Particle size and surface adsorption

A geochemically important property of small particles is their tendency to adsorb ions from solution onto their surfaces. Adsorption events can involve ions that constitute the bulk (i.e., a step in crystal growth) or can involve other ions. The driving force for adsorption is reduction of surface energy (easily appreciated for the special case of crystal growth). 

Given a number of possible alternative explanations for experimental results involving adsorption onto small particles could be advanced (see Navrotsky, this volume), it is perhaps useful to consider the thermodynamic basis for the prediction of a size-dependence of adsorption. 

The concept stems of size dependence of the adsorption coefficient stems from structural and energetic considerations. For small particles, an adsorption event decreases the energy of a system composed of small particles of radius rA by AGAo=3V ,x[y(S)-y(S-I)]/rA 

It follows that AGao AGbo, since rA rB. Because an adsorption event on smaller crystals decreases the energy of a system more than adsorption on particles in a system composed of larger particles, the driving force for adsorption is larger, thus adsorption should be favored on small particles. 

Another way of viewing this is to consider the effect of particle size on the equilibrium between adsorption and desorption. From the above we deduce that the activation barrier for desorption from smaller particles will be higher than from larger particles, implying a smaller rate constant. Thus, under conditions when adsorption is favored, at equilibrium ions should partition more strongly onto the surfaces of small particles than larger particles. The activation barrier for adsorption is probably size- 

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