Attraction of a repulsive way to stop things getting too close


January 16, 2004

Isaac Newton's interests in gravitation and the nature of light and colour are well known, and his involvement with alchemy has also become widely known in recent years. But these were not his only interests. One phenomenon that puzzled him was capillarity: when a glass tube is dipped into water, the water rises up the tube to a level above the surrounding liquid. The level of mercury in a capillary tube, on the other hand, is lower than that of the surrounding liquid.

Newton remained interested in this phenomenon for years. His assistant at the Royal Society carried out experiments that showed that the rise was higher in narrower tubes. But neither Newton nor any other scientist of the time was able to find a satisfactory explanation. The problem remained unsolved for a century until Thomas Young showed that it could be explained in terms of a new property that he called "surface tension", and Laplace independently showed that attractive forces between the particles of the liquid could account for it.

Laplace did not know what the particles of the liquid were, or whether they were discrete particles or just notional elements of a continuous fluid.

Nor did he have any idea what the force was. The idea that substances were composed of molecules built from atoms of a limited number of distinct elements was by then beginning to emerge, but the forces remained mysterious for much longer. Eventually van der Waals was able to describe the relationship between gases and liquids using two ideas: that the molecules could not overlap but occupied a finite volume; and that attractive forces between them would reduce the pressure by an amount proportional to the square of the density. The resulting equation of state described brilliantly the equilibrium between liquid and gas, and the fact that at high enough temperatures, above the "critical point", liquid and gas are indistinguishable.

But only with the advent of quantum mechanics in the 1920s did it become possible to understand them from first principles. In the first few years after the formulation of quantum mechanics, Heitler and London were able to explain the repulsive force that kept molecules from getting too close to each other, and London deduced the dispersion force that attracted them to each other at greater distances.

John Rowlinson's Cohesion gives a detailed account of the story. This is a scientific history, and a sound knowledge of physical chemistry to degree level is a prerequisite for much of the material. Rowlinson remarks that the field of intermolecular forces has only intermittently been a major focus of scientific activity over the past three centuries, but modern research on the behaviour of proteins and other biological molecules has increasingly come to depend on a better understanding of this very fundamental subject. Rowlinson provides a fascinating historical background to it.

Anthony J. Stone is reader in theoretical chemistry, University of Cambridge.

Cohesion: A Scientific History of Intermolecular Forces

Author - J. S. Rowlinson
Publisher - Cambridge University Press
Pages - 333
Price - £70.00
ISBN - 0 521 81008 6

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