Effect of surfactant on dissolution of spherical particles in micellar systems.

The influence of micelle-drug solubilization on the dissolution rate of monodisperse particles of benzocaine has been investigated. A model describing and predicting the initial dissolution rates of spherical particles was derived starting from the boundary layer theory. The dissolution rate of benzocaine spherical particles was determined in water and in solutions of sodium lauryl sulfate (SLS) under static conditions. The derived model was applied to the experimental data. The diffusion coefficients and the aqueous diffusion layer values were estimated from the experimental results and the aforementioned model. The diffusion coefficients and the boundary layer thickness values were also obtained experimentally from the rotating disk method and were used to predict the initial dissolution rates. Excellent correlations were obtained between the experimental and the calculated values at low micellar concentrations. However, obvious deviation was observed at high micellar concentrations. The results obtained from this study suggest that it is possible to predict the initial dissolution rates of monodisperse particles in micellar systems.

The application of the convective diffusion model and the film equilibrium model to surfactant-facilitated dissolution of gliclazide.

Gliclazide is practically insoluble in water, and has low dissolution rate. Therefore, it was of interest to improve its dissolution rate using anionic and cationic surfactants. The intrinsic dissolution rates of gliclazide in solutions of sodium dodecyl sulfate (SDS) and in solutions of tetradecyltrimethyl ammonium bromide (TDTMAB) were measured using the rotating disk method to study the convective diffusion transport of drug-loaded micelles. Two different approaches were applied to the experimental data; the convective diffusion model and the film equilibrium model. The two approaches are based on the same fundamental assumptions differing only in their interpretation of the diffusional boundary layer. The results obtained from the film equilibrium model were less satisfactory, and in case of TDTMAB the model was inapplicable (negative diffusion coefficient). While excellent results were obtained from the convective diffusion model. The free solute diffusion coefficient (D(s)) obtained experimentally was 2.47 x 10(-5) cm(2)/s, and the diffusion coefficient of the drug-loaded SDS micelle (D(sm)) estimated was 1.74 x 10(-6) cm(2)/s. The drug-loaded SDS micelle radius was 14 A. The thickness of the diffusional boundary layer was 54 and 22 microm for the free solute and the drug-loaded SDS micelle, respectively. TDTMAB showed lower effect in improving the dissolution rate of gliclazide than SDS. The drug-loaded TDTMAB micelle diffusion coefficient was 1.03 x 10(-6) cm(2)/s. The radius of the drug-loaded TDTMAB micelle and the boundary layer thickness were 24 A and 19 microm, respectively.

Study of the solubilization of gliclazide by aqueous micellar solutions.

It was of interest to increase the solubility of gliclazide in aqueous media. Therefore, solubilization of gliclazide in a variety of surfactants was investigated. Anionic and cationic surfactants exhibited dramatic solubilizing ability for gliclazide, whereas nonionic surfactants showed significantly lower solubilizing ability. It was found that gliclazide solubility increases with increasing the carbon chain length of cationic surfactants and decreases with increasing the carbon chain length of anionic surfactants. The solubilization data were analyzed on the basis of a pseudo-phase model with gliclazide exhibiting moderate partition coefficients into the micellar phase. The possible sites of solubilization of gliclazide in the micelle were examined by studying the effect of NaCl on solubilization and by comparing the absorption spectra of gliclazide in different solvents. The results obtained from these two experiments indicated that gliclazide is solubilized mainly in the inner core of the cationic surfactant micelles and in the outer regions of the anionic surfactant micelles.