Interactions between hydration spheres of two different solutes in solution: The least squares fitting with constraints as a tool to determine water properties in ternary systems

Biological systems are complex and the problem of their description lies in mutual interactions between their components. This paper is focused on model experiment-based studies which can reduce these difficulties. The ternary aqueous N-methylacetamide (NMA)-Na2HPO4 system has been studied by means of the FTIR spectroscopy. A novel difference spectra method aimed to extract the spectral contribution of water affected simultaneously by two solutes is proposed. The key improvement is the additional least squares curve fitting procedure with carefully selected constraints which allow obtaining information on the composition of water affected in complex solutions. The fitting parameters are optimized with the Monte Carlo method and the human influence on parameter selection is minimized. The simple method allows determining individual contributions originating in hydration of multiple solution components. FTIR spectra of water in the NMA-Na2HPO4 system gave an insight into the interactions and susceptibility for interactions between solvent and co-solutes or hydrated co-solutes. (C) 2020 The Authors. Published by Elsevier B.V.