Title: Thermodynamics and Kinetics of Silicate Glasses submitted to Binary Ion Exchange: Equilibrium Conditions and Interdiffusion Kinetics

Abstract: Ion exchange processes between an ions reservoir and a solid matrix are modeled under the assumption that near interface volumes achieve equilibrium in a much faster time than the overall process time, while in the bulk of the solid matrix, ions are transported by an interdiffusion kinetic process. Ion exchange equilibrium conditions are established within the framework of classical thermodynamics. The result is defined in terms of the chemical potentials of exchanging ionic species. The proposed original derivation is performed making use of the thermodynamics of subsystems. This result is fundamental for the development of the equilibrium theory to determine near-surface equilibrium concentrations and ion exchange isotherms. Interaction energies of the exchanging ions in the glass can be determined from the thermodynamic factor n, which is a parameter of the ion exchange isotherm. The kinetics of the ion exchange process in silicate glass are discussed to find connections with the near-surface equilibrium condition. The resulting flux equation for the incoming ions in the glass is written in the form of a Fick equation with a variable, concentration-dependent, interdiffusion coefficient incorporating the thermodynamic factor n. It has been found that the thermodynamic factor of the interdiffusion coefficient is related to the interaction energy of the exchanging ions in the glass allowing a new approach to the interpretation of past experimental results. Surface concentration has been found substantially connected to the second parameter of the ion-exchange isotherm which is the equilibrium constant K and significatively influenced by the chemical composition of the ions reservoir.