Effect on dissolution rate

It 1s well known that water absorption bears a direct relation with the number of polar groups in the polymer and that Ionic diffusion occur via "hopping" along hydrophilic sites. Therefore, the hydrophilic/ hydrophobic characteristics of the Inhibitor exert a profound effect on dissolution rate. 

Lerk, C.F., Bolhuis, G.K., Smallenbroek, A.J., and Zuurman, K., Interaction of tablet disintegrants and magnesium stearate during mixing. 11. Effect on dissolution rate, Pharm. Acta Helv., 57 282-286 (1982). 

Making a polymer relief image commonly requires two processes. First, there is a photochemical process which alters the solubility of the exposed areas relative to the unexposed areas. This is followed by the actual dissolution of the most soluble areas during development. Historically, studies of photoresists have emphasized the photochemical aspects of image formation rather than the dissolution process. The central theme of this paper is the very Icurge effects on dissolution rate that can result from adding small molecules to the matrix. 

Sanghavi, N.M. Munot, D.S. Kamath, P.R. Shaikh, F.AR. Solvent deposition of griseofulvin on excipients and its effects on dissolution rate. Indian Drugs 1988,26, 23-27. 

Particle Size. For substances to be used in certain formulations such as solid or suspension products, the particle size can have a significant effect on dissolution rates, bioavailability, and stability. A decision tree in the guideline provides guidance as to when particle size testing should be considered. 

For some new drug substances intended for use in solid or suspension drug products, particle size can have a significant effect on dissolution rates, bioavailability. 

Apart from the above effect on dissolution rate, surfactant micelles also affect the membrane permeability of the solute [8j. Solubilization can, under certain circumstances, help the transport of an insoluble chemical across a membrane. The driving force for transporting the substance through an aqueous system is always the difference in its cdiemical potential (or to a first approximation the difference in its relative saturation) between the starting point and its destination. The principal steps involved are dissolution, diffusion or convection in bulk liquid and crossing of a membrane. As mentioned above, solubilization will enhance the diffusion rate by affecting transport away from the boundary layer adjacent to the crystal [8j. However, to enhance transport the solution should remain saturated, i.e. excess solid particles must be present since an unsaturated solution has a lower activity. 

Higuchi s analysis [24] predicts that substantial effects on dissolution rate will only be evident when the drug concentration in solution approaches or exceeds saturation solubility. The dissolution model used by Higuchi assumes that an equilibrium exists between the solid and the solution at the interface and that the rate is controlled by the diffusion of free and solubilized solute across the diffusion layer which has a thickness S. 

In Sec. 2 the overall surface dissolution was considered. Except in the case of surface diffusion, we assumed that a dissolving surface is free from defects. Since real crystals usually contain dislocations and other defects, it Is necessary to know their effect on dissolution rates and the mechanism of formation of etch pits at their emergence points. 

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