Langmuir 1996,11, 1415
1415
Comments Silicone “Superwetters”
In a recent publication’ devoted to a study of the phase behavior of a group of silicone surfactants, the authors use the opportunity to assail a simple model2we advanced some years ago to explain the unusually efficient wetting action of the silicone surfactant M(DE7,5Me)M,where M is Me3SiO-, D’ is MeSi(CH2)3-, and E represents the ethylene oxide group (EO). We find their criticism unfortunate on two counts: (1)Although (presumably) a major goal of their research, an alternative explanation of this superior wetting is not offered. (2) The authors aver (incorrectly) that our model postulates a “ T or “umbrella” molecular shape as being a prerequisite for superwetting activity. We point out that we used neither the designation ”T” nor “umbrella” in detailing our model. Instead, we described an effective molecule as possessing a compact (hydrophobic group) structure, which allows efficient adsorption, packing, and, by implication, the right hydrophilic-lipophilic balance. At the time of our research the above surfactant, among those available to us, was found to be unique in its wetting characteristics. That related structures, satisfying the above description, can lead to superior wettingis not inconsistent with our simple model. For example, the efficient wetting material’ MDMEsOH (Dis MeSiMe and M’ is -Si(Me)2(CH2)30i), in which the hydrophilic group is terminal to the compact hydrophobic group, can be readily accommodated by our pictorial model if represented by structure b instead of our original structure (a) given in Figure 1. (1)Hill, R. M.; He, M.; Davis, H. T.; Scriven, L. E. Langmuir 1994, 10,1724. (2) Ananthapadmanabhan, K. P.; Goddard, E. D.; Chandar, P. C. Colloids Surf. 1990, 44, 281.
0743-7463/95/2411-1415$09.00/0
EO Grows
Figure 1. Nonetheless, it is ofinterest to us to read that molecules of structure a are still apparently found to be the fastest spreaders and that terminal capping of the EO chain, as in the first surfactant structure listed above, is still found beneficial. Finally, we emphasize that the structure depicted in Figure l b is not the same as that shown on the right hand side of Figure 6 of our original paper.2 The rectangular areain Figure l b endeavors to depict the compact (methylrich silicone) structure we hypothesized as favoring superior wetting, while in the latter it depicts a conventional “noncompact” hydrocarbon group.
E. D. Goddard 349 Pleasant Lane Haworth, New Jersey 07641-1311 K. P. Ananthapadmanabhan*and P. Chandar Unilever Research U.S. Edgewater Laboratory 45 River Road Edgewater, New Jersey 07020 Received July 15, 1994 LA940562U
0 1995 American Chemical Society
