A predictable solution

The solubility of a particular material is an important physical characteristic. Whole tomes and databases exists citing solubilities for given substances in different solvents, at different temperatures and pressures. It would be useful if there were a way to predict solubility and such a tool has been sought for many years, particularly in the pharmaceutical and petrochemical industries but in other areas of chemical and related sciences too. After all, solubility in water is half the battle in making an orally active medicine, while the low solubility of petroleum products can be the cause of unwanted deposits or low yields from oil wells and fracking efforts.

Brute force simulations have been tried, but these require long computation times, while faster techniques give inaccurate estimates. Now, Daan Frenkel of the University of Cambridge, UK and colleague Lunna Li have worked with Tim Totton of British Petroleum to develop an open source code. This software can give a precise solubility estimate for almost any molecular substance over a wide range of temperatures and pressures.

"A general-purpose tool to compute solubilities has been lacking for a long time," explains Frenkel, "The underlying methodology was there, but nobody had actually created a working program. He and his colleague chose to use well-documented, freely available software to build their code so that it could be made available to the whole scientific community.

The software uses thermodynamic expressions regarding vapor pressure and other properties that were well known even to scientists in the middle of 19th century. Indeed, the calculations are based on the fact that when a solid and liquid phase are in equilibrium, their vapor pressures are equal. When a liquid or solid are heated, molecules escape and form vapor. Vapor pressure can be readily calculated using a computer model that works out the energy required to move solute from the solid state to the completely solvated state based on vapor pressure of each of the two phases.

In order to model the solid phase, the team based their calculations on an Einstein crystal in which non-interacting solute molecules are placed on a lattice and tethered hypothetically to a lattice point. The vapor pressure of the crystal is computed by calculating the work needed to disconnect the tethers and allow the molecules to interact. To model the dissolved solute molecule, the researchers used the standard energy potential for the solvent in question. This procedure eliminates a number of errors and produces accurate estimates of the vapor pressure and, thus, the solubility. The team demonstrated proof of principle by determining the solubility of naphthalene in water and obtained a value close to the experimental measurement. The next step will be to validate the software with other solutes and solvents and to extend it so that it can cope with larger solute molecules. [Lunna Li et al. J Chem Phys (2017); DOI: 10.1063/1.4983754].

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom" you can see his ever-growing gallery of birds on his Imaging Storm website here.