MULn-PRONGED IN-SITU CHARACTERIZATION OF ADSORBED SURFACTANT

AND POLYMERIC MOLECULAR FILMS AT SOUD-UQUID INTERFACE

P. Somasurmran and Joy T. Kunjappu

langnui' Center for CoIoids and InterfacesCdumbia UniversityNew York, N.Y.10027

Abstract

Thin layers d surfactant and polymeric materials In mono and multlayer thickness at sd~iqukjnetface b800g to an area d continued active Investigation. The reason for such an unprecedentedttvust may be traced to the known and potential applications of ffiOOlfied surfaces In ~ucing specWicbenevolent effects k1 techooogically and Industriailly relevant fields d microelectrOOcs, proteinadSOfptOO and mneral processing, to name a few. All of these processes requi"e effective contra of theadsorbed layer thickness, an understanding of wtMch may be sought by Integrating the dasslca ~kmethods poptjar k1 coIlokt science and the newer roecular~eve/-information yielding spectroscopictechniques. The present paper comprises the use of absorption, emission, ma~1c resonance andscattering spectroscopic techniques (fluorescence, electron spin resonance, excited state resonanceRaman, MC.) along with adSOfptkln, ftotatioo, ftocculation and electrokinetic studies to gather informationabout the microscopic properties d the adsorbed surfactant and polymeric films. The interaction dsdkjs with surfactants and polymers (ethOxyiated and electrolyte types) indivkiuallyand In ~ures arealso examined here. Aside from unraveling the microstructure and ev~~ of the adsorbed films, newprobing techniques are kientlfled as an off-shoot of these studies.

Inoovations in Materials ~ng UsingAqueous. Colloid and &rfsa Chemiatry

Edited by F .1(. Doyle. S. Raghavan.P. Somasundaran aDd G.W. WarreD

The Minerals, Metals" Materials Society, 1988

J1

Introduction

Thin 11m technology Is a fast-developing research from with proven aOO ~entIaJ applications In manytraditk>naJ aoo emergi1g flekts. In particular, the thrust areas ci microelectronics and ~k:aI devkeshave drawn heavly from the spate ci recent research on thkt IIrns (1) taking advantage ci the plasmaarcs, as well as laser or k>n beam deposition methOOs to /OOuce corrosion resistance aOO improvedmechanical properties. Here, the gtJf between the d~ aoo the allied technology Is being brkjgedat an kx:redibly fast pace. ThIs Is a sequel to the development ci special techniques capable ciinterrogating the IIrns on a mOec~ar level, thus being able to dk:tate effective contm on the Irteffaclalproperties ci the ~ specialized substrate materials.

In this resped. the Irxjustrles of mineral engineeringaoo processing are lagging behktd, chiefty on twocounts: the lack ci sensklve techni<JJes to stOOy the sdkj~lqukj Interfaces aOO the Ilmbtklns k1 makk1guse of the existing techrjCPJeS due to dlfflct*les In halding the sdkj~lqukj samples ~er in-situequi/ibrkJm cond~. mn layers ci surfactant and polymeric materials in mono- aOO ~~yerthickness at sdkj nineraljwater ~erface formed as a resUt of adsorption from aqueous sd.-kJns rnodlythe surface propefties ci m~s. The Impact ci these adsorbed layers ci surfactants aoo polymers onflotation and flocc~ation (2), the major processes goverring the efficiencyaoo ease of mineraiprocessing and recovery, Is tremeOOous.

Surfactant aggregates as a dass ci organized assembly, though well studied In sd~ Is nct Uyunderst(xxj at the sdkl-ll~ Interface. The discussions on their mcrOStrlJctures are fUI ci spec~atlonswhk:h are ~ substantiated wkh experimenta ~ Theoretk:aI treatment ci these systems appearingsporadically In literature has ~ yiekjed any convincing and ~~ support to the existk1g modelsfor the ad~1on process. The situatk)n Is stli more complex when the thin coatings d ~ymerk;materials on minerals are to be underst(xxj, the worse case being the ~ed assemtJIy ci adsorbed~ymers and SlXfactants at the minerai/water Interface. In this paper, we present the res(jts ci sorne of(XK recn studies on the adsorbed layers d surfaC1ants and polymers at the oxide mneral/waterInterface as deI~ted by ~r-level-k1formatlon yiekjk1g spectroscopic techniques assI$ted byconventional ~k methods. The spectrosco~ techniques employed are of luminescence emissk>n(lluorescence and phosphore~). magnetk: resonance (electron spk1). and scattering (excled stateresonance Raman) types. The ~k properties studied made use ci adsorption densky, ft~ftoccUation and eledrokk1etlc measurements. Important Insight "'0 the strIx:tl.-e and evdutk>n ci theadsorbed layers was ~ined by Regrating the potentials ci spedroscopic tectw1iques with those ofbulk methods. SlmRarty, properties ci the adsorbed layers d two types d ~ymers - ~yacrylic ackl and~ene oxide - have been studied by lluorescence spectroscopy. Importance d the cmormatlonalaspects of the ~ymer mQec~es at the interface in relatk>n to their conformations In Ixjk sdutJon Is alsoassessed by evaluating the pH-dependent ftocc~ behavior ci the adsorbed suspensloos. In allthese cases, pure alumina was chosen as the mOOeI compound.

SoIloids & 5-F Isothenn

A geneI'k: name to represeft the thk1 coating of surfactants ao%r polymers on sakis deYokt of anyunrealistic ~kx18 has been sought by adsorption scientists. An appropriate term suggested torepresent any type of colloid on ~ces is soIloid. Such a term wQtjd elininate some of theInconguies aOO inconsistencies associated with some of the well knCPNn aOO the less ~ twms:hemlmk:ele. a tenn representing the surfacta~ aggegates on charged minerals Is ~ r_1yhemlspherk:al, but. ~ tenn has ~ined wOe a~ncc irl the above cor1text; the tenn admk:ellesuff8fS from the drawback that the adsorbed aggregates have litde to do wkh mlcelar adsorption. Thus,sdIokI can be ge~ enough to comprise the adsorbed aggregates of any shape aoo structure ofsurfactam (Ionic or not) aOO/or polymers (electrolytes or~) at the sakl~~ukllnterface.

A model m~-mactant system described hefe k1IIdves alumlna-sOOium d~ S(jfate (SDS)~e. A typical adsorption Isotherm ~ tor ~ system Is s"""'" it Rgure 1. The shape d sidar

3t

~henns was the fOl'en.V1er to intfjtjve mechanistic models fOf the adSOC'ptk>n process. Thesign.1Cance of such an Isothenn referred to as S-F ~hem1 as pinpointed by Somasundaran andFuerstenau (a) has been recognized by many workers (4). An S-F Isotherm Is characterized by fourdistinct regions - I, II, III & IV. and is exemplified by the Al20a-SDS system: region I is dornNted byelectrostatic adSOC'ptk>n: region II is marked by the onset ()f ~tIon process (hemmk:ellizatlon);region III cOITesponds to 1c7N, ~ continued adsorption due to the inteffaclal charge reversal; region IVrep-eseru the maximum surface CC1t/erage and Is marked by micele formatk>n it the tMjk (5). Fig.1represents the S-F Isaherm for Al20a/SDS system, and the electrokinetic pr~rties of the -.ertacecorres~1ng to the points in the isdhenn are shown in Fig.2.

~:~56-- - 258 m:

196 m u166 ~

50S/ALUMINAO.lM NoCl, pH 6.5

128

n49

.;.t~

N 10-9E~...,!!.0

E. 10-10Z0

~Q.~

g 10-110-cI

t&J

~~ 10-12:>cn-J>-U!oJg 10-130

I!ISDS ONLY(!J" ASDS WITH

PYRENE. ... 1 .,.. \ 1 ...~

10-5 10-4 10-3 10-2

RESIDUAL DODECYLSULFATE. moles/liter

10-14

Figure 1 - AdSOl'ption ~herm ci sodkRn dodecyl sjjfate (SDS) on alumina at pH 6.5 in 10-1 kmd/m3Naa with and wkhout pyrene; nunm-s marked on the Isotherm correspond to the aggregatkJn number.

Intrinsic and ExtrInsic Probes

To explore the moIeclAar neighborhood within soIoids. one has to depend e*her on the intrinsicresponse d' the surfactant molec~es to an external stim~us (intrinsic probing) or on the response ci areporter moIecUe incorporated in the assembly (extrinsic probing). For exampe, NMR and FTlR areintrinsk: probing ~s; but, the experimental diffic~ties have discouraged their use to any greatextent, though some headway Is being made on the IR front. Most ci the methods mille the behavior ofexternal molecules (report8fS) whose emission, spin or some other special properties provide maecuarinlormation aoo. their mlcroenvir~ h is Important to prove the inertness d' the probe moIec~eson the aggregation process lest one should be proting artefacts. ~,electromagnetic ~ Isnot consktered to have any perturang elfect on the aggregation process.

33

Figure 2 - Zeta potential ci alumina as a function ci equBi~ concentration ci SOS (designation ciregions based on shape ci Isothem1ln fig. 1).

Materials and EguiRment

Uncle A alumina (particle size = 0.3 m, surface area = 15 m2/g, 90% alpha form); sodium dodec}ois(jfate (Bk>-Rad. electrophoresis grade); pyrene (Aldrich, recrystallised from ethanol-water mixture);doxy! stearic acOs (MaectJar Probes); tris (2,2' - blpyrkl}oi) ruthenium (II) cNOfkie (A/fa); di~}oipropane (synthesized as In ref. 6); pyrena labelled po/yaCryfK: ackj (M.W. = 40,000, pyrene = 1.5 mde %.synthesized as i1 ref.7); pyrene labeled poIyethyI~ oxkje (M.W. = 6000, pyrene labeling by MolecularProbes). These materials were used for the various tests. The equi~ used Is as fdlCNis:

LN /VIS ~ion spectrophotometer - Beckman;Ruorescence spectr~ometer - SPEX Ruoraog;Tkne-f"esdved fluorescence spectrometer - PRA single photon countIng;EIedron spin resonance spectrometer - Bruker-100;Tkne-f"esdved Resonance Raman spectrometer - as in ref.8;EIedrokInetk: mea&.-em~ - Zetarneter.

Exoloratlon of the Microstructure of the AI~O..;/SDS Sollold

The mk:roslnJCtlKe aro evdution of the soIloki may be studied by the folkM'lng specW spectroscopc

probingt~

Lumlnescel1C8

lumi~ emission after photo-excitation of a 11g~ absorbing moIectAe ocan either asftuorescence orphos phorescence (9). The parameters of relevance are (i) intensiy of emIssb1(quantum yWd). (Ii) emission maxknum. (Iii) life tine of emission. aoo (IV) ~arizatk>n properties. Thelatter property Is not of ~ use In the soIid/liqukl systems due to the interference frOO1 scatteringradiations. The emission intensity If[o] of fluorescence at zero time Is related to that at time t (1f{tV' amlife time f by the relation

'f[t) = If[OJ exp(-t/T)

14

The str\Idwes of the three photdumlnophOl'es used ., 0tX stlxIy are given b8~

,+

~ Dinaphthyl propane RU(bpy)302

Detals ci the specific methods and treatment of the data falow:

Pvrene as a PolaritY Probe

Pyrene Is an organic moIec{je whose fluorescence emlssk>n Is higNy sensitive to the polarity d theenvironmeoc The region from -370 to -400 nm d.s fluorescence nsslon spectrum is remarkable withfive well-resdved line vibrational features (373, 379. 384, 389 aoo 393nm). Of these. the intensMies d thefVst (11) am the third (13) respond to the polarity dthe medkJrn almost in a linear way (10). This ~pI~liooing. though lacmg a precise quantitative uOOel'standing, Is a useful too i1 arriving at themlcropoiarlles d lnaccessl~ regk>ns as i1 the Interior of micelles am soDoids. The '3/11 value dpyrene changes from -0.6 in wat~ to a value of > 1 in non-polar hydrocarbons. A value d -1, the sameas In SOS mlceRes. was observed In Al203/SDS saloid indicating a non-poiar envi'onment within thesalcido Also, the 13/!1 values were relatively constart throughout the higher regions In the S-F isotherm(5).

It may be noted that the S-F isothem1 remained invariant in the preserw:e d sub-m1croo1darconcentrations d pyrene (fig.1).

1.3 - Din8Dhthvl Prooane lDNP\ as a Microvlscositx Probe

Auorescence poiarizatk>n studies offer the best means to measure the micr~riies, but Is of IMdeavail ., the context d soIlokIs. Hence, the excimer fom1lng ability of DNP which Is a sensitive function ofthe microviscosily is exploited to stlxly the soIloid. This property expressed as the ratk> d the yWdS dthe monomer and excmer (lmll~ is determined in saurian 800 in the Al203-SDS doid COITespondlngto various regions d the adsorption isotherm. Mixtures of ethanol and !Jycem were used as theviscosity calibration standards. Fig. 3 shows the spectra of DNP ., SDS micelles 800 A:'2°3/SDS sdIokI.This stlxly yielded 8 m~roviscosiIyvalue of -120 cp for the soIlokI aoo -10 cp for SDS miCeles (11).

)-t-

~ZI&Jt-Z

I&J>t-c:t-JI&J~

320 480340 360 380 400 420 440

WAVELENGTH,nm

460 500

FIgtn 3 - ~ p,q.me (DNP) lluorescence spedJa k1 (A) 80S rr*:-. 8d~ am SDS-8~slurry; ([SDS]/(ONP]) refers to ratio of mk:ellized or adsorbed SDS to added DNP.

Pvren8 Emis8ion DYnamics for Agg[IgItIon Number

The tkne-lesdved lluorescence emission of pyrene ~ed k1 the sdIokt stOOled uoo..ccxxi.o. d monc.n.- aoo exctner errissk)ns can yWd ~ ~ the ~ ,..,.. d~ ~ A knoMedge d n. the average ~ ,..,.. d the P'OO8S I88d8 to the~ NmiJ.- N from the foIowing expressm:

n = [PJ / (AggJ = [PJ N / «(SJ - (SeqI)

where [PJ is the t~ PY'ef1e ~ (AggJ is the ~ d the aggI'8gId:es, «(SJ-(S~) Isthe conc..-ratoo d the edsorbed surfactant. n Is (X)takled from a kinetic scheme which a-.nes ap~ dIstrft)utIoo d the probe k1 fragmemed micellar ~es (12). Fig.4(a) 8h<7Ns a typk:aI decay(:t.w for the monc.n.- at two pyrene ~ (~ A & B) aI.t the exckner «:t.w C) ~ to(:t.w B. .. the $OS ~ sd.-1on. The ~k'O II~ spedra .. repr...ced k1fig.4(b). The abC7I8 methoddogy was awled to ~ regions k1 the S-F ~ am the c8CtJatedaggregation Nnbers are marl<ed In fig.1. A mecharistIc mOOells suggested to re~ the evdutlonam gr(Wt'Ih d the sUIfadaR layers 00 aI~ am Is 8ustrated k1fig.5.

36

F9JI'e 4~) - Pyrene monon:Ier aOO ~r decay ~, In SOS ~ sa~ (SOS) = 8.2 X 10-2knO/m ; (NaO) = 1x 10.1 knO/m . CMC - 1.5 x 10 knO/m; W-levela" ~_theratk> d mk:eIHzed SOS to added ~ (enasi<WI ~ored at 383 rwn b ~ n 48J rwn torexcimer); (A) monomer emlssloo for SOS/Py ; 2160. (8) monomer emission for SDS/Py a 108. (C)

exckner emIsskx\ b SDS/Py s 1~.

figure 4(b) - Pyrene emI88Ion spectra In SDS micellar sdutlons ~er varying levels d pyrene (A aM B1:c:IO.~~ to pyrerIe levels mlcated In fig.4(a)).

37

-..Ltt._L"-."-'.'-..'-'-'- -" ..

~NO~GATIOII

~llUUKR OFAGGREGAnSINCREASES-1Z0-13OMOLEC~Pt:R AGGREGATE

l ,Ii>

~SIZE OF AGGREGAT£SINCREASES~MO ~EC\A.ESPER AGGREGATE

~ 5 - Schematk; representation of the correlation cI surface charge aOO the grCM1h cI aggregates forvarious regions cI the ad~kJn ~ depicted in fig.1.

PhOSDhorescence Emission of TriA(2.2' -bJDvridvl\ nrthenium an chloride - (RUBP)

RUBP Is a luminescent compourxl with an emission quantum yield of about unity. the emissb1properties of wtlich are very sensitive to changes i1 the envWonment. This compound may be conskteredto be the lnorgarjc analogue of pyrena In teRnS of Its versatility as a luminophore. Moreover. being look:.It Is h~ water sau~e. The charge and the pl'esence of hydrophobic bypyrkjl1e ~ mpart to kspecIfk: properties with respect to saubllzatlon sites In aggregates and sdIokjs.

luminescence spectral analysis d the salak! containing RUBP was perforn1ed at dHferem ~ onthe adsorJjIon ~ (13). The emission ~ and the Intensky d eIrissIon COfrespOr'Idlng to thepOnts on the ad~ Isotherm are plotted In figs.6 & 7. The concentration d the probe incorporatedWI the solak! was dMermned In each case from Its a~k)n at 450 nm and Is represented WlIig.8. Itmay be rued that the Wendies d emssIon at the waveler9h maximum corresponding to dlterent~ WI the ~ k1Crease even above the transitoo between regions II and III (1ig.6) wt1ere the

38

probe adsorption reaches a maximum (fig.S). The variation k1 the luminescence maximum (flg.6) ~soshaNs a continuous Increase as obs8fVed In the case of intensity changes. MCKeover. the probeadsorptoo closely talONed the changes In the zeta potential at the interface (flg.2). The signWicance ofthese res(jts wli be discussed wt1Ie evaltatlng the different adsorp«:ion models. In ~ case ~so. theprobe dkl not affect the adsorp«:oolsotherm.

10-'

1j lO-m

t0:..c 10-0-.0c

~ 10-12

......~'"...~ 10-1'...000

SDS/ALI*ItIA0.1 M NoCl, ,HU .--G"j

E.5

~~2

650;(c~......Z.......,...

640 ~~~-'

CMC

:r-°~\~ 1YJ:...t<~L 00

10-14' t ...1"630

10""$ 10""4 10-5 10""2

RESIDUAl. DOOECYLSULFATE. mole./llte,

FIgure 6 - ~ eIrission maxinMJm d Ru(bpy)32 + k1 aI~-SDS sol<*! COI'respondI1g to

selected corM:entrations on the S-F isothenn.

Figure 7 - Ircensity of luminescence emssion of RU(bpY)32 + at emission maximum In alumna-SDS

sdlokl corresponding to selected ~tlons on the S-F Isotherm.

39

FiglKe 8. Am~ cX RU(bpY)32 + 81COrporated in alumina-SDS dokt.

Electron Soln Resonance

Surfactants may not be stLKIied as such by ~ectron spin resonance techn~ue since they do notpossess free electron spin. Spin labelling technique Is the popular method adoIXed to crcumvert ttjsdiffic(jty. The Intrinsk: angular momentum of a free electron (spin) splits In an external magnetic field(zeeman splitting) which upon the inftuence of secondary magnetk: moments of nelg1boring ~undergoes hyperfine splming. A system with a spin ; 1/2 can have two zeeman energy levels. Thisinteracts with the spin (j a nucleus like nitrogen (5; 1) to produce a hyperfine triplet (j equal ~ensity.This Is the type of spectral pattern exhibited by a nitroxkte probe as employed In the present study. In theESR experiments, the soIloid was mixed with doxy! stearic acid (stearic ackllabelled with a ~bearing moiety), and the spectra were analyzed (14) to calculate the hyperfine splitting constant ~) andthe rotational cooeIation tine (c), The rotational correlation time is suscept~e to the vIscosiy (j theenvironment. Here again, mixtures of ethaoo and giycer~ were used as calibration standards (j vlscosiyto arrive at the viscosity of the soIIoid. Tivee different doxy! stearic acids were chosen so as to report themk:rOYIscosityç at three differem locations within the soIIoid. The structure 01 the probes may berepresented as f~I(7NS:

n=14 16-0m=1

n=10 12-0m:5

~N-O.

CH3(CHJ)~ (CHJ).cOOHm=12 n= 3 s-o

Doxy! stearic ackts

40

The vlscosky d~ence of the trYee doxy! steark: acid probes In various ethaoo--giycerd mixtures Isdepicted In fig.g. A comparison of ESR Spectra of 16-doxy! stearic ackj In SOS sdlokl. miceles aMethaoo~ycerd mixtures Is sh<M'n kI fig. 10. It may be seen from fig. 10 that the sdI<*j repol1ed a~tIve/y high vIscosky of -120 cp near the peripf1ery. Other two prOOes, viz. 12- and 5- doxy! stearicackjs respectively reported rrboviscosltJes of 100 and >300 cp In the soIoo dose to the sdkj SINce.These ~ are indicative of an adSOl'p(lon mecha~ in which the carboxylate group of SOS Isattached to the surface and the 8kyi chailS are In a state of two dimensional awegation. as predictedearlier .

figure 9 - ~tiOnaI correlation times of 5-, 12-. and 16-doxyi steark: acid as a fundoo d vI-=oIIy d

ethanol-glycerd nMxtures.

:~.OJS;:

~~..JOJE

§

:0;:~0E

FIgure 10 - Comparison of ESR spectra of 16.00xyl stearic ackj in hemkniceles. micelles, and ethaOO.glycerol mixtures and COITesponding rQtatk>na1 correla1ion times and corresponding viscosity.

41

Time-Resolved Resonance Raman lTR~ SDeclr~y

Raman spectroscopy Is a powerfU too capable of yielding Information on the Vlbratk)nal energylevels of mdec(jes. Normal laser Raman spectroscopy of sulfactant aggregates has not yielded muchInformation about the nature of the aggregates. Recently, there is a grCWling Interest k1 the surfaceenhanced Raman spectroscopy of surfactant aggregates. In the presert wor1<, we employed a probe -RUBP- to report the changes in the saloldallayer by f~owIng the frequency changes and the kttensiychanges of the Raman lines collected by tlme-resaved resonance Raman specttOscOfJy k1 a silVe-<:dor~p.probe mode(15). The ~ analysis was carried w at dilfefert soIokjaI regions.ong withthose In the surfactant saution .so. ThIrd harmonk: of a Nd- Y AG laser ex~ed and scattered the excledRUBP mdec(jes (pulse energy = 5 mJ; PIJse wklth = 6 ns and wavelength = 354.5 nm). The excitedstate Raman spectrum of RUBP consisting of 14 lines (7 lines each from the ground and the excitedstates) was calibrated using an authentic spectrum.

Figure 11 - Excted state resonance Raman spectra of RU(bpy)32 +

(A) in water, (8) In SDS micelles and. (0) difference spectrum.

Fig. 11 ~ the excited state Raman spectra of RUBP In water aoo in 8DS mmles. The spectra Indifferent salokiaJ regions are sh<7tYn in fig.12. The shWts in frequencies aoo the varlatk)n in intMsiIies ascalaMted for the soIIoids are depicted In figures 13 aoo 14. InterestintJy enough, the latter two plotsroughly ~ the same shape as the adsorption Isotherm kself reflecting the senskMiy d the probetONards the evaution of the saloid.

42

.=c~

.A~.

-!..c::c

....

.?

:!.c

Figtn 12. Excked state resonance Raman spectra of Ru(bPY)32 + (A) in alu~na slurry. (8) r~ I. (C)

region II. (0) regioo III. (E) regk>n IV of alumina-50S Isothenn.

TNs Is the fi'St report of the excited state Raman spectrum of a probe roecUe I~ed ~n thesurfactant aggregates In solution aoo the soII~. This stLKiy loolcates the potential d TR spectroscopyto expore the soIkI/llqukllnterface aoo to interpret the adsorption mechanism by yet another sensMive

probing techn~ue.

Reverse Orientation Model

Informatk>n gathered from aM these techniques may be collated to reevaluate the comprehensivepicture presented earlier to rationalize the growth of surfactant aggregates on the saki. The modeldepicted in flg.5 may be termed as the reverse orientation model since it envisages the adsorption dsurfacta~ roecUes as a growing monda~r with the polar head facing the aqueous bulk. This model Isin conftict with the bila~r model postulated to account for the hydrophl~.hydropt1ilic Interactk>n betweenthe polar head of the surfactant and the aqueous phase. The bila~r model assumes the adsorptionprocess to occur throogh a bRayer of the surfactant aggregates from the onset of the aggregation

43

process. Fig.15 provides a convenient overlap of these twin mod~s for the different regions d the S-FIsothenn.

FIgtn 13 - Ff8CJJenCY shifts d resonance Raman IkIes d RU(bpy)32 + * as a functk)n ~ 80s

~tlon for alumina-SDS system.

figure 14 -Intenslles of Ra~ lines (nom1aIlzed with respect to 1286 cm-1I1ne) for dlferert regions daI~k18..sDS syst~

44

Figure 15 . Representation of reverse orieftation model (left) aoo the blayer mOOeI (~) for theevolution of SOS sdlOO on alumina (~ shONs the regions k1 the ~).

The esserjial arguments In defence of the reverse orientation mods based on experimental res(jtsmay be listed as fdlows:

1. ESR spin probing within the soIlokj at three different locations. using doxyt steark: acid nltroxkleprobes, reported very high values of mlcrovlscosltles - one order of magnitude higher than those inmcenes. The blayer mod~ supports the view that the tail groups of the upper layer cj the !>layeredaggregates do ~ sign.lcandy Interpenetrate the tail group region of the bottom layer. This W(Xjd predictdokjal mlcroviscosities ~rable to those existing in micellar soI~k>ns.

2. Analysis of luminescence spectra of RUBP in differer-. saloklal regk>ns sh<M'ed that the microstructureof the adSOfbed surfactant changes continuously as the adsorption density increases. Ct1anges WIJurrinescence maximum and Intensity monitored at various points in the isotherm corroborated theadsorption data of the pos.ively charged probe at the soIkI/liqukllnterface.

3. Time-resaved resonance Raman spectra of RUBP in the same system registered systematic changesWI the frequencies and variatk>n i1 the intensities, again indicative of gradually changing ~s~n the dokJ.

4. Flotation studies on skniar systems $h(7Ned maximum flotation property for the soIloo after the onsetc:i hemimicellization. Bilayer formation would have left behind increasir9Y hydrophllc solid particles.making them flotation resistant.

All of these findings suggest that the microstructure of the adsOfbecl saIOOaJ layer does rd remainthe same as Implied In the bilayer model.

45

Polvmer-SOOaciant SoIIold.

~ of polymers ao%r surtactants onto minerai surface JKoduces thkl coatklgs of~cromdec~es COfUIning detergents. The k1ftuence of these thin fims on many processes oft~ogk:alimportance such 8S floccwtion. adhesk>n 800 synthetic JKGcesses Is remarl<able (16). Theadsorption of pdymeric ~terials Is basically dBferent from the adsorption of smaI mdecules. ThIsdllfererM:8 stems from the possIbIlty of the existence of polymers In different sizes and conformations.ThIs Is In addition to the JKesence of mtJtifunctiora grol4)s In polymers, each of which having the~ to adsorb on 8 surface.

Pvrene-taaaed PolYmers

The structures fA the polymers used In this study are rep'es8nt8d bekM:

t atl).tatlatIO,.tata' i00 00

00

J:::h:!..~", ,~I

PyPAA PyPEO

The PyPAA had 1.5 mOe % of pyrene bearktg moiety, mdecUar we~ of 40,000 and degree ofpofymerIzatIon of 510. The PyPEO ~ a nQecwr weight of -&X)O contained two pyrena ~ at thetwo ends of the pdymer ~In.

The ~ beh~ the mplementatlon of this technique is the obS9fvatoo that the extent of excWnerformatk)n ~ depends on the Interactions of an excled state pyrene of paymer pendant grOl4> wIhardher pyrene group In the gr~ state, has a drect bearing with the polymer confOfmation. The pHdepencin confmnatiora changes of PyPAA payrner and the SUbsequn exclrner fonnatlon arerep-esned i1 fig.16.

It may be noted that the exclmer formation Is higher at IaN pH than at high pH as seen fr<Xn theirsteady state emssIon spectra (fIg.16). At low pH, there Is a better prOOabilky for Intramolecdar excimer~ between pyrene groups res(jting fr<Xn a ~e coIIed~. Smlarty, a IaNpI'obabiity for the exckner formation at ~h pH may be ~ersto<xl as a consequence c1 ~b«ween higNy ionized carboxylate groups on the polymer and the subsequern stretci1k1g c1 the paymer

46

d'8k\. The ~ 8pedra d PyPEO ~ resporxted to chang88 k1 the CCiI ~ -.. IIIIkJri ... toPyP M; ka8aed ~ ~ d18 exc8ner ~ ~ to meased jio"'-'-';;- f d d18 er.t pyrene~ atd vk:e-\'ef-. ~k:atkJn d d18 ~ ~ to akri1a-PyPM soIokt showed that thead~ pdymer retained the same conformatloo as k1 sdutlon from which It goes to d18 8dki~lqukik1terface. The excinB eIIk:Iency ~ these cases Is sh<7Nn k1 fig. 17. It R8Y be ~ ttlel d18 ~~ was rapki 81OUgh to retak1 d18 CCiI~ atk)n ~ sd.-kJn ~ after adsorpkJn.

---6I... pH

Hie" pH

Figure 16(8) - Schematic ~ a the corr8Iatkx1 a the.xr..- a exckner fomwion arvJ WItTasIf8OO coIing a pyrene-Iab8I8d ~ acki.

f9n 16(b) . A~ ~ 8pedra d ~sorbed ~. two J*I Y8~

. .

I-. 4-

,1 J-

I-

1-

O..lulie.

. .. .1

-, . . . . . . . .4 . . 10 11

.N

FJgtn 17 - Monomer to excimer ratk> as a U1Ction ci pH for aqueous ~ ci pcjymeI' ...t adsc:.bed

pcjymeI' on TWIa.

47

The polymer sdlokt was subjected to pH shook to Induce confOl"matkxW changes In the adsorbedpo/ymef. It was obsefVed that the polymer adSOl'ptIon at high pH is rathellrr8Vefsi~e since changing pHto k7N values did not slgnllk:a~y alter the excJmer efficiency. But. adSOfPtJon at low pH is ~e. tosome ext&rc. with pH change since the polymer chain in this state CQtjd be further stretched byincreasing the pH. A mechanistk: representation of these processes is given in fig. 18.

io) ""'-"- - l-... ""'=--~~--. . . .

dld/,.,."

..W .H

~ ~ ~/-. . . .,.p.ftd,d'.d,.,','

#

FIgI.-e 18 - SchematIc representation of the adsorption process of pyrene-iabeIed poIyacryIlc ackt onsumn.

The knplk:atkJn of the ab<7o'e ~ to the lIoccuatk>n behavior of alumk18 dispersk>nS wasInvestigated k1 detaR. The paymer was adsorbed at a fixed pH am the soI~ was perturbed with pHchanges k1 cycles. le., a polymer sdlokt at IaN pH was brought to ~ pH and then retraced the pH tothe k11t1a1 value. Various ftocCtJation responses Il<e sediment vdume, % sdkl settled, transmltance andsettling rate were studied In conjunction with the ftuoresce~ response. Untreated alumina at differentpH values SefVed as the contra.

The fluorescence emission behavk>r of the above samples Is represented In fig. 19. In theseexperimerts. the pH was changed to high values causing charge reversal ci the sdkl-liqukt Interface. Inthe earliei' 8Ch8me shONn for the pH changing situation (fig. 18), the pH was not high enough to farexceed the PZC ci the mineral. In the present case, a m<x11"1ed scheme Is postulated (18) in ~with aI the loccUation responses am fluorescence data (FIg.20).

48

E

~

Figure 19 - Excimerto roonomer ratio.lefllT)' for alumna with 20 PJXn PM as function dflnal pH IxKJerchanging pH conditkX1S {I.S. - 0.03 M NaC!, S/l = 10g /200m).

-",~I(S)j~ ""'-: - ++++++

(b) medium pH, 5-7

"'c~i1b)D++++++

(a) low pH, 4 (c) high pH, lC

~ -~~

(d) hi~h plIo 10

~ ++++++

(f) low pll. 4

++++++

(e) medium pll, 5-7

~ 20 - Schematic representatk>n of dle variation of ~ymer conformatk>n at dle sdkl/liqukf ifefface

~er d1anging pH conditions.

49

In contrast to ~ayllc ackl type polymers. poIymeIS aOO surfactam based on ethylene oxk1e unitsdo not adsorb on oxkIe minerals like alumina and tItanIa. Pdymel'S cX ttjs type nmy be forced 10 adSa'bon alumina mediated by the agency of an ankJnic surfa~ Ii<e SOS. Such soIoos ~ Influence thefloccUation behavior cX the substrate substantially. albeit an exact understalxling cX the confOI'mation cXthe poiymer-surfaaant composite at the Interface Is unknc:M'n. PyPEO has been put to SefVk:e tounderstand this system. In~ studies in bldk sdutlons cX PyPEO and SDS sh<Med that the polymerstretches out ~ SOS m~les. When the polymer was force-adsorbed onto Al20~SDS sdlold above thehemimlcelar concenIratk>n, . was Inferred to be In the stretched state as rev8etlby the dwindli1gexcmer emission (19). No polymer was observed In the aqueous ~k cX these ad~kx1 sampessuggesting the indusion of the polymer In the SDS sdloid. This is deemed to be an Importa.- finding Incomection wkh the adsorptkJn cX polymers at sdkj/liqukl interface.

The above methOOs prOYkie a means to i1Vestigate structural features ~ adsorbed layers on andecular level uooer in-situ corxf~k)ns for the &st time. Correlation ~ such feaues ~ interfacialproperties such as fttXation aOO IIocctjation determk18d for the same system ~es one to formtjateschemes to arrive at optI~ performance ~ cdlokial systems in vaOOus processes. It Is dear that thereIs potential for developmert ~ add~ tectwliques to gain information on a mdectJar scae ~ static as"I as dynamic behavior ~ surface active species and rums that have major ~k:atIons In many novelaoo traditional technQogy.

References

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2. 'nterfaciaJ Ct1emistry of ParticUate AcXatk)n," Advances In l~erfacIaI Phenomena cA Part~e fsolution! gls Imems." ed. P. SomaslJlxlaran, and R. B. Grieves (AiChE, symposium series, 1975),71,1-15.

3. P. SomasuOOaran and D. W. Fuerstenau, "Mechanism of Alkyl Slifonate Adsorption at d19 Alumina.Water Intefface.. Jourra of Phvsk:aI Che!I!m. 70 (1~) 00.

4. J. H. Harwell, R. Scf1ecfter and W. H. Wade, "Recent Developments In the Theory of SurfactantAdsorption 00 Oxkjes.. SdkJ..LJaukJ Interactions WI pQr(XJS Media. Techn~, Paris, J]t). 371-410.

5. P .Chardar. P .Somasundaran and N. J. Turro, "R~ Probe St1Xfies 00 d19 Structure of d19Adsorbed Layer of DocIecyI Sulfate at the Alumina-Water Intefface.. J~ of CofIokj and I~erfaceScience." 117 (1~7) 31.

6. E. A. Chandross and C. J. ~er, 'rwamaeaJar Excimer Formatk)n and Ruoresc«1ce QuenchingWI DIna~kanes." Jatma/ of AmerIcan ChemIcal Soc!glx. 92 (1970) 3586.

7. N. J. turro and K S. Arora. "Pyrena as a Ptdophysical Probe ffX Intel'rOOeclUr I~ of Water.S~u~e P~ymers In Dlute Sdutions," P~~. 27 (1~) 783.

8. C. V. Kumar, J. K Bartoo, I. R. G()tj(j, N. J. Turro, and J. V. HWen, "Energy Redistribution andLocaizatk)n WI d19 ~ed State of Ruthenium 01) P~ C<Xnplexes.. l~nIc ChemIst[X. 27(1~) 648.

9. J. R. Lak(M'\cz. "PrlncDes cl Fluorescence Soedroscoov." (New YOfk. Pt~) 1983.

10. J. K Thomas, "The Chemlstrv of Excitalm at I~.. (ACS monograph) 1984.

so

". P. Soma.maran. N. J. Turro aOO P. Chandar, "Auorescence Probing d M~ukily d Surfacta.-Layers at the Soio-Lquki I,.erface; CdloOs arxf I"~. 20 (1~) 145.

12. P. P. Infelta, "Auorescence Ouenchilg In Micellar SokJions arxf b Application to the D«~NtIon ~Aggregation Nunms. . Chemk:aI PhYSIcs Lett8fB 61 (1979) 88.

13. J. T. Kunjappu aOO P. ~ "Tris (2,2'-bipyrkiyt) R~um (II) ~e as a Probe dAdsorption Characteristic ~ S<Xtkm DodecyI ~e on Alumina, . to appear k1 CoIIokis aOO Surfaces.

14. P. Chandar, P. Somasundaran, K C. Wate~nn aOO N. J. Turro, "Variation k1 NWxkie Probe ChainFlexiblity WMhi1 S<Xtlum DOOecyt Sjjfate Hemlmicelles.. Journal ~ PhvsIcaI Chem~. 91 (1~7) 150.

15. P. Somasundaran, J. T. Kunjappu, Kumar, C. V., Turro, N. J. aOO Barton, J. K, "Excled StateResonance Raman Spectroscopy to Probe Alu~na-Sodlum DOOecyt Sjjfate Hem~es; to appear inLa!]Sl!!uir.

16. Y. S. ~ov arxf L M. SeIgeeva, .Adsorntion of poIvm8fB.. (John WIey aOO Sons) 1974.

17. P. Charxfar, P. Somasundaran, N. J. Turro arxf K C. Watermann. "Excimer RU«escenceDeteI'nMnation ~ SoIki-li<Jlid I,.erfaclal Pyre~ed Pdy(acrytk: ackI) C<XiormatJons; Langmuir. 3(1~7) 298.

18.P. Somasundaran arxf J. T. Kunjappu, 'n-Sku Investigation ~ Adsorbed Surfactants arxf Polymefs onSoIkis In Solution; (paper presented at the International Symposium on Adsorption, Kyoto, Japan. June,1968; to be ~Ished In CoIIoOs arxf Surfaces).

19. P. Somasundaran..A study ~ Interactions of Surfactants arxf Polymers In BUk arxf at the SoIo-LiqukiInterface. (Annual Report Su~ed to Department ~ Energy, May, 1988)

51