Experimental Techniques for Studying Adsorption Kinetics

The two most suitable techniques for studying adsorption kinetics are the drop volume method and the maximum bubble pressure method. The first method can obtain information on adsorption kinetics in the range of seconds to some minutes. It has the advantage of measurement both at the air/liquid and hquid/liquid interfaces. The maximum bubble pressure method allows one to obtain measurements in the millisecond range, but it is restricted to the air/hquid interface. Both techniques are described below. 

As mentioned above, the drop volume method is of dynamic character and it can be used for adsorption processes in the time interval of seconds up to some minutes. At small drop time, the so alled hydrodynamic effect has to be considered [27]. This gives rise to apparently higher surface tension. Kloubek et al. [28] used an empirical equation to account for this effect. 

Ve is the unaffected drop volume and V t) is the measured drop volume. JQ is a proportionality factor that depends on surface tension y, density difference Ap and 

Miller [20] obtained the following equation for the variation of V(f) with time, 

The drop volume technique is limited in its application. Under conditions of fast drop formation and larger tip radii, drop formation shows irregular behaviour. 

At t = 0 (initial state), the pressure is low (note that the pressure is equal to 2y/r since r of the bubble is large, p will be small). At t = r (the smallest bubble radius that is equal to the tube radius), p reaches a maximum, whilst at 

The measuring cell, which is equipped with a water jacket for temperature control, simultaneously holds the measuring capillary and two platinum electrodes, one of which is immersed in the liquid under study while the second is situated exactly opposite to the capillary and controls the size of the bubble. The electric signals from the gas flow sensor PSj and pressure transducer PS2, the microphone and the electrodes, as well as the compressor, are connected to a personal computer which operates the apparatus and acquires the data. 

The value of Tj, equivalent to the time interval necessary to form a bubble of radius R, can be calculated using Poiseuille s law. 

At the right-hand side of the critical point, the dependence of p on I is linear, in accordance with Poiseuille s law. Under these conditions. 

The critical point in the dependence of p and L can be easily located, and is included in the software of the computer program. 

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