Solubilization-emulsification mechanisms of detergency

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210 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 ionic and anionic-non-ionic surfactant systems occurs near an appropriately defined PIT because solubilization of hydrocarbon into intermediate microemulsion or liquid crystalline phases is high and interfacial tensions are low. The low tensions facilitate emulsification of the intermediate phases into the agitated washing bath. Hydrocarbon soil removal is low at temperatures significantly below the PIT because, as is well known, the amount of oil that can be solubilized by microemulsion phases under these conditions is well below the solubilization capacity of middlephase microemulsions near the PIT, and interfacial tensions are significantly higher. Moreover, the initial rate of solubilization is less than that found near the PIT, as shown in Fig. 24 above. At temperatures significantly above the PIT, the explanation for the poor detergency is different. In this case both surfactant and water diffuse into the oil phase, so that it actually increases in volume instead of being solubilized. Moreover, extensive spontaneous emulsification of water in the oil occurs, as seen, for example, in Fig. 25. Solans and Azemar [19] reported that polyester-cotton fabric washed at temperatures above the PIT often experienced a net gain in weight, a result consistent with the above statements. They further observed that "aggregates", which apparently contained both oil and surfactant, remained on the fabric surfaces after washing. They suggested that additional water drops might be incorporated into the initial spontaneously formed emulsion as a result of agitation during the washing process, the result being an emulsion with a viscosity so high that it is not easily removed. Indeed, they proposed that the disperse phase content could become so large that a so-called high internal phase ratio (HIPR) emulsion could be formed on the fabric with the drops of water becoming polyhedral and the emulsion developing a yield stress. Yang and Rathman [88] confirmed that increases in weight occurred for polyestercotton fabrics washed above the PIT, and they found a surfactant-to-oil ratio of about 0.5 in material extracted from the washed fabric in one system. They also found that clean fabric became soiled if it was washed with soiled fabric above the PIT. Thus, redeposition of soil, probably in the form of an oil-continuous emulsion or microemulsion, must be considered a significant factor in the washing process under these conditions. In the studies discussed so far, the effects of electrolytes on detergency performance were not investigated. Recent results have shown that the optimum detergency temperature and the PIT are equivalent but lower when salts such as sodium citrate, a common builder for liquid detergent formulations, are present in appreciable amounts in the washing solution [89]. Similar effects with other salts would be expected. For the hydrocarbon soils and surfactants of interest for detergency, the PIT is invariably above the surfactant cloud point, so that the washing bath is a dispersion of the surfactant-rich liquid L1 or the lamellar liquid crystalline phase La in water. In most contacting experiments near the PIT an intermediate microemulsion phase was observed near the surface of contact. However, as mentioned above in connection with the vertical cellcontacting experiments for C12E4 and n-hexadecane at 30ºC, no microemulsion phase was seen when the dispersion of the lamellar phase was sufficiently dilute. In this case, small particles of liquid crystal striking the oil-water interface apparently dissolved in the oil, although it is likely that tiny drops of microemulsion quickly formed and dissolved during the process. However, if only a small quantity of oil had been present in this experiment, as in practical washing situations, it would have rapidly become saturated with surfactant by taking up such particles, and subsequent particles striking the interface would have produced an intermediate microemulsion phase. The experiment where the pure lamellar phase was contacted with oil for the same system at the same temperature is also of interest. Here considerable oil was solubilized directly by the liquid crystal although, as the dark region in Fig. 27 and the diffusion path of Fig. 28 indicate, some microemulsion did form as a dispersion with the liquid crystal at a location away from
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 211 the oil-liquid crystal interface. This experiment demonstrates that the formation of an intermediate phase is not necessary for high solubilization if sufficient quantities of a surfactant-rich phase are present initially. Indeed, such solubilization into the lamellar phase was apparently responsible for the moderate removal of pure triolein from polyester-cotton fabrics by C 12E 4 at temperatures between about 25 and 50ºC (Fig. 45). The poor detergency performance at high temperatures in this case is, as with hydrocarbon soils, the result of conversion of the oil phase into a water-in-oil microemulsion accompanied by spontaneous emulsification of the water there. If two different types of hydrocarbon soils are present, a useful strategy with a single pure surfactant would be to wash near the higher of the two PIT values and rinse near the lower. Experiments have shown that substantial soil removal does take place when washing occurs above the PIT and rinsing near the PIT [83,88], as would be the case in this example for the soil with the lower PIT. One reason this method works is that substantial surfactant remains on fabric washed above the PIT, as mentioned above, much of it probably dissolved in the unremoved soil. If there are multiple hydrocarbon soils and if a commercial surfactant or surfactant mixture is used, washing should again start at the highest PIT. In this case, however, it would be best to decrease temperature with time during the latter stages of washing or during rinsing, so that the PIT values for the various soils would all be reached at some time during the process. Note that during rinsing the PIT values will not be the same as those for the initial washing bath owing to differences in surfactant composition and surfactant-to-oil ratio during washing and rinsing. The concept of designing a process so that the PIT is achieved at some time as the system is gradually made more hydrophilic is related to certain strategies that have been proposed for enhanced oil recovery processes, e.g. variation of the injected salinity [90]. The concept of washing at high and rinsing at low temperatures to improve detergency was proposed by Rubingh and Stevens [91] although their explanation did not involve the PIT and the formation of intermediate phases. Achieving excellent detergency for pure liquid triglyceride soils and synthetic fabrics is difficult because the large triglyceride molecules are not readily solubilized. Indeed, substantial solubilization apparently requires conditions where an intermediate D phase forms. With straight-chain ethoxylated alcohols, this seems not to be possible except at undesirably high temperatures, e.g. about 65ºC with C12E5 and triolein (Fig. 10). Adding a short-chain alcohol does seem to make the surfactant films more disordered and thereby promote D phase formation at slightly lower temperatures (Fig. 11), but the relatively high solubility of the alcohols in water makes them ineffective for this purpose when the dilute surfactant solutions of interest for detergency are used. As indicated previously, preliminary results with secondary alcohol ethoxylate surfactants, whose double-tailed structure with different chain lengths also creates disordered films, look more promising as a means of forming the D phase at temperatures existing during warm and cold water washing [55]. Further study of these systems is in progress. Of course, the D phase could also be formed by using washing baths containing solubilized hydrocarbon (see Fig. 12), but this scheme seems less attractive than using the secondary alcohol ethoxylate surfactants. Results presented above demonstrate that liquid mixtures of hydrocarbons and long-chain alcohols or undissociated fatty acids can be removed effectivcly at temperatures below the cloud point of the surfactant but equal to or above the PIT of a system whose excess oil phase coexisting with the microemulsion has the same composition as the soil. Under these conditions, the soil takes up surfactant and water from the washing bath, as shown in the oil drop-contacting experiment of Fig. 35, the results being the lowering of the interfacial tension between the soil and surfactant solution and the eventual formation of an intermediate lamellar liquid crystalline phase as filaments or myelinic figures. These, presumably, are broken off and dispersed into the washing bath as a
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Solubilization-emulsification mechanisms of detergency

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