Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

5 THE LIQUID-GAS INTERFACE 267balances have been constructed; one version described in detail by Gaine~~’~incorporates a linear variable differential transformer driving the sweep devicewhich restores the surface pressure to a previously set value. Alternatively, thesweep device may be driven at a constant speed and the output of the transformerrecorded directly, as a surface pressure change.The two major problems which arise in using a film balance are leakage pastthe ends of the barrier and contamination. There are several ways to preventleakage; Langmuir’s original method was to direct air jets on the gaps at the endof the float, but this method is unsatisfactory at high surface pressures. Thinribbons of gold or platinum foil, waxed to prevent wetting, are more convenient,although the use of plastics such as Teflon is increasing. Because the ribbons canaffect the sensitivity, other workers have used fine threads of nylon. Slight irregularitiesin the ribbons or threads can lead to incomplete contact with the surface.The resulting leaks can usually be overcome by stroking the water surface up to theribbon, or sprinkling talc or sulphur powder in front of the ribbon. One way ofdetecting leaks is to sweep the rear part of the trough towards the float at the endof the experiment. Any deflection of the float is strong evidence for leaks or contamination.Troughs are generally made of metal, coated with paraffin wax to make themhydrophobic. Stainless steel or chromium plated troughs coated in this mannerare probably the most satisfactory. Glass or silica troughs give less contamination,but do not coat as easily with wax, are more fragile, and are difficult to produceto accurate dimensions. One disadvantage of waxed troughs is that the wax issoluble in most solvents used for spreading the film, thus providing a large possiblesource of contamination. A variety of plastic materials have been used to overcomethis problem, but Teflon is superior to most of the others. It is sufficientlyhydrophobic, but lacks dimensional stability and cannot be easily joined together.5.3 SURFACE POTENTIAL MEASUREMENTSThe potentiaI change AV which occurs when an insoluble monolayer is spreadat a liquid-gas (or liquid-liquid) interface is called .the surface potential of thefilm. It is frequently expressed in terms of a surface dipole moment p, wherep = AV/4nnn being the number of molecules per cm2 in the monolayer. An alternative form iswhere A is expressed in A’ . molecule-’, AV in millivolts and p is obtained inReferences pp. 270-278