Confinement effects on the phase transition temperature of aqueous NaCl solutions: The extended Gibbs–Thomson equation

• Melting temperatures of confined NaCl solutions were determined.
• Solutions experience both confinement- and solute-induced melting point depression.
• An extended version of the Gibbs–Thomson equation is proposed to explain the data.
• Concentration dependence of the confined ice–solution interfacial energy is important.

Thermal properties of confined, single-component liquids have been extensively investigated over the past few decades, and the Gibbs–Thomson equation has emerged as a relatively good model capable of explaining the general relationship between pore size of the confinement host material and magnitude of the confinement-induced melting point depression. However, relatively little research is focused on thermal properties of confined solutions, despite aggressive research initiatives to adapt nano-engineered materials to applications that will almost certainly employ solutions instead of pure liquids. Therefore, we have chosen to explore the phase transition temperatures of dilute aqueous NaCl solutions confined within mesoporous silicas. We also derive an extended version of the Gibbs–Thomson equation to provide a theoretical framework for understanding our experimental data. The confined solutions have melting points that are lower than those of pure water confined within the same porous host, and the magnitude of the melting point depression increases with increasing salt concentration. Moreover, the slope of the ΔT vs. molal solute concentration is found to be pore-size dependent. Both of these features are in agreement with the theoretical model that we provide.

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