Effective Interaction of Charged Platelets in Aqueous Solution: Investigations of Colloid Laponite Suspensions by Static Light Scattering and Small-Angle X-Ray Scattering

TL;DR

This study investigates the interactions of charged disk-like Laponite particles in dilute aqueous solutions using scattering techniques, revealing a balance of short-range attraction and long-range repulsion that influences stability and gelation.

Contribution

It provides the first detailed analysis of the equilibrium scattering functions of Laponite, demonstrating their effective pair potential and limited stability at low salt concentrations.

Findings

Laponite particles interact via a short-range attractive and long-range repulsive potential.

Solutions are in a well-defined equilibrium state at low concentrations.

Increased ionic strength leads to flocculation and potential gelation.

Abstract

We study dilute aqueous solutions of charged disk-like mineral particles (Laponite) by a combination of static light scattering (SLS) and small-angle x-ray scattering (SAXS). Laponite solutions are known to form gels above a certain critical concentration that must be described as non-equilibrium states. Here we focus on the investigation by SLS and SAXS at concentrations below gelation (c < 0.016 g/L) and at low concentrations of added salt (0.001 and 0.005 mM). Thus, we have obtained the scattering function of single Laponite platelets as well as the structure factor describing their interaction at finite concentration. A detailed analysis of the combined sets of data proves that the solutions are in a well-defined equilibrium state. Moreover, this analysis demonstrates the internal consistency and accuracy of the scattering functions obtained at finite concentrations. We find that Laponite particles interact through an effective pair potential that is attractive on short range but repulsive on longer range. This finding demonstrates that Laponite solutions exhibit only a limited stability at the concentration of added salt used herein. Raising the ionic strength to 0.005 mM already leads to slow flocculation as is evidenced from the enhanced scattering intensity at smallest scattering angles. All data strongly suggest that the gelation occurring at higher concentration is related to aggregation.