ESTABLISHMENT OF PHASE-EQUILIBRIA - TEMPERATURE-JUMP EXPERIMENTS IN A SPINNING-DROP APPARATUS

In a spinning-drop apparatus, used for the determination of interfacial tensions, it was investigated how phase-separated polymer solutions react on a rapid rise in temperature. These measurements, yielding an apparent interfacial tension as a function of time, demonstrate that new equilibria are achieved in four clearly separable steps. The first, very rapid one consists in the establishment of a local equilibrium within the interfacial area dividing the two coexisting liquids. During the second stage of much longer duration, solvent and solute are exchanged between the bulk material as a result of the gradients in the chemical potential that exist for both components in both phases; due to the higher mobility of the solvent, its flux dominates. This stage comes to an end as one of the coexisting phases assumes its equilibrium composition, i.e., falls on one end of the corresponding tie line. Despite this situation the entire system has not yet reached equilibrium since the composition of the other coexisting phase is not identical with that of the other end of the tie line. The third step, which is again long-lasting, is therefore characterized by the transport of only one component over the phase boundary, namely for one for which still exists a driving force. If the droplet consists of the dilute polymer solution and the matrix of the concentrated polymer solution, it is the solvent which migrates exclusively during that stage; in the opposite case it is the polymer. If the overall composition of the system corresponds to a point within the homogeneous region of the phase diagram, this third step is followed by a fourth. Ultimately, the new equilibrium is achieved by mere diffusion in the absence of a discontinuity in concentration. Since the spinning-drop experiments also allow the determination of the volume of the phases and the interfacial area between them, it is possible to determine the net fluxes for the first three stages; some preliminary data are given. Furthermore, it is possible to visualize the volume elements within which the composition has changed during equilibrium by jumping back to the lower starting temperature by means of the phase separation that takes place in these zones.