Nitrofurantoin is chemically N-(5-nitro-2-furfurylidene)-1-aminohyantoin. Its molecule consists of several polar groups, C=O, NH, C=NN, NO2, and COC, as well as an apolar cyclic ring. This reagent is used in pharmacy in the treatment of urinary-track infections. It is frequently prescribed in the dispersed form. It is obvious that the stability or instability of dispersed systems is of great practical importance, and some theoretical predictions of this behavior can be made if one knows the surface free energy components of the dispersed solid and the dispersant liquid. The presence of the above-mentioned apolar and polar groups in the nitrofurantoin molecules gives rise to the Lifshitz-van der Waals, γLWs, and acid-base, γABs], free energy interactions (Van Oss et al., Langmuir 4, 884 (1988); J. Colloid Interface Sci. 128, 313 (1989); Chem Rev. 88, 927 (1988)). The acid-base interactions are due to electron donor, γ-s, and electron acceptor, γ+s, interactions, and in many cases they are hydrogen bonding (Van Oss et al., Langmuir 4, 884 (1988); J. Colloid Interface Sci 128, 313 (1989); Chem Rev. 88, 927 (1988)). They may contribute to, in addition to the electrostatic repulsive term, the repulsive forces between particles suspended in a liquid (Van Oss et al., Chem Rev. 88, 927 (1988)). Copyright 1993, 1999 Academic Press