several ether groups in a given PPG chain interact withthe silica surface. Even though the individual ether-MMT surfaceinteractions are weak, desorption of the PPG will only occur ifsimultaneous detachment of all interacting ether groups occurs.As the probability of such an occurrence is small, the overallretention of PPG on the Na-MMT is high. Although the thermaldata indicated little or no desorption of PPG, determination ofamounts desorbed using the differential refractometer datarevealed that some desorption did in fact occur and was mostnoticeable in the case of the dried Na-MMT. This may be relatedto removal of PPG that is weakly associated with the initialmonolayer via loose entanglement and/or intermolecular interac-tion. No significant energy change could be resolved duringdesorption because of the very weak nature of these interactions.The molar heats of adsorption (per repeat unit) of the PPGs ontodried Na-MMT are lower than that for diethyl ether; increasingthe molar mass of the PPG gave rise to a further reduction inmolar heat of adsorption. Generally, conventional anionic ring-opening polymerization of propylene oxide results in an atacticpolymer.27 Therefore, even if the PPG could adsorb flat as anextended chain, it will not be possible for all the ether groupsto make close contact with the silicate surface, because of sterichindrance by the methyl groups. Bearing in mind that the PPG340 contains on average 4.6 propylene glycol-derived repeatunits per chain, it is conceivable that such a short chain couldlie relatively flat on the surface and hence interact with it viamost of the accessible ether linkages and the terminal OH group;the lower heat of adsorption in this case may therefore be at leastpartly due to increased steric hindrance of the ether groups relativeto that of diethyl ether. The PPG 1000 contains on average 16propylene glycol-derived units per chain; the greatly increasednumber of conformations will inevitably result in only a fractionof the ether linkages being in contact with the surface at a giventime, and therefore a reduction in the molar repeat unit heat ofadsorption of PPG 1000 (relative to PPG 340) is expected.

Although preadsorbed water reduced the molar heat ofadsorption of diethyl ether, the molar repeat unit heat of adsorptionof PPG was relatively unaffected by preadsorbed water. Thelatter is somewhat surprising considering steric issues and thegeneral effect of water on the adsorption of ether (Figure 3).Because of the latter, and the indication that adsorption of alcoholis less affected by preadsorbed water, it is highly likely that therelative contribution of the terminal OH group of PPG toadsorption activity is greater on undried Na-MMT.

Preadsorbed water on Na-MMT (i.e., undried Na-MMT) ledto higher levels of PPG adsorption and a greater proportion ofthe PPG being retained (Figure 2 and confirmed by DRIFTS; not

(25) MacEwan, D. M. C.Trans. Faraday Soc.1948, 44, 349-367.(26) Vali, H.; Koster, H. M.Clay Miner.1986, 21, 827-859.

(27) Shibatani, K.; Lyman, D. J.; Shieh, D. F.; Knutson, K.J. Polym. Sci.1977, 15, 1655-1666.

1948 Langmuir, Vol. 24, No. 5, 2008 Kato et al.

shown). This observation is particularly true of PPG 340 and,on first consideration, is somewhat unexpected based on thehigher adsorption activities of both alcohols and diethyl etheronto dried Na-MMT. Although it is acknowledged that furtherwork is required to properly explain this apparent discrepancy,it may be proposed that removal of the weakly adsorbed wateris likely to cause the ether groups to be adsorbed with similarlyhigh energy to the terminal OH group of PPG (Figure 3). Thisset of circumstances is likely to lead to multipoint contact ofPPG chains with the surface (i.e., relatively flat adsorption) andhence a large area of surface coverage; the level of adsorptionwill therefore be low. However, PPG adsorption onto undriedNa-MMT is likely to be dominated by the terminal hydroxylgroup (Figure 3). This is likely to lead to a change in orientationof adsorption from relatively flat (i.e., dried Na-MMT) to moresingle point in nature. In the case of PPG 340, it could be arguedthat near-vertical adsorption could occur, thus enabling closerpacking of the PPG and a higher level of adsorption. Strongerintermolecular interaction between the adsorbed molecules mayoccur as a result of closer packing and could lead to greaterretention of PPG on undried Na-MMT. The increased numberof weak adsorption sites on the latter (formed by the looselybound water, perhaps mostly on exposed basal surfaces) cannotbe ignored either and may also contribute to the closer packingof PPG and the generally higher level of adsorption.

WAXS data can assess the occurrence of intercalation of PPGinto the gallery of Na-MMT (Figure 4). Both undried Na-MMTand dried Na-MMT after adsorption/attempted desorption of PPG340 did not show any change in the shape or position of the 001reflection relative to pristine Na-MMT. The same was also trueafter adsorption/attempted desorption of PPG 1000 onto undriedNa-MMT. The space-filling model for atactic PPG calculated byMOPAC computation is illustrated in Figure 5. The modelstructure for atactic PPG indicates a minimum thickness of 0.58nm, which is greater than the wide end of the interlayer distancedistribution of the Na-MMT (i.e., 0.53 nm). Therefore, whenNa-MMT is undried, PPG was not intercalated into the galleryto any significant (WAXS resolvable) extent, but was most likelyto be adsorbed on the external basal and edge surfaces. On theother hand, the WAXS pattern of dried Na-MMT after adsorption/attempted desorption of PPG 1000 has a broad peak at 2θ )5-6°, which corresponds to an interlayer spacing between 0.53and 0.83 nm. The latter is consistent with the theoretical minimumthickness of 0.58 nm of an atactic PPG chain. Therefore, a

measurable fraction of the Na-MMT was intercalated with PPG1000. It is somewhat surprising that intercalation of dried Na-MMT with PPG 340 could not be resolved. The difference inbehavior indicates that interaction between the terminal hydroxyland gallery adsorption sites is not driving the intercalation process;if this was the case, intercalation would not be so dependent onPPG molar mass and could have resulted in higher levels ofintercalation with PPG 340. In the case of PPG 340, it may beproposed that interaction between the ether oxygen atoms andedge hydroxyl groups resulted in segments that were too shortto enable intercalation. With the PPG 1000, the reduced frequencyof ether group interaction (as indicated by the FMC data) resultedin longer nonadsorbed chain segments (loops) that may possesssufficient mobility to enter the galleries and adsorb, hence openingthe way for intercalation of further PPG chains. The inability ofPPG 1000 to intercalate undried Na-MMT was most probablydue to the interactions between the ether groups of the PPGchain and water adsorbed within the gallery being too weak toresult in further separation of the platelets and intercalation.

Adsorption Behavior of 4-EPI. It is apparent that adsorptionenergy of 4-EPI (per unit mass of substrate) is low relative tothat of the alcohol probes; however, it must be appreciated thatthe adsorption process of 4-EPI was revealed to be completelydifferent and mainly involves the weakly adsorbed water (in thecase of undried Na-MMT) and platelet edge hydroxyl groups inthe case of dried Na-MMT. The relatively small edge surfacearea (ca. 5% of the total) explains the low substrate mass-relatedadsorption energy. The thermal FMC data for adsorption of 4-EPIonto undried Na-MMT (Figure 6a) were unusual in that after theexothermic peak the data did not return to the baseline, thuscomplicating determination of a peak area associated with heatof adsorption. The refractometry data, however, appeared normalin reaching a limiting value after the initial peak. Continuedreaction in the cell after adsorption saturation appeared to betaking place; the limiting value of cell effluent refractive indexmay be misleading as reaction products of similar refractive

Figure 4. WAXS patterns of Na-MMT ((a) undried and (b) dried)collected from the FMC cell after an adsorption/desorption cyclefrom heptane with PPG 340 and PPG 1000 as probes. WAXS patternsof undried and dried Na-MMT are shown for comparison.

Figure 5. Cross-sectional (end) view of a space-filling model ofan atactic PPG chain (determined by MOPAC computation).

Figure 6. FMC data for adsorption of 4-EPI on (a) undried Na-MMT and (b) dried Na-MMT.

Interfacial Interactions in PLSN Langmuir, Vol. 24, No. 5, 20081949

index to 4-EPI could also be present. Further work to resolve thisaspect is ongoing. Because of the difficulty in obtaining a trueheat of interaction relating to adsorption, it is not possible toobtain an accurate value of molar heat of adsorption. A valuefor the overall heat of reaction over the run time of the experimenttogether with the amount initially adsorbed will give an impossiblylarge value of molar heat of adsorption. The value of heat ofinteraction obtained from the small initial thermal peak (dottedline in Figure 6a) combined with the amount initially adsorbedgave a molar heat of adsorption of-160 kJ mol-1, which couldbe realistic as it is similar to that obtained by Artavia28for reactionof isocyanate with water to produce soluble urea. No desorptionof 4-EPI from undried Na-MMT was observed and thus confirmedstrong interaction. As it was considered that 4-EAN may resultfrom reaction between 4-EPI and hydroxyl groups, adsorptionof 4-EAN onto undried Na-MMT was also investigated and wasfound to be weakly retained but, nevertheless, yielded a molarheat of adsorption similar to that of the octylamine and alcohols.

DRIFTS of undried Na-MMT treated with 4-EPI (Figure 7)revealed the disappearance of the isocyanate (NdCdO) stretchingband (2273 cm-1)29 together with the development of strongN-H stretching bands at 3411 and 3301 cm-1. These resultsindicated that the 4-EPI may react to form an amine or amide,and it is well known that isocyanate groups will react with waterto produce amine groups. Within the functional group region,the characteristic IR absorption bands of 4-EPI adsorbed onundried Na-MMT are the CdO stretching (amideΙ) band at1634 cm-1, the amideΙΙ combination band at 1559 cm-1, andthe amideΙΙΙ combination band at 1234 cm-1, while the bandsat 1608 and 1514 cm-1 are due to the C-H bending and C-Cstretching in the aromatic ring, respectively. From the assignmentof the amide bands,29 the product is identified as a urea group.The isocyanate group of 4-EPI will react with water to producean amine (4-ethylaniline) (Scheme 1); the latter will then reactwith the isocyanate group of an adjacent 4-EPI molecule, therebyjoining the two molecules via a urea linkage. It is highly likelythat adsorption (possibly via adsorbed water) of the 4-EPI, andsubsequent formation of 4-EAN, facilitated formation of theurea linkage. The resultantN,N′-bis(4-ethylphenyl) urea may

have less affinity for the Na-MMT surface than the reactants andtherefore readily desorbs, leaving vacant surface available forfurther reaction. This process may continue until all accessiblewater is consumed. The continued generation of heat within theFMC cell, even after apparent saturation adsorption, may supportthe idea that the urea is easily desorbed. After desorption, onlythe most strongly adsorbedN,N′-bis(4-ethylphenyl) urea remainedon the undried Na-MMT. The CdO stretching band of “free”(i.e., non-hydrogen-bonded) urea appears at 1710-1715 cm-1,while 1640 cm-1 is the band of the hydrogen-bonded urea.29Theurea group adsorbed on Na-MMT is hydrogen bonded, asindicated by a strong peak at 1634 cm-1. Such hydrogen bondingmay be characteristic of the small strongly retained fraction ofN,N′-bis(4-ethylphenyl) urea observed in the DRIFTS spectra.WAXS of undried Na-MMT treated in the FMC cell with 4-EPIand 4-EAN revealed no increase in interlayer spacing relativeto pristine MMT. This in itself does entirely rule out intercalation,as theN,N′-bis(4-ethylphenyl) urea can probably be accom-modated within the more widely spaced Na-MMT platelets foundclose to the edges and in the top and bottom regions of tactoids.The lack of observable intercalation together with the relativelylow intensity of IR absorption bands indicated that very littleN,N′-bis(4-ethylphenyl) urea was retained.

Thermal FMC data for adsorption of 4-EPI onto dried Na-MMT (Figure 6b) were typical of adsorption to a saturationlevel; the adsorption exothermic peak returned to the baselineand the refractometry data reached a stable limiting value. Therewas no evidence of further reaction after the saturation adsorptionlevel was obtained. The molar heat of adsorption (-112 kJ mol-1)was the highest recorded onto dried Na-MMT and could beindicative of chemical reaction; Artavia obtained a similar valuefor reaction of isocyanate with a hydroxyl group.28The adsorptionof 4-EAN onto dried Na-MMT was also investigated and wasfound to be retained slightly more strongly relative to that ofundried Na-MMT, but with similar molar heat of adsorption.

As with undried Na-MMT, DRIFTS spectra of dried Na-MMTtreated with 4-EPI revealed elimination of the isocyanate (NdCdO) stretching band (2273 cm-1) together with the developmentof weak N-H stretching bands (3418 and 3308 cm-1). TheDRIFTS spectrum of 4-EAN adsorbed on dried Na-MMT wascompared with that of 4-EPI adsorbed on Na-MMT. The N-Hstretching and bending bands (3429 cm-1/3350 cm-1 and 1622cm-1, respectively) present in unbound 4-EAN disappeared when

(28) Artavia, L. D. Ph.D. Thesis, University of Minnesota, Minneapolis, MN,1991; pp 23-25.

(29) Dillon, J. G.Infrared Spectroscopical Atlas of Polyurethanes;TechnomicPublishing: Lancaster, PA, 1990; pp 167-190.

Figure 7. DRIFTS spectra for (a) pure 4-EPI, (b) pure 4-EAN, (c) undried Na-MMT after adsorption/desorption of 4-EPI, (d) dried Na-MMTafter adsorption/desorption of 4-EPI, and (e) dried Na-MMT after adsorption/desorption of 4-EAN.

Scheme 1. Chemical Reaction of 4-EPI with Water Adsorbed on Undried Na-MMT

1950 Langmuir, Vol. 24, No. 5, 2008 Kato et al.

adsorbed, leaving only the aromatic ring C-C stretching bands(1516 cm-1). These differences suggest that the product formedon adsorption of 4-EPI onto dried Na-MMT was not 4-EAN.The characteristic peaks of 4-EPI adsorbed on dried Na-MMTare the CdO stretching band at 1684 cm-1, the amideΙΙcombination band at 1538 cm-1, and the amideΙΙΙ combinationband at 1231 cm-1, whereas the bands at 1593 and 1514 cm-1

are due to C-H bending band and C-C stretching band of thearomatic ring, respectively. From the assignment of the amidecarbonyl bands,29 the product most certainly features a urethanegroup. The latter was formed as a result of reaction between theisocyanate of 4-EPI and hydroxyl groups on the platelet edgesof Na-MMT (Scheme 2). The CdO stretching band of the freeurethane appears at 1730 cm-1, while hydrogen-bonded urethaneappears at 1710 cm-1.29 In this case, the CdO stretching bandat 1684 cm-1 of the urethane group on dried Na-MMT is oflower energy because the urethane group is attached directly toNa-MMT. The concept of chemical adsorption of 4-EPI at theplatelet edge hydroxyl groups is certainly supported by the FMCdata as saturation adsorption would be obtained once all accessibleedge hydroxyl groups have reacted. WAXS of dried Na-MMTtreated in the FMC cell with 4-EPI and 4-EAN revealed noincrease in interlayer distance relative to pristine MMT.

Conclusions

The adsorption of a range of probes, intended to the modelcomponents of a polyurethane system, onto Na-MMT has beenstudied. Particular attention was given to the effect of ambientmoisture (adsorbed on external and internal surfaces) on theadsorption characteristics of the probe set using FMC, inconjunction with DRIFTS and WAXS. Undried Na-MMT refersto the as-received sample stored in ambient humidity (containingca. 10% w/w water). Dried Na-MMT refers to Na-MMTcontaining less than ca. 0.3% w/w adsorbed water. Watermolecules adsorbed on Na-MMT affected the adsorption activitiesof probes from heptane solution, especially the alcohols. The

adsorption of alcohols was hindered by the preadsorbed watermolecules and led to weak interaction with undried Na-MMT,whereas amines could strongly interact with undried Na-MMTvia the loosely bound water molecules. The adsorption activitiesof the probes on dried Na-MMT were increased because of theexposure of edge hydroxyl groups (mainly Si-OH and Al-OH)and inner hydration sphere water surrounding Na+ ions on thesilica basal surfaces of Na-MMT. Heats of wetting (also measuredusing the FMC), and swelling volume in the neat alcohols,revealed similar energies of interaction and zero macroscopicswelling, respectively. WAXS data supported the latter, althoughflat adsorption of alcohols within the more widely spaced anddisordered tactoid edges cannot be ruled out. In contrast, the heatof wetting by the neat amines was higher than the energy ofamine interaction from solution in heptane. This indicated ageneration of higher interfacial area in the neat amine. Swellingof the Na-MMT in the neat amines was a clear manifestation ofthe latter and arose because of intercalation of the galleries bythe amines. In FMC studies, the amine concentration was toolow to give rise to such intercalation effects. The interaction ofNa-MMT with diethyl ether was weak, but was strong with PPGbecause of multipoint adsorption. WAXS revealed interestingmolar mass-related effects; with dried Na-MMT the PPG 1000was partly intercalated into the gallery, whereas PPG 340 did notgive rise to intercalation. This was thought to be due to theincreased length of chain between attachment points with PPG1000, allowing sufficient mobility to enter the gallery. When4-EPI was adsorbed on undried Na-MMT, the loosely boundwater promoted formation of 4-ethyl aniline followed by reactionof the latter with further 4-EPI to formN,N′-bis(4-ethylphenyl)urea. The FMC thermal data indicated that this reaction continuedafter saturation adsorption and therefore suggested that formationof N,N′-bis(4-ethylphenyl) urea was quickly followed by de-sorption, thereby re-exposing the MMT surface, thus enablingfurther reaction. Interaction of 4-EPI with exposed platelet edgehydroxyl groups of dried Na-MMT led to chemical adsorptionvia the formation of a urethane linkage.

Acknowledgment. We thank KUREHA Corporation, Japan,for providing funding to R.K.

LA702620C

Scheme 2. Chemical Reaction of 4-EPI with DriedNa-MMT

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