Page 123 - Introduction to Colloid and Surface Chemistry

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.
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Biological membranes "

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
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primarily the result of hydrophobic bonding . The permeability