Linking the physics of the relaxation behavior of visco-elastic fluids as they form arrested gel states to the underlying chemical controls is essential for developing predictive controls. In this study, laponite suspensions at concentrations of 3 wt%, are studied as model systems to probe the influence of salt-induced relaxation behavior arising from the assembly of laponite discs. X-Ray Photon Correlation Spectroscopy (XPCS) measurements show that laponite suspensions prepared in the presence of 5 mM concentrations of CaCl2, MgCl2 and CsCl salts accelerate the formation of arrested gel states, with CaCl2 having a significant impact followed by CsCl and MgCl2 salts. The competing effects of ion size and charge on the relaxation behavior are noted. For example, the relaxation times of laponite suspensions in the presence of Mg2+ ions are slower compared to Cs+ ions despite the higher charge, suggesting that cation size dominates in this scenario. Faster relaxation behavior of laponite suspensions in the presence of Ca2+ ions compared to Cs+ ions shows that higher charge dominates the size of the ion. The trends in the relaxation behavior are consistent with the cluster formation behavior of laponite suspensions and the electrostatic interactions predicted from MD simulations. Charge balance is achieved by the intercalation of the cations at the negatively charged surfaces of laponite suspensions. These studies show that the arrested, gel-state of laponite suspensions is accelerated in the presence of salts, with ion sizes and charges having a competing effect on the relaxation behavior.

Probing accelerated salt-induced relaxation dynamics of Laponite suspensions using X-ray photon correlation spectroscopy measurements and molecular dynamics simulations

ACS Fall 2020 Virtual Meeting

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