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Adsorption of Anionic-Nonionic and Cationic-Nonionic Surfactant Mixtureson Kaolinite
QUN XU, T. V. VASUDEVAN, AND P. SOMASUNDARAN1Henry Krumb School of Mines, Columbia University, NN' York. New York 10027
Received May II, 1990-. accepted September 18. 1990
Adsorption on Na-kaolinite from surfactant mixtures of nonionic - ()ctaethylene glycol mono-n-dodecyl
ether (C.zEo.) with the anionic sodium dodccyl sulfate (C.zSO.Na) and the cationic dodecyltrimethyJ..ammonium cbloride [C.z(CH)hNa) was studied in the surfactant concentration range varying frombelow CMC to above. It was found that in both systems, the adsorption of the ionic surfactant wasenhanced by the presence of the nonionic surfactant and vice versa in the pre-CMC region. Chain-chaininteraction between the adjacent ionic and nonionic surfactants on the partidcs has been proposed tobe responsible for the enhanced adsorption. Such chain-chain interaction leads to the formation ofsurfactant clusteB at the interface which abstract the hydrocarbon chains of the highly surface activeC,zEO. into them. Under these conditions hydrophobicity or the particles as measured by skin Ilotationwas found to be lower due to the exposed ethylene oxide poups. ~ in the adsorpCion levd ofC.zSO.Na with i~ in CllEo. in the region above CMC has been attributed to the decrease ofC.zSO.Na monomer concentration due to mixed micellization with C.zEO.. The adsorption density ofC,zEO. in plateau region passes through a ma."imum at equimolar ratio of the two surfactants. This hasbeen suaested to be due to the most compact arrangement of surfactants in the cluster formed 3t theinterface at the equimolar composition of the surfactants. Similar adsorption behavior from anionic-nonionic and cationic-nonionic surfactant mixtures suggesU that the synergism arises mainly from thechain-chain interaction between the two surfactants in the adsorbed state. C 1991 Academic ~ IK.
Surfactant depletion due to adsorption onreservoir minerals and precipitation with dis-solved cations is a major problem in chemicalOooding for enhanced oil recovery (I). Use ofmixtures of anionic and nonionic surfactantsis receiving increasing attention since precip-itation is significantly less in these systems (2).An understanding of the adsorption behaviorof each surfactant component from mixturesis helpful for development of such applicationsince chromatographic / selective adsorptionwill alter the compositional balance requiredfor optimum oil recovery. Adsorption fromsurfactant mixtures on minerals such as alu-mina and silica has been reported in a fewstudies (3, 4). However, information on theadsorption on kaolinite, one of the major res-
ITo whom co~ndence should be addressed.
5280021-9797191 S3.00copy;.-. c 1991 by NadcIIIk Pr-.1~.AI ol, oac-.. i..., iIn8.-r.-. ~.c~~ ~Sc;-.y. Vol 142. No. 2. ~ 15. 1991
ervoir minerals with more complex minera-logical composition and surface propertiesthan either alumina or silica, has been scanty(5). Understanding of the adsorption mech-anism from previous studies (5) was made dif-ficult since adsorption was conducted only inthe surfactant concentration range well aboveCMC and the nonionic sur;factants used werepolydispersed in nature.
In this work. adsorption of ionic-nonionicsurfactant mixtures on kaolinite was studiedusing isomerically pure surfactants. Adsorp-tion tests were carried out in a .wide range ofsurfactant concentration varying from wellbelow CMC to above. Since kaolinite possessespositive and negative sites (6. 7) adsorptioncharacteristics of both cationic-nonionic andanionic-nonionic surfactant mixtures werestudied to elucidate the role of electrostaticadsorption.
SURFACTANT ADSORPTION ON KAOLINITE
.', . the suspension to stand for 72 h at room tem-perature (25 :t 2C). The vials were periodi-cally shaken manually every 24 h. The sus-pensions were then centrifuged and the su-pernatants were analyzed for residualsurfactant concentration. Surfactant depletionfrom solution was used to calculate the ad-~rption density.
~um dodecyl sulfate concentration was~etermined using a two phase titration method
(9,10). Dodecyltrimethylammonium chlorideconcentration was determined by complexingthe surfactant _wi~~x~ CtlSOo4l'la andmeasuring the noncomple-xed Ct2S0~a usingthe two phase titration method (9, 10). Anal-ysis of octaethylene glycol mono-n-dodecylether (Ct2EO.) was conducted using a highpressure liquid chromatograph provided witha refractive index detector.
RESULTS A~"D DISCUSSION
Adsorption from Single Surfactant Solutions
Isotherms for the adsorption of anionic s0-dium dodecyl sulfate and cationic dodecyltri-methylammonium chloride are sbo~-n in Fig.1, while that of the nonionic octaeth~"ene gly-col mono-n-dodecyl ether is given in Fig. 2.It can be seen from Fig. 1 that both the anionicCszSO.Na and the cationic Cs2(CH3)3NOadsorb on kaolinite at the same pH value. This
Well-crystallized Georgia kaolinite waspurchased from the clay repository at the Uni-'lenity of Missouri and was converted to Na-kaolinite by ion-exchange ( 8 ). Specific surfacearea of the sample obtained by nitro-gen BET method using a Quantasorb was8.2 m2/g.'
The anionic sodium dodecylsulfate(C'2S04Na) and cationic dodecyltrimethyl-ammonium chloride [C'2(CH)~a] of>99% purity were purchased from FlukaChemicals and Tokyo Kasai Chemicals, re-spectively. The nonionic octaethylene glycolmono-n-dodecyl ether, CH)(CH2),,(OCHrCH2)aOH or C'2EOa, purchased from NikkoChemicals, was of >97% purity. Since nominimum was found in the surface tensioncurves of these surfactants they were used asreceived.
'~i . ,-r---~x
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Adsorption tests. Three gram samples ofNa-kaolinite were contacted with 15 ml of0.03 M NaG solutions for 2 h in Tefloncapped glass vials. Surfactant solutions of de-sired concentration were then added to makethe total volume to 30 mI. The solids wereconditioned with the surfactant by allowing
D- - - oJ1 10 ,~ ,.RESIDUAL ICI2Eq.J k8D1/.3.IOS
fJO. 2. lsothenns for octaethyleoe glycol mooo-n-do-decyl ether on kaolinite. Ionic stIeolth = 0.03 .\i NaCi.2S.C.
c...,.., ~ Vol. 142. No. 2. ~ 15.1"
would suggest a hydrogen Ifor C'2EOa adsorption as proposed for(14) and nonionic TX-lOO (15)on silica. Reduced adsorption at highervalues can be attributed toof the surface hydroxyl groups ( 16,will result in a decrease in the number ofdrogen bonding sites availablesorption. Using a moleculararea of 65 A 2 ( 11) for C'2EOa in the adsorbedstate, the surface coverage at saturation canbe calculated to be 12% which would also rep-resent the percentage of the kaolinite surfacecovered by groups capable of hydrogen bo~d.ing-with me nonionk surfactant.
An important inference that can be drawnfrom the adsorption studies discussed aboveis that, unlike alumina which barely adsormethoxylated alcohols (3), and silica which ad-~ethoxy1ated alcohol (3) but notC,~.Na,kaolinite [(A1(OHhh.O.(Si~h], adsormboth C'2SO.Na and CI2EOs to about the sameorder of magnitude indicating that the kaolinitesurface exh1bits the characteristics of both alu-"mina and silica with respect to the adsorptionof these surfactants.
is consistent with an electrostatic adsorptionmechanism for this system since both positiveand negative sites coexist on kaolinite surface(6. 7). The above adsorption mechanism isalso supported by the observed decrease inC1~.Na adsorption as the pH is increasedfrom 5 to 10. since the number of positivesites on kaolinite does decrease with increasingpH (6.7).
FUrther examination of the isotherms showsthat the saturation adsorption density in thecase of cationic CI2(CHJ)JNO is 4.35 X 10-6mol/m2 which coincides with the monolayercapacity (4.37 X 10-6 mol/m2) calculatedbased on a-cross-seetional--area ofJ8-A-1-ttl}:This suggests that the cationic surfactant ad-sorbs not only on the negative sites but alsocovers the positive sites indicating the possi-bility of two-dimensional condensation. Sucha mechanism for cationic surfactant adsorp-tion has also been suggested by Cases el al.(12).ln the case of anionic C12S0.Nathe sur-face coverage at plateau. calculated based ona cross-sectional area of 53 A 2 (II). is 56%which indicates that about half of the surfaceis covered by positive sites.
It has been reported by Cases el al. (12)that the positive sites are present only on thebroken edges and that even in most crystallinesamples the broken edges constitute only 40%of the total surface area. Since the surface cov-erage of the anionic C1zSO.Na is estimated tobe above 50% it appears that part of the sur-factant adsorbs on negative sites through Na +
counter ions ( 13) or positive sites are presentalso on the basal faces. The positive sites onbasal faces can arise from the precipitation ofdissolved aluminum hydroxide complexes.
It can be seen from Fig. 2 that the adsorptiondensity of octaethylene glycol mono-n-dodecyiether (CI2EO.). as in the case of sodium do-decyl sulfate (CI2SO.Na). also decreases withan increase in pH. Similar decrease in adsorp-tion with increase in pH has been reported forpoly(ethylene) oxide (PEO) on silica (14).This similarity coupled with the fact that theethylene oxide groups are the common hy-drophilic moiety for both PEa and CI2Eo.
Adsorption from Surfactant Mixtures
The effect of miring the ionic (anionic andcationic) and nonionic surfactants on the ad-sorption behavior of individual componentsis shown in Figs. 3-7. The isotherms in thesefigures have been divided in two regions belowand above mixed CMCs of the solution, inorder to facilitate explanation of the results.
Anionic-nonionic surfactants system. Ad-sorption isotherms. Preplatea:u region: Theisotherms for the adsorption of sodium do-decyl sulfate (C 12SO.Na) from mixtures of dif-ferent composition are shown in Fig. 3. whilethose of octaethylene glycol mon.o-n-dodecylether (CI2EOs) are plotted in Fig. 4. (t can beseen from Fig. 3 that the affinity ofC12S0.Nato the surface is enhanced by the presence ofCI2EOs as indicated by the continuous shiftof CI2S0.Na isotherms toward the left withincrease of the molar ratio of CI2EOs. This
~ .c~ ~ ~ Scw-. Vol 142. No. 2. w-. 1 S. 1"1
SURFACTANT ADSORPTION ON KAOUNITE 531
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tain level of adsorption is reduced resulting ina shift of the isotherm toward the. left.
Another feature to be noted on the sodiumdodecyl sulfate (C 12SO.Na) adsorption iso-therm is the decrease in the slope of the iso-therm as the molar ratio of octaethylene glycolmono-n-dodecyl ether (CI2EO.) is increased.For adsorption of pure C12S0.Na on kaolinite,a slope of I is obtained on a logarithmic plot.indicating the absence of lateral chain-chaininteractions (18). A value less than I for theslope.u-seen in ng. 3, -suggests electrostatichindrance for CI2S0.Na adsorption resultingpossibly from the masking of the positive sitesby adsorbed ethylene oxide chains (fig. 8b).
The adsorption of CI2EO.. similar to thatofC1~.Na. is enhanced by the presence ofC1~.Na as seen from a continuous shift inthe isotherm to the left with increase in themolar ratio of CI2SO.Na. This can a~n beattributed to the chain-chain interaction be-tween C1~.Na and CI2EO.. It is conceivablethat due to chain-chain interaction betweenadsorbed CI2SO.Na and CI2Eo.. mixed sur-factant clusters will be formed at the interface.These mixed surfactant clusters, serving ashydrophobic POOls. can enhance the adsorp-tion ofCI2EO. by abstracting the hydrocarbon
!!:.-~I~ . - --", =
FKi. 3.lSOthenns for the adsorption ofC,~.Na fromC,~.Na/ClzEO. mixtures on kaolinite. Ionic strength= 0.03 M Naa. pH S, 2SoC. Arrows indicate critical mi-cdJe concentration of the conaponding surfactan1S.
can be explained in tenDs of the coadsorptionof C1zSO~a and CI2EO. as shown schemat-:c::lly in Fag. 8; CI2SO.Na adsorption is as-sisted, in addition to its electrostatic interac-tion with the positive sites on kaolinite, by thechain-chain interaction with CI2Eo. adsorbedon adjacent sites. Because of the free energygain from such chain-chain interactions theresidual concentration, or the chemical p0-tential, ofCI2SO4Na required to achieve a cer-
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AG. 4. Isotbenns for the adsorption of CllEo. from C1~.Na/C'lEo. mixtures on kaolinite. Ionicstrenath - 0.03 M NaC. pH S. 2S.C.
~.;C~.v '~Sc--. Vol 142. No. 2. M-" IS. 1991
xu. V ASUDEV AN. AND SOMASUNDARAN532
~"'2 .. x"-
I I~. .
-Ct2S0.Ha AOSOAPTICM JI3 ~-Ct2E~ ADSORPTION 5
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---C==-~~~c"2Eco;J, k801 '~x"iP
FKi. 7. Isotherms for CI1EO. adsorption on kaolinitefrom C,1(CH3),NO/CI1Eo. mixtures. Ionic strength= 0.03 MNaO, pH 4.5-5, 25C.
FIG. S. Skin flotation of kaolinite after mixed surfactantadsorption. C1~4Na concentration fixed at 1.0 mi\.!.Ionic strength = 0.03 M NaG. 25C. pH 5.
crease above CMC, while that of nonionicsurfactant does decrease (3, 19). A consistentdecrease of CI2S0.Na adsorption in plateauregion is observed as the molar ratio ofCI2EOsin the mixture is increased. This can be at-tributed to (a) reduced CI2S04Na monomerconcentration due to mixed micellization withCI2EOa as indicated by the decrease in CMCwith increase in the molar ratio ofCI2EOa and(b) masking of the positive sites that are re-sponsible forCI2S04Na adsorption by theeth-ylene oxide chains of the adsorbed CI2EOa.
chains of the highly surface active nonionicsurfactant into the clusters leaving the ethyleneoxide segments protruding into the bulk s0-lution, as shown schematically in Fig. 8c.
Adsorption isotherms. Plateau region. It canbe seen from Fig. 3 that in plateau region(above CMC) the adsorption of sodium do-decyl sulfate (CI2S04Na) while alone reachesa plateau, whereas in the presence of octaeth-ylene glycol mono-n-dodecyl ether (CI2EOs)it continues to increase even above CMC. Thisis because in mixed surfactant systems, unlikein single surfactant solutions, the monomerconcentration of anionic surfactant does in-
... I.0.~ I~ ;== ;Do ,. I0 iat .
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RES[mIfIt. [c.2(~)3NCIJ. 1c_1/83x1r1
fIG. 6.lsothem1 fordodecyltrimethylammonium chlo-ride adsorption on kaolinite from CI2(CHJhNC/C1,Eo.mixtures. Ionic strength = 0.03 ,\1 NaO, pH 4.5-5, 25C.
Jounlal ~Collo;da..J /",~Srinoc'. v... 142. No.2. Match IS. 199\
FIG. 8. Schematic representation of the mechanism ofsurfactant adsorption on kaolinite. (a) Chain-chain in-teraction between adjacent C1~4Na and CI2EO, mole-cules results in enhanced adsorption in the lower concen-tration range (pre-CMC region) for both surfactants; (b)electrostatic hindrance for further C1~4Na adsorptionresults from the masking of positive sites on kaolinite byethylene oxide chain adsorption; (c) formation of mixedsurfactant clusters and abstraction ofCl2Eo. hydrocarbonchains into the clusters.
533SURFACTANT ADSORPTION ON KAOLINITE
ether (CI2EOs) into the hydrophobic poolfonned by the clusters of mixed surfactant isalso supported by the (skin) flotation results.Since such an abstraction would result in ori-entation of ethylene oxide chains toward thebulk, the floatability of kaolinite decreases asthe adsorption of CI2EOs is increased in thepresence ofC12SO.Na (Fig. 5).
Cationic-non ionic surfactants system: Ad-sorption tests for this system were carried outat 1:1 molar ratio of the two surfactants. It~nbe-seenfrom-Figs.-6-and 7 thatthe-~thenns obtained for the adsorption of cationicdodecyltrimethylammonium chloride (C1~-(CH3)~a) and nonionic octaethylene glycolmono-n-dodecyl ether (CI2EOs) exhibit fea-tures similar to that found for C12S04Na-CI2EOs system indicating that the adsorptionmechanism is the same for both systems.
In the case of the nonionic surfactant, sincethe monomer concentration does decreasewith increase in the total surfactant concen-tration (3, 15), the isotherm of octaethyleneglycol mono-n-dodecyl ether (CI2EO8) will beexpected to pass through a maximum at theCMC of mixed surfactants. However, it canbe seen from Fig. 5 that CI2EO8 adsorptionisotherms in plateau region are either flat orhave a positive slope. This is attributed to theincreased affinity ofCI2EO8 toward the surface-mooLfied-by-tBeadsorbed Ci2S0.Na.-As can-be seen from Fig. 3, the adsorption ofCI2S0.Na increases even above mixed CMC.The effect of such continued modification ofthe interface by CI2S0.Na adsorption appar-ently outweighs the effect of decrease inCI2EO8 monomer concentration. It is also seenfrom Fig. 4 that the plateau adsorption densityvalue of CI2EO8 is, in all cases. enhanced bythe presence ofC'2S04Na. Such an enhance-ment can be accounted for, as explained ear-lier, by the abstraction of additional C1~EO8chains into the mixed surfactant clusters atthe interface (see Fig. 8c). Such hydrocarbonchain abstraction is apparently highly depen-dent on the organization of the mixed surfac-tant clusters at the interface and thereforemaximum adsorption increase occurs at acertain molar ratio between the two surfac-~nts. As is seen in Fig. 4, the maximumCI2EOs adsorption in plateau region occurs atabout 1:1 molar ratio between CI2S04Na andCI2EOs. The dependence of CI2EO8 hydro-carbon chain abstraction on the organizationof mixed surfactant clusters is also revealed bythe change in slope of the CI2EO8 isothermsin preplateau region. As seen in Fig. 4, theslope of the adsorption isotherm increases inhe presence ofC'2S04Na with the maximum
slope occurring around I: I molar ratio, indi-cating a more compact arrangement of the ad-sorbed surfactants in the cluster, which even-tually leads to maximum C,~EOs abstractionas shown by the highest plateau adsorptiondensity value at this ratio.
The theory of abstraction of hydrocarbonchain of octaethylene glycol mono-n-dodecyl
Synergistic interaction between surfactantswas observed during the adsorption on ka-olinite for both anionic-non ionic and cat-ionic-nonionic surfactant mixtures. In thesebinary surfactant systems, affinity of the ionicand the nonionic surfactant toward the surfacewas found to be enhanced by the presence ofone another in the pre-CMC region. Increasedchain-chain interaction and the formation ofmixed surfactant clusters have been proposedto be the main reasons for the observed syn-ergism. Above CMC, sodium dodecyl sulfate(CI2S04Na) adsorption decreased with in-creasing octaethylene glycol mono-n-dodecylether (CI2EOa) composition and this was at-tributed to the decrease in CI2S04Na mono-mer because of the decrease in CMC for themixed surfactants. The abstraction of hydro-carbon chains OfCI2EOa into the hydrophobicpools formed by mixed surfactant clusters isproposed to be responsible for the observedadsorption increase above mixed CMC. Hy-drophobicity of the particle was found to belowered under such conditions due to the ori-entation of the ethylene oxide chains in thebulk solution. Such abstraction is apparently
}""noal.JfC.JII"id cutJ I"'..,(at:~ s.:i~~. Vol. 142. No.2. March IS. 1991
dependent on the organization of the surfac-tants in the cluster and was therefore depen-dent on the composition of the surfactantmixtures with the maximum abstraction ofCl2EOs occurring at the equimolar ratio of thesurfactants. Similarities observed between theanionic-nonionic and the cationic-nonionicsurfactant mixtures suggest that the synergismobserved in mixed surfactant system is mainlydue to hydrophobic interactions between thehydrocarbon chains of the ionic and the non-ionic surfactants.
Financial support of the National Science Foundation(NSF Contract NSF-CBT-86-ISS24-AO2), Department ofEnergy (DOE Contract DE-FG22-89BCl4432), UnileverResearch, Engelhard, and Standard Oil Company ofObiois gratefully acknowledged.
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