Molecular Structure and Ice Nucleation of Some Organics

Abstract

The ice nucleating ability of a series of substituted phenols and benzoic acids was determined. Two different measures of activity were utilized: the onset temperature of freezing, as determined on a cold stage microscope; and the relative number of ice nuclei formed at a fixed temperature in a cold chamber. Reactivity was generally found to vary with the potential strength of a hydrogen bond between the hydroxyl or carboxyl group and a water molecule. In a given series of compounds (i.e., either the phenols or the benzoic acids), it was found that the onset temperature could be predicted from the sigma value of Hammett for the individual substituent, so long as the substituent was in the meta and para position, and was not more strongly hydrogen bonding than the functional group of the parent compound. This suggests a free-energy relationship between molecular structure and nucleating power. These observations are offered for further discussion; it is clearly premature to offer a detailed mechanism for the effect.