48 BIOLOGICAL CHEMISTRY 



there is no attraction from outside, hence the layer of liquid 

 is slightly more condensed on the surface. This condensation 

 produces a stress across the surface of the liquid. Most 

 observations have been made on air-liquid surfaces, but the 

 liquid-liquid surfaces are of greater importance in Biology. 



Owing to the stress in the surface there is a tendency to 

 reduce the amount of surface. Hence a liquid suspended in 

 another liquid of about the same specific gravity assumes the 

 shape of a sphere as this has the smallest surface for a given 

 volume. 



Substances which decrease the surface tension accumulate 

 on the surface whilst substances which raise the surface tension 

 become less concentrated on the surface. This is known as 

 the Gibbs-Thompson law. The differences in concentration 

 at the surface are spoken of as adsorption. 



The spherical form and the changes in concentration are 

 both explained by the Law of Minimum Energy which states 

 that a system assumes the condition of least available energy. 

 Thus by reducing the extent of surface or by decreasing the 

 surface tension, the total available energy (surface X surface 

 tension) is decreased. 



Owing to the surface tension a pressure is produced. It is 

 well known that the pressure inside a soap bubble is greater 

 than that of the surrounding air because the tension of the two 

 surfaces (one inside and one outside) of contact between the 

 soap film and the air causes the soap film to contract on the 

 contained air. The pressure on the concave side of a sur- 



T T 



face is greater than that on the convex side by -f 



v\ ?i 



where T=surf ace tension and 7^ and y 2 are the radii of curva- 

 ture in two planes at right angles to each other.* Thus in a 

 sphere with uniform internal pressure a local change of surface 

 tension will cause a change of curvature. If the pressure 

 inside is to remain the same, a decrease in surface tension 

 must be accompanied by a decrease in the radii of curvature, 

 hence a projection or pseudopodium is produced. 



An electrical charge on the surface lowers the surface 

 tension. This is explained as follows : If the particles of 

 the surface are charged they repel each other owing to the 

 mutual repulsions of similarly charged bodies. This repulsion 

 counteracts the attraction between the molecules, hence the 

 surface tension is less. 



The electrical charge on the surface also affects adsorption 

 * W. H. White, A Handbook of Physics, Methuen & Co., p. 235. 



