CoUoids and Surfaces, 20 (1986) Else..ier Science Publishers B. V., Amsterdam - Printed in The ~etherlands

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1 CoUoids and Surfaces, 20 (1986) Elseier Science Publishers B. V., Amsterdam - Printed in The ~etherlands 145 FLUORESCE.~CE PROBING OF MICROFLUIDITY OF SURFACTANT LAYERS AT THE SOLID-UQUID INTERFACE P. SOMASUN1)A1{AN*. N.J. lurru"" A:."V I'.Cll, ",Dr'u, School of Engineering and Applied Science* and Depanment of Chemistry Columbia University. New York. NY Received May 27th 1986; accepted July 16th 1986 ABSTRACT Excimer formation of dinaphthylpropane (DNP) and pyrene fluorescence probes is used to monitor the microfluidity of dodecylsulfate hemimicelles formed on alumina particles. The efficiency of excimer formation of both probes indicate that the hemimicelle environment is considerably less fluid than that of SDS micelles suggesting a higher structural organization inside the bemimicellar aggregate. I NTRODUCTI ON Surfactant adsorption on particles in suspensions has been studied in the past by determining adsorption. zeta potential. panicle wenability. etc. These studies have been helpful in formulating mechanisms that govern the formation of surfactant layers on solids. In panicular. a major driving force for adsorption of long-chain surfactants arises from the selfaggregation of the surfactant molecules on the solid surface resulting in the formation of localized surfactant aggregates called hemimicelles [I). Hemimicellization occurs at concentrations well below the critical micelle concentration (CMC) of the surfactant owing to favorable electrostatic surface-surfactan[ interactions. Even though the mechanism of hemimicellization has been well discussed in the past. hemimicelle structural characteristics have not. however. yielded to investigations using the conventional techniques. Spectroscopic techniques capable of insitu probing of the adsorbed layer at a molecular level should. on the other hand. help to study these structures directly. Structural differences. if any. between hemimicelles and micelles could also be elucidated. The hemi~icelle is often envisioned as existing on the surface as a two-dimensional. closely-packed aggregate [1). If this structural representation is valid. significant differences in the microfluidity of tbe hemimicellar environment compared to that of the micellar environment can be expected. The micronuidity of micelles can be measured by fluorescence probing methods such as fluorescence polarization and excimer formation [2.3]. In this communication. we demonstrate that the excimer formation technique can be extended to obtain microfluidities of hemimicelles formed by dodecylsulfate (SDS) on alumina. We have chosen as the excimer forming probes. dinaphthylpropane and pyrene. which owing to their hydrophobic nature. are solubilized exclusively in the SDS micelles and /86/$03.50 <g) 1986 Elsevier Science Publishers B. V.

2 1.46, hemimicelles,lexcimer formation is known to occur in'~nno/~cularly in the case of pyrene or. ;n'ramo/~cularly in the case of the bichromophoric probe. dinaphthylpropane (DNP) [4]. RESULTS AND DISCUSSION Intramolecular Excimer Formation of Dlnaphth~"propane (DNP) The e~cimer formation of DNP. depends upon the motion of the arene groups in order to achieve the e~cimer conformation as illustrated below [3]: MONOMER EXCIMER c6~~)~ro Figure 1: Schematic representation of intramolecular excimer formation of dinaphthylpropane (DNP) Excimer formation at low probe concentrations. can be considered to be exclusively unimolecular (intramolecular) so that only the medium viscosity determines the extent of excimer formation [4]. Thus the monomer to excimer intensity ratios, Imlle obtained from the steadystate emission spectrum of DNP can be used to estimate the effective fluidity of the microenvironment of the probe using values obtained in solvents of known viscosity [7] The emission spectra obtained for the DNP solubilized in SDS micelles and SDS-alumina hemimicelles were identical except for the relative intensities of the monomer (A -340nm) max and excimer (A mix =420nm). The 1m/Ie values obtained and the corresponding viscosities for the various systems studied are summarized in Table 1. I A known ~olume?f 10-IM I soclium doc}4«yisulf~'f (SD~~ Auka OIemiclts) solutio~ contajning IO-4M or 4~IO-5M DNP (synthesized IS In reference 4) or 5~IO. 8~IO. 4~IO M pyrene (Aldrich OIemlcals. r«rystauized from ethanolwater) was added 10 I finll volum1 of 15Om M NaCl solution containing S g of alumina (Linde A grade. Union Clrtlide Corp OJ micron5. 1SIn Ig 5urface Irel) al rh 65 After mi~ing ttle suspension for 12 hours. the su(ie[natant "IS analyzed (or ttle residual SDS concentration IS) The steady-slate emission spectrum of DNP in the slllrry was measured in I 2ml ceo with an excitation wavelength of 280nm The emission spectrum (JIOnm to SOOnm) of DNP in the slurry was otltained and corr«ted for ~llitred incident light by scanning I solid slurry without any DNP. Sleady-state and t1uorescence d«ay (single photon countingl method~ 161 were used to monitor the fluorescence of pyrene. Comparative studies were performed Mth halh D~P and pyrene in 0 1M SDS (0 1M NaCl) solutions In addition. the cenlriftlgrd soluticlns from the adsorrtion ItStS were also examined; in an cases repoft~d here. no probe was d~tected in solution

3 147 TABLE 1 Monomer to excimer ratio of DNP and corresponding viscosities in various systems T~' 1) SDS MICrLE: CO.1M NaCl) {SDS].M CKC. M - -2 " 1-j" r 1,,-3 ~ 8OS/PY' - 7~;) 1./Ie 1., II,cP 8 ;. Dote: DO DNP 1D 8UperDataat) Test' 3) SDS-SILICA <a.1m NaC1. ph 8) AdsorptloD 2 Equl11brlU8 DeDS1~y. eo1/= SDS codc.,m SDS/PY" lalle 0 8.2x (Dote: 1./le 1D.uperDa~aD~ - 1.7) 1.*. ratio of a1cel11zed SDS to added DNP ratio of Adsorbed SDS to added DNP V1sco.1t1es based OD Imile in etbadol-ll,ce~ol ~1x~ures It is evident that the effective viscosity reponed by the probe in the SDS-alumina hemimicelles (90 cp) is significantly greater than that in SDS micelles (8cP). It is to be noted from the results obtained in test 2 that varying the level of DNP had no effect on the 1m/Ie value thus supporting the assumption that excimer formation occurs unimolecularly (intramolecular). Also. SDS-DNP micellar solution added to non-adsorbing silica particles (test 3) gave almost the same 1m/Ie value as that of the supernatant as well as that of the SDS micellar solution (test I) suggesting that the presence of solid particles does not interfere with the measurement; DNP is apparently present exclusively in micelles. Based on the above data. DNP response in,the alumina slurry can be considered to reflect its mobility in the adsorbed layer.2 We thus interpret the higher viscosity (or lower probe mobility) in SDS-alumina system as being due to a higher structural order (rigidity) of the surfactant molecules in hemimicelles than in micelles, Intennolecular Exdmer Fonnation of P}Tene The intemlolecular excimer formation of pyrene occurs intramicellarly owing to the slow exit of pyrene from micelles compared to its fluorescence lifetime [3]. In micellar solutions. in ~here remains the question as 10 whether DNP interacts with the alumina surface direc1ly even though the hydrophobic J1robe is not expected to interact with the hydrophilic alumina surface. The ex\temely low solubilities of DNP in water made it not JX)ssible to determine the ext~nt of adsorption of DNP on to alumina in the absence of 5DS so a control test was conducted in fouowing mann~r 8S suggested tly th~ reviewer: A con~tant reservoir of DNP crystal~ W8~ maintained on the inside of a filter p8per which in turn was imlnersed in an alumina-water suspension. Mer 4 day~ of stirring. weak nuorescence due 10 DNP wa~ observed in the supernatant of thi~ suspension showing th., some DNP had indeed dissolved. How~ver. no delectable nllorescence was observed when the solid slurry was excited suggesting that DNP does not have a high 8frInity for the alumina sulface.

4 148 addition to local fluidity. the. nature of the distribution of the probe among micelles affects the efficiency and extent of excimer formation: excimer formation occurs in only those micelles with probe occupancies greater than one (B). This distribution complicates analysis of the extent of excimer formation so that the monomer to excimer ratios cannot be readily related to local fluidity as in the case of intramolecular excimer formation (9]. The fluidity of the hemimicellar and micellar environments as reponed by pyrene. can however. be compared on the basis of the excimer formation rates obtained through fluorcsccnce decay r.lcasurements i i \ij. Kinetic mooels uased on Poisson slatistics tor the probe distribution among aggregates. have been developed to account for the decay profiles obtained in micelles and hemimicelles and the equation describing the time dependence of monomer fluorescence is given below (11.12]: \ I (t) - I (O)exp[-kot + n(exp(-k t)-i)] -I m m e where ko is the reciprocal fluorescence lifetime of the excited monomer ke is the 1 st order excimer formation rate constant, and ii is the Poisson average occupation number. Figures 2a and 3a show the emission spectra for varying levels of pyrene in SDS micelles and SDS-alumina hemimicelles respectively. It is noted from the emission spectra, that values of the 13/11 fluorescence parameter, which has been used as a monitor of environment polarity (10,13], are the same for both systems; pyrene thus reports a micelle-like polarity in the hemimicelle which is consistent with the formation of hydrocarbon chain aggregates at the surface. The corresponding monomer (383nm) decay profiles obtained in micelles and hemimicelles are given in Figures 2b and 3b respectively. In the absence of excimer formation the profiles (Curves A) are single exponentials as expected. The decay profiles under excimeric conditions (curve B in Figure 2b and curves Band C in Figure 3b) which are multiexponential due to the formation of excimer in some of the aggregates (micelles or hemimicelles with occupancy> 1) were fined to equation 1.3 These results are summarized in Table 2. TABLE 2 Summary of Kinetic Analysis of Pyrene Excimer Formation Test' l)sds MICELLE CO,lM lfacl) -!- 1.1 D &)4.5 a b)., 8.0xl xl0' 2.lxl0 6 0.$7 1.$5. ratio of 8icellized SDS to added pyreae ratio of adsorbed SDS to added pyreae J A non-linear regre5~ion.naly~i5 involving minimilation of weightetl ~um of ~uare5 was used to obtained from curve~ A wa~ u~ a~ a known parameter 10 fit C\1rv~ 8 an<l/nr C Filling was begun froln SO ns without del"l'nvt\i\1lio" A Nckground nt\ise «I ~ of ma(imum intensity) Clt-tained froln the last 2S channels was subtracted from the actual dala.

5 149 )- ~ Z &oj Z ~ 0.J S0 SOO WAV(L(NGTH. ~ ~ Figure 2 (a) pyrene emission spectra in SDS micelles (0~1 W NaCl); ratio of micellized SDS to added pyrene, 2160 (A). 108 (8); excitation at 332nm. (b) ~onomer decay profiles under conditions corresponding to 2&; excitation at 320nm. emission at 383nm.. w! )- t: CI\ Z &6J. Ẕ ~ 0 J TIME. nslc Figure 3 (a)pyrene emission spectra in SDS-alumina hemimicellar srs~em; ratio of adsorbed SDS ~O added PYTene (A). 336 (B) 139 (C); exci~a~lon at 332nm. (b)yonomer decay profiles under conditions corresponding to 3a; excitation at 320nm, emission at 383nm.

6 150, CONCLUDING REMARKS ACKNOWLEDGEMENTS REFERENCES t. P. Somasundaran and D. W. Fuerstenau. J. Phys. Chern. Vol , pp N.J. Turro, M. Gratzel and A.M. Braun. Allgew. Chern. lilt. Ed. EIIgl., Vol t9, pp , L.A Singer. Solution Behavior of Surfactants. K.L. Minai and E.J. Fendler, eds Vol I. Plenum ~s~-:-~~~pp 73~'12~ 4. N.J. Turro and Tsuneo Okubo. J. Am. ChL"rn. Soc p V. W. Reid. G.F. Longman and E. Heinherth. Tenside. 5. (3-4) pp J.R. Lakowicz. Principles of Fluorescence Soect~. Plenum Press. New York P. Avouris. J. Kordas. M.A. EI-Bayoumi. Chern. Phys. Lett p P. Infelta. Ch~m. Phys. Lett., p R. C. Dorrance and T. F. Hunter. J. Chern Soc Faraday Trans. I, J. K. Thomas Chern. Rev p283. II. S. Atik. M. Nam and L. Singer. Chern. Phys. Letl pp P. Levitz. H. van Damme and D. Keravis. J. Phys. Chern Vol. 88 No. II. f985. pp K.P. Ananthapadmanabhan. E.D. Goddard. P. L. Kuo and N.J. Turro. Langtnllir. I. 1985, p P. Chandar. P. Somasundaran and N.J. Turro. submined for publication to J. CoIl. IlIter}: 5<'i