Solubilization-emulsification mechanisms of detergency

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212 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 result of agitation. Note that Fig. 48 and 49 show no significant change in detergency between 40 and 60ºC for C 12E 7 and the soil containing 75% cetane even though the contacting experiments mentioned above for this system indicated that an intermediate D' phase formed first and only later the liquid crystal formed. Below the PIT no intermediate phase was seen in all the systems, the drops were slowly solubilized into micelles in the washing bath, and soil removal was lower. For all the systems discussed above for which both videomicroscopy observations and soil removal measurements have been performed, intermediate phase formation and low interfacial tensions leading to good detergency occurred when the hydrophilic and lipophilic properties of the surfactant or surfactant -alcohol films of the intermediate phase were approximately balanced. This balance may be present initially, as in the experiments with hydrocarbon soils near the PIT, or it may develop during washing as a result of mass transfer, as in the experiments with mixtures of hydrocarbons and long-chain alcohols. Note that if mixed surfactants and lower surfactant-to-oil ratios had been used in the hydrocarbon washing experiments, mass transfer effects would most likely have been important there as well, i.e. the system could have moved closer to or further from the PIT over time. A similar conclusion on the need for balance was reached by Malmsten and Lindman [92], although without the proviso that a system not balanced initially may achieve balance during washing. It should also be recognized that, even when balance is achieved, the intermediate phase formed must have the capability of solubilizing considerable soil, a property clearly lacking in some of the non-ionic surfactant-triolein systems discussed above. If a system contains soils with several different mixtures of hydrocarbons and long-chain polar compounds on the fabrics, an effective strategy would seem to be to choose a washing temperature and surfactant so that the temperature is initially above the PIT of all soil compositions. The surfactant must be hydrophilic enough to be somewhat below its cloud point but lipophilic enough to meet the above PIT criterion. Mass transfer during the washing process will bring the various soils to a balanced condition (though at different times) at which they can be removed by some combination of intermediate phase formation and emulsification promoted by low interfacial tensions. This strategy is, of course, similar to that suggested above for multiple hydrocarbon soils except that no variation of temperature is required. For hydrocarbon soils the phase behavior of the surfactant-water mixture in the initial washing bath did not seem to have a significant influence on detergency. Maximum soil removal occurred at the system PIT whether the washing bath consisted of a dispersion of the L 1 phase or of the Lα phase. In contrast, soil removal dropped sharply when the cloud point temperature of the surfactant solution was reached for the mixed hydrocarbon-alcohol soils above their PITs. It is likely that water formed as in the intermediate phase for temperatures above the cloud point temperature and hindered the mass transfer necessary to achieve the balanced condition discussed above. That is, the diffusion path is most likely similar to that of Fig. 26 except that the surfactant-rich phase is L 1 instead of La. In the oil drop-contacting experiments it was not unusual for 20 min or even more to elapse before liquid crystal formation began. Such long times are unsatisfactory for washing processes. It should be kept in mind, however, that the contacting experiments were deliberately conducted with no imposed mixing to facilitate the interpretation of the observations in terms of diffusion theory. In actual washing situations, mixing would greatly speed up mass transfer, and liquid crystal could be expected to form after much shorter times. Further discussion, including some quantitative estimates of the enhancement in mass transfer rates, may be found in the original papers [60,74]. When mixing occurs, the times required for liquid crystal formation were found to be reasonable for practical processes, a conclusion confirmed by the excellent soil removal obtained in the washing experiments described above.
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 213 In the preceding section, it was mentioned that the time required for removal of mixed hydrocarbon-long-chain alcohol soils increases when the initial state of the system is further above the PIT. This result is consistent with the intuitive expectation that surfactant would have to diffuse into the oil for a longer time before liquid crystal formation occurred for systems further above the PIT as well as with the quantitative values of the parameter K s in Eq. (6) found for soils with different compositions. It is also noteworthy that in some of the contacting experiments, convection arose spontaneously near the interface, an effect that would also speed up mass transfer. The Marangoni flow mentioned in connection with Fig. 19 for the C12E5-water-n-tetradecane system at 20ºC is one example, although it occurred for a situation when soil removal is minimal owing to a low solubilization capacity of the surfactant solution. Perhaps more relevant to detergency, vigorous convection was sometimes seen during the early stages of formation of a non-wetting intermediate phase, i.e. one that forms preferentially as lenses rather than as a continuous layer. Intermediate phase formation in the C 12E 5-water-n-tetradecane system at temperatures just below and above the cloud point is an example. If non-uniformities in intermediate phase thickness develop immediately following the initial contact when the phase is a still a thin liquid film, disjoining pressure effects will promote flow from thin to thick portions of the film, thus increasing the discrepancies in thickness. Diffusion processes will continue to form more of the intermediate phase in the thin regions, and disjoining pressure gradients will continue to drive the newly formed material to thicker regions. No soil removal experiments are currently available for the systems discussed above containing mixed triolein-oleyl alcohol soils and a pure nonionic surfactant. The contacting experiments indicate, however, that the first intermediate phase formed is D', which solubilizes little triolein although readily incorporating alcohol. It may be that some of the same ideas suggested above to improve detergency with pure triolein, e.g. use of secondary alcohol ethoxylate surfactants, will be needed to achieve high degrees of removal of the triolein portion of these mixed soils. Of course, liquid triglyceride soils normally contain some fatty acids formed by triglyceride hydrolysis. In addition, lipase enzymes are incorporated into some detergent formulations to promote the breakdown of triglycerides into monoglycerides, diglycerides and fatty acids. Increasing the pH can convert these acids to soaps, which could well improve detergency. This behavior is currently being studied by the contacting techniques discussed above. 12. Conclusions Solubilization-emulsification is a key mechanism in the removal of oily liquid soils from polyester and polyester-cotton fabrics. Systematic studies, discussed above, utilizing several model soils indicate that solubilization into an intermediate phase formed during washing is generally much more extensive and rapid than solubilization into a micellar solution. Accordingly, knowledge of the phase behavior of surfactant-soil-water systems is needed to make a rational choice of optimum surfactant compositions and washing conditions. With hydrocarbon soils, for instance, washing at temperatures near the PIT is best. The intermediate phase is a microemulsion although the lamellar liquid crystalline phase may form as well if it is not already present in the initial washing bath. For commercial non-ionic surfactants and their mixtures with anionic surfactants, the appropriate PIT in the usual case of washing with a high surfactantto-oil ratio is that for which the composition of the surfactant films in the middle-phase microemulsion is that of the surfactant mixture in the initial washing bath. For mixed soils containing hydrocarbons and long-chain alcohols or fatty acids, good soil removal is achieved at temperatures above the PIT but below the cloud point of the surfactant. In this case the PIT is evaluated at high surfactant-to-oil ratios as above with the additional condition that the excess oil phase in equilibrium with the microemulsion has the
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Solubilization-emulsification mechanisms of detergency

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