Hydrophobic (interaction) chromatography
Hydrophobic interactions have been described as “the unusually strong attraction between non-polar molecules and surfaces in water” (Israelachvili, 1985). For two contacting methane molecules the attraction energy is ca. 6-fold higher in water than the van der Waals interaction energy in vacuum. This energy, which has been estimated to be ca. −8.5kJ mol−1 for two methane molecules (Ben-Naim et al., 1973; Israelachvili, 1985) is due to the extrusion of ordered water on two adjacent hydrophobic surfaces into lessordered bulk water with a concomitant increase in entropy. This entropy driven attraction between non-polar groups in water (Israelachvili, 1985; Israelachvili and Pashley, 1984; Kauzmann, 1959; Lewin, 1974; Tanford, 1973) is the basis for hydrophobic interaction chromatography as summarized in several reviews (Arakawa and Narhi, 1991; Eriksson, 1989; Halperin et al., 1981; Hjerten, 1981; Hofstee and Otillio, 1978; Hubert and Dellacherie, 1993; Jennissen, 1988; Mohr and Pommerening, 1986; Ochoa, 1978; Oscarsson, 1997; Shaltiel, 1984; Yon, 1977). For an interesting distinction between definitions of “hydrophobicity” and “hydrophobic interactions” see the technical comment by Dill (Dill, 1990). Recently the terms ultra-hydrophilic and ultra-hydrophobic have extended the above range of definitions to new applications in the area of biomaterials (Jennissen, 2001).