J. Chem. SOC., Faraday Trans. I , 1986,82, 2011-2023 Micellar Inhibition of the Aquation of Tris-( 3,4,7,8-tetramethyl- 1 , 10-phenanthroline)iron(II) by Sodium Dodecyl Sulphate in Aqueous Acid Medium Jide Ige" and 0. Soriyan Department of Chemistry, University of Ife, IEe-rfe, Nigeria The inhibition of the aquation of Fe(Me,phen):+ by sodium dodecyl sulphate (SIX) in aqueous acid media has been investigated and a mechanism which explains the pronounced inhibition and pre-micellar activity at low [SDS], has been proposed. Inhibition is due to favourable thermodynamic- /hydrophobic/electrostatic binding between the Fell complex and SDS monomer aggregates. The bound FeII complex is stabilised with respect to dissociation and binding takes place between the ridges of the Stern layer.The partitioning of the substrate between the bulk-water phase and the micellar phase is in favour of the latter at low [H+], and low [SDS],. From the rate law obtained and the observed kinetic data, the micellesomplex binding constant (KJ and micelle-acid binding constant (K3) were calculated to be (2.8 1 f 0.08) x 1 O5 and (1 3.80 f 0.16) dm3 mol-l, respectively, in acid media. Using the Scatchard method, Kl values of (3.95 k0.08) x lo5 and (3.04 f 0.16) x lo5 dm3 mol-1 were calculated for the binding in neutral medium (distilled water) and 2 x 10-5 mol dm-3 H+, respectively. The decrease in Kl in acid media is attributed to competition between H+ and the complex ion for the binding sites on the micelle. The k,-[SDS], profiles are structured owing to the evolution (size, geometry, aggregation number, etc.) of the micelle.The inhibition of the aquation rate by HSOh and SO:- ions which form a negative field around the Fe" complex is only significant at high acid concentrations. The micelle-bound complex and micelle-bound protons have opposing effects on the aquation rate. The degree of inhibition is therefore sensitive to the ratio of the concentration of these bound species. The parallel between the catalytic behaviour of macromolecles and enzymes has led to a renewed interest of physical chemists and biochemists in chemical reactions which take place on charged surfaces in solution. A large volume of work has been done on micellar catalysis of organic reactions1*2 such as solvolyses, rearrangements and decarboxylations, but there has also been an increasing level of activity in the study of aqueous and reverse micelles on metal-ligand complex f~rmation,~ metal redox reactions4 and metal complex diss~ciation.~-~ One important advantage of some of the metal dissociation studies performed in this laboratory is the relative simplicity of the system investigated coupled with the fact that the reactions are sufficiently slow to be monitored by conventional techniques.These features allow for a fair qualitative insight into the extent of electrostatic and hydrophobic interactions, substrate partitioning and substrate solubilisation effects. These effects were investigated in the present study of the aquation of tris-(3,4,7,8-tetramethyl- 1,lO- phenanthroline) iron@) sulphate in aqueous SDS. A previous report from this laboratory8 showed the aquation of Fe(phen)2+ to be catalysed by dodecyl pyrazinium chloride (DPC) (a cationic micelle-forming surfactant) in aqueous perchloric acid.The k,-[DPC] profiles were structured and this was attributed to the evolution of the micelle. For the present study, the effect of the increased hydrophobic character of the ligand (Me,phen) and 201 1 67-22012 Micellar Inhibition of Aquation the change to an anionic surfactant (SDS) on the aquation rate is of interest. Whereas in the Fe(phen)i+-DPC system no mechanism was proposed, the present work looks at the mechanistic aspect of the aquation process in more detai1.T Experiment a1 Materials Fe(Me,phen),SO, (G. F. Smith chemical company) was used as supplied.It was characterised by its visible spectrum which gave E = 13700 dm3 mot1 cm-l, in excellent agreement with the literatureg value of 13800 dm3 mol-l (A, = 500 nm). Specially purified SDS (B.D.H.) was used as supplied and its purity was ascertained by determination of the critical micelle concentration (c.m.c.) in aqueous solution at 25 "C, which correlated with the literaturelo value of 8.1 x mol dm-3. All other reagents were AnalaR grade. Kinetics Kinetic measurements involved monitoring the change in absorbance of Fe(Me,phen)i+ at Amax = 500 nm as a function of time using a Pye-Unicam SP500 series 2 spectro- photometer fitted with an automatic cell changer and four thermostable cell compartments. The reaction components were mixed in 1 cm quartz cells in the sequence water, acid, surfactant and complex to ensure minimum pre-mixing aquation.The concentration of the complex was maintained at 1.459 x mol dm-3 (except where stated otherwise) to avoid complications that may arise from the dependence of the aquation rate on the concentration of the complex in surfactant solution. Such a dependence was established for the Fe(phen);+-DPC system reportedg earlier. Owing to its inertness, the complex does not aquate appreciably in the absence of acid, hence all runs were carried out in acidified solution. Sulphuric acid was used because Fe(Me,phen);+ is precipitated from the solution of other mineral acids. The acid concentration range was 0.005-1 .OO mol dmW3 H,SO,. The acid was treated as essentially monobasic, but corrections were made for the slight dissociation of HSO, within the conditions of the experiment.A dissociation constant of 1.20 x lop2 was used for HSO,. A constant temperature was maintained using the combination of cryocool CC-60T compressor, a Gallenkamp thermostatting unit and a fast Austen pump which supplied the cell compartments. All the stock solutions except that of the complex were placed in a thermostatted water bath. All runs were performed assuming pseudo-first-order kinetics. The aquation rate constant k , was obtained from the slope of plots of In (A, -A,) vs. time. The plots were always linear to more than three half-lives. The investigation of binding of the FeII complex with SDS in a neutral medium and in 1-00 x lod5 mol dm-3 H+ at 25 "C used an SP6-400 spectophotometer and analysis by the Scatchard method.ll Results Dependence of the Observed Rate Constant (k,) on Complex Concentration At 25 "C the observed rate constant (k,) increases with increase in [Fe(Me,phen)i+] at fixed surfactant and acid concentrations. [Complex], was in the range (0.3623-3.623) x mol dm-3 and [€I+], was fixed at 0.017 mol dm-3 in all the runs.The choice of I A more logical extension of the Fe(phen)i+-DPC system is an investigation of the behaviour of Fe(phen)g+-SDS. Initial data show that SDS inhibits the aquation of Fe(phen)g+. More data are necessary for the comparison of mechanisms. We hope to report our detailed findings in the future.J . Ige and 0. Soriyan 2013 1.6- ( b ) 1.5 - 1.4 - - 1.3- m p, 1 . 2 - .-( \ 3 1.1 1.0 0.9 - * - - 0 0.5 1 .o 1.5 2 .o 0 1.0 2.0 3.0 L.0 [complex J T/ 1 O-' mol d ~ n - ~ Fig.1. Variation of k, with [Fe(Me,phen)i+], at fixed SDS concentrations [1.0 x lov4 mol dmP3 (a); 2.0 x lod4 mol dmd3 (0); 3.0 x mol dm-3 (a)] in the presence of Hy0.017 mol dm-3) at 25 "C. surfactant concentrations was based on the structure in the k,-[SDS], profile. Fig. 1 is a plot of k, us. [complex], at (0.1, 0.2 and 3.0) x mol dm-3 SDS. The first two concentrations represent the region of the minimum in the k,-[SDS], profile (fig. 2). Fig. 1 is representative for any surfactant concentration and shows that k, varies linearly with [complex],. The slope of the k , vs. [complex], plots increases with increase in [SDS],. Fig. 1 shows that [complex], must be kept constant in any study of the dependence of k , on [SDS], to allow for meaningful interpretation of the experimental data.Effect of [SDSIT on k , at Constant w + ] T and [c~mplex]~ Overall data at constant [H+],, [complex], and varying [SDS], show that aquation of the complex is inhibited by the addition of SDS. Fig. 2 shows a plot of k , us. [SDS], at 25 "C with [H+], = 0.017 and 1.1 16 mol dm-3. There is a minimum in k , at 2.0 x lo-* mol dm-3 SDS. At [SDS], < 3.0 x lo-* mol dm-3, k, values at 1.1 16 mol dm-3 H+ are higher than the corresponding k , values at 0.017 mol dm-3 H+. At surfactant concentrations > 3.00 x rnol dm-3, k , values obtained at 1.116 rnol dm-3 [H'], are lower. These results indicate acid catalysis at fixed low surfactant concentrations and acid inhibition at fixed higher surfactant concentrations. At both acid concentrations the absolute initial inhibition of the aquation rate by the surfactant is markedly sharp prior to the minimum point.At each fixed acid concentration the k,-[SDS], profile shows saturation at [SDSIT > 1.75 x mol dm-3 (data at higher [SDS], are summarised in table 1). The observed inversion in the comparative trend of the relative magnitudes of k , at high and low surfactant concentrations for the two fixed H+ concentrations necessitates a study of the acid dependence of the aquation rate at fixed surfactant concentrations. The acid dependence was studied at two SDS concentrations chosen from the regions of acid catalysis and acid inhibition suggested by the k,-[SDS], profile.The results are stated below.2014 Micellar Inhibition of Aquation 1 v) I 0 I,,,,,, 0 1.0 2.0 3.0 4.0 5.0 6.0 [ SDS],/ 1 0-3 rnol dm-3 Fig. 2. Variation of k , with [SDS], at 25 "C. [H+IT = 0.017 mol dm-3 (A); [HfIT = 1.1 16 mol dmP3 (0); [Fe(Me,phen):+] = 1.459 x mol dm-3. Table 1. Observed aquation rate constants (k,) at 25 "Ca 0 8 .o 0.993 0.607 10.0 0.994 0.606 12.0 0.998 0.584 14.0 - 0.583 16.0 - 0.591 20.0 - 0.575 25.0 - 0.556 30.0 - 0.561 a [FE(Me,phen):+], = 1.459 x 1 0-5 rnol dmP3. [H'], = 0.017 mOl dm-3. [H+IT = 1.1 16 mol dm-3. Effect of Acid on k , at Constant [SDSIT and [c~mplex]~ The aquation rate was determined at 25 OC, [SDS], = 1.0 x rnol dm-3 and [H+], = 0.007-0.129 rnol dm-3. The results (fig. 3) show that the aquation is catalysed by acid at this surfactant concentration.We could not determine k , at acid concentrations < 0.007 mol d ~ n - ~ because of the establishment of a rapid equilibrium. The initial kinetic data also show mixed first- and second-order kinetics. The observed complexities here are left for extended future investigation of this work at [HS], < 0.007 mol dmP3. At acid concentration 2 0.007 mol dm-3, the reaction shows first-order kinetics for more than three half-lives. mol dmP3, k , decreases with increase in [HfIT, contrary to the observed catalysis at low surfactant concentration discussed above, but consistent When [SDS], = 3.0 x4.0 3.9 - I ," 3 . 8 - 2 *3 3 . 7 - I . 3.6 3.5 2015 #. " - n A - - " I I I I I I I 0 0.02 0.04 0.06 0.08 0.10 0.12 [ H+IT/mol dm-3 [Fe(Me,phen)l+], = 1.459 x Fig.4. Variation of k , with [H+], at 25 "C. [SDS], = 3.0 x mol dm-3, with the observed trend in k,-[SDS], profile (fig. 2). This is a significant result. Fig. 4 shows the plot of k , us. [€I+], at 3.0 x mol dm-3 SDS. It exhibits an exponential decrease in the value of k, as the acid concentration is increased (kv approaching saturation at [H+], > 0.057 mol dm-3). The exponential decay of k , is observed to be valid for high [SDSIT (1.50 x < [SDS], < 3 x c.m.c.)t Binding of Fe(Me,phen)g+ by SDS The binding constant Kl was determined both in neutral medium and in 1.0 x mol dm-3 H+ at 25 "C. For each set of runs the absorbances of solutions containing f The c.m.c. decreases from 8.3 x to 7.04 x mol dmP3 for [H+IT in the range (0.0-1.0) x loP3 rnol dm-3.2016 Micellar Inhibition of Aquation 0 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0 [ SDS],/ lo-' rnol dm-3 Fig.5. Typical plot of fractional saturation 6 = ( A -A,,)/(& -Ao) against [SDS],. 14.0 12.0 10.0 5 8.0 6.0 2.0 I I I I I I I I I I J 0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 l/[Fe(Me4Phen)s2'lf/ lo4 mol dm-3 Fig. 6. Reciprocal of average number of molecules of bound Fel* complex (1 /v) us. reciprocal of free [Fe(Me,phen)g+] in acid-free medium (0) and in [H+IT = 1.0 x mol dm-3 (A) at 25 "C. 1.459 x mol dm-3 Fe(Me,phen)g+ were measured at varying concentrations of SDS (< 2.00 x mol dm-3). Fig. 5 shows a typical plot of fraction of Fe(Me,phen):+ bound (6) us. [SDS],. 6 is given by the ratio (A-Ao)/(Aa-Ao).Using the Scatchard methodll [SDS]/ [Fe(Me,phen)f+]b,,,d, i.e. 1 /v was plotted against 1 /[Fe(Me,phen);+],,,, (fig. 6). Kl and the number of binding sites per molecule of SDS (n,) were calculated from the slope and intercept, respectively. K , values of (3.95k0.05) x lo5 and (3.04f0.16) x lo5 mol-1 dm-3 were obtained in neutral medium and 2.0 x mol-1 dm-3 H+, respect- ively; the corresponding values of n, were 0.25 and 0.20. Kl could not be determined at higher acid concentrations by this method owing to significant initial reaction which prevents accurate determination of the initial absorbance of the reaction mixture. Mechanism Some of the above results can be explained by the following mechanism. We suggest an equilibrium step involving the binding of the substrate [S2+ = Fe(Me,phen):+] with the micelle or monomer aggregates (M) of the surfactant, SDS K , (1) Mn- + S2+ MS2-nJ.Ige and 0. Soriyan 2017 where Kl is the binding equilibrium or association constant and n is the magnitude of the average charge on the micelle. The bound complex (MS2-") then reacts with H+ K2 present in solution MS2-" + H+ + MH1-n + S2+ where MH1-n is the protonated micelle. H+ competes with Fe(Me,phen)i+ for the binding sites on the micelle. Unbound micelle in solution can be protonated directly through the equilibrium pathway K3 Mn- + H+ e MHl-" where K3 is the acid-micelle binding constant. The dissociation of the substrate, S2+ to products occurs in two phases; in the bulk-water region where the complex molecules are unbound or free and in the micellar phase where the complex is bound to the SDS aggregates: (2) (3) kw S2+ -products MS2-" -products slow km slow where k, is the first-order dissociation constant of the substrate or complex in bulk water and k, is the first-order dissociation constant of the substrate or complex in the micellar phase.There is thermodynamic stabilisation of the complex with respect to dissociation via equilibrium 1. Hence k, would be less than k,, i.e. there will be inhibition of the aquation process due to the relative thermodynamic stability of the MS2-" micelle- complex ion, with respect to its dissociation. This is a reasonable explanation for the observed inhibitions in the present work. The micelle+omplex ion is either solubilised in the micellar core (as in the suggested SDS solubilisation of haemin)12 or the complex may be bound in the Stern layer of the micellar region.We define the following equilibrium concentrations : [S2+] = a [equilibrium concentration of Fe(Me,phen)I+] [MS2-"] = /3 (equilibrium concentration of bound complex) [MH1-n] = A (equilibrium concentration of micelle-bound proton) [H+] = y (equilibrium concentration of free hydrogen ions) [Mn-] = 8 (equilibrium concentration of free micelle). It follows from eqn (1)-(5) that [s2+]T = a+B K1K2 = K3 (1 1) where subscript T denotes total or initial concentrations. The rate of disappearance or dissociation of the complex is given by d -- + = kw[S2+] + k,[MS2-"] dt2018 Micellar Inhibition of Aquation and the observed rate constant k , is given by rate [S2+] [MS2-"] k , = - = k w T + k m p [s2+lT [s IT LS2+t]T i.e.k , = kw 4 r e e +km Ftmund (13) where is the fraction of free complex, lj',ound is the fraction of bound complex and but therefore From eqn (7) and (10) K2 B([H'lT - A) A = a Since A 6 [H+IT, or assuming K2P 4 a K2 PIHSIT A = a Substituting for a from eqn (6), eqn (19) yields From eqn (6) and (9) From eqn (8), (20) and (21) which can be rearranged to give At low surfactant concentrations the term p2 is negligible. Eqn (22) can then be solved for p to give From eqn (17) and (23) Eqn (24) can be rearranged to yieldJ . Ige and 0. Soriyan 2019 Table 2. Observed aquation rate constants (k,) at 25 OCa LSDS1T k; / lop3 mol dm-3 110-4 s-1 /10-4 s-1 0.00 1.733 (k,) 2.014 (kw) 0.02 1.114 - 0.04 0.835 0.896 0.06 0.748 0.687 0.08 0.639 0.544 0.10 0.385 0.423 a [Fe(Me,phen):+], = 1.459 x mol dm-3.mol dm-3. [H+IT = 0.017 mol dm-3. [H'], = 0.129 In 5 8.0 - n I I I I I I I I I I k - 0 2.0 4 .O 6.0 8 .O 10.0 [ M - " ] ~ / l o - ~ mOl dm-3 I I I I I I I Fig. 7. [Mn-IT (km-kw)/(kv-kw) against [Mn-IT at 25 "C. [H+] = 0.017 mol dm-3 (0) and 0.129 mol dm-3 (A). A plot Of [Mn-]~ (k, - k,)/(k, - k,) against [Mn-]~t at constant [S2+]~ and [H+]T should be linear with unit slope. To check the validity of this prediction, more data (table 2) were taken at surfactant concentrations < 1.0 x lop4 mol dmP3. Fig. 7 shows the required plot, at two acid concentrations (0.017 and 0.129 mol dmP3). The two plots are linear with slopes of 0.9 0.2 and 0.8 f 0.2, respectively, in excellent agreement with the prediction of eqn (25) within the limits of experimental error.From eqn (25), the intercept is given by 1 intercept = [s2+lT + K2[H+], +:. Kl [S2+], was fixed at 1.459 x mol dmP3 and [H+IT was constant for each set of runs. Substitution of the above acid concentrations and the extrapolated intercepts in fig. 7 into eqn (26) yields two simultaneous equations from which the binding constant Kl [(2.81+0.08) x lo5 dm3 mol-l], the equilibrium constant K2 [(4.91 k0.08) x dm3 mol-l] and hence K3 (= Kl K2) (13.80k0.16 dm3 mol-l) were obtained. It is important to note that the values of the above equilibrium constants are sensitive to variations in k,. E.g. a 10 % variation in k,, at 0.0 17 mol dm-3 H+, results in a 3.5-fold variation in K,, 6-fold variation in K, and 21-fold variation in K3.k , is the limiting value of k , at high surfactant concentrations. For the present calculation k , values were obtained by measuring k , at a surfactant concentration of 20 x c.m.c. t For [Mn-IT we have assumed [SDSIT. Because only a few monomer aggregates are involved at the low surfactant concentrations, we expect the error resulting from this assumption to be within acceptable limits.2020 Micellar Inhibition of Aquation concentration (fig. 1). Rearrangement of this equation yields Eqn (24) can be used to explain the observed dependence of k , on complex which can be reduced to the form 41 + kW[S2+1T 4 2 + [S2+IT k, = which then reduces to assuming [S2+] 6 42 where k , = 41/42 + kw[S2+1,/42 41 = K2 kw[H+lT+ km[M"-lT+ kw/K, 4 2 = [M"-]T + K~[H']T + 1 /K1.and #1 and d2 are constants at fixed surfactant and acid concentrations. From eqn (29) a plot of k , against [S2+] should be linear, in agreement with our observed experimental data (fig. 1). Kl, K2 and K3 can also be calculated from the intercept and slope, if these are known, at two or more surfactant and/or acid concentrations. Using eqn (27) it can also be shown that where 6 3 = kw[S2'~T+km[Mn-l~+kw/Ki (33) and 4 4 = [ s 2 + ] ~ + [Mn-]~ + 1 /Ki (34) within the range of acid concentration 0.007 0.129 mol dm-3 in this experiment and [SDS] < 2.0 x lo-* mol dm-3, d4 % K2[H+IT and 43 4 kWK2[H+lT. Under these conditions, eqn (32) reduces to (35) k , zz -[H+], kw K2 4 4 which predicts catalysis by acid at fixed low surfactant concentrations, in agreement with the observed data (fig.3). Discussion Binding Constant The binding constant K,, (2.81 +0.08) x lo5 dm3 mol-1 in acid medium (from fig. 7) is lower than the value of (3.95 Ifi 0.05) x lo5 dm3 mol-1 in acid-free medium (from fig. 6). From experimental observation, a significant increase in initial absorbance is obtained when SDS is added to a neutral aqueous solution of complex, while the change in initial absorbance is much smaller in the presence of acid. This experimental observation is in agreement with the higher binding constant obtained in an acid-free medium. Within the limits of experimental error, the agreement between Kl values determined in an acid medium using a Scatchard plot (3.04 0.16) x lo5 dm3 mob1 and the value obtained from kinetic data in an acid medium (2.81 -+0.08) x lo5 dm3 mol-l is reassuring.The number of binding sites per SDS molecule (0.25 and 0.20 in neutral and acid medium, respectively) suggest there are four SDS molecules in the aggregate in neutral medium and five SDS molecules in acid medium. This is a confirmation of premicellar activity and the higher value of monomer aggregates in acid medium is in agreement with the postulated binding of H+ to some sites originally occupied by Fe(Me4phen)i+. In addition, the plots of fractional bound complex against [SDS], (e.g. fig. 6) showJ. Ige and 0. Soriyan 202 1 no breaks, confirming that there is only one type of binding site for the complex at the surfactant concentration range covered; ([SDS], $ c.m.c.).With only one type of binding site and the given competition between H+ and complex molecules for this site, k , should increase with increase in [€€+IT at constant [SDS],. The reasoning is that as [H+], increases, more H+ is bound to the micelle via equilibrium ( 3 ) and the net surface negative charge of the micelle is reduced, i.e. there is a change in surface potential. This decrease in the net negative charge-density on the micellar surface reduces the strength of the electrostatic binding between the monomer aggregates and the FeII complex. Since fewer molecules of the complex are bound to the monomer aggregates, more of the complex will be found in the bulk-water phase where the aquation reaction is faster. If the postulation of electrostatic binding effect is correct, the bound complex molecules must be located on the micellar surface between the rough ridges of the Stern layer.Solubilisation of the complex in the micellar core, as postulated SDS for the solubilisation of haernin,l2 is ruled out. If there is solubilisation of the complex in the micellar core, inhibition of aquation should be more marked due to the low activity of water in this region and k , should be significantly insensitive to changes in [Hf],. This does not rule out hydrophobic interaction between the micelle and complex a few atoms below the Stern layer, with part of the complex still in the Stern layer. We propose, however, that electrostatic binding in the Stern layer has a predominant effect. There is evidence in the literature to support the type of binding proposed above.E.g. in the photolytic and radiolytic studies of Ru(bpy):+ in SDS micelle13 the kinetic results indicate that essentially all the Ru(bpy)g+ is bound to the micelle. In addition, the SDS catalysis of Ni(PADA)2+ formation14 has been postulated to be due to stabilisation of the Ni(PADA)i+ by binding on the micellar surface. In either case binding through both electrostatic and hydrophobic interactions were postulated. Complex Dependence The increase of k, with [complex], (fig. 1) at fixed [El+], and [SDS],, is due to changes in the distribution of the complex between the micellar phase and the bulk-water phase: [CompleXlrnicellar phase * [CompleXlbulk-water phase. (36) For the runs in fig. 1, [H+], was in at least 170-fold excess of [SDS],.Under this condition, most of the binding sites are already taken up by H+, leaving most of the complex molecules in the bulk-water solution. At fixed [SDS], and excess [H+IT, equilibrium (36) is therefore shifted to the right with increase in [complex], i.e. the distribution of the complex between the micellar phase and bulk-water phase is in favour of the later, where the aquation rate is faster. Rate - [Surfactant] Profiles At fixed [H+], and [Fe(Me,phen)t+],, we cannot readily explain the marked structure in k,-[SDS], profiles. However, we suggest that the evolution (size, geometry, etc.) of the micelle is an important factor. All the profiles show strong premicellar activity. This is attributed to the presence of premicellar aggregates consisting of few surfactant monomers which are more effective in the solubilisation/binding of the complex molecules. The increase in k, following the initial strong inhibition is the result of the expulsion of some of the solubilised complex molecules into the bulk water phase as the size of the aggregates increases.The equilibrium distribution of the complex molecules is therefore shifted progressively to the bulk-water region [equilibrium (36)]. This effect becomes less important once micellisation becomes significant. With [SDS], > 2.25 x lop3 mol dm-3 we observed that there is no significant change in k,,, with increase in [SDS],,2022 Micellar Inhibition of Aquation i.e. k, tends to its limiting value, k,. More data up to 3 x c.m.c. show little or no variation i n k .TKe observed overall inhibition suggests that solubilisation of the dissociated tetra- methylphenanthroline ligand is not a predominant factor as it was the case in the PDC-Fe(phen)i+ system,* where overall rate enhancement was observed and the unsubstituted phenanthroline ligand was used. Comparatively, the bulky tetramethyl- phenanthroline ligand may not be so readily solubilised in the present work. However, we have no supporting evidence in the literature for this type of steric effect. Acid Dependence Keeping [SDS], and [Fe(Me,phen)i+] constant, fig. 3 shows that the aquation is catalysed by acid at very low surfactant concentration (< 1 .OO x mol dm-3), while at higher surfactant concentration (3.0 x lop3 mol dm-3) k, decreases with increase in [H+] mol dm-3 SDS, Hf is in 70-fold excess at the lowest acid concentration.As [H+IT is increased, the ratio of the concentration of micelle-bound complex M W 2 to micelle-bound proton MHlPn decreases; i.e. less complex molecules are bound by the micelle. There is a shift in the partitioning of the complex between the micellar phase and the bulk-water phase, in favour of the later. As explained earlier, aquation is faster in the bulk-water phase. The presence of HSO; and SO:- ions produced from the addition of H,SO, will also inhibit the aquation rate. These ions form a negative field around the complex molecules and hence retard its attack by water molecules. This effect is, however, only significant at the higher acid concentrations. mol dm-3 SDS, [H+IT is in excess by only a factor of 2.3 to 13.3 (in the acid range from 7.0 x to 4.0 x lo-, mol dm-3).[H+], [SDS], is suf- ficiently low that [MS2-n]/[MH1-n] is significantly higher than at (1.0 and 2.0) x lo-* mol dm+ SDS. It is obvious that the rate of aquation will depend on the ratio [MS2-n]/[MH1-n] because of the opposing effect of the presence of and MH1-n on the aquation rate. Ordinarily, in acidified micelle-free media the aquation process is described by the following equilibria : (fig. 4). For fig. 3 at 1.0 x For fig. 1 at 3.0 x Fe(Me,phen)i+ + aq t- Fe(Me,phen)if,,, + Me,phen (37) Fe(Me,phen)i+ + aq Fe(Me,phen):; + Me,phen (38) Fe(Me,~hen)~+ + aq Fe&, + Me,phen (39) where the first dissociation is the rate-determining step. This dissociative mechanism is well established for iron(I1) phenanthroline complexes8 and is catalysed by acid through Me,phen + H+ Me,phen * H+ (40) Me,phen + 2H+ Me,phen - 2H2+. (41) A priori one would expect that in acidified SDS an additional catalytic pathway is the solubilisation of the hydrophobic dissociated ligand by SDS micelles. The absence of this effect and the observed overall inhibition are strong indications that Fe(Me,phen)$+ is stabilised with respect to dissociation in acidified SDS by favourable thermodynamic/ electrostatic/hydrophobic interactions between Fe(Me,phen)g+ molecules and SDS aggregates.J. Ige and 0. Soriyan 2023 References 1 E. D. Cordes and R. B. Dunlap, Acc. Chem. Res., 1969, 2, 329. 2 E. J. Fendler and J. H. Fendler, Adt.. Phys. Org. Chem., 1970, 3, 271. 3 S. Diekmann and J. Franhm, J. Chem. SOC., Faraday Trans. I , 1979,75, 2199. 4 E. Pelizetti and E. Pramauro, Inorg. Chem., 1979, 18, 1882. 5 C. J. O’Connor, E. J. Fendler and J. H. Fendler, J. Am. Chem. SOC., 1973, 95, 600. 6 C. J. O’Connor, E. J. Fendler and J. H. Fendler, J. Chem. SOC., Dalton Trans., 1974, 625. 7 J. Ige, J. N. Lambi and J. Jeje, to be published. 8 H. D. Burrow, J. Ige and S. A. Umoh, J . Chem. SOC., Faraday Trans. I , 1982,78, 947. 9 W. W. Brandt and G. F. Smith, Anal. Chem., 1949,21, 1313. 10 R. J. Williams, J. N. Phillips and K. J. Mysels, Trans. Faraday SOC., 1955, 51, 728. 11 G. Scatchard, J. S. Coleman and A. Shen, J. Am. Chem. SOC., 1957, 79, 12. 12 J. Simplicio, Biochemistry, 1972, 11, 2525. 13 P. Meisel, M. S. Matheso and J. Rabani, J. Am. Chem. SOC., 1978, 100, 117. 14 A. D. James and B. H. Robinson, J. Chem. SOC., Faraday Trans. I , 1978, 174, 10. Paper 4/2 I9 1 ; Receioed 3 1 st December, 1984