Some common solvents include acetone, methanol, tetrahydrofuran, dimethylsulfoxide, and water among countless others.
However, as with many biological rules exceptions are common and hydrophilic residues are frequently found to be buried in the native structure and vice versa.
Other measures are for example: Lee B, Richards F. (1971) The interpretation of protein structures: estimation of static accessibility.
Science 221:709-713 Chakravarty S, Varadarajan R. (1999) Residue depth: a novel parameter for the analysis of protein structure and stability.
Pintar A, Carugo O, Pongor S. (2003) Atom depth in protein structure and function.
Hamelryck T. (2005) An amino acid has two sides: A new 2D measure provides a different view of solvent exposure.