We request 5 days on NIMROD to collect total scattering data of aspirin and caffeine at pH7 and pH3 in order to determine the origin of the effect of the hydrotrope caffeine on the aqueous solubility of aspirin. Hydrotropes are used to improve the solubility of various pharmaceutically active compounds that are poorly soluble in aqueous solution. In order to understand the change in the solubility through the interactions between the solvent, solute and hydrotrope, the experiment will be performed in aq solutions (pH 7) and also at pH 3 (achieved using 0.5% HCl) thus simulating the behaviour of the drugs in the stomach. Using isotopic substitution a pair distribution function can be generated that is centred on different atoms in the molecule, thus enabling the solute-solute and solute-solvent interactions to be identified, including the formation of pi-stacks of phenyl groups.