Langmuir 1996, 12, 1107
1107
Dissolution of Solid Detergents Cheryl L. Figge and P. Neogi* Chemical Engineering Department, University of MissourisRolla, Rolla, Missouri 65401 Received September 22, 1995
Dissolution rates of a commercial soap made from a mixture of surfactants were measured. Thin sections were cut from the solid using a razor blade and put on a glass slide, momentarily covered with once distilled water, and sealed off with a cover slip. The sliver of soap was held tightly between the slide and the cover slip with only the edges exposed to water. It could be viewed from the top using a microscope. The graduated eyepiece made it possible to measure the position of the interface as a function of time. The results are plotted in Figure 1 against the square root of time for seven different cases. Two features are apparent and striking. The first is that all plots become linear at large times, showing that diffusion dominates. The fact that diffusion is important in surfactant systems even at large concentrations appears to have been pointed out first by Kielman and van Steen.1,2 The fact that slopes are different in all cases reflects the differences in the interfacial areas of mass transfer. The second feature is that there is a significant gestation time before the mass transfer becomes important. This is more difficult to explain but has been observed earlier.3 One possible explanation lies in the fact that when a solid surfactant is contacted with water, a number of phases form to eventually give rise to lamellar liquid crystals, and finally to a micellar solution. Liquid crystals and systems high in surfactants have some degree of order. When water (or oil) comes into the system, it takes some time to rearrange to the appropriate structure, which is little more complex than just the phase transition. Raney et al.4 have reported the formation of exaggerated myelin figures in the liquid crystalline regions as seen under the (1) Kielman, H. S.; van Steen, P. J. F. In Surface Active Agents; Proc. Conf. Soc. Chem. Ind., Nottingham Univ., England, 1979. (2) Kielman, H. S.; van Steen, P. J. F. J. Phys. (Paris) 1979, 40, 447. (3) Ma, Z.; Friberg, S. E.; Neogi, P. AIChE J 35; 1989; 1678. (4) Raney, K. H.; Benton, W. J.; Miller, C. A. In Macro- and Microemulsions; Shah, D. O., Ed.; American Chemical Society: Washington, DC, 1985; p 193.
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Figure 1. Change in the location of the interface ∆x as a function of the square root of time.
microscope at greater magnification. It is suggested here that such rearrangements initially give rise to a lag in mass transfer. That both complexity of phase change and the simple phenomenology of diffusion can have roles to play is intriguing. In a different experiment, shaved chips were layered at the bottom of a large test tube and once distilled water was introduced on the top. The test tube was immersed in a water bath, but the water was not thermostated. A slight increase in turbidity was observed in a week’s time but nothing else. However, an opaque band 1 cm thick formed 2 cm above the soap after about a month. McGreevy and Schechter5 have shown that in surfactant systems dissolution can be accompanied by a reversal because of the competition between diffusion to disperse the material and the thermodynamics to retain the appropriate form of structural identity (microemulsions in their case). Such arguments can also hold here. Kapinsky et al.6 have shown that, if the reactions are fast and reversible, then reprecipitation can occur during dissolution. The process of surfactant aggregation is generally taken to be a fast reversible reaction. Thus even though diffusion is an important mechanism behind dissolution, the fact that surfactants form a variety of microstructures does influence the process. LA9507904 (5) McGreevy, R. J.; Schechter, R. S. AIChE J. 1991, 37, 169. (6) Kopinsky, J.; Aris, R.; Cussler, E. L. AIChE J. 1988, 34, 2005.
© 1996 American Chemical Society
