Adsorption-Desorption and Hysteresis ot Sultonateson Kaolinite: pH Effects

PAUL A. SIRACUSA AND P. SOMASUNDARAN1Henry Krumb School of Mines. Columbia University. New York. New York 10027

Received May 13, 1985; revised January 22, 1986

Adsorption and desorption behavior of dodecylbenzenesulfonate on kaolinite was investigated undercarefully controlled conditions of pH, ionic strength, and dissolved mineral species. The nature of theadsorption maximum and the hysteresis obtained is found to strongly depend on the pH change thatoccurs during equilibration and dilution. Interestingly, hysteresis can be positive or negative dependingmainly on the pH perturbations that take place during the experiment. The results are examined on thebasis of available data for the pH-dependent dissolution equilibria for kaolinite. Kaolinite dissolutionand subsequent sulfonate precipitation and redissolution are identified to be the major factors in deter-mining abstraction in this system. Many complex interactions are involved in determining surfactantadsorption; studies of desorption behavior along with that of adsorption have proved to be a powerfultool for studying the mechanisms involved. 0 1986 Academic ~ I~

INTRODUCTION

Adsorption isotherms of sulfonates onminerals such as kaolinite have been shownto exhibit special features such as a maximumin the critical micelle concentration region (I-S). While a number of interactions have beenconsidered for the existence of a ~aximum(2-4, 6-11), none can adequately explain theexistence of hysteresis that these systems havebeen found to exhibit upon surfactant dilution.An approach involving the study of adsorptionsimultaneously with that of desorption isadopted in this work since the adsorption pro-cess in these systems has been clearly shownto exhibit many nonequilibrium effects (1,3).

Surfactant adsorption on oxides and claysis dependent on such solution properties aspH and ionic strength (12-15), and a precisecontrol of these variables is necessary for theisolation and identification of various mech-anistic features of these systems. In this study,adsorption and desorption experiments wereconducted for the dodecylbenzenesulfonate-

I To whom all correspondence should be addressed.

1840021-9797/86 $3.00CopyriIIIt C 1986 by Acldcnlio: "-. IIIC.

AM riII* of ~- i. ...y r- '--.C""" _/~ Scw-. Vol 114, No. I. ~ 1986

kaolinite system as a function of the sulfonateconcentration with emphasis on the identifi-cation of the role of pH changes that invariablytake place in these systems. The measuredsurfactant depletion can include in additionto the actual adsorption, precipitation and en-trapment, and all these processes will have sig-nificant dependence on mineral dissolutionwhich in turn is pH dependent. Adsorption isexamined here in light of the mineral-solutionequilibria and its dependence on pH.

MATERIALS AND METHODS

Kaolinite. A well crystallized sample ofGeorgia kaolinite purchased from the clay re-pository at the University of Missouri wassubjected to an ion-exchange treatment (16)to produce the monoionic sodium kaolinite(Na-kaolinite) used in this study. BET surfacearea of the treated sample was determinedby N2 adsorption (using a Quantasorb) to be9.8 m2/g.

Surfactant and chemicals. Sodium dodec-ylbenzenesulfonate purchased from LachatChemicals, Inc., was purified by deoiling withpetroleum ether. desalting with hot isopro-

18'SULFONA TES ON KAOUNrrE: pH EffECI'S

panol. and recrystallization from cold acetone(17). High-pressure liquid chromatographicanalysis revealed the presence of not only Cl2isomers, but also CIa isomers and other lesshydrophobic components as shown in Fig. I.

Inorganic salts used to adjust the ionicstrength and pH were of A.R. grade. Tripledistilled water was used for all tests.

Adsorpt;on-desorpt;on procedure. The ad-sorption-desorption tests involved equilibra-tion of the mineral in the desired surfactantsolution and detennination of the initial sur-factant depletion (adsorption density) by re-moving a specific volume of supernatant andanalyzing for residual concentration. Desorp-tion tests were conducted by adding a diluentadjusted to the System ionic strength and pHat the same volume as supernatant removed.The total volume and thus solid to liquid ratioremain constant as the dilution stage is re-peated for many cycles with the surfactant

tdsorption in mollsnitial sulfonate concentration;esidual sulfonate concentration:otal solution test volumemass of adsorbent solids.

[Cj- C.]o£V/W]. . . initial adsorption.

~i - C,J = ~C.

,= [«C 1)a+ ~C_1)- C.",][V/W]

[(CrA-l)a + dC"-I] = CiJI

dilution factor [I - volume removedl V]

- initial adsorptionr ~ ~ adsorption after n dilutions.

Sulfonate concentrations were determinedeither by two-phase titration using dimidiumbromide/disulphine blue mixed indicator with10-) kmol/m) hexadecyltributyl ammoniumbromide (18, 19), or by UV absorbance at 223nm. Titrations were conducted for samples ofsulfonate concentration greater than 10-.kmol/m) with optical methods employed fordilute solutions.

The adsorption-desorption procedural flowdiagram is given in Fig. 2.

RESUl1'5 AND DISCUSSION

Adsorption-desorption results obtained forthe purified dodecylbenzenesulfonate/Na-ka-olinite system at pH 3.8 and an ionic strengthof 10-1 kmol/mJ Naa are given in Fig. 3. Inagreement with the results obtained earlier forthis type of system (1.3). an abstraction~ max-imum is exhibited. with the maximum locatedin the region of CMC as determined by thedye solubilization technique (20). Also. the

z A~ ~ts all ~bIe means of SUrtt1anl

depietion-adSO(1)tion. precipitation. and/or entrapment.

J.-1I.i.,C-"'N.l8...I ","".So' , Vol 114. No I. _1916

concentration monitored at each stage. Thusthe adsorption density at each step can be cal-culated from the foUowing:

r.[mol/g]=[Ci-Cr]X [VIWJ

where

r. = ~Ci =iC. = IV=1

W=I

ro=

Let [(

r,with

a-Ir.-i

SIRACUSA AND SOMASUNDARAN186

desorption pathways are found to depend onthe location of the initial (prior to dilution)abstraction point with respect to the maxi-

mum; further, the point at which dilution iscommenced from the position of the maxi-mum, larger is the hysteresis.

. .. , , , , , . , . . , . . . . . .'. . . jJ ,HoDGeS (V-RC-41/No-KAOL.INITE (CH-I) T. 3Otl.C

pH . 3..~0.3S/L . 0.2.

I . 10-1 W_I/",3 NoCI

'*' "~

. INITIAL ABSTRACTION

: }~O'" VOLUME DILUTION WITI1.a 10-1 kMOI/",3 HoCl AT pH. 3.B

. . . . . . . . . . . . . . . . ... . ...0 2 4 6 8 10 12 14 16 18 30 32 "40

RESIOUAt. SUt.FONATE CONCENTRATIOH, 11101,..3

FIG. 3. Adsorption-dcsorption iSOtherms for DOBS-Na-kaoIinite at pH 3.8 :t 0.3 in 10-1 kmol/mJ NaCl

at )O°C.

~cfC~_I-t-Sc"-. Vol. "4. No. I. - 1986

187SULFONA TES ON KAOUNITE: pH EFFECTS

For adsorption of ionic surfactants on min-erals which undergo hydroxylation and ion-ization, pH is an important variable since itcan affect the electrostatic interactions gov-erning the adsorption process (12-15). It isnecessary, therefore, for systems made up ofminerals such as kaolinite to take into accountpossible effects of pH change during equili-bration while interpreting adsorption data. Inthe present case, while the pH of the solutionswas adjusted to 3.0 prior to mixing with ka-olinite, the pH drifted to higher values duringthe equilibration. The pH 3.8 reported in Fig.3 is the average of the final pH values obtainedfor all tests. An examination of the pH changesfor each test along with the corresponding ab-straction values (Fig. 4) shows a systematicdecrease in the equilibrium pH values abovethe concentrations corresponding to the ab-straction maximum. It is to be noted that theabstraction will increase with any decrease inpH and therefore the abstraction isotherm inFig. 4 can be expected to exhibit an evensharper maximum if the pH was maintainedat constant values during the equilibration.

~~0e~

>-I-c;;Z1MQZ

QI-tjC~I-'"G"

XA

( . . .. I . . . . I ... . .. ,. I ... . . . .. f10-4 10-' 10-2 KS"1

RESIDUAL SULFONATE CONCENTRATION. ~I/""

FK8. 4. Abstraction density and pH :IS a function of ~dua1 DDBS concentration for Na-kaolinite in10-1 kmol/mJ NaCl at JOoC. Initial pH - 3.0.

J I..C ..,..~ Sci~. Val. 114. No. I. - 1-

Concentration dependence of abstractionunder constant pH conditions was detenninedby producing a family of abstraction versuspH curves and estimating, by interpolation,abstraction as a function of surfactant con-centration at the desired constant pH values.The resultant data given in Fig. 5 show all iso-thenns in the pH range 3 to 7 to exhibit asharp maximum, with the maximum appear-ing in the region 3 X 104 to 10-3 kmol/m:3sulfonate. The CMC of the system was deter-mined by dye solubilization to be at 2.6 X 10-4kmol/m3 sulfonate; therefore, in all cases, thedecrease in abstraction can be stated to ac-tually occur in miceUar solutions.

A comparison of the experimental isothermin Fig. 4, pH = 3.8 :t 0.3, with abstractionisotherms interpolated for constant pH con-ditions is made in Fig. 6. The experimentalisotherm has abstraction levels between theisotherms at constant pH 3.0 and 3.5 ratherthan between the isothenns at pH 3.5 and 4.0,even though the equilibrium pH was betweenthe latter values for all the experimental tests.This result suggests that most of the abstraction

188 SIRACUSA AND SOMASUNDARAN

the desorption isotherms were generated fromdilutions with solutions adjusted to the equi-librium or final pH of 3.8. Since the pH canalso be expected to playa major role in theprocess of desorption, the effect of such dif-ferences in the pH of the diluent was nextinvestigated. Toward this purpose, desorp-tion tests were conducted by adding to iden-tical systems diluents adjusted to different pHvalues.

Results given in Fig. 7 show that dilutionswith a more acidic diluent (pH = 3.0) thanthe equilibrium value (pH = 3.8) produce asharper increase in abstraction, yielding ab-straction values that are even larger than theinitial abstraction isotherm itself and thus apositive hysteresis. This increase in abstractioncould be due to both a greater electrostatic at-traction between sulfonate and kaolinite andalso an increased level of precipitation due toincreased kaolinite dissolution which is ex-pected with lower pH. On the other hand, useof the diluent adjusted to pH 3.8 yielded anegative hysteresis; the more alkaline diluent(pH = 9.0) results not only in negative hys-teresis but also in a significant decrease in ab-straction from the initial values. This effect ofdiluent pH can be more clearly seen by si-

V'

~..>-...Uiz...0

z2

~c..on~..

apparently occurred at pH values closer to theinitial pH of 3.0 and remained irreversible asthe system subsequently drifted to higher pHvalues. Thus. adsorption densities as a func-tion of residual concentration measured underconstant final pH do not correlate with ad-sorption densities generated from samples atconstant initial pH. A system under equilib-rium thennodynamic conditions. which im-plies reversibility. should exhibit a correlationbetween adsorption and pH. It is clear that analternative mechanism such as precipitationwith dissolved kaolinite species is also respon-sible for sulfonate depletion. Sulfonate deple-tion by precipitation would depend on thelevel of dissolved kaolinite species in solutionwhich in turn is dependent on pH. The pHhistory of the system becomes extremely im-portant and could account for the variation inadsorption isotherms for the experimental andinterpolated isotherms in Fig. 6.

ft is to be noted at this point that eventhough most of the abstraction has apparentlytaken place at close to pH 3.0 rather than 3.8.

~ ..(~ eMt-r- ofc;.,.,.-o

189SULFONA TES ON KAOUNITE: pH EFFECTS

importantly, desorption hysteresis can also becaused by pH perturbations, with either a re-sulting positive or negative hysteresis depend-ing on the pH of the diluent. These results alsopoint out the need for precise control of pHduring adsorption tests, a criterion that israrely mentioned as having received attentionin the adsorption literature.

The present desorption results were ob-tained by dilution with electrolytes adjustedfor pH and ionic strength. Removal of kaolin-ite-electrolyte-sulfonate supernatant and re-placement with just electrolyte will cause a de-crease in concentration not only of the sul-fonate in the system, but also of any dissolvedmineral species. This effect was tested by con-ducting desorption tests using an electrolytesolution precontacted with kaolinite in the ab-sence of sulfonate as the diluent. Results ob-tained under natural pH conditions (nopreadjustment as to minimize kaolinite dis-solution) at 27°C are shown in Fig. 9. A max-

0 2 4 6 8 10 12 14 16RESIDUAL StJ..FOfIATE CONCENTRATION, moI/m'

FIG. 7. Desorption isotherms generated from pH-ad-justed dilutions ofDDBS-kaoIinite supernatants at pH 3.8in 10-1 kmol/m' NaO at )O°c.

multaneously examining the changes in boththe abstraction and the system pH as dilutionis made (Fig. 8). Here, while the pH 3.0 diluentwas found to result in a decrease in the systempH (3.8 to 3.3) with abstraction increasingfrom 15.6 to 20.4 micromol/g before decreas-ing, the pH 3.8 diluent was seen to produceonly a slight increase in pH and also a shal-lower abstraction maximum. Dilution with thealkaline solution produced the largest changesin both the system pH and the abstraction.The pH increased sharply from 3.8 to 7.9 dur-ing the dilution process while the abstractionremained similar to those obtained at pH 3.8until the fifth dilution and then decreased bya factor of 2. This decrease in abstraction par-alleled the increase in pH to 7.9. a pH whichis well above the isoelectric point (pH = 4.5)of kaolinite (21). These results show that de-sorption of the anionic sulfonate from kaolin-ite can be achieved by pH adjustment alone,but only after the system has been driven topH values well above its isoelectric point. More

JCIW.w.fu~IINiI"' s.,-". Va No.I._I916

SIRACUSA AND SOMASUNDARAN190

to.."b.,

.-DO8S (Y-RC-41/No-KAOLINITE (CH-1)

T. 27"C51\.. 0.2

1. t(j'lkmol/.,l HoCI

~."

~. INITIAL ABSTRACTION (pH. 4.25 t 0.1)""

""'- -',~~- ".

"'0."~,.

30I

281D

26''I i.!-Mj

-22~- 201I-u.. 18E L"I-~ 16m4... 14I-4 12z

f 10..I

i 8

6

4.200 4 8 12 16 20 24 28 32 36 40 44

R£SIOUAL NaOOBS CONCENTRATION, mol/m'

FKJ. 9. Adsorption-desorption isothenns for ODDS-kaolinite at pH 4.3 :t: 0.1 in 10-1 kmol/mJ NaG at27°C. Dilutions were made with kaolinite-NaCl supernatant at natural pH 3.9.

a

8} eo% ~LUME DILUTIONS WITH~ Na-KAo.-INITE SUPERNATANT:0 [pH . 3~]

electrolyte-sulfonate supernatant it replaced(pH = 4.3), although both were generated un-der identical conditions. As discussed earlier,the presence of sulfonate results in an increasein the system pH. Therefore, the change in pHof the system due to the use of a more acidicsolution as the diluent will also produce in-creased abstraction (compare results in Fig. 7to those in Fig. 9). With decreasing pH. therecan also be an increase in the amount of sul-fonate depleted due to precipitation. sincemore dissolved metallic ions will be presentin the system at lower pH values. The extentof precipitation in the system was tested bycontacting the kaolinite supernatant with sul-fonate. Results given in Fig. 10 show precip-itation of almost 70% of the sulfonate at lowsulfonate concentrations and redissolution ofthe precipitate at higher concentrations. Suchprecipitation-redissolution has also been ob-served previously with dodecylbenzenesul-fonate in CaO2 and A1CI3 solutions (22, 23).

In the case of the kaolinite supernatant,there will be dissolved aluminum in solution

imum in both the abstraction isotherm andthe dilution isotherms is again observed. Thedilution isotherms show an increased abstrac-tion upon dilution or positive hysteresis withmemory effects also apparent, since the fartherthe- maximum is from the sulfonate concen-tration at the onset of dilution, the larger thehysteresis will be.

The difference in the behavior of the systemdiluted with NaG-kaolinite supernatant andthat diluted with NaCI solution alone pointsout the significance of the effect of dissolvedmineral species in the supernatant on pro-cesses responsible for abstraction. Positivehysteresis obtained with the kaolinite super-natant clearly suggests that precipitation of thesulfonate by dissolved kaolinite species maycontribute significantly to the total abstraction.Such precipitation, resulting in a positive hys-teresis, can indeed also contribute toward theoccurrence of the abstraction maximum.

It is to be noted that the natural pH (3.9)of the kaolinite-electrolyte supernatant usedfor dilution is lower than that of the kaolinite-

"-884(C"""",--Sc'-Y. V~. "4, No. I. - 1916

191SULFONA TES ON KAOUNITE: pH EFFECtS

000

_0~o.

o.O.0.1

~ (V-RC~)/UAOI.INlTE (CR-l),-31ft 1°C

$/l-o'~l 31-10 k801/8 ~l

-0 4 8 12 16 31 24 28 32 36 1103

AESlOOAl SULF~TE CQNC. 1101/11

FIG. 10. Ratio of ~ua1 (C.) to initial (Ci) concentrationas a function of ~ua1 DDBS concentration for DDBScontacted with kaolinite-NaO supernatant at pH 3.9.

Al3+ + 2H2O ~ Al(O8)! + 2H+

pKs = 9.249

Al3+ + 3820 ~ Al(0H)~ + 38+

p~= 14.936

Al3+ + 4820 ~ Al(08k + 4H+

pK7 = 23.255

Al3+ + 3820 ~ Al(08Ms) + 38+

pK. = 8.052.

The above equations have been solved usingthe appropriate mass balance equation to o~tain the data given in Fig. II for A13+ in equi-librium with kaolinite as a function of pH.Using this data and the concentration product.K~, estimated earlier for aluminum sulfonate[2.5 X 10-17 (22»), the pH of precipitation ofAI(DDBS») can be calculated for various sul-

in amounts dictated by the following equilib-ria:

AI2Si2Os(0H).(s) + 6H+ ~

2Al3+ + 2~Si04(aq) + H2O

with the solubility product, K"" related to theactivities, a;:

2 2a AlJ+a H.s;ogaH..nK.., ='- -6'-aA~itO,(OH),a H+

pKsp = -5.958.3

Assuming the concentration of dissolved spe.cies will be low such that the activity of wateris 1.0, and the kaolinite is pure (ai = 1.0). the

equilibrium can be simplified to

Ksp = ai.J+a~at.+.

The dissolution of kaolinite consumes H+while releasing aluminum and silica speciesand is therefore pH dependent. The releasedaluminum and silica species can also undergohydrolysis reactions that must be considered:

H4Si04(aq) ~ H3Si04 + H+ pK, = 9.838

H~i04(aq) ~ H2SiO~- + 2H+ pK2 = 22.938

H..si04(aq) ~ Si~s) + 2H2O pK3 = -4.051

A13+ + H2O ~ AI(OHY+ + H+ pK4 = 5.005

pH

FKi. 11. Log activity of Ar+ as a function of pH com-puted from the dissolution of kaolinite.

) Equilibrium constants were calculated from thenna-

dynamic data gathered in Refs. (24-26).

s-.- ,.ClAM ,...,...~. Vol. 114. No. I. -- 1-

192 SIRACUSA AND SOMASUNDARAN

fonate additions. Assuming that the highestpossible surfactant monomer concentration isCMC (2.6 X 10-4 kmol/m3), the pH limit forbulk precipitation was estimated for the pres-ent system to be 4.9. However, since the sur-face pH can be expected to be lower than thebulk pH (due to the negative charge of thekaolinite surface), precipitation of AI(DDBS)3in the surface region can be expected to occurat bulk pH values higher than 4.9. Also, assuggested by earlier work (22), precipitationof various other aluminum sulfonate com-plexes [in addition to AI(DDBS)3] can also oc-cur in the pH region 6 to 9.8, all this resultingin increased sulfonate depletion due to pre-cipitation.

The abstraction given in Fig. 5 for theDDBS/kaolinite systems under constant pHconditions of 3 to 7 may all indeed includesurfactant loss due to precipitation. Redisso-lution of such precipitates in micellar solutionsshould produce a maximum in the isotherms.Furthermore, since A13+ from kaolinite dis-solution does increase with decreasing pH, theDDBS/kaolinite system with an initial pH ad-justed to 3.0 and a final value of 3.8 (Fig. 6)would have released higher amounts of A13+and hence caused more precipitation in thesystem than if dissolution and abstraction hadoccurred at a constant pH of 3.8. The higherabstraction levels obtained in the systems withmore acidic diluents can also be attributed toincreased kaolinite dissolution and subsequentsulfonate precipitation.

SUMMARY

Abstraction of sulfonates by kaolinite hasbeen studied here along with desorption as afunction of pH. The nature of the maximumand the hysteresis exhibited by the isothermsis found to strongly depend upon the pH his-tory of the system. Both positive and negativehysteresis can result depending upon the pHchange of the system during equilibration orduring dilution. These effects are explained interms of, in addition to adsorption, precipi-

.--"'C""-I-r-s.:'-r.Volll..No.I.~berl986

tation (and redissolution) of aluminum sul-fonate which in turn is governed by pH-de-pendent dissolution of the mineral.

A comparison of iso-pH isothenns withthose under usual conditions where pH is al-lowed to drift shows most of the abstraction(resulting from precipitation) to occur close tothe initial more acidic pH values and to remainirreversible. Significant desorption occurredupon pH adjustment alone and only whendiluents at pH levels above the isoelectric pointwere used. The implications of the effect ofpH change on adsorption and precipitationshould be noted since pH control during theentire adsorption process itself is rarely re-ported in the literature.

The effect of the decrease in the concentra-tion of the dissolved mineral species upon theaddition of the diluent was determined by us-ing kaolinite supernatant as the diluent. Thedissolved species in these systems are suggestedto contribute significantly to the abstractionas well as the occurrence of the maximum.

The abstraction data has been treated usingthe dissolution equilibria for kaolinite, and itis seen that precipitation of aluminum sulfo-nate can be a major factor for this system evenunder natural pH conditions. Tests conductedby contacting the sulfonate with kaolinite su-pernatant shows in fact almost 70% of the sul-fonate to precipitate in the present case at lowconcentrations and all of it to redissolve athigher concentrations.

This study has clearly shown the effect ofpH and pH perturbations on abstraction aswell as desorption of ionic surfactants such assulfonates, and the major role of precipitation-redissolution of the surfactants in determiningthe extent of abstraction and the hysteresis in-volved. Also, the study of desorption is foundto be a necessary means for developing a fullunderstanding of adsorption phenomena.

ACKNOWLEDGMENTS

Support from the Department of Energy (DE-ACI9-79BC-IOO82), the National Science Foundation (CPE-8201216), Amoco Production Co., OIevron Oil Field Re-

193SULFONA TES ON KAOUNITE: pH EFFECI'S

sean:b, Exxon Research aDd Engineering, Gulf Researchand Development, Sbel1 Development Co" Standard OilCompany, Texaco, Inc" and Union Oil Company orCa!-ifomia is gratefully acknowledged.

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IS. Scamerbom, J. F., Schechter, R. S., and Wade, W. H.,J. Colloid Inlet/ace Sci. 85. 463 (1982).

16. HoDander, A. F., Somasundaran, P., and GIYte, C. C.,J. App/. Polymer ScL 26 (1981).

17. Somasundaran, P., .. Adsorption from Aooding S0-

lutions in Porous Media." Annual Repon sub-mitted to the Depanment or Energy, ColumbiaUniversity. New York, 1982.

18. U, Z., and Rosen, M. J., Anal. Chern. 53, SI6 (1981).19. Reid, V. W., Longman, G. F., and Heinerth, E., Ten-

side., 292 (1967).. 20. Corrin, M. L., and Harkins, W. D., J. Amer. Chern.

Soc. 69, 683 (1947).21. Van Olpnen, H., "An IntroduCtion to Oay Colloid

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masundaran, P., SPE Paper 11796, Presented atInt1 Symp. on Oilfield and Geothennal O1emistry,Denver, 1983.

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2S. Stumm, W., and MOrIan,J. J., "Aquatic Chemistry,"2nd cd. Wiley, New York, 1981.

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(1961).9. Mukeljee, P., and Anavil, A., in "Adsorption at In-

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.CIJBDid _1.-1- &'--. Val 114. No. I. NoWI8bcr 1986