The Molecular Universe

Like melting, the process of dissolving is commonplace; yet, like melting, it is quite remarkable. We are all familiar with the fact that a hard crystalline solid like salt, when placed in contact with water, apparently disappears in a short space of time; the crystalline structure breaks up and the atoms enter into the water. How and why does this process occur? And how do the atoms behave once they have dissolved? More puzzles soon arise when we start to think more closely about dissolving and solutions. Salt, for example, dissolves in water, but it will not dissolve in petrol; camphor (used in moth balls) dissolves, however, quite easily in petrol but not in water. While crystal like diamond and silicon will not dissolve in any liquid. So what controls whether a solid dissolves and in what it dissolves?

Let's consider more carefully the example of salt dissolving in water. We recall that salt (sodium chloride) has a simple crystal structure in which positive sodium ions and negative chloride ions are stacked together in a regular array. The electrical interactions between the positive and negative ions means that they are strongly bound at their sites in the crystal. To break up the crystal requires a large amount of energy; or the energy of interaction between the ions must be replaced by some other form of interaction. The latter is the key to understanding what happens when the ions dissolve in water; the interaction between the ions in the solid is replaced by the interaction between the ions and the water molecules in the solution. Water is a polar liquid; the oxygen atoms have a negative charge while the hydrogens have a positive charge. When the sodium ion enters the liquid water, the water molecules cluster round it so that the negative oxygens are next to the positive sodiums. Similarly the water molecules group around the chloride ions so that the positively charged hydrogens are directed toward the negative charge of these ions. And it is these ion-water interactions in the solution that replace the ion-ion interactions in the solid. The ions can break away from their partners in the crystal because they have found almost equally congenial ones in the solution. Such behavior is quite general: high solubility requires that the interaction between the atoms, ions or molecules in the dissolving solid be replaced by equivalent interactions between these species and the molecules of the solvent (i.e. the liquid into which the solid is dissolving). Normally, it still costs some energy for a solid to dissolve in a solvent. But if this is small enough, its effect can be outweighed by that of the increased disorder of the solution compared with the solid. The process is driven by the entropy of solution.

We are now in a position to understand an old chemical maxim: "Like dissolves like. Solids like salt which are built up from ions dissolve in polar solvents like water in which the atoms have high charges, because the electrical interactions between the ions and the solvent are needed to replace those in the solid. In crystals built up out of molecules like camphor, the forces are different; the molecules are bound in the crystal by weak 'van der Waals" interactions. Similar forces predominate between the molecule sin a solvent like petrol. So again the interactions between the molecules in the solid can be nicely replaced by those between the molecule and solvent and camphor dissolves in petrol.

Just as we saw that liquids, although disordered, have greater or lesser degrees of structure, so do solutions. Indeed, this feature should be apparent from our discussion of sodium and chloride ions in solution. The structures of ions in solution may be very well defined, as with the ions of, for example, the metallic elements iron and nickel have a well defined octahedron of water molecules around metal ions. Such hydrated ions may also be found in solids. (Indeed, solids like copper sulfate adsorb water from the atmosphere to form such species in the crystalline state.)They are, however, an example of the general class of chemical species known as "complex ions" involving metal ions and small molecules.

Much of what we have described so far relates to water as a solvent. Water is, of course, the most widespread liquid on the surface of our planet. And water is also an excellent solvent, as we have seen, for polar solids. Polar solids include not only those that are constructed from ions like sodium chloride, but those containing 'polar groups' like OH and CO. So sugar molecules like glucose and sucrose dissolve in water because the OH groups of the molecule can interact strongly with those of the water molecules; hydrogen bonding interaction plays a major role here. Indeed, molecules or parts of molecules (groups of atoms) can be classified as hydrophilic (water loving) or hydrophobic (water hating) depending on whether they contain polar groups. And the extent to which a molecule or group tends to either of these extremes will, of course, strongly influence its behavior in an aqueous environment (i.e. when surrounded by water). An important class of molecules in this context are those present in soaps and detergents, which have both hydrophilic and hydrophobic ends, leading to a range of useful and remarkable properties. Of even greater importance is the behavior, in the respect of biological molecules, especially proteins, whose shape (and hence function) is to a large extent controlled by the relative hydrophobicity of different parts of the molecule.

Solid Solutions

We do not normally think of solids like copper or silicon as being 'soluble' (i.e. as being able to dissolve), because there are no common liquids in which these solids will dissolve (although strong acids will "dissolve" metals like iron, but this involves a chemical reaction). Remember that for a solid to dissolve, the interaction between the atoms or molecules in the solid must be replaced by comparable ones in the solution; and there simply are not substances which are liquids at normal temperatures, in which the atoms or molecules interact sufficiently strongly with those of iron or silicon (without there being a chemical reaction). However, it is possible for these solids to dissolve in other solids forming solid solutions. So copper will dissolve in zinc to form an alloy (simply a solution of one metal in another) known as brass. Like most alloys, brass is crystalline, that is we have a regular arrangement of metal sites; but some are occupied by copper and some by zinc atoms. Alloy formation is very common, with other examples including pewter (tin and zinc) and bronze (iron and copper). Dissolving a small amount of one metal in another can also have drastic effects on physical and chemical properties. Stainless steel is essentially iron into which a small amount of chromium is dissolved, with drastic changes in the rate at which it corrodes (i.e. reacts with oxygen in the atmosphere to form a metal oxide). Alloying iron with copper in bronze results in a much tougher, less brittle material.

Solid solutions are widespread. Silicon, as we have seen, will not dissolve in any common liquid; but it will dissolve in germanium -a solid with the same crystal structure - in much the same way that copper dissolves in zinc. These solids further illustrate the point just made regarding alloys, namely that dissolving small amounts of one solid substance in another is a vitally important way of altering the properties of materials - and one that is used on an enormous scale in contemporary technology. The classic example is the semiconductor silicon: dissolving tiny amounts (less than one part per million) of phosphorus has a drastic effect on its ability to conduct electricity (making the material that is known as an 'n-type' semiconductor); similar amounts of arsenic have equally large effects but result in different electrical characteristics (the material becomes a 'p-type' semiconductor). Putting the two types of material together creates the famous p/n junction which has 'rectifying action', that is, when included in an electrical circuit, it allows electricity to flow only in one direction - a vital feature of electrical circuitry. Silicon with tiny quantities of deliberately introduced impurities is therefore the material basis of the technology on which the modern electronics revolution is based.