Reprinted rrom LANGMUIR, 1989, 5, 215.Copyright @ 1989 by the American Chemical Society and reprinted by permission or the copyright owner.

Excited-State Resonance Raman Spectroscopy as a Probe ofAlumina-Sodium Dodecyl Sulfate Hemimicelles

P. Somasundaran,.,t Joy T. Kunjappu,t,t Challa V. Kumar,S Nicholas J. Turro,'and Jacqueline K. Barton'

Langmuir Center for Colloids and Interfaces and Chemistry Department, ColumbiaUniversity, New York, New York 10027

Received May 2, 1988. In Final Form: September 12, 1988

Excited-state resonance Raman spectroscopy has been shown to be a sensitive technique to monitorthe formation of hemimicelles. The alumina-sodium dodecyl sulfate hemimicelles are examined byexcited-state Raman spectroscopy, for the first time, by observing the Raman spectrum of tris{2,2'-bi-pyridyi)ruthenium(Il)* incorporated in the solid-liquid interface under in situ equilibrium conditions. Thestudy clearly shows several transitions are sensitive to the evolution and structure of hemimicelles.

marked increase which was about 10 times more than themicellar viscosity. Here, the alumina-SDS system is in-vestigated by excited-state resonance Raman spectroscopy,which is convenient in an aqueous environment as in thepresent system. Tris(2,2' -bipyridyl)rutJ1enium(ll) chloride,Ru(bpY)32+, was chosen as the reporter molecule. It hasbeen shown that ruthenium polypyridyl complexes serveas excellent photophysical probes for biopolymers like

IntroductionAdsorption of ionic surfactants on alumina from aqueous

solutions results in the aggregation of surfactant moleculeson the solid-water interface forming two-dimensionalsurfactant structures called hemimicelles. Hemimicellestend to change the surface properties of solids and areexploited in many technologically important processes suchas flotation, flocculation, and oil recovery.1

The internal structure of hemimicelles formed by so-dium dodecyl sulfate on alumina was recently studied byfluorescence2 and electron spin resonance3 spectroscopicmethods. These results substantiated the earlier obser-vations involving bulk property measurements.. Micro-scopic properties like polarity and viscosity as well as ag-gregation number for different regions in the adsorptionisotherm were determined5 by these spectroscopic mea-surements. Tbese studies indicated that hemimicellarmicropolarity is comparable to that in a micellar envi-ronment; but the hemimicellar microviscosity showed

(1) (a) Reagents in Mineral Technology; Somaaundaran, P., Moudgil,B. M., Eda.; Marcel Dekker: New York, 1987. (b) Hanna, H. S.; Soma-8undaran, P. Improved Oil Recovery by Surfactant and polymer Flood-ing; Shah, D.O.. SclIecter. R. S.. Ed&.; Academic Press: New York, 1977.(c) Fuerstenau, D. W. Principles of Flotation; King, R. P., Ed.; SouthAfrican Inst. Min. Metall: JobanDe8berg, 1982-

(2) Somasundaran, P.; Turro. N. J.; Chandar. P. Colloids Surf. 1'".20, 145.

(3) (a) Wa~ K. C.; Turro, N. J.; Chandar. P.; Somasundaran, P.J. Phys. Chem. 1986.90,6830. (b) Cbandar. P.; Somasundaran. P.,Waterman, K. C.: Turro, N. J. J. Phys. Chem. 1987,91,150.

(4) (a) Somasundaran. P.; Chandar, P.; Chari, K. Colloids Surf. 1983.8, 121. (b) Hough, D. B.; Rendall, H. M. Adsorption from Solution atthe Solid-Liquid Interface; Parfitt, G. D.. Rochester, C. H.. FJIs.; Aca-demic P~ New York, 1983. (c) SoIDa8Wldaran. P.; Fuentenau, D. W.J. Phy. Chem. I_, 70, 90.

(5) Chandar. P.; Somasundsran, P.; Turro. N. J. J. Colloid InterfaceSci. 1981. 117,31.

t Langmuir Center for Colloids and Interfaces.I Postdoctoral research scientist from Chemistry division, BARC.

India-400 085.I Chemistry Department.

0743-7463/89/2405-0215$01.50/0 @ 1989 American Chemical Society

216 Langmuir, Vol. 5, No.1, 1989 Somasundaran et al.

~~~~

Table I. Normalized Intensities for Various RamanTransitions in Aqueous Solution and SDS Micelles

~~

normalized intensitiesaqueous solution

(%3%)SDSmiceUea

(%3%)peak position,

cm-l

1213128613241426149915471605

0.661.000.110.420.470.850.26

0.841.000.200.420.730.920.39

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onto a diode array detector (PARC Model 1420, 1024 elements).The data were collected and processed by homemade softwarein Heminway Basic. This allowed convenient subtraction ofbackground spectra and calibration of the data. The spectra werecalibrated by using the spectrum of Ru(bPY)a2+ *.10 The slurryof alumina and SDS was allowed to drop as a smooth continuousstream under gravity to intercept the laser path.

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Wav.Ru r (em-1)Figure 1. ~ Raman spectza of Ru(bPY)32+ (A) in waterand (8) in SDS micelles. (C) Difference spectrum.

nucleic acids.s The excited state of Ru(bpY)32+ showsstrong resonance-enhanced Raman transitions whenprobed at 355 nm.' Furthermore, it has been shown thatbinding of this ion to clay particles results in substantialchanges in the ground-state transitions of the excited-state~nance Raman Spectrum.8 For these reasons, this probewas chosen to study the hemimicelles formed at the alu-mina-water interface with excited-state ~nance Ramanspectroscopy.

Experimental SectionRu(bpy),CI2 was used after repeated crystallization from

methanol (Alfa Products). Adsorption samples were preparedby shaking alumina (Linde A from Union Carbide, specific surfacearea 15 m2/ g particle size 0.3 JI.n1) with a solution of sodiumdodecylsulfate (SDS; Biorad, electrophoresis grade) to get therequisite fmal concentration when made upto 100 mL. All s0-lutions were prepared with triple distilled water at constant ionicstrength (0.1 M sodium chloride). Alumina (10 g) was suspendedin a solution of SDS, and 20 mg of Ru(bpY)32+ was added to it.The samples were diluted to 100 mL with sodium chloride solution(0.1 M), and the pH of the solution was adjusted to 6.5 with 1N HCL They were shaken for 13 h and then used for Ramanstudia Solutions of RU(bPY)32+ (1.25 X 10-4 M) without aluminawere also prepared in SDS as blanks under identical conditions.

Raman spectra were run on a homemade time-resolved RamanspectrOmeter.8 The samples were excited by the Y AG thirdharmonic at 355 nm (5-mJ, 6-ns fwhm). The same laser pulseserved as the probe pulse. Back-scattered Raman light wascollected and dispersed through a SPEX triplemate spectrograph

Results and Discussion

Raman spectra of Ru(bPY)a2+* in water and SDS mi-cellar solution (9.5 X 10-a M SDS) above its critical micellarconcentrationll are shown in Figure 1. Spectrum of Ru-(bPY)a2+ * at premicellar region of SDS was a1m~t identicalwith its spectrum in water. However, Raman spectra ofRu(bPY)a2+ * above the cmc show frequency shifts as wellas intensity changes as compared to its spectrum in water.The 1213-cm-l transition is shifted to 1208 cm-l, and smallshifts in other bands are also noticeable. A resolvable newtransition at 1563 cm-l is present in SDS micellar media.A ground-state transition around this frequency is re-ported, but other possibilities cannot be ruled out. It isto be pointed out that the 1015- and 1036-cm-l transitionsare better resolved in the spectra with SDS than in water.This could be due to a decrease in the intensity of theground-state transition at 1028 cm-l, which appears in theexcited-state resonance Raman spectrum in water. Therelative intensities of various transitions with respect tothe line at 1286 cm-l in water are listed in Table I for theaqueous and SDS micellar cases. Several transitions aremore intense in the micellar environment. In particular,peaks at 1213, 1499, 1547, and 1605 cm-l are enhanced.The peak at 1425 cm-l is substantially broadened, whereasa shoulder is observable at 1563 cm-l. These changes canbe clearly seen in the difference spectrum. We attributethese transitions to the perturbation of the excited stateby the SDS micelles.

It has been shown earlierS that the adsorption isothermof SDS / Al2Oa system contains for distinct regions. Hem-imicelle formation starts at region II, and continued ad-sorption of SDS leads to surface charge reversal on thealumina particle at the onset of region III. Surfactantadsorption saturation occurs at the beginning of region IV.From these earlier observations, it can be expected thatthe interaction of the positively charged Ru(bPY)a2+ probewith the hemimicelles may be significant in the transitionof regions II and III, and in region IV only whereas inregions I and II the probe may be present predominantlyin the aqueous bulk. Excited-state resonance Ramanspectra of this probe in various regions of adsorption iso-therm of SDS on alumina are shown in Figure 2.

The spectrom of Ru(bPY)a2+ * on alumina in the absenceof SDS was very much similar to its spectrum in water~

(6) (a) Kumar, C. V.; Barton, J. K.; TunG, N. J.lnorg. Chem. 1981,26, 1455. (b) Kumar, C. V.; Barton, J. K.; Turro, N. J. J. Am. Chem. Soc.1985, 107, 5518.

(7) Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1979,101,4391.

(8) Turro, N. J.; Kumar, C. V.; Grauer, z.; 8arWI1. J. K. Langmuir1181, 3, 1056.

(9) Kumar, C. V.; Barton, J. K.; Gould, I. R.; TunG, N. J.; Houten, J.V. lnorg. Chem. 1988,27,648.

(10) Bradley, P. G.; Kre88, N.; Hornberger, B. A.; Dallint.er, R. F.Woodruff, W. H. J. Am. Chern. Soc. 1981, 103,7441.

(11) Coli, H. J. PhYB. Chern. 1970, 74, 520.(12) Beck, S. M.; BnJa, L. E. J. Chern. PhYB. 1981, 75, 1031.

Resonance Raman Spectroscopy To Probe Hemimicelles Langmuir, VoL 5, No.1, 1989 217

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both in terms of frequencies and relative intensities. Thistrend is continued into region ll, where the hemimicellaraggregation process starts (Figure 2). In regions III andIV, the Raman spectra show dramatic changes. The fre-quency shifts are more pronounced here than in micelles.A plot of change in wavenumbers for some of the lines inthe four different rgions of the adsorption isotherm areshown in Figure 3. These curves resemble the typical,S-shaped adsorption isotherm for the SDS/ Al2Os system.A similar plot of the relative intensities under these con-ditions is shown in Figure 4, which also shows a similar

set of S-shaped curves. These changes in Raman frequencyand intensity assume substantial significance in the tran-sition of regions II and III and onwards only. This couldbe due to the change of net charge on the alumina surfacefrom positive to net negative charge. The favorable netnegative charge developed on alumina surface at thissurfactant concentration enhances the electrostatic in-teraction with the positively charged Ru(bpy)S2+ at thesolid-liquid interface. Accordingly, no adsorption of Ru-(bpY)s2+ on alumina was observed when the supernatantof adsorption samples was analyzed in region I and thebeginning of region II or in the absence of SDS. Theseresults clearly indicate that adsorption of RU(bPY)32+ ontothe hemimicelles on alumina surface becomes significantonly close to the point of zero charge. The transitions at1213, 1286, and 1428 cm-l show substantial increase infrequencies The relative intensities of some of thesetransitions also show substantial increments. More in-terestingly, these trends clearly follow the adsorption ofthe probe molecule to the alumina-hemimicellar struc-tures.

There are some characteristic features which distingujgbthe Ru(bpy)s2+ spectrum of micelles from that of hemi-micelles: the 1286-cm-l peak shifts to 1281 cm-l in hem-imicelles whereas this peak is practically unaffected forSDS micelles. Most of the transitions for hemimicellesundergo small. but definite high-frequency shifts whereasmost of the transitions in SDS micelles remain unchanged.The intensity changes in both cases appear to be of thesame order. It may be speculated that Ru(bpY)s2+ may besensing different environments within SDS micelles andAl2Os/SDS hemimicelles.

Even though the enhancement of the Raman signals ofcompounds like chrysene and anthracene has been knownwithin SDS micelles,l1 there is no record of any variationof relative intensities of frequencies of Raman lines in thosecases. In the present case we clearly show that the Ramanfrequencies as well as intensities can be utilized to probethe microenvironments of hemimicelles and interfacialstructures. These results are in addition to the enhance-ment of ground-state transitions observed for Ru(bpY)s2+bound to aqueous laponite clay mineral, 8 where the spectrain the adsorbed state remained unaltered irrespective ofsurface stacking. But, in the present system, the relativeintensity of Raman lines increases as the adsorption den-sity of SDS on alumina is also increased, indicating thesensitivity of the Ru(bpY)32+ Raman probe to respond to

218

changes in the hemimicellar environment.To summarize, this work reports the excited-state Ra-

man spectrum of Ru(bpY)s2+ in the adsorbed layers of asurfactant on a solid at a solid-liquid interface under insitu equilibrium conditions. This opens the general pos-sibility of observing the adsorption phenomenon by yetanother sensitive technique to provide basic information

on adsorbed layers at solid-liquid interfaces.

Acknowledgment. P.s. and J. T .K thank the NSF andDOE for financial support. N.J. T. and J.K.B. thank theNSF, AFOSR, and NIH for financial support.

Registry No. SDS, 151-21-3; Ru(bPY)32+, 15153-62-0; alumina,1344-28-1.