Solubilization-emulsification mechanisms of detergency

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178 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 balance between hydrophilic and lipophilic properties at high temperatures. The PIT is also influenced by the composition of the oil phase, being higher for hydrocarbons with longer chains. The reason is that their penetration into the hydrocarbon chain region of the surfactant films tends to make the films curve toward a water-in-oil configuration. Such penetration is less for longer-chain hydrocarbons [40], probably due primarily to an entropic effect [41] except for short-chain hydrocarbons where energy effects have recently been shown to be important as well [42]. Reed and Puerto [43] have developed a scheme relating optimal conditions to the molar volume of the oil and the solubilization at optimal conditions. The other factor mentioned above as being necessary for microemulsion formation is the existence of flexible films. Films are most rigid for surfactants having long, straight hydrocarbon chains. Flexibility can be increased by promoting less ordered packing in the hydrocarbon chain region of the films, e.g. by using branched-chain surfactants or mixtures of surfactants with different chain lengths, or by adding short-chain alcohols. Increasing the temperature also promotes flexibility. Too much flexibility can be undesirable, however, because it reduces the solubilization capacity of a microemulsion. However, when the surfactant films become too rigid, the lamellar liquid crystalline phase forms. Barakat et al. determined the conditions in several systems for which the lamellar phase formed instead of a middle-phase microemulsion because of film rigidity [44]. Hackett and Miller investigated the detailed phase behavior near the transition [45]. The liquid crystal can also form when the amount of oil or water present becomes too low [46] or when the surfactant concentration of a middle-phase microemulsion becomes too high [47]. Kunieda and Shinoda [47] have presented ternary diagrams at several temperatures ranging from below to above the PIT for the C12E5-water-n-tetradecane system. We discuss below the use of such diagrams in interpreting the dynamic behavior which occurs when surfactantwater mixtures contact oil. 3.3. Water-non-ionic surfactant- triglyceride systems Extensive studies have been made of microemulsions in hydrocarbon systems. Much less information is available for oils which are liquid triglycerides. An important difference is that triglycerides such as triolein which are of interest for detergency are of much higher molecular weight than simple straight-chain liquid hydrocarbons such as n-hexadecane. The higher molecular weight makes it much more difficult to incorporate such triglycerides into surfactant films, the result being a major reduction in solubilization in many systems. Figure 8 shows the general form of a water-nonionic surfactant-liquid triglyceride ternary diagram [48-50]. Deviations from this behavior occur at sufficiently high temperatures, a matter discussed further below. As indicated on the diagram, the phase designated D' is the same as that usually called L3 in binary surfactant-water systems, mentioned in section 3.1. While, like the middle-phase microemulsions discussed above, the D' phase coexists with both excess water and excess oil for suitable overall system compositions, it differs from the microemulsions in that it can Fig. 8. Schematic phase diagram for non-ionic surfactant-water-triolein system at low temperatures [48]. O denotes a triolein-rich phase.
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 179 solubilize only small amounts of the oil phase. Note that solubilization of triglyceride in the lamellar liquid crystalline phase is also low. Behavior of the W + D' + O three-phase triangle has been studied as a function of temperature for pure non-ionic surfactants and triolein. Figure 9 shows the results for C12E3 [48], a lipophilic surfactant that is already above its cloud point at 0ºC, well below the experimental temperature range. The surfactant content of the D' phase increases rapidly with temperature, the same as for the L3 phase in binary surfactant-water systems, but solubilization of triolein remains low. The solubility of surfactant in the triolein phase is substantial - more than 10% by volume even at the lowest temperature studied (30.5ºC) - and increases with temperature. For comparison, we note that the solubility of C12E3 in excess n-hexadecane in equilibrium with microemulsions at 30ºC is about 3% by volume (see Fig. 7). At about 40ºC the rate of increase with temperature of surfactant solubility in the triolein phase increases significantly. Simultaneously, water solubilization in this phase, previously rather low, rises dramatically. Such formation of water-in-oil microemulsions is one way the system can depart at high temperatures from the behavior shown in Fig. 8. Another way is illustrated in Fig. 10 by the Fig. 9. The W + D' + 0 region at several temperatures in the C12E3-water-triolein system [48]. Fig. 10. The W + D' + O and W + D + O regions at several temperatures in the C 12E 5-water-triolein system [48]. corresponding C 12E 5 diagram [48]. Just above 64ºC a phase transformation occurs, and at higher temperatures the diagram shows a new W + D + O three-phase region instead of the W + D' + O region. The D phase is able to solubilize considerable triolein and is thus more favorable for detergency than D' if it forms as an intermediate phase during washing. According to Fig. 10, the composition of the D phase shifts to become richer in oil with increasing temperature in a manner similar to that seen for microemulsion systems [47] although the surfactant concentration of about 40% is well above that observed in typical microemulsions. Details of the phase behavior in the temperature region where the D phase first appears, including the existence of two four-phase regions at two closely spaced temperatures, are given for another system by Kunieda and Haishima [50]. As indicated above, the inability of the hydrocarbon chain region of the surfactant films to incorporate the large triglyceride molecules is the chief reason for the poor solubilization. Similar poor solubilization and phase behavior have been seen in systems containing the anionic surfactant Aerosol OT, hydrocarbons with chain lengths of twelve and above, and NaCl brine [26]. Recently, Binks [51] has investigated further the phase behavior of some of these systems. If the surfactant films were made more flexi-
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Solubilization-emulsification mechanisms of detergency

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