Joel Henry Hildebrand - Legacy

Legacy

His study of the solubility of non electrolytes led to his formation of the 'Hildebrand solubility parameter'.

The general idea is that a potential solute will be soluble in a solvent which has a comparable value for .

This work was then used in the formation of the more comprehensive 'Hansen solubility parameter', which accounts not just for dispersion interactions between solvent and solute (as the Hildebrand parameter does) but also for hydrogen bonding and polar interactions - thus lifting the restriction of application to just non-polar species. Hansen shows great respect for Hildebrand and his work and indeed acknowledges that his work of the Hansen solubility parameter would not have been possible without the great contribution that Hildebrand made to this field.

Hildebrand was also outspoken on the manner in which small non-polar species exist in water. The dissolution of species such as methane in water is accompanied by both a negative enthalpy and a negative entropy. A common model for this behavior is the iceberg or clathrate type model, in which a network or cage of hydrogen bonded water develops around the methane molecule. This explains the drop in enthalpy since hydrogen bonding is increased compared to pure water and the drop in entropy since a solvent excluded volume has come in to existence along with an ordered network of water molecules.

Hildebrand challenged this popular view in a series of papers in the late 1960s and 1970s and concluded that methane has a just a 40% lower diffusivity in water than in carbon tetrachloride. If water was enclatherated or in an iceberg type structure then he predicted that this diffusivity difference between water and carbon tetrachloride ought to be significantly larger.

This conflict of ideas still exists in the literature with publications between 2000-2010 for the clathrate type hydrophobic hydration still being submitted in computer simulations of various types. There are papers however which cite Hildebrand's earlier criticisms of this model and suggest that hydrophobicity arises from the small size of water increasing the free energy required to develop a suitable cavity for certain solutes to occupy.

Given the conflict in this field and the high level of interest involved it seems that Hildebrand may continue contributing to the scientific community for quite some time yet.