of the properties of water at interfaces of organized

assemblies at the microscopic level is a prerequisite for understanding the effect of these species on chemical reactivity and equilibria. Interfacial water is a reactant in spontaneous hydrolyses, but it is involved in, inter alia, solvation of reactants and transition states, solva- tion of surfactant headgroups and counterions, and pro- ton transfer.

At one time or another, water was believed to pen- etrate the micelle completely, not to penetrate at all, or to reach to any intermediate depth. Neither of the two extremes, picturesquely described as the ‘‘fjord’’ and the ‘‘reef’’ models, is likely to represent the real situ- ation. For example, Menger in 1979 [62] used space- filling models to demonstrate that there will be large voids in the micelle if eclipsing is to be avoided in the alkyl groups, and he concluded that these voids will accommodate water molecules, which penetrate deeply into the micelle. Right after him, Fromherz [63] pos- tulated a different packing of monomers that also leads to partial exposure of alkyl groups to water, and Dill and Flory [64] showed how a micelle could be con- structed with eclipsing of some of the alkyl groups and contact between methylene groups and water. A mod- ified Dill-Flory model has been reported to be consis- tent with results of small-angle neutron scattering that exclude water penetration into the micellar interior [65]. However, the structure of the micelle is so dy- namic that every segment of the hydrophobic alkyl group is exposed to water at some time, and in a small micelle over half the chain segments will be at the mi- cellar surface at any given time. This theoretical pre- diction agrees with proton and fluorine NMR spectros- copy [66].

Properties of aqueous micelles are typically de- scribed by pseudophase models with a distinction being made between ionic and nonionic surfactant monomers in the bulk solvent and in the micellar pseudophase. Distinction between interfacial water at the micellar surface and that in the bulk has been made. A consid- erable amount of work on the properties of interfacial water has depended on probes [67] or the study of re- actions whose rates are sensitive to the micellar micro- environment (see Section IV). A new method based on chemical trapping based on a dediazoniation reaction has been developed by Romsted and coworkers and used for determination of hydration numbers [68–70]. An interesting new method for investigating the properties of interfacial water is based on measure- ments of the deuterium isotope effect on the1

H chem- ical shift of the solvent (H2O plus D2O), developed by El Seoud et al. [71,72]. It is evident that ionic and

zwitterionic micelles generally increase structuring of the interfacial region, with values of the fractional fac- tor,⌽, being lower for zwitterionic than for ionic mi- celles of otherwise similar structure.

C. Substrate Solubilization

One of the most important processes leading to micel- lar effects on reactions is the solubilization of organic compounds at the micellar surface. It is possible to sol- ubilize water-insoluble substances or to increase the solubility of slightly soluble ones in aqueous micellar solutions. Solubilization has been applied empirically for centuries, but the fundamental bases of micellar sol- ubilization were established only in the 1950s by McBain and Hutchinson [73] and by Elworthy [74].

The most realistic model for analyzing solute incor- poration is undoubtedly that based on the mass action approach. In its general form [75,76], the solubilization process can be treated in terms of the stepwise addition of solute molecules, S, to aggregates containing -i-sol- ute molecules. Making the assumptions that (1) the mi- cellar aggregation number is independent of the pres- ence of the solute, (2) the solute entry rate is independent of the number of solute molecules present in the micelle, (3) the solute exit rate is directly pro- portional to the number of solutes present in the mi- celle, and (4) the solubilization capacity of the micelle is ‘‘infinite,’’ an interesting form of expressing the equi- librium constant for the incorporation of the first solute into an empty micelle, KS, has been defined in terms of the concentration of micellized surfactant [Dn]:

[SDn]

K =S (1)

[S ][Dn]W

An analogous solubilization constant can be written in terms of molarity of micelles, KM; the two constants differ in magnitude by the aggregation number of the micelles. This definition of KS, which in essence re- duces the solubilization process to a pseudophase equi- librium of the solute between a micellized surfactant and the aqueous phase (Scheme 1), has played a central role in pseudophase models for analyzing micellar ef- fects on reaction kinetics [11,12,77].

A wide range of experimental techniques and meth- ods are available to measure the partitioning of non- ionic solutes between water and micelles, such as cal-

orimetry; ultraviolet-visible (UV-VIS), NMR, and ESR spectroscopies; laser light scattering; and luminescence probing [78,79]. Reliable values of binding constants are generally obtained provided that the solute is not very hydrophobic, that the amphiphile is in excess over substrate, and that the amphiphile concentration is well above the cmc, because solutes may induce micelliza- tion or bind to premicelles. Fluorescence quenching provides information on properties of association col- loids and partitioning of solutes between the colloids and water, and this general method has been reviewed [80]. The binding of nonionic solutes has been treated by applying a linear solvation free energy relationship (LSER) [79,81–83]. The structural features that govern binding have been identified, with hydrophobic effects and solute basicity being the dominant terms. Distri- butions of solutes that cannot be examined experimen- tally, for example, because of their high reactivity in water, can be predicted on the basis of this approach.

Unlike homogeneous solvents, the inherently mi- croheterogeneous micellar solutions provide a variety of solubilization environments, ranging (in principle) from the ‘‘hydrocarbon-like’’ core to the bulk water. Indeed, the distinctive feature of a micelle as a solvent is that it can provide not only different microenviron- ments for different molecules but also different micro- environments for different parts of the same molecule. Thus, polar molecules encounter a polar environment, hydrophobic ones have available a hydrocarbon-like medium, and amphipathic molecules should be able to orient themselves at the micellar surface with their hy- drophobic portion extending into the hydrocarbon-like core.

The pathways and rates of reactions in micelles are affected by how deeply the solubilized species is lo- cated within the micelle. Both electrostatic and hydro- phobic factors play a role in determining the binding site of a solute inside the micelle, and both the structure of the amphiphile and the solute are of great impor- tance in determining the extent of solubilization and the penetration of solute into the micelle [31]. NMR is extensively used to elucidate locations of solubilized species. A large number of NMR parameters are sig- nificantly affected on adding a solubilizate to a micellar solution and are thus capable of giving some infor- mation on molecular aspects of solubilization [84]. Ar- omatic compounds are useful because of their sizable so-called ring current shifts in NMR, and such mole- cules as benzene and its derivatives were the first sol- utes studied by Eriksson and Gillberg [85]. Subsequent studies have provided more details on the interfacial location of aromatic solubilizates. An increase in the

size of the surfactant headgroup also increases the depth of penetration of methyl naphthalene-2-sulfonate [86]. Kang et al., in their study of n-alkylphenothia- zines, show that similarly sulfonated N-alkylphenothia- zines reside near the micellar interface, unsulfonated ones penetrate more deeply into the micelle, and the degree of penetration depends on the length of the alkyl chain [87,88].

D. Ion Binding

A fundamental process in micellar catalysis or inhibi- tion is ion binding to micelles. The major problem in the analysis of micellar rate effects is modeling the interfacial concentration and distribution of ionic re- actants, although in favorable cases they can be directly measured [12,89].

Packing of headgroups and counterions at aggregate interfaces produces a high local counterion concentra- tion; typical estimates are >1 M, much greater than counterion concentration in the surrounding aqueous pseudophase, which is usually in the range 0.001–0.01 M [1,11,12].

There are two models describing the ionic distribu- tion at charged aqueous interfaces. In the pseudophase ion exchange (PIE) model, micellar surfaces are treated as selective ion exchangers saturated with counterions; using the Poisson-Boltzmann equation (PBE) modified for specific ion interactions, ion distributions are com- puted within a reaction region at the micelle surface [89] (see also Section III).

Distribution of counterions is usually characterized by the micellar degree of ionization, ␣, and for most ionic micelles␣is in the range 0.1–0.3 and is seen to depend little on the overall concentration of counter- ions; in other words, the surface of an ionic micelle is treated as if it is saturated with counterions [11]. The remaining counterions are distributed in the diffuse Gouy-Chapman layer, and their distribution is gov- erned by their nonspecific electrostatic interactions with the micelle, which can be regarded as a bulky macroion [12]. Values of ␣ are determined for many ions, including different values for the same ion, by a variety of methods [31]. A new method is based on chemical trapping of ‘‘free’’ counterions in the aqueous pseudophase, based on the dediazoniation reaction de- veloped by Romsted et al. [68,90]. Extensive kinetic data have been fitted by the PIE model with data on the ion-exchange parameters for pairs of ions [89], in- cluding estimates for Cl⫺and Br⫺and estimates based on the method of ion flotation of Warr et al. [91–93].

Counterions are bound primarily by the strong elec- trical field created by the headgroup, but also by spe- cific interactions that are dependent on headgroup and counterion type [9,89,94]. The nature of the counterion has an important bearing on␣for cationic surfactants: ions that have a lower hydration enthalpy, higher affin- ity for Dowex 2 (a tetralkylammonium ion resin), and a higher lyotropic number are more strongly bound to cationic micelles and highly effective in charge neu- tralization of the micellar headgroups. Generally speak- ing, we can say that micelles bind counterions selec- tively, and several properties such as size, shape, phase stability, binding of ions and neutral molecules, and their effects on the rates and equilibria of chemical re- actions are sensitive to counterion concentration and type. Some lyotropic series or affinity orders have been established for the relative affinities of both anions and cations to micelles [31,94].

When salts are added to micellar solutions, the coun- terions of the salts compete for the ionic headgroup of micelles with the surfactant counterions that already exist in solution. Thus, displacement can occur, de- pending on the relative affinities of counterions for the headgroups. Interesting kinetic effects can arise. If the added salt is reactive, micellar rate enhancements are observed after displacement. If added ion is inert and the reactive ion is the surfactant counterion, the addi- tion can cause inhibition. Kinetic salt effects are pe- culiar in micellar systems [31].

Ion selectivity has been treated thermodynamically, and it was concluded that partial ionic dehydration at the micellar interface is of crucial importance [95]. Ev- idence indicates that strongly hydrated ions, such as OH⫺ and F⫺ are readily displaced from cationic mi- celles by relatively weakly hydrated anions such as Br⫺ and Cl⫺ [89]. Results of a chemical trapping method are consistent with partial dehydration of counterions of CTABr and CTACl micelles at the sphere-to-rod transition [96].

Morini et al. [97] used ion-selective electrodes to demonstrate that in mixtures of DTABr and DTAOH, the breakpoints used to estimate the cmc differ for Br⫺, OH⫺, and the amphiphile cation; that the degree of ion- ization decreases continuously up to about 0.2 M am- phiphile; and that the exchange constant for OH⫺and Br⫺ depends on solution composition, consistent with theoretical models [98].

As for organic neutral substrates and also for organic anions, the structure of both the surfactant and the ion can affect location at the interface. Binding of salicy- late and o-, m-, and p-nitrobenzoate anions to MTABr micelles has been examined by1

H NMR spectroscopy

[99]. Competition is treated by the PIE model, although these polarizable anions may perturb the micellar struc- ture. With organic counterions there is often micellar growth, which is sensitive to counterion type. Two-di- mensional NMR spectroscopy, capable of revealing spatial relationships among proximal protons (NOESY, ROESY), can be very useful, giving insight into struc- tural details of micelle-solute interactions. Results with both 1D and 2D NMR show that the 3,5-dichloroben- zoate ion intercalates further into cationic rodlike mi- celles than does the 2,6-iosmeric ion into spherical mi- celles [100]. Other 1

H NMR data show that micellar growth in alkylpyridinium micelles to form entangled wormlike micelles depends on counterion structure; it was observed for o-hydroxybenzoate but not for the p- hydroxy isomer and was observed for p-chlorobenzoate but not for benzenesulfonates [101]. Growth induced by added organic anions is probably not due to charge reversal of the micelle surface, because the surface po- tential, estimated by using an indicator, approaches zero with added salicylate ion but does not change sign [102], consistent with Poisson-Boltzmann equation (PBE) treatments [77,89]. For the aromatic anion 2- naphthoate in cationic micelles,1

H chemical shifts and NOESY spectra led to the conclusions that the aromatic section of the molecule is embedded in the palisade layer whereas the charged parts are located near the micellar interface, so they can still be solvated by water [103].

In spite of the neutral nature of zwitterionic surfac- tants and their well-documented insensitivity to ionic strength, electrolytes do bind to zwitterionic micelles. Binding of various ions to micelles was suggested in several papers [104–108], and it was unambiguously established from radioactive tracer, self-diffusion, and fluorescence quenching data [109,110]. Micellar aggre- gates of zwitterionic carboxybetaine, sulfobetaine, or phosphobetaine surfactant can be represented as a hy- drophobic sphere surrounded by a spherical shell where positive charges are distributed on the inner layer and negative charges on the outer layer. In this purely elec- trostatic model, a positive electrostatic potential exists inside the shell as a function of the distance from the center of the sphere. The number of positive and neg- ative charges is the same, but the surface where the positive charges lie is less extended. Positive charge density is consequently greater [110,111]. This model explains the strongest anion binding to zwitterionic mi- celles but does not explain the strongest binding of the softer anions [112]. Another failure of the model is the dependence of the electrostatic potential on the thick- ness of the double-charged shell, corresponding to the

SCHEME 2