Catalysis of the alkaline hydrolysis of 4-nitrophenyl diethyl phosphonate by cationic dimeric surfactant micelles

Abstract

Dimeric (gemini) surfactants containing hydroxyl functional groups have been synthesized and studied. These new surfactants have low critical micelle formation concentrations (<5·10 -5 mol/L) and Krafft temperatures (£0°C). Depending on the pH of the medium, the alkaline hydrolysis of 4-nitrophenyl diethyl phosphonate, which is a model of organophosphorus ecotoxicants, proceeds 20-80 times more rapidly in the presence of gemini surfactant micelles than in water.The enormous interest in recent decades concerning the preparation and study of dimeric (gemini) surfactants and derived organized molecular systems has been due to the unique physicochemical characteristics of these compounds [1, 2], primarily, their anomalously low critical micelle concentrations (CMC » 10 -5 -10 -6 mol/L), which are one or two orders of magnitude less than for the corresponding monomeric surfactants, more efficient lowering of the surface tension of aqueous solutions, better solubilization of compounds with low solubility in water, and micelle polymorphism [2].Gemini surfactants may be seen as a promising basis for organized molecular systems corresponding to the requirements of "green chemistry" for the detoxification and utilization of toxic phosphorus acid esters [3]. The decomposition of ecotoxicants should be carried out under mild conditions using inexpensive and nontoxic solvents. Water is the preferred solvent from this viewpoint but most organophosphorus compounds have low aqueous solubility. This problem may be solved using surfactant additives providing for the solubilization of hydrophobic substrates and an increased rate of nucleophilic substitution reactions, including alkaline hydrolysis. While the effect of monomeric surfactants on the rate of acyl group transfer reactions has been studied in considerable detail [3][4][5], the reactivity of nucleophilic reagents in the presence of gemini surfactant micelles has been studied much less extensively [2,6,7].In the present work, we studied the effect of new gemini surfactants I and II on the rate of alkaline hydrolysis of 4-nitrophenyl diethyl phosphonate (NPDEPS), which is a model analog of highly toxic organophosphorus pesticides and chemical warfare agents. The micellar effects of I and II are comparable to those for cetyltrimethylammonium bromide (CTAB), which is a monomeric surfactant.New gemini surfactants I and II feature a bridging unit (spacer) functionalized by hydroxyl groups. Modification of the spacer by the introduction of hydroxyl groups improves the solubility of these compounds in water since gemini surfactants with the general formula m-s-m [2] have relatively high Krafft temperatures and low solubility in water, which hinders both the 108 0040-5760/11/4702-0108

Reactivity studies of carbon, phosphorus and sulfur‐based acyl sites with tertiary oximes in gemini surfactants

“…The latter structure modifications allow us to use gemini surfactant at considerably high concentrations without a co‐surfactant at room temperature. Another factor is the change of the microenvironment due to the presence of OH − groups which makes micellar interface less water deficient and therefore facilitates reactions involving charged species …”

“…This allows nucleophiles, both OH − ion and oximate ions, to cleave esters in micelle with the observed rates of almost one order of magnitude higher than in water. The OH − ion binding is low and can be assumed to be 30 to 50 M −1 that makes oxymolysis the main route in the micellar pseudophase. As it can seen from the Table , binding constant of oximate ions is lower for 16‐6‐16 MEA almost for all the systems, irrespective of substrate, that makes 16‐4‐16 MEA more efficient surfactant in the studied reactions.…”

“…A comparative study of catalytic hydrolysis of carboxylate esters using conventional and cationic gemini surfactant has been monitored by Zeng and co‐workers. Gemini surfactants containing hydroxyl and ester groups in spacer have recently been synthesized, and alkaline hydrolysis of paraoxon (4‐nitrophenyl diethyl phosphate) and arminum (4‐nitrophenyl diethyl phosphonate) in the presence of gemini micelles was studied by Popov and research group.…”