Electrophoretic behavior of α-Zr(HPO4)2 and its phosphonated derivatives

Electrophoretic mobility studies demonstrate that the charging mechanism of zirconium phosphate–phosphonate materials is similar to that involved in ion exchange; i.e. the ionization of hydrogenphosphate groups. Participation of surface ZrOH at the crystallites edges is also central. An isoelectric point at pHiep≈1.3 was determined for nearly all α-Zr(HPO4)2−x(C6H5PO3)x. Zirconium pyrophosphate, obtained by thermal treatment of these materials, exhibits less acid properties; the iep shifts to 3.0±0.2. On the other hand, materials intercalated with 3-aminopropyltriethoxysilane (APTEOSi) are noticeably basic, with an iep at pHiep=9.6±0.2. In this case the behavior is governed by the ionization of the amino groups derived from APTEOSi. Thermal treatment of intercalated materials leads to the formation of hydrous silica pillars, with release of ammonia, that inhibits the condensation of the hydrogenphosphate groups. Therefore, the acid properties of these pillared solids are close to those of the host materials. The pillars themselves may also contribute to the observed electrophoretic behavior.