Diffusion Behavior Of Nicotinic Acid In Model Medium Of Controlled Release Agents

Some effective components of drugs are weak organic electrolytes. The degree of dissociation is determined mainly by the medium. In pharmaceutical techniques, materials of pharmaceutical aids are usually introduced to enhance the bioavailability of drugs. The effect of the composition of drug carrier on the ionization equilibrium and the molecular diffusion of drugs is our interest in this paper.In this paper, nicotinic acid was chosen as a model drug of weak electrolyte. The diffusion properties of nicotinic acid in some model mediums of controlled release agents were studied. This paper is arranged in the following:1. Poly(vinyl alcohol) (PVA) was chosen to simulate the hydrogel environment of drug. Viscosity, density and electric conductance for the solution of nicotinic acid in PVA aqueous solution at the temperatures from 293.15 to 313.15K are reported. By using Eyring's transition state treatment, the activation free energy of the viscous flow is evaluated. Our result shows that, theΔμ3≠increases with the addition of PVA into water. This leads to the dissociation and molecular diffusion of nicotinic acid to be more difficult. However with more PVA is added, the capacities of PVA to improve the drug solubility is appeared. This in turn leads the decrease inΔμ3≠. The interaction between PVA and nicotinic acid improves the diffusion of the nicotinic acid.2. Dextran solution was chosen as a model environment natural sugar media. Viscosity, density and electric conductance for the solution of nicotinic acid dissolves in dextran aqueous solution at the temperatures from 293.15 to 313.15K are reported. The degree of dissociation of nicotinic acid in dextran aqueous solution is reported and the activation free energy of the viscous flow is evaluated. It is found that, in dilution concentration region, dextran promotes the dissociation of nicotinic acid, and increase the value ofΔμ3≠. That is dextran demotes the diffusion of nicotinic acid in water. The interaction between nicotinic acid and dextran, leads the entropy effect great than the enthalpy effect.3. PVA hydrogel was prepared to study the release behavior of nicotinic acid. The gel-sol phase transition temperature of PVA hydrogel was measured by a ball dropping method. Our result indicates that, the diffusion and release behavior of nicotinic acid in PVA gels follows the Weibull diffusion model, and a diffusion- controlled mechanism is assumed.