JOURNAL OF ax.wm AND INTERFACE SCIENCE 192, 179-183 (1997)AaTK:I.B NO. CS975005

Desorption Behavior of Surfactant Mixturesat the Alumina-Water Interface

Lei Huang, S. Shrotri, and P. Somasundaran J

(.anglmlir Center for CoUoids and Inteifaces. Columbia University. New York, New York 10027

Received February 17, 1997; Kcepted May 23, 1997

ful to understand the adsorption and solution behavior ofsurfactant mixtures (7 -10) .

While the adsorption of surfactants has been studiedwidely, the desorption behavior of surfactant system hasreceived only scant attention ( 11-15). Although the adsorp-tion of most surfactants on mineral oxides is usually consid-ered as a process of physical adsorption, the results obtainedfor the desorption of single surfactants indicate that the ad-sorption! desorption is not completely reversible, and non-equilibrium effects due to changes in system variables suchas pH, ionic strength, and dissolved mineral species, canmake adsorption! desorption processes more complex. Asindicated above, in contrast to the information available onadsorption, the desorption behavior, especially of mixed sur-factants, is relatively unknown despite its practical signifi-cance on many applications. In this work, desorption behav-ior of cationic and nonionic surfactant mixtures has beenstudied systematically, and the effect of mixed micelles onthe desorption has been discussed.

MATERIALS AND EXPERIMENTAL PROCEDURFS

The desorption behavior of cationic-nonionic surfactant mix-tures, tetradecyltrimethylammonium chloride (TTAC) and penta-decylethoxylated nonyl phenol (NP-15), at the alumina-waterinterface was studied. It has been found that while nonionic NP-15 itself does not adsorb at alumina-water interface, it will doso with cationic TTAC preadsorbed at the interface. During thedesorption process, however, the presence of NP-15 in the systemwas discovered to cause desorption of TT AC. This is attributedto changes in equilibrium among surfactant monomers, mixed mi-celles. hemimicelIes, and solloids upon the addition of NP-15: Thecationic TTAC species at the interface is solubilized in NP-15 richmicelles in the bulk and perturb the equilibrium. This in tum isproposed to cause the desorption of NP-15 significant. The desorp-tion behavior in the mixed surfactant system depends on the mix-ing ratio of surfactants in the mixtures. In nonionic NP-15 richmixtures, negative hysteresis is observed for the desorption of bothTTAC and NP-15. In cationic TTAC rich mixtures, the effect ofNP-15 on the desorption of TTAC is limited and the desorptionof TTAC is similar to that in single TTAC system; in this casesome positive hysteresis is observed for both TTAC and NP-15.C 1997 Academk Press

Key Words: adsorption , desorption; cationic surfactant; nonionicsurfactant; alumina interface; surfactant mixtures.

INTRODUCTION

Adsorption of surfactants at solid-liquid interface playsa crucial role in many important industrial applications. Mostof these applications of SUrfactants involve use of mixturesof surfactants (1- 3). Since technical-grade surfactants aremixtures of chain length/isomers, which often perform bet-ter than the individual components alone: when differentsurfactants are mixed together, they usually exhibit syner-gistic or antagonistic effects (4-6). This makes the adsorp-tion from mixed surfactants and their effects a more complexprocess. On the other hand, this also makes it possible tocontrol the behavior of surfactants at interfaces and utilizethem more efficiently. Toward this purpose it becomes help-

Alumina. Linde A alumina purchased from Union Car-bide Corp. was specified to be 90% a-Al2O3 and 10% y-Al2O3 and to have a mean diameter of 0.3 jJ.m. The specificsurface measured by N2 BET adsorption using a Quantasorbsystem was 15 m2/g.

Surfactants. The cationic surfactant. n-tetradecyltri-methylammonium chloride (lTAC) , [CH3(CH2)13N-(CH3)3)Cl, from American Tokyo Kasei, Inc., and the non-ionic surfactant, pentadecyl ethoxylated nonyl phenol (NP-15), ~HI9~O(CH2CH2OhsH, from Nikko Chemicals,Japan, were used as received.

Reagents. NaCl, used for controlling the ionic strength,was purchased from Fisher Scientific Co. and was ACS certi-fied-grade (purity >99.9%). NaOH, used for adjusting thepH, was purchased from Fisher Scientific Co. and was certi-fied as the volumetric standard solution (0.1 M). Water usedin all experiments was triply distilled water. Conductivityof triply distilled water was measured to be in the range of(1-2) X 10-60-1 cm-l.t To whom correspondence should be addressed.

179 0021-9797197 $25.00Copyright C 1997 by AcDmiC Press

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180 HUANG. SHROTRI. AND SOMASUNDARAN

Adsorption. Adsorption experiments were conducted incapped 20 ml vials. 2-g samples of alumina were first mixedin 10 ml of 0.03 M NaCI solution for I h, and then the pHwas adjusted to 10 and allowed to further equilibrate at theroom temperature (22 :t 2°C) for I h. 10 ml of 0.03 M NaCIsolution containing the surfactants was then added, and thesamples were allowed to equilibrate for 15 h. pH was thenmeasured and. if necessary, adjusted using 0.1 M NaOH.The samples were allowed to equilibrate for about 3 h afterthe final pH adjustment, and then centrifuged for 25 min at5<XK> rpm. About 20 ml of the supernatant was pipeted outfor analysis.

Desorption. After adsorption, desorption tests were con-ducted by adding the same volume of diluent adjusted to thesystem ionic strength and pH (0.03 M NaCl and pH 10) asthe volume of the supernatant removed. The total volumeand thus the solid to liquid ratio remained constant as thedilution step was repeated many times with the surfactantconcentration monitored at each step. The equilibrium timefor desorption process is the same as that for adsorptionprocess. Adsorption density at each step was calculated fromthe changes in concentrations of the surfactants.

Surfactant concentration analyses. The TT AC concen-tration was determined using a two-phase titration technique(16). Pentadecyl ethoxylated nonyl phenol (NP-15) wasanalyzed by determining UV absorbance at 223 or 275 omusing a Shimadzu 1201 UV -vis spectrophotometer.

RESULTS AND DISCUSSION

Adsorption behavior of a cationic surfactant (1TAC) anda nonionic surfactant (NP-15) at the solid-liquid interface

was studied in our previous work (17). While the cationiclTAC adsorbed on the negatively charged alumina as ex-pected, the nonionic NP-15 adsorbed only in the presence ofthe cationic surfactant. The interaction parameter detenninedusing the regular solution theory (18,19) showed molecularlevel association between IT AC and NP-15 to be weakerthan that between an anionic and a nonionic surfactant. Nev-ertheless, significant adsorption of the nonionic NP-15 oc-curred as a result of these interactions. Presence of IT ACinduced adsorption of NP-15 on alumina surface where thelatter normally does not adsorb. The adsorption densities ofboth lTAC and NP-15 were dependent upon the composi-tion of the surfactant mixture. Presence of coadsorbed NP-15 increased the adsorption of lTAC below saturation ad-sorption and decreased it above. While the increase undersubmonolayer coverage was attributed to cooperation dueto reduced repulsion between the cationic heads owing toshielding by the nonionic surfactant, the decrease under satu-ration conditions was attributed to competition between NP-15 and the lTAC for adsorption sites.

The desorption behavior of single cationic IT AC has alsobeen discussed in our early work (20). It has been shownthat the desorption of IT AC from the alumina-water inter-face was reversible at high concentrations but was irrevers-ible at lower concentrations. This was attributed to changesin the structure of the adsorbed layer upon dilution and theactivation energy needed for the formation of surfactant sol-liodal aggregates (22).

The results obtained for the adsorption! desorption behav-ior of mixed surfactants (4:] lTAC:NP-15) are shown inFigs. ] and 2. The solid line represents the initial adsorptionisotherm. It is noted that the desomtion of IT AC from a

181DESORPTION OF SURFACTANT MIX1URES

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FIG. 3. Desorption of 1TAC at alumina-water interface upon dilutionfrom different residual concentrations (C,),1TAC:NP-IS ratio 1:1, pH 10,ionic strength 0.03 M NaCi.

the desorption isothenns of both 1TAC and NP-15 shownegative hysteresis, i.e. the surfactant adsorption at the inter-face is lower after desorption than that before at the sameresidual concentration. This suggests that although the de-sorption behavior of NP-15 is controlled by the co-adsorbed1TAC molecules acting as anchors, the presence of NP-15in the mixtures does also affect the affinity of 1T AC to thealumina - water interface.

The desorption behavior of 1:41TAC:NP-15 mixed sys-tem is illustrated in Figs. 5 and 6. At this ratio adsorptionof1TAC is very low. Nevertheless, desorption of1TAC inthis system shows some negative hysteresis and the desorp-tion of NP-15 is mostly reversible. Clearly the results ob-tained at different mixing ratios indicate that mechanism ofdesorption in this mixed system is more complex than thatfor single surfactant system.

It is evident that the desorption behavior of this mixedsystem depends upon the ratio of surfactants in the mixtures.With an increase of NP-15 in the mixture the desorption of1TAC becomes easy. A reason for this kind of behavior isthat as the NP-15 content in the mixture increases, the posi-tively charged head group of the 1T AC will be partiallyshielded from each other by the coadsorbed nonionic NP-15 molecules. This will decrease the electrostatic attractionbetween the cationic 1T AC molecules and the negativelycharged alumina surface and facilitate desorption.

On the other hand, it is well known that adsorption ofsurfactants at interfaces is directly related to the monomerconcentrations in the system. For single surfactant systems,solloid will usually fonn at a concentration much lower than

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4:1 mixture is similar to that for TfAC alone(20). At highconcentrations the adsorption of Tf AC is reversible, butat low concentrations the desorption shows some positivehysteresis, i.e. at the same residual concentrations, the ad-sorption density is higher after desorption than that before.This is possibly due to the hysteresis involved in the deag-gregation of the solloids from the interface. The desorptionisotherms of NP-15 suggest that the fully grown NP-15 sol-loidal aggregates do not come off the surface easily upondilution whereas the incipient ones do. At high concentra-tions, the adsorption density of Tf AC does not change mea-surably upon dilution, and neither does NP-15 desorb fromthe interface under these conditions. However in cases whereTf AC desorbs from the interface, desorption of NP-15 issignificant and S-shape desorption isotherms are obtained.It can be concluded that the desorption of NP-15 is facilitatedin this case by the desorption of Tf AC. For dilutions startingfrom high concentrations, the desorption of NP-15 showssome positive hysteresis (similar to the positive hysteresisof Tf AC) but only when the residual concentration becomeslow due to dilution. Since the adsorption of NP-15 on alu-mina requires the TfAC to coadsorb and act as an anchor,it is then reasonable to expect the desorption behavior of theNP-15 to be controlled by the desorption of TfAC, and thisis discussed in detail later.

The desorption isotherms for 1:1 TfAC:NP-15 mixturesare shown in Figs. 3 and 4. Similar to the desorption in 4: 1Tf AC:NP-15 mixtures, the desorption of NP-15 is facilitatedby the desorption of Tf AC significantly. At this mixingratio, the desorption of Tf AC also becomes significant and

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182 HUANG, SHROTRL AND SOMASUNDARAN

Ix10"' will always be enhanced, therefore, whenever NP-lS mi-celles exist in the bulk. In other words, synergism between17AC and NP-lS will occur for the desorption process also.This is supported by the fact that negative hysteresis occursin the 1:I17AC:NP-lS mixture system. Similar desorptionbehavior has also been observed in the 1:4 17AC:NP-lSmixture system. But at this mixing ratio, the solution is NP-IS rich and the adsorption density of17AC is very low, sothe effect of 17AC on the desorption of NP-lS is minimaland the desorption of NP-lS is not enhanced significantly.

In the case of the 4:117AC:NP-lS mixture, the CMC is2.7 X 10-4 M. At this CMC, the NP-lS concentration is S.4X 10-5 M. Above this concentration, the NP-15 micelleswill facilitate desorption of NP-lS, and S-shape desorptionisotherms have been observed for NP-15 due to significantdesorption around this concentration range. For 17 AC, sincebulk and the interface are rich in 17 AC, the effect of NP-15 micelles on the desorption of 17AC is relatively small.When the desorption process starts from a concentrationcorresponding to plateau of 17AC adsorption, there will bemany 17 AC hemimicelles still at the interface as NP-lSmicelles are depleted from the bulk solution upon dilution.Hence the desorption of 17 AC becomes similar to that fromsingle 17AC system, and as discussed before (20) positivehysteresis observed. Under these desorption conditions,some NP-15 molecules can be trapped in 17 AC hemimicel-les and this will indeed result in positive hysteresis for NP-15. When desorption process starts from a concentration atwhich adsorption density of 17 AC is lower than the plateauvalue, the effect of NP-15 micelles on the desorption be-comes significant. Desorption ofNP-15 then shows negative

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the CMC of the surfactant (21). Once the CMC is reached,the monomer concentration will be constant. and the adsorp-tion will reach a plateau. In contrast, for mixed surfactantsystems, the monomer concentrations of surfactants continueto change even above the CMC of the mixed systems, andhence the adsorption quantities of surfactants at interfacesalso continue to change above mixed CMC. In other words,the equilibrium among micelles, solloids, and monomers willchange over a much wider concentration range than that ina single-surfactant system. For example, in the 1 : 1TI'AC:NP-15 mixture system, the mixed CMC is 1.7 X10-4 M total concentration. At CMC, the concentrations ofcationic TI'AC and nonionic NP-15 in the bulk are about8.5 X 10-4 M. From Figs. 3 and 4, it can be seen that solloidsof TI' AC and NP-15 are formed just at this concentration.It is obvious that the adsorption! desorption in this mixturewill be controlled by the micellization processes in the bulk.As mentioned earlier, NP-15 molecules do not adsorb atthe alumina-water interface themselves, but once NP-15 isadsorbed at the interface with the help of preadsorbed TI' AC,the synergism between NP-15 and TI'AC in turn enhancesthe adsorption of TI' AC. It is expected that desorption ofNP-15 from the interface can also facilitate the desorptionof TI' AC.

Since NP-15 is more surface active, NP-15 micelles format lower concentration in the bulk than that of TI' AC. Duringthe dilution process, new equilibria among solloid, micelle,and monomers will be established with the only source ofsurfactants for desorption being from that at the interface.Some of the TI' AC molecules at the interface may be solubi-lized in NP-15 micelles. The desorption of n"' AC molecules

FIG. 6. Desorption of NP-15 at alumina-water interface upon dilutionfrom different residual concentrations (C,). TTAC:NP-15 ratio 1:4. pH 10.ionic strength 0.03 M NaCI.

183DESORPTION OF SURFACfANf MIXTURES

hysteresis, and desorption of TT AC is reversible. Consider-ing the positive hysteresis characteristics in the single TT ACsystem, this suggests that the desorption of TT AC is en-hanced by the NP-15 in the mixed system. From abovediscussion, it is clear that the desorption and the hysteresisare dependent upon the relative ratio of surfactants in themixtures. It is also clear that adsorption! desorption in thesystem are not completely reversible.

ACKNOWLEDGMENTS

We acknowledge the suppon of the National Science Foundation (cfs-9212759) and Depanment of Energy (DE-AC22-92BCI4884).

REFERENC~

CONCLUSIONS

The desorption behavior of cationic-nonionic surfactantmixtures oflT AC and NP-15 at the alumina-water interfaceshows unique features which throw light on interactions be-tween the species at the interface and in the bulk. While thenonionic NP-15 itself does not adsorb at the alumina-waterinterface, it is induced to do so by preadsorption of thecationic lTAC at the interface. During the desorption pro-cess, however, NP-15 in the system facilitates the desorptionof IT AC due to the changes in equilibrium among surfactantmonomers, mixed micelles, and solloids. Since NP-15 ismore surface active than lTAC, NP-15 micelles form in alower concentration range in the bulk solution, and the cat-ionic lTAC species at the interface is solubilized by NP-15rich micelles. This in turn enhances the desorption of NP-15 also. Thus the synergism between the cationic lTACand nonionic NP-15 is observed not only in the adsorptionprocess, but also in the desorption process. The desorptionbehavior of the mixed surfactant system is dependent on therelative amounts of surfactants in the mixtures. In nonionicNP-15 rich mixtures, negative hysteresis is seen for the de-sorption of both lTAC and NP-15. In cationic lTAC richmixtures, the effect of NP-15 on the desorption of IT AC islimited and the desorption of IT AC is similar to that insingle lTAC system, positive hysteresis is observed for bothlTAC and NP-15. It is clear that desorption behavior ofsurfactant mixtures is much more complex than that of singlesurfactant systems, and it can be controlled by adjusting therelative ratio of surfactants in the mixtures. This is veryimportant in many industrial applications such as enhancedoil recovery processes in which surfactant loss due to adsorp-tion should be minimized.

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