Effect of surfactants, pH and water hardness on the surface properties and agglomeration behavior of engineered TiO2 nanoparticles

Abstract

The influence of sodium dodecyl sulfate (SDS) on the stability of TiO₂ engineered nanoparticle (ENP) dispersions is investigated under various pH conditions, SDS and divalent cation concentrations. Based on different scenarios and systematic measurements of surface charges and z-average sizes, a detailed mechanistic approach is proposed assuming surfactant adsorption/desorption, charge inversion, cation bridging, specific adsorption, hydrophobic effects, agglomeration and disagglomeration. Adsorption of SDS on oppositely charged TiO₂ nanoparticles is found to strongly modify their stability. Formation of large agglomerates can be achieved via several routes: i) in the absence of SDS and by adjusting the pH close to the TiO₂ point of zero charge, and ii) in the presence of SDS at a concentration where the positive surface charges of the TiO₂ nanoparticles are counterbalanced by the SDS negative charges (charge neutralization). It is also found that hydrophobic interaction mechanisms between the SDS molecules can also promote the formation of large structures. The influence of pH variations on TiO₂–SDS electrostatic complexes, formed at low pH, indicates that an excess of SDS is required to prevent the formation of large agglomerates upon pH changes. At low or intermediate SDS concentrations, TiO₂ stability is governed by the subtle interplay of SDS adsorption and TiO₂ surface charge acid–base properties. The presence of divalent electrolytes (water hardness) is found to reduce the SDS amount adsorbed on TiO₂ ENPs and to promote the formation of large micron-sized agglomerates by cation bridging. Our results also indicate that the dispersion preparation protocol is an important issue to consider when ENPs and SDS mixtures have to be prepared and that for the formation of “individually” coated and stable nanoparticles, punctual addition of SDS is required at concentrations higher than the isoelectric point.