Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

5 THE LIQUID-GAS INTERFACE 265The film pressure is given byn = AW/pwhere A W is the difference between the total weight and the weight of the slide,p is the perimeter of the slide. A W can be determined by prior calibration of weightchanges in terms of a scale deflection, or by changing the balancing weight torestore the balance to the null position. It is recommended that films should alwaysbe studied in compre~sion~'~. Glass slides can be cleaned with chromic acid, micaslides by rinsing in alcohol after roughening with carborundum paper. The disadvantagesof the Wilhelmy slide method are that the contact angle must remainconstant during the experiment and that the reference surface tension, i.e. theclean surface value, cannot be measured during the experiment.(b) Film balanceIn most experiments, film pressures have been measured directly by means of afilm balance or Langmuir-Adam trough. The principle of the method involvesthe direct measurement of the horizontal force on a float separating the film fromwater level qh k 3WFig. 46. A modern film balance. 1, torsion wire control; 2, sweep control; 3, sweep holder; 4,trough; 5, sweep; 6, float; 7, mirror; 8, calibration arm; 9, head; 10. main torsion wire; 11, goldfoil; 12, wire for mirror; 13, elevation control; 14, guide; 15, traverse. From ref. 1.References pp. 270-278(C)