Adsorption Behavior of Alkylarylethoxylated Alcohols on Silica

P. SOMASUNDARAN, 1 E. D. SNELL, AND QUN XU

Langmuir Center for Colloids and Interfaces. Henry Knlmb School of .!.lines.Columbia Unillersily. New York, New York 10027

Received June \3. \990: accepted December 6. \990

The effect of ethoxylation of 10l1g chain alcohols on their adsorption behavior is investigated in thisstudy for a series of alkylarylethoxylated alcohols. The degree of ethoxylation affects markedly both themaximum plateau adsorption and the uptake of the surfactant at lower concentrations. Hydrogen bondingis considered to provide the initial driving force for adsorption. Surfactants with lower ethoxylation showpreferential adsorption on silica compared with those with higher ethoxylation because of the steric effectof ethylene oxide chains on the hydrocarbon chain-chain interaction at the solid/liquid interface. Thehydrophobicity of silica surface after surfactant adsorption has been found to be dependent on thestructure of surfactant as well as the adsorption density. While both C,EOIO and C,EOoo impart hydro-phobicity to silica surface above a threshold concentration, C,EOoo shows a return to hydrophilicity nearthe CMC. These changes in hydrophobicity are interpreted by considering changes in orientation of thehydrocarbon chain of the adsorbed surfactant. C 199\ Ac8demic Pnoa. IJIC.

INTRODUCTION

Adsorption ofnonionic ethoxylated surfac-tants has been of increased interest recently(1-3) because of their many possible appli-cations in process such as detergency ( 4), cos-metics (5), and enhanced oil recovery (2).Nonionic ethoxylated alcohols show strongadsorption on silica (1-2,6) but not on someother minerals such as alumina (2, 7). Suchpreferential adsorption may find use also inseparation process. In this regard, a systematicstudy of adsorption of commercially availablenonionic surfactant on silica is highly desir-able. Since in these process the degree ofethoxylation does playa role, it becomes im-portant to determine the dependence of ad-sorption behavior on ethoxylation.

Partyka et aI. (I) have studied adsorptionof the Triton series nonionic alkylarylethoxy-lated alcohols on silica under varying condi-tions of electrolyte, temperature, hydrocarbonchain length, and degree of ethoxylation. Itwas found that for a given series of surfactants,the cross-sectional surface area occupied by

I To whom correspondence should be addressed.

is0021-9797191 $3.00COP7riPl C 1991 by Ac8demic "- IK.~ 01 ~ in any form ~.~qCoBoM altdl-,*" s.-~. Val. 144, No. I. June I'"

one unit of the oxyethylenic chain (OCHrCH2) in the adsorbed state is constant and in-dependent of the overall length of the poly-oxyethylene chain. However, a full under-standing of the adsorption mechanism is lack-ing since this study was done mainly abovethe critical micelle concentration. Levitz et aI.( 8) studied the structure of the adsorptionlayer of nonionic Triton X-IOO on silica inaqueous solution using fluorescence tech-nique. The main result is that the adsorbedmolecules form aggregates on silica in therange of surface coverage between 0.07 and0.8. At higher surface coverage, the adsorptionlayer is more like a continuous medium. It isreasonable to assume that the structure of thesurfactant, say, the hydrophilic ethylene oxidechainlength, plays a role in determining theaggregation behavior. But this has not beenstudied so far.

In this study, the adsorption behavior ofseveral nonionic surfactants on silica was in-vestigated along with their effect on the wen-ability of silica to develop an understandingof the mechanism of adsorption includingpossible conformation of the adsorbed species.

166 SOMASUNDARAN. SNELL AND XU

EXPERIMENTAL

"'/aleria/s

Silica. The silica used for this study wasSpherosil XOB IS supplied by Rhone-Pouiene.France. It had a particle size distribution be-tween 40 and 100 Ilm. The average pore sizewas specified to be 1200 A and the surfacearea as determined by BET nitrogen adsorp-tion was 2S m2/g.

Surjacranls. The nonionic surfactants usedin this study were IGEPALs supplied by GAFCorp. These are alkylphenoxypolyethoxy-ethylalcohols. The structures of the surfactantsare given in Table I.

J.,[ethods

Adsorption. Silica samples of 0.2 g weremixed with 10 ml of surfactant solutions ofknown concentrations for 10 h and the solidswere then separated by centrifugation. The re-sidual concentration of the surfactant in thesupernatant was determined by UV absorptionanalysis at 223 or 275 nm using a BeckmanDU-8 spectrophotometer.

Sur/ace tension. Surface tension of the sur-factant solutions and supernatants after ad-sorption was measured using a Fisher DuNuoy ring tensiometer.

Hydrophobicity. After the UV analysis andsurface tension measurements, the superna-tant was poured back into the vial containingthe solids to reconstitute the slurry. The slurrywas transferred to a separatory funnel to which3 ml of toluene was added. The silica-surfac-

TABLE I

Structure or Alkylphenoxypoiyetboxyiethyl Aicobois

Product ~ .4bb1wYi8bofo

R -C.H. -(OCHzCHz).. -OHR=CaH., m - 10R=C.HI7 m - 40R=C,H.9 m - 10R=C.H.9 m - 20R=C,H'9 m - 40R=C.zHu m - 10

(C"EOR)C,EO..C,EO.C.EO..C.E~C.EO.C1JEOIo

CA630CA890CO66OCOBSOCO890RC630

Effect of Alkyl Chainlength

The adsorption isotherms for surfactantswith different effective hydrocarbon chain-lengths but the same degree of ethoxylationare given in Figs. 4 and 5 for EOlo and EO40.respectively. Some features different from

~.c~-I-t-~ v.."- No. I.J_I991

tant-toluene dispersion was shaken for 4 minand then allowed to settle overnight. The bulkof the aqueous phase and hydrophilic solidswas then allowed to flow out of the funnel bygravity. The toluene phase and the toluene-aqueous interfacial layer containing the hy-drophobic solid was rotevaporated in a I~mlround-bottom flask and the weight of the silicawas recorded. The relative hydrophobicity wasdetermined as follows:

% Hydrophobicity - (Weight of silica

in toluene extract/Total weight) X 100.

RE.~UL TS

Effect of Ethoxylation

The effect of ethoxylation of the nonionicsurfactants on adsorption is shown in rigs. 1and 2 using surfactants with three different EOnumbers but the same alkyl chainlength. It isclearly seen that the slope of the ascending partof the isotherms decreases with increase inethoxylation. However. the extent of adsorp-tion in the ascending part at low concentrationis greater for the surfactants with the higherethoxylation than for those with the lowerethoxylation. This trend is. however. reversedat higher concentrations (near the plateau).The plateau adsorption of the surfactant de-creases progressively as surfactant ethoxylationincreases. When the parking area of each sur-factant calculated from the plateau adsorptionvalue is plotted as a function of ethoxylationnumber. a linear relationship is obtained withthe slope giving a parlcing area of 9.2 A 2 for

each -OCH~H2 segment (Fig. 3). This iscomparable with the value of 8.S A 2 per

-OCH~2 segment obtained by Partyka etaJ. for Triton series surfactants (I).

ADSORPTION OF ALCOHOLS ON SIUCA 167

,0-; i., , .., "..i . c.~-C.H. - toeaH.'. -oHiSILICA

T 8 13-278C... 8 7.0-7.1 .. ~

S/1. 8 0.2I 80

/6 A ~ '6- . ~

...d'-dod~.tI'-o~-o.Q e-,

:.A ~-;;"""'7 . 0 ' . :0 00

6.010

Om 040

.I

1612\ . .. 1 /.. 1 '" ,...,1 ... .10.1 10-5 10-4 10-1 10-1

RESIDUAL ALCOtfOL COHCENT~TION. '..01/.1

Fki. I. AdIorIMion ol1atiaryoctylpilenoxyethoxylated akoboIs (EO.. aDd EO.> on silica.

those observed for ethoxylation effect arenoted. rmt. the slopes of the ascending partof the isotherms are comparable. Second, themwmum plateau adsorption density is rela-tively independent of the hydrocarbon chain-length. The onset of the plateau adsorptionhowever is found to shift to lower concentra-tions in line with the decrease in CMC withincrease in the hydrocarbon chainlength.

Chromatographic Adsorption fromSurfactant Solutions

Since all the surfactants used in the presentstudy are polydispel'sed with respect to the de-gree of ethoxylation, it is po&DIe that differentcomponents may adsorb differently resultingin chromatographic separation. Such chro-matographic adsorption behavior is indicated

1=A.~0UIQ4

.J0

~10~

168' SOMASUNDARAN. SNELL AND XU

by the shift of the surface tension curve afteradsorption (Fig. 6).

For the C9 series of surfactants, a linear re-lationship between the CMC values and theethoxylation number, m, is observed (Fig. 7).Linearity is also assumed for the C. and C'2series based on the assumption that at a highdegree of ethoxylation (m ~ 50) CMC will berelatively independent of the hydrophobe.From the CMC data obtained for solutionsafter adsorption and the corresponding cali-bration curves for each series of surfactants,an estimate of the average degree ofethoxyla-tion of surfactants in the supernatant after ad-sorption can be made. The results, given inTable II, show preferential adsorption of lowerEO surfactants at the silica/liquid interfaceresulting in the enrichment of surfactant withhigher EO number in the supernatant.

.-:.'0E-'OC

~.C

...

..

C

..

A.

..

C

C

...=

C

=

C

..

~

U

...

..

0

2

Silica Hydrophobicity upon Adsorption

Information on changes in wettability ofsilica due to surfactant adsorption can be use-ful for developing an understanding of theirinteraction on particle surfaces. The effect of

0 10 20 30 40ETHOXYLATION NU..eE~. '"

FIG. 3. Effect of dearee of ethoxylation on molecular

parking areas calculated from plateau adsorption densitiesfor aikyphenoxyethoxytated alcohols on silica.

~ _/~~. v~ 144, No. 1.1- 1991

169ADSORPTION OF ALCOHOLS ON SILICA

the results obtained at higher surface coverage.Whereas silica particles in CsEOlo solutionsremain hydrophobic as adsorption is in-creased, the CSEO40 surfactant makes the par-ticles progressively hydrophilic above a certainadsorption level. At saturation adsorption ofCSEO40 the hydrophilicity of silica particles istotally restored.

degree of ethoxylation of the surfactant on thehydrophobicity changes of silica due to its ad-sorption is illustrated in Figs. 8 and 9. In theabsence of the surfactant the silica surface ex-hibits complete hydrophilicity. With increasein adsorption of both CaEO1o and CaEO40 onsilica, the surface becomes hydrophobic. Mostimportantly, there is a marked difference in

--.C~_I-t-Sd-..VG.I44.No.I.J_I991

170 SOMASUNDARAN. SNELL. AND XU

Figs. 3 and 4 where surfactants with the sameethoxylation number but different alkylchainlength have essentially the same plateauadsorption value. This would also suggest thatat plateau adsorption the alkyl chains of theadsorbed surfactants are oriented vertical tothe surface. The constant parking area of 9.2A 2 obtained for each -OCH2CH2 segment

would on the other hand suggest that all theethylene oxide chains of the adsorbed surfac-tant molecules are directly adsorbed on silicasurface. These observations are in agreementwith reports in the literature suggesting a flatconfiguration for the adsorbed ethylene oxidechains adsorbed on silica particles (12-15).

Although hydrogen bonding provides theinitial primary driving force for nonionic sur-factant adsorption on silica, hydrophobicchain-chain interaction between the adsorbedsurfactants is proposed to become more sig-nificant at higher concentration regions.Chain-chain interaction usually leads to theformation of surfactant aggregates at solidIliquid interfaces and consequently causes achange in the slope of the adsorption isotherm( 16 ). The progressive increase in the slope ofthe adsorption isotherm with decrease in theethoxylation number (Figs. I and 2) is a clearindication of the increased chain-chain inter-action at the interface arising from the de-creased hindrance by the ethoxyl groups. Inthe case of CgEO40, the long ethylene oxide

DISCUSSION

Adsorption of surfactants at solid/liquidinterfaces is the result of one or more contrib-uting forces arising from electrostatic attrac-tion, chemisorption, hydrogen bonding, hy-drophobic chain-chain interaction, and sol-vation effects (9). Because of the absence ofionic and chemisorbing components in thenonionic surfactants used, hydrogen bondingmechanisms may be considered as the initialdriving force for the adsorption (1). Hydrogenbonding is the mechanism proposed also forthe adsorption of polyethylene oxide (1O, 11)on silica. The higher uptake of surfactants withlarger EO number in the low concentrationregion, where factors such as chain-chain in-teraction at the interface are not significant, isclearly due to stronger cumulative hydrogenbonding interaction of long EO chains withthe hydroxylated silica surface. Another in-dication of adsorption through interaction be-tween EO chain and the substrate is seen in

TABLE II

Change in Ethyoxy Composition ofSolutions after Adsorption

"'" m ~ '. '. 'c

Wore~ion

AfteradsorPIionSurfactant

C.EO,oC.EO.,C,EO,oC.EOmC,EO.,C,zEOlo

104010204010

27SS273S

SS-603O-3S

J C"""""'.u.t-~. Vol.l44.No.I.J,,~ 1991

171ADSORPTION OF ALCOHOLS ON SIliCA

"!

:aE

z2.,.GoC0..0c

oJ0~0

~

chain makes surface aggregation virtually im-possible, as is indicated by a slope of less thanI of the adsorption isotherms in the log-logplots in Figs. I and 2. The predominatingdriving force for chain-chain interaction at theinterface can be considered to be the reasonfor the chromatographic adsorption of lowerEO surfactants from the polydispersed surfac-tant solutions (Table II).

It is interesting that the hydrophilicity ofthe silica surface is restored as the adsorption

~.:. ... . .. ~iflf -C_"4 -IOCzU4140-0HISILICA

100

.0

10, ,..

70 ~

160 mI50

40 ,..

x30 ~i .20

10

1......E

~~Go

~'"co4oJ0%0UoJ4

T -13-ZT'CpH - 1.0-7..

Sit. - 002. 0.21-0

: ~/"t~""<

10-6 10- 5 10- 10-1 10-1

RESIDUAL ALCOHOL CONCrNTRATION, t_l/m1

f)(;. 9. Diagram illustrating changes in the hydrophobicity of silica particles with adsorption ofC.EO.oOH.

IO.Z

J C~_/N"'~Val.l".~ I.JUM 1991

of CSEO40 is increased above a certain value.The possibility ofbilayered adsorption, whichcan conveniently impart hydrophilicity to thesurface, can be ruled out if, as discussed above,all the ethylene oxide chains of the adsorbedsurfactant molecules can be considered to beadsorbed directly on the silica surface. Evenifbilayered adsorption were to occur, it shouldbe expected to be more predominant forCsEO1o than for CSEO40 because the strongerchain-chain aggregation ofCsEO1o will cause

10""

1()-11

72 SO~IASUNDARA."l. SNELL. AND XU

more effective abstraction of the hydrocarbonchains of the second layer into the interior ofsuch a layer leaving the hydrophilic chainsprotruding into the bulk solution. Thus on thebasis of a bilayer adsorption model, restorationto hydrophilicity of the surface should be morepronounced for the silica/ CsEO\o than for thesilica/CsEO40 system. The persistent hydro-phobic nature of silica at high CsEO\o adsorp-tion clearly does support a bilayered adsorp-tion model.

There is always a possibility that the pres-ence of toluene could disturb the adsorptionlayer due to solubilization of the nonionic sur-factant molecules into the toluene phase andthus diminish the value of the hydrophobicitytest used in this study. Should the presence oftoluene pose a severe perturbance to the ad-sorption layer, however, silica particles mayremain in the aqueous solution in the case ofCsEO\o adsorption and in the toluene phasein the case ofCsEO40 adsorption-contrary towhat was experimentally observed. The logicbehind this reasoning is that the CsEO\o ad-sorption layer will be more easily stripped offby the toluene phase than the CSEO40 adsorp-tion layer, first because CsEO\o is more solublein the toluene phase and second, because thecumulative hydrogen bonding with the silicasurface for CsEO\o adsorption is weaker thanfor CSEO40 adsorption. Since the experimentalresults are contrary to the above, we may con-clude that the presence of toluene does notconstitute a serious disturbance to the adsorp-tion layer and consequently the method canstill be used for testing the hydrophobicity ofsilica particles. However, it may be helpful todevelop other more direct methods to com-pletely avoid the possible influence of tolueneon the adsorption layer.

We propose that the change in wettabilityof particles is the result of the correspondingchanges in the orientation of the hydrocarbonchains at the interface. At low concentrations,the hydrocarbon chains of the adsorbed sur-factant molecules are separated from eachother, with many of them possibly lying flat

""'..~_I~sa VG.I"".~.I.June IY'JI

on the silica surface (1). Such an orientationwould render the silica surface hydrophobic.As adsorption proceeds, however, the hydro-carbon chains will tend to dangle into the s0-lution. This would reduce the effective surfacearea of silica covered by hydrocarbon chainsand thus make the surfaces less hydrophobic.It has been shown (17) that incipient flotationof silica was obtained at equal adsorption den-sities for surfactants with different chainlength.This supports the view that it is the effectivecoverage of the surface rather than the thick-ness of the hydrophobic layer that primarilydetermines the hydrophobicity. At high ad-sorption densities, the long ethoxyl groups canpossibly be squeezed out to protrude into thesolution; this can lead to large hydrophilic areaexposed on the particle surface in the case ofsurfactant with the long ethoxyl chain. How-ever, such a process is not expected to occurin the present system since there are no otherfunctional groups on the surfactant with strongattraction toward the adsorption sites. This issuggested by the fact that the parking area oc-cupied by ethoxyl groups is independent ofthe ethoxyl chain length.

CONCLUSION

Nonionic alkyarylethoxylated alcohols ad-sorb on silica through ethoxylated chains andthe constant parking area of 9.2 A 2 per

( -OCH2CH2) segment obtained for all surfac-tants at maximum adsorption suggests directadsorption of the complete ethylene oxidechains on the silica surface. Based on thechange in the slope of the adsorption isothermsand the preferential adsorption of surfactantwith lower EO number from the polydispersedsurfactant solutions. it is proposed that aggre-gation of the adsorbed surfactants is hinderedas the ethoxylation number is increased. Atlow concentrations, when chain-chain inter-action is not prominent, surfactants withlonger ethylene oxide chains show higher ad-sorption on silica because of stronger cumu-lative interaction of the EO groups with thesilica surface. For surfactants with longer

ADSORP110N Of' ALCOHOLS ON SIUCA

cthoxyl chains, such as CsEO«J, flat adsorptionof the hydrocarbon chains is considered nec-essary to impart sufficient hydrophobicity tothe silica surface. Such hydrophobicity is di-minished as the hydrocarbon chains arepushed up by further adsorption of the hydro-philic ethylene oxide chains at the interface.

A CKN 0 WLEOO MENis

SuPPOrt of tile National Scie!.ce FouDdatioD (CTS-89-21.170), UDiteYer Research U.s., loc., the Department ofEIIerJY, won, and StaDdard Oil of Ohio is patefuDy -=-kDOWted&ed-

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2. Fu, E., .. ~ ot Anjoaic-NoDionic Surfactant

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1'. Killmann, E., Eisenlauer, J., and Korn, M., J. Poiym.Sd. PolY'"- Symp. 61. 413 (1977).

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~ _1-- ~ Va 144. No. I, J- 1991