Destabilization of Titania Nanosheet Suspensions by Inorganic Salts: Hofmeister Series and Schulze-Hardy Rule

Abstract

Ion specific effects on colloidal stability of titania nanosheets (TNS) were investigated in aqueous suspensions. The charge of the particles was varied by the pH of the solutions, therefore, the influence of mono- and multivalent anions on the charging and aggregation behavior could be studied when they were present either as counter or co-ions in the systems. The aggregation processes in the presence of inorganic salts were mainly driven by interparticle forces of electrostatic origin, however, chemical interactions between more complex ions and the surface led to additional attractive forces. The adsorption of anions significantly changed the surface charge properties and hence, the resistance of the TNS against salt-induced aggregation. On the basis of their ability in destabilization of the dispersions, the monovalent ions could be ordered according to the Hofmeister series in acidic solutions, where they act as counterions. However, the behavior of the biphosphate anion was atypical and its adsorption induced charge reversal of the particles. The multivalent anions destabilized the oppositely charged TNS more effectively and the aggregation processes followed the Schulze-Hardy rule. Only weak or negligible interactions were observed between the anions and the particles in alkaline suspensions, where the TNS possessed negative charge.

Effect of Ionic Compounds of Different Valences on the Stability of Titanium Oxide Colloids

“…The PZC values of numerous titanium oxide particles were determined with various techniques in the past decades [23,31,[53][54][55][56][57][58] and they were found to be in the range of 4-7 [51]. In addition, adsorption of ionic species may also change the charge and the position of the PZC of the titanium oxide materials [19,59]. To demonstrate this feature, the pH profile of the electrokinetic potentials (equal to zeta potential) of titanate nanowires and titania nanosheets are shown in Figure 1 [56,59].…”

“…In addition, adsorption of ionic species may also change the charge and the position of the PZC of the titanium oxide materials [19,59]. To demonstrate this feature, the pH profile of the electrokinetic potentials (equal to zeta potential) of titanate nanowires and titania nanosheets are shown in Figure 1 [56,59].…”

“…However, this observation is only valid for indifferent electrolytes, that is, adsorption of ions is negligible, which do not change the position of the PZC. The presence of anions or cations with significant affinity towards the surface may induce different changes in the surface charge density and the corresponding surface potential [59]. 2) as a function of the pH at 1 mM ionic strength.…”

“…Salt-induced aggregation of titanium oxide materials was investigated by several authors and CCC values between 1-100 mM [7,59,[62][63][64][65][66] were determined by various methods. For instance, a colloid stability study performed on spherical titania particles was carried out at different pH in KCl solutions by following particle aggregation with light scattering technique [67].…”

“…Nevertheless, it was also found that ion specific effects for positively charged titania below the PZC are not significant once the surface underwent calcination [68]. Electrokinetic potentials and stability ratios were determined with titania nanosheets under acidic and alkaline conditions, that is, with negatively and positively charged surfaces, in the presence of various monovalent anions by light scattering methods [59]. Therefore, the anions were applied as counterions below the PZC, which was reported to be 5.2 [31] and as coions above it.…”