Hofmeister Effects in the Stability and Electrophoretic Mobility of Polystyrene Latex Particles

Abstract

Specific ionic effects on numerous solid-aqueous solution interfaces have usually been ranked in the socalled Hofmeister series. Ion specificity has experimentally been manifested in many ways. In this work, the colloidal stability and electrophoretic mobility of three different polystyrene latex samples are analyzed in the presence of different electrolytes. The effects on their colloidal stability and electrokinetic behavior provoked by four anions (SO 4 2-, Cl -, NO 3 -, and SCN -) and three cations (NH 4 + , Na + , and Ca 2+ ) located in different positions in the Hofmeister series are shown. Some of these ions specifically modified the pair potential interaction of the colloidal particles yielding results not explained by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. A modification in the repulsive electrostatic term of this theory is then proposed to consider the ionic specificity. In addition, general but qualitative explanations about the subjacent mechanisms involved in the Hofmeister effects are also summarized.

Hofmeister effects on the colloidal stability of an IgG-coated polystyrene latex

“…Consequently, the surface charge density (σ 0 ) changed as the pH value varied. In fact, σ 0 value was 18.3 ± 1.2 µC/cm 2 at pH 4 and 4.0 ± 0.6 µC/cm 2 at pH 10, respectively [21]. The mean diameter, obtained by TEM, was 320 ± 15 nm, with a polydispersity index extremely close to unity (1.007).…”

“…[21]. It had an amphoteric nature with positive charge in the pH range 4 to 10 and an isoelectric point (pI ) at pH 10.5.…”

Electrostatic heteroaggregation regimes in colloidal suspensions

“…As can be seen, the CCC is approximately 180 mM for cationic particles and 270 mM for anionic particles. It should be mentioned that ion-specific effects (also known as Hofmeister effects) could affect the value of the CCC [49], although potassium bromide (KBr) is generally regarded as an unspecific electrolyte [50].…”

Adsorption of different amphiphilic molecules onto polystyrene latices

“…However, strong acid groups are difficult to determine directly by means of conductometric titration. Therefore an alternative method was used for Lx(SO 3 H): adsorbing a cationic lipid (distearoyl dimethylammonium bromide, DSDMA) (Fluka) in a 50% (v/v) ethanol/water mixture as explained elsewhere [15,16]. The calculated values of the maximum surface charge density are presented in Table 1.…”