Interactions between Adsorbed Layers of Cationic Gemini Surfactants †

Interactions between Adsorbed Layers of Cationic Gemini Surfactants†

Eva Blomberg,*,‡,§ Ronald Verrall,| and Per M. Claesson‡,§

Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning KristinasVag 51,SE 100 44 Stockholm, Sweden, Institute for Surface Chemistry, Box 5607,

SE-114 86 Stockholm, Sweden, and Department of Chemistry, UniVersity of Saskatchewan,110 Science Place, Saskatoon SK S7N 5C9, Canada

ReceiVed September 21, 2007. In Final Form: October 22, 2007

The forces acting between glass and between mica surfaces in the presence of two cationic gemini surfactants, 1,4diDDAB (1,4-butyl-bis(dimethyldodecylammonium bromide)) and 1,12 diDDAB (1,12-dodecyl-bis(dimethyldode-cylammonium bromide)), have been investigated below the critical micelle concentration (cmc) of the surfactants usingtwo different surface force techniques. In both cases, it was found that a recharging of the surfaces occurred at asurfactant concentration of about 0.1× cmc, and at all surfactant concentrations investigated repulsive double-layerforces dominated the interaction at large separations. At smaller separations, attractive forces, or regions of separationwith (close to) constant force, were observed. This was interpreted as being due to desorption and rearrangement inthe adsorbed layer induced by the proximity of a second surface. Analysis of the decay length of the repulsivedouble-layer force showed that the majority of the gemini surfactants were fully dissociated. However, the degreeof ion pair formation, between a gemini surfactant and a bromide counterion, increased with increasing surfactantconcentration and was larger for the gemini surfactant with a shorter spacer length.

Introduction

Surfactants containing two hydrophilic and two hydrophobicgroups (so-called “gemini” surfactants) have been the focus ofconsiderable research interest since the early 1990s. Severalcomprehensive reviews,1-3 and even an entire book,4 have beendevoted to the behavior of gemini surfactants. Their uniquestructures, consisting of two typical single-head, single-tailsurfactant molecules linked chemically at or near the head group,show a rich array of aggregate morphologies and solutionproperties that are dependent upon the nature and size of thelinking spacer.3,5-7

Their critical micelle concentration (cmc) is typically 1-2orders of magnitude lower, and they are considerably moreefficient at reducing surface or interfacial tensions as comparedto traditional single-tail, single-head group surfactants. Variationsin the length of the spacer group are known to have a significanteffect on their aggregation properties. Gemini surfactants haveattracted considerable attention due to their intriguing and richproperties and potential applications, for example, in soilremediation, enhanced oil recovery, drug entrapment and release,and the construction of high-porosity materials.8 It has also been

shown that certain cationic gemini surfactants with low toxicityhave a superior ability to introduce genes into cells.9-12

In a previous study, the mechanism of adsorption of the cationicgemini surfactant 1,2-bis(n-dodecyldimethylammonium)ethanedibromide, abbreviated as 12-2-12, on mica was followed bysurface force and atomic force microscopy (AFM) measure-ments.13 It was observed that, well below the cmc, isolatedsurfactant molecules were adsorbed up to charge neutralizationwhereupon surfactant monolayer domains began to appear. Afurther slight increase in surfactant concentration caused thediscrete monolayer patches to join together. A continued increasein surfactant concentration led to a significant amount of materialadsorbed on top of the monolayer, as confirmed by AFM imaging.At 1.8 × 10-4 M, a bilayer was observed from surface forcemeasurements, and above this concentration an extra repulsiveforce was measured and interpreted as being due to adsorptionof a limited amount of surfactant outside the bilayer.

One objective of the present study was to examine, usingsurface force techniques, what effect lengthening the spacer grouphas on the mechanism of adsorption and the interactions inducedby the adsorbed surfactant layers. Another objective was todetermine the state of dissociation of the gemini compounds inaqueous solution below the cmc. This issue has been a topic ofdebate in the literature. Ion pairing, the possibility of binding ofa counterion by a gemini cation, and premicelle association havebeen invoked to explain results obtained in surface tension,electrical conductivity, and neutron reflectivity studies. Zana14

has attempted to show from electrical conductivity studies underwhat conditions one or the other of these factors may prevail for

† Part of the Molecular and Surface Forces special issue.* To whom correspondence should be addressed. E-mail eva.blomberg@

surfchem.kth.se.‡ Royal Institute of Technology.§ Institute for Surface Chemistry.| University of Saskatchewan.(1) Kirby, A. J.; Camilleri, P.; Engberts, J. B. F. N.; Feiters, M. C.; Nolte, R.

J. M.; Soderman, O.; Bergsma, M.; Bell, P. C.; Fielden, M. L.; Rodriguez, C. L.G.; Guedat, P.; Kremer, A.; McGregor, C.; Perrin, C.; Ronsin, G.; van Eijk, M.C. P.Angew. Chem., Int. Ed.2003, 42, 1448-1457.

(2) Menger, F. M.; Keiper, J. S.Angew. Chem., Int. Ed.2000, 39, 1906-1920.(3) Zana, R.J. Colloid Interface Sci.2002, 248, 203-220.(4) Gemini surfactants: Synthesis, Interfacial and Solution-Phase BehaVior,

and Applications; Zana, R., Xia, J., Eds.; Marcel Dekker: New York, 2003.(5) Zana, R.; Benrraou, M.; Rueff, R.Langmuir1991, 7, 1072-1075.(6) Danino, D.; Talmon, Y.; Zana, R.Langmuir1995, 11, 1448-1456.(7) Zana, R.; Talmon, Y.Nature1993, 362, 228-230.(8) Lyu, Y. Y.; Yi, S. H.; Shon, J. K.; Chang, S.; Pu, L. S.; Lee, S. Y.; Yie,

J. E.; Char, K.; Stucky, G. D.; Kim, J. M.J. Am. Chem. Soc.2004, 126, 2310-2311.

(9) Fielden, M. L.; Perrin, C.; Kremer, A.; Bergsma, M.; Stuart, M. C.; Camilleri,P.; Engberts, J. B. F. N.Eur. J. Biochem.2001, 268, 1269-1279.

(10) Ronsin, G.; Perrin, C.; Guedat, P.; Kremer, A.; Camilleri, P.; Kirby, A.J. Chem. Commun.2001, 2234-2235.

(11) Rosenzweig, H. S.; Rakhmanova, V. A.; MacDonald, R. C.BioconjugateChem.2001, 12, 258-263.

(12) Badea, I.; Verrall, R.; Baca-Estrada, M.; Tikoo, S.; Rosenberg, A.; Kumar,P.; Foldvari, M.J. Gene Med.2005, 7, 1200-1214.

(13) Fielden, M. L.; Claesson, P. M.; Verrall, R. E.Langmuir1999, 15, 3924-3934.

(14) Zana, R.J. Colloid Interface Sci.2002, 246, 182-190.

1133Langmuir2008,24, 1133-1140

10.1021/la702940p CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 12/04/2007

gemini surfactants of the m-s-m series. Nevertheless, differencesstill prevail between the conclusions reached in these experimentsand by neutron reflectivity studies.15

In an attempt to shed further light on these matters, wehypothesized that the use of an independent technique in whicha comparison of the decay lengths of the repulsive double-layerforces derived from surface force measurements of members ofthe 12-s-12 gemini series with calculated Debye lengths for a1:1 and 2:1 electrolyte, using the nonlinear Poisson-Boltzmanntheory, might be helpful in explaining these differences. Membersof the gemini series m-s-m with m) 12 and s) 4 and 12 werechosen because conductance studies showed no evidence of ionpairing for s) 4 and 12 but evidence of premicelle associationfor s ) 12,14 while a comparison of neutron reflectivity resultswith those from surface tension measurements could be interpretedas indicating that ion pairing occurs for s) 4 and 12.15

Materials and Methods

Materials. The synthesis of the 12-4-12 (1,4 diDDAB, 1,4-butyl-bis(dimethyldodecylammonium bromide)) and 12-12-12 (1,12 diD-DAB, 1,12-dodecyl-bis(dimethyldodecylammonium bromide)) gem-ini surfactants has been reported previously.5,16 The purity of thesurfactants was assessed from aqueous surface tension measurements;the absence of a minimum in the post cmc region of a gamma vslog C plot indicated a pure surfactant devoid of surface-activeimpurities. Elemental analysis showed better than 99.5% agreementwith the theoretical molecular formula. The chemical structures wereconfirmed by using 1H NMR in CDCl3 (Bruker 500 MHz). KBr(Merck, spectroscopy grade) was used as received. Milli-Q gradewater, with a resistivity of 18.2 MΩ cm and total organic carboncontent of less than 10 ppb, was obtained from a Millipore systemcomprising RiOs-8 and Milli-Q+ 185 units.

Surface Preparation.For use in the interferometric surface forceapparatus (SFA), thin muscovite mica sheets (1-3 µm) were silver-coated on one side by metal vapor deposition and glued (silver sidedown) onto polished cylindrical silica discs. The handling of thesesurfaces was done in a laminar flow cabinet to minimize the riskof contamination. The surfaces for the non-interferometric surfaceforce apparatus (MASIF) were prepared by melting the end of aborosilicate glass rod in a butane/oxygen flame until a sphericaldrop with a radius of ca. 2 mm was formed. The surfaces weremounted in the force-measuring instrument immediately afterpreparation. Each set of measurements was performed on freshlyprepared surfaces.

Surface Force Measurements.Forces between adsorbed layersof one of the gemini surfactants (1,12 diDDAB or 12-12-12) wereinvestigated by the interferometric SFA,17 employing the Mark IVmodel.18 The surfaces were mounted inside the measuring chamberin crossed cylinder geometry. The separation between the surfaceswas controlled by a motor or by applying a voltage to a piezoelectriccrystal to which the upper surface was attached. The absolute surfaceseparation was determined, interferometrically, by using fringes ofequal chromatic order (FECO). The force acting between the surfaceswas obtained from the deflection of a double cantilever spring holdingthe lower surface. In the beginning of each experiment, the baremica surfaces were brought into contact in dry air to obtain the zerocontact separation in the absence of the adsorbed layers. The absolutesurface separation and the thickness of the surfactant films werethen obtained from the shift of the FECO interference pattern relativeto the contact between bare mica surfaces in air. Experiments inaqueous surfactant solutions were performed by filling the gapbetween the surfaces with a drop of the surfactant solution.

A non-interferometric surface force apparatus (measurements andanalysis of surface interactions and forces, MASIF)19 was also usedfor measuring interactions between adsorbed surfactant layers. TheMASIF instrument, like atomic force microscopy, uses indirectsurface separation detection where the distance between the surfacesis determined relative to a hard wall contact by fitting a straight lineto the region of constant compliance. Thus, this method does notallow the thickness of a firmly adsorbed layer to be determined. Theforce is determined by the charge output from a piezoelectric bimorphforce sensor, which is proportional to its deflection. The forcemeasurements were carried out at low driving speeds to allow oneto ignore hydrodynamic interactions.20Details concerning the MASIFtechnique can be found elsewhere.19,21

The force profiles obtained in 1:1 electrolyte were compared withthe electrostatic double-layer force calculated in the nonlinearPoisson-Boltzmann (PB) approximation assuming interaction atconstant surface charge. The calculations were performed accordingto the algorithm of Chan et al.22 After adding the gemini surfactant,the solution contained both monovalent and divalent ions. In thiscase, we focused on comparing the measured decay length of thedouble-layer force with the Debye length calculated in the PBapproximation.

Results

1,4 diDDAB (12-4-12) on Glass Surfaces.The interactionsbetween two bare glass surfaces immersed in a 40× 10-6 M KBrsolution are shown in Figure 1, together with the correspondingDLVO fit. KBr was added to have a well-defined backgroundelectrolyte concentration, providing a base for comparing themeasured decay length with the theoretical Debye length. Themeasured decay length of the repulsive double-layer force wasfound to be 48 nm, in good agreement with the theoretical valueof 48 nm. At large distances, a repulsive double-layer force ispresent and dominates the interaction. The fitting process returneda value of the double-layer potential of-80 mV (sign not givenby the fitting process). We emphasize that the nonlinear Poisson-Boltzmann model does not consider ion-ion correlation effects

(15) Li, Z. X.; Dong, C. C.; Thomas, R. K.Langmuir1999, 15, 4392-4396.(16) Wettig, S. D.; Verrall, R.J. Colloid Interface Sci.2001, 235, 310-316.(17) Israelachvili, J. N.; Adams, G. E.J. Chem. Soc., Faraday Trans. 11978,

74, 975-1001.(18) Parker, J. L.; Christenson, H. K.; Ninham, B. W.ReV. Sci. Instrum.1989,

60, 3135-3138.

(19) Parker, J.Prog. Surf. Sci.1994, 47, 205-271.(20) Stubenrauch, C.; Rojas, O. J.; Schlarmann, J.; Claesson, P. M.Langmuir

2004, 20, 4977-4988.(21) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W.AdV. Colloid

Interface Sci.1996, 67, 119-183.(22) Chan, D. Y.; Pashley, R. M.; White, L. R.J. Colloid Interface Sci.1980,

77, 283-285.

Figure 1. Force scaled by the radius of curvature versus distancebetween two bare glass surfaces in 40× 10-6 M KBr. The dashedline represents a fit to DLVO theory with constant surface chargeboundary conditions, using the Hamaker constant for glass interactingacross water (A ) 1 × 10-20 J).

1134 Langmuir, Vol. 24, No. 4, 2008 Blomberg et al.

or ion size effects and the fitted double-layer potential is expectedto be somewhat lower than the surface potential,23 with thisbeing due to the neglect of ion-ion correlations. No adhesionis observed when the bare glass surfaces are separated, indicatingthe presence of a short-range hydration force. These results areconsistent with previous measurements.24,25

The interactions between glass surfaces across aqueoussolutions containing 40× 10-6 M KBr as background electrolyteand different concentrations of 1,4 diDDAB are illustrated inFigure 2. The repulsive double-layer force dominates theinteraction at larger separations. The repulsive double-layer forceat the lowest surfactant concentration, 0.1× cmc (cmc) 1.2

mM15), is significantly lower than that obtained for bare glass,and from these data we cannot determine the sign of the double-layer potential. However, the magnitude of the double-layer forceincreases with surfactant concentration, which allows us toconclude that at 0.2× cmc and at higher concentrations the signof the double-layer potential has changed and is becomingincreasingly positive as more 12-4-12 surfactant is adsorbed.The decay length of the double-layer force decreases withincreasing surfactant concentration (for details, see Table 1and Figure 6a), since the ionic strength of the solutionincreases. This will be elaborated on in more detail in theDiscussion section.

At surfactant concentrations up to 0.6× cmc, an attractiveforce overcomes the repulsive double-layer force as the separationbetween the surfaces becomes smaller. In the surfactantconcentration range investigated, the onset of the attraction isobserved at larger separations the smaller the surfactantconcentration (for details, see Table 1). The attraction results in

(23) Attard, P.; Mitchell, J.; Ninham, B. W.J. Chem. Phys.1988, 89, 4358-4367.

(24) Rojas, O. J.; Macakova, L.; Blomberg, E.; Emmer, Å.; Claesson, P. M.Langmuir2002, 18, 8085-8095.

(25) Vanegyte, P.; Leyh, B.; Rojas, O. J.; Claesson, P. M.; Heinrich, M.;Auvray, L.; Willet, N.; Jerome, R.Langmuir2005, 21, 2930-2940.

Figure 2. Force scaled by the radius of curvature versus distancebetween two glass surfaces across aqueous solutions containing 0.1-0.8× cmc 1,4 diDDAB. Black circles) 0.1× cmc, unfilled circles) 0.2 × cmc, black squares) 0.4 × cmc, unfilled squares) 0.6× cmc, and black tilted squares) 0.8 × cmc.

Figure 3. Force scaled by the radius of curvature versus distancebetween two glass surfaces across aqueous solutions containing 0.1-0.8× cmc 1,12 diDDAB. Black circles) 0.1× cmc, unfilled circles) 0.2 × cmc, black squares) 0.4 × cmc, unfilled squares) 0.6× cmc, and black tilted squares) 0.8 × cmc.

Figure 4. Adhesion force scaled by the radius of curvature as afunction of the surfactant concentration, scaled by the cmc of thesurfactants (for cmc values, see Table 1). Squares and circles representthe forces measured in 1,4 diDDAB and 1,12 diDDAB, respectively.

Figure 5. Force scaled by the radius of curvature versus distancebetween two mica surfaces across aqueous solutions containing 0.1-0.4 × cmc 1,12 diDDAB. Filled squares) 0.1 × cmc, unfilledsquares) 0.2× cmc, filled circles) 0.3× cmc, and unfilled circles) 0.4 × cmc.

Cationic Gemini Surfactant Layer Interactions Langmuir, Vol. 24, No. 4, 20081135

a spring instability that causes the surfaces to jump into adhesivecontact (Figure 2). From Figure 2 and Table 1, it can also be seenthat the force barrier that has to be overcome before entering intothe attractive force regime increases with increasing surfactantconcentration, which reflects the increase in the surface chargedensity. At the highest surfactant concentration, 0.8× cmc, noattraction is observed on approach but the repulsive force displaystwo steps in the small distance regime. We interpret this as beingdue to a slow removal of the outer surfactant layers from thecontact zone as the surfaces are brought together. On separation,a strong adhesive force, of a magnitude similar to what has beenobserved for other surfactant layers in hydrophobic monolayer-monolayer contact,26,27 is present. Thus, we conclude that thezero separation in Figure 2 corresponds to the situation with a12-4-12 surfactant monolayer on each surface. The adhesionforce observed on separation is roughly independent of thesurfactant concentration, in the 1,4 diDDAB concentration range0.1-0.8 × cmc, as demonstrated in Figure 4.

1,12 diDDAB (12-12-12) on Glass Surfaces.The interactionsencountered in aqueous 1,12 diDDAB solutions containing 40× 10-6 M KBr as a background electrolyte are summarized inFigure 3. Again, the repulsive electrostatic double-layer forcedominates the interaction at large separations (Figure 3), and themeasured decay lengths at large separations as a function of 1,12diDDAB concentration are shown in Figure 6b and Table 1. Therepulsive double-layer force in the 0.1× cmc (cmc) 0.4 mM15)1,12 diDDAB solution is significantly smaller than that at highersurfactant concentrations. Thus, just as for 1,4 diDDAB, weconclude that adsorption of 1,12 diDDAB onto the silica surfaceresults in charge reversal, at least at concentrations of 0.2× cmcand above. In the case of 1,12 diDDAB, the force barrierencountered at low surfactant concentrations is larger than for1,4 diDDAB (Figure 3 and Table 1). Thus, the magnitude of thesurface charge density at low surfactant concentrations, 0.1-0.2

× cmc, is higher for the surfactant with the longer spacer. Athigher surfactant concentrations, however, the magnitude of theforce barrier is similar for the two gemini surfactants. A clearjump into contact is observed at the two lowest surfactantconcentrations. However, in the 1,12 diDDAB concentrationrange 0.4-0.6× cmc, a region of constant force is encounteredat small separations rather than a “jump”. This feature resemblesa pressure-induced phase transition in the adsorbed layer (aconstant force in curved geometry corresponds to a constant freeenergy in flat geometry). At 0.8× cmc, a small inward step isobserved at measurable forces, and a second one occurs at veryhigh forces. For this reason, this curve in Figure 3 is displacedby the monolayer thickness, 2 nm, determined by SFA. Theadhesion force between the 1,12 diDDAB-coated surfaces isshown in Figure 4. It is high, but significantly lower than thatfor 1,4 diDDAB, and it decreases with increasing surfactantconcentration in the 1,12 diDDAB concentration range 0.1-0.8× cmc. These findings demonstrate that the monolayer of 1,12diDDAB is less hydrophobic than that of 1,4 diDDAB andthat this difference increases with increasing surfactant con-centration. It is conceivable that the large spacer length of 1,12diDDAB allows some of the surfactant head groups to be alsodirected away from the surface when the surfaces are broughtinto contact. Finally, we conclude that the zero separation inFigure 3 corresponds to contact between 1,12 diDDAB mono-layers.

1,12 diDDAB on Mica. The forces between mica surfacesinteracting across a 1,12 diDDAB solution (10× 10-6 M KBras background electrolyte) are illustrated in Figure 5. As expected,the decay length and the range of the electrostatic double-layerforce decrease with increasing surfactant concentration. Fur-thermore, the surfaces are pulled into adhesive contact from asurface separation of about 7 nm (Table 1). The adhesion forcemeasured on separation was between 120 and 160 mN/m, andno dependence on the surfactant concentration was observed.With the interferometric surface force apparatus, the absolutesurface separation can be measured, and the compressed layer

(26) Herder, P. C.J. Colloid Interface Sci.1990, 134, 346-356.(27) Claesson, P. M.; Herder, P. C.; Rutland, M. W.; Waltermo, Å.; Anhede,

B. Prog. Colloid Polym. Sci.1992, 88, 64-73.

Table 1. Decay Length, Force Maximum (F/R Max), Distance at Which the Force Maximum Is Located (D Max), the Range of theDouble-Layer Repulsion, and the Adhesion Force for Glass and Mica Surfaces Interacting Across Aqueous Solutions Containing 1,4

diDDAB and 1,12 diDDAB

1,4 diDDAB on glass; cmc) 1.17 mM

conc(×cmc)

decay length(nm)

D max(nm)

F/Rmax(mN/m) jump in

range of rep(nm)

adhesion force(mN/m)

0.1 15.6 6.2( 0.1 0.5( 0.2 yes ≈73 2540.2 11.8 5.4( 0.1 1.0( 0.1 yes ≈69 2380.4 8.9 4.4( 0.1 2.1( 0.1 yes ≈54 2760.6 7.8 3.5( 0.2 2.8( 0.1 yes ≈46 2660.8 7.0 steps ≈55 231

1,12 diDDAB on glass; cmc) 0.37 mM

conc(×cmc)

decay length(nm)

D max(nm)


range of rep(nm)

adhesion force(mN/m)

0.1 22.5 4.8( 0.2 1.1( 0.1 yes ≈93 2290.2 18.3 4.2( 0.2 1.7( 0.1 yes ≈82 2120.4 14.0 2.4( 0.2 2.1( 0.1 no ≈66 1810.6 12.0 2.0( 0.2 2.7( 0.2 no ≈53 1680.8 11.3 1.6( 0.2 4.5( 0.5 no ≈54 155

1,12 diDDAB on mica; cmc) 0.37 mM

conc(×cmc)

decay length(nm)

D max(nm)


range of rep(nm)

0.1 27.1 5.9( 0.2 7.0( 0.1 yes ≈1460.2 20.0 6.9( 0.2 6.4( 0.1 yes ≈1170.3 17.9 6.1( 0.3 5.7( 0.5 yes ≈1010.4 16.1 5.4( 0.6 5.4( 0.3 yes ≈95


thickness between the two mica surfaces after they were pulledinto adhesive contact was determined to be about 2 nm, whichmeans that the surfactant layer is about 1 nm thick on eachsurface. This is somewhat shorter than a fully extended C12chain+ head group, and this means that the surfactant moleculesare not directed perpendicular to the surface once the surfacesare brought into contact.

Discussion

Adsorption of Gemini Surfactants on Silica.In general, theadsorption of cationic surfactants on silica is initially driven byelectrostatic interactions, and, provided the silica surface hassome hydrophobic character, the surfactant adsorbs with boththe head and the tail toward the surface.28 At higher surfactantconcentrations, surfactant self-assembly driven by hydrophobicinteractions between surfactant tails is the main driving force,resulting in the formation of bilayer aggregates. We note thatadsorption of surfactants to the silica surface promotes dissociationof additional surface silanol groups, meaning that the chargeregulating capability of silica is important for the adsorptionprocess.28,29

The adsorption of cationic gemini surfactants on silica hasbeen investigated in several studies.30-34 In general, the samefeatures are observed as for simple (single chain, singleheadgroup) surfactants.34 However, due to the dimeric nature ofthe headgroup, the possibility of coadsorption of the bromidecounterion in the layer next to the surface has been considered.In the initial part of the adsorption isotherm of gemini surfactantswith a small spacer (s) 2), it was found that only one sodiumion bound to the sodium silicate surface was released for eachadsorbing surfactant, whereas two sodium ions were releaseddue to the adsorption of surfactants with a long spacer (s) 12).33

This indicates that one bromide ion is coadsorbed with the dimericsurfactant when the spacer length is small, whereas no bromidecoadsorption occurs for longer spacers. Considering this dif-ference, it is somewhat surprising that the amount of surfactantadsorbed at the initial part of the adsorption isotherm (up to 0.4mg/m2 at 0.3× cmc) shows insignificant dependence on thespacer length.34 At higher surfactant concentrations, however,the extent of adsorption increases significantly with decreasingspacer length.31,34

AFM images of the adsorbed layers have been presented inonly a few cases. For instance, Manne et al. showed that longcylindrical aggregates of the 12-4-12 surfactant formed on mica.30

Fielden et al. demonstrated that by decreasing the spacer lengtha rather featureless bilayer structure of the surfactant 12-2-12was formed on mica,13whereas Atkin et al. showed that the samesurfactant on silica formed small ellipsoidal bilayer aggregates.34

This difference can be rationalized by the higher negative surfacecharge density of mica as compared to silica. Atkin et al. alsonoted that it was exceedingly difficult to image surfactant layersformed with gemini surfactants having long spacers (s> 4) dueto the presence of long-range attractive forces between the tip

(28) Goloub, T. P.; Koopal, L. K.; Bijsterbosch, B. H.Langmuir1996, 12,3188-3194.

(29) Goloub, T. P.; Koopal, L. K.Langmuir1997, 13, 673-681.(30) Manne, S.; Scha¨ffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky,

G. D.; Aksay, I. A.Langmuir1997, 13, 6382-6387.(31) Chorro, C.; Chorro, M.; Dolladille, O.; Partyka, S.; Zana, R.J. Colloid

Interface Sci.1998, 199, 169-176.(32) Grosmaire, L.; Chorro, M.; Chorro, C.; Partyka, S.J. Colloid Interface

Sci.2001, 242, 395-403.(33) Grosmaire, L.; Chorro, C.; Chorro, M.; Partyka, S.; Zana, R.J. Colloid

Interface Sci.2001, 243, 525-527.(34) Atkin, R.; Craig, V. S. J.; Wanless, E.; Biggs, S.J. Phys. Chem. B2003,

107, 2978-2985.

Figure 6. (a) Calculated and measured Debye lengths plotted asa function of surfactant concentration for 1,4 diDDAB, scaled bythe cmc of the surfactant (for cmc values, see Table 1). (b) Calculatedand measured Debye lengths plotted as a function of surfactantconcentration for 1,12 diDDAB, scaled by the cmc of the surfactant(for cmc values, see Table 1). (c) Calculated and measured Debyelengths as a function of surfactant concentration for 1,12 diDDABadsorbed on mica, scaled by the cmc of the surfactant (for cmcvalues, see Table 1). Unfilled squares represent the calculated Debyelength for a 1:1 electrolyte, and unfilled circles represent the calculatedDebye length for a 1:1 background electrolyte plus a 2:1 surfactant.Filled squares represent the measured Debye length for the differentsurfactant concentrations.


and the surface. However, they argued that the layer structurein these cases consisted of bilayer aggregates with interdigitatedchains.34

Based on the observations recapitulated above, we concludethat the structure of the adsorbed surfactant layers in our study,when the surfaces are far apart, consists of bilayer aggregates(possibly with the exception of the lowest concentration, 0.1×cmc, where monolayer aggregates may still dominate). Withincreasing surfactant concentration, more bilayer aggregates areformed, increasing the excess charge brought to the surface bythe surfactants.

Changes in Layer Structure with Surface Separation.Theadhesion force between surfactant bilayers with quaternaryammonium groups is expected to be zero as observed forhexadecyltrimethylammonium bromide (CTAB) on mica,35 orrelatively small as for the gemini surfactant 12-2-12 on micawhere an adhesion force of 5-10 mN/m between the bilayerswas reported.13 In contrast, bilayer structures formed by CTABor dodecyltrimethylammonium bromide (DTAB) on glass arenot stable under high compression, and on this substrate strongadhesion forces are observed on separation, indicating contactbetween hydrophobic monolayers.36-38 The adhesion forcesencountered in this study are even larger than those reported forCTAB on glass and are comparable in magnitude to thoseobserved for dodecylammonium chloride on mica.26,27Thus, itis clear that once the surfaces are in contact, a hydrophobicmonolayer remains on each surface, meaning that as the distanceis decreased the adsorbed layer structure is transformed frombilayer aggregates to monolayer aggregates. This change inadsorption and layer structure, termed proximal adsorption/desorption, resulting from surface interaction has been inves-tigated in detail by Lokar et al., Subramanian and Ducker, Koopalet al., and Leermakers et al., and the theoretical modeling hasprovided a detailed molecular description of this process.38-44

In brief, the dramatic proximal change in adsorption andlayer structure observed for surfactants is a consequence of ashort-range attraction between surfactant tails (resulting incooperative adsorption and desorption) combined with the changein electrostatic potential that occurs at the two surfaces as theyare brought together. This leads to increased surfactantadsorption at decreasing separations below the charge neutraliza-tion point (cnp) and decreased adsorption at decreasing separationabove the cnp. It is the latter case that is relevant for theconcentration interval investigated here. Thus, the structuralchanges inferred from the force measurements are in line withtheoretical predictions. In addition, we noted in several cases(see, e.g., Figures 2 and 3) that a region of close to constant forceis observed in the short-distance regime. In fact, a (close to)constant force regime has been predicted to occur for surfactant

layers undergoing a separation induced phase transition,42

in the present case from bilayer aggregates to monolayeraggregates.

Ion Pairing in the Electrical Double-Layer. The surfaceexcess of surfactants at the air-water interface is normallydetermined by surface tension measurements employing theGibbs equation:

whereR is the gas constant,T is the absolute temperature,γ isthe surface tension,a is the activity, andΓ is the surface excess.For nonionic surfactants, the prefactor,n, is 1, and for simpleionic surfactants it is 2. Indeed, evaluations based on eq 1 anddirect measurements of the surface excess by neutron reflectivityagree well for such compounds.45 For gemini surfactants, theexpected prefactor is 3, provided the surfactants are fullydissociated. However, the use of this prefactor gives unexpectedlylarge areas per molecule at the solution-air interface, and it hasbeen argued that a counterion (Br-) may be tightly associatedwith the gemini surfactant in bulk solution, thereby changing theprefactor to 2.46This notion was supported by neutron reflectivitymeasurements that provided good agreement with directlymeasured values of the surface excess and calculated valuesusingn) 2 in eq 1 for a series of gemini surfactants with differentalkyl spacers.15Even so, Li et al. restrained from firmly concludingthat gemini surfactant-bromide ion pairs were formed,15mainlydue to difficulties associated with accurate surface tensionmeasurement determinations in dilute cationic surfactant solu-tions. Indeed, the conductivity measurements of Zana did notgive any indication of ion pair formation for the 12-s-12 series(but ion pair formation was suggested for m-s-m geminisurfactants with me 10) but rather suggested that premicellaraggregates form when the spacer length is high enough, that is,at and above 12.14

Another quantity that is affected by ion pairing and by theformation of premicellar aggregates is the Debye length thatdescribes the decay of the repulsive double-layer force at largeseparations. The Debye length,κ-1, is given by47

whereε0 is the permittivity of vacuum,εr is the dielectric constant,k is the Boltzmann constant,e is the elementary charge,n is theion concentration (number density), andz is the ion valency. Ascompared to the situation of full dissociation, ion pairing wouldresult in an increase in the Debye length. On the other hand, theformation of higher charged species due to premicellar aggregates(without further association of counterions) would result in adecrease in the Debye length. Thus, by comparing the slope ofthe double-layer force determined at large separations with theexpected Debye length assuming full dissociation, it is possibleto draw conclusions about the degree of ion pairing within thediffuse part of the electrical double-layer.

(35) Kekicheff, P.; Christensson, H. K.; Ninham, B. W.Colloids Surf.1989,40, 31-41.

(36) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M.J. Phys. Chem.1993, 97,7706-7710.

(37) Rutland, M. W.; Parker, J. L.Langmuir1994, 10, 1110-1121.(38) Lokar, W. J.; Ducker, W. A.Langmuir2002, 18, 3167-3175.(39) Subramanian, V.; Ducker, W.J. Phys. Chem. B2001, 105, 1389-

1402.(40) Lokar, W. J.; Koopal, L. K.; Leermakers, F. A. M.; Ducker, W. A.J. Phys.

Chem. B2004, 108, 3633-3643.(41) Lokar, W. J.; Koopal, L. K.; Leermakers, F. A. M.; Ducker, W. A.J. Phys.

Chem. B2004, 108, 15033-15042.(42) Koopal, L. K.; Leermakers, F. A. M.; Lokar, W. J.; Ducker, W. A.Langmuir

2005, 21, 10089-10095.(43) Leermakers, F. A. M.; Koopal, L. K.; Lokar, W. J.; Ducker, W. A.Langmuir

2005, 21, 11534-11545.(44) Leermakers, F. A. M.; Koopal, L. K.; Goloub, T. P.; Vermeer, A. W. P.;

Kijlstra, J. J. Phys. Chem. B2006, 110, 8756-8763.

(45) Eastoe, J.; Nave, S.; Downer, A.; Paul, A.; Rankin, A.; Tribe, K.; Penfold,J. Langmuir2000, 16, 4511-4518.

(46) Alami, E.; Beinert, P.; Marie, P.; Zana, R.Langmuir 1993, 9, 1465-1467.

(47) Israelachvili, J. N.Intermolecular and Surface Forces; Academic Press:London, 1991.

Γ ) - 1nRT

dγdln a

(1)

κ-1 ) x ε0εrkT

e2∑i

nizi2

(2)


The calculated decay lengths of the repulsive double-layerforces encountered in this investigation are listed in Table 1. Asexpected, the decay length decreases with increasing surfactantconcentration. By comparing the measured decay lengths withthe calculated Debye lengths for 1:1 and 2:1 electrolyte solutions,it is evident that the gemini surfactants 1,4 diDDAB (12-4-12)and 1,12 diDDAB (12-12-12) behave more like a 2:1 electrolytethan a 1:1 electrolyte (see Figure 6). This finding is consistentwith the conductivity measurements of Zana.14 However, as thecmc isapproached, somedeviations indicating ionpairingbetweenthe gemini surfactant and the bromide ion become evident. Thiseffect is quantified in Figure 7 that illustrates the amount ofsurfactant in solution that behaves as a 1:1 and a 2:1 electrolyteas a function of surfactant concentration. The degree of ion pairingremains below 50% at all surfactant concentrations, and it islarger for 1,4 diDDAB than for 1,12 diDDAB. We note that theincrease in ion pairing with concentration is expected from anycomplex formation model that describes the association with anequilibrium constant. In the simplest case, the ion pairing betweenthe surfactant, S2+, and the bromide counterion can be describedby the following set of equations:

whereK is the equilibrium constant andx is the fraction of ionpairs. The prediction of this model is shown in Figure 7 as dashedlines. The values of the equilibrium constants used in thecalculations were 0.45 and 0.2 mM-1 for 1,4 diDDAB and 1,12diDDAB, respectively. The results for 1,4 diDDAB are inreasonable agreement with the model predictions, whereas thedegree of ion pairing for 1,12 diDDAB changes more rapidlywith concentration than predicted by eq 3. Clearly, in this case,the association process is more complex and may involve theformation of premicellar aggregates and counterion binding tothese.

Atkin et al. carried out force measurements above the cmcfor a series of gemini surfactants and tentatively concludedthat at these concentrations 50% of the bromide ions were boundto the gemini surfactants.34 The interpretation of the decaylength of the double-layer force in their experiments is notstraightforward due to the presence of micelles, but neverthelessour results extrapolate to similar values at the cmc. Thus,within the assumptions of the nonlinear Poisson-Boltzmannmodel used to estimate decay lengths of the repulsive double-layer for fully dissociated 1:1 and 2:1 electrolytes, a comparisonof these values with the measured decay lengths suggeststhat a change in the degree of ion pairing with surfactantconcentration should be considered when evaluating thesurface excess of gemini surfactants from surface tensiondata.

The Magnitude of the Double-Layer Force.At the shortseparation limit, the double-layer force between two crossedcylinders,Fc, is given by47

whereR is the geometrical mean radius,Wf is the free energyof interaction per unit area between flat surfaces,σ is thesurface charge density,D is the separation, andc is a constantof integration. Thus, the magnitude of the double-layer force ata given sufficiently small separation is proportional to thesurface charge density. From the work of Atkin et al., weknow that the adsorbed amount is close to being independent ofthe spacer length for concentrations below 0.3× cmc. Never-theless, in this concentration range, we observe a significantlylarger double-layer force for 1,12 diDDAB than for 1,4 diDDAB(compare Figures 2 and 3). This is consistent with the con-clusion of Zana et al. that gemini surfactants with short spacersadsorb to silica with one of the bromide counterions whereas1,12 diDDAB does not retain any of its counterions during theinitial state of adsorption.33 At higher surfactant concen-trations, the adsorption increases significantly and is morepronounced for the surfactant with the shorter spacer.34 Themagnitude of the double-layer force at small concentrationsincreases significantly for 1,4 diDDAB (Figure 2), whereas it isapproximately constant for 1,12 diDDAB (Figure 3). Themagnitude of the double-layer force is, despite the larger adsorbedamount for1,4diDDAB,34rathersimilar.This indicatessignificantcounterion binding to the outer surfactant layer, and more so forthe surfactant with shorter spacer. This is consistent with thesignificant bromide counterion binding to gemini surfactantmicelles, increasing with decreasing spacer length, that has beenreported recently.48

Conclusions

The forces acting between glass and mica surfaces in thepresence of two gemini surfactants, 1,4 diDDAB and 1,12diDDAB, below their cmc’s have been investigated. Inboth cases, a significant recharging of the surfaces is ob-served. The surface force data is consistent with the formationof bilayer aggregates that are transformed into monolayeraggregates at small surface separations as predicted by theo-retical studies of changes in the adsorption and layer structureof surfactants confined to thin gaps. Moreover, in several cases,a range of distances of (close to) constant force is observed at

(48) Geng, Y.; Romsted, L. S.; Menger, F. M.J. Am. Chem. Soc.2006, 128,492-501.

Figure 7. Amount of surfactant that behaves as a 1:1 (filled symbols)or 2:1 electrolyte (unfilled symbols) as a function of the surfactantconcentration, scaled by the cmc of the surfactants (for cmc values,see Table 1). Tilted squares illustrate the amount for 1,4 diDDABand squares represent the amount for 1,12 diDDAB on glass. Thelines are results obtained from a basic association model (see text).

S2+ + Br- S SBr+

K )[SBr+]

[S2+][Br-]) x

(1 - x)[Br-]

x )K[Br-]

1 + K[Br-](3)

Fc(D)

R) 2πWf(D) ) -2π|2σkT

ze| ln(D) + c (4)


small separations, indicating a phase transition in the adsorbedsurfactant layer. We have shown that the difference in double-layer forces and their variation with surfactant concentrationcan be rationalized by considering that bromide counterionsbind together with the 1,4 diDDAB surfactant, but not (or to alesser degree) with 1,12 diDDAB, at the initial state of adsorp-tion as also suggested by Grosmaire et al.33 For both surfactants,significant bromide coadsorption occurs at higher surfactantconcentrations. By analyzing the decay length of the double-layer force, it was concluded that at all concentrations the maj-

ority of the gemini surfactants are fully dissociated, but the degreeof ion pair formation increases with increasing surfactantconcentration.

Acknowledgment. Financial assistance for E.B. was providedby the Natural Sciences and Engineering Research Council ofCanada (NSERC), and the Swedish Research Council for E.B.and P.C. is gratefully acknowledged.

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Interactions between Adsorbed Layers of Cationic Gemini Surfactants †