Chapter 16

Interactions Between PentadecylethoxylatedNonylphenol and Tetradecyltrimethylammonium

Chloride Mixtures at the Alumina-WaterInterface iI

!!

IL Huang, c. Maltesb, and P. Somasundaran

Langmuir Center for Colloids and Interfaces, School of Engin~ringand Applied Science, Columbia University, New York, NY 10027

Adsorption behavior of surfactants in mixtures was studied usingtetradecyltrimethylarnrnonium chloride (TT AC) and pentadecyl-ethoxyiated oonyl phenol (NP-l 5). In the single surfactant system. onlythe cationic lTAC was found to adsorb at the alumina-water interface atpH 10. In the mixed surfactant system. the cationic tetradecyl-trimethylarnrnonium chloride (TT AC) was found to induce significantooadsorption of the oonionic NP-1S. This was attributed to hydrophobicinteractions between the lTAC and NP-1S. The adsorption behaviorvaried with the ratio of the two surfactants in the mixture. In lTAC richmixtures, adsorption ofNP-l 5 was higher and the adsorption isothermsofNP-1S shifted to lower concentrations. On the other hand, in NP-1Srich mixtures adsorption of IT AC was suppressed due to competitiveadsorption and steric hindrance by the larger NP-lS molecules. Zetapotential measurements also showed that with the increase of NP-lS inthe mixtures, IT AC molecules could not reach the alumina-waterinterface easily as their positive charge was partially screened by the co-adsorbed NP-lS. Regular solution theory has been used to understandthe nature of the int~on betWea1 surfiI.ctants in the mixtures and therelation between the monomer concentratiton and adsorption behaviors.

Adsorption of surfactants at solid-liquid interfaces plays a critical role in many important"mdustria1 processes. In many of these processes, the use of mixed surfactant systems cansignificantly improve the performance over that of single component systems. Inapplications such as enhanced oil recovery processes, surfactant adsorption is notdesirable and mixed surfactant systems, if properly designed, may offer some promisingadvantages over pure components in this regard and reduce the adsorption. In othercases the adsorption of surfactants from mixtures can be higher and past work hasclearly shown that the adsorption behavior is much more complex than that of singlesurfactants (1-3).

0097 -61S6,9S~ 15-0241$ 12.00,1)@ 1995 American Chemical Society

t

i

242 SURFACTANT ADSORPTION AND SURFACE SOLUBII.IZATJON

Surfactant adsorption from solution is related to the chemical potential of theRlrfactant molecules in the solution, and the nature of solid. It is widely accepted thatsurfactant mixtures can form mixed micelles in the solution and that the CMC of asurfactant mixture varies with the mixture composition. This may in turn reduce thedtenMcaI potential of the monomers from that for the single surfactant system, therebyreducing its adsorption as well. But on the other hand, the hemimicelle concentration(HMC) may also vary with the mixture composition. It is clear that adsorption fromsolution containing mixtures of surfactants can be understood only by taking intoaccount the equilibrium anK>ng monomers, micelles and hemimicelles. So far theinteraction of surf act ants in bulk solutions have been studied for many cases (4-8) .Literature on the solution behavior of surfactant mixtures suggests significant deviationfrom ideal mixing behavior. Generally, mixtures of surfactants with similarly chargedhead groups behave more ideally in solutions, and very large deviations from ideality areobserved for mixtures of oppositely charged surfactants in solutions. However, there isvery little information on mixed surfactant interactions at solid/liquid interface,partiQJlarly for the case of cationic- nonionic surfactant mixtures. Regular solutionapproximation has been used to treat nonideal mixing micelles in solution and thisapproach has been extended to model also adsorption at interface (9-10). Althoughregular solution theory has been criticized by some researchers (II), it has provided avery useful way to describe the behavior of mixed surfactant aggregates. In this work,the adsorption of binary mixtures of the isometrically pure cationictdradecyttrimethylammonium cltIoride(TT AC) and pentadecylethoxylated nonylphenol(NP-l S) at alumina-water interface was investigated. The effects of synergism and sterichindrance in this mixed system are discussed, and the possible relationship betweenadsorption behaviors and monomer concentrations is discussed using regular solution

approximation.

Materials and Experimental Procedure

Alumina. Linde A alumina was purchased from Union Carbide Corp. It was specifiedto be 900/0 a-Al1O) and loe/. y-Al1O) and to have a mean diameter of O.3J1m. Thespecific surface area was measured to be 15 m1/g by N1 BET adsorption using a

Quantasorb system.Surfattanil: The cationic surfactant, n-tetradecyltrimethylammonium chloride,[CH~CHJ,~(CHJJCI (TT AC), from American Tokyo Kasei, Inc., and the nonionicpentadecylethoxylated nonylphenol, C,H"C,H40(CH1CH1O),sH (NP-15), from NikkoChemicals, Japan were used u rC(:eived.Rtagtnts: NaCl, used for controlling the ionic strength, was purchased from FisherScientific Co. and was of A.C.S. grade certified (purity >99.98/.). NaOH, used foradjusting the pH, was purchased from Fisher Scientific Co. and was certified as thevolumetric standard solution(O.1 M). Water used in aU experiments ,vas triple distilledwater. Conductivity of triple distilled water was measured to be in the range of(I-2)xIO..0.'.Adsorption: Adsorption experiments were conducted in capped 20-mL vials. First. 2g

/nteract;oR.\' Between NP-/S and 7TAC16. IIUANG~AIA 243

of alumina and 10ml of 0.03 M NaCI solution were mixed and conditioned for I hourand then the pH value was adjusted as desired and allowed to equilibrate at the roomtemperature (22:t2°C) for I hour. Then, 10 ml ofO.03M NaCI solution containing thesurfactants was added, and the samples were allowed to ~uilibrate at room temperature(22:t2 °C) for 15 hours. pH measurements were then made and, if necessary, adjustedusing 0.1 M NaOH. The change in pH during equilibration was usually between 0.2 and0.4 units. The samples were allowed to equilibrate for about 3 hours after the final pHadjustment, and then centrifuged for 25 rnin at 5000 rpm. About 20 ml of the supernatantwas pi petted out for analysis.Surfactant analyses: Tetradecyltrimethylammonium chloride (TT AC) concentrationwas measured by a two-phase titration technique(12). Pentadecylethoxylatednonylphenol (NP-15) concentration was analyzed by UV absorbance using a ShimadzuUV-1201 UV-Vis spectrophotometer. Wavelength scans of NP-15 solution shows twoabsorbance peaks at 223nm and 275nm. NP-15 concentrations can be measured at eitherof these wavelengths.Surface tension: Surface tension was measured using a Fisher Model 20 ringtensiometer. A 15-20 mI portion of the solution was poured into a 25 mI beaker at roomtemperature (22:t2 OC). Three minutes were allowed for equilibration before surfacetension measurements.Electrokinetic measurements: Zeta potential measurements were made using a PENKEM, Inc. Laser zee meter model 50 I system.

Results and Discussions

Interactions between tetradecvltrimethxlammonium chloride (Tf AC) and ~ntad~1ethoxxlated nonvlRhenol fNP-15) in solution

In an attempt to correlate changes in monomer concentrations of the individualsurfactants in the mixtures with their adsorption behavior, mixtures oftetradecyltrimethylammonium clIloride (Tf AC) and pentadecyl ethoxylated nonylphenol(NP-15) were studied using surface tensiometJy and regular solution theory (9). Resultsobtained for the surface tension of aqueous solutions of pure tetradecyl-trimethylammonium chloride (Tf AC) and ~re pentadecylethoxylated nonylphenol (NP.15) showed. as expected, theCMC of the nonionic NP-15 to be lower than that for theTf AC, oot the reaJlts obtained for mixtures showed them to be not as surface active asthe nonionic surfactant. The critical micelle concentrations (CMC) of the mixtures areplotted in figure I as a function ofNP-15 composition. Regular solution model was usedto fit these data (figure I) and an interaction parameter (r3) between -1.5 and -1.2 wasobtained. A considerable number of binary mixed micellar system have been studiedusing this procedure (4-8) and the interaction parameters obtained are about -25.5 forcationic-anionic systems (4), -4.6 to .1.0 (16-17) for cationic-nonionic mixtures, and-0.2 for cationic-cationic mixtures (6). Compared with these, the interaction parameterobtained for the present system is not very large. Nevertheless, the adsorption behaviorof these surfactants was modified measurably as a result of these interactions. This willbe discussed later in this paper.

144 SURFACTANT ADSORPTION AND SURFACE SOLUBILIZATION

Measured CMC values of mixtures of tetradecyltrimethylammoniumchloride (IT AC) and pentadecylethoxylated nonylphenol (NP-15) andtheir relation to ideal mixing and regular mixing theories.

Figure

Adso[Rtion of surfactant mixturesThe adsorption isothenn ofTfAC on alumina at pH 10 is shown in figure 2. It

is ~ that Tf AC adsorbs significantly at the alumina-water interface at pH 10. Atthis pH the alumina surface is negatively charged(isoelectric point of alumina is pH 8.5-9.0) arK! the electrostatic attraction with the cationic TfAC will be dominant. There isa sharp increase in the adsorption density around Sx I 0" kmoVm) which is attributed tothe formation of surfactant aggregates(solloids) at the solid-liquid interface. Themaximum adsorption density ofTfAC on alumina at pH 10 is about 2.SxI0"moVm2.This translates to roughly 66A02/moIecu1e which is similar to the molecular area atair/solution interface(6IADZ) reported in the literature (15): This suggests that theadsorption layer on alumina is composed mainly of a monolayer than a bilayer or

micelles.Tests with non ionic pentadecylethoxylated nonylphenol (NP-15) showed it

not to adsorb at the alumina-water interface. In a study on the adsorption of

InJeractions Between NP-15 and 1TAC 24516. IIUANG ETA!...

SxIO-6

ry-- u u o~

'""s

~ lxlO-6e:i-

.~"0

lxlO.7

g"l:Je-o

i~~

; ~::::::~::: , , ...

lxlOlxlO.S lxlO-4 lxlO.) lxlO.r . lxlO.

Tf AC residual cooc., kmol/nl

Adsorption of tetradecyltrimethylammonium chloride (TT AC) aloneon alumina at pH 10, 0.03M NaCI.

Figure 2

polyethoxylated monolaurate (ML-n) and polyethoxyllauryl ether (LE-n) on titania,Fukushima and Kumagai (13) concluded that for adsorption on polar surfaces it isnecessary that the nonionic surfactant have a radical which has sufficient adsorption forceto overcome the strong interaction between water molecules and the ethylene oxidegroups. This observation is similar to that in the literature on the adsorption ofpolyethylene oxide (PEO) on different minerals (14). It has been reported that PEaadsorbs on silica but not on alumina or hematite. It was proposed that for PEa to adsorbon oxide surfaces, the polymer has to displace enough water molecules bonded to thesolid surface and create a strong entropic effect. In the case of the strongly hydratedalumina surface the polymer molecule is unable to adsorb on it to displace sufficientnumber of water molecules from the interface. It is reasonable then that NP-15 does notadsorb on the alumina-water interface. Experiments conducted with mixtures show thatin the presence of TT AC, NP-15 is forced to adsorb at the alumina-water interface. Theadsorption ofNP-15 is plotted in figure 3 as a function ofTT AC residual concentration.In this series of experiments, the initial NP-15 concentration was the same for all thetests.

246 SUU"ACfANT ADSORPTION AND SURFACE SOLUBIUlATION

1T AC residual Cooc., kmoVm )

Figure 3 Adsorption of pentadecyletooxylated nonyiphenol (NP-15) as a functionof tetradecyttrimethyianunoniwn chloride (IT AC) raMIual concentration,NP-15 imtial concentration 2.13x 10.' kmoVm'-

It can be seen that adsorption of NP-I 5 increases with increase in n ACconcentration. It is also to be noted that when n AC mol~les fOnD hemimicelles atthe interface, NP-IS adsorption iocreased markedly. Above the hemimicelleconcentration however, there is no further effect of any increase in the n ACadsorption density on the adsorption ofNP-IS.

Figure 4 shows the adsorption isotherms of pentadecylethoxyiated nonyJphenolwith pro-adsorbed n AC on it along with one isotherm for adsorption of it when addedalong with n AC. It is noted that tetradecyl trimethylammonium chloride(Tf AC) doesforce the adsorption ofNP-IS on alumina. Pre-adsorbed nAC mol~les function asanchors for the adsorption ofNP-IS. With an increase in nAC initial concentration,the adsorption of NP-IS increases and the isotherm is shifted to lower NP-ISconcentrations. 11te results also show that the order of addition of the surfactants has amarked effect on the adsorption of NP-I s. If NP-I 5 and n AC are pre-mixed andadded to the alumina suspension, the adsorption density ofNP-IS is markedly higherbelow the saturation co~e. This indicates that NP-I 5 adsorption from mixtures withn AC is not completely reversible, but is controUec:t; by the nature of moI~lar packingat the intenace. AJthough NP-I 5 itself does not adsorb on alumina, coadsorbed NP-I 5does affect the adsorption ofnAC as shown in figure s.

16. HUANG ET AL InlmldiolU BdW«II NP-1S 4IId 7TAC 24

.1~IO

~ ~ . .

.... )( D.

.x.-6x ---

~-~--'X

~

tJ '-

-'7IxlO -

f ..1 IxlO -

V\~ IxIO'"

Figure 4

0 TTAC: 0.91mM

0 TTAC: .5.2.5 DIM

A TTAC: 0.2 aIM

X ~ IO8eIJ.: 1T AC 2 RIMIxIO..I. , , , IxIO"' IxIO.' IxIO-4 Ix10.3 IxIO-2

NP.I.5 18idual-.,blMll/m3Adsorption ofNP-1 5 on alumina in the presence of preadsorbed IT ACor when added together with lTAC. pH 10, 0.03M NaC!.

IxIO"

D

00

1UI:S'8

J

II

D

I TTN:...; 0.97811

Figure 5

IxIO" .. .-"".

IxIO" IxIO.S IxIO-4 IxIO.] Ix10.2 IxIO.1

NP.IS 1aidl81_., kmoIhn)

Effect ofcoadsorbed NP-IS on the adsorption of1TAC. pH 10,0.03M NaCI

11

dO"

(10"

248 SURFACTANT ADSORPTION AND SURFACE SOLUBIUZATION

In this experiment ~adsorbed IT AC amounts are the same for all the samples,but with the increase ofNP-1S concentration the adsorption oflTAC first increased,reached a maximum, and then decreased. This suggests both cooperation andcompetitive interactions between IT AC and NP-1 S can take place depending on the

concentration region.The adsorption isotherms of IT AC in the presence of different amounts of NP-

1 S are shown in figure 6 along with that in the absence of it. In all these experiments,the tctradecyl trimethyl ammonium chloride (IT AC) and pentadecylcthoxylated nonylphenol (NP-1 S) were pre-mixed and then equilibrated with alumina for 1 S hours at pH10. It is ~ that tetradecyitrimethylammonium chloride (1T AC) solloid aggregationoccurs at lower IT AC concentrations in the presence of the nonionic NP-1 S but only atthe4:1 and 1:llTAC:NP-IS ratios. For the 1:4 ratio, however, the sharp increase inadsorption density corresponding to such aggregation is not observed. It is to be notedthat the plateau adsorption density decreases continuously upon the addition of thenonionic surfactant. This is attributed to the steric hinderance and competition providedby the bulky nonionic pentadecylcthoxylated nonylphenol (NP-15) for the adsorption

sites.

4x1O-6

!O.::-~.6IxIO~

Na:~

~~] IxIO.7

Jr' 6,, 6-'~.-) . 6, II.

!..' ,

tA~

, 00-- ~

-~---o-~ IxlOf::

.. c::;? D-? . ~ '. .1)...

A 4, ,,0---- .

. o,~,.-

0-'.9 1 0- -1.10.1 1 1 1 1 IxIO~ IxIO.' IxIO~ IxIO.) lxI04 Sx10.2

Tr AC raidual COlIC., kmol/rJ

Adsorption of tetradecyltrimethylammonium chloride (Tf AC) in thepresence and absence of pentadecylethoxylated nonylphenol (NP-15),pH 10, 0.03M NaCI

Figure 6

Interaction.\' Between NP-IS and 7TAC 24916. IIUANG Jo:f AI..

The adsorption isotherms ofNP-15 from different mixtures with TTAC areshown in figure 7. As mentioned earlier whereas the pure nonionic surfactant NP-15 didnot adsorb on alumina, the presence of TT AC causes significant adsorption of the NP-15. With an increase in TT AC content in the mixtures, the adsorption of NP-15 isenhanced significantly, and the adsorption isotherms are shifted to lower concentration

ranges.

Adsorption of pentadecylethoxylated nonylphenol (NP-15) on aluminain the presence of different amounts of tetradecyltrimethylammoniumchloride (TfAC), pH 10, 0.03M NaCI.

Figure 7

The peculiar S-shape isothenn obtained for NP-15 adsorption as a function of itsresidual concentration should be noted and is in agreement with our previous work (2).This is attributed to the more effective coadsorption ofNP-15 as the number ofTTACsolloids at the alumina-water interface increase, since the concentration of tetradecyltrimethyl ammonium chloride (IT AC) does vary from point to point along the NP-15adsorption isotherm.

The NP-15 molecules are much larger than the TT AC molecules and theadsorbed NP-15 molecules will occupy more area than the TT AC spaces thus reducingthe area available for TT AC. The structure of the solloids and micelles will be differentin the single surfactant and the mixed surfactant systems and the charge of the ionic head

250 SURFACTANT ADSORP110N AND SURFACE SOLUBIUZATION

of the t~ trimethyl armX)niI,Hn chloride (IT AC) will be shielded by the NP-I S asit~ content in the nKxtures Ux:reases. ThU in twn will reduce electrostatic attractionbetween the negatively charged alumina surface arxI the positively charged 1T ACnnJecua thereby reducing the Tf AC adsorption ~ity. This hypothesis was testedby ~"U18 the zeta potential after adsorption arxJ the resuks are shown in fIgUre 8.The zeta potential of alumina particles will be decided mainly by the adsorption of theoppositely charged Tf AC nnlecules. even though the adsorbed nonionic species canalso ~e to reduction in zeta potential by masking the surface arxJ by shifting theshear plane

so

040 ...D0

0/10

30 0

>20a)10

I 0

~ -10

~~~-6Ar/

I

-20

-301

, ...t

'P, A .,0" ..",.

0",-,0 - -~,o-~ 0,.

o..~- ,-- - ~- -0 ,q'. '

-40..1 :---':-'-.,: 1 1 , 1 .0-

lxlO.7 lxlO-6 lxlO.S IxlO" IxIO.3 lxlO.2 SxlO.2

1T AC residual cooo..knX>Vm 3

Figure I Zeta potential ofalurnina particles after adsorption oflTAC:NP-ISmixtures of different ~ition.

From figure 8 it is seen thai with an Mease of NP-IS in the mixture. the zetapoter-.iaI of aIunW. ~ «BicalJy, especially in the high concentration range. Thisis in agreement with the adsorption isot~ of n AC atxJ NP-IS in figures 6 &. 7respectively. Con1)aring the isoelectric point (IEP) for alumina in the presence ofmxt~ to that m the presence of n AC alone it can be seen that the IEP is shifted tohigher lTAC concemrations with an increMe in the NP-IS. Upon examining theadsorption dersity of IT AC at the isoelectric point, it is evident that the adsorption<Ien,ity ofn AC from mxtures i1 higher than that for n AC alone. This ~ that theeffect ofn AC in n.:xtures on zeta potential reduction is less than that oflT AC when

~0

251Interactions Bdwnn NP-IS and 1TAC16. IIUANG ET AI..

present alone. It can be concluded that the positive charge of the n'AC ionic head ispartially screened by the ~ nonionic NP-l S molecule. The fad that adsorptionof the cationic tetra decyi trimethyl ammonium chloride (n' AC) continues to take placeleading to monolayer coverage even after the particles have become similarly charged(figure 2) suggests the predominating role of hydrophobic interactions between thehydrocarbon tails in causing adsorption. On the other hand, lack of adsorption of thenonionic pentadecylethoxylated nonyl phenol (NP-l S) without the synergism of thecationic n' AC shows the essential role of the electrostatic interaction as well.

The ratio of adsorption densities of n' AC and NP-I S at the alumina-waterintelface is examined in figure 9 as a function of the total residual concentration. It canbe ~ that the ability of the two surfactants to cooperattlcompete at the alumina-waterinterface keeps changing over the entire concentration range. This suggests that themooomer composition and adsorption mechanism are changing over the concentrationrange studied.

6

s"""In

~ 4~

b3

2

s.~~~'g0

~

lxlO') lxlO-4 IxlO.J lxlO.2 9xl0.2.

Figure 9

To better understand the relation between the adsorption mechanism and themooornerconcaltrat ions, the monomer concentrations of solutions of different milrturesin this system were calculated using the regular solution theory with an interactionparameter of -1.5. The results are shown in Figures 10 and II.

Total Residual Conc.. kmol/mJ

Ratio of adsorption densities ofTT AC:NP-I S for different surfactant

mixtures.

SURFACTANT ADSORPTION AND SURFACE SOLUBILIZATION151

-4IxlO

f'\

~] ~u8u

~80~'f'\

~

IxIO' -2

SxlOIxIO.2IxIO.' lxlO-4 lxlO.3

Total Concentration, kmoVni

Monomer concentrations ofNP-15 in different TTAC:NP-15 mixturescalculated from regular mixing theory with interaction parameter 13=-1.5

Figure 10

Although the initial mixed ratio of studied surfactant system will have somechanges after adsorption, it is still interesting to compare the adsorption isothenns inFigures 6 and 7 with the results for the monomer concentrations, which were calculatedfrom regular solution with initial mixed ratio. It can be found that the adsorption ofNP-15 does not depend upon its monomer corx:entrations in the feed mixtures. For example,the adsorption ofNP-15 in 4: IlT AC:NP-15 mixture is the highest in all these mixtures,but its monomer concentrations are the lowest. This further confinns that the adsorptionof NP-15 is very much controlled by the ~re-adsorbed IT AC as anchors. For theadsorption of Tf AC, the adsorption quantities corresponds to its monomerconcentrations in the mixtures. The higher the monomer concentration, the higher is theadsorption density. On examining the synergistic and competitive effects in this system,it becomes evident that these phenomena are decided mostly by the relative as well as theabsolute quantities ofNP-15. Only when the relative and absolute quantities ofNP-15are low, synergism between these two surfactants can be seen, and the maximumexhibited in Figure 8 is attributed to this behavior. There is no this kind of maximumexhibited in the case of 1 :4 TT AC:NP-15 mixture because the relative quantity of NP-15is too high. In the high concentration range the absolute quantity ofNP-15 is higher. andsteric hindrance will be dominant thus suppressing the adsorption ofTTAC.lt will be

Interactions Between NP-I5 and 1TAC 25316. HUANG U AlA

interesting to determine the monomer concentrations experimentally using suitabletechniques to confirm the results of regular solution theory.

Monomer concentrations of lTAC in different lTAC:NP-15mixtures calculated from regular mix.ing theory with interactionparameter p=-l. 5

Figure 11

SUMMARY & CONCLUSIONS

Interactions between a cationic surfactant and a nonionic surfactant were studiedusing tetradecyl trimethylammonium chloride and pentadet;ylethoxlated nonylphenol. Theinteraction parameter determined using the regular solution theory indicated themolecular level associations between tetradecyl trimethyl ammonium chloride (n' AC)and pentadecylethoxylated nonyl phenol (NP-15) to be much weaker than thosebetween an anionic surfactant and a nonionic surfactant. Nevertheless, the adsorptionbehavior of the nonionic NP-15 was modified significantly as a result of theseinteractions. Presence oflT AC in this system forced adsorption ofNP-15 on aJuminawhere the latter does not normally adsorb. The adsorption density of both IT AC andNP-15 could be varied by altering the composition of the surfactant mixture. Presenceof coadsorbed NP-15 lowered the adsorption of IT AC due to competition and steric

SURFACTANT ADSORPrION AND SU rACE SOLUBn.IZAnON254:1\.

hindrance. As a result of mixed aggregation between the nonionic and the cationicsurfactant, electrostatic attraction between Tf AC and the negatively charged aluminasurface was reduced which also contributed to the lower adsorption of Tf AC from itsmixtures with NP-15. Importantly the adsorption of NP-15 on alumina was notdependent on its monomer concentration but on the [TTAC]:[NP-lS] ratio. The modeof addition of the TfAC:NP-IS mixture also had a significant effect on the adsorptiondensities of both surfactants. Adsorption was higher when the two surfactants werepremixed than when the Tf AC was preadsorbed. This is attributed to more favorableinteractions and more poSSIole ways to arrange surfactants coaggregates at the alumina-water interface.

Acknowledgments: Support of the Department of Energy (DE-AC22-92BC 14884) andthe National Science Foundation (CTS-9212759) is gratefully acknowledged.

i,Is....i~

~~"'11"

t

REFERENCES

1 Harwell, J. H, Roberts, B. L., and Scamehom, J. F., Colloids and Surfaces1983, vol. 32, pp.l

2. Somasundaran, P., Fu, E., and Xu, Q., Langmuir 1991.,vol. 8, pp. 10653. Esumi. K., Sakamoto, Y., and Meguro, K.,.l Colloid Interface Sci. 1990,vol.

134, pp.283Zhu, B. Y., and Rosen, M J.,.l Colloid Interface Sci. 1984,vol. 99, pp.435Holland, P. M., and Rubingh. D. N.,.l PhySo Chem.1983,vol. 87, pp.1984Nguyen, C. M., Ratiunan, J. F., and Scamehom, J. F.,.l Colloid Interface Sci.1986, vol. 112, pp.438Rathman. J. M., and Scamehom, J. F., Langmuir 1987,vol. 3, pp. 372Shinoda, K.,.J: Phys. Chern. 1954, vol. 58, pp.541Rubingh. D. N., in -Solution Chemistry of Surfactants.-(K. L. Mittal , ed.),Plenum Press. New York. 1979 Vol. I, pp.337,

10. Rosen, M. J., and Hua, X. Y.,.l Colloid Interface Sci.1981., vol. 86, pp. 16411. Hall. D.G., arxt Huddleston. R W., Colloids and Surfaces, 1985,vol. 13, pp.20912. Li. Z., and Rosen, M. J., AnaL Chern. 1981,vol. 53, pp. 151613. Fukushima. S., and Kumagai. S.,.l Colloid Interface Sci. 1973 vol. ,42, pp. 53914. Koksal, E., Ramachandran. R., Somasundaran. P., and Maltesh, C., Powder

Tech., 1990, vol. 62, pp.25315. Venabl, R L., and Naumann. R V., J. Phys. Chem.1964, vol. 68, pp.349816. Lange, H., and Beck. K. H., KolL -z.u.Z Poiym.1973, vol. 251, pp.42417. Rosen, M J., in -PbenomenainMixed Surfactant System- (J.F. Scamehom, ed.)

ACS. Symposium Series 1986 Vol. 311, pp. 144

RECBIVED May 3, 1995

4S6

789

..

j