Hydrophobic Interactions in Concentrated Colloidal Suspensions: A Rheological Investigation

Rheological investigations have determined the influence of particle hydrophobicity and solution pH on particle interactions in concentrated suspensions of colloidal silica (advancing water particle contact angles in the range ∼35° to ∼90° have been explored and obtained through surface chemical modification). Suspensions of hydrophilic silica (φ = 0.55) showed negligible viscoelasticity and a pH dependent apparent yield stress. The inverse relationship between the yield stress and the particle ζ-potential squared confirmed that electrical double layer and dispersion forces control particle interactions. On increasing the particle contact angle of silica there is a substantial increase in suspension rheology, i.e., pronounced viscoelasticity and apparent yield stresses, thus suggesting the action of an attractive hydrophobic force. Attempts have been made to deconvolute the various interaction forces, quantify the hydrophobic interaction, and explain the apparent pH dependent non-DLVO behavior.