92168.pdf

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.

Vapour

phase

v Aqueous

phase

—0=53-

• Hyd roca rbon —^V~
  phase
  ZCfc^~
  -JP—

WflW^WI

"~ *"" ~"'" C-^^^1 ^ " ""

-v.Aqueous.

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)