Open discussion of papers by Lahav, Nuzzo, and McBride.

PAUL HANSMA (University of California, Santa Barbara): I have a question for Mike McBride. I was really interested in the data you showed (and you gave one number of 20 to 30 kilobars), because it kind of relates to the atomic force microscope. The question is, how hard can you push on a crystal before it deforms? Would it be possible to get from your data a graph of how hard you can push, in newtons, versus temperature? DR. MCBRIDE: The problem is that the stress is very anisotropic, of course, and we only measure the component that is along the long axis of CO2. So, all we have is a lower limit on the maximum stress. It is difficult to get the kind of number you would like to have from our data. We know that the interface gives because, as we warm the sample, we see the successive relaxations. But we have not pursued the quantitative relation to try to get a very reliable number, I am sorry to say. STEPHEN GAROFF (Schlumberger-Doll Research Center, Richville, CT): Ralph [Nuzzo], one crude question and one elegant question, elegant first. You showed a pretty picture of clusters of water on a surface. Is there any chance of getting an infrared spectrum of those clusters of water? Crude question: The surfaces you make are probably about as homogeneous as you are going to get. Have you ever breathed on them, doing the "breath test" to see if the nucleation and growth ofwater looks uniform on these almost single crystals of gold with highly dense surfactant layers? DR. Nuzzo: We clearly need to do structural work. I have been having many conversations with various people, one of whom [David Allara, AT&T Bell Laboratories, Murray Hill, NJ] is sitting right back there. The whole point is, in order to determine the structure of clusters of water, we will have to do experiments that we are currently not capable of doing. I think we would have to devote some effort to figure out what is the best way of doing that. But it is possible. I think that is really an important thing to go after. There is another bit in all these data that I did not have a chance to talk about; they also seem to bound considerations of which portions of the substrate molecules are, in fact, accessible to the adsorbate overlayer. For example, I suspect that no one noticed that on an ester surface, as regards water adsorption, water does not seem to see the carbonyl group. There is a lot of other information involved therein. That is why we really need to get some more detailed information as regards the structure ofthese overlayers. As regards the breath test, they are very uniform in their behavior when you breathe on them. THEODORE MADEY (National Bureau of Standards, Gaithersburg, MD): I also have a comment and a question concerning the water. In thermal desorption studies of water on single-crystal metals which water does not wet, such as close-packed surfaces of copper and silver, you see similar behavior to that which you reported here, that is, an increasing temperature of desorption as the water coverage increases. That indicates zero-order kinetics or fractionalorder kinetics which is consistent with clustering. First of all, did you ever look at thermal desorption ofwater from gold alone to compare it with the desorption from these overlayers and, secondly, do you really think that the outer moieties of the overlayer are sufficiently closely packed that you would expect clusters of water to form under your experimental conditions? DR. Nuzzo: That is a really hard question. The answer is, we did look at water on gold(111), and it shows similar characteristics. It is sort of intermediate in character between a hydrocarbon and the more polar surfaces. The data I showed were in terms of one particular coverage, and what you see are desorption peak maxima which shift to higher temperature with increasing coverage. I think the problem that exists in all of this in terms ofwhere these peak maxima occur is that the heats involved are also sensitive to the cluster size, and the cluster size is responding somehow to how these things nucleate. This is indicated in the one data point I put in about water adsorption on the carboxylic acid surfaces. If we dose under one set of conditions, the peaks are all isothermal. Ifwe dose under another set of conditions, they do, in fact, show coverage shifts. Interpreting all that, then, is notjust a simple analysis based on activation energies or even the order of the desorption process. There has to be more information. That is one of the problems we are having in the interpretation. JACOB ISRAELACHVILI (University of California, Santa Barbara): You mentioned the contact angle. I presume the adsorption of the surfactant on the gold is from solution. Are you talking about adsorption of monolayers? DR. Nuzzo: That is right. DR. ISRAELACHVILI: And you measure the contact angle of 1100. Was that actually a measurement? DR. Nuzzo: Yes. DR. ISRAELACHVILI: I will tell you why I find that interesting. We have found, from our experiments, when we adsorb surfactants from solution onto mica, for example, we rarely get a contact angle of 110°; we often get something significantly less. If you look in the literature, you find, also, that others have always got values less than 110°, even though the surfaces should be nominally CH3 or CH2 groups. However, with Langmuir-Blodgett deposition, one does get 1100, which is what you expect. Subsequently, from the force measurements, we found that adsorption from solution usually occurs with some of the molecules inverted, because, of course, they are adsorbing from a hydrophilic surface, and there is no reason for all the molecules to have the hydrophilic group attached to the gold, in this case, or mica, in our case, but there is a greater tendency to have some-maybe 10%, maybe 5%-facing the other way and, therefore, you get a lower contact angle. Yours is the first time where I have seen a case where you have adsorption of a single-chained surfactant from solution where you have a contact angle of 110°, and I find that fascinating, because you still have all the hydrophilic head groups, the sulfur, going on to the gold, and you have absolutely no molecules facing the other way. I do not know of any other example of that. Ijust wanted to clarify that that