Open Discussion of Papers by Israelachvili, Tirrell, and GaroffSource: Proceedings of the National Academy of Sciences of the United States of America,Vol. 84, No. 14 (Jul. 15, 1987), pp. 4733-4736Published by: National Academy of SciencesStable URL: http://www.jstor.org/stable/30260 .

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Proc. Nail. Acad. Sci. USA Vol. 84, pp. 4733-4736, July 1987 Symposium Paper

This discussion was a portion of the symposium "Interfaces and Thin Films," organized by John Armstrong, Dean E. Eastman, and George M. Whitesides, held March 23 and 24, 1987, at the National Academy of Sciences, Washington, D.C.

Open discussion of papers by Israelachvili, Tirrell, and Garoff DONALD HAMANN (AT&T Bell Laboratories, Murray Hill, NJ): I have what I guess is a very naive question to Professor Israelachvili, which is a lack of understanding on my part of one aspect about how the force-measuring apparatus works. I gather from your schematic that at least one of the mica surfaces has to be curved. Otherwise, the parallelism prob- lem on the angstrom scale would be insoluble. However, as the curved surface approaches a flat surface, you have to have a distribution of distances, and yet all of your results were plotted in terms of force versus distance, as if there were a unique distance between two parallel planes. I wonder if you would clarify these issues for me?

DR. ISRAELACHVILI: The surfaces are cylindrical. Mica comes in the form of sheets, so you cannot curve it into a sphere. And you would not want to, either. But if you have two cylinders, like two beer cans, you bring the surfaces like that, so one is curved that way and one is curved the other way.

It so happens that for identical radii, locally the curvature is uniform. It is like bringing a sphere onto a flat, and the distance of closest approach is the distance that has been measured. I should say, experimentally, it makes it very easy to do these experiments. If you have some contamination problem-and that is usually the worst problem in these experiments-you have a particle in the way, and at the angstrom level, a particle is a nuisance, to say the least. With two crossed cylinders you can move the top surface to the right and then a little bit to the left and you get a new position for both surfaces, and hence in one experiment you can actually do many different experiments with many different positions.

But they are curved surfaces, both of them are curved, and with this geometry of curvature, it is very easy to make a transformation from the force between curved surfaces to the interaction energy between two flat surfaces. That is known as the Derjaguin approximation; you divide the force by 2,rR [R = radius] and that gives you, immediately, the energy of interaction between two flat surfaces. In fact, if you divide the forces that are measured by 2rrR, you can immediately compare them with any theory giving the energy between two flat surfaces.

DR. TIRRELL: And that is how, in fact, all the results in Jacob [Israelachvili]'s and in my talk were plotted, although you might not have recognized it. Mine were all plotted as force divided by the mean radius of curvature of these cylinders. This is also a way of normalizing different exper- iments, since you do not always have the two cylinders exactly orthogonal. You use the fringes to determine the principal radii of curvature.

DAVID MCCALL (AT&T Bell Laboratories, Murray Hill, NJ): Matt [Tirrell], did you try varying the distribution of chain lengths of your styrene segments?

DR. TIRRELL: You mean make a mixed layer of these things with different chain lengths? No, we have not done that.

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DR. MCCALL: It seems like you could approach the interpretation of your data as though maybe the longer chains were dominating, because you do have a distribution there.

DR. TIRRELL: Yes, but we do not have a very strong distribution. What the effects of the molecular weight distri- bution that we do have are, are not entirely clear. I do not think they are very important, in our case, but I do think it would be interesting to make a mixture.

DAVID ANDELMAN (Exxon Research Co., Annandale, NJ): I have a question for Jacob Israelachvili. Could you elaborate more on the origin of the oscillation of the force between repulsion and attraction for the simple liquid case?

DR. ISRAELACHVILI: There has been quite a bit of theoret- ical work on this over the years and, in fact, they were predicted before they were measured, apparently, by Van Megen and Snook.

Anyway, the thing is, imagine you are separating two surfaces in a liquid, and you do not have to have interactions. Of course, the liquid will evaporate immediately, so you put a cork in your test tube to stop them evaporating, and then you separate them. Up until the surfaces are one molecular diameter apart, no liquid molecules will enter between the surfaces. So, the density of molecules between the surfaces up to that point is zero.

At that point, there will be an inrush of liquid molecules and the density of molecules will now reach some value compa- rable to bulk, but may be higher or lower. Then separate the surfaces 1? molecular diameters. You will find that you can no longer pack them at that density, you just cannot avoid having large void regions.

Then, at 2 molecular diameters, you get good packing again. The density is bound to go up and down, up and down, and eventually be the same as bulk. Since the osmotic pressure on each surface is simply kT times the density of molecules on each surface, the pressure between those two surfaces must equal kT times the density at any distance of separation minus the density at infinity, and therefore must be oscillatory, and it is as simple as that.

If you add interactions between the molecules and the wall, it sort of modulates that, but basically that is their origin. It is quite a straightforward, simple one. Maybe Stuart Rice could confirm whether what I have just said is basically a correct "Readers' Digest version."

STUART RICE (University of Chicago, Chicago, IL): It is better than Readers' Digest.

CYNTHIA FRIEND (Harvard University, Cambridge, MA): Jacob [Israelachvili], it would seem reasonable, in your monolayer measurements, that you might be inducing some subtle structural transformations during the course of your measurement. Do you see any evidence of that, or how would it be manifested in the measurement?

DR. ISRAELACHVILI: Do you mean monolayers of surfactants?

DR. FRIEND: Yes. DR. ISRAELACHVILI: Bilayers or monolayers? DR. FRIEND: Monolayers. DR. ISRAELACHVILI: As you bring the surfaces together?

4733

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4734 Symposium Paper: Discussion Proc. Natl. Acad. Sci. USA 84 (1987)

DR. FRIEND: Yes. I mean, could you get some slight distortion or force things into islands that you would be able to measure differences in the force? Have you considered that?

DR. ISRAELACHVILI: No, I have not considered it. The monolayers are basically close-packed monolayers.

DR. FRIEND: And how do you know that? DR. ISRAELACHVILI: Well, either they adsorb naturally

from solution, and by having cationic surfactants, you get adsorption from solution, and from the fringe wavelength at contact, you can tell how much is adsorbed and what the thickness of that monolayer is. Or we can deposit, using the Langmuir-Blodgett technique, insoluble monolayers, for example, double-chained cationic surfactants. There are a number of different ways of doing it, using different surfactants, adsorption from solution or by Langmuir-Blod- gett, and the results are always at least consistent with each other, as those results showed.

I do not know if you get islands. We have also done electron microscopy of the surfaces, I should say, and, also, the fringes do not indicate that there are any islands.

DR. FRIEND: I have one other quick question that is sort of a technical nature. In your studies, where you have alkali ion adsorption, do you actually get adsorption on both surfaces? In other words, do you have the ions in solution? Is some of the repulsive part just from ions on the two surfaces?

DR. ISRAELACHVILI: Oh, yes. DR. FRIEND: Is that entirely what it is? DR. ISRAELACHVILI: That is what I think, that the ions

adsorb (they are hydrated ions) and, in fact, what I did not mention, because I did not have time, is that the strength of this short-range repulsion correlates exactly with the known hydration of the adsorbing ions. It is weakest for cesium, stronger for potassium, sodium, lithium, then stronger for calcium, and then stronger for magnesium. It happens at different concentrations, but when you get it at its maximum coverage, it correlates exactly with the known hydration of the adsorbing species, which is very nice. And it correlates with other studies in colloid science, where you get lyotropic series effects determining the behavior of colloidal systems.

DR. RICE: I take it that the very long range of the interactions between the polymeric-covered surfaces is pri- marily an entropic effect, the reorganization of the chain via the intermediary of the solvent as you push the plates together. Now, presumably, the configuration of the chain depends, in part, on the excluded volume along the chain, and in part on the interaction of the chain with the surface. That is, you can make the chain with the radius of gyration greater than in the homogeneous solution or less than in the homo- geneous solution. It would depend on the balance of those internal and external interactions.

Have you done any experiments which explore the rela- tionship between the range of the force, possibly even its functional form, and the playoff between the internal exclud- ed volume versus the surface-polymer interaction?

DR. TIRRELL: Those things have not been done in a very thorough way. The simplest answer is to confine ourselves to the block copolymers, first. There we can play with the solvent quality and one sees that in going from toluene to cyclohexane, there is a collapse of the layer [Hadziioannou, G., Patel, S., Granick, S. & Tirrell, M. (1986) J. Am. Chem. Soc. 108, 2869-2876].

So, there, what we are doing is actually just playing with that part of the molecule that is experiencing the excluded volume in interaction and not doing much with the anchored part. You find a contraction of the layer on going to a poor solvent-if you do the same molecular weight series in cyclohexane near the 6 point as we have done in toluene, you find that the range is still linear in molecular weight in these solvents, so they are still packed to a density such that even

near 6 chains are stretched out, to some extent, and the range of the forces is consistent with the stretching of about half of what you see. So, there is still a lot of stretching due to the lateral interaction.

Now, the case of a homopolymer has not been explored thoroughly in this apparatus, because it is complicated. It is not totally understood, even just from other kinds of adsorp- tion measurements, but let's say you start out near the 0 condition, as with polystyrene and cyclohexane, and you start making the solvent better. Typically, what you will find is the layer will swell. That is, somehow, by some means of measurement of the thickness, ellipsometry, or some other way, you will see a swelling of the layer, but then you may see a diminution afterwards, which is probably due to the fact that you get desorption-assuming that you really do wait long enough for things to equilibrate.

So, there is a competition, also, between the quality of the solvent and the attraction of the surface. That is sort of how things look, but that latter aspect has not been explored in this apparatus.

DR. AR,MSTRONG: I have a question for the panel generally. If I understand the apparatus, you have metallized films over the mica sheets?

DR. TIRRELL: The metal is in the back. DR. ISRAELACHVILI: It is for the optics. DR. ARMSTRONG: I understand it is for the optics. My

question, though, is, it is of enormous interest to study the interaction of the forces, the repulsion and the attraction, between a metal surface and polymers, and to what extent can your apparatus be adapted to that?

DR. ISRAELACHVILI: This is being done at the moment in Minnesota.

DR. TIRRELL: Henry White in our department at Minnesota has been reasonably successful in depositing relatively smooth platinum layers on the surface [White, H. S., Maeda, M. & McClure, D. J. (1986) J. Electroanal. Chem. 200, 383]. When I say "relatively" that is . . . as we heard yesterday, some precise measures of this roughness would be very nice and important to get.

If you want to use them as substrates, then, for polymer adsorption, that is pretty forgiving, the amount of roughness you can have, if we are measuring forces of 600 A, I think we can tolerate 20 A of roughness.

The detailed characterization of the platinum surfaces that Henry has made is not complete, but it is possible to do the experiments with metal surfaces in there. The first problem you encounter is if you just put metal over the two front surfaces of the mica sheets, you get a lot less intensity, so those fringes are, depending on how much metal you put down, a big factor more dim, and then there is also another set of interference fringes superimposed on the set that Jacob [Israelachvili] showed that come from the interference be- tween the front and the back of any individual mica sheet. But by appropriate manipulation of what fringe you measure on, you can do an experiment like this. When we get to the point that we have smooth enough metal surfaces, these kinds of experiments can be done on metals, I am certain.

I would like to measure polymers against metals, and we are, but there are a lot of other things that one can do. There is no need, in these experiments, to work with symmetrical surfaces, for one thing. One could put a polymer on one surface and a metal on the other surface and use this apparatus to measure the interaction.

DR. ARMSTRONG: I think you should look for adhesion as well as for repulsion. I am surprised by the lack of adhesion showing up in most of these measurements. You just do not want to get things stuck together?

DR. ISRAELACHVILI: Oh, there is lots of adhesion. I just did not talk much about it. I showed you some fringe patterns of what happens to the shapes of the surfaces when you get

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Symposium Paper: Discussion Proc. Natl. Acad. Sci. USA 84 (1987) 4735

adhesion, and I could have talked about both adhesion in the presence of vapors and capillary condensation effects of adhesion, but there was no time.

DR. TIRRELL: For example, if you make the solvent bad, in our case, that is, you go way below the polystyrene and cyclohexane 6 temperature, then you start to see an adhesion between these two surfaces [Hadzioannou, G., Patel, S., Granick, S. & Tirrell, M. (1986) J. Am. Chem. Soc. 108, 2869-2876]. That comes from the fact that the second virial coefficient for mixing of polystyrene segments and cyclohex- ane at 20? or 5l is negative and you see a significant adhesion.

The other thing that we have done, getting back to this thing of asymmetry, is put polystyrene only on one surface and brought it up against a bare mica surface [Granick, S., Patel, S. & Tirrell, M. (1986) J. Chem. Phys. 85, 5370-5371]. Then you get enormous adhesion, a lot stronger than the levels of adhesion that Jacob [Israelachvili] showed when both surfaces are covered with polystyrene.

HARDEN MCCONNELL (Stanford University, Stanford, CA): You showed an attractive curve in aqueous systems for two hydrophobic surfaces. For two flat surfaces, at what areas would the binding energies be comparable to kTat room temperature?

DR. ISRAELACHVILI: Our results showed that if you take the known hydrocarbon-water interfacial energy per unit area of about 50 mJ/m2 or so for most hydrocarbons and water, then, as a function of distance apart, that will simply fall exponentially with a decay length of about 10 A. So, you can calculate that immediately from that.

At contact, we know what the answer is, and, in fact, we measured that adhesion as exactly the same as the thermo- dynamic value.

DR. MCCONNELL: Let me ask it, again. You had a curve that was going out to 20 A, an attraction curve. How big would the areas have to be in order for the one-half energy distance to go down halfway on the curve, be comparable to kT, how big would two surfaces have to be so that they would start to bind to each other?

DR. ISRAELACHVILI: At what distance? DR. MCCONNELL: So you can measure out to 20 A. If I

have two surfaces, two molecules, two large molecules, when will they for sure run down the hill and join each other at this distance? I am trying to see what the molecules really do, what two molecules would do.

DR. ISRAELACHVILI: It is of the order of two molecules. DR. MCCONNELL: Not two tyrosine molecules, not two

benzene rings. DR. ISRAELACHVILI: Two water molecules. I would have to

sit and do a calculation. I am just telling you how easy it is to do that calculation. If I had a calculator, I could do it almost immediately. Closer than about two water molecules, or something of that order, two hydrocarbon chains or segments of chains are experiencing something greater than kT. Even though we can measure it to 100 A because we have this tremendous sensitivity in measuring forces between macro- scopic surfaces, but kT per molecule, say, per water mole- cule, it only reaches kT in the last one or two water molecules.

One has to appreciate that the range of force and the magnitude are two different things. We can measure forces which correspond to 10-5 kT per molecule, which would be totally unimportant in many systems, but you can measure it, because the surfaces are basically large.

RALPH Nuzzo (AT&T Bell Laboratories, Murray Hill, NJ): I have to apologize, I am an organic chemist. I cannot think in forces, electron volts, or anything like that, but in terms of kilocalories per mole, it is on the order of one kilocalorie per mole, more or less. In terms of kT, you are talking about very low temperatures, indeed.

DR. WHITESIDES: But Harden [McConnell]'s question, as I understood it, is either what magnitude, what area, or what

distance, is it when two cells are spontaneously going to go "smash"?

DR. ISRAELACHVILI: That depends on the area. But in that case, it would be, of course, much more than two water molecules.

DR. MCCONNELL: That is what I am driving at. Suppose you take two hydrophobic surfaces that are a square micron?

DR. ISRAELACHVILI: Oh, but then that would be about 100 A, say 50 to 150 A.

CEDRIC POWELL (National Bureau of Standards, Gaithers- burg, MD): Professor Israelachvili, you made a brief remark at the end of your talk on the important effect of impurities. I was curious to know what experimental techniques do you use to ensure that you do not have a significant amount of unwanted impurities. Also, how do you measure the con- centration of desired impurities, and what are some of these impurity effects?

DR. ISRAELACHVILI: Each system is different, and one cannot generalize. I can just give you specific examples. We found that if we have forces between hydrocarbon liquids, organic solvents, such as decane, then a very small amount of water, 10 parts per million, is already enough to totally change the forces. Because we have mica surfaces (they are basically sort of hydrophilic) and because 50 parts per million of water is already saturation, is already an activity of 1. At that point, you have a monolayer or submonolayer of water on those surfaces, even though it is only 10 or 20 parts per million of water in the hydrocarbon, and that changes the packing or the way the molecules are arranged on the surface, and it totally changes the forces. You can get strong adhe- sion, and everything can change.

Apparently, this has been known for many decades, but has always been buried in the colloid literature, that small amounts of impurities of immiscible liquids can have pro- found effects.

If you have miscible liquids, say you add a bit of ethanol to water, there is hardly any effect at all, because they mix. You would need to have quite a significant amount of one to affect the force. But if things are immiscible, especially oil and water, then a small amount of one can have a very dramatic effect. So, it does depend on the system.

The way we find these things out is actually to go and do experiments and make mixtures, add impurities and see what the effect is.

DR. POWELL: How about impurities on the mica itself? DR. ISRAELACHVILI: At the start, mica will have a mono-

layer in air of water, it will also have some carbon adsorbed on it, as we know from ESCA [electron spectroscopy for chemical analysis] studies, and so on, but this seems to dissolve away. It is sort of a 3-A layer, on average. That has been studied. You cannot get pure mica. It is a too-high energy surface. It has really got a layer of water or some carbonaceous material-but we know that it dissolves away in water, so we do not worry about it.

SYLVIA CEYER (Massachusetts Institute of Technology, Cambridge, MA): I just wanted to extend this discussion a little bit about what happens if you keep pushing. In other words, you have one molecule stuck between your two surfaces, and if you keep pushing, can you make that molecule fall apart?

The origin of my question comes from some experiments that we have recently done where we physically adsorbed methane on a surface of 400 Kelvin, and then took an inert gas beam, and the impact of the argon atom beam deforms the molecule sufficiently to make it fall apart.

DR. ISRAELACHVILI: If you have two flat surfaces with a layer of molecules in between them and you keep on pushing, you do not collapse the molecules into a black hole, or something. What happens is that they slide out, and that we see on many different systems, like a dislocation. The last

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4736 Symposium Paper: Discussion Proc. Natl. Acad. Sci. USA 84 (1987)

layer, you get a local breakthrough at one point, and then all the molecules file out. It is an elastic deformation that causes that last layer to leave. Are you with me?

DR. CEYER: No, I understand that, but I also think it is possible for you to actually break these bonds.

DR. ISRAELACHVILI: Maybe we can even break bonds. MEIR LAHAV (Weizmann Institute, Rehovot, Israel): The

question is to Dr. Garoff. How are you preparing the surfaces of the solid support on which you deposit your monolayers, question number one. And, secondly, can you elaborate a little bit on the size of the monolayers on the solid supports?

DR. GAROFF: The studies we have done show trends th'at are independent of the particular material systems we used. The vibrational spectroscopy was done on spontaneously adsorbed molecules such as sulfides on gold. The Langmuir- Blodgett diffraction study was done on a surface we came upon by accident: When we had just a carbon-coated electron microscopy grid, we did not get good enough adhesion of the monolayer, so we deposited about 20 A of SiO on it. We found substantial substrate effects. We observe strong pin- ning centers microns apart that seem to be suspending the monolayer between them.

The size of the tilt domains? We can take Langmuir's estimate of 100 A, and that is not too far off from my in-plane correlation length. However I have found several pieces of evidence that say that the tilt order seems to be decoupled from that lattice, so I have no guarantee that the tilt domains are as small as the in-plane correlation length.

CLAYTON TEAGUE (National Bureau of Standards, Gaithersburg, MD): Just going back to the area question very briefly, are your estimates of the actual area or average area of force interaction comparable with what you would get from theoretical crossed cylinders, or what kind of areas do you find?

DR. ISRAELACHVILI: The problem does not arise. If you have two curved surfaces like you mention, if we divide the force by the 27rR, that immediately gives you the energy between two flat surfaces. However, if you want to ask the question of what is the effective area over which the inter- action is, you can do something known as the Langbein approximation, which nobody knows about, except for Langbein. But it usually works pretty well-it is a rule of thumb. If you have two curved surfaces like that [shown on transparency], and they are distance D apart, and you work your way into one surface an equal distance, D, and draw a line across the axis, that area is your effective area of interaction, and you will find that for most power laws and exponential forces, that will give you the effective area over which things are happening.

If we are talking about two radii of 1 cm, which is roughly the radii we are dealing with, separated a few tens of angstroms, the effective interaction areas would therefore be hundreds of square microns.

DR. WHITESIDES: I have to say that, as an organic chemist, that is my idea of really useful deep theory. I am very pleased with that.

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