Kinetics of Adsorption for Hydrophobically Modified Poly(Acrylic Acids) at Cyclohexane/Water Interfaces

Abstract

Hydrophobically modified poly(acrylic acids) (HMPAA’s) are commonly used as primary, pH sensitive, stabilizers for oil-in-water emulsions. It has been shown that electrosteric stabilization is one of the mechanisms. An important factor in steric stabilization is the breadth of the extension of stabilizing molecules from the discontinuous phase interface into the continuous phase. It must be substantial. This dictates that the stabilizing molecules be of substantial hydrodynamic volume. However, polymers with large hydrodynamic volumes diffuse slowly through solution (Brownian diffusion coefficients on the order of 10⁻¹² to 10⁻¹³ m²/s). This can be a restriction to their use as emulsifiers, since the kinetics of emulsion coalescence begin to compete with the kinetics of interfacial adsorption of the stabilizing species. With these things in mind, we have developed a technique, based on drop volume tensiometery, which can be used to determine the “adsorption/diffusion” coefficients that characterize the kinetics of polymers adsorbing at oil/water interfaces. We have previously used this technique to show that increased hydrophobic content and increased crosslink density enhance adsorption efficiency for HMPAA’S. The current work is a pH dependence study which shows that neutralizing HMPAA in aqueous solution decreases its adsorption efficiency.