I()URN AL OF COLLOID AND INI1!RF ACE SCIENCE 177, 222 - 228 ( 1996)

Article No. 0024

Adsorption Behavior of Cationic and Nonionic Surfactant Mixturesat the Alumina-Water InterfaceLEI HUANG, C. MAL TESH, AND P. SOMASUNDARAN 1

Langmuir Center for Colloids and Inte/fGCe.s, CollIIIIhiD University. New Yort. New Yort 10027

Received March 6, 1995; KcqJted J- 7. 1995

The solution and interfacial behavior of surfactant mixtureswas investigated using a cationic surfactant, tetradecyl trimethylammonium chloride (IT AC), and a nonionic surfactant, pentade-cylethoxylated nonyl phenol (NP-IS), with alumina as the sub-strate. The cationic lTAC adsorbed at the alumina-water inter-face as a result of electrostatic attraction at pH -10 whereas thenonionic NP-1S did not. Interestingly, in the mixed surfactantsystem, the tetradecyl trimethyl ammonium chloride forced ad-sorption of the NP-IS as a result of hydrophobic interactions be-tween the adsorbed surfactant chains at the alumina-water inter-face. The adsorption behavior was dependent upon the ratio ofthe two surfactants in the mixture as well as the order of addition.With an increase in Tf AC content, the adsorption density of NP-IS increased and the isotherm shifted to lower surfactant concen-trations. The adsorption of IT AC decreased under conditions ofsaturation adsorption due to the bulkiness of the coadsorbed NP-I S as well as competition for common adsorption sites. However.below saturation adsorption conditions, the adsorption of IT ACwas increased due to synergistic interactions between the cationicand nonionic heads leading to reduced repulsion among the cat-ionic head groups. Zeta potential measurements showed that, wjthan increase of NP-IS in the mixtures, the positive charge of thelTAC was partially screened by the coadsorbed NP-IS. Surfacetension of the surfactant mixtures was also measured, and regularsolution theory was used to model the interactions between thesurfactants. Monomer concentrations obtained using the theorywere correlated with the adsorption tendencies and it was seenthat IT AC adsorption corresponded wjth Its monomer concentra-tion but that of NP-IS did not, suggesting the need for adequatetheories for interactions between surfactants. Q 1996 Academic ~ Inc.

Key Words: ethoxylated alcohols; cationic-nonionic surfactantmixtures; surfactant mixture adsorption; alumina-water inter-face; tetradecyl trimethyl ammonium chloride (lTAC).

nificantly the performance over those of single componentsystems. In processes such as enhanced oil recovery, how-ever, surfactant adsorption on reservoir minerals has to beavoided. since it contributes to undesirable reagent loss.Mixed surfactant systems offer several advantages over sin-gle components since the adsorption of surfactants on thereservoir minerals can be controlled using appropriate com-position and solution conditions. As a result, studies on theadsorption and solution behavior of surfactant mixtures haveincreased significantly in the recent past ( 1- 3).

Surfactant adsorption is related to the chemical potentialof the surfactant molecules (monomers) in solution and thenature of the solid. When surfactants of different characteris-tics are present together they will form mixed micelles withthe critical micelle concentration (CMC) varying with thecomposition of the mixture. Under mixed micellization con-ditions the chemical potential of monomers will be lowerthan that for the single surfactant system and this in turn canreduce adsorption at the solid-liquid interface. Interactionbetween surfactants in mixtures has been studied for severalsystems (4-8). Literature on solution behavior of surfactantmixtures suggests significant deviation in their behaviorfrom ideal mixing. Generally, mixtures of surfactants withsimilar head groups behave more ideally in solution, whereaslarge deviations from ideality are observed for mixtures ofdissimilar surfactants. In contrast to the case of solutions,there is very little information on interaction of surfactantmixtures at solid/liquid interfaces, especially for the cat-ionic-nonionic surfactant system, which bas wide applica-tion in several consumer products. The regular solution ap-proximation that has been applied to treat nonideal mixingin solutions has also been extended to model adsorption atthe liquid-air interface (9, 10). Although regular solutiontheory has been criticized in the past ( II ), it has provideda very useful means to describe the adsorption behavior ofmixed surfactant aggregates. In our study, adsorption of bi-nary mixtures of a cationic surfactant, tetradecyl trimethylammonium chloride (TfAC), and a nonionic surfactant.pentadecylethoxylated nonylphenol (NP-15), was investi-gated at alumina-water interface. The synergism and com-petition between the two surfactants are discussed and rela-

INTRODUCTION

The adsorption of surfactants at the solid -liquid interfaceplays a crucial role in many important industrial processessuch as detergency, flotation, and deinking. In many of theseapplications. the use of surfactant mixtures improves sig-

I To whom C(¥IeSpondeoce should be a(Idressed.

2220021-97971X> SI2.00Copyright C 1996 by Acadcmjc Press. Inc.AU riahIJ of repmduction in lilY fann ~ed.

CAnONIC AND NONIONIC SURFACTANT MIXTURES 223

tionships between adsorption behavior and monomer con-centration are developed using the regular solutionapproximation.

f - - -- -'""lxlO.-

00'MATERIALS AND METHODS

~I;: J

Ntj -7~ lxlO

.... .- . . ,A

IxIO"

'6IxIO" 1,.1.1 1...

IxIO-6 IxIO'" IxIO-4 IxIO-J IxIO-2

TTAC ~ CaIC.,bMII/mJ

FIG. I. Adsorption of tetradecyl trimethyl ammonium chloride (Tf AC)on alumina in the pesence and absence of pentadecylethoxylaled nonylphenol (NP-15). pH 10, I.S.0.03 M NaCl.

RESULTS AND DISCUSSION

The adsorption of mixtures of the cationic tetradecyl tri-methyl ammonium chloride and the nonionic pentadecyleth-oxylated nonyl phenol was studied at the alumina-liquidinterface at several ratios. The nonionic pentadecylethoxy-lated nonyl phenol does not adsorb at the alumina-waterinterface. In a study on the adsorption of polyethoxylatedmonolaurate (ML-n) and polyethoxyl lauryl ether (LE-n)on titania, Fukushima and Kumagai (13) concluded that foradsorption on polar surfaces it is necessary that the nonionicsurfactant have a moiety which has sufficient adsorptionforce to overcome the strong interaction between water mol-ecules and the ethylene oxide groups. This observation issimilar to our earlier results on the adsorption of polyethyl-ene oxide (PEa) on different minerals (14). It was reportedthat PEa adsorbs on silica but not on alumina or hematite,and it was proposed that for PEa to adsorb on oxide surfaces,the polymer has to displace enough water molecules bondedto the solid surface and create a strong entropic effect. Inthe case of the strongly hydrated alumina surface the poly-mer cannot displace sufficient water molecules necessary foradsorption. It is therefore reasonable that NP-15 also doesnot adsorb on alumina.

The cationic surfactant tetradecyl trimethyl ammoniumchloride, however, does adsorb on alumina at pH 10 (Fig.1). At this pH the alumina surface is negatively charged(isoelectric point of alumina is pH 8.9) and hence the elec-trostatic attraction with the cationic 1T AC will be dominant.There is a sharp increase in the adsorption density at around 5X 10-4 kmol!m3 which is due to the formation of surfactantaggregates (solloids) at the solid -liquid interface. The maxi-

Alumina. The substrate for adsorption was Linde A alu-mina from Union Carbide, specified to be 90% a-Al2O3 and10% y-A12O3 with a mean diameter of 0.3 J.tm and wasused as received. The specific surface area was 15 m2/g, asmeasured by the BET technique using nitrogen adsorptionon a Quantasorb system. In this measurement the samplewas degassed at 2500C for 12 h.

Surfactants. The cationic surfactant n -tetradecyltrimeth-ylammoniumchloride (TrAC), [CH3(CH2)13N(CH3)3]CI,from American Tokyo Kasei, Inc., and the nonionic pentade-cylethoxylated nonyl phenol (NP-15). C9HI9C~O(CH2-CH2O )15H, from Nikko Chemicals, Japan, were used as re-ceived. Surface tension measurements confirmed the highpurity [>99%] of these samples.

Reagents. Sooium chloride and sooium hydroxide fromFisher Scientific were of A.C.S. reagent grade.

Adsorption. Adsorption experiments were conducted incapped 20-ml vials. Two-gram samples of alumina weremixed with 10 ml of 0.03 M NaO solutions for I h atroom te.mperature. The pH was adjusted as desired and thesuspension allowed to equilibrate further for I h. Then, 10ml of 0.03 M NaCl solution containing the surfactant(s)at desired concentration was added and the satnples wereequilibrated for 15 h. The pH was measured and, if neces-sary, adjusted using 0.1 M NaOH. The samples were allowedto equilibrate for about three more hours after final pH ad-justment and then centrifuged for 25 min at 5000 rpm; about20 ml of supernatant was then removed with a pipette foranalysis.

Surfactant concentration analyses. Tetradecyltrimethy-lammoniumchloride concentration was measured using atwo-phase titration technique (12). Pentadecylethoxylatednonyl phenol concentration was analyzed by UV absorbanceat 223 or 275 nm using a Shimadzu 1201 UV-Vis spectro-photometer.

Surface tension. Surface tension of aqueous surfactantsolutions was measured using a Fisher Mooel 20 ring tensi-ometer and applying appropriate correction factors.

Electrokinetic studies. Zeta potential measurementswere made using a PEN KEM, Inc. Laser zee meter Model50 I system.

Solution conditions. All experiments were conducted atan ionic strength of 0.03 M NaCI and at a pH of 10.0 :!: 0.5.Triply distilled water of conductivity 1-2 X 10-60-1 cm-1was used for preparing all solutions.

224 HUANG. MALTESH, AND SOMASUNDARAN

lxlO.

~ 74' ~- B0"'" ,~/ ,.

;l°A,or~ ""° 0

,

the more effective coadsorption of NP-15 as the number oflTAC aggregates at the alumina-water interface increases.It is to be noted that the concentration of tetradecyltrim:ethyl-ammoniumchloride does vary from point to point along theNP-15 adsorption isotherm.

The NP-15 molecules are much larger than the TI'AC'molecules and the adsorption of the pentadecylethoxylatednonyl phenol will reduce the area available for TI' AC. Themicrostructure of the hemimicelles and micelles will be dif-ferent in the single surfactant and the mixed surfactant sys-tems particularly due to the shielding of the ionic heads ofthe tetradecyl trim:ethyl ammonium chloride by the coads-orbed NP-15. This in turn will reduce electrostatic attractionbetween the negatively charged alumina surface and the pos-itively charged lTAC molecules, thereby reducing lTACadsorption density. This hypothesis was tested by monitoringthe zeta potential after adsorption and the results are shownin Fig. 3. The zeta potential of alumina particles will bedecided mainly by the adsorption of the oppositely chargedTI'AC molecules. It is recognized that the nonionic surfac-tant species at the alumina - water interface can also mask thesurface charge and also cause a reduction in the magnitude ofthe zeta potential but will not alter the isoelectric point ofthe alumina. From Fig. 3 it is seen that with an increase ofNP-15 in the mixture, the zeta potential of alumina decreasesdrastically, especially in the high concentration range. Thisobservation is in agreement with the adsorption isothermsofTl'ACand NP-15 in Figs. I and 2, respectively. Compar-ing the isoelectric point (IEP) for alumina in the presenceof mixtures to that in the presence of TI' AC alone, it canbe seen that the IEP is shifted to higher IT AC concentrationswith increase in the NP-15 concentration. Upon examiningthe adsorption density of IT AC at the isoelectric point, it isevident that the amount of lTAC necessary to cause chargereversal of the alumina particles is higher in mixtures than

rI

._.L ./ - 0/ 4,[1 /

b'- ,,6~IxlO'"

- ..J .. . .".' ,

lxlO" lxlO" lxlO-3 lxlO-2

NP-lS Rea-. ~.. kmoJIm3

FIG. 2. Adsorption of pentadecylethoxylated nonyl phenol (NP-15) on

alumina in the presence of varying amounts of tetradecyl trimethyl amm0-

nium chloride (WAC). pH 10. I.S. 0.03 M NaCi.

mum adsorption density of 1T AC on alumina at pH 10 is-2.5 X 10-6 mol/m2. This translates to roughly 66 A2/molecule which agrees well with values reported for themolecular area at the air/solution interface (61 A2) (15),which suggests that the adsorption layer is composed of amonolayer rather than a bilayer.

The .adsorption isothenns of 1T AC in the presence ofdifferent amounts of NP-15 are also shown in Fig. 1. Inall these experiments the tetradecyl trimethyl ammoniumchloride and pentadecylethoxylated nonyl phenol were pre-mixed and then equilibrated with the alumina for 15 h at pH10. It is seen that tetradecyl trimethyl ammonium chloridehemimicelle formation occurs at lower 1T AC concentrationsin the presence of the nonionic NP-15 but only at the 4: 1and 1:11TAC:NP-15 ratios. At the 1:4 ratio, however, thesharp increase in adsorption density corresponding to suchaggregation is not observed over the entire concentrationrange studied. In all cases, the plateau adsorption densitydecreases markedly upon the addition of the nonionic surfac-tant. This is attributed to the competition of the bulky non-ionic pentadecylethoxylated nonyl phenol with 1T AC forthe adsorption sites under saturated adsorption conditions.

The adsorption isotherms of NP-15 from different mix-tures with 1TAC are shown in Fig. 2. As mentioned earlier,the pure nonionic surfactant NP-15 does not adsorb on alu-mina, but it is seen here that the presence of 1T AC causessignificant adsorption of the NP-15. With an increase in1T AC content in the mixtures, the adsorption of NP-15 isenhanced significantly, and the adsorption isotherms areshifted to lower concentration ranges. The peculiar S-shapedisotherm obtained for NP-15 adsorption as a function of NP-15 residual concentration should be noted and is inagreement with our previous work (2). This is attributed to

AG. 3. Zeta potential of alumina particles after ~on ofTrAC:NP-15 mixtures of different composition. pH 10.1.5.0.03 M NaC!o

"i lxlO.7

f lxlO"

225CATIONIC AND NONIONIC SURFACTANT MIXTURES

and the results obtained are shown in Fig. 5. Since the NP-15 does not adsorb on alumina by itself, only the TTACcould be preadsorbed and its subsequent effect on NP-15adsorption determined.

Preadsorbed TT AC functions as anchor molecules for NP-15 and causes adsorption of the nonionic surfactant .Withan increase in preadsorbed TT AC density the adsorption ofNP-15 also increases and the adsorption isotherms are shiftedto lower NP-15 concentrations. It is also observed that oncethe TTAC forms hemimice11es at the alumina-water inter-face there is no further effect of increasing the preadsorbedTT AC density on the adsorption of NP-15. Desorption andreorientation of surfactants adsorbed as aggregates will de-pend to a large extent on the activation barrier involved indehemimicellization. The results also show that the order ofaddition of the surfactants has a marked effect on the adsorp-tion ofNP-15. IfNP-15 and TTAC were premixed and addedto the alumina suspension together, the adsorption densityof NP-15 is higher, particularly in the low concentrationrange. This indicates that NP-15 adsorption from mixtureswith TT AC is controlled by the packing of the molecules atthe alumina-water interface.

q

0

:;,

~ 4-~l:-

f'0 .J 1 0

D

0 -::=:::=:::::~:::::::::~=~ ~ ~ . .IxIO" IxIO.' IxIO" b1O.3 IxIO~ 9xIO-2

T oI8l a-.. ~ bMJUaI3

FIG. 4. Ratio of adsorption densities of TrAC:NP-15 on alumina fordifferent surfactant mixtures. pH 10. 1.5. 0.03 M NaCL

that required with IT AC alone. This means that the effectof IT AC in mixtures on zeta potential reduction is less thanthat of IT AC when present alone. It can be concluded fromthis that the positive charge of the ITAC ionic head is par-tially screened by the coadsorbed nonionic NP-15 molecule.For the case of the 1:4 ITAC:NP-15 system, there is nocharge reversal obtained which lends support to our hypothe-sis. At this mixture ratio, the concentration of the IT AC inthe adsorbed layer is low and it is possible that the adsorbedaggregate consists predominantly of NP-15, preventing anypossibility of charge reversal. It is interesting to note forIT AC alone that adsorption continues to take place, leadingto monolayer coverage even after the particles have becomesimilarly charged (Figs. 1 and 3). This suggests the predomi-nant role of hydrophobic interactions between the hydrocar-bon tails in causing adsorption. On the other hand, lackof adsorption of the nonionic pentadecylethoxylated nonylphenol without the synergism of the cationic IT AC showsthe essential role of the electrostatic interaction as well.

The ratio of adsorption densities of ITAC and NP-15 atthe alumina-water interface is examined in Fig. 4 as a func-tion of the total residual concentration. It can be seen thatthe ability of the two surfactants to cooperate! compete atthe alumina-water interface changes over the entire concen-tration range. This suggests that the monomer compositionand adsorption mechanism are changing over the concentra-tion range studied.

Interactions between Tetradecyl Trimethyl AmmoniumChloride and Pentadecyl Ethoxylated Nonyl Phenolin Solution

In an attempt to correlate changes in monomer concentra-tion of the individual surfactant~ in the mixtures with theiradsorption behavior, interactions between tetradecyl tri-methyl ammonium chloride and pentadecyl ethoxylatednonyl phenol in bulk solution were studied using surface

Order of Addition

In the results discussed so far, the cationic and nonionicsurfactants were mixed together in solution prior to additionto the alumina suspension for adsorption. The effect of theorder of addition on the adsorption behavior was determined

NP-IS ResiMI~..kDKIVmJ

FIG. S. Effect of TfAC order of addition on NP-IS adsorption onalumina, varying amounts of preadsorbed Tf AC and NP-15 added togetherwith TfAC. pH 10, I.S. 0.03 M NaQ.

226 HUANG, MALTESH. AND SOMASUNDARAN

1.3xIO.3TABLE 1Critical MiceUe Concentrations (CMCs) of Tetradecyl Tri-

methyl Ammonium Chloride (TTAC) and PentadecylethoxylatedNonyl Phenol (NP-15) Mixtures of Varying Composition

0 ExperimcDIaJdaIa.JUxlO

CMC (1anoVrnJ)Surfactant mixture ratio

1TAC alone (0% NP-lS)4:11TAC:NP-IS (20% NP-IS)l:I1TAC:NP-lS (50% NP-IS)1:41TAC:NP-IS (80% NP-IS)NP-lS alone (100% NP-lS)

1.2 )2.7 ).1.7 )1.1 )9.8 )

\\\ /,,81 mixiDg

. \'. \

§U 6.SxlO"

4.Sx10"

-42.SxlO

VI.I""

/'~":---p 1.5 . .~~;&~~---~---

5.OxI0" .,.,."..., ..0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0..9 1

NP-15 Mme FIKtMa

FIG. 6. Measured CMC and theoretical CMC calculated from idealmixing and regular mixing theories for mixtures of TI' AC and NP-15. I.S.0.03 M NaG.

tensiometry and regular solution theory. The surface tensionof aqueous solutions of tetradecyl trimethyl ammonium chlo-ride alone and pentadecylethoxylated nonyl phenol alonealong with that of mixtures of varying composition weredetermined and the critical micelle concentrations obtainedfrom these measurements are listed in Table I. As can beexpected, the CMC of the nonionic NP-15 is lower than thatfor the 1T AC, but the mixtures are not as surface active asthe nonionic surfactant alone (higher CMC).

Regular solution approximation (5). The regular solu-tion approximation is introduced by assuming that the excessentropy of mixing is zero. In the case of mixed surfactantsystems, an interaction parameter, 13, is defined which canbe interpreted as a parameter representing the excess heatof mixipg. The net interaction parameter, 13, in a binarymixed surfactant system can be determined from a singlemeasured value of the mixed CMC and the CMCs of the puresurfactant components. This is directly obtained followingiterative solution of the following equation:

nonionic mixtures, and -0.2 for cationic-cationic mixtures( 6 ). Compared with these, the interaction parameter (.B)obtained for this system is not very large. In addition to thetype of the mixture, the interaction parameter will also beaffected by the structure of the surfactant molecule~ as wellas the ionic strength. The monomer concentrations of differ-ent mixtures in this system were detennined using regularsolution theory and an interaction parameter of -1.5. Theresults obtained are shown in Figs. 7-9. Comparing the

.8 = In(~ ]/ <1 - X.)2

x.'C.

In[ a2'C:'x ]/ 1 = XI.(1 - X.)-C:

where lXl and lX2 are mole fractions of the component surfac-tants in the mixture. C~ and ci are CMCs of the puresurfactant components. C:,;. is the CMC of the mixed surfac-tant system, and Xl is the mole fraction of surfactant I inthe mixed micelle.

The critical micelle concentrations of the mixtures areplotted in Fig. 6 as a function of NP-15 composition. Theregular solution model was used to fit these, and an interac-tion parameter ({3) between -1.5 and -1.2 was obtained.A considerable number of binary mixed micellar systemshave been studied using this procedure (4-8) and the inter-action parameters obtained are about -25.5 for cationic-anionic systems (4), -4.6 to -1.0 (16, 17) for anionic-

lxlO'S IxlO" IxlO.3 IxlO.Z SxlO.Z3

Tolal C-.. kmol/m

FIG. 7. NP-15 monomer concentrations in different lTAC:NP-15 mix.tures calculated using regular solution theory with an interaction parameter({3) = -1.5.

< 10-3< 10-4< 10-4<10-4< 10-'

CATIONIC AND NONIONIC SURFACTANT MIXTURES 227

quantity of NP-15. If the relative and absolute quantity ofNP-15 is lower, synergism between these two surfactants isseen. At high concentrations the absolute quantity of NP-15is high, and steric hindrance will be dominant thus sup-pressing the adsorption of 1T AC. This is proposed to bethe reason for the maximum in the ratio in Fig. 4 at .lowconcentration range. For the 1:4 1T AC:NP-15 mixture thereis no maximum because the relative quantity of NP-15 isvery high.

"

j~

J~t::: SUMMARY AND CONCLUSIONS

Interactions between a cationic surfactant and a nonionicsurfactant at the solid-liquid interface were studied by de-tennining the adsorption isothentlS of individual and mixedsurfactants at the alumina-water interface and modeling thesame using the regular solution theory. While the cationictetradecyl trimethyl ammonium chloride adsorbed on thenegatively charged alumina as expected, the pentadecyleth-oxylated nonyl phenol adsorbed only in the presence of thecationic surfactant The interaction parameterdetenninedin-dicated that molecular-level association between tetradecyltrimethyl ammonium chloride and pentadecylethoxylatednonyl phenol is weaker than that betWeen an anionic and anonionic surfactant Nevertheless, significant adsorption ofthe nonionic NP-15 occurred as a result of these interactions.Presence of TT AC forced the adsorption of NP-15 on asurface (alumina) where the latter normally does not adsorb.The adsorption density of both TTAC and NP-15 was depen-dent upon the composition of the surfactant mixture. Pres-ence of coadsorbed NP-15 increased the adsorption of TT ACbelow saturation adsorption and decreaSed it above. Theincrease under submonolayer coverage is attributed to re-

lxIO-' lxlO" lxlO-) lxlO.2 SxIO-2)

ToCaI Cooc.. bId/m

AG.8. TfAC monomer concenb'ations in different TfAC:NP-15 mix-tures calculated using regular solution theory with an interaction parameter(.8) = -1.5.

30 ...,.--

'"

~u~~.90

.~.

~

adsorption isothenns with these monomer concentrations, itcan be seen that the adsorption of NP-15 does not dependupon its monomer concentrations in the mixtures. For exam-ple, the adsorption of NP-15 is more significant in 4:1IT AC:NP-15 mixtures than in other mixtures, but its mono-mer concentrations are the lowest. This further confinns thepostulate that the adsorption of NP-15 requires the anchoringeffect of the preadsorbed IT AC. It is accepted that theresidual surfactant concentration at which maximum ( or pla-teau) adsorption density is attained usually corresponds tothe critical micelle concentration of the surfactant under theoperating conditions. The isothenn shifts to lower concentra-,tions if the CMC of the surfactant is lowered. In the presentsystem, the onset of the plateau for adsorption of NP-l5from its mixtures with IT AC does not correspond to theCMC of the mixtures. For example the CMC of the 4: 1mixture is the highest of the mixtures (Table l) but theisothenn of NP-15 from this mixture is located in the lowestconcentration region (Fig. 2). This implies that the adsorp-tion of NP-l5 is controlled not only by the CMC of it.~mixture but also by the adsorption of IT AC, the change inmonomer composition, and the structure of the adsorbedlayer.

The adsorption density oflTAC corresponds to its mono-mer concentrations in the mixtures. The higher the IT ACmonomer concentrations, the higher is its adsorption density.It is interesting to compare the curves of monomer concen-tration ratios (Fig. 9) with the curves of adsorption densityratios in this mixture system (Fig. 4). The adsorption behav-ior in this system depends on the relative ratio of the mono-mer concentrations. Considering the synergism and sterichindrance in this system, it may be concluded that all thesephenomena will be decided by the relative and absolute

s

~-~~:~~~~~::::==:=~:~~ .'.0

IxIO.' IxIO" Ixl0-3 lxl0-2 5xl0-23

Total Ccmc.. kmoI/IIi

FIG. 9. Ratio of NP-15 to lTAC moIIomer concentrations in differentmixtures calculated using regular solution theory with an interaction param-eter (.8) = -1.5.

2S

20

l.s

10

228 HUANG, MALTESH. AND SOMASUNDARAN

duced repulsion between the cationic heads owing toshielding by the nonionic surfactant, and the decrease undersaturation conditions is attributed to competition betweenNP-15 and TfAC for adsorption sites. Interestingly the ad-sorption of NP-15 on alumina was not dependent on itsmonomer concentration but on the [TfAC):[NP-15) ratio.The mode of addition of the Tf AC:NP-15 mixture also hada significant effect on the adsorption densities of both surfac-tants. Adsorption was higher when the two surfactants werepremixed than that when the Tf AC was preadsorbed. Evi-dently coadsorption of NP-15 is not as easily achieved oncethe cationic surfactant forms solloidal aggregates on the solidparticles. Thus the amount of each surfactant adsorbed andpossibly the nature of the adsorbed layer are dependent bothon the surfactant mix and the mode of addition.

ACKNOWLEDGMENTS

REFERENC~1. Harwell, J: H., Roberts, B. L., and Scamebom, J. F., CoUoids Sulj:

32, 1 (1988).2. Somasundaran, P., Fu, E., and XU, Q., Langmuir 8, 1065 (1992).3. Esumi, K., Sakamoto, Y., and Meguro, K., J. Conoid Inteiface Sci.

134,283 (1990).4. Zhu, B. Y., and Rosen, M. J., J. Colloid Imeiface Sci. 99,435 (1984 ).'5. Holland, P. M., and Rubingh. D. N., J. Phys. Chem. 87,1984 (1983).6. Nguyen, C. M., Rathman, J. F., and Scamehom, J. F., J. Colloid

Inteiface Sci. 112,438 (1986).7. Rathman, J. M., and Scamebom, 1. F., Langmuir 3, 372 (1987).8. Shinoda, K., J. Phys. Chem. 58, 541 (1954).9. Rubingh, D. N., in "Solution Chemistry ofSurfactants" (K. L. Mittal,

Ed.), Vol. I, p. 337. Plenum, New York, 1979.10. Rosen, M. J., and Hua, X.Y., J. Colloid Inleiface Sci. 86, 164 (1982).11. Hall, D. G., and Huddleston, R. W., Colloids Sulj: 13,209 (1985).12. Li, z., and Rosen, M. I., Anal. Chem. 53, 1516 (1981).13. Fukushima, S., and Kumagai, S., J. Colloid Imeiface Sci. 42, 539

(1973).14. Koksal, E., Ramachandrnn,R., Somasundaran, P., and Maltesh, C.,

Powder Technol. 62, 253 (1990).15. Venabl, R. L., and Naumann, R. V., J. Phys. Chem. 68, 3498 (1964).16. Lange, H., and Beck:, K. H., Kolloid Z Z Polym. 251,424 (1973).17. Rosen, M. J., in "'Phenomena in Mixed Surfactant Systems" (J. F.

Scamehom, Ed.) ACS Symposium Series, Vol. 311, p. 144, 1986.

The Department of Energy (DE-AC22-92BCI4884) and the NationalScience Foundation (crs-9212759) provided support for Ihiswork.