Aqueous solubility-molecular size relationships:: A mechanistic case study using C10- to C19-alkanes

Saturated hydrocarbons are important constituents of petroleum products. Their behavior in water, the most prevalent environmental solvent, is of relevance with regard to environmental partitioning. Due to their negligible attractive interactions with water, they are suitable compounds for a mechanism-based validation of the relationship between molecular size and the solubility in water. To that end, we measured the aqueous solubility of aliphatic and alicyclic hydrocarbons with 10 to 19 carbon atoms employing the slow-stirring experiment. Moreover, we compiled data on molecular weight and molar volume at the boiling point as macroscopic size parameters and calculated quantum-chemical molecular size parameters. The aqueous solubility data span a range from 6 x 10(-6) M to 4 x 10(-11) M with coefficients of variation of less than 15% except for 2,6,10,14-tetramethylpentadecane (39%). The relationships of the experimentally determined solubility values with the macroscopic reflected the general trend of decreasing solubility with increasing molecular size, but discriminated between n- and branched alkanes. This indicates that these parameters do not reflect the solute-solvent interactions at the microscopic level. Interpretation of the experimentally observed solubility data based on theoretical considerations of the conformation and the constitution of alkanes is consistent with the following overall picture: For a given n-alkane in aqueous solution, the all-trans conformation is preferred over folded geometries. Within alkanes, molecular size is the primary determinant of their solubility in water, and increasing molecular size results in a decrease in water solubility mainly due to the increased free energy penalty for cavity formation in water. The solvent-accessible molecular volume and surface area appeared to be valid reflections of the molecular size.