CONCENTRATION-DEPENDENCE OF SOLVENT DIFFUSION IN A COLLOIDAL SUSPENSION

We have used the large-pulsed-gradient NMR method supplemented by NMR relaxation to study the diffusivity D of water and cosolvent in a commercial poly(vinyl acetate)-water emulsion (Elmer's Glue-All, Borden, Inc.), in samples with volatiles concentrations c between (water added) 100 and (drying) 5 wt%, in concentration equilibrium attained over several months. At high c, D(c) has the mild variation characteristic of obstruction by particles, while below c = 30% D drops by almost two orders of magnitude and cosolvent diffusion begins to dominate as water depletes. Diffusion becomes unmeasurable with this method near c = 6%, where the adhesive is translucent and rigid. We interpret the sharp drop in D(c) in terms of a topological transition between water-based emulsion and PV A-based porous but nearly impermeable solid. An elaboration of the Maxwell-Fricke model quantitatively accounts for the full c-dependence of D. © 1990.