ABSTRACT
II. Almost-Pure Organic Solvents 345
IV. Mixtures Rich in Water 355 A. Surface charging 356 B. Dissociation of weak acids 359 C. Zeta potential 362
v . Solvent Water 363 A. Temperature 363 B. Water at extremely high ionic strengths 364 c. Heavy water 364 References 366
The surface-charging behavior of silica in water has been extensively studied. Compared with metal oxides, silica shows a very low pristine point of zero charge (PPZC): most sources report PPZC of silica at pH between 2 and 3. Therefore, the surface charge density a0 and the ~ potential of silica is negative over the usually studied pH range, and the positive branch of surface charging and electrokinetic curves is rarely visited. In other words, silica is more acidic than most metal oxides. This property is attributed to the presence of surface hydroxyl groups which behave like a weak acid:
• pK. is calculated from the fraction of ionized surface hydroxyl groups, thus the total concentration of surface hydroxyl groups Ns must be known. Although most authors agree that there are a few such groups per nm2 of fully hydroxylated silica surface, Ns can vary from one sample of silica to another, and most authors do not determine the Ns themselves but rather they accept Ns values from literature.
