Crystal Growth from Supersaturated Aqueous Salt Solutions on a Polymer Nafion Substrate

Abstract

Features of the crystallization of various compounds on the Nafion polymer substrate from supersaturated aqueous solutions is studied by XRD. The solutions are prepared from natural deionized and deuterium-depleted water. It was earlier established that the swelling of Nafion in natural deionized water clearly demonstrates effects caused by the partial uncoiling of polymer fibers into the liquid volume, but no such effects were observed in deuterium-depleted water. The ab initio calculations performed in the present work show that the uncoiling of lateral Nafion chains with terminal –SO₃H groups and the dissociation of these groups with subsequent formation of hydronium ions and \(-\text{SO}_3^{-}\) structural anionic residues are energetically favorable in the case of the contact with a sufficient amount of water molecules. The influence of these factors on the process of crystal formation on polymer substrates is studied. It is established that the effect of partial uncoiling of polymer chains indeed affects the crystallization, and does it in different ways, depending on the crystal structure. Thus, no differences between XRD patterns of crystals grown on smooth and polymer substrates are observed for the monoclinic lattice of sodium acetate trihydrate. In the case of copper sulfate (with either CuSO₄·5H₂O pentahydrate crystals (triclinic crystal system) or CuSO₄·3H₂O trihydrate crystals (monoclinic crystal system)), copper sulfate trihydrate and pentahydrate are formed on the polymer and smooth substrates, respectively. None of the studied crystals exhibited differences between smooth and polymer substrates for the supersaturated solutions based on deuterium-depleted water, and in both cases a triclinic crystal system of the CuSO₄·5H₂O pentahydrate precipitate is identified for the CuSO₄ aqueous solution. For sucrose, the precipitate amorphization upon the deposition onto the Nafion plate is detected. A theoretical model is proposed to explain the effect of partial polymer chain uncoiling on the growth of crystals, depending on their crystal system, unit cell parameters, and the volumetric ratio of hydrophilic and hydrophobic fragments in the molecular structure.