Fast Reactions in Micelles

Modeling counterion binding in ionic-nonionic and ionic-zwitterionic binary surfactant mixtures

A predictive molecular-thermodynamic theory is developed to model the effect of counterion binding on micellar solution properties of binary surfactant mixtures of ionic and nonionic (or zwitterionic) surfactants. The theory combines a molecular-thermodynamic description of micellization in binary surfactant mixtures with a recently developed model of counterion binding to single-component ionic surfactant micelles. The thermodynamic component of the theory models the equilibrium between the surfactant monomers, the counterions, and the mixed micelles. The molecular component of the theory models the various contributions to the free-energy change associated with forming a mixed micelle from ionic surfactants, nonionic (or zwitterionic) surfactants, and bound counterions (referred to as the free energy of mixed micellization). Specifically, the various molecular contributions to the free energy of mixed micellization model the underlying physics associated with the assembly of, and the interactions between, the surfactant polar heads, the surfactant nonpolar tails, and the bound counterions. Utilizing known structural characteristics of the surfactants and the counterions, along with the solution conditions, the free energy of mixed micellization is minimized to predict various optimal micelle characteristics, including the degree of counterion binding, the micelle composition, and the micelle shape and size. These predicted optimal micelle characteristics are then used to predict the critical micelle concentration (cmc) and the average micelle aggregation number. Our predictions of the degree of counterion binding, the cmc, and the average micelle aggregation number show good agreement with available experimental results from the literature for several binary surfactant mixtures. In addition, the theory is used to shed light on the relationship between the micelle composition, counterion binding and ion condensation, and the micelle shape transition.

Study of the derivatization of n-alkylamines with 1-fluoro-2,4-dinitrobenzene in the presence of aqueous cetyltrimethylammonium bromide micelles

The use of aqueous cetyltrimethylammonium bromide micelles in the derivatization of n-alkylamines with 1-fluoro-2,4-dinitrobenzene was investigated systematically. The rate constants of derivatization of the n-alkylamines (C1-C8) were analysed using liquid chromatography. Up to butylamine the micellar rate enhancement depends on the electrostatic interactions between the amines and cetyltrimethylammonium bromide, and beyond C4 it depends mainly on the hydrophobic interactions. The reaction rates are also enhanced by a micelle-induced decrease of the pKa of the amines, but to a lesser extent. The derivatization rates for the longer alkylamines are comparable with those in dipolar aprotic solvents. Pharmaceutical and biomedical science is likely to benefit from the use of micellar systems in pre-column derivatization reactions in aqueous solutions.

Interactions of neutral molecules with ionic micelles

The interactions of neutral molecules with ionic micelles are analyzed. The cell and mass action models are presented in order to provide a semi-quantitative description of the solubilization process. Both approaches are discussed from a thermodynamic and kinetic point of view and the different definitions of solute incorporation constants are also discussed and compared. An extensive compilation of standard free energies of transfer from water to micelles is provided and the basis of the employed methods to obtain them is presented. Several aspects of the solubilization process such as its dynamics, the effect of additives, the probe microenvironment and its dependence with the solute mean occupation number are reviewed and critically discussed. The effect of solute incorporation upon the micelle shape and size is also briefly reviewed.

Incorporation kinetics of lysolecithin into lecithin vesicles. Kinetics of lysolecithin-induced vesicle fusion

The incorporation kinetics of L-palmitoylphosphatidylcholine (lysolecithin) into dimyristoyl- and dipalmitoylphosphatidylcholine vesicles and the subsequent aggregation and fusion of the vesicles into larger aggregates were studied by using stopped-flow rapid-mixing techniques. The half-times for the lysolecithin incorporation vary between 50 and 500 ms. The incorporation rate has a maximum in the temperature range of the vesicle phase transition. This process is not diffusion controlled. The rate-limiting step is the incorporation of the lysolecithin monomer into the liquid bilayer. After this fast process, a slow reaction in the 10-50-min time range is observed. The large irreversible increase in turbidity indicates aggregation and fusion of the vesicles. The initial step is a second-order reaction with respect to the vesicle concentration, indicating aggregation or fusion of two vesicles. The aggregation rate passes through a maximum at the phase transition temperature.

Interaction of lysophosphatidylcholine with phosphatidylcholine bilayers. A photo-physical and NMR study

Several photo-physical methods together with 31P-NMR have been used to investigate the effect of lysophosphatidylcholine on phosphatidylcholine bilayers. 31P-NMR shows that the permeability of the vesicle to Eu3+ increases sharply above approx. 40% lysophosphatidylcholine: fluorescence-quenching studies also show this type of behavior. Similar sharp changes in vesicle properties are observed via the photo-physical technique at this lysophosphatidylcholine/phosphatidylcholine composition. Fluorescence spectra of pyrene and pyrene carboxaldehyde show that increasing lysophosphatidylcholine composition increases the polarity of the environments of these probes up to 40% lysocompound. Above this composition the photo-physical properties of the probes slowly revert to those characteristic of the micellar lyso-compound. The pyrene fluorescence lifetime, the fine structure of the fluorescence, and the case of formation of pyrene excimer in these bilayer mixtures suggest that pyrene complexes weakly with the charged nitrogen of the choline group of the phosphatidylcholine and that the physical state of the system has a striking effect on this complexation process. Similar experiments with simple quaternary compounds lend strong support to this suggestion. The studies monitor in several ways the effect of bilayer composition on movement of molecules in these systems. The degree or site of solubilization of carcinogens is also uniquely affected by composition.