A Convenient Stopped-Flow Experiment Demonstrating the Influence of Micelles on Reaction Kinetics Vincent C. Reinsborough' Mount Allison University, Sackville, N.B., Canada Brian H. Robinson University of Kent, Canterbury. U.K. Fast reaction techniques are discussed now in most standard texthooks in physical chemistry, hut demonstrations in these techniques suitable for the undergraduate laboratory are few. We describe in this article a kinetic system which we have found to be ideal for study by the stoppedflow technique. Good results can he ohtained easily with a relatively simple experimental arrangement using optical detection. Recently, there has been an increased interest in the influence of micelles and other tvoes .. of colloidal svstems on reaction kinetics particularly as aids to synthesis and reaction control. The reaction we describe -aives dramatic rate enhancements in micellar solution and is amenable to a particularly simple quantitative treatment. The results clearly demonstrate that the micelles promote this chemical reactivity hy increasing the local reactant concentrations. An appropriate kinetic analysis for surface reactions in micellar solution is developed and shown to offer a reasonable explanation. The powerful link hetween kinetics and mechanism which is often only poorly demonstrated thus comes across strongly to the student. Furthermore, the system described in this article is capahle of further development and some suggested avenues to explore are offered at the end. Nature of Micelles
Molecules or ions which possess both water-insoluble (hyd r o ~ h o h i cand ) water-soluhle (hydrophilic) groups are surfaceactive and are known variwdy as surfactants, soaps, detergents, and tensides. In aqueous solution above a certain concentration, known as the critical micelle concentration or cmr, these species can aggregate to form micelles. Such a micelle is composed typically of about 100 monomer surfactant units and is rouehlv. soherical in shaoe. A schematic di. agram of the micelle'fnrmed hy the surfaitant sodium dodecylsulfate (SDS) is shown in Figure 1A. Recent reviews on the structure and properties of micelles have been puhlished by Fisher and Oakenfull (1). Murkeriee (2), and Menger ( 3 ) .It has been shown recently that aqueous micelles have a "dynamic" structure such that monomers are exdhanging extremely rnpidl? a,ith micrllrz in the mia.rosrc,md time ranyr and n>iwllvs;arc d~wntecraring and r r l m n ~ n y In ) the rnillisecond to second time [ange 14). The extent of penetration of water into the micellar core ( 5 ) and the location of soluhilized molecules within the micelles (6) are still controversial topics. Micelles can form also in non-aqueous solvents of low permittivity such as benzene and n-heptane. The resulting aggregate structures are known as reversed micelles (Fig. 1B). When small amounts of water are added to the system, this is soluhilized within the charged core, and water-in-oil microemulsions are formed. Dramatic "catalysis" effects are often observed in both aqueous and non-aqueous micellar solutions (7). This is of interest hecause it has been suggested that these micellar
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Figure 1. a, Micells formed in aqueous solution from an anionic surfactant 8.g. ~Ddiumdecyisulfate.b, Reversed micelle famed from a doubly-chained anionic surfactant e g . Aerosol OT.
Figure 2. Complex formed between Ni,.,fi
and PAOA.
systems can mimic reaction processes in enzymes and membranes. Berezin ( 8 ) and Romsted (9)have descrihed a general theory of reactivity based on simultaneous reaction in the aqueous and micellar "phases" assuming fast rommunicarim oireartants between the tu~,~"ohaae*." In this article we develop a more specific treatment which is applicable to reactions occurring.exclusively in the charged micelle surface region. In principle a range of fundamental processes might he selected for study and two such reactions have been reported (10)(11). The reaction studied previously inTHlS JOURNAL here is that between Ni(.q,2+ and the azo dye ligand pyridine-2-azo-p-dimethylaniline or PADA.(Fig. 2). Most divalent metal aquo-ions react too rapidly to he studied by stoppedflow methods, but Nic,,f2+ reacts more slowly. The progress of the reaction is convenientlv monitored soectroohotometrically using PADA as the ligand. Furthermore, tile mechanism of this reaction in aaueous solution is well understood (12)(13)(14). In water, the first step in the mechanism is the diffusioncontrolled formation of an outer-sphere complex I in which the first hydration shell of the ion is intact. Since the overall rate of complex formation does not depend on the nature of the ligand when the ligands have the same charge, the rate-determining step is assumed to he the loss of a first water molecule adjacent to the Ni2+ ion to form the complex 11. The rate constant for this process (k.,) can also he determined independently from NMR measurements. The kinetic scheme is therefore
Since I is present only a t very low concentrations this scheme can be simplified to NiZ+ + L
A NiLZt (11)
Under pseudo-first-order conditions ([Ni2+] >> [L]), -d[L]/dt = k,b,[L] and, we have k,~,,(s-') = k l [ N i Z + ]+~ kLl (1) in which [ N i 2 + ](mol ~ dm-" is the total (initial) concentration of Ni(,,I2+ in solution and kl (dm" mol-' sSL)= KOshex. When micelles which have a negatively charged surface (as for SDSI are nresent. the reactants Ni2+ and L will no loneer he distributed homogeneously throughout the solution prior to reaction since both species will interact strongly with the micelles. Ni(J+ is attracted so strongly to the micelle surface region that for surfactant concentrations greater than twice the crnc over 90% of the Ni2+ ions are located within 1 nm of the micelle surface (15). Similarlv from solr~bilitvmeasurements in aqueous and micellar solutions, it can be shown that PADA is also partitioned strongly into the micelles probably in a more hydrophobic environment than the Ni2+ hut still close to the micelle-water interface (16). (Even molecules like benzene are thought to be solubilized close to the micelle surface.) The Ni2+-PADAcomplexation reaction in SDS solutions will, therefore, occur exclusively in the micelle surface region and should then be visualized as a surface phenomenon with surface concentration units being used. The surface concentration of the excess reactant, [Ni'"]s, in moles per m2 of micelle surface is calculated easily since all the NilC is bound for [SDS] > 2 crnc and the ionic part (head-group) of each micellar surfactant molecule is in contact with water, so that where C(mol dm-" is the total surfactant concentration, A is the area per head group of the surfactant molecule, and NAV is Avoeadro's Number. (It should be noted that the crnc of SDS iidependent on the added Ni,.,," concentration; for examole. when INir..Pl = mo1 dm-3. the crnc of SDS l in pure water to 4 x lo-" is rediceh from k x ' - f ~ - ~ r n odm-" mol dm-"16). It is assumed also that the monomer surfactant roncentrati'm in a mirnllnr sdltt~tmis p v e n hy I hewnc valur. While this is not strirtlv true, i t i*asatirfart,rs. nppn,ximntim .. in eqns. (2) and (3).) Then by analogy with eqn. (2), we obtain for the surface reaction for C > 2 cmc:
where kl'(m2 mol-I S-I) is a second-order surface rate constant and h-l'(S-l) is the first-order rate constant for the micellar surface dissociation of NiL2+ com~lex. The form of eqn. (3) suggests the behaviur shown below (Fie. 31 when the micelle concentration is varied a t fixed Nil+ L concentrations. It should be noted that the rate of complex formation decreases as C increases and, that in the region crnc < C < 2 cmc, the analysis is more complex since some Nit,,12+ and PADA will be present in the aqueous phase 117) \ - . ,.
We predict from eqn. (3) that a linear plot of h,,b, against [Ni'+IT/(C - cmc) should he ohtained. A reasonable estimate for A is 60 X 10-2"m2 for SDS micelles (18) from which k i c a n he readily evaluated.
now availahle commercially for teaching purposes. T h e constructiun of such an instrument has been described previously in THIS J O I I R NAI. (191. and a stopped~flowinstrument with conductivity detection has been described also ( 2 0 ) . A particular advantage of the Ni2+/ PADA reaction is the prunounred color change on complexation (AMAx for PADA is 470 nm: AMAX for the complex is 540 nml so that it is not necessary t o use a high resolution grating monochromator: a cheap intprference or color filter is perfectly adequate. A low power pnljeetor lamp is ideal as the light source and stabilized voltage supplies for these lamps are available cheaply. (We use WOTAN RV, 50W pre-focus hulhs). Hydrated nickel nitrate, PADA, and SDS are all rendilv availahle commerciallv and reasonahlv orieed. Hence the
tails. Values of k l and kl' can he obtained in an afternoon's work, or it may he desired t o extend the study in the form of a longer term project. T o familiarize the students with the operation of the stopped-flow instrument, we recommend, if time permits, that the kinetics of the Ni"t/PADA reaction he examined first in the ahsence of SDS. Suitable concentrations would he t o mix Nil,,12+ solutions over the range 2 X I O P t o 5 X 10F3moldm-Jwith PADA solutions a t 1 X 10-"01 dm-:'. (Note that the final concentrationsare half the pre-mixed ones.) The pH of the sdutians hefhre mixing should be adjusted to pH 7 to avoid protonation of the ligand (pK. of PADAHt 4.5) and hvdrolvsis of Nii..iz+ laK. 9.5) which un-
-
-
-
about 0.1 s-' (24). Reactions in the micellar solutions can be studied by variation of the NiZ+concentratian as above o r by variation of the SDS concentration keeping in either instance the PADA concentration constant. T h e pH should he adjusted to 8.2 since the pK.'s of PADAHt and Nii.,{'+ areshifted to higher values on hinding 10 the micelle (241. The SDS concentration after miring should he kept within the 1 0 F - 10-' mol dm-," range. A value of / , , J ~ ' d l m dI n # ~ ~ r / o ,Sc8 c ~ c, 17, 47:! 119741. ,I:BI MII.PIII.H I . I . H L M . E I ~ ~ I ~ . . s~ ~ .m i ~ s m , . 12111 P11t.1. R. (.. Atkmxm. i;.. and HIIF.K . I . .I. l'HPM. KnI1T..II,R*i 119701. i 2 l l Srh~lly,Z A and liyrm&E. M . . l I'HI'Y. EI~I'C..4X,E!II II'Jill. i??! ?ahlin. E. F.. "Fast K e a r l l m r # nR i l u t ~ ~ ~ n Hlaukurll*. ; OxPlrd. 1104. im tiurtln. K.." ~ ~ f i nh~ d~ ~~ ~ mvill~ XVI. ~ l ~~a s~R t o ga aym .a . " cade emir PIWS, Y w k . 1989. 1241 d i m e s . A . I>.and Hcii~innmR. H.. J i'hpni .Sni Forodn, I. 74, 10 lIQ7RI
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Literature Cited
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Journal of Chemical Education
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