Influence of anions on the solubility of polymers. Free Technical Library News

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Effect of anions on the solubility of polymers

06.11.2021

Chemists from the USA and the Czech Republic have established a relationship between the curvature of the conditional surface of a macromolecule and dissociation constants for various cations and anions. It turned out that ions interact more strongly with a polymer if its planar structure deforms the network of water hydrogen bonds. This interaction helps the polymer to be in solution.

A group of scientists led by Jan Heyda from the University of Chemistry and Technology in Prague and Paul S. Cremer from the University of Pennsylvania investigated the interaction of weakly coordinating anions with polyethylene oxide polymer of different lengths. They suggested that molecules with different surface curvature affect the network of hydrogen bonds of surrounding water molecules in different ways, which, in turn, leads to a change in the degree of ion adsorption.

The authors showed the relationship between the ion adsorption energy and the degree of distortion of the hydrogen bond network. The results obtained confirmed the hypothesis that macromolecules with a flat surface strongly distort the structure of water and, therefore, interact well with weakly coordinating anions, which easily penetrate to the polymer surface through a deformed network of hydrogen bonds. On the contrary, the surface of most small molecules cannot be called flat, which results in no energy gain, high hydrophobicity, and poor solubility.

In this case, the main interaction of polymers with anions occurs precisely in the middle of the chain, since the hemispherical surface of the end groups poorly distorts the surrounding water. The average dissociation constants of SCN- were about 0,1 mol per liter for the middle of the chain and more than 2,4 mol per liter for its ends. The maximum energy gain resulting from the interaction of the polymer with thiocyanate was 5,3 kilojoules per mol.

The data obtained will affect not only the study of hydrophobic interactions and solution chemistry of polymers, but also other areas of science, for example, the study of supramolecular compounds of the guest-host type.

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