Page 108 - Introduction to Colloid and Surface Chemistry

98 Liquid-gas and liquid-liquid interfaces
for containing and manoeuvring insoluble monolayers between
barriers for the direct determination of ir-A curves. The film is
contained (Figure 4.18) between a movable barrier and a float
attached to a torsion-wire arrangement. The surface pressure of the
film is measured directly in terms of the horizontal force which it
exerts on the float and the area of the film is varied by means of the
movable barrier.

Movable n
barrier »• Float
/Ar \
_
— — — — Aqueous substrate — —

Figure 4.18 The principle of the Langmuir-Adam surface balance

The film must be contained entirely between the barrier and the
float without any leakage. To achieve this, the trough walls, barrier
and float must be hydrophobic and the liquid level must be slightly
above the brim of the trough. Teflon troughs and accessories are ideal
in this respect. Silica or glass apparatus which has been made
hydrophobic with a light coating of purified paraffin wax or silicone
waterproofing material is also satisfactory. Waxed threads prevent
leakage past the ends of the float.
Before spreading, the surface must be carefully cleaned and freed
from contamination. Redistilled water should be used. The liquid
surface in front of and behind the float can be swept clean by moving
barriers towards the float and sucking away any surface impurities
with a capillary joined to a water pump.
To achieve uniform spreading, the material in question is normally
predissolved in a solvent such as petroleum ether to give a c, 0.1 per
cent solution. A total of c. 0.01 cm 3 of this spreading solution is
ejected in small amounts from a micrometer syringe burette at
various points on the liquid surface. The spreading solvent evaporates
away, leaving a uniformly spread film. Benzene, although frequently
used as a spreading solvent, is not entirely suitable, owing to its slight
solubility in water and long residence time at the interface (see page
96).