Surface Tension of Aqueous Solutions of Small-Chain Amino and Organic Acids

Surface tension was measured for aqueous solutions of dl-malic acid, l-glutamine, l-serine, l-proline, l-methionine, l-valine, and l-lactic acid as a function of concentration using a Du Noüy ring over a temperature range between 298 and 328 K at 101 kPa atmospheric pressure. Surface tensions of several concentrations of each organic acid were measured to develop four isotherms with a temperature ramp of 10 K starting from 298 K. Cohesive forces between molecules were weakened by temperature rise, resulting in decreased surface tension. Mass concentration and temperature effects on surface tension were reflected by the slopes of the relationships between surface tension and either mass concentration or temperature, respectively. In this study, l-serine and l-glutamine showed hydrophilic behavior, while l-proline, l-methionine, dl-malic acid, l-valine, and l-lactic acid had hydrophobic behavior. For all acids studied here, increased temperature caused decreased surface tension of aqueous solutions of solute.