131

Adsorption of Alkylxylenesulfonates on Alumina: AFluorescence Probe Study

A. Sivakumart and P. Somasundaran.

Langmuir Center for Colloids and Interfaces. Henry Krumb School of Mine,. ColumbiaUniversity, New York, New York 10027

Received March 22, 1993. In Final Form: September 18, 19938

Changes in the positions of the sulfonate and the methyl groups on the aromatic ring of alkylxylene-sulfonate were found to produce a marked effect on the micellization and adsorption at the solid/liquidinterface of alkylxylenesulfonatea. Fluorescence spectroscopy was used to probe the micr~tructure of themicelles and hemimicelles of the surfactanta. The studies showed the p-xyieneaulfonate micelles to beless polar and the p-xylenesulfoante micelles and hemimicellea to have higher aggregation number thanthe m-xylenesulfonate micelles and hemimicelles, respectively. Baaed on results obtained, steric constraintto the packing of these molecules in the aggregates was detennined to be the main reason for the differencesin the micellization and hemimicellization of the surfactant!.

techniques provide information on the structure of theadsorbed layers in terms of the aggregate size, the aggregateviscosity, and the orientation of the molecules and, incombination with adsorption isotherms, help in under-standing the evolution of the adsorbed layer.

In this work. the effect of the position of the sulfonateand the methyl groups on the aromatic ring of threealkyl.xylenesulfonatea on adsorption at the alumin/waterinterface and on micellization has been examined usingfluorescence spectroscopy. Earlier work using microcal-orimetry and zeta potential measurements on the samesystem showed steric constraints to the packing of themolecules in micelles and hemimicelles to be the mainmechanism- for the differences in the adsorption of thethree surfactants. Fluorescence spectr~py has beenused to investigate such steric constrints and to gain moreinsight into the nature of the adsorbed layers.

Introduction

Adsorption of surfactants plays an important role inprocesses such as flotation, detergency, enhanced oilrecovery, paint formulation, lubrication, and micro-electronics.l-6 Also, surfactant aasembliessuch as micellesand hemimicelles have potential applications in novelseparation and reaction schemes such as magnetic isotopeseparation and polymer synthesis. 6,7 An understandingof surfactant adsorption at the solid/liquid interface andmicellization is important for improving the efficiency ofthe above processes.

Apart from measuring adsorption isotherms, techniquessuch as calorimetry, electrophoesis, and infrared spec-troscopy have been used in the past to understand themechanisms of adsorption. S,9 An important aspect ofadsorption studies that is lacking is in situ characterizationof the microstructure of the adsorbedlayers, even thoughit is recognized that the microstructure controls theinterfacial properties of the solid. Experimental tech-niques such as fluorescence, electron spin resonance,resonance Raman, and neutron scattering, which have beenused to characterize micelles in situ, have recently beenextended to the study of adsorbed layers.10-12 These

Experimental Section

Materiala. Surfactants. 5-(4-Undecyl)-2,4-xylen_u1fonate(Meta), 4-(4-undecyl)-3,5-xylen_u1fonate (Paral), aDd 4-(4-undecyi)-2,5-xylenesulfonate (Para2) obtained from ARCO Oiland Gas Co. were uaed in this work. All the surfactant. werespecified to be at least 97 % iIOmerically pure and were used ..received. The ItnIctures of theae surfactant. are IhoWD below:

.,-..",

803-

Para!

. To whom all co~poDdeoce abou1d be 8d~.'Currently at NaIco Chemical Company, Naperville, n. 60563.. Abetract published in Advance ACS Abltrocta, December 15,

1993.(1) SoIDMUDdaraD, P., MoudciJ. B. M. Eda. ReGlenta in Mineral

TeclanolClf)'; M. Dekker: N- York. 1987.(2) 8hilliDI, G. L; Brilht, G. S. Lubrication 1t77, 63, 13.(~) H.~h. D. B.; Rendall, H. M. In Adsorption from Solution at t~

Solid-Liquid Interface; Pufitt, G. D., Rochester, C. H., Eda.; AcadelDJCPr-= New York. 1983.

(4) Sc:hwucr, M. J. In Anionic Surfactonta; Surf8CtantScieoceSeriea;LUCaI8en-R.yDder8, E. H., Ed.; Marcel Dekker: New York. 1981; VatU.

(5) Swaien,J. D.; Allar.. D. L.; Andrad..J. D.; ChaDdI.-, E. A.; Garoff,S.; laraelachvill, J.; McCarthy, T. J.; Murray, R.; P_, R. F.; Rabolt, J.F.; Wynne, K. J.; Yu, H.l..anImuir 1187,3,932.

(6) Wrllhtoo. M. 8., Ed. Interfacial Procear. Enerry Conversion andSynthe8i8; AdVaDcee in CheIni8try Seri8; Amerie8D Chemical Society:W MhiDIt4n. DC, 198); P 184-

(7) Turro, N. J.; KraeutJer, B. In Iaotopea in QrIanil: Chemiatry; B~l,E., Lee. C. C., Eda.; Ellevier: Am8terdam, 1984; Vol 6-

(8) Woodbury, G. W.; Noll, LA. CoUoida Surf. 1188, 33, ~1.(9) P8rtyb. 8.; Rlvtn...ki, W.; Brua, B.; Clint, J. H. L4n1muir I_,

5,297. .(10) Chandar, P.; Somuundaran, P.; Tuno, N. J. J. Colloid Interface

Sci. ItIT, 111,31.(11) WatenDaD, K. C.; Turro, N. J.; ChaJMIar, P.; So-~~~ran, P.

J. ~.Y8. CMm. 1_, 00, 68.'K).(12) Som~~~~~ P.; Kunjappu, J. T.; KUJDar, C. V.; Tuno, N. J.;

Barton. J. K. Langmuir Itn, 5, 216.

0743- 7 463/94/ 241 0-0 131 $04.00/ 0

~

503-

Para2

Experimental Conditio... All the experiments were carriedout at 43 °C aDd at a constant ionic strength of 0.03 kmol/mlNaCl.

Methods. Surface Tenaiometry. Surface tenaion was mea-sured using a water jacketed du Nuoy riDC tensiometer set to thetest temperature.

Ad.orption. A cram of alumina was conditioned in 5 cml of0.03 kmol/ml NaClaolution for 1 hat 43 °C in a glaaa vial. Then

C 1994 American Chemical Society

e.o ..-0

xN.

0

a~

>-0-

cnzwc

zI:)

0-~a:I:)U1Ccc

-c-.I;",P'p~)~:::;:~~-O'~ '~,,0

, 0 0

.i C.O

z0;;; :E.OzW0-

WU

~ :1).0 I

l'4 Parol

0 Meta

0 Para2

~ Paral

0 Hela

a Paral~p,

I .I~

).5 1.0 10.0 100.0 ICOO.O

RESIDUAL SULFONATE CONC.. K.ol/.3x 105

Figure z. Adsorption of alkylIylenesulfonates on alumina.

1.31

VI m.ol..

0.91 1.00 10.00 \00.00 200.00

SURFACTANT CONC.. kmol/.3x 104

l'iCUre 1. Surface tension of alkyhylenesulfonate solutions.

~~/ Ci"//~ .r

o'J '"iu /

O.Y--M-- tII

)I

0.8

0.

0.8

0.5

0.40.7 1.0 10.0 :KI.O

SURFACTANT CONC.. kmol/m3x 104

Figure 3. Polarity of alky}xylenesulfonate solutions in terms of13/11,

a greater extent than the m-xylenesulfonate. By use of acombination of microcalorimetry, electrophoresis, andhigh-performance liquid chromatography (HPLC), thelower surface activity and adsorption of the m-xylene-sulfonate were explained in terms of its lower hydropho-bicity and higher steric constraints to the packing of itsmolecules in micelles and hemimicelles.15 The results ofthe fluorescent probe characterization of the micelles andthe hemimicelles of the three surfactants are reportedbelow. .

Micropolarity Studies. Micelles. The micropolarityof the surfactant solutions was measured in terms of theratio of the intensities of the third to rust peak, Ia/l1. TheIa/l1 values for the three surfactant solutions as a functionof concentration are shown in Figure 3. At low surfactantconcentrations, the value of la/I 1 is 0.55, which is the sameas that observed in water. With the onset of cmc, thevalue increases slowly and reaches a value of 1.1 for thep-xylenesulfonates and 0.95 for the m-xylenesulfonate.The slow increase in Ia/l1 is in contrast to the abruptincrease above cmc observed for dodecyl sulfate.10 Theslow increase can be attributed to the fact that due totheir bulky nature, these surfactant aggregates evolveslowly, i.e., the aggregate size increases slowly with theincrease in concentration and rmally becomes constantabove a certain concentration. The plateau la/I 1 value of1 is in between that for water (-0.6) and for organicsolvents such as hexane (-1.6). This is attributed to the

5 cm3 of the surfactant solution at the desired concentration wasadded to the slurry and the mixture conditioned for 24 h at theset temperature. After conditioning, solid-liquid separation wasachieved using centrifugation. A Beckman DU-8 UV - Vis spec-trophotometer was used to analyze the surfactant at lowconcentrations and two phase titrationl3 was used to analyzehigh surfactant Concentrations. The absorbance was recordedat a wavelength of 254 DID. The equilibrium pH of adsorptionwas 8.2.

Steady-State Fluorescence. Steady-state emission spectra formicropolarity measurements were obtained using a PTI LS-l00fluorescence spectrophotometer. The slurry from the adsorptionsamples was transferred to a 2 mm path length quartz cuvetteand the solutions to a 10 mm path length cuvette and the spectrawere recorded. The excitation wavelength used for pyrene was335 DID. The ratio of surfactant to pyrene concentration in thestock solution used for adsorption studies was 2000.

Fluorescence Decay. The decay measurements were also madeusing the PTI ~ 1 00 nanosecond lifetime system which consistsof a thyratron gated lamp source filled with a gas mixture ofnitrocen (70%) and helium (30%) and excitation and emissionmonochromator with mirror on the opposite side for efficientzero-order illumination. The samples were excited at 335 DIDand the emission was recorded at 385 and 480 DID for the monomerand excimer, respectively.

The fluorescence decay behavior of pyrene under excimerforming conditions was analyzed using the intramicellar kineticsmodell' to determine the lifetime, rate of excimer formation, andthe aggregation number of the micelles and hemimicelles of thealkylxyleneaulfonates. The equation that describes the decaybehavior of the monomer is

1.(0) = 1.(t) exp[-kof + n.(exp(-k.t -1»] (1)

where ko is the reciprocal lifetime of the monomer in the excitedstate, he is the rate of intramicellar excimer formation, and n isthe average Poisson occupancy number of the probe in theaggregates. The aggregation number N was calculated from nusing the following equations:

n = [P]N/[C - cmc] for micelles (2)

~ = [P]N/[C - Ceq] for adsorbed layer (3)

where [P] is the pyrene concentration, C is the total surfactantconcentration, cmc is the critical micelle concentration, and Ceqis the residual surfactant concentration.

Results and Discussion

Surface Tension and Adsorption. Surface tensionsof the three surfactant solutions and their adsorptionisotherms are shown in Figures 1 and 2, respectively. Thep-xylenesulfonates are more surface active and adsorb to

(15) Sivakumar, A. D.E.8. The8i8, Colwnbia University, New York,1991.

(13) Li, z.; ROlen, M. J. AnGI. Chern. 1981, 53, 516.(14) AUk, S.; Nam, A.; SiDler, L Chern. PlaY', LeU. 1m, 67, 75.

~.o

100.0

10.0

1.0

0.1I

0 Parol

c Parol

6 Me la

t..

..

..

07

I...

i

...

0.'

U.

Q2

fJ

~~~~

,~ - -

--

(")-

.. -. -. kq -q ft-Wo..I..-q." In.' W...I"",," (~i

Ficure 6. Pyrene monomer and excimer decay profiles in mi~lIarsolutions of alkyhylenesulfoDates, surfactant concentration =0.01 M: (A) monomer emission for surf/py = 2089; (B) monomeremission or surf/py == 108; (C) excimer emission for surf!py -108.

Table I. Kinetic Analysis of Pyrene Decay in Micella ofAlkyhylene8ulfonate

aurf/Py ko (ar1) k. (nrl) Naurfactant "Para1 105 0.0056 0.015 0.33 35Para2 110 0.0054 0.016 0.32 35Meta 108 0.0057 0.018 0.24 26

model, the monomer decay (curve B) is multiexponential.The excimer decay profile (curve C) shows the typicalgrowth and decay behavior. The aggregation number forthe three surfactants was calculated by fitting the decayprofiles and the results obtained are tabulated in Table1.

0.40.1 1.0 10.0 00.0

ADSORPTION DENSITY. Hol/c.2x 1012

Figu~ 5. Polarity-of adsorbed layers of alkylxylenesulfonatesin terms of la/It as a function of surfactant adsorption density.

possible location of the probe in the palisade layer of themicelles which contains some penetrated water. Thedifference in the 13/11 plateau values for the p-xylene-sulfonates and m-xylenesu1fonate is proposed to be dueto the different degree of water penetration. The lesserwater penetration in the p-xylenesulfonate micelles in-dicates a tighter packing of the moelcules in this case.

Adsorbed Layers. The 13/11 is plotted in Figure 4 as afunction of the equilibrium suifactant concentration ofthe three surfactants. At low adsorption densities, priorto the onset of hemimicellization, the 13/11 is -0.6. Oncethe hemimicelles form, the value increases to 1, indicatinga behavior similar to that of micellization. In order tocompare the polarities of the adsorbed layers, 13/11 isplotted as a function of the adsorption density in Figure5. As seen from the figure, there is no difference in thepolarities of the adsorbed layers unlike in the case ofmicelles. This lack of difference could be due to the factthat the adsorbed layers are more compact than themicelles and hence the water penetration is the same forall the surfactants.

Fluoresence Decay and Excimer Formation. Mi-celles. Typical monomer and excimer decay profllesobtained for one of the surfactants are shown in Figure 6.At low pyrene concentrations, there is no excimer for-mation and the monomer decay (curve A) is singleexponential. At high pyrene concentrations there isexcimer formation and as per the intramicellar kinetic

The aggregation numbers of the micelles of p-xylene-sulfonate are similar and higher than that of the m-xy-lenesulfonate micelles. The values obtained are compa-rable to those obtained for other alkylbenzenesulfonatesin literature.16.17 The lower aggregation number of them-xylenesulfonate micelles suggests that fewer moleculesare able to pack into the micelles which in turn reflectsthe higher steric constraints to such packing. ko. whichis the reciproca1lifetime of the monomer in the excitedstate, is the same for all the surfactants suggesting a similarenvironment around the three surfactants. The excimerformation rate constant, k., is also similar for the threesurfactants. The K. depends on the size, shape, and therigidity of the aggregates and evidently the difference inthese parameters is not large enough to produce measur-able changes in k..

Adsorbed Layers. Monomer and excimer decay profilesof pyrene in the adsorbed layers were obtained at differentadsorption densities. The decay profiles below an ad-sorption density of 2.5 x 10-11 mol/cm2 could not beobtained due to the low pyrene concentration in theadsorbed layer. Typical monomer and excimer decayprofiles for one of the alkylxylenesulfonates are shown inFigure 7. These profiles are similar to those obtained forthe micelles, indicating the presence of fragmented ag-

(16) Chenc. D. C. H.; GuJari, E. J. Colloid Interface Sci. 198%,~. 410.(17) Sinton, S. W.; Huff, S. L. J. Colloid Interface Sci. 1m ,1:6>,358.

00

AA

.?

~~ ~ f

-

:~-. -. -. -- -- ~~I.nq." (~) W.,.lonQ'" I_'Ficure 7. Pyrene monomer and excimer decay profiles inadsorbed layers of alkylxylenesulfonates, adsorption density =3.1 X 10-" moVcm2: (A) monomer emission for surf/py = 545;(B) monomer emission or suf/py a 102; (C) excimer emission forsurf/py = 102.

Table 2. KiDetic Analysis of Pyrene Decay inp-XyleDesuifoDate (Panl)! AlWDina Adsorbed Layer

ad.. density (moVcm2) aurf!Py ko (ns-l) k. (ns-l) Ni\

3.3 X 10-" 105 0.0053 0.015 0.16 176.2 X 10-" 108 0.0058 0.015 0.32 341.9 X 10-10 110 0.0053 0.009 0.5 553.8 X 10-10 116 0.0054 0.006 0.64 74

Table 3. KiDetic Analysis of Pyrene Decay in,..Xylen88ulfonate (Pard)/ AIWDiDa Adsorbed Layer

ads density Cmol/cmf) aurf/Py ko Cort) k. C.-I) NA

3.1 x 10-" 102 0.0056 0.015 0.2 206.7 X 10-" 107 0.0057 0.015 0.31 333.1 x 10-10 111 0.0054 0.008 0.58 643.9 x 10-10 112 0.0054 0.0062 0.7 78

Table 4. KiDetic Analysis of PyreDe Decay inm-XyleDesulfonate (Meta)/ Alumina Adsorbed Layer

ads density (moVcm2) surf/Py ko (ns-l) k. (ns-l) N"

sured. However, at higher adsorption densities, theaggreation numbers of the m-xylenesulfonate are lowerthan that of the p-xylenesulfonates. This suggests highersteric hindrance to the packing of the surfactant moelculesin the hemimicelles of the m-xylenesulfonate.

k. values for the thfee surfactants decrease graduallywith an increase in the amount adsorbed. The valuedecreases from 0.015 (same as that of the micelles) to 0.006ns-l at the highest adsorption density studied. Thisdecrease in k. is attributed to increase in the aggregatesize or due to increased rigidity of the adsorbed layers.

SummaryFluorescence spectroscopy was used to probe the mi-

crostructure of the micelles and adsorbed layers ofalkylxylenesulfonates. The studies show that the micro-structure of the hemimicelles of these surfactants is similarto that of these micelles. Micropolarity studies showedthe polarity in the interior of the p-xylenesulfonate micellesto be lower than that of the m-xylenesulfonate micellesdue to lesser water penetration in the case of the former.The reduced penetration suggests tighter packing of themolecules in the p-xylenesulfonate micelles. No differencewas observed between the polarities of the adsorbed layersof the three surfactants. Fluorescence decay studiesshowed the aggregation number of the p-xylenesulfonatemicelles (-35) was higher than that of the m-xylene-sulfonate (-26). Also, at higher adsorption densities theaggregation number of the p-xylenesulfonate hemimicelles(-76) was higher than that of the m-xylenesulfonate(-63). The lower aggregation number of the m-xylene-sulfonate micelles and hemimicelles confirm the postulateof increased steric hindrance to the packing of thesurfactant molecules in the case of m-xylenesulfonatemicelles and hemimicelles.

Based on the evidence provided here, the effect of changein the position of the functional groups on the aromaticring of alkylxylenesulfonates on micellization and hemi-micellization can be explained in terms of steric constraintsto the packing of the surfactant molecules in theiraggregates.

3.6 xl&-" 82 0.0058 0.018 0.2 175.9 xl&-" 87 0.0065 0.0092 0.4 351.0 X 1&-10 90 0.0053 0.0082 0.55 493.8 X 1&-10 94 0.0056 0.0075 0.66 62

gregates at the solid/liquid interface. The results of thekinetic analysis of pyrene decay for the three surfactantsare given in Tables 2-4 for selected adsorption densities.

The error in the calculated value of the aggregationnumbers was - 25 % at adsorption densities below 5 x10-11 mol/cm2 and -10% above that.

The ko values are the same as a function of adsorptiondensity with the experimental error for the three surfac-tant&. Also, sin1ilar. ko vlaues in the micelles and theadsorbed layers indicate a similar environment in boththe aggregates.

Average aggregation numbers of the three surfactanthemimicelles increase with increase in adsorption from 17to 76 (Figure 8). The aggregation numbers of the twop-xylenesulfonates are similar throughout the range mea-

Acknowledgment. The authors acknowledge the Na-tional Science Foundation, Department of Energy, ARCO,and BP America for support of this work and S. Thach forsupplying the surfactant samples.