Liquids and gases are both fluids, meaning that both flow. The characteristic that distinguishes a gas from a liquid is that the latter has a definite boundary. If you put quantities of oxygen gas and carbon dioxide gas into a container at room temperature and pressure, then even though the carbon dioxide is heavier and tends to stay at the bottom, the interface between the two is diffuse, and after a while they will mix completely. This is the case for all pairs of gases. However with oxygen gas and liquid water in the container, the water always stays at the bottom and the mixing is never complete. Even though the molecules of the water are moving at speeds even higher than those of the CO₂ in the first case, the boundary between gas and liquid does not become diffuse.

Another way of stating the same fact is that it takes energy to create more liquid interface. The energy required to create unit area of interface is a well-defined quantity and is called the surface tension of the liquid, symbolised g. The surface tension of common liquids ranges from about 20 mN/m for petrol to 75 mN/m for water.

Although the random thermal movements of the molecules do not cause the interface to become diffuse, they do perturb it. The surface is continually distorted by millions of tiny capillary ripples. The resultant roughness is well understood and has been measured by X-rays. The amplitude of the ripples on a water surface at room temperature is approximately 0.3 nm RMS, i.e. two molecular diameters.

The surface tension g which is responsible for the existence of sharp molecular interfaces is closely connected to the existence of first-order phase transitions. If a cylinder full of steam (gaseous water) is slowly cooled, the first result is just a continuous decrease in the pressure exerted on the walls, while the contents of the cylinder remain uniform. However at a specific temperature, drops of liquid water start to appear with a density ~1000 times that of the gas phase. The molecules in the liquid droplets are in equilibrium with those in the gas and hence have the same chemical potential (free energy per molecule), although the molecular packings in the two phases have almost nothing in common. All other molecular states, including halfway stages with intermediate density, show a higher chemical potential than either stable phase. The molecules of water at the interface are in effect forced to be in such a strained halfway stage. Hence it takes energy to create more interface.

The value of surface tension is inherently positive. If the g of a liquid interface ever became negative, then it would be energetically advantageous to create more interface. This is easily done by increasing the roughness of the interface. Within a short period of time, a sharp interface would cease to exist.

What is an amphiphile?

Paraffin oil and water do not mix, and this is a result of the interactions between their individual molecules. Molecules of water prefer other molecules of water, and molecules of oil prefer other molecules of oil. There are many substances showing this like prefers like behaviour. Substances which mix in water tend not to mix with oils, and vice versa. The former are called hydrophilic (Greek for water-loving), the latter hydrophobic (Greek for water-fearing).

An amphiphile is what results when you chemically link a water-fearing and a water-loving part. This apparently contradictory mixture of behaviours in the one molecule gives amphiphiles unusual properties and makes them very important both in industry and in science. The oldest known amphiphile is soap, and its cleaning action on your hands is closely related to its amphiphilic structure. All detergents are amphiphiles.

Soap is made by cooking either animal fats or vegetable oils with soda. The fatty or oily substances in animals and plants are all amphiphiles, and are called lipids (Greek for fat). The fact that there are amphiphiles in living things is no mere coincidence - the properties of amphiphiles are vitally important for life, and they are what all living things make their cell membranes out of. Without a membrane, a cell could have no individual identity, and molecules required by the cell would just diffuse away into the environment.

What is self assembly?

Self assembly is a way of fabricating ordered molecular structures with the ultimate spatial precision. It is a phenomenon in which a number of independent molecules suspended in an isotropic (disordered) solution spontaneously come together to form an ordered aggregate which is of molecular size in at least one dimension. Whilst we have learnt to emulate Nature to some extent, the most impressive examples of self assembly are still biological. For example the ribosome is quite a complicated machine for building up a polypeptide chain according to the set of instructions encoded in a chain of messenger RNA. Just as a lathe for machining metal knobs, say, is built up from a lot of individual components themselves made out of different sorts of metal, so a ribosome is built up from a large number of different sorts of polypeptides and RNA chains.

There is however a very important difference. The lathe is built up by its manufacturer from its component parts by step-by-step assembling the parts in their correct configuration and then fastening them together by welding or bolting to form the final unit. A cell does not do this. All it needs to do is to synthesize the polynucleotides and polypeptides of which the ribosome is made. The component parts then come together spontaneously into the correct configuration to form a functional aggregate. Closely related self-assembly techniques are used in many different ways by all living things to build up their internal structure.

Self assembly is similar in some respects to crystallization, for example of copper sulfate from a saturated solution in water. While the concentration of CuSO₄ is kept above the solubility limit, crystal nuclei will slowly form and grow. The crystal aggregates certainly show much more order than the original isotropic solution. However this is not self assembly, because they eventually grow to reach macroscopic size in all directions. The size restriction is not artificial but results naturally from the requirement for functionality.

The hydration of a lipid is now recognized as self assembly. Water is spontaneously taken up by an anhydrous lipid to form fantastic shapes called myelin figures. When the mixture is well agitated, the shapes become rounder and are called micelles, vesicles or liposomes. You may have heard of liposomes being used to encapsulate bioactive substances in cosmetics and medicines for slow release.

The driving force for this self-assembly is the attempt by the molecules to hide their hydrophobic parts from the water. At very low concentrations the amphiphile dissolves completely in water to form a true solution. At higher concentrations the molecules start to aggregate together in little balls with the hydrophilic headgroups pointing outwards in contact with the water, and the hydrophilic tails hidden away from it in the interior. These are micelles, which have molecular size in all dimensions over a range of concentrations of the lipid in the surrounding solution. The phase diagram on the left of sodium oleate/water mixtures shows these two regions. When there is more amphiphile and less water, the micelles become distorted, distending either to form a disc or a rod. Eventually they become infinitely extended. In the region of the phase diagram marked C the micelles have stretched into threads or columns which pack in a hexagonal array. In the regions L and L' of the phase diagram the micelles have flattened into sheets, or lamellae. Since a lamella consists of two individual layers of molecules, it is also called a bilayer.

What are Langmuir monolayers?

A Langmuir monolayer is a sheet of amphiphilic molecules all oriented with their hydrophilic heads on one side of the sheet and their hydrophobic tails on the other. This means that, unlike the bilayers of biomembranes, Langmuir monolayers cannot exist in a stable condition when completely surrounded by water. Instead, Langmuir monolayers assemble and exist at pre-existing interfaces between phases with different degrees of hydrophilicity.

The most hydrophilic molecular medium is water, whilst the most hydrophobic media are gases, e.g. air. Hence the most extreme example of a hydrophilicity interface is a water surface. Liquid amphiphiles, for example cooking oils, spontaneously spread over such a surface until the film is just one molecule thick. The calming effect of oil on a rough sea was known to the ancients, including Pliny the Elder and Plutarch. The earliest scientific observation of this effect was reported by the great American statesman Benjamin Franklin[1], who on a trip to London performed a famous experiment on the pond at Clapham Common:

"...the oil, though not more than a teaspoonful, produced an instant calm over a space several yards square, which spread amazingly, and extended itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass."

Simple arithmetic shows that the thickness of Ben's oil film was indeed only one molecule, although the significance of this was only realized over a century later.

The calming of waves used by Franklin to detect the presence of a monolayer on a water surface is still studied scientifically today. However the most convenient indication that there is a monolayer on the surface is its effect on the surface tension. Pure water has a surface tension of approximately 72 mN/m, but a monolayer can cause this to drop nearly to zero.

What is an isotherm?

The change of surface tension caused by the presence of a Langmuir monolayer on a water surface is called its surface pressure, symbolized , and for any given substance it depends on the total area of the water surface divided by the number of molecules. The curve of the surface pressure against area per molecule at a constant temperature is called an isotherm, and it often contains sharp bends or kinks indicative of phase transitions in the two-dimensional layer[2]. The figure shows the isotherm of stearic acid at 20°C, in which three distinct regions of differing compressibility can be seen. At the lowest pressure, close to the area-axis, is the region of coexistence of the gas and one of the liquid-condensed phases. Then comes the liquid-condensed region (L2), a group of hexatic mesophases in which the molecules are tilted. The near-vertical part of the isotherm is the superliquid phase (LS), sometimes erroneously called the solid phase, another hexatic mesophase in which the molecules stand upright. Finally at a higher pressure c the monolayer becomes unstable with respect to bulk phase and it collapses. It should be kept in mind that the two phase transitions visible in the stearic acid isotherm are not the same as the main phase transition which is of most importance for the lipids in biological cell membranes. Together with two colleagues, Dr. C. Mingotaud has compiled a useful reference book[3] giving the isotherms for many amphiphiles.

What is a Langmuir-Blodgett film?

In 1918 Langmuir discovered that a single water-surface monolayer can be transferred to a solid substrate, and it was his assistant, Katharine Blodgett, who 16 years later discovered that the process could be repeated[4], leading to a multilayer stack of any desired thickness. A film made in this way, from one monolayer to many wavelengths of light in thickness, is called a Langmuir-Blodgett film.

Monolayers on the water surface are rather fragile, and their constituent molecules are quite mobile. After transfer to a solid substrate the molecules are largely frozen in place, and the layers are much more robust. An LB film can, for example, be mailed through the post, or placed in a vacuum. Nevertheless the LB film preserves many of the structural aspects of its constituent monolayer(s) prior to deposition.

Modern technological interest in Langmuir-Blodgett (LB) films began with the work of Kuhn, based initially in Marburg, Germany in the late 1960's[5] and later in Göttingen (where his group gave birth to Nima's partner company NFT). Over a decade, Kuhn and his colleagues showed that LB films could be fabricated with all the characteristics required for an information-processing technology in which individual molecules perform distinct functions. They showed not only that the films could be made with molecular-scale patterning, but that previously deposited 'sub-assemblies' could be manipulated to build up more complicated systems[6]. They showed that the built-up films could remain stable for long periods, and that their defect levels were acceptably low[7]. Kuhn's exciting results and his effective communication of them not only around Europe but also around the world inspired many groups to carry on his research.

There are now regular conferences both local and international on this topic. The next International Conference, LB10, will be in China in the year 2003. The field has evolved over the years. The International Conference is no longer limited to the LB technique, but covers a whole range of techniques of organic nanotechnology, and the delegates include physicists, chemists, biologists and electronic engineers.

What is a Gibbs monolayer?

A Gibbs monolayer is structurally identical to a Langmuir monolayer, with the only difference being the solubility of the amphiphile. The substances which form Langmuir monolayers are essentially insoluble, so that the molecules are trapped at the air-water interface. In a Gibbs monolayer, the molecules can hop in and out from the water. Since there is no such thing as absolute insolubility, there is no hard dividing line between the two types of monolayer, the distinction depending, amongst other things, on the depth of the water under the monolayer and the time scale on which the experiment is performed. The amphiphiles giving Gibbs monolayers are the ones useful for detergents.

Distinct experimental techniques are used on the two types of monolayer. There is no point in directly measuring the isotherm of a Gibbs monolayer, because on reducing the area available to the surface molecules on one side of a barrier they will just dissolve into the water on that side and re-emerge on the other. The surface pressure, and the area of water surface per molecule at the interface, depend on the concentration in the solution. Instead, Gibbs monolayers are characterized using a tensiometer. The surface tension is usually plotted versus the logarithm of the concentration. The concentration at which the surface tension bottoms out is the critical micelle concentration, or CMC.

The isotherm can be derived from this plot using the Gibbs surface isotherm equation. The surface concentration is in fact its gradient, and the area per molecule its inverse. Hence when both are rotated through 90°, the isotherm is the derivative of the Gibbs plot. Beyond the CMC the Gibbs surface isotherm is not applicable.

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