Hydrophobicity vs Hydrophilicity:  Effects of Poly(ethylene glycol) and tert-Butyl Alcohol on H₂O as Probed by 1-Propanol

The enthalpic interaction between 1-propanol (1P) molecules, , was evaluated in 1P−poly(ethylene glycol) (PEG)−H₂O and 1P−tert-butyl alcohol (TBA)−H₂O ternary mixtures. The model-free and experimentally accessible quantity, , indicates the effect of an additional 1P on the actual enthalpic situation of 1P in the mixture. It was shown earlier that the composition dependence of reflects the process how 1P modifies H₂O. This pattern changes in the presence of a third component, PEG or TBA. The effects of PEG or TBA on the molecular organization of H₂O were elucidated from these induced changes. Together with previous similar studies for the effects of methanol (ME), 2-propanol (2P), ethylene glycol (EG), and glycerol (Gly), we suggest a method and hence a possible scaling for sorting out hydrophobicity vs hydrophilicity of these alcohols by the changes induced to the loci of the maxima in . We show that hydrophilicity scales with the number of oxygen, regardless of whether O is the ether −O− or the hydroxyl −OH. Hydrophobicity also scales with the number of carbon atoms for alcohols without a methyl group. For those with methyl groups, the hydrophobicity seems proportional to the total number of carbon with a different proportionality factor from those without methyl group.