C.J. Gommes, Chim0698

CEDRICJ.GOMMES PHYSICALCHEMISTRY OFINTERFACES UNIVERSITYOFLIÈGE Copyright © 2014 Cedric J. Gommes First printing, December 2014 THEREISNOEASYWAYTOTRAINANAPPRENTICE.MYTWOTOOLSARE EXAMPLEANDNAGGING.IWILLSHOWYOUWHATITISIDO,ANDTHENI WILL TELL YOU TO DO OTHER THINGS YOURSELF. DO YOU UNDERSTAND? LEMONYSNICKET, ALLTHEWRONGQUESTIONS NOUSPRENONSENGARDELESOPINIONSETLESAVOIRD’AUTRUI,ETPUIS C’ESTTOUT:ILLESFAUTFAIRENÔTRES.NOUSSEMBLONSPROPREMENT CELUI,QUIAYANTBESOINDEFEU,ENIRAITQUÉRIRCHEZSONVOISIN,ET YENAYANTTROUVÉUNBEAUETGRAND,S’ARRÊTERAITLÀÀSECHAUF- FER,SANSPLUSSESOUVENIRD’ENRAPPORTERCHEZSOI.QUENOUS SERT-IL D’AVOIR LA PANSE PLEINE DE VIANDE, SI ELLE NE SE DIGÈRE, SI ELLENESETRANSFORMEENNOUS?SIELLENENOUSAUGMENTEETFOR- TIFIE? MICHELDEMONTAIGNE, ESSAIS, I, 25, DU PÉDANTISME Contents Introduction 0-1 Surface energies and Laplace’s pressure I–1 Wetting, a short introduction II–1 Solved problems involving wetting III–1 Intermolecular forces IV–1 Gibbs-Thomson Effects V–1 Thermal fluctuations VI–1 Metastability and nucleation VII–1 Adsorption on surfaces VIII–1 Introduction Gold is known as a shiny, yellow noble metal that does not tarnish, has a face centred cubic structure, is non-magnetic and melts at 1336 K. However, a small sample of the same gold is quite different, providing it is tiny enough: 10 nm particles absorb green light and thus appear red. The melting temperature decreases dramatically as the size goes down. Moreover, gold ceases to be noble, and 2-3 nm nanoparticles are excellent catalysts which also exhibit considerable magnetism. At this size they are still metallic, but smaller ones turn into insulators. Their equilibrium structure changes to icosahedral symmetry, or they are even hollow or planar, depending on size. Emil Roduner, Size matters: why nano materials are different. Everything goes nano these days. Examples from the marketing world are myriads, from cars to watches, shoes and toy helicopters. I personally own a music player bearing that name, to which I am listening at the very moment where I am writing these lines. Poli- cymakers even seem to be anticipating a nano-revolution. So what exactly is this hype all about? Figure 0–1: Thomas Graham (1805- 1869) Let us not fool ourselves: nanomaterials aren’t anything new. The oldest known example of a nanotechnology is probably the Lycurgus Cup, which is a 4th century roman cup currently exposed at the British Museum. The cup exhibits dichroism due to the presence of colloidal silver within the glass. Another spectacular example is that of so-called Damascus blades, which are made up of steel with specific inclusions, which some researchers clim to be carbon nanotubes. Closer to us, the entire field of colloids, born at the end of the nineteenth century under the influence of people such as Thomas Graham and Michael Faraday, among others, is about nanometer-sized objects. The fact is that nanometer-sized objects have properties that are distinctly different from chemically identical, Figure 0–2: Michael Faraday (1791-1867) but larger objects. The quote at the beginning of the chapter is the spectacular example of gold. 0-2 c.j. gommes, chim0698 A first difference between today and say, twenty years ago is the fantastic progress made in the field of electron microscopy, which enables us to visualise nanometer-sized objects with unprecedented accuracy. Advanced techniques such as electron tomography even Bottom-up and top-down methods enable one a obtain 3D reconstructions of structures with nanome- refer to whether the larger structures are created last or first. Typically, ter resolution. The second difference is the progress in the synthesis lithography is top-down. Chemical of nano structured materials by a variety of techniques, both phys- methods, by which molecules assemble into nano structures is bottom-up. ical and chemical, top-down and bottom-up. The fields of synthesis of nano materials, and that of fundamental understanding of nanometer-scale phenomena are clearly feeding on each other. It is because we understand better nanometer-scale phenomena that we can create structures on that scale in a more controlled way. In turn, those better-defined structures enable us to better understand nanometer-scale phenomena. This is incidentally the reason why sci- entific or technological progresses are exponential: because they feed on themselves. There are, however, still some problems on the horizon. For example, in the field of imaging of nanostructures one is often limited to studying materials in vacuum, which is a very different environment from that in which most nanomaterials are generally used. The de- velopment of so-called in situ imaging techniques at the nanometer- Figure 0–3: One of the colloidal gold suspension prepared by Faraday in the scale, to visualise nano structures in working conditions, is still a 1850’s, which is still kept by the Royal challenge. One therefore often relies on indirect methods, such as Insitution. Read more about it here small-angle X-ray scattering. In the field of synthesis too, our current skills are limited. Although the word nano fabrication is sometime used, our current abilities are far from the self-multiplying robots that terrorised Great Britain in 2001 under the ominous name of "Grey Goo". A positive way to look at this is to think that there are many things left to be developed by new scientists and engineers in that field. The word colloidal was coined by Graham to characterise "glue-like" sub- stances. The official IUPAC definition When people express their enthusiasm about nanoma- of that word is as follows: "State of subdivision such that the molecules or terials, they often say that it is fantastic that we can control the polymolecular particles dispersed in a shape of materials on a scale as small as a few nanometers. Before medium have at least one dimension we comment on this, we should first emphasise that there is already between approximately 1 nm and 1 mm, or that in a system discontinuities an existing and successful science, the purpose of which is to control are found at distances of that order." matter over a still smaller scale, that of the Angstrœm. That science In other words, colloidal is almost synonymous with nano. is naturally chemistry. So size per se is not the issue. The real diffi- culty with nano structures is that they at the interstice of two worlds: It is important that you become familiar with a few orders of magnitude. For the microscopic and the macroscopic. Systems in the former world example: the number of molecules are generally made of a few molecules, interacting with very strong or atoms in a macroscopic amount of matter is 1023, the size of an atom is covalent bonds, and subject to thermal fluctuations. By contrast, in 1 Å, the energy involved in chemical the latter world systems comprise a huge number of molecules (say, bonding is 1 eV, the thermal energy at one mole) interacting through very weak forces, and thermal fluctu- room temperature is 25 meV, etc. introduction 0-3 ation is generally negligible. What is difficult and interesting about naometer-sized systems is not really that they are small, it is that they are intermediate between two worlds. It is often unclear which type Because nano materials are squeezed of physics applies: would a 5 nm particle better be thought of as a between the macroscopic and the microscopic worlds, one often refers to small crystal or a large molecule? The answer is obvious "it depends". them as being mesoscopic. The greek The purpose of CHIM0698-1 is to guide you into mesoscopic physics. prefix "meso" means "in between". Figure 0–4: When a given object (here a cube) is cut into objects of half the ini- tial size, the total surface area exposed is multiplied by two. At this stage, you may wonder why the title of the course is "Physical Chemistry of Interfaces" and not, say "Intro- duction to nano sciences". The reason is that all many properties of nanostructured materials result from their having a large surface-to- volume ratios. In other words, interfaces play an important role at the nanometer-scale. This is illustrated in Fig. 0–4 with the unfolding of a paper cube, compared to that of eight smaller cubes. The surface is two times larger in the latter case. To put this in a more quantitative way, try to estimate the specific surface area of a material made up of silica particles with a diameter of 10 nm. Obviously, if you double the mass of material, you double the surface. So the only sensible quantity to evaluate is expressed in m2/g, for example. The word "specific" means "per unit of mass". Now, when I wrote "try to estimate.." I really meant it: before reading what follows try to do it! A possible way to calculate this is to estimate first the area of a single nanoparticle, and then to calculate how many nanoparticles there are in one unit of mass. The area of one spherical particle of radius R is 4pR2 so that the total area of the N particles in the material is A = N 4pR2 (0–1) × The mass of a single particle is 4/3pR3 r where r is the density × (the specific mass) of the considered material. The number of parti- It is useful to know the order of mag- cles in a mass m of material is therefore nitude of the density of a few common materials. The density of water, and of most ordinary solvents is 1 g/cm2, that 4 N = m/ pR3r (0–2) of noble metals is around 20 g/cm3. 3 The density of silica (glass) and of graphite are both close to 2 g/cm2. 0-4 c.j. gommes, chim0698 The specific surface area of the material is finally A 3 = (0–3) m rR In the particular case of R = 5 nm, and r = 2 g/cm3, one finds A/m = 300 m2/g.

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