Concentration and Medium Micellar Kinetic Effects Caused by Morphological Transitions

Abstract

The reaction methyl naphthalene-2-sulfonate + Br(-) was investigated in several alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br(-) (with s = 2, 3, 4, 5, 6, 8, 10, 12), micellar solutions in the absence and in the presence of various additives. The additives were 1,2-propylene glycol, which remains in the bulk phase, N-decyl N-methylglucamide, MEGA10, which forms mixed micelles with the dimeric surfactants, and 1-butanol, which distributes between the aqueous and micellar phases. Information about the micellar reaction media was obtained by using conductivity and fluorescence measurements. In all cases, with the exception of water-1,2-prop 12-5-12,2Br(-) micellar solutions, with 30% weight percentage of the organic solvent, a sphere-to-rod transition takes place upon increasing surfactant concentration. In order to quantitatively explain the experimental data within the whole surfactant concentration range, a kinetic equation based on the pseudophase kinetic model was considered, together with the decrease in the micellar ionization degree accompanying micellar growth. However, theoretical predictions did not agree with the experimental kinetic data for surfactant concentrations above the morphological transition. An empirical kinetic equation was proposed in order to explain the data. It contains a parameter b which is assumed to account for the medium micellar kinetic effects caused by the morphological transition. The use of this empirical equation permits the quantitative rationalization of the kinetic micellar effects in the whole surfactant concentration range.

Effects of nonionic micelles on the rate of alkaline hydrolysis of N-(2′-methoxyphenyl)phthalimide (1): Kinetic and rheometric evidence for a transition from spherical to rodlike micelles under the typical reaction conditions

“…The value(s) of either k M or K S or both k M and K S may be expected to be sensitive to the micellar structural changes [7,8,34]. Thus, the observed kinetic data may not be expected to fit to Eq.…”

“…The detection of a micellar structural transition by kinetic measurements appears to be rather recent [3][4][5][6][7][8]34]. It appears that no single technique can fully characterize a complex fluid system, and hence the best approach to rule out shear artifacts is to combine more than one technique.…”

“…1 H NMR [3] and rheometric [7,34] measurements supported the cationic micelle-catalyzed kinetic data for sphere-to-rodlike micellar structural transition with increasing values of the total concentrations of cationic surfactants. But the studies [4][5][6]8] provided only the micelle-catalyzed kinetic data as the evidence for sphere-to-rodlike micellar structural transition. In this report, kinetic and rheological measurements lead to the same conclusion regarding the nonionic micellar structural transition with increasing concentration of micelle -forming surfactants.…”

“…But in recent years, a few studies reported systematic deviations of the observed data from the corresponding predicted data (based upon the use of PM or one of its related extended models) at considerably higher values of the concentration of micelle-forming surfactants (surf). These systematic deviations have been attributed to the micellar structural transition under such typical experimental conditions [3][4][5][6][7][8]. However, in all these studies [3][4][5][6][7][8], except the two [3,7] the proposed micellar structural transition was not substantiated by any other physicochemical technique such as rheometry and 1 H NMR.…”

Study of the Reaction 2-(p-Nitrophenyl)ethyl Bromide + OH− in Dimeric Micellar Solutions

“…The sphere-to-rod transition is followed by variations in the characteristics of the micellar aggregates which can affect the rate of reactions. In a previous work [ 11 ], the reaction methyl naphthalene-2-sulfonate + Br − was investigated in several alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br − (with s = 2, 3, 4, 5, 6, 8, 10, 12), micellar solutions. The kinetic data within the whole surfactant concentration range were quantitatively explained by using a modified pseudophase model which took into account the micellar kinetic effects caused by morphological transitions.…”