Interfacial barrier

Referring to Fig. 9, the effect of the shear is to catalyze the reaction, presumably through suppression of the interfacial barrier by stretching the flow. The latter is believed to reduce the diffusion path, promoting the reaction rate, and hence the rate of increase in the viscosity. A similar effect is produced with temperature as a parameter, which also augments the reaction rate. The modified reaction rate constant in case of any external stimulus or perturbation acting on the system may be computed from the scalar K, where ... 

A Ghanem, W Higuchi, A Simonelli. Interfacial barriers in interphase transport III Transport of cholesterol and other organic solutes into hexadecane-gelatin-water matrices. J Pharm Sci 59 659, 1970. 

AH Goldberg, WI Higuchi, NFH Ho, G Zografi. Mechanism of interphase transport I Theoretical considerations of diffusion and interfacial barriers in transport of solubilized systems. J Pharm Sci 56(11) 1432-1437, 1967. 

AB Bikhazi, WI Higuchi. Interfacial barrier limited interphase transport of cholesterol in the aqueous polysorbate 80-hexadecane system. J Pharm Sci 59 744-748, 1970. 

Fig. 15 Two of the simplest theories for the dissolution of solids (A) the interfacial barrier model, and (B) the diffusion layer model, in the simple form of Nemst [105] and Brunner [106] (dashed trace) and in the more exact form of Levich [104] (solid trace). c is the concentration of the dissolving solid, cs is the solubility, cb is the concentration in the bulk solution, and x is the distance from the solid-liquid interface of thickness h or 8, depending on how it is defined. (Reproduced with permission of the copyright owner, John Wiley and Sons, Inc., from Ref. 1, p. 478.)...

According to the diffusion layer theory, for which the transport process is rate-limiting, kT kR, so that k = kT. According to the interfacial barrier theory, for which the surface reaction is rate-limiting, kR kT, so that it, = R-... 

The interfacial barrier theory is illustrated in Fig. 15A. Since transport does not control the dissolution rate, the solute concentration falls precipitously from the surface value, cs, to the bulk value, cb, over an infinitesimal distance. The interfacial barrier model is probably applicable when the dissolution rate is limited by a condensed film absorbed at the solid-liquid interface this gives rise to a high activation energy barrier to the surface reaction, so that kR kj. Reaction-controlled dissolution is somewhat rare for organic compounds. Examples include the dissolution of gallstones, which consist mostly of cholesterol,... 

The conclusions we may draw from these results are that, in general, interfacial turbulence will occur, and that it will increase the rate of mass transfer in these otherwise unstirred systems. Monolayers will prevent this turbulence, and theory and experiment are then in good agreement, in spite of spontaneously formed emulsion. There are no interfacial barriers greater than 1000 sec. cm. due to the presence of a mono-layer, though polymolecular films can set up quite considerable barriers. Usually there are no appreciable barriers due to re-solvation however, in the passage of Hg from the liquid metal into water, the change between the metallic state and the Hg2++ (aq) ion reduces the transfer rate by a factor of the order 1000. 

The effects of surface-active agents on the motion of and transfer from bubbles and drops have been discussed in earlier chapters. The main effect is to reduce the mobility of all or part of the interface. In this section we consider briefly two other interfacial phenomena interfacial convection during mass transfer and interfacial barriers to mass transfer. 

The existence of interfacial barriers to mass transfer caused by films of surface-active materials has long been recognized (LI). When surfactants are added... 

FIGURE 17.1 (a) Diffusion-layer model of dissolution, (b) Interfacial barrier model of dissolution. 

Two of the simplest theories to explain the dissolution rate of solutes are the interfacial barrier model and the diffusion-layer model (Figures 17.1 and 17.2). Both of these theories make the following two assumptions ... 

T. Yotsuyanagi, W. I. Higuchi, and A. H. Ghanem. Theoretical treatment of diffusional transport into and through an oil-water emulsion with an interfacial barrier at the oil-water interface. J. Pharm. Sci. 62 40-43, 1973. 

It is worth repeating the above. T o date, the effects of the presence of interfacial barriers or interface regions have not been explicitly considered in models of charge injection at such interfaces. The... 

Fig. 8.6 Ca— PPV interfacial barriers. The figures are drawn with alignment to the vacuum level of energy, so that the band edge energies can be seen. In this illustration, the electro-chemical potential is not a constant throughout the sample. The band edge values used for the polymer are for MEHPPV.

Figure 5.2 Schematic representation of the dissolution mechanisms according to (A) the diffusion layer model, and (B) the interfacial barrier model.

In the interfacial barrier model of dissolution it is assumed that the reaction at the solid-liquid interface is not rapid due to the high free energy of activation requirement and therefore the reaction becomes the rate-limiting step for the dissolution process (Figure 5.1), thus, drug dissolution is considered as a reaction-limited process for the interfacial barrier model. Although the diffusion layer model enjoys widespread acceptance since it provides a rather simplistic interpretation of dissolution with a well-defined mathematical description, the interfacial barrier model is not widely used because of the lack of a physically-based mathematical description. 

Stochastic variation may be introduced in other models as well. In this context, Lansky and Weiss [130] have also considered random variation for the parameter k of the interfacial barrier model (5.20). 

It should be noted that the droplet sizes in Hallworth s emulsions are considerably greater than those investigated by Davis and Smith. The importance of the two possible routes of degradation of the emulsions, coalescence or molecular diffusion, may be dependent upon the droplet size and size distribution. Also an interfacial coherent film may reduce the demulsification by either mechanism, i.e. by reducing the rate of coalescence or by presenting an interfacial barrier to... 

---