Solubilization-emulsification mechanisms of detergency

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188 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 Upon addition of the oil, the drops of the L1 phase rapidly solubilized the hydrocarbon to form an oil-in-water microemulsion containing an appreciable quantity of hydrocarbon. After depletion of the larger surfactant-containing drops, a front developed as smaller drops were incorporated into the microemulsion phase. This behavior is shown schematically in Fig. 21. Unlike the experiments carried out below the cloud point temperature, an appreciable solubilization of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the phase inversion temperatures of the hydrocarbons with C12E5 (45ºC for tetradecane and 50ºC for hexadecane), two intermediate phases formed when the initial dispersion of L1 drops in the water contacted the oil. One was the lamellar liquid crystalline phase Lα (probably containing some dispersed water). Above it was a middle-phase microemulsion. In contrast to the studies below the cloud point temperature, appreciable solubilization of hydrocarbon into the two intermediate phases, shown 4 min after contacting in Fig. 22, was observed. A diagram of the phenomena observed is shown in Fig. 23. A similar progression of phases was found at 35ºC using n-decane as the hydrocarbon. At this temperature, which is near the phase inversion Fig. 21. Schematic diagram showing conversion of the L 1 phase into an oil-in-water microemulsion at temperatures above the cloud point [72]. The symbol me denotes a microemulsion. Reprinted with permission of Academic Press. Fig. 22. Video frame showing intermediate phases 4 min after contact in C 12E 5-water-n-tetradecane system at 45ºC near the PIT [72]. Reprinted with permission of Academic Press. Fig. 23. Schematic diagram showing the conversion of L 1 phase into a middle-phase microemulsion and a liquid crystal dispersion, for the experiment depicted in Fig. 22 [72]. Reprinted with permission of Academic Press. temperature of the water-C12E5-decane system, the existence of a two-phase dispersion of Lα and water below the middle-phase microemulsion was clearly evident. To compare the rate of tetradecane solubilization at temperatures below and near the phase inversion temperature, verticalcontacting experiments were performed between the L1 phase and oil at 40 and 48ºC. In both cases, the surfactant-rich phase was the equilibrium phase that separated from a 10% aqueous solution at that temperature. At 40ºC a liquid crystalline phase and an oil-in-water
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 189 micro emulsion formed between the L1 and oil phases. At 48 º C, a similar phase progression was observed, with a middle-phase microemulsion forming in place of the oil-in-water microemulsion. As shown in Fig. 24, plots of the position of the oil-micro emulsion interface versus the square root of time were straight lines in both cases indicating diffusion-controlled mass transfer. In Fig. 24, an arbitrary constant has been added to each position for ease of comparison, i.e. x = 0 is not the initial contact position. At any given time, the relative slopes of the two lines are indicative of the relative rates of oil solubilization. In this case, the rate of solubilization near the PIT is 2.2 times greater than at 40ºC. Well above the phase inversion temperatures for both hexadecane and tetradecane, 1 wt.% C12E5 exists as a dispersion of liquid crystal Lα in water. Rapid movement of the liquid crystal to the oil occurred upon contacting, causing extensive spontaneous emulsification of water in the oil phase. Eventually a layer depleted in liquid crystal formed near the oil interface. Figure 25 shows the spontaneous emulsification that occurred. Interpretation of the phenomena Fig. 24. Variation of oil- microemulsion interface with time at 40ºC and 48ºC following contact of the L 1 phase with oil for the C 12E 5-water-n-tetradecane system. Fig. 25. Video frame showing spontaneous emulsification observed 18 min after initial contact in the C 12E 5-water-n-hexadecane system at 60ºC [72]. Reprinted with permission of Academic Press. by diffusion path analysis indicated that the oil was being converted into a waterin-oil microemulsion at this high temperature; this means that very little solubilization of oil into the aqueous phase was taking place. The spontaneous emulsification occurred due to the passage of the oil-phase diffusion path segment across the twophase water-micro-emulsion region, as shown in Fig. 26. Experiments similar to those described above were also performed using C12E4 as the surfactant [18]. This surfactant is more hydrophobic than C12E5 and therefore has a lower Fig. 26. Schematic diffusion path for the experiment depicted in Fig. 25. Point d represents the composition of the initial water-surfactant mixture; HC, S, and tie denote hydrocarbon, surfactant, and microemulsion. The last is oil-continuous in this case. Some multiphase regions are not shown [72]. Reprinted with permission of Academic Press.
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Solubilization-emulsification mechanisms of detergency

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