Experimental Solvation of KCl and NaCl in N-Methylacetamide

by Mazzanti, Christopher L.; Koeppe, Roger E., II

N-methylacetamide (NMA) as a solvent has attracted considerable interest due to its similarity to the amide bond in proteins. For the modeling of ion/protein interactions, therefore, it is important to establish exptl. free energies of transfer of selected ions and salt from water to NMA. For this purpose, we sought to measure the solubilities of KCl and of NaCl in pure, dry NMA as a function of temp. NMA is a difficult solvent because it is hygroscopic and furthermore melts at 28-32 °C. Com. (99%) NMA was therefore purified by a classical method of fractional freezing and its purity was monitored by f.p. anal. NaCl and KCl were oven dried and then were equilibrated 2-4 wk in satd. solns. of purified NMA at several temps. between 30-50 °C. Using gravimetric methods, we find solubilities of 0.32 +/- 0.01 M for NaCl and 0.09 +/- 0.01 M for KCl in NMA at 30 °C, increasing to 0.40 M and 0.16 M, resp., at 50 °C. The corresponding transfer free energies from water to NMA at 30 °C are 2.3 kcal/mol for NaCl and 1.7 kcal/mol for KCl. These results will be compared with theor. predictions from ab initio calcns. and other methods.