ELECTROKINETIC BEHAVIOR OF FLUORIDE SALTS AS EXPLAINED FROM WATER-STRUCTURE CONSIDERATIONS

Unlike the other silver halides, silver fluoride is positively charged in its saturated solution as determined by nonequilibrium electrophor esis measurements. In the absence of surface hydrolysis reactions, oth er fluoride salts (LiF, CaF2, and MgF2) also are positively charged in their saturated solutions. Furthermore, the electrokinetic behavior o f these fluoride salts is rather insensitive to the fluoride ion activ ity in neutral or acidic solutions, and reversal of the sign of the su rface charge by fluoride addition is not possible. Based on FTIR trans mission spectra to describe the water structure of ionic solutions, in situ FTIR/internal reflection spectroscopy (FTIR/IRS) has been used t o spectroscopically characterize interfacial water at fluoride salt su rfaces. The experimental spectra were examined by consideration of the O-H stretching region (3000-3800 cm(-1)) associated with the vibratio nal spectra of interfacial water. These results reveal a unique hydrat ion state for fluorides and explain the anomalous electrokinetic behav ior of fluoride salts such as LiF, CaF2, and MgF2, which show an unexp ected insensitivity to the fluoride ion concentration in solution. It appears that this insensitivity is due to the formation of strong hydr ogen bonding of the fluoride ions with water molecules. This hydration state prevents the accommodation of excess fluoride ions at surface l attice sites and accounts for the observed electrokinetic behavior. (C ) 1997 Academic Press.