Surfactants alkaline hydrolysis

Enzyme Sta.bihty, Loss of enzyme-catalytic activity may be caused by physical denaturation, eg, high temperature, drying/freezing, etc or by chemical denaturation, eg, acidic or alkaline hydrolysis, proteolysis, oxidation, denaturants such as surfactants or solvents, etc. pH has a strong influence on enzyme stabiHty, and must be adjusted to a range suitable for the particular enzyme. If the enzyme is not sufficiendy stable in aqueous solution, it can be stabilized by certain additives a comprehensive treatment with additional examples is available (27). 

A fluid loss additive for hard brine environments has been developed [1685], which consists of hydrocarbon, an anionic surfactant, an alcohol, a sulfonated asphalt, a biopolymer, and optionally an organophilic clay, a copolymer of N-vinyl-2-pyrrolidone and sodium-2-acrylamido-2-methylpropane sulfonate. Methylene-bis-acrylamide can be used as a crosslinker [1398]. Crosslinking imparts thermal stability and resistance to alkaline hydrolysis. 

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]. 

The effects of dilution of the micellar surface charge on the rate of alkaline hydrolysis of a betaine ester surfactant have been investigated for a mixture of decyl betainate and a nonionic surfactant with a similar CMC. It was shown that the relation between micellar composition and the hydrolysis rate essentially parallels the relation between micellar composition and counterion binding to mixed micelles made up of ionic and nonionic surfactants [20]. 

Biodegradation tests have shown that surfactants containing a carbonate bond between the hydrophobic tail and the polar head group are readily biodegradable. In comparative tests such carbonate surfactants biodegrade somewhat faster than the corresponding surfactants containing an ester bond [35]. The carbonate bond is not only susceptible to alkaline hydrolysis... 

A wide range of hydrolyzable surfactants exist that are suitable for different applications. The decisive factor for choosing the best hydrolyzable surfactant is the pH of the apphcation Fig. 19 illustrates the susceptibility of the different types of surfactants that have been discussed in this review to acid and alkaline hydrolysis. As can be seen from the figure, the formulator has a tool box of surfactants with different hydrolysis characteristics to choose from. The figure also illustrates that for some surfactants, e.g., betaine esters, the susceptibility to hydrolysis increases above the CMC, i.e., when the surfactant is present in aggregated form, while for other surfactants, e.g., normal esters, the opposite holds true. This behavior is practically important and must be taken into account in the formulation work. 

A quantitative assessment of the effects of head group bulk on, S k2 and E2 reactions in cationic micelles has been made.148 The kinetics of the acid-catalysed hydrolysis of methyl acetate in the presence of cationic, anionic, and non-ionic surfactants has been reported on.149 The alkaline hydrolysis of -butyl acetate with cetyltrimethylammonium bromide has also been investigated.150 The alkaline hydrolysis of aromatic and aliphatic ethyl esters in anionic and non-ionic surfactants has been studied.151 Specific salting-in effects that lead to striking substrate selectivity were observed for the hydrolysis of /j-nitrophenyl alkanoates (185 n = 2-16) catalysed by the 4-(dialkylamino)pyridine-fimctionalized polymer (186) in aqueous Tris buffer solution at pH 8 and 30 °C. The formation of a reactive catalyst-substrate complex, (185)-(186), seems to be promoted by the presence of tris(hydroxymethyl)methylammonium ion.152... 

The effects of cationic head groups on the alkaline hydrolysis of the quinoxaline (192) to give (193) have been looked at using the surfactants (194 R = Me, Et, n-Pr, n-Bu X = Cl, OH).164 The reactivity increases with increasing head-group size and is related to the disruption of the hydration of the HO ion. An earlier paper from the same group describes the synthesis of (192) and some micellar effects on its basic hydrolysis.165 A novel site-selection functionalization reaction is facilitated by histidine side-chains in helical structures which can catalyse the acylation by mono-p-nitrophenyl... 

Non-ionic surfactants either decrease or have insignificant effects on the rate constants for hydrolysis of carboxylic esters (Lach and Pauh, 1959 Riegelman, 1960 Nogami et al., 1960, 1962 Kakemi et al., 1962 Mitchell, 1963 Behme et al., 1965 Mitchell and Broadhead, 1967 Saheki et al., 1968 Ullmann et al., 1968). The available data do not warrant conclusions on the relationship between substrate or surfactant structure on the magnitude or nature of catalysis by non-ionic micelles, but it should be noted that synthetic and naturally occurring amphiphiles cause very similar retardations of the rate of alkaline hydrolysis of ethyl p-aminobenzoate (Lach and Pauli, 1969). 

KOH affects the size of the changed micelles (12.16-18). For mixed surfactant systems, this effect may be different for different values of surfactant ratios of the mixed surfactants. Furthermore, it was observed, during the earlier period of the present study, that the dilute solutions of sodium lauryl sulfate beccime turbid in the presence of KOH, presumably due to alkaline hydrolysis. Therefore, KOH was not used in this study. 

Three papers from Ghosh s group deal with the hydrolysis of benzohydroxamic acids in acidic and alkaline conditions. A pre-equihbrium protonation followed by a slow A-2 type nucleophilic attack by water is seen as the mechanism of the acid-catalysed hydrolysis of j -chlorophenylbenzohydroxamic acid (107 R = / -ClCgEU) by mineral acids (HCl, HCIO4) in 20% aqueous dioxane. An A-2 mechanism was also supported for the reaction of (107 R = Me) imder comparable conditions. The alkaline hydrolysis imder micellar conditions of (107 R = Ph) and a series of para-substituted derivatives has been investigated in the presence of cationic and anionic micelles in 5% dioxane-water medium at 55 °C. Cationic surfactants exerted a catalytic effect and anionic surfactants were inhibitory. The rate-surfactant profiles were analysed in terms of the pseudophase and Piszkiewicz models. The detection of N2O in the products of the oxidation of hydroxamic acids suggests the intermediacy of nitroxyl, HNO, in the process. Scheme 9 may represent the pathway followed. 

The aminolysis and methanolysis of ionized phenyl salicylate (189) have been examined under micellar conditions. The effect of CTABr on the rates of aminolysis of (189) by -butylamine, piperidine, and pyrrolidine is to bring about a rate decrease (up to 17-fold with pyrrolidine). The results are interpreted in terms of binding constants for the amines with CTABr and the pseudo-phase model.The effects of mixed surfactants SDS and CTABr on the methanolysis of (189) and the alkaline hydrolysis of phenyl benzoate suggest that micellar aggregates are involved in the processes.The effects of NaOH and KBr on the intramolecular general base-catalysed methanolysis of (189) in the presence of CTABr has been investigated. Pseudo-first-order rate constants were not affected by either additive but other changes were noted. The effect of mixed MeCN-water solvents on the same reaction has also been probed. 

Alkaline hydrolysis of forskolin is inhibited in a lipid emulsion.550 Indomethacin in an oil-water gel is solubilized in the surfactant aggregates and stabilized against acid- and base-catalyzed hydrolysis.551... 

L. Ya. Zakharova, D. B. Kudryavtsev, F. G. Valeeva, L. A. Kudryavtseva, Inhibition of alkaline hydrolysis of ethyl p-nitrophenyl (chloromethyl)phosphonate in the system cationic surfactant-water-electrolyte, Russ. J. Gen. Ghent., 2002, 72, 1215-1221. 

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