Page 86 - Introduction to Colloid and Surface Chemistry

76 Liquid-gas and liquid-liquid interfaces
dimensions (obtained photographically), flow rate, etc. The age of
the surface can be controlled to some extent by altering the flow rate.

Adsorption and orientation at interfaces

Surface activity
Materials such as short-chain fatty acids and alcohols are soluble in
both water and oil (e.g. paraffin hydrocarbon) solvents. The
hydrocarbon part of the molecule is responsible for its solubility in
oil, while the polar —COOH or -OH group has sufficient affinity to
water to drag a short-length non-polar hydrocarbon chain into
aqueous solution with it. If these molecules become located at an air-
water or an oil-water interface, they are able to locate their
hydrophilic head groups in the aqueous phase and allow the lipophilic
hydrocarbon chains to escape into the vapour or oil phase (Figure
4.9). This situation is energetically more favourable than complete
solution in either phase. Further discussion of this point in terms of
the intermolecular forces which are involved is given on page 85,
where the related phenomenon of micellisation is considered.

—0=53-
Vapour - Hyd roca rbon —^V~
phase phase
-JP—
ZCfc^~
WflW^WI
1
"~ *"" ~"'" C-^^ ^ " ""
v Aqueous -v .Aqueous.
phase phase o —
~^° ^ ~

Figure 4.9 Adsorption of surface-active molecules as an orientated monolayer at air-
water and oil-water interfaces. The circular part of the molecules represents the
hydrophilic polar head group and the rectangular part represents the non-polar
hydrocarbon tail. At the air-water interface, the hydrocarbon chains will tend to lie
horizontally on top of the water surface at low coverage, but will tend to assume more
upright configurations at high coverage (see page 103)