Solubilization-emulsification mechanisms of detergency

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192 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 Fig. 29. Diffusion path corresponding to observed behavior for the C 12E 5-water-triolein system at 64.5ºC [48]. complete microemulsion layer during the course of the experiment probably resulted from the high solubility of the surfactant in the oil mixture. The latter experiment was repeated with tertiary amyl alcohol (TAA) added to the concentrated La dispersion at three different TAA/C12E4 ratios: 0.05, 0.10, and 0.20 by weight [52]. All three experiments showed the rapid formation of an intermediate phase, presumably the D or microemulsion phase. The higher the ratio of TAA to surfactant, the faster was the formation of the intermediate phase as a continuous layer. As indicated in the previous paragraph, the formation of a continuous middle-phase microemulsion layer did not occur in the absence of TAA. 7. Oil drop-contacting experiments As discussed in the preceding section, contacting experiments using vertically oriented cells, and their interpretation using diffusion path theory, have provided a fundamental understanding of the conditions for the occurrence of intermediate phase formation and spontaneous emulsification for a variety of systems containing water, pure surfactants, and non-polar oils. However, when either surfactant mixtures, e.g. the C12E5-additive systems discussed above, or commercial surfactants that are themselves complex mixtures, are used, the conditions for which intermediate phases form during a vertical cell-contacting experiment are frequently rather different from those expected during washing experiments in the same system at the same temperature. The reason is differential partitioning of different surfactant species into the oil phase. As explained previously in section 3, the extent of differential partitioning depends on both the overall surfactant concentration in the aqueous phase and the water-to-oil ratio (see Eq. (2)). In particular, the surfactant remaining after some partitioning into the oil has occurred, the factor which largely controls whether an intermediate phase will form, is less hydrophilic for a washing experiment in which the water-to-oil ratio is very large than for a contacting experiment in which the volumes of oil and water are comparable. It should be noted that this problem does not arise for systems containing mixtures of anionic surfactants and hydrocarbon oils, because none of the individual surfactant species has appreciable solubility in the hydrocarbon phase. When the oil consists of one or more polar components or of both polar and non-polar components, there is another limitation of the vertical cell-contacting technique that is significant even when pure surfactants are used. Basically, both experiment and diffusion-path theory yield information on the behavior of the system during the early stages of contact when the oil phase remains at its initial composition except in a region near the surface of contact. However, when the volume of the oil phase is small, the diffusion of polar material out of and/or diffusion of surfactant into the oil can cause the composition of the entire oil phase to vary with time, i.e. no location exists where the oil has its initial composition. This effect is especially important when inverse micelles or other aggregates form in the oil that have mixed films of surfactant and polar compounds. As a result, situations occur in which an intermediate phase does not form on initial contact but de-
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 193 Fig. 30. Schematic illustration of contacting experiment in which a small oil drop is injected into an aqueous surfactant solution. velops later in the experiment when the oil composition becomes suitable. Examples of such behavior are discussed below. An oil drop-contacting technique was developed in which the water-to-oil ratio is large, as in practical washing situations. As shown in Fig. 30, a drop of oil, usually some 10-100 mm in diameter, is injected into a horizontal rectangular glass cell by means of a very thin hypodermic needle. The cell, which is 400 Rm thick, is inside a thermal stage modified to enable the drop to be observed by videomicroscopy from the moment of injection [24,60]. Since the drop must be viewed through the surfactant solution in which it is immersed, this technique works best when the surfactant solution is below its cloud point temperature. However, some experiments have been successfully carried out in which the initial surfactant solution was a dispersion of the lamellar liquid crystal in water, as discussed below. It is noteworthy that no similar limitation exists for the vertical cell technique, and indeed almost all of the experiments described above for non-polar oils were conducted above the cloud point temperature. 7. 1. Experiments with surfactant mixtures and nonPolar oils Mixtures of anionic and non-ionic surfactants are now almost universally used in liquid detergents for laundry applications since they are more effective than anionics alone for washing synthetic fabrics at low temperatures. The oil dropcontacting technique was used to determine whether an intermediate microemulsion phase would form near the PIT for these mixed surfactant systems with hydrocarbon soils [24] in a manner similar to that described above for pure non-ionic surfactants with the vertical cell technique. The pure non-ionic surfactant C12E3 and the commercial anionic surfactant Neodol 23-3S were used in this study, i.e. the same combination as discussed above in the phase behavior section. The use of a commercial mixture rather than a pure anionic surfactant had minimal effect on the differential partitioning since all the individual anionic species in the mixture had very low solubilities in n-hexadecane, the hydrocarbon used. Because the volume of the oil drop injected was small, it dissolved little non-ionic surfactant, and the relevant PIT was that for which the surfactant composition Ssn in the films within the microemulsion phase was the same as the overall surfactant composition in the system. Data on Ssn for this system when the aqueous phase contains 1 wt.% NaCl are given in Fig. 7. As may be seen from Fig. 4, the initial washing bath, i.e. the oilfree mixture of the surfactant and a 1 wt.% NaCl solution, forms a dispersion of the lamellar liquid crystal in brine for surfactant compositions equal to the relevant values of Ssn. Contacting experiments were conducted for a surfactant mixture containing 78 wt.% of the nonionic surfactant [24]. According to Fig. 4, the relevant PIT is 30ºC. At 25ºC, no intermediate phase was observed and the drop diameter did not decrease appreciably with time. Thus, solubilization of hydrocarbon by the liquid crystalline phase was very slow at this temperature below the PIT. At 30ºC, an intermediate microemulsion phase was observed. Its volume continued to increase until the oil phase disappeared. At 40ºC, the drop diameter increased with time, the expected behavior above the PIT as the oil phase takes up surfactant and water. The liquid crystalline particles surrounding the drop made it difficult to discern whether spontaneous emulsification
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Solubilization-emulsification mechanisms of detergency

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