92168.pdf

Liquid-gas and liquid-liquid interfaces 113

interaction one component displaces the other, usually at the collapse

pressure of the displaced material; or (b) interacting mixed films

collapse as a whole at a surface pressure different from, and usually

greater than, the collapse pressure of either component.

Another type of interaction is the penetration of a surface-active

constituent of the substrate into a spread monolayer. Penetration

effects can be studied by injecting a solution of the surface-active

material into the substrate immediately beneath the monolayer: (a) if

there is no association between the injected material and the

monolayer, ir and AF will both remain unaltered; (b) if the injected

material adsorbs on to the underside of the monolayer without actual

penetration, AV will change appreciably but irwill alter very little; (c)

if the injected material penetrates into the monolayer (i.e. when

there is association between both polar and non-polar parts of the

injected and original monolayer materials), TT will change significantly

and A V will assume an intermediate value between A V of the original

monolayer and A V of a monolayer of injected material. Penetration

is less likely to occur when the monolayer is tightly packed.

It is often desirable (e.g. in emulsions) to have a surface film of

charged species; however, as such, repulsion between the head

groups usually makes such a film non-coherent. Surface films which

are both charged and coherent can be produced by use of a mixture of

ionic and non-ionic surfactants, especially where the structural

characteristics of the surfactant molecules are such that they pack

efficiently between one another.

Biological membranes^59 "^61

Biological membranes consist mainly of lipoprotein material. The
classic experiment relating to membrane structure was first performed
by Gorter and Grendel^160. They extracted the Hpid from erythrocyte
membranes, spread it at an air-water interface and found that
the film compressed to a limiting area which corresponded to twice
the external area of the cells from which the Hpid was derived. The
results of experiments such as this led Danielli and Davson^161 to
propose that the cell membrane consists essentially of a bimolecular
layer of Hpid with the hydrocarbon chains orientated towards the
interior and the hydrophilic groups on the outside. As with micelles
(see pages 85 and 88), the organisation of such a cell membrane is
primarily the result of hydrophobic bonding^49. The permeability