Interactions between small organic molecules and water measured using pressure perturbation calorimetry

Abstract

Aqueous liquid mixtures play a critical role in many biological and chemical processes. Solutes including sugars, sugar alcohols, carboxylic acids, alcohols and acetone can affect the hydrogen-bonded structure of water and this can be measured using pressure perturbation calorimetry (PPC). In binary water–solute mixtures, Δ(∂CP/∂P)T is a measure of the structure of the water component. At low alcohol concentrations, negative Δ(∂CP/∂P)T values are consistent with clathrate-like water cages around the alkyl moieties. Conversely, when solutes hydrogen bond with water it interferes in the formation of “ice-like” water and is observable as a positive Δ(∂CP/∂P)T. The Δ(∂CP/∂P)T at increasing concentrations of ethanol, acetone and acetic acid in water displayed very different behaviors. Ethanol–water mixtures had three distinct concentration dependent phases; the first, with ethanol surrounded by water molecules, followed by the ethyl groups self-associating breaking the clathrate-like cages, and the ethanol–water network displacing all of the bulk water. Acetic acid–water mixtures display nonlinearity in Δ(∂CP/∂P)T versus acetic acid concentration consistent with acetic acid self-interaction which interferes with acetic acid capacity to disrupt water structure. Acetone-water mixtures display linearity in Δ(∂CP/∂P)T versus acetone concentration which is consistent with acetone’s inability to hydrogen bond with other acetone molecules. The lack of negative Δ(∂CP/∂P)T values in acetic acid-water and acetone-water mixtures suggests there is sufficient self-association between these solutes to prevent clathrate-like water cage formation. PPC can provide invaluable insight into the behavior of aqueous binary mixtures.