(187s) Evidence and Relevance of a Structural Barrier for Coalescence of Soft Sphere Colloids
Coalescence phenomena are widely known for heterogeneous systems with liquid-gas and liquid-liquid interfaces. Coalescence of particles is much less intuitive but occurs for elastomer colloids under certain conditions. As for bubbles or droplets, coalescence of elastomer particles depends in general on bulk viscosity and surface tension (surface energy) as driving force. Thus, prerequisite for coalescence is high polymer chain mobility as can be found for elastomer materials at temperatures sufficiently above so called glass transition temperature. In cases of very fast extrusion or immediate coalescence, drainage of liquid film between approaching surfaces and rupture of the liquid film will control the coalescence event. Our investigations aim at contributing to the fundamental understanding of elastomer particle coalescence on the basis of aggregation studies of soft elastomer particles.
Coalescence of elastomer particles can be induced by destabilization of the colloid. Modeling of the aggregation kinetics allows determination of the cluster structure where fractal structure is indicated by a low fractal dimension of i.e. 1.7 to 1.8 for diffusion limited aggregation and full coalescence by a fractal dimension of 3.0. Both situations have been found for elastomer latex of the same polymeric material but different surface characteristics. At 25°C anionic surfactant stabilized particles always coalesce, forming spherical clusters. Particles carrying fixed polar surface groups in contrast form fractal clusters upon aggregation. The coalescence behavior of the first particle type indicates no limitation with respect to flow ability of bulk material. Since both latexes are of identical bulk material, drainage and rupture of the separating liquid film are assumed to control the coalescence process. Intuitively, one may expect that any physicochemical parameters that reduce the mobility of water molecules in the contact region would restrict coalescence. On the other hand, it is known [1, 2] that any ionic species can affect the water structure at interface. Then, polar surface groups or cationic surfactants at the solid-liquid interface may raise a structural barrier between interacting particles. Such a non-DLVO, short range repulsive interaction is often referred to as hydration interaction. Although understanding of the complex phenomenon ?hydration barrier' is far from complete  it is generally accepted that an increase in thermal energy is associated with easier dehydration of ionic surface groups or counter ions respectively a reduced order or smaller extension of hydration layer at the particle surface. In this way the structural barrier between attractive particles should be reduced. Indeed, we find full coalescence for the particles with fixed polar surface groups above a certain temperature, which might indicate the collapse or sufficient reduction of the structural barrier. Coalescence of the surfactant stabilized latex already at ambient temperature is explained by the mobility of surfactants that might get squeezed out of the interaction gap readily when particles approach.
 J. N. Israelachvili, Intermolecular & Surface Forces, 2nd Ed., Academic Press, London, 1992
 R. R. Lessard, S. A. Zieminski, Ind. Eng. Chem. Fund., 1971, 10, 260-269.