(195i) Interaction Forces between Colliding Emulsion Drops (Oil or Water) Coated with Non-Ionic Surfactants (C12E5 and PGPR)

We measured the interaction forces between colliding emulsion drops (Oil or Water) coated with non-ionic surfactants C₁₂E₅ (penta-ethylene-glycol-mono-dodecyl-ether) and PGPR (polyglycerol polyricinoleate). Non-ionic surfactants are often used in combination with ionic surfactants in detergency, oil recovery and mineral flotation applications because they increase surface activity of surfactant mixtures. The emulsion systems tht we studied are oil-in-water (O/W) and water-in-oil (W/O) in the cases of C₁₂E₅ and PGPR respectively. The oils used are Tetradecane and Canola oil for O/W and W/O emulsions respectively. The interaction force between Tetradecane drops were measured in the range of pre-micellar compositions of C₁₂E₅ in various salt solutions. The Tetradecane drops were charged even though the surfactant was non-ionic in nature. The surface potential of the oil drops decreased with increase in C₁₂E₅ concentration in agreement with the experimentally measured electrophoretic mobilities and zeta potentials. The surface potential decreased with increasing electrolyte concentration that confirmed the presence of a charged oil-water interface. We also measured the non-equilibrium film drainage between tetradecane drops coated with C₁₂E₅ as a function of magnitude of the force of interaction and velocity. We found that the tetradecane drops were stable against coalescence even when the drops were deformed on the order of their radii. This observation is in contrast to some existing experiments in literature.

Unlike the case of O/W emulsion with C₁₂E₅, in the case of W/O emulsion where water drops were emulsified into Canola Oil (with PGPR), we found that the interaction forces were dominated by steric forces. PGPR is a fat soluble non-ionic surfactant widely used in chocolate industry. Hydrophile lipophile balance (HLB) number for PGPR is between 2 and 4; hence it is known to be the best food grade emulsifier for stabilizing water-in-oil emulsions. Interfacial tension measurement and adsorption isotherm fitting showed that the water-canola oil interface gets saturated with PGPR for concentrations above 0.2 wt%. In order to probe meta-stability of emulsified water drops coated with PGPR for concentrations below 0.2 wt%, we measured the interaction force between the drops as a function of PGPR concentration and velocity of collisions. We also measured forces between hydrophilic rigid solid surfaces coated with PGPR in canola oil in order to get qualitative and quantitative understanding of repulsive forces resulting from adsorption of PGPR. There was a long range attraction between the hydrophilic rigid surfaces for PGPR concentrations below 0.02 wt% due to insufficient coverage of PGPR. Steric repulsion was observed for PGPR concentration of 0.05 wt% and above. Estimated thickness of adsorbed brush layer of PGPR on hydrophilic surfaces was found to increase from 10 to 64 nm when PGPR concentration increased from 0 to 0.2 wt%. In the case of force between PGPR coated water drops in canola oil, hysteresis between the approach and retraction curves increased with increase in the maximum force to which the water drops are pushed to. We observed that during the retract phase of drop collision there was a kink in the force curves and sudden jump out. We attributed such a force jump outs to bridging and extension of adsorbed PGPR molecules on water drops. In order to rule out the role of water transport in the form of PGPR reverse micelles to observed force jump outs, we carried out water drop collision experiments with 1 wt% salt in water. Salt was introduced to arrest the water transport across oil layer. However, we found similar force jump outs even in the presence of salt. This observation indicated that bridging and extension of PGPR molecules was the mechanism behind the force jumps observed during retraction phase of drop collision.

The above finding relevant to O/W and W/O emulsions have significant implications in the stability of emulsions in food, personal care and detergent industries.