Coadsorption of the sodium salts of two steroid molecules at a silica/interface as induced by the adsorption of a cationic surfactant.

The incorporation of two ionic steroids, namely the sodium salts of hydrocortisone 21-hemisuccinate (HNa) and prednisolone 21-succinate (PNa), at a silica/water interface in the presence of adsorbed cetyltrimethylammonium bromide has been investigated first at a constant pH value. It is shown that this coadsorption effect is qualitatively similar to the adsolubilization effect which is described for neutral molecules. The adsorption of the cationic surfactant induces the coadsorption of the anionic drug molecules although the silica surface is negatively charged. At surfactant equilibrium concentration above the critical micelle concentration the drug molecules are distributed between the adsorbed aggregates and the free micelles. At larger surfactant concentration, the drugs may be completely depleted from the silica/water interface. Based upon Langmuir-type isotherms, the equilibrium constants of the drug molecules with the adsorbed aggregates and the free micelles are calculated. The constants are about three times larger for the former than for the latter aggregates. The signification of such results is discussed. The coadsorption of HNa at low surfactant surface coverage was also investigated in the pH interval between 3 and 9. HNa is strongly coadsorbed at lower pH onto the silica surface. The coadsorption goes through a maximum at a pH value which may be considered as equal to the apparent pK of the drug and decreases to zero at higher pH values. A pK value equal to 4.2 is proposed for HNa. This behaviour is interpreted as the result of the interplay of the drug dissociation and that of the surface silanol groups upon the change of pH.

Emulsion multiple de type L/H/L: étude de l'obtention et du mécanisme de libération.

Synopsis An O/W/O multiple emulsion was formulated using almond oil as the inner phase and liquid paraffin as the outer phase. The properties of the formulation ingredients were explored: effect of the type of primary emulsifier, and the nature and concentration of the secondary emulsifier. The volume fraction of the internal phase in the multiple emulsion was also optimized with regard to the stability. The multiple systems were assessed by evaluating several parameters such as the macroscopic aspect, the droplet size, the direction of the dispersion and the accelerated stability under centrifugation or elevated temperature. The effect of some additives (ozokerite, benzyl alcohol and glycerol) was examined. Whatever additive was used, a decrease in the formulation stability, which could be attributed to different phenomena, was observed. Furthermore, an analytical procedure was developed for studying the release of an active principle, alpha-tocopherol (vitamin E). This release could be a result of the instability of the multiple droplets or a diffusion through the water layer membrane. This procedure was based on the difference of chemical type between the two oil phases: mineral outer phase and fatty inner phase. Two chromatographic methods were used: a gas chromatographic assay of oleic acid which is the main component of the fatty phase, and a liquid chromatographic assay for the alpha-tocopherol, encapsulated in the inner phase. The presence of oleic acid in the paraffin phase originated from a rupture of the multiple droplets, whereas the presence of alpha-tocopherol in the outer phase could be due both to a diffusion phenomenon and to the instability of the formulation. The results obtained showed that only the rupture of the droplets occurred. Moreover, a part of the alpha-tocopherol was retained by Hypermer A 60 used as an emulsifier in the formulation.

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Synopsis The main purpose of this article is to define multiple emulsions, their preparation (new materials and manufacturing processes) methods of characterisation and analysis, and their applications in cosmetology and pharmacy. The second part is devoted to a presentation of the investigation in our laboratory into the optimal manufacturing process to provide stable formulations. The study of release mechanisms from such formulations mainly shows a breakdown of multiple droplets which could be controlled by variation of solute concentration.

Equilibrium dialysis of barbituric acids up to high concentrations of aqueous sodium alkylsulfonate solutions.

The partition coefficient K of four barbituric acids has been determined by the equilibrium dialysis method in aqueous solutions of sodium alkylsulfonate at 25 degrees C. The results obtained from dilute solutions to 0.3 M surfactant concentration are compared with solubility data for the same systems. The K values as deduced from Langmuir or Freundlich isotherms decrease with increasing surfactant concentration. The detergent used was an impure commercial product of known composition, which could be considered as a mixed surfactant. The change of partition coefficient with surfactant concentration as obtained from equilibrium dialysis experiments has been interpreted by assuming a continuous change of micelle composition.

Solubility behavior of barbituric acids in aqueous solution of sodium alkyl sulfonate as a function of concentration and temperature.

The solubility of 13 barbituric acids was determined in aqueous solutions of sodium alkyl sulfonate. The effects of concentration and temperature were investigated, and the thermodynamic functions of the solubilization process were calculated. An analysis of the location of a solubilized species within a micelle is suggested in terms of the sign and amplitude of the standard entropy of solubilization, which is strongly positive for micelle penetration and negative for adsorption. A solubilization mechanism through adsorption onto the micellar surface is suggested for most of the barbituric acids studied. The enthalpy/entropy compensation phenomenon was identical for barbituric acids in ionic and nonionic (polyoxyethylene lauryl ether )surfactant solutions with a compensation temperature of 270 degrees K, indicating common behavior of these compounds with respect to micellar solubilization. The concept of molecular surface area was used to correlate the free energy of solubilization of the solutes to their size and structure. A linear relationship was found with an excellent correlation factor for the alkane derivatives of the 5-ethyl-barbituric acids. The specific behavior of some of the barbituric acids investigated is discussed.