Role of the hydrophobic properties of functional detergents on micellar effects in the dissociation of environmental toxicants

Abstract

we should take into account not only their hydrophobic properties but also the efficiency of substrate solubilization as well as the reactivity of the oximate group in the surfactant micelles.The unflagging interest during recent decades in the synthesis and reactivity of surfactants with nucleophilic functional groups is a consequence of a number of factors. Firstly, microheterogeneous systems derived from functional detergents serve, to some extent, as models for studying the action of hydrolytic enzymes, which provides for the investigation of various features of enzymatic catalysis in simpler systems. The increase in the reaction rate in micelles of functional surfactants in comparison with water may reach factors of 10 2 -10 7 , which makes such systems comparable in efficiency with enzyme systems [1,2]. The observed effects are largely a function of the role of the microenvironment of that part of the micelles, in which the chemical reaction proceeds and the hydrophobic binding of the substrate by the micelle core [1][2][3][4]. Secondly, the intensive study of the reactivity of functional detergents has had and still has marked applied significance related to the decomposition and utilization of environmental toxicants, in particular the highly toxic esters of organophosphorus acids [1,[5][6][7][8].Modification of functional surfactants in order to obtain highly efficient supernucleophilic systems requires us to establish the factors responsible for the micelle effects of detergents. In previous work [9], we showed that, in the case of surfactants with an oximate group, nucleophilicity is directly related to the acid-base properties of the oximate group. In this instance, as in the case of oximes, which do not form micelles, the Brönsted catalysis equation is not linear and has an inflection point at pK a » 8.5-9.0 [9, 10]. Since the concentration of the substrate in the micellar pseudophase makes the major contribution to the increase in the observed reaction rate [10], the hydrophobic properties of the functional surfactants have fundamental importance in the manifestation of their micellar effects [1,[11][12][13].

Reactivity of micelle-forming 1-alkyl-3(1-oximinoethyl)pyridinium bromides in acyl group transfer reactions

“…The effect of functional detergents II and III* on the rate of the cleavage of esters is analogous to the behavior already established for surfactants containing the oximate group [4][5][6][7][8]. Firstly, at constant detergent concentration, the observed reaction rate constant (k obs , s -1 ) increases with increasing pH of the medium and reaches a limiting value k obs max (Fig.…”

“…The differences in the activation free energies for reactions in micelles of functional detergents or functional/cationic surfactant comicelles and in water are negligibly small, as indicated by data for a series of functional surfactants and comicelle systems [4][5][6][7][8]. The second factor involving concentration of the substrate in the micellar pseudophase, as a rule, plays the determining role in increasing the observed reaction rates relative to the rates for their structural analogs not forming micelles [4][5][6]9]. This property is directly linked to the hydrophobic properties of both the substrate itself and the functional detergent [4,[9][10][11].…”

“…The first factor may lead to change in both nucleophilicity and the acid-base properties of the functional group [4,5]. The differences in the activation free energies for reactions in micelles of functional detergents or functional/cationic surfactant comicelles and in water are negligibly small, as indicated by data for a series of functional surfactants and comicelle systems [4][5][6][7][8]. The second factor involving concentration of the substrate in the micellar pseudophase, as a rule, plays the determining role in increasing the observed reaction rates relative to the rates for their structural analogs not forming micelles [4][5][6]9].…”

“…The second factor involving concentration of the substrate in the micellar pseudophase, as a rule, plays the determining role in increasing the observed reaction rates relative to the rates for their structural analogs not forming micelles [4][5][6]9]. This property is directly linked to the hydrophobic properties of both the substrate itself and the functional detergent [4,[9][10][11]. These hydrophobic interactions are specifically responsible for transfer of the nonpolar electroneutral substrates from water into the surfactant micelles.…”

“…The first factor may lead to change in both nucleophilicity and the acid-base properties of the functional group [4,5]. The differences in the activation free energies for reactions in micelles of functional detergents or functional/cationic surfactant comicelles and in water are negligibly small, as indicated by data for a series of functional surfactants and comicelle systems [4][5][6][7][8].…”