Basu, Sadhan

Gupta, Debasish

English

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Indian Journal of ChemistryVol. 14A. August 1976. pp. 549-552Excimer Probe in the Study of Pyrene Solubilization by Fatty Acid(Capric, Lauric & Palmitic Acids) SoapsDEBASISH GUPTA & SADHAN BASUDepartment of Pure Chemistry. University College of Science, Calcutta 700009Received 2 February 1976Using excimer emission from pyrene solubilized by capriate, laurate and palmitate as aprobe, it has been established, (i) that micellar size increases as the length of the fatty aeidc hainis increased, and (ii) as the soap concentration is increased. In the presence of low concentrationof neutral. salts micellar size at first diminishes, and then increases. The quenching effect ofneutral salt has been attributed to the adsorption of the neutral salts on the micelles and sub-sequent polarization of the micelles. It has further been established that the pyrene moleculefrom one micelle can pass on to another micelle by diffusion.SINCE the discovery of excimer emission frombimeric pyrene by Forster and Kasper", thisphenomenon has been used as a probe todetect the hydrophobic regions in proteins andrnembranes-, as well as to determine the viscosity"of the hydrocarbon region of the micelles. Thephotochemistry of excirner emission from pyrenesolubilized by detergents has also been reported+".We have been interes.ed in the s.udy of the solubi-lization phenomenon itself and we have used excimeremission as a probe to study the change of micellarstructure in soap SOIUjOll under different conditions.We report here the results of our inves.iga.ionson the solubilization of pyrene by fatty acid soaps.Materials and MethodsThe fatty acids, capric (ClO). lauric (C12) andpalmitic (C14) acids were of 99·00% purity and foundfree from unsaturated fatty acids. The corres-ponding sodium and potassium salts were obtainedby neutralization with aqueous alkali. The pyrene(Eastman Kodak) was purified by chromatographyover silica gel. The inorganic salts used were ofGR quality (E. Merck).Preparation - Finely powdered pyrene was sus-pended in appropriate soap solution and was shakenin mechanical shaker for 12 hr. The solution wascentrifuged and then filtered through fine filter paper.The optical clarity of the solution was checked byturbidity measurement.Fluorescence and absorption measurement - Thefluorescence spectra were measured in a right anglescattering set-up, using a Beckman monochromatorand a IP 28 photomultiplier tube used in conjunc-tion with a de amplifier of 100 mV sensitivity.The exciting light was 365 nm mercury lineisolated from a high pressure mercury arc using achance filter showing cut off at 370 and 350 nm.The concentration of pyrene was determined byabsorption measurement on a Beckman spectro-photometer model DU at 360 nm. The concentra-tion determination from the extinction measure-ments at 334 urn gave the same result.Results and DiscussionIt is well known that hydrocarbons which areinsoluble in water may be solubilized by solutionsof fatty acid soap like potassium laurat es (KCn).Extensive X-ray investigations indicate that thesolubilizate is incorporated into the hyr'rocarboncentre of the soap micelle. In a 0·5% solution ofKC12 there are 0·68 molecules of pyrene per micelleon an average. It has been assumed that one pyrenemolecule can enter a micelle at most. Fig. 1 showsthe emission spectrum of 2xl0-4M pyrene in 1·0%KC12. It is evident that even at this low overallconcentration of pyrene both monomer band at1500~4~0~0--------~50~O---A.nm50.,.--........o·~~ __ ::==============~ _400 500A,nmFig. 1 - Emission spectra of (1) 1·6 x 10-6M pyrene in5% aq. Kvcapri vte (KC10); (2) 2·0 x 1O-4M pyrene in 1%aq. K-la.ur,~te (KCu); and (3) 2·3 x 10-4M pyrene in 0'5%aq. Kvpalmitate (KC14)549INDIAN J. CHEM., VOL. 14A, AUGUST 1976390 nm and excimer band at 470 nm are quiteprominent. This can only happen if there aremicelles of different sizes, the smaller one accom-modating one pyrene molecule and the larger oneaccommodating more than one. In Fig. 1 is alsogiven the emission spectrum of pyrene in 5% RClD•It may be observed that even at this high soap con-centration no excimer emission is observed. Onthe other hand, in a 0'5% RCl4 (palmitate) theexcimer emission is larger than monomer emission.This means that the micellar size is determined bythe length of the fatty acid chain, the longer thechain length, the bigger the micellar size. It wasfurther observed that in 1% NaCu at a pyrere con-centration of about 3 X 10-2M no excimer emissior-is observed. This indicates that the potassium soapgives larger micelles than sodium soaps. Thesefindings are consistent with the results obtained byprevious workers from solubility studies",It is desirable at this stage to discuss themechanism of excimer emission from micellar system.Since the micellar collision is 103 mole/see only.the excimer emission cannot arise due to collisionof excited and unexcited pyrene molecule indifferent micelles. In a 10-4M pyrene in 1% soapsolution there can exist one pyrene molecule permicelle of 200 soap molecules. Even if we assumethat there are micelles containing more than 500soap molecules then even, the number of pyreneper micelle cannot exceed three. In such a systemthe collision of a pyrene molecule with the micellarwall is much more frequent than its collision withanother pyrene molecule, so that the intensity ofexcirner emission will be insignificant compared tothat of monomer emission. Experimentally we findthat excimer emission is comparable to that ofmonomer emission and in some cases it is evenTABLE 1 - EXCIMER EMISSIONS AT DIFFERENT SOAPCONCENTRA nONS[K-laurate] [Pyrone] 1&70/1390% lO-'xM1·0 1·32 0·4212·0 5·30 0·4385·0 5·54 0·470higher than monomer emission. The collisionmodel, therefore, does not appear to holel a good forthese systems. A better model will be to assumethat a non-bond dimeric pair exists in the micelle,the electronic excitation of one of the molecule inthe pair takes the system directly to the excimerstate. Such a model has been assumed for excimeremission from perylene crystals",As the concentration of RCl2 is increased, moreand more pyrene dissolves and the intensity of boththe monomer and excimer bands increase. Butthe ratio 1470/1390 increases as the soap concentrationis increased (Table 1). This may be explained ifwe assume that addition of more soap to a soapsolution results in an increase in not only micellarnumber but also micellar size.It is also well known that the addition of neutralinorganic salts to a soap solution to a certain extentincreases the solubility of hydrocarbons". Thelight scattering" data indicate that the micellar sizeis increased on the audition of neutral salt like KCIto a soap solution. In the Fig. 2 are shown theemission spectra of 1·63 X lO-"M pyrene in 1% RCawith KCI concentration in the range of 0·01 to O·OSM.It may be observed that as the salt concentrationis increased the intensity of excimer band diminishes20~------~4~onO---------------------4-5~O5001, ,n rnFig. 2 - Emission spectra of (1) 1·63 x 10-4M pyrene in 1% aq. K-laurate and (2)-(4) in the 'presence of 0·01, 0·04 and0'08M KCl solutions s: :; . , . ..;550GUPTA & BASU: EXCIMER PROBE IN PYRENE SOLUBILIZATION BY FATTY ACID SOAPSbut that of monomer band increases. That is,micelles containing two pyrene molecules are break-ing up into smaller ones containing one pyrene mole-cule in the presence of neutral salts. This findingcannot be attributed to the quenching effect ofneutral salt on excimer emission because themonomer and excimer emissions are equally quenchedby quenchers. In this range of KCI concentrationthe solubility of pyrene in soap solution is not muchaffec+ed. When, however, the salt concentrationis in the range of 0·1 to O·SM, the solubility ofpyrene in KC12 is increased and the monomer andexcimer emissions are strongly quenched (Table 2).The intensity measurements at constant [soap] and[pyrene], but varying [KCl] in the range of 0·1 toO·SM showed (Fig. 3) that the quenching effect onmonomer emission follows the Stern-Volmer law,1110 = 1/(1 +Q[A]) , where 10 and 1 are fluorescenceintensity without and with Kel concentration [A],and Q is the so called quenching constant. Con-sidering the quenching effect of neutral salts in othersystems, it may be assumed that the quenching isdue to electron transfer from Cl"to the excited pyrenemolecule. Similar quenching effects have been ob-served by Gratzel and Thomas" on the emissionTABLE 2 - EXCIMEREMISSIONAT DIFFERENTKCICONCENTRATIONSIN 0·1% Ag. KC'2 MEDIUM( [Pyrene]-1 0-4M)[KCl] I39o{fl 1470M0·00 70·5 22·00·10 64·5 17·50'20 62·0 16·50·30 57·0 14·5spectrum of pyrene solubilized by cetyl-trimethylammonium bromide in the presence of I- ion. Theyassume that the added electrolyte can move acrossthe micelle. It is expected, therefore, that if KCl,KBr and KI are used as quenchers and if the ionscan enter into the micelles, then the quenching con-stant Q should follow the order I- > Br- > ci-,With pyrene in KC12 it was observed that thequenching constant Q was constant irrespective ofwhether the neutral ion is chloride, bromide, iodideor sulphate. The quenching effect of cations couldnot be studied as sodium chloride of 0·1M causedhigh turbidity in the soap solution. We believe9"J o,·OlL.----c;-O.71-......-.,O"·-::-2--""O ...•.·3-Cone. I<CI (H)20400 450~ ,n mFig. 3 - Emission spectra of (1) 1O-'M pyrene in 1% aq.Kslaurate and (2)-(4) in the presence of 0·1, 0·2 and 0'3MKCI solutionsz2---400n---------------------4~5~O~--------------------L~500A,nmFig. 4 - Emission spectra of (1) 2·3 X lO-'M pyrene in 0,5% aq, K-palmitate and (2) 1.15 X lO-'M pyrene in0,5% aq. K-palmitate551INDIAN J. CHEM .• VOL. 14A. AUGUST 1976that the quenching effect is due to adsorption ofneutral ion on to the micellar surface causingpolarization of the micelle. As a matter of fact it hasbeen observed that both excimer and monomeremissions are quenched in polar mediumw,An interesting observation was made with pyrenesolubilized by KC14• It is expected that if a solutionof pyrene solubilized by KC14 is diluted with thesoap solution of the same concentration, then theintensity of both monomer and excimer bands willgo down. It was observed that on lowering pyreneconcentration in this way, the intensity of excimerband went down but that of monomer band increased(Fig. 4). This observation is in direct contradictionto the fin-lings of Forster and Selinger'! with solutionsof pyrene solubilized by detergent. This Can onlyhappen if the pyrene from one micelle can diffuseinto the hydrocarbon centre of another micelle. Forthis process to take place the pyrene molecule mustbe able to reside at all parts of the micelle-surface,palisade and hydrocarbon centre - and not justin the hydrocarbon centre. The micelles, therefore,cannot have a rigid structure, but must be swollenenough for the pyrene molecule to be mobile.Dorrance and Hunters, however, have found thatfor detergent micelles, the interior appears to bevery rigid at room temperature (20°). Our observa-tion also indicates that pyrene molecule can enter552into the hydrocarbon centre of a soap micelle andnot that the micelles are formed around a pyrenemolecule from monomeric soap molecules.We may conclude that excimer probe corroboratesa number of observations on the soap solution ob-tained by other methods. In addition it Gives anumber of new and interesting results.References1. FORSTEI<, T. & KASPER, K, Z. Elccltochem., 59 (1955),976.2. nilAND, L. & GOHLKE, J. R, Am. Rev. Biochem., 41(1972), 843; STTIVER, 1,., Science, 162 (1968), 526;YANDERKOOl, J. & MAJITTNOSE, A., Arch. Biochem.Biophys., 133 (1969), 153; CHANCE, n., Proc, natnl.Aced. ss: U.S., 67 (1970). 560.3. SHINITZKY, M., I>IANOUX, ,\. c., eITLER, C. S: \VELEH,G., Biochem., 10 (1971), 2106; ]'OWNALJ., 1 .. J. & SMITH,L. C., J. Am. chcm, soc.. 95 (1973), 3136.4. DOHRANCE, R. C. & .1.UNTEI<, T. F., J. chem. soc., Farad.Tmns., 68 (1972), 1312.5. GI<ATZEL, M. & THO~IAS, O. K, J. Am. ahem, Soc., 95(1973), 68856. KLEVENS, B., Chem. Rev., 47 (1950), 1.7. KOLTHOFF, T. M. & ~'TRICKS, Vi., J. phys. coil. cu«;53 (1949),424; McBAIN, J. W. & jounsox, K, J. Ant.chem. s«.. 66 (1944), 9.8. TANAKA, ].. Bull. chem . .';'}(.., Ja/Jan, 36 (1936), 1248.9. DEBYE, I'., Ann. N. Y. A':lId. sa., 51 (1949), 575.10. MATAGA, N., EZUMJ, K & OKADA, T., Molee. Phys., 10(1965), 201.11. FOI<STER, T. & SEI.INGEI<, B., Z. Naturf" A. 19(A)(1964), 39.

Excimer Probe in the Study of Pyrene Solubilization by Fatty Acid (Capric, Lauric & Palmitic Acids) Soaps

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Excimer Probe in the Study of Pyrene Solubilization by Fatty Acid (Capric, Lauric & Palmitic Acids) Soaps

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