Effect of water structure modifiers on hydrotropic behavior

Anita Taylor, Purdue University

Abstract

The term hydrotropy refers to the significant increase in the solubility of a poorly soluble substance in the presence of large concentrations of a second component. The mechanism by which hydrotropy occurs remains uncertain. The aim of this project was to investigate the effect of water structure modifiers on hydrotropic behavior. ^ Water structure modifiers are substances that cause a change in the structure of water by affecting the degree of hydrogen bonding. The degree of hydrogen bonding is decreased by structure breakers and increased by structure formers. Nicotinamide and urea were investigated for their water structure breaking effects, while the structure forming effects of sucrose and sodium chloride were studied. Riboflavin was selected as the sparingly soluble solute. The effect of temperature on riboflavin solubility in water and in solutions containing each water structure modifier was investigated. Also, the relationship between structure of a solute and the extent to which it can be solubilized hydrotropically was studied using three solutes structurally similar to riboflavin; namely, alloxazine, lumichrome, and lumiflavin. Finally, the effect of two water structure modifiers on the solubility of riboflavin in aqueous solutions was determined. ^ The solubility of each solute increased in solutions of increasing nicotinamide and urea concentrations. In sucrose, the solubility of each solute showed a slight increase with increasing concentration. This behavior contradicts the behavior expected for a structure forming material. Riboflavin concentration showed very little change with increasing sodium chloride concentration as expected. The combined use of water structure breaking agents in solution revealed a synergistic effect in their capacity to solubilize riboflavin. Aqueous solutions containing nicotinamide and sucrose exerted a sub-additive effect on the predicted solubility enhancements of riboflavin, while sodium chloride exhibited a synergistic effect. ^ Though a specific mechanism to explain the phenomenon of hydrotropy still remains elusive, this work indicates that hydrotropy is closely aligned with water structure breaking. The temperature data for riboflavin indicated that the heat of solution may be useful in determining structure breaker-structure former interactions. Also, the combined use of certain water structure modifiers can provide an even greater hydrotropic effect than is possible with one agent. ^

Degree

Ph.D.

Advisors

Major Professor: Dane O. Kildsig, Purdue University.

Subject Area

Chemistry, Pharmaceutical|Chemistry, Physical

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