Micellar Catalysis hydrolysis

A particularly interesting type of micellar catalysis is the autocatalytic self-replication of micelles [58]. Various examples have been described, but a particularly interesting case is the biphasic self-reproduction of aqueous caprylate micelles [59]. In this system ethyl caprylate undergoes hydroxyl catalysed hydrolysis to produce the free carboxylate anion, caprylate. Caprylate micelles then fonn. As these micelles fonn, they solubilize ethylcaprylate and catalyse further production of caprylate anion and caprylate micelles. 

A kinetic study of the basic hydrolysis in a water/AOT/decane system has shown a change in the reactivity of p-nitrophenyl ethyl chloromethyl phosphonate above the percolation threshold. The applicability of the pseudophase model of micellar catalysis, below and above the percolation threshold, was also shown [285],... 

The use of a lipophilic zinc(II) macrocycle complex, 1-hexadecyl-1,4,7,10-tetraazacyclododecane, to catalyze hydrolysis of lipophilic esters, both phosphate and carboxy (425), links this Section to the previous Section. Here, and in studies of the catalysis of hydrolysis of 4-nitrophenyl acetate by the Zn2+ and Co2+ complexes of tris(4,5-di-n-propyl-2 -imidazolyl)phosphine (426) and of a phosphate triester, a phos-phonate diester, and O-isopropyl methylfluorophosphonate (Sarin) by [Cu(A(A(A/,-trimethyl-A/,-tetradecylethylenediamine)l (427), various micellar effects have been brought into play. Catalysis of carboxylic ester hydrolysis is more effectively catalyzed by A"-methylimidazole-functionalized gold nanoparticles than by micellar catalysis (428). Other reports on mechanisms of metal-assisted carboxy ester hydrolyses deal with copper(II) (429), zinc(II) (430,431), and palladium(II) (432). 

R = Et) were hydrolysed in micellar solutions of the prepared ketoximes under pseudo-first-order reaction conditions.In the alkaline hydrolysis of / -nitrophenyl ethyl chloromethylphosphonate (254), micellar catalysis by cetylpyridinium bromide is much reduced when KCl and KBr are present. ... 

The hydrolysis of lipids rarely occurs in a single homogeneous phase, and the behavior of lipases at membrane-solvent and micelle-solvent interfaces has been discussed in detail by Verger and Jain et aP See Micellar Catalysis... 

Abstract There is a growing demand for hydrolyzable surfactants, i.e., sirnfactants that break down in a controlled way by changing the pH. Environmental concern is the main driving force behind current interest in these sirnfactants, but they are also of interest in applications where sirnfactants are needed in one stage but later undesirable at another stage of a process. This chapter summarizes the field of hydrolyzable sirnfactants with an emphasis on their more recent development. Surfactants that break down either on the acid or on the alkaline side are described. It is shown that the susceptibility to hydrolysis for many surfactants depends on whether or not the surfactant is in the form of micelles or as free unimers in solution. It is shown that whereas nonionic ester sirnfactants are more stable above the CMC (micellar retardation), cationic ester surfactants break down more readily when aggregated than when present as unimers (micellar catalysis). 

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

Take as an example the experimentally most studied reaction in micellar catalysis, ester hydrolysis in a basic medium... 

Substrate-catalyst interaction is also essential for micellar catalysis, the principles of which have long been established and consistently described in detail [63-66]. The main feature of micellar catalysis is the ability of reacting species to concentrate inside micelles, which leads to a considerable acceleration of the reaction. The same principle may apply for polymer systems. An interesting way to concentrate the substrate inside polymer catalysts is the use of cross-linked amphiphilic polymer latexes [67-69]. Liu et al. [67] synthesized a histidine-containing resin which was active in hydrolysis of p-nitrophenyl acetate (NPA). The kinetics curve of NPA decomposition in the presence of the resin was of Michaelis-Menten type, indicating that the catalytic act was accompanied by sorption of the substrate. However, no discussion of the possible sorption mechanisms (i.e., sorption by the interfaces or by the core of the resin beads) was presented. 

Micro emulsion droplets and micellar aggregates can catalyse or inhibit chemical reactions by compartmentalization and by concentration of reactants and products. The catalytic effect in micelles has been widely studied, a typical reaction being base catalysed hydrolysis of lipophilic esters. This rate enhancement is normally referred to as micellar catalysis. The analogous effect occurring in microemulsions may be called microemulsion catalysis. 

Since phosphates and sulfates with long chain alkyl substituents form micelles at concentrations above their CMC, the hydrolysis of these esters can be subject to micellar catalysis thereby providing a simplified system in which micelle formation and structure are not alfected by the presence of a foreign solubilizate. The hydrolysis of such surfactants must be considered, however, in investigations of their effects on reaction rates. Fortunately, the rate constants for the neutral hydrolysis of esters such as sodium dodecyl sulfate are extremely slow at 90° = 296 days at pH = 8-63), and the acid-catalyzed hydrolysis of the same ester is some three orders of magnitude faster and thus is still negligible in most cases (Kurz, 1962). 

The electrostatic model for the micellar effect on the hydrolysis of phosphate monoesters is also consistent with the results of inhibition studies (Bunton et al., 1968, 1970). The CTAB catalyzed hydrolysis of the dinitrophenyl phosphate dianions was found to be inhibited by low concentrations of a number of salts (Fig. 9). Simple electrolytes such as sodium chloride, sodium phosphate, and disodium tetraborate had little effect on the micellar catalysis, but salts with bulky organic anions such as sodium p-toluenesulfonate and sodium salts of aryl carboxylic and phosphoric acids dramatically inhibited the micelle catalysis by CTAB. From equation 14 and Fig. 10, the inhibitor constants, K, were calculated (Bunton et al., 1968) and are given in Table 9. The linearity of the plots in Fig. 10 justifies the assumption that the inhibition is competitive and that incorporation of an inhibitor molecule in a micelle prevents incorporation of the substrate (see Section III). Comparison of the value of for phenyl phosphate and the values of K for 2,4-and 2,6-dinitrophenyl phosphates suggests that nitro groups assist the... 

Catalysis arising solely from hydrophobic interactions between the reactants in model systems has been investigated recently by Knowles and Parsons (1967, 1969). The effects of hydrophobic interactions on the rate of hydrolysis, aminolysis, and imidazole-catalyzed hydrolysis of p-nitrophenyl esters were elucidated by varying the hydrocarbon chain length of the -nitrophenyl ester, the primary amine, and the N-substituted imidazole and determining the second order rate constants at concentrations well below the CMCs of the reactants, conditions under which cationic (amine) and neutral (ester) micellar catalysis is... 

Croce and Okamoto [109] described a cationic micellar catalysis (Chapter IV) of aqueous alkaline hydrolysis of cyclonite (and octogene). Denitration occurred in the presence of ethylhexadecyldimethylammonium bromide. Liquid chromatography was used as the analytical method (32) ... 

It is of interest to note the resemblance of our data to that reported by Friberg et al. (24). They have investigated the rate of hydrolysis of p-nitrophenol laurate in the microemulsion system consisting of ceryltrimethyl ammonium bromide, butanol and water. Two pronounced and broad peaks of the reaction rate were observed. The enhancements have been ascribed to the conventional micellar catalysis effect in which the micellar surface charge density plays a dominant role. However, this seems unlikely to be the reasons for the enhancements observed in our studies in view of the sharpness of the peak at 0.855 as compared to that reported by Friberg et al. (24). 

Micellar catalysis of acid hydrolysis of the ester linkage may occur. 

Micelles concentrate reactants from the surrounding medium and provide microenvironments favorable to reaction. Rate enhancement arises from electrostatic and hydrophobic interactions between reactants and micelles. Rates may be strongly dependent on the struture of the reactant. For example, the hydrolysis of methyl ort/iobenzoate is catalyzed by micellar sodium dodecylsulfate, whereas the hydrolysis of methyl ortho-formate (which has less hydrophobic character and less affinity for the micelle core) is not ". The more pronounced the hydrophobic nature of the reactant, the more rapid is the micellar catalysis. ... 

T. J. Broxton, T. Ryan, and S. R. Morrison, Micellar catalysis of organic reactions. XIV. Hydrolysis of some 1.4-benzodiazepin-2-one drugs in acidic solution. Ansi. J. Chem. 37, 1895-1902(1984). 

One of the most widely discussed processes involving micellar catalysis deals with hydrolysis reactions of the form... 

Metallomicellar-catalyzed reactions, inclnding hydrolysis, oxidorednction, and C—C bond formation, might characterize these snpramolecular objects as metalloenzyme mimics that use hydrophobic microenvironment and active centers in constrained domains. In this direction, formal approaches using Michaelis-Menten methods known for enzyme chemistry were applied to kinetics characterization of micellar catalysis. In addition, recent achievements of advanced organometallic reactions such as Heck, Suzuki, and Sonogashira couplings as well as olefin metathesis, directly in water and at room temperature, make the use of surfactants particularly promising to lower the environmental impact, which has become a requirement for the chemical industry in the past years. ... 

Alkaline hydrolysis of ethyl caprylate (itself insoluble in water) yields sodium caprylate, initially at a very slow rate bnt as soon as sufficient caprylate was formed for aggregation into micelles to take place, the authors observed an exponential increase in reaction rate owing to micellar catalysis. These self-assembling surfactant strucmres may consequently provide a model system for studies of pre-biotic chemistry. The possible relevance of this process to prebiotic chemistry was emphasized by their observation that the micelles can be converted into more robust vesicles by a pH change induced by dissolved CO2, and latter on, Luisi extended this approach to vesicular systems (see Section 3.3). Kinetic models for this kind of autocatalytic dynamic systems were also developed in the literature." ... 

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