Advances in Colloid and Interface Science (v.86, #3)
Does capillarity influence chemical reaction in drops and bubbles? A thermodynamic approach by A. Sanfeld; K. Sefiane; D. Benielli; A. Steinchen (153-193).
After a brief introduction on the variables which describe the physico-chemical properties of a fluid surface, this paper compares, in a very simple way, the equilibrium constant of homogeneous and heterogeneous reactions taking place in spherical micro-objects (uncharged and charged droplets and bubbles) and in media bordered by a flat interface. This quantity is by definition the exponential of the dimensionless standard chemical affinity whose values (≤0, ≥0) may indicate the direction and the importance of the reaction (strictly true when the mixing term of the affinity is zero). The classical thermodynamic approach combined with the Laplace equation shows that: (i) high surface tension and high curvature influence the equilibrium constant, this effect being, however, much more important for bubbles than for droplets; (ii) charges on droplets reduce this effect; (iii) the constant of reaction taking place in the vapour in contact with a charged droplet depends significantly on the electric field pressure; (iv) reactions in droplets dispersed in the liquid phase are discussed and, in particular, capillarity seems to play a negligible role on reactions in micro-emulsions; (v) the surface amount of a gas bubble component transferred in the continuous liquid can be related to capillary quantities; (vi) expanding (or shrinking) bubble induced by a chemical reaction is analysed by using an extended Laplace law which includes the volumetric flow rate; (vii) the Laplace law is discussed in the frame of the choice of the dividing surface. Numerous actual examples from the atmosphere, sonochemistry and metallurgy illustrate the theory proposed. One of the interest, among other points, is that small objects (specially bubbles) give the potentiality to obtain, for steady or (near) equilibrium states, large amount of components which would not be possible when dealing with large reservoirs.
Keywords: Capillarity influence; Physico-chemical properties; Thermodynamics;
An overview of polymer latex film formation and properties by P.A. Steward; J. Hearn; M.C. Wilkinson (195-267).
The literature on polymer latex film formation has grown enormously in recent times — driven by the need to find alternatives for solvent-based systems with their adverse environmental impacts. Although greater insight has been shown by the use of modern instrumental techniques such as small angle neutron scattering, direct non-radiative energy transfer and atomic force microscopy, the actual mechanisms involved in deforming spherical particles into void-free films are still the subject of controversy and debate. Surfactant-free homopolymer model colloid latices, favoured in academic studies, together with latices containing surfactants whose redistribution can influence film properties, and the more complex copolymer, blended, core-shell and pigmented systems needed to satisfy a full range of film properties are all considered.
Keywords: Polymer latex; Drying; Film formation; Morphology; Surfactants; Plasticisers; Pigment volume fraction;