Solubilization-emulsification mechanisms of detergency

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198 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 containing at least 50 wt.% hydrocarbon, were injected into dilute aqueous solutions of pure non-ionic surfactants at temperatures below their cloud points [60]. PIT measurements for these systems were given previously (Fig. 15). A general pattern of behavior was observed. At temperatures below the PIT, as given by Fig. 15, no intermediate phase was seen at any time during the experiment and the drop volume decreased very slowly with time owing to solubilization of the alcohol and hydrocarbon into the micellar solution. Above the PIT, the behavior was similar to that described in the preceding section for the C12E5-water-oleyl alcohol system. That is, the drop would swell, and, at a particular time, the lamellar liquid crystalline phase could be seen growing as myelinic figures. However, the myelinic figures were shorter, smaller in diameter and more numerous, and they grew faster than in the pure alcohol system. The differences can be seen by comparing Fig. 35 for a drop initially containing 85 wt.% hydrocarbon and 15 wt.% alcohol immersed in an aqueous solution containing 0.05 Wt-% C12E8, with Fig. 33 for the pure alcohol system. The low surfactant concentration for the mixed oil experiment is typical of those used in household laundry processes. Coexistence curves for the L1-L2 region were determined at 30ºC for systems containing n-dodecyl heptaoxyethylene monoether (C12E7) and oils with 50% and 75% n-hexadecane, respectively [60]. In both cases the curves extended to water contents of 75-80 wt.%. Contacting experiments for various initial drop sizes and surfactant concentrations in the latter system confirmed that the dynamic behavior was consistent with Eq. (6). Here, too, liquid crystalline intermediate phases were seen for drops in contact with solutions containing only 0.05 wt.% surfactant. It was also noted that the rate of swelling of the drops increased markedly as the limiting tie line was approached. Since the coexistence curves exhibited nearly constant alcohol-tosurfactant ratios near the limiting tie lines, this behavior was expected as little surfactant must diffuse into the drop for it to experience a substantial increase in volume. The basic analysis leading to Eq. (6) confirmed quantitatively that rapid swelling should occur for these conditions. Values of the parameter K s were found from the phase behavior data to be 0.313 and 0.0516 for the two systems. That is, for a given initial drop size and surfactant concentration, formation of the liquid crystal occurred more rapidly in the system containing less alcohol, presumably because less surfactant had to diffuse into the drop to balance hydrophilic and lipophilic properties of the surfactant films -to the extent that conditions favorable for formation of the lamellar phase were created. Indeed, a general result of the contacting experiments with various surfactants and oils was that more time was required for liquid crystal formation when the system was made less hydrophilic by increasing temperature or the alcohol content of the drop or by reducing the ethylene oxide chain length of the surfactant [60]. One may ask why the intermediate phase formed during the contacting experiments should be the lamellar liquid crystal instead of a microemulsion, in cases where the drops contained substantial amounts of hydrocarbon. After all, diffusion of surfactant into the drop should cause the surfactantalcohol films within it to become more hydrophilic, as indicated above. Eventually, one might expect film composition to approach that corresponding to the PIT at the experimental temperature and thereby cause formation of a middle-phase microemulsion. The answer is that the drop itself becomes a microemulsion as it takes up water and surfactant. The occurrence of low interfacial tensions supports this conclusion. For instance. a tension of about 0.03 mN m -1 was measured by the spinning drop technique about 50 min after the start of the experiment for an oil phase initially containing 75% n-hexadecane and 25% oleyl alcohol and an aqueous solution containing 0.1 wt% C 12E 7 at 30-C [60]. The coexistence curve for this system indicates that the water-to-hydrocarbon ratio during an oil drop contacting experiment varies
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 199 Fig. 35. Video frames showing dynamic behavior following the contact of 0.05 Wt'% C12E8 solution with a drop of 5.67/1 n-hexadecane-oleyl alcohol at 50ºC [60]: (a) oil drop about 13 min after injection; (b) the same about 8 min later; (c) growth of myelinic figures less than 6 s later; (d) further growth of myelinic figures into the aqueous phase 12 s later. Reprinted with permission of the American Chemical Society. from its initial value of zero to about 5 when the drop composition eventually reaches the end of the coexistence curve (corresponding to point F of Fig. 32). No intermediate phase forms until point F is reached because the hydrocarbon content does not exceed the solubilization limit of the microemulsion. However, at point F, where the hydrocarbon-to-amphiphile ratio has dropped to about 1.5, the microemulsion structure apparently cannot be sustained and an intermediate lamellar phase begins to form. That it would form at such ratios of the various components and when the composition of the surfactant-alcohol films is approximately balanced between the hydrophilic and lipophilic properties is generally consistent with phase behavior results for other systems reported by Ghosh and Miller [46]. Indeed, lamellar phases with even higher oil contents have been reported near the PIT of the C 12E 4-water-n-hexadecane system [75].
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Solubilization-emulsification mechanisms of detergency

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