Effect Of Local Concentration Research Articles

Modification of the order of reaction and reaction rate of nucleophilic aromatic substitution in micellar solutions

The rate of formation of 2,4,6-trinitrodiphenylamine following the attack of aniline on 1-methoxy-2,4,6-trinitrobenzene has been studied in micellar media. The partial order with reference to the nucleophilic reagent (aniline) is unity in solutions of cationic detergents (positive micelles) and 3/2 in water or solutions of anionic detergents. For such reactions there are two main steps in the reaction scheme: first, the formation of an adduct between reagents, favoured by the effect of local higher concentration in the two kinds of micellar solutions and secondly, ejection of a proton from the adduct formed. The latter reaction is catalysed largely by positive micelles and in this case the kinetics are not limited by the deprotonation step. On the other hand, negative micelles inhibit the ejection of a proton and this opposes, in part, the effect of higher local concentrations.

Kinetics of drug release from hydrogel matrices

The release of water soluble drugs from initially dehydrated hydrogel matrices generally involves the simultaneous absorption of water and desorption of drug via a swelling-controlled diffusion mechanism. Thus, as water penetrates a glassy hydrogel matrix containing dispersed drug, the polymer swells and its glass transition temperature is lowered. At the same time, the dissolved drug diffuses through this swollen rubbery region into the external releasing medium. Such diffusion and swelling generally do not follow a Fickian diffusion mechanism. The existence of some molecular relaxation process in addition to diffusion is believed to be responsible for the observed non-Fickian behavior. In this article, the kinetics of drug release from hydrogel matrices will be examined both experimentally and theoretically. Special emphasis will be placed on the effect of local drug concentration on the swelling behavior of hydrogel drug carriers. A novel approach to constant-rate drug delivery from glassy hydrogel matrices via an immobilized non-uniform drug concentration distribution will also be described.

Insulator‐Induced Conduction

The presence of insulating particles, even those of higher resistivity than the liquid, was found to increase electrical conduction in several dielectrical liquids. We believe this to be a general effect which will occur if the particles have a higher dielectric constant than the liquid. In particular, we observed in several highly purified liquids ( and toluene) that the current conducted across a cylindrical electrode system was considerably enhanced when highly purified solids (e.g., polyvinyl chloride, quartz, or sulfur) were held against the smaller inner electrode by dielectrophoretic force. A theory for the effect is given. It emphasizes the effect of local field concentration (field‐focussing) by the particles. This “focussing” of the field in the liquid acts to enhance carrier concentration and conduction since the conduction is in the nonohmic space‐charge‐limited regime.