Is a Methyl Group Always Hydrophobic? Hydrophilicity of Trimethylamine-N-oxide, Tetramethyl Urea and Tetramethylammonium Ion

We have developed what we call the 1-propanol (1P) probing methodology. By using it, we determined the relative hydrophobicity/hydrophilicity indices of 2-butoxyethanol, a typical hydrophobe; urea, a typical hydrophile; and trimethylamine-N-oxide, an amphiphile. By comparing these indices with those for other solutes studied earlier, including tetramethyl urea, tetramethylammonium ion, mono-ols, and polyols, we suggest that methyl groups attached to an N atom, or N-methyl groups, do not promote hydrophobicity but, rather, enhance hydrophilicity.