Solubilization-emulsification mechanisms of detergency

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200 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 When the alcohol content of the oil is low and the temperature is near or only slightly above the PIT, only a small amount of surfactant need diffuse into the drop in order for hydrophilic and lipophilic properties of the surfactant-alcohol films formed there to be almost balanced. In this case, the hydrocarbon solubilization limit is exceeded and drops of oil form within the original drop, which has become a middle-phase microemulsion. Such behavior was observed, for example, when a drop containing 90 wt.% n-hexadecane was injected into a solution containing 0.05 wt.% C12E7 at 40ºC, which is near the PIT for an oil of this composition. As the oil content of the microemulsion decreased during the experiment owing to this spontaneous emulsification, and as the surfactant content continued to increase, a point was eventually reached when the hydrocarbon-to-amphiphile ratio was too low to sustain the microemulsion structure and the lamellar phase again developed as myelinic figures [76]. Finally, it is noteworthy that when drops of oil containing 75% n-hexadecane and 25% oleyl alcohol were contacted with a surfactant solution containing 0.05 Wt-% C12E7 at temperatures above 40ºC, it appeared that the first intermediate phase formed was a liquid, presumably D' [76]. That is the system apparently experienced between 30. and 400 the transition discussed in the phase behavior section above which D' replaces La as the third phase in the three-phase region bounding the region of coexistence between La and L2. Detailed phase behavior at 40º was not determined, however. The intermediate D' phase in the contacting experiment was later converted to liquid crystal (myelinic figures). 7.4. Experiments with mixtures of triolein and longchain alcohols Drops containing various mixtures of triolein and oleyl alcohol were injected into 0.1 wt.% solutions of C 12E 6 at 40ºC, which is about 10º below the cloud point temperature of dilute solutions of this surfactant [61]. As discussed in section 3 and shown in Fig. 16, the sequence of phases seen with increasing temperature in this system is similar to that found with pure triolein, the various transition temperatures being lower for higher oleyl alcohol contents. For drops containing less than 25 wt.% oleyl alcohol, a scarcely perceptible change in drop size with time was observed, an indication that the oil was being slowly solubilized into the surfactant solution. As indicated above, similar behavior was seen for hydrocarbon- alcohol drops below the PIT. Figure 16 confirms that the system is quite hydrophilic under these conditions. A liquid intermediate phase, presumably D', formed after a few minutes for a drop containing 50 wt.% oleyl alcohol (Fig. 36(b)). Careful study of the image at various depths of focus revealed that this phase did not surround the original drop but instead formed a drop in contact with the original drop but larger in diameter, as the figure shows. Later, myelinic figures began to grow outward into the aqueous solution from the D'phase (Fig. 36(c)). Ultimately, what was left of the original oil drop - its volume was only about a third of the initial value - became detached from the intermediate liquid phase and showed no further changes with time. Although this system has four components, with the result that its phase behavior cannot be adequately described by a triangular diagram similar to Fig. 32, an explanation can be given for the dynamic behavior observed. As in the hydrocarbon-alcohol systems, surfactant diffuses into the drop, and the surfactant-alcohol films which form there become more hydrophilic with time. Eventually, the boundary of the L1-L2 coexistence region is reached, and an intermediate D' phase forms as surfactant continues to diffuse into the drop. As this new phase solubilizes little triolein, the alcohol-to-triolein ratio in the original drop decreases greatly. However, the D' phase is itself in contact with the surfactant solution and accordingly experiences an increase in surfactant content with time. When the hydrophilic and lipophilic properties of its surfactant - alcohol films are balanced, the D'
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 201 Fig. 36. Video frames from an experiment in which a drop containing equal amounts of triolein and oleyl alcohol was injected into a 0.1 Wt'% C 12E 6 solution at 40ºC [61]. (a) Shortly after injection; (b) approximately 9 min later (the intermediate phase has formed); (c) about 5 min later (the intermediate phase has grown, myelinic figures are starting to form); (d) about I min later (a drop of unsolubilized oil separates from the intermediate phase); (e) the drop is never completely solubilized. Reprinted with permission of Marcel Dekker publishers.
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Solubilization-emulsification mechanisms of detergency

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