Effect of sodium pyrophosphate and understanding microstructure of aqueous LAPONITE(R) dispersion using dissolution study

TL;DR

This study explores how sodium pyrophosphate influences the microstructure and ergodicity breaking in aqueous LAPONITE(R) dispersions, revealing the microstructural role of attractive interactions and gel formation through dissolution, rheology, and imaging experiments.

Contribution

It provides new insights into the microstructural mechanisms and effects of sodium pyrophosphate on LAPONITE(R) dispersions, especially regarding gel formation and ergodicity breaking at different pH levels.

Findings

Sodium pyrophosphate dissolves colloidal dispersions at pH 10.

Attractive edge-to-face interactions form fractal gel networks.

Water and pyrophosphate effects are pH-dependent, with no effect at pH 13.

Abstract

We investigate physical origin of ergodicity breaking in an aqueous colloidal dispersion of synthetic hectorite clay, LAPONITE(R), by performing dissolution and rheological experiments with monovalent salt and tetrasodium pyrophosphate solution. We also study the effect of interface, nitrogen and paraffin oil on the same. Dissolution experiments carried out for dispersions with both the interfaces show similar results. However, for samples with nitrogen interface, all the effects are observed to get expedited in time compared to paraffin oil interface. When kept in contact with water, 1.5 wt. % and 2.8 wt. % colloidal dispersion at pH 10 swell at small ages, while do not swell at large ages. The solution of tetrasodium pyrophosphate, interestingly, dissolves the entire colloidal dispersion samples with pH 10 irrespective of the clay concentration. Experiments carried out on colloidal dispersions prepared in water having pH 13 demonstrate no effect of water as well as sodium pyrophosphate solution on the same suggesting a possibility of the presence of negative charge on edge at that pH. We believe that all the behaviors observed for samples at pH 10 can be explained by an attractive gel microstructure formed by edge-to-face contact. Furthermore, the absence of swelling in old colloidal dispersion at pH 10 and dissolution of the same by sodium pyrophosphate solution cannot be explained by merely repulsive interactions. This behavior suggests that attractive interactions play an important role in causing ergodicity breaking in the colloidal dispersions at pH 10 at all the ages irrespective of the clay concentration. We substantiate the presence of fractal network structure formed by interparticle edge-face association using rheological tools and cryo-TEM imaging. We also conduct a comprehensive study of the effect of sodium pyrophosphate in the sol-gel transition of LAPONITE(R) dispersion.