J, Indian ~ Soc., IVol, 66, Aogust-october 1989, pp 639-646

Raman Spectroscopy of Surfactant Aggregates inSolution and as AdsorbatesJOY T. KUNJAPPU.. and P. SOMASUNDARAN.

Langmuir Oente~ for Colloids and Interfaces, Columbia University. Now York. N.Y. lOO~7. U.S.A.

resonance in that the latter two depends on theservices of an externally added or labelled lumine-scent and free radical bearing compounds ormoieties. Those methods are often criticised ascausing perturbation to the environment they are toprobe. although important information on thesolution and interfacial properties of the surfactantmolecules nnd aggregates are derived from thesestudies.

Among vibrational spectroscopic techniques,Raman spectroscopy has an edge over infraredabsorption spectroscopy in that it is ideal for anaqueous environment. versatile in ease of sample-handling and remarkable due to the wide raoge(50 - 5 000 cm-~) over which surfactant spectra canbe recorded. However. infrared techniques are alsogaining importance with the combined popularity ofattenuated total reflectance (A TR) and diffusereflectance (DRIFT) methods.

In this article, we outline some of the Ramanstudies on surfactant aggregates in solution and atthe solid -liquid interface. While sketching develop-ments in the Raman spectroscopy of surfactantaggregates in solution and at ioterfaces. we describein detail the relatively novel time-resolved resonanceRaman spectroscopic technique as adopted by us togain insight ioto the evolution and structure ofsurfactant aggregates on solids.

Principles of Raman spectroscopy:

Raman and infrared spectroscopies are twopopular methods in the area of vibrational spectros-copy. They have an edge over other relatively newtechniques of vibrational spectroscopy like inelasticelectron energy loss and inelastic electron tunneling

g URFACTANTS. a clas5 of amphiphilic molecules,

show remarkable solution properties which arise

from their inhere;1t ability to aggregate in solution.The well-known micellisation process is triggered bythe hydrophobic interaction of alkyl chains of thesurfactant molecules leading to aggregation. In thecase of ionic surfactants, micellisation occurringat critical micellar concentration (CMC) rendersthe micellar aggregates a spherical shape1 inaccordance with the large surface area and minimalv(')lumc requirements. Such spherical micellarparticles would have maximum interfacial area andare stabilised by the hydration of polar head groupsand interaction between head groups as well as alkylchain interaction and binding of the counter ions.At concentrations much higher than the CMC. the~urfactant species f"rllls rod-like micelles and atstill higher concentrations bilayered structuress.

Parameters of primary significance to micellarsy~tems are the CMC of the surfactant and theaggregation number of micelles. They are normally

. estimated by a variety of physical methods whichincludcs measurement of surface tension, electricalconductivity. dye soiubilisation. osmotic pressureetc. Such methods though useful in generating agross picture of the surfactant assembly cannot yieldany molecular level information about the interac-tion between individual surfactant molecules.Vibrational and magnetic resonance spectroscopicmethods are considered to be eminently suited toprovide fundamental information about the interiorof the micelles.

Methods such as infrared, Raman and nuclearmagnetic resonance as intrinsic probing techniqueshave an advantage over extrinsic spectroscopictechniques like luminescence and electron spin

..Poet-doctoral reeearch aclent.ist from Ohemist.ry DIvision, B.A.B.O.. India.

639SICS-IS

J. INDIAN CHEM. SOC., YOLo 66. AUOUST-oCYOBBR 1989

bility tensor is represented as

,,'z=

~UNJAPPU & SOMASUNDARAN : RAMAN SPBCTROSCOPY OF SURfACTANT AGORBGATES elc

of terminal methyl groups of the hydrocarbonchains of th~ surfactant molecules in the smallcentral volume clement. Accordingly. bending orself.coiling of the surfactant chains might result.Such changes should be reflected in the Ramanspectra of micelles.

CW la$er Raman studies:Majority of Raman studies of surfactants have

been done under continuous wave id:;~r illumination.Vibrational bands of C - H stretching modes ar~not much useful in judging the extent of the abovementioned self-coiling- accompanying aggregationof surfactant molecules. The diagnostic bandsbelong to a special category which arc indicativeof the relative conformation of the individualmethylene segments. Thus. a Irans-conformationof methylene chajos would be indicated by theappearance of accordion modes below 500 cm-~.Such longitudinal acoustic vibrations were earlier\deotified for polymcthyleDe chains by Schaufeleand Shimanoucbi8 who represented this mode ofvibration as shown in Fig. I.

~ ~~ ""

'I"\.~~~~~A/FtB. 1. LoualwdfDal accordion mode or an ,x~nd4d uormal

hydrocarbon moJ~ole.

This originates from a symmetrical, longitudinalaccordion like motion of an extended zig-z'lg carbonback-bone and is treated similar to the longitudinalacoustical fundamental frequency of a continuouselastic roo. Accordion frequencies in solid stateand to some extent in aqueous solution follow therelation,

These results are in contrast to the results otother workers. for sodium dodecyl sulphate (SDS)and cetyl trimetbyl ammonium bromide (CT AD)in different phases (solid, micelle and invertedmicelle). While an all-trans conformation isreported in the solid state, concentration dependentRan1an spectra in solution progressively showedkinking of the alkyl chain and the presence ofgauche conformers in both pre-micellar and post-ti1icI:lldf ';unc~ntratiol1 r2nge';. This iJ1dicates thechange of surfactant molecules from an essentiallyall-trans chain geometry (solid state) into one withdetectable gauche conformation (solution). Thesame trend was observed even in inverted micellesof CT AB in chloroform - water despite the absenceof packing constraints of the peripheral alkyl chainsin those cases. They employed the accordion mode('*'*' 220 cm-J.) and other trans markers at 891 and928 cm-l (doublet-methylene wag) and 1063 and1 127 cm- J. (doublet-skeletal C - C stretching) toassess the contribution of trans-gauche conforma-tions.

While examining the conformational state ofsurfactants in the solid state and in micellar form,Kalyanasundaram and Thomas. also observed thationic surfactants like CTAB and SDS, in solid state.exist in ail-trailS conformation and show additionalRaman lines in liquid state which correspond tothe liquid like nature of the micellar core. Similarly,electrolyte-induced sphere-rod transition in micellarsystems 31so indicatcd greater ordering of thehydrocarbon chain in the rod-shaped than in thesphere-sha~d agg£egates.

Among the. non ionic surfactaots, Triton X-IOOhas been the subject of Raman spectroscopic investi-gations by several groups to elucidate the natureof the conformations of hydrocarbon and ethyleneoxide chains. Matsura and Fukuhara8 showedthat the l.l.3,3-tetramethylbutyl group in TritonX-IOO, i.e. t(.-[p-( l,l.3.3-tetramethylbutyl)phenyl)-OJ-hydroxypuly(oxyethylene) exists more in the gaucheconformation in the liquid state and in aqueoussolution, than in the trans form. These structuresare schematically represented in Fig. 2. Anotherstudy. on Triton X-tOO indicated that ethylene

-. ~ tin

where-; is the frequency in wavenumbers and n thenumber of carbon atoms. For a chain containing12 carbon atoms as in SDS.

;~ 200 cm-J.

Okabayashi et al.. studied the concentrationdependence of the molecular conformation ofsurfactant. in aqueous solutions by Raman spectros-copy. These authors found that the Ramanintensities of the accordion vibrations of the all-trans form relative to the skeletal deformationvibrations of the gauche isomers increased withsurfactant concentration; marked intensity changewas observed for aqueous solutions of potassiumn-bexanoate (accordion: 327 cm-J. i gauche:397 cm-J.) and potassium n-pcntanoate (accordion:362 cm-J. ; gauche: 432 cm-1) at CMC. The studyalso i'ldicated that the percentage of all-trans formof the surfactant molecule increases with an increasein concentration above the CMC.

c(0) (bl

I'll. i. Bchoma~IQ .tfUo~Ur08 01 ,-{1.1.8.8-Wrame~1IbQt11)-phenoxJl groop In Trl&ou X-lOO: (a) Cron, and(b) gauch, conformation..

641

I. INDIAN CHEAl. 800., VOL. 66, AUGUST-oCTOBSR 1989

surfaces provided a new handle to surface chemiststo closely observe the adsorbed surfactants also.Colloids of silver. gold and copper have beenchosen by different workers I 0;. to observe theorientatioll of adsorbates. Moskovits and Suhlainvestigated the ~urface geometry and conformationof a number of carboxylic acids (valeric to decanoic)adsorbed on .Iqueous silver sol particles andconcluded that these acids adopt an all-Irons con-forn:::tioil vi. th.:: "jlver 5vl :iurfact:. Their conclusionswere derived from the absence of gouche markersin the skeletal C - C stretching region of the Ramanspectrum and the presence of tbe low frequencyaccordion mode in the C - C - C deformation region.Moreover, alternation of the intensity of theasymmetric CH a stretching vibration was observedto be dependent on the number of carbon atomsin the alkyl chain; higher intensity was noted withodd-number carbon chains than with even-numbercarbon chain. This is in line with an all-trans con-formation. In anothcr investigation1-, long chainedthiols adsorbed on silver colloid particles havebeen studied by Raman spectroscopy. In a recentreportlG, the terminally functionalised surfactant,sodium-IO-undecenoate is shown to adsorb oncolloidal silver as a bidentate through the carboxy-late and olefinic functiollal groups. Surface-enhanced Raman spectra of molecules like pyrazine,pyrimidine and pyridazine adsorbed on silver solhave been compared with data from the correspond-ing experiments on Ag electrode. Results from theabove two method:)'t almost converged except forpyrazine in which case differences observed in thespectrum on silver electrode is attributed to thoreduction pruducts co-adsorbed on the silversurface.

oxide chain ill it is in an open coil form ill pre-micellar c;)ncentrations with micellisation bringingpartial ordering of the ethylene oxide chains.Reporting the Raman spectra of non ionic surfactantslike Triton X-IOO, Coon~y et a/.1o concluded theexistence of four H,O-s.:nsitive vibrational regions« 600.790-850, 1 090-1 ISO, 2700-3 100 cm-1)and interpreted the markedly different depolarisationratios of Raman lines ",s due to the three differentconformat1~!!s ,,~t~~ p:)!)'(oxyethylene) ch:li~.

Though the C- H ,stretching Raman lines ofsurfactants could not be usefully exploited in theearly studies due to a lack of general consensusabout the C - H band assignments and difficultiesfrom the interference of solvent bands in thisregion, Kamogawa et 0/.11 describe a method tomake use of the profiles of the C - H spectra ofalkyl sulphate surfactants. They reported appre-ciable high-frequency shifts of the 2 860 and2 900 cm-1 bands and an intensity rise around2 950 cm-1. in the post-micellar concentrationregion. The frequency shifts associated with thechange from the micellar to molecular solvationare attributed to an intermolecular vibrationalcoupling of C-H bonds rather than solely due tochanges in the polarity of the medium. In addition,the frequency shifts in aqueous solutions of sodiumoc'tanoate due to change in polarity of the surfactantenvironment were shown l' to vary monotonouslywith concentration without registering any remark-able transition at CMC.

Some studies18.1.A have been devoted to themicroemulsification property of Aerosol-OT. Tileformation of inverted micelles of this surfactant inoil is cbaracterised by bending and kinking of thehydrocarbon chains cre~ting an atmosphere con-ducive to the formatIon of reverse micellaraggregates. The ratio of intensities at 2910 and2870 cm-1. is observed to be a function of theamount of water in the AOT/oil/water system.

Raman spectroscopic studies of surfactants,though promising, have not made deep inroadsinto conformational aspects of surfactant molecules.Tbe much proclaimed virtue of its being a probelesstechnique, bas not generated definitive informationon the surfactant assembly. In fact, some of the

.studies do not agree with each other with respectto tbe Irans-gauche ratio, an indicator of the con-formation of the alkyl chain. Furthermore, thechangcs observed do not show, in general, any sharptransition at CMC.

As noted earlier, R.\man spectroscopy, essentiallya scattering process, suffers from inherently weaksensitivity due to the very low probability of theRaman scatterillg process. Tlii:; limitation, to someextent, is overcome in SEI~S. but this has beenfound to be suited chielly for gaseous and lo\vmolecular weight molecules .ldsorbed on speciallyprepared metal surfaces, and cannot be recognisedas a universal method to study the adsorptionphenomenon. The SERS effect is generally attri-buted to the classical electromagnetic enhancementSSwhiclt is related to the fourth power of localmacroscopic field with rcspect to the incident laserfield. The chemical effects are found to be of lessimportant in SERS mechanismss.

[n another variation of the Raman technique,the sensitivity problem is overcome by choosing anappropriate light frequency to selectively excite anarrow absorption band (Resonance RamanScattering). Other modified forms of Ramanspectroscopy such as inverse Raman spectroscopyfind limi.ted applicability as a general Ramantechnique. Hence, scarch was on for new forms ofRaman spectroscopy which lcd to the use of Time-Resolvcd Resonance Raman (TR 8) spectroscopy.

Surfactants at solid/liquid illterface :Adsorption of surfactants at solid -liquid inter-

face1.. is actively discussed due to its applicationin many technologically important areas coveringthe classical froth flotation and novel processesrelated to microchip-fabrication. Recognition ofthe high sensitivity of surface-enhanced Ramanspectroscopy (SERS)1.8 of small org~nic moleculesin the absorbed state on specially preparcd metallic

642

K.UMJAPPU & SOMASUNDARAN : RAMAN SPBCTROSCOPY Oil SURfACTANT AGGRBGATBS e~t.

IS mw/g; particle size: 0.3 micron) with a solutionof sodium dodecyl sulphate (SDS; Biorad, electro-phoresis grade) to get the requisite final concentra-tion when made upto 100 mi. All the solutions wereprepared using triple-distilled water at constantionic strength containing 0.1 M sodium chloride.Alumin:1 (10 g) was suspcnded in SDS solution andthen Ru(bpy):t- (20 mg) was added to it. Thesamples were diluted to 100 ml with 0.1 M sodiumchloride solution and the pH ot- the' solution wasadjusted to 6.S with IN HCI. They were shakenfor 13 h aDd then used for Raman studies. Solutionsof Ru(bpy):+ (1.2Sx 10-. M) without alumina werealso prepared in SDS as blanks under identicalconditions.

The third harmonic of n Nd-Y AG laser(354.5 nm, 5 mJ per pulse, 6 ns duration) served asthe pump and probe source in a one-color experi-ment. Fig. 4 shows the absorption spectra ofRu(bpy):+, Ru(bpy»)+. and emission spectrum ofRu(bpy):+ in aqueous solution... Overlapping ofabsorption spectra of ground state and excited stateof Ru(bpy):+ at the laSer wavelength can be clearlyseen in It. The scattered light was collected byappropriate optics, detected by a diode-array

TR' spectroscopy:In TR I spectroscopy, the scattering property of

a molecule in the excited state is made use of. Thisis normally practised to gain insight into the mole-cular dynamics in solution and also to sort outphotochemical reaction mechanisms proceedingthrough excited stat~~s.. The requisite high con-centration of excited state is achieved by using alaser p~i~ of ~hor( uu..;4tilJll and thc c...cit~d statcproduced .is scattered by the same laser pulse or byanother laser pulse. Accordingly, we have a one-color or two-color laser pump-probe experiment.The temporal choice of the second laser pulsepermits a time-resolved study of the excited state.Since the metastable excited state of a species islikely to undergo more changes than its groundstate, a time resolved resonance Raman study ofadsorbate can yield more information than theground state resonanc~ Raman study. This, in turn,can yield information on the microenvironment oftbe scattering species.

To test the feasipility of adopting the TR atechnique to investigate solid/liquid problems, aconvenient Raman probe was chosen. This gave usenough flexibility to employ the common lasersources. Thus, the present study documents thefirst successful probing of an adsorbed surfactantlayer on a solid by TR8 spectroscopy of a reportermolecule incorporated into the layer. Here, tris(2.2'-bipyridine)rutbenium(n) chloride [Ru(bpy):+],a complex with well resolved ground and excitedstate Raman spectra, has been incorporated in theadsorbed SOS layer on alumina and studied by TR Ispectroscopy. Experiments were also performed inaqueous SOS solution with the same probe whosestructure is given in Fig. 3.

"Ec~A.;::..c.!c...c0

-=E..,

2+

Fig. .).,"18

(AI Ground state ab:lorption, (B) exolted-I'-te absorp'tlon and (C) emisllon of Bu(bpy):+.

detector and the data processed by a computer. Thedetails of this Raman spectrometer has beendiscussed elsewhere..; Fig. 5 shows a schematicrepresentation of it. Aqueous solutions werepumped and alumina/80S slurries were allowed todrop under gravity as a smooth continuous streamto intercept the laser path. The Raman spectrumof water was routinely subtracted from the data.The Ru(bpy):+ spectrum was calibrated using its

Pia. 3. Struoture oftrb-(!J.i'-bipyrldIDeJru~nlum(ll) chloride[Ru(bpy):+).

Experimenta'

Ru(bpy).CI. was used after repeated crystallisa-tion from methanol (Alfa Products). Adsorptionsamples were prepared by shaking alumina (LindeA from Union Carbide; specific surface area,

643

). INDIAN ~M. SOC.. YOLo 66. AUGUST-OCTOBER 1989

TIME-RESOLVED RAMAN SPfCTROM£TER

[~~=J-J~--~:::!:i!]-- I..-,.. II -{ElCiaaerl

1$1'.0'",~AO"'O""

Ea

Pig. 5. Schema~io repreaeutr.tioD of a ~ime-r501ved Ramaalpectrometer..

A qualitative understanding of the Raman lines inthe ground and excited state spectra can be hadfrom literature. Dcpolarisation and excitationprofile studies of the Raman lines.Y'.' iodicate thatthe seven-line Raman spectrum in the region 900-1600 cm-J. arises from the C-C and C-N sym-metric stretching vibrations of the bipyridyl ring.The excited state spectrum contains an additional~~t of seve'"' lilIes \vh()

~UNJAPPU & SOMASUNDARAN : aAMAN SPBCTROSCOPY OP SURfACTANT AGGREOATES etc.

.-i~..~

.~

..a........?;..C

1 213. I 499. 1 547 and I 605 cm-J. are enhanced.The peak at 1 425 cm-J. is substantially broadenedwhereas a shoulder is observed at I 563 cm-J.. Weattribute these transitions to tbe perturbation of tbeexcited state by the SOS micelles. Since the probeappears to be responding to the surfactant aggrega-tion process in solution. the study was extendedto aggregates at the solid -liquid interface also.

AluminalSDS hemimicel/cs:The adsorption isotherm of SOS/AI.O. system

contains four distinct regions (Fig. 8). This isotherm(S - F isotherm.O) is usually divided into four distinctregions. Each region is attributed to a differentmechanism of adsorption: region I corresponds toelectrostatic adsorption, region II to surfactant

10-'sos,aLUMI..aO.IM NoCl, pH6.~ ~

cuc10.~

~..~... 10oQr I'4 f

~ 10.I'r/ '

u ~ NU(~ .0 .Q ~

10-14 I ... 1 .. .. ...1 ... . . ... . .-10-\ 10-. 10-] 10-'

A[SIOUAL OOOECYLSIAFATE. _I.' I IiI..

Fl. 8. 8 - P lso~rm for sodlom dodeoyl sulpha'. &dsorp'lonon .lumina.

r 11C)o 1300 1~ 1700WQ.en..m~., Icm-')

Pis- 9. B%olted-state Raman lpec~ra of Ru(b~y):+ In sodiumdod8Cyl sulphate/alumina hemimlcellel for ~he yarioulregIons of ~he 8 - F ad80rp~lon Iso~herm-

dramatic changes. The frequtncy shifts are morepronounced here than in the case of micelles. Aplot of change in the wave number (or some of thelines or their intensities in the four different regionsof the adsorption i50therm shows a gradual changewith adsorption density. These curves resemble thetypical S-shaped adsorption isotherm for SOS!AI.O. system. However. cbanges in Raman fre-quency and intensity assume substantial significancein the transition of regions If and III and aboveonly. This could be due to the change in net chargeon the alumina surface from positive to net negativecharge. Tbe favorable net negative charge deve-loped on alumina surface at this surfactant con-centration enhances the electrostatic interactionwith the positively charged Ru(bpy):+ at the solid-liquid interface. Accordingly. no adsorption orRu(bpy):+ on alumina was observed when thesupernatant of adsorption samples were aoalysed inregion I and beginning of region II. or in theabsence of SOS. These results clearly indicate thatadsorption of Ru(bpy):+ onto the hemimicelles onalumina surface becomes significant only close tothe isoelectric point. Interestingly enough. thesetrends clearly follow the adsorption of the probemolecule to the alumina - hemimlccUar structure.O.

aggregation (hemimiceUisation), region II[ toadsorption of surfactants with a reverse orientation,and region IV for constant surfactant adsorption.These generalisations were reached from a con-sideration of zeta potential, flotation and otherhydrophobicity studies. Excited state resonanceRaman spectra of this probe in various regions oradsorption isotherm of SDS on alumina are shownin Fig. 9. It can be expected that tbe interaction ofthe positively charged Ru(bpy):+ probe with thehemimicelles may be significant in the transition ofregions II and III. and region IV only, whereas inregions I and II, the probe may be present pre-dominantly in tbe aqueous bulk. This is manifestedin the Raman spectra of Ru(bpy):i-8 in the aboveregions of the SDS/alumina isotherm.

The spectrum of Ru(bpy):i-8 on alumina in theabsence of SDS was very much similar to itsspectrum in water both in terms of frequenciesand relative intensities. This trend is continuedinto the region II where hemimiccllar aggregationprocess starts (Fig. 9 I panels 2 and 3 from the top).In regions III and IV, the Raman spectrum shows

64'

J. INDIAN CREM. Soc.. VOL. 66, AUGUST-oCTOBER 1989

Even though the enhancement of the Ramansignals of compounds like chrysene and anthracenebave been known within SOS.I. micelles, tbere is norecord of any variation of relative intensities ofRaman lines in those cases. In the present case weclearly show that the Raman frequencies as w~il asintensities can be utilised to probe the microenviron-ments of hemimicelles and interfacial structures.The re!ativ~ intensity of Ra~~:'. !inc3 ::-:::-e:1.ses asthe adsorption.density of SOS on alumina is alsoincreased'., indicating the sensitivity of Ru(bpy):+Raman probe to changes in tbe hemimicellarenvironment.

COr'lC1USIOILf :

The immense potential of excited state resonanceRaman spectroscopy may be exploited in studyingthe dynamics of micellar, bemimicellar and solloidalprocesses, especially with a two-color pump.probeexperiment under varying temporal resolution ofthe probe laser. Judicious choice of properlyfunctionalised surfactants suitable for common lasersmay permit TR. studies without extrinsic probes.

The Raman spectroscopic analysis or surfactantaggregates has yielded information on the relativeconformation of alkyl chains of tbe surfactant fromtheir trau-gauche ratio. Our own work discussedhere reports tbe excited state Raman spectrum ofRu(bpy):+ in micelles and in the adsorbed SDSlayers on alumina under in .fitu equilibrium con-ditions. The study clearly shows tbat severaltransitions are sensitive to the evolution andstructure of hemimicelles. Implicit in the resultsis tbe potential of TR. spectroscopy as a powerfuldiagnostic tool to explore solidliiquid interfaces.

Ackno\YledaemeotThe authors thank the National Science Founda-

tion and tbe Department of Energy for theirfinancial assi£tance.

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646