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CO1"FOR.'tATIO~AL CHANGES OF POLY (E11IYLENEOXIDE) DL"RING ASSOCIAnON wmI SODlt;~DODECYLSULFATE

c. ~ IA L TES H. P . S 0 ~ tAS U 1'- 'D ARAN and R. RAMACHANDRAN

LDngmuir Center for Colloids and Int~rfaces, Columbia Uni~'enity, New York.N~w York 10027..Pn-sent Address: Union Carbide Corporation Limited, Tecitnical Center,

Bo"nd Brook. .\'ew Jersey 08805. -

STh'OPSIS

Conformational aspects of the inter:1ctions between pyrene end-labeled poly(ethylene oxide) (Py-PEO) and sodium dodecyl sulfate (50S) in bulk and atthe solid-liquid interface have been investigated us.ing fluorescencespectroscopy. Intr:l.molecu1ar excimcr fluorescence of the l:1beledpolyethylene o..ude was monitored 3S a function of ionic strength and sodiumdodecyl sulfate co~centration and excimer intensity was used 3S a me3Sure orpolymer coiling. [Ii 0.5 M sodium sulfate solutions. the polymer is coiled inthe bulk and retains this conformation in the presence of surfact:l.ntmolecules bound to it. but it is found to stretch out upon the forltt~tion ofsurf:1cr:lnt micelles in solution. The conformation or the polymer C\t the

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JJO MALTESH. SOMASUNDAR.-\N. AND RAMA("H..\~DRA,"

Ollumina/watcr intcrface was monitorcd: 3S a function of thc SOSc:>nccntration. In thc :1bscnce of SOS, no adsorption of polyethyt~ne oxideonto alumina was detectcd. However, formation or 50S hcmi-miccllcs on thesolid rcsulted in the :1dsorption of the polyethylene oxidc in thc strctcnedform at the alumina/water interface.

INrRODUcrlON

Inter3ctions betWeen polymers and surfact3nts in 3queous solutionsh3VC important applications in chemical. petrochemical. and pharm3ceulicalindustrics. for achieving desired solubility, rheology. or colloid31 stability oidiffcrent systems. The understanding of polymer-surfactant interactions willprove hclpful in the fonnulation of optimum surfactant-polymer systems forusc particularly in enhanced oil recovery. In mineral beneticiation. selecti\ityof nocculation and notation processes can be enhanced by controiling thcnature or interactions in the solid-liquid- interfacial region betwecn Ihcpol:omcric and surfactant specieslll. More recently, it has been reco~nizcdlhat the special fcatures of the microstructural environment oi I~C poiymcr-surfac~ant complexes can be utilized in apolications of drug dcii\'cry. solarenergy conversion. and isotope separationt~. As a consequcncc 0'- this wiucrange or applications, polymer-surractant interactions have bc~n ~'~dc.lvinvestig3ted for many years and have becn the topic of sc\'cral rc\icws..:J.l~r.For these studies. classical techniques such as dial~"Sis. surf3cc tcnsion.viscosity, electrical conducti\ity, dye solubilization. specific iQn :lcti\;ty, and.rcccntly, nuclear ma2netic resonance (mIR), neutron SC3tlcring andnuorescence spectr~py have been uscd. ~

- .Fluorescence techniques have long bcen used to study lhc

microstructural environments of molecular species and have proved lO bc avery useful tool lo study in-sitU among several othcr processes. thccharacteristics o( adsorbed specicslsJ. Pyrene is a widely used nuoresccntprobe mainly because of its long excited-state lifetime and spectr:ll sensiti\ityto the surrounding medium. Two aspects of pyrene nuorescence commonlyemployed in fluorescence studies are (a) monomer fluorescence and (b)excimer fluorescence. Monomer fluorescence is widely used to ch:lractcrizethe micropol:lrity of the environment whcreas excimcr lluorescence is uscdfor the analysis of aggregation and conformational studies.

CO:\"FOR~IA nO:-.'AL CHANGES OF P~I) 331

In this study. We ha\'e used nuorescenc= 5pectrosco~y to investigatethe interactions bet\veen p:VTene end-labeled poly (ethylene o~de) rPy-PEOIand sodium dodecyl sulfate [SDSI. We discuss the eifeCl of 3dded SUri.1ct3nton the conformation of the polymer both in bulk solution and 3t the solid-liquid interf:lce. The e:ttent of intra-molecul:lr e:tcimer formation is used as amc~surc of tbe pol:vmer strand coilin~. The r3tio of the intensities nf thecxcimer and the monomer (Ie/1m> indiQtcs the extent of coiJin~ of thepolymer and hence is tenDed the .coiling index". It is to be noted that excimerCorm.1tion in the Py-PEO can be due to both inter- and intra-molecularinteractions. Whereas intra-molecular excimer fonnation is a true indiQtionof the polymer coiling. inter-molecular excimer formation docs .not give ame.1sure of polymer couing. A simple way to isolate the tWo is to ,!ener:lte aplot of the -coiiing indeX"' as a function of pol:vmer concentr.1tion. Only theintr.1-molecul~r excimer r:ltios will be independent oi the poiymcrconcentration. A more aCCUr:ltc method for distinguisnin~ between inter- antiintr.1-molecular excimer formation is to carrv out tluorcscence dCC41Yme.1Surements .)S suggested by Oyama et :1li61. In the present study. unGer th~pol~mer concentrations used, oniy intra-molecular excirner was iormed.

E~~ERI.\JENTAL SECTION

Materials

, ,

Poly (ethylene o:ade) [PEG] supplied by Union Carbide was cndlabeled wirb pyrene by Molecular Probes, Inc., using :1 proccdure devclopcdby Cuniberti and PericboliJ. The strucrure or ~he label.ed polymer is sho\1tubclow. The weigbr..average molecular weight was 6(XX). 7500.

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The coiling o( p~Tene end-!abeled polycthylcne oxidcs in aqueoussolutions ~n be increased by hydrophobic attractions bctWe~n the cndryrcne groups. but tbis is more significant (or ~I.vmers wiih molecularwci~hts in the range o( 4SOO as reported in literatureIS!. .

SDS of greater than 99 % purity purchascd from ICNPh:1rm:1ceu(icals was used as received. ACS certified anhydrous sodiumsulf:1(c (rasher Scientific) was used as received. Unde alumina from UnionC:1rbide was used. The particle \ize was specified (0 be 0.3 pm with a B.E. T.specific suri3ce area of 15 m- /g. AU solu(ion! were prep~red in triplydis(illed water. All experiments were performed at an ionic strength of 0.5 Msodium sulf3te. e.~ept in C3Ses men(ioned.

Methods

F1uorcscence s~ctra were recorded using a SPE.\:-Auorolog ($PE..\:Industries Inc.. ~ew Jersey) s~Ctrophotometer. The sample cell for .solutionsh:ld a path length of lO-mm. where3S Cor scanning a solid suspcnsion a ccll of2-mm path length was used. i.e.. solution sampies were analyzcd in.transmission or. right angle geometry and solid samplcs by frunt iilCCgeometry. The solution s:lmple:s were frce from solid impuritics thuseliminilting the possibility oi sC:lttered light ~nd appropri~te .corrcaion wasmade Cor l:l.mp scattering thus ensuring that the me~suremcms werc only offluorescence from the sample. The excitation wavelengti} in all C3Ses was 3:0nm. The monomer intensity was determined at 376 nm and the cxcimerintensity at 480 nIn. The surface tension was me3Sured as a function ofsurfactant concentration according to the \Vtlhelmy plate method using aplatinum plate. The me3Surements were orried out 30 min. aftcr aging lhcsolution. Due to the influence of the concentration on the time dcpcndenccof surface tension. only points beyond the critical micelle conccntr:llion of thcsurf3ctant az:e exact equilibrium values. This procedure was adoplcd bccausconly the CMC value was desired and not tbe exact surf:lce tension :It th:ltconcemratio!1-

~CO:-.lFOR~tA TtO:-:AL CHANGES OF PEa

RESULTS ~'lD DISCUSSION

PEa . SOS Interactions in Bulk

Surf:1CC tension as established by Jones{91 has been tr:1ditionaJlv usedto study polymer-surf:1ct:1Dt inter:1Ctions. Figure 1 shows a plot of suri:1cetcnsion of SOS solutions ior polymer-tOree :1nd polymer-containiD2 soiutions.

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Fig. 1: Surf3ce Tension or PEO-50S ;\tix(u~~

As seen from the figure. the C~(C of SDS in t>.s M socJium sulf.1tesolution is be"tween 1 and 2 x lO-~ M. In the presencc of the polymcr. thesurf.1ce 3ctivity of the surf.1ct.1nt is found to be diffcrcnt. Thrce regions .1Sdcscribcd by Joncs(91 3re evidcnt in this plot: region I \vhere "-° inter.1ctionoccurs between SDS and PEa; region II whcrc therc is bincJing of the ~DSonro the polymer indiC.1ted by (be const.1nt suriace tcnsion: rcgiun III wherefree SDS exists in solution 3nd the curve joins th-:.t of (he polymcr.ircesystcm. These results confirm tbe bincJing of (hc surfactant onto the pol~mer.

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J).a MAL TESH. SOMASU~DARAN. AND RA~I/\CHANDRA~

but ~vc 00 indiC.1tion as to the cffcct on the pnl:-mcr conform:ltion. Fi~rc ~shows thc plot of thc coiling indcx of thc pol:-mcr :IS :I functinn of surl:lct:lntconccotr:ltion. Comp:lred to Figure 1. it C:1n be sccn th:lt :It I~~ point :1t\\Ohich pol:omer and surf:lctant bcgin to inter:lct. thcre is a cl~r ch:ln~e in thcconli~r:ltion of the polymer. Thc polymcr bccomes more coilc'd :1Ssuri:lct~nt binds onto it. The binding nf the surf:lct:lnt onto Ihc. pol:-merC.1USCS tbe polymer coil app:lreatly to co\l:lpse owing to an incrc:lse in Ihehydrophobicity of the polymer coil. 5imil:lr results were obtained byPrud'bomme aDd Uhll1Ol in tbeir experiments where the viscosity of pol~'mer-osurf:lct:1nt miXtures was determined. The point at which the polymer i5 mostcoilcd is just below the suriact:mt concentration :It which tree suri:lctantexists in bulk solution :lad tbe stretchinl! out of the Py-PEO cnrrcsnonus tothc Ir:lnsilion from PEO/50S agJ;1'c~lcs plus exccss PEa to PEO/SOSa~e~:ltes plus excess $DS (Fig. 1). At Ihc prcscnl tilne. \vc t-.:I\'C nosalisiactory expl;1n:llion ior this pnenomcn:l.

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by eleClrost:1lic rcpulsion among thc SDS micelles att:1chcd lO thc polymer

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CO:-'-FORMA TIO~,\L CHANGES OF p~O ~J~!

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Thc adsorption isothcrm of SDS on alumina wa.~ first dctermincd :Ita s.1lt conccntr.1tion of 0.5 M sodium sulfat~ and is shown in Fi~urc 4. Thcprc~cncc of hemimicellcs at th,c ~Iumin~/water intcrf~cc :lOOVC a SOSrcsidu.11 conccntr;1tion of 4 x 10'" kmnlfm.) was vcrilicd by pyrcnc .cmis~innusing ~ procedure est~blished earlierlSI. To dctcrmine the effcct of surfact~ntprcco.1ting. :1Iumina was conditioned lirst w;th SOS solutions at diffcrcntconccntr~tions for 15 h aDd then w;th PED solutions for 12 h. Auol'csccncespectr.1 of the :1Iumina slurry were then recorded to check for prescnce ot anyPED at the alumina-solution interface.

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Figure 5 shows the co.oiormalioo of thc Py.PEO lm the ai~minasurf.1ce. The presence oc the surf.1ct.1nt 00 the alumina surfacc c3uscdadsorption of the polymer,and furthcrmore,. the Corin3tion or. surf3ct3nt.1ggreg.1les has changed the configur61tioD of the 61dsorbing polymer, to astrctched form. It is noted th61t both surf61ct61nt miccllcs and hcmi-miccllesafCcct the polyrlter conrormation in a similar m61nner. ftuoresccncc spcctr61 orthe supcrnatants showed very little polymer to be rrcscnt, indicatin~ ncarcomplel~ 61dsoiplion or PEO onto 61lumina in thc prcsencc of preadsorbcd50S. This result can be significant in rorcing thc' 61dsorplion or pol~'tncrs ontosolids where they may not othcn\;se adsorb. Inslc61d of drastically allcrin~ thesolid surf61ce, a simple altcrnalive is to rorm hcmimiccllcs at Ihc solid-liquidinlcrfacc 61nd thcn to 611l0W lhc pol:-mcr lO adsorb.

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."CO~FOR~1."T10~AL CH..\:-'-GES OF PF:O

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CO~CLl"SIO"'S

- PED - SOS interactions :Ire seen to be m:1rK~ botil in bulk :lnc.i :Itthe solid-liquid interfa~. The binding of suriac(~nt (0 the pol~mcr causcu itto coil- but (he formation of free surfactant micelles c:luscd (he polymcr (0strc(ch out. -

\a.At the solid-liquid interf:lce. pre3dsorb~d SOS 00 alumina furccd

polymer to adsorb on it and in addition surf3tt.1Dt hcmimicellcs :1llcrcd thcconformation of lhe adsorbed polymer from coiled to ~tretchcd. Surf:1ct:lnt-micelles in bulk solution and surf3ctant hemi-miccll~s :It .(hc solid/liquidinterface affect the polymer conformation in a similar m:1nner: .

.YFin:Jnci:JI support from the N:Jtion:J1 Sciencl: F\)undation (NSF-CBT-

86-1552.$), Department of Encrgy, Standard Oil Comp:1nY'oC Ohio (SOHlb)~nd E.'(."(ON is gr:1tcrully ~cknowledgcd. . .

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