Solubilization-emulsification mechanisms of detergency

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194 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 in the oil occurred under these conditions, as would be expected based on observations above the PIT described above for the vertical cell experiments. The conclusion reached from these experiments is that mixed surfactant systems behave in basically the same manner as pure surfactant systems for hydrocarbon oils, provided that the PIT used to interpret the behavior is as defined above. Valuable results were also obtained by the oil drop technique for the C12E4-water-triolein-TAA system. As discussed in Section 3, the addition of TAA reduced the temperature at which the D phase formed in this system and also promoted formation of the D instead of the D' phase. The latter has considerably less ability to solubilize triolein than the former. When a drop of pure triolein was injected into an alcohol-free mixture of C12E4 and water at 50ºC, spontaneous emulsification was observed within the drop as well as the growth of small myelinic figures at the drop surface [52]. The drop volume decreased slowly with time, an indication that some triolein was being solubilized into the liquid crystalline particles present initially. This behavior is, as expected, the same as was seen with the vertical cell technique for the same system and temperature. In contrast, with TAA added to the surfactantwater mixture in an amount equal to 20 wt.% of the surfactant, an intermediate liquid phase, presumably the D phase, formed when a triolein drop was injected at the same temperature. As shown in Fig. 31, the drop shrank considerably during the first few minutes of the experiment as triolein was solubilized into the D phase. After about 5 min, the triolein drop had disappeared completely. The contacting experiments thus confirmed the conclusion reached from the phase behavior results that TAA promoted the formation of the D phase, which is capable of solubilizing considerable triolein. 7.2. Experiments with pure long-chain alcohols It can be shown using diffusion path theory that when the initial surfactant concentration in Fig. 31. Video frames showing the dynamic behavior of a drop of triolein contacted with 5 Wt-% C 12E 4 with added TAA at 50-C (TAA/C12E7=0.20). Obtained from Ref. 52. an aqueous micellar solution is sufficiently low, no intermediate phases form upon initial contact of this solution with a long-chain alcohol. However, an intermediate phase does form when the surfactant mass fraction exceeds a
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 195 critical value ws* given by the following equation [73] w s* = w sB F 1(w oC) + w sC F 2 (w oC) (5) Here w sB and w sC are the surfactant mass fractions at the L 1 and L 2 ends of the limiting tie line forming one boundary of the two-phase region for these phases, and w oC is the oil mass fraction at the L 2 end. The quantity w sB is often called the "limiting association concentration" or LAC [56]. The functions F 1 and F 2 are defined as follows where D o and D s are the diffusivities of the oil and the surfactant in the L2 phase, D's is the diffusivity of the surfactant in the L1 phase and hoC is the (constant) value of the similarity parameter [x/(4D.t) 1/2 ] at the L2 end of the limiting tie line (see discussion of diffusion path analysis above). Vertical cell-contacting experiments gave results in reasonable agreement with Eq. (3) for the sodium octanoate-n-decanol-water system [73]. One might expect that detergency would be improved when the intermediate phase - in this case the lamellar liquid crystal - is formed since more of the alcohol is solubilized. Indeed, Kielman and van Steen [11] observed such behavior in the potassium octanoate-n-decanolwater system. The focus of this section is the time-dependent behavior of the system if a drop of alcohol is injected into a solution whose surfactant concentration is below the critical value. The question to be answered is whether an intermediate phase will form at some time after initial contact. A series of such experiments was performed for the C 12E 5-water-oleyl alcohol system at 30ºC [74]. The L 1-L 2 coexistence curve and some interfacial tensions between these two phases are shown in Fig. 32. Note that the coexistence curve terminates at a point F corresponding to a water content of about 70 wt.%, which is one vertex of the L 1-Lα-L 2 three-phase triangle. Video frames taken at various times during an experiment in which the surfactant concentration was 1 wt.% are shown in Fig. 33. Note that the drop, initially some 70 mm in diameter, swells as it takes up water and surfactant. After about 23 min, the lamellar (La) phase begins to develop as myelinic figures which grow into the aqueous solution. Eventually, nearly all the alcohol is converted to liquid crystal. The order of magnitude of the time required for diffusion within the drop is the ratio of the square of its radius to the diffusion coefficient. As this time is much less than that of the experiment, a quasi-steady state scheme may be used to model drop behavior. Basically, this means that drop composition may be viewed as TIE -LINE INTERFACIAL TENSION (dyne/cm) 1 2.52 2 1.03 3 0.25 4 0.03 Fig. 32. Partial ternary phase diagram for the C 12E 5-water- alcohol system at 30ºC showing the L 1-L 2 coexistence curve and the limiting tie line EF. The oil drop composition varies as indicated by the arrows. The interfacial tensions are between pre-equilibrated phases for the four tie lines shown [74]. Reprinted with permission of Plenum Press.
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Solubilization-emulsification mechanisms of detergency

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