Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

5 THE LIQUID4AS INTERFACE 269osc.Fil.scopeFig. 48. Surface potential measurements by the vibrating plate method. The oscillator (Osc.)and amplifier (Audio amp.) are commercial units which drive the electrode. The oscilloscope(Scope) and high gain, low noise amplifier (Amp.) are also standard commercial instruments.From ref. 374.(b) The vibrating condenser methodThis method is similar to that used for measuring contact potential differencesat the solid-gas interface (see Section 2.1 S). It was applied to monomolecular filmsby Yamins and Zi~man~'~. An audiofrequency current is used to vibrate anelectrode attached to the voice coil of a loudspeaker at between 200 and lo00 cps.The electrode is normally about 1 cm2 in area and placed about 0.5 mm from thesurface. The vibration of the electrode causes a variation in the capacity of the airgap, so that an alternating current is set up in the second circuit whose magnitudedepends on the contact potential difference. This signal is amplified and detectedby earphones. An opposing voltage is applied with a potentiometer until the humdisappears, giving the contact potential of the surface. A more refined electronicarrangement described by Gaine~~'~ is shown in Fig. 48. Here, the signal isamplified, filtered to improve the signal-to-noise ratio, and displayed on anoscilloscope.Surface potential variations of about 0.1 mV can be detected, which is ratherbetter than the ionising electrode method. However, the method is subject to moredisturbances. Vibrations can be induced in the liquid surface which may disrupt themonolayer and give a noisy signal. Surface ripples on the liquid produce the sameeffect. The noise can be reduced by increasing the electrode-liquid spacing or byreducing the amplitude of vibration. Both of these alternatives reduce the sensitivityof the apparatus.5.4 OTHER METHODSThere are a number of other methods which have been used for the study ofmonolayer properties, but not applied at present to the study of reaction kinetics.References pp. 270-278