Apparent Hamaker Constants and Characteristic Interaction Distances Governing Sol–Gel Transitions in Aqueous Smectite Clay Dispersions

Aqueous smectite clay dispersions undergo sol–gel transitions and form three-dimensional networks at relatively low clay concentrations. However, the clay and electrolyte concentrations required for gelation differ markedly among clay species, and the physical origin of these differences remains unclear. In this study, sol–gel state diagrams were constructed for four smectite clays based on rheological measurements, and DLVO analysis was subsequently applied to the experimentally determined sol–gel transition boundaries using the corresponding zeta potentials and Debye lengths. The apparent Hamaker constant, AH,app, decreased with increasing NaCl concentration for all clays. In addition, ln(AH,app/10−20 J) showed approximately linear relationships with the reciprocal Debye length, 1/LD, with distinct trends in the low- and high-salt regions. A characteristic interaction distance, H*, was defined as the negative of the fitted slope. H* was larger and more strongly clay-dependent in the low-salt region, whereas it became smaller and less clay-dependent in the high-salt region. The crossover in H*, observed at approximately 0.01–0.03 mol/L NaCl, occurred in the same broad concentration range as the onset of decreased transmittance and previously reported rheological changes. Because these datasets were acquired at non-identical clay volume fractions, this agreement should be regarded as qualitative rather than as a direct one-to-one correspondence.