Does capillarity influence chemical reaction in drops and bubbles? A thermodynamic approach

After a brief introduction on the variables which describe the physico-chem ical properties of a fluid surface, this paper compares, in a very simple w ay, the equilibrium constant of homogeneous and heterogeneous reactions tak ing place in spherical micro-objects (uncharged and charged droplets and bu bbles) and in media bordered by a flat interface. This quantity is by defin ition the exponential of the dimensionless standard chemical affinity whose values (less than or equal to 0, greater than or equal to 0) may indicate the direction and the importance of the reaction (strictly true when the mi xing term of the affinity is zero). The classical thermodynamic approach co mbined with the Laplace equation shows that: (i) high surface tension and h igh curvature influence the equilibrium constant, this effect being, howeve r, much more important for bubbles than for droplets; (ii) charges on dropl ets reduce this effect; (iii) the constant of reaction taking place in the vapour in contact with a charged droplet depends significantly on the elect ric field pressure; (iv) reactions in droplets dispersed in the liquid phas e are discussed and, in particular, capillarity seems to play a negligible role on reactions in micro-emulsions; (v) the surface amount of a gas bubbl e component transferred in the continuous liquid can be related to capillar y quantities; (vi) expanding (or shrinking) bubble induced by a chemical re action is analysed by using an extended Laplace law which includes the volu metric now rate; (vii) the Laplace law is discussed in the frame of the cho ice of the dividing surface. Numerous actual examples from the atmosphere, sonochemistry and metallurgy illustrate the theory proposed. One of the int erest, among other points, is that small objects (specially bubbles) give t he potentiality to obtain, for steady or (near) equilibrium states, large a mount of components which would not be possible when dealing with large res ervoirs. (C) 2000 Elsevier Science B.V. All rights reserved.