Stability study of single water-in-oil-in-water and oil-in-water-in-oil globules in microcapillary
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Description
A novel technique for producing single water-in-oil-in-water (W$\sb1$/O/W$\sb2$) and oil-in-water-in-oil (O$\sb1$/W/O$\sb2$) type globules is used to study their stability microscopically, by bringing the internal droplets in contact with each other and with the external interface between water and oil. Each experiment lasted twenty four hours unless there was rupture within a shorter period, and the parameters that were studied are pH, ionic strength and the concentration of surfactants. Under all experimental conditions that produced limited stability, the internal droplets did not coalesce among themselves, and when the globule broke down, it was because the internal droplets coalesced with the external phase. Theoretical analyses are based on models with an 'effective thickness' of adsorbed surfactant layer, a parameter which is used in the calculation of interaction energy between particles. Theoretical results are in agreement with experiments qualitatively For W$\sb1$/O/W$\sb2$ globules, it was found that in the absence of surfactant Span 80 in the oil phase the globule was found intrinsically unstable at all pHs and salinities. Experimental results are explained through a model for the interaction energy, van der Waals and Coulombic, between a single internal water droplet and the external aqueous phase as well as between two internal water droplets. It is found that the effective thickness of the adsorbed layer of surfactant determines whether the net interaction energy is attractive or repulsive Similarly, O$\sb1$/W/O$\sb2$ globules were made and studied in a microcapillary. Experimental studies showed that among many factors that affect the stability of globules the concentration of surfactant Span 80 is the primary influence. A simple sphere-plate model is developed. The electrical double layer repulsion and van der Waals attraction are taken into account for the interaction of the inner oil compartments with the outer oil phase across the aqueous medium. Theoretical results showed that the thickness of the adsorbed surfactant has much stronger affect on the interaction energy curve than pH and electrolyte do A comparison of the stability of both W$\sb1$/O/W$\sb2$ and O$\sb1$/W/O$\sb2$ globules under similar conditions revealed that Span 80 has a far superior ability to stabilize globules than Tween 80. The two surfactants examined had little or negative synergistic action in stabilizing globules. The coalescence of water droplets in oil phase is also investigated visually in a microcapillary. It is found that the coalescence is controlled by the nature and concentration of surfactants used in both oil and water phases. Water droplets can be stabilized by oil soluble surfactant Span 80 when its concentration is over certain critical point. Other surfactants, such as Tween 80 and SDS, do not have the ability to stabilize the water droplets against coalescence. The experimental results show that pH value and ionic strength have no obvious effects on the coalescence. Interaction energy is calculated by use of the surfactant coated model