Role of Acid-Base Interactions in Colloid Adhesion and Stability of Aqueous Thin Films

Experiments were performed to evaluate the hydrodynamic force required to detach colloidal particles from different types of substrates by fluids in simple shear flow. The role of DL VO and acid-base interactions on the process of adhesion and removal of colloidal particles, from model surfaces, in various fluid environments was investigated. Particles and substrates with varying degrees of hydrophobicity were used. The free energy of interaction between these surfaces in a number of fluid media was calculated using the acid-base approach. Good qualitative agreement was obtained between calculated values of free energy of interaction using the acid-base approach and the critical hydrodynamic force required to detach the particles if long range electrostatic repulsion is properly accounted for. Results for a range of particles and substrates of varying hydrophobicity and for a range of solvents can be consistently explained using this approach. Interactions between hydrophilic and hydrophobic surfaces in aqueous medium at various pH and ionic strengths as well as in some organic solvents were measured using atomic force microscopy. In hydrophilic systems the forces observed were found to be well described by DL VO theory at large separation distances. Very long-range hydrophobic forces were not observed in hydrophilic hydrophobic systems. The interaction between two hydrophobic surfaces was dominated by the long-range attraction due to hydrophobic forces. This interaction was found to be sensitive to the type of substrate as well as to the pH and electrolyte concentration. Measured pull-off forces showed poor reproducibility. However, average values showed clear trends and were used to estimate interfacial energies or work of adhesion for all systems studied by means of the Derjaguin approximation. These values were compared to those calculated by the acid-base approach. Good qualitative agreement was obtained, giving support for the usefulness of this approach in estimating interfacial energies between surfaces in liquid media. A comparison of the measured adhesion force with hydrodynamic detachment experiments showed good qualitative agreement. The wetting behavior of model systems and crude oils was investigated by means of the "extended DLVO theory". The model oil is octadecane and the acidic or basic character of the oil is obtained by addition of a fatty acid or an amine. Good agreement between AFM measurements and acid-base calculations was obtained only for pure octadecane. We also applied a novel methodology to use contact angle measurements to compute the acid-base component of disjoining pressure in crude oil systems. The calculated disjoining pressure isotherms were compared with experimental data. from atomic force microscopy measurements. The method was shown to work for three crude oils tested.