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Langmuir 2003, 19, 4050-4050
Reply to Comment on Effect of Temperature on Surfactant-Driven Water Movement in Wet “Unsaturated” Sand
We are pleased that Henry, Smith, and Warrick1,2 have adopted our technique3,4 to follow capillary pressure driven water movement in wet “unsaturated” systems and have duplicated our experimental results with tetradecanol, an insoluble surfactant that spreads to form condensed monolayer films at the air-water interface. Their results with 7% butanol in water solutions, a system we did not study (and representative of mixed solvent systems we did not study) are interesting. On the basis of our tests of 33 long chain surfactants, we established criteria for effective water movers. These criteria were (1) water insolubility, (2) high equilibrium spreading pressure at the air-water interface, and (3) formation of a solid condensed monolayer at the air-water interface.3-5 Henry and Smith6 infer that because butanol (which they have shown to move water) does not meet these criteria, the criteria are wrong. It seems to us that the issue is one of system definition and what is meant by “effective”: All the surfactants we studied were long-chain compounds that were insoluble or very slightly soluble in water. Our criteria apply absolutely to surfactants that have this character. Very much larger quantities of soluble than of insoluble surfactants are required to move equivalent amounts of water. As an example, we have shown7 that a close-packed monolayer of tetradecanol is required to move water in wet “unsaturated” sand columns and that the air-water interfacial area in a column that contains 0.12 g of water/g of dry sand is 80 cm2/g of dry sand. To form a close packed monolayer of tetradecanol on this area requires (80 cm2/g of sand)/(22 × 10-16 cm2/molecule)/(6.02 × 1023 molecules/ mol) × (214.4 g of tetradecanol/mol) ) 1.29 × 10-5 g of tetradecanol/g of sand. In contrast, we calculate the quantity of butanol in a similar sand column that contains 0.12 g of 7% butanol-in-water solution/g of sand to be (0.12 g of solution/g of sand) × (0.07 g of butanol/g of solution) ) 8.4 × 10-3 g of butanol /g of sand. On comparing these numbers, we see that over 600 times more butanol than tetradecanol is required to move similar amounts of water. Soluble and insoluble surfactants behave very differently during capillary pressure driven flows. Soluble surfactants such as butanol flow with the water as it moves while insoluble surfactants such as tetradecanol are (1) Henry, E. J.; Smith, J. E.; Warrick, A. W. J. Hydrol. 1999, 223, 164. (2) Henry, E. J.; Smith, J. E.; Warrick, A. W. J. Hydrol. 2001, 245, 73. (3) Karkare, M. V.; La, H. T.; Fort, T. Langmuir 1993, 9, 1684. (4) Karkare, M. V.; Fort, T. Langmuir 1993, 9, 2398. (5) Karkare, M. V.; Fort, T. Langmuir 2002, 18, 2190. (6) Henry, E. J.; Smith, J. E. Langmuir 2003, 19, 4047. (7) Karkare, M. V.; Fort, T. Langmuir 1996, 12, 2041.
constrained to the air-water interface. Because of this different flow behavior, backflow occurs in soluble surfactant (butanol)-water systems6 while significant backflow does not occur with insoluble surfactants such as tetradecanol.3,4 Henry and Smith6 state that our calculation of “specific water movement”3,4 is inadequate for soluble surfactant systems such as butanol-water. This is true. We never applied it to soluble surfactant systems. Our calculation of specific water movement gives an accurate measure of water movement by insoluble surfactant monolayers that effect “permanent” movement of water. For soluble surfactant systems where water movement is not permanent and backflow occurs, more detail is helpful. We showed4 that water movement in wet “unsaturated” porous media systems may be caused by differences in capillary pressure within the systems. Capillary pressure differences are directly related to surface tension differences through the classical equation of Young and Laplace, and so it is no surprise that butanol solutions may cause water movement as demonstrated by Henry, Smith, and Warrick.1,2 Capilllaary pressure differences, and water movement, may be generated in other ways. If, for example, one side of a sand-filled horizontal tube contained water and the other side contained a water-miscible fluid of lower surface tension, and both tube sides were “unsaturated”, transient differences in capillary pressure would pull the fluid toward the water just as the waterbutanol solution is pulled toward the water in Henry, Smith, and Warrick’s experiments. As another example, our experiments with glass beads4 showed how differences in bead size in different halves of a column could overcome and even reverse the water movement caused by differences in surface tension. The means chosen to move water should depend on the application. For example, insoluble surfactants are uniquely suitable for determining the area of the airwater interface in wet unsaturated systems.7 In other situations, insoluble surfactants, soluble surfactants, or particle size differences may be preferred. We do caution that in any application based on the equation of Young and Laplace, large capillary pressure differences are necessary to effect significant water movement.4 Very small quantities of properly chosen insoluble surfactants can achieve these differences. Much larger quantities of soluble surfactants are required. These larger quantities may be less economical and more prone to undesirably contaminate the water and the solid matrix in practical systems than are classical mechanical methods. Milind V. Karkare and Tomlinson Fort*
Department of Chemical Engineering, Vanderbilt University, Nashville, Tennessee 37235 Received October 29, 2002 In Final Form: February 7, 2003
10.1021/la020882a CCC: $25.00 © 2003 American Chemical Society Published on Web 04/04/2003
LA020882A
