Solubilization-emulsification mechanisms of detergency

More documents

Recommendations

Info

190 C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 cloud point (7ºC) and PIT with hexadecane (30ºC). Contacting experiments were performed below, at, and above the PIT. In contrast to C12E5, the structure of the 1 wt% C12E4 solution was a lamellar liquid crystalline dispersion in water at all the temperatures studied. With regard to the intermediate phases which formed at the different temperatures, the following results were obtained. Below the PIT, an oil-in-water microemulsion formed between the lamellar liquid crystalline dispersion and oil. At the PIT, the behavior was dependent upon the concentration of liquid crystalline material at the initial oil-surfactant solution interface. At the low concentration of liquid crystal present at a 1% surfactant level, no intermediate phase formation was observed when the aqueous dispersion was contacted with hexadecane. To provide a higher level of liquid crystal at the initial point of contact, La drops were allowed to cream to the air-water interface prior to the contacting studies. These drops converted to concentrated La domains upon heating to 30ºC. With this initial aqueous phase structure, the formation of a middle-phase microemulsion layer was observed upon contacting with oil. Finally, if a pure lamellar liquid crystalline phase containing 25% surfactant was contacted with oil, swelling of the liquid crystalline phase was observed, as shown in Fig. 27. An interpretation of these results was made by comparing the qualitative diffusion paths, shown in Fig. 28, resulting from the different aqueous starting compositions [18]. The differences in behavior for the dilute (C), concentrated (B) and pure liquid crystal (A) cases were attributed to the relatively high solubility of C12E4 in the oil phase at this temperature, which influenced whether the diffusion path passed above or below the various three-phase regions that are present on the phase diagram. At temperatures of 40-50ºC, which are above the PIT, behavior similar to that for C12E5 was observed. Dissolution of the lamellar liquid crystalline phase into the oil resulted in the formation of a water-in-oil microemulsion and spontaneous emulsification of water in the oil phase. Fig. 27. Video frame showing swelling of the lamellar liquid crystalline phase 43 min after initial contact for the C 12E 4-water-n-hexadecane system at the PIT of 30ºC [18], Reprinted with permission of Academic Press. Dynamic contacting studies were also performed with hydrophobic additives combined with C 12E 5 [30]. C 12E 3 and n-dodecanol were added to C 12E 5 in proportions to yield cloud points near that of C 12E 4 (approximately 7ºC). This study was conducted Fig. 28. Schematic diffusion paths representing different behavior observed at different surfactant concentrations for the C 12E 4-water-n-hexadecane system at the PIT of 30ºC [18] The symbol (mp µe) denotes a middle-phase microemulsion. Reprinted with permission of Academic Press.
C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 191 ducted to determine whether formation of intermediate microemulsion phases containing a high proportion of oil could be obtained at temperatures lower than those for C 12E 5 alone. However, despite exhibiting phase behavior in the absence of oil similar to that of C 12E 4, the C 12E 5-C 12E 3 mixture behaved in a way intermediate to the behavior seen with C 12E 4 and C 12E 5 upon being contacted with hexadecane. Also, the microemulsion phases formed at various temperatures when the C 12E 5-dodecanol system contacted oil were essentially unchanged from those seen in the C 12E 5 system without any additive present. For example, rather than forming a middle-phase microemulsion with hexadecane at 30ºC, the two systems formed oil-in-water micro-emulsions. The contacting temperature had to be increased to 40ºC in the case of the C 12E 5-C 12E 3 System and 50ºC in the case of the C 12E 5-dodecanol system before middlephase microemulsion formation was observed. These differences between the two systems were attributed to differences in partitioning of the additive and the more water-soluble C 12E 5 between the oil and the microemulsion phases. The observed differences between the two additive systems would be less if smaller quantities of oil relative to the surfactant solution had been present. 6.2. Water-alcohol ethoxylate-triglyceride (+ hydrocarbon) systems Triolein is a pure triglyceride suitable for use as a model for kitchen soils such as vegetable oils. Dynamic contacting studies similar to those described above for hydrocarbons were performed with triolein using aqueous solutions containing the three alcohol ethoxylates C 12E 3, C 12E 4 and C 12E 5 [48]. As described in the phase behavior section, ternary triolein - water-non- ionic surfactant systems exhibit different phase behavior than those containing straight-chain hydrocarbons. Specifically, the large size of the triolein molecules inhibits solubilization and formation of microemulsion phases. At low temperatures, schematic phase behavior like that shown in Fig. 8 is observed in which two three-phase regions are present in the ternary diagram. At these temperatures, the surfactantwater mixture is a dispersion of the liquid crystal La in water. When this dispersion is contacted with triolein, a water layer forms between the liquid crystal and the oil, and extensive spontaneous emulsification occurs in the oil phase [48]. This behavior can be explained in terms of a diffusion path which passes below the bottom three-phase region in Fig. 8. Spontaneous emulsification occurs in the oil phase due to passage of that diffusion path segment across the two-phase water-oil region. In general, insufficient surfactant is available at the interface to form an intermediate L 3 or D' phase due to the high solubility of non-ionic surfactants in triolein at these conditions. At still higher temperatures where C 12E 3 and C 12E 4 are in the form of aqueous dispersions of L 3 (greater than 30ºC for C 12E 3 and 55ºC for C 12E 4), similar behavior occurs except that even more vigorous spontaneous emulsification is observed. C 12E 5 exhibited behavior at approximately 65ºC quite comparable to that observed with hydrocarbon systems near the PIT. In fact, as shown in Fig. 10, the D phase in that system contains almost equal volumes of triolein and water at 65ºC. When a concentrated lamellar liquid crystalline dispersion contacted triolein at that temperature, the D phase formed, first as lenses along the water-oil interface, then as a continuous layer. A schematic diffusion path corresponding to this behavior is shown in Fig. 29. The dynamic contacting of C 12E 4. solutions with oil mixtures containing varying proportions of triolein and hexadecane was also studied [48]. For 3/1 hexadecane-triolein mixtures, behavior comparable to that found with the pure hydrocarbon system was obtained. Similarly, 3/1 triolein-hexadecane systems behaved in the contacting experiments like the pure triglyceride system. However, contacting of the concentrated liquid crystalline dispersion at 38ºC with a 1/1 mixture of triolein and hexadecane resulted in the formation of transient middle-phase microemulsion droplets. The failure to form a
Page 1 and 2: Colloids and Surfaces A: Physicoche
Page 3 and 4: C.A. Miller and K.H. Raney/Colloids
Page 21: C.A. Miller and K.H. Raney/Colloids
Page 47: C.A. Miller and K.H. Raney/Colloids

Solubilization-emulsification mechanisms of detergency

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?