Freezing points of aqueous alcohols. Free energy of interaction of the CHOH, CH2, CONH and CC functional groups in dilute aqueous solutions

Abstract

The freezing temperatures of dilute aqueous solutions of methanol, ethanol, 2-propanol, butanol, t-butanol, cyclohexanol and ethylene glycol were measured over the concentration range 0.1 to 1 mol kg^–1. Osmotic coefficients at 0°C were calculated. The limiting pairwise interaction coefficients of the alcohols, plus a variety of polyhydroxy compounds and carbohydrates, were calculated at 25°C from the available data and then correlated using the additivity principle of Savage and Wood. This correlation approximates effective free energies of CH₂ and CHOH group interactions with themselves and with each other. Literature data were used to estimate interactions between CONH and CC groups. The CONH—CONH interaction appears to be large, consistent with a strong stabilizing effect of these on native protein structures. The CH₂…CH₂ interaction also indicates attractive forces between these groups. The present model for the hydrophobic interaction is most appropriate for small molecular interactions whereas previous treatments are best for situations involving site binding.

The CHOH…CHOH and CH₂…CONH interactions are small, while the CHOH…CH₂ free energy of interaction is positive, due either to volume exclusion or net repulsive forces.

The entropy change associated with the CH₂…CH₂ interaction is large and positive as expected and is not completely compensated by a corresponding enthalpy change. The entropy change associated with the CONH…CONH interaction indicates that few degrees of freedom are involved, which is consistent with the formation of a strong hydrogen bond.

The correlation can be used to estimate thermodynamic properties of dilute non-electrolyte solutions and can also predict the effect of solutes on the solubility of solids and gases.