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Langmuir 1995,11, 1416
Reply to Comment on Silicone “Superwetters” The critique of our work by K. P. Ananthapadmanabhan is welcome, but does not accord with our readings of his paper and ours. To argue the weaknesses of a published theory before we are prepared to propose a more adequate explanation is reasonable in our view. We intend to address the explanation in a forthcoming publication. In collaboration with Dr. Michael Ward of the University of Minnesota, we have recently developed an accurate and rapid method for measuring the rate of surfactant solution spreading’ that allows us to measure the rate of spreading as a function of substrate surface energy and surfactant concentration, structure, etc. These measurements will allow us, we feel, to establish a mechanism of supenvetting with much greater confidence. In their original paper Ananthapadmanabhan et al.2 state that “the special wetting properties of SS1 are attributed to ...its unique structure, ability to lower liquidair surface tension to extremely low values, fast kinetics of adsorption a t the liquid-air and solid-liquid interfaces, high affinity of the surfactant for low-energy surfaces, and favorable orientation and structure of its adsorbed molecules.” Later in the paper the authors state that “the structure itself of the SS1molecule plays a governing role in determining its superior properties”, and, in particular, “we believe that the unique ability of SS1 solutions to spread on hydrophobic surfaces is related to its compact structure which in turn facilitates a transfer of surfactant molecules from the liquid-air interface to the solid surface.” Their next sentence refers to their Figure 6 to illustrate this point. An examination of this figure shows the same two molecular structures as Ananthapadmanabhan presents in his Comment as Figure la,b! We call the structure depicted in Figure l a “Tshaped or (preferably) “branched” and the structure depicted in Figure l b “linear”.3 Clearly, the authors intend the reader to conclude that “compact” and “unique))refer to the structures depicted in Figure 6 as “T-shaped”.Figure 6 differentiates the transfer of surfactant molecules from the liquid-air interface to the solid surface on the basis of the molecule’s “T-shape”; the “linear” structure (duplicated in Figure l b of the Comment) is asserted to represent the structure of “conventional nonionic surfactants” and not to be associated with supenvetting. We (1)Lin, Z.; Hill, R. M.; Davis, H. T.; Ward, M. D. Determination of the wetting velocities of surfactant superspreaders with the quartz crystal microbalance. Submitted to Langmuir. (2) Ananthapadmanabhan, K. P.; Goddard, E. D.; Chandar, P. Colloids Surf. 1990,44, 281. (3) Hill, R. M.; He, M.; Davis, H. T.; Scriven, L. E. Langmuir 1994, 10, 1724.
think that the superwetting effect is related to solution microstructure rather than interfacial ~ r i e n t a t i o n . ~ In our papel.3 we presented the counterexample of a trisiloxane surfactant which superwets but which does not have the “T-shaped” structure, thus demonstrating that this unique structure cannot be critical to superwetting. In addition, we have recently given several other reasons for objecting to the model of Ananthapadmanabhan et al. and reported a second counterexample of several “T-shaped” organic surfactants that do not superwet.4 In his comment Ananthapadmanabhan insists that our “linear” trisiloxane can be accommodated to their model (see his Figure la,b), yet these are the same two figures Ananthapadmanabhanet al.used in their original paper (see Figure 6 of [2]) to argue just the opposite, that the trisiloxane surfactant had a substantially different shape and because of this shape oriented itself differently at the liquid-solid interface. Contrary to what is stated in the above Comment, we made no statements in our papel.3 implying that the ulinear)) structure did not spread as rapidly as the “branched” or “T-shaped” structure. Various claims are made in the agricultural adjuvancy literature and in the patent literature regarding the effects of end-capping of the polyoxyethylene group on the rate of wetting. We are intrigued by these claims and, as mentioned above, are presently investigating the effects of such structural parameters on the rates of spreading.l Randal M. Hill*
Dow Corning Corporation Midland, Michigan 48686-0994 Mengtao He
Unilever Research, Inc. 45 River Road Edgewater, New Jersey 07020 H. Ted Davis and L. E. Scriven Department of Chemical Engineering and Materials Science University of Minnesota 421 Washington Avenue S.E. Minneapolis, Minnesota 55455 Received August 19, 1994 LA940728R (4) Zhu, S.;Miller, W. G.; Scriven, L. E.; Davis, H. T. Superspreading of water-silicone surfactant on hydrophobic surfaces. Colloids Surf. A. In press.
0743-746319512411-1416$09.00/00 1995 American Chemical Society
