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page 102
The tension is calculated using the following equation:
F
σ
=
⋅
L
cos
(σ = surface or interfacial tension; F = force acting on the balance; L = wetted length,
θ = contact angle).
The plate is made of roughened platinum and is optimally wetted so that the contact
angle is virtually 0¡ã. This means that the term cos θ has a value of 1, so that only the
measured force and the length of the plate need to be taken into consideration.
Correction calculations are not necessary with the plate method.
15.1.3
Ring and plate methods in comparison
To make the selection of the method for measuring surface or interfacial tensions
easier the advantages of the ring and plate methods are listed here.
Advantages of the plate method
•
In contrast to the ring method, no correction is required for measurement values
obtained by the plate method.
•
The densities of the liquids don't have to be known as they have to be with the ring
method.
•
In an interfacial tension measurement the surface is only touched and not pressed
into/pulled out of the other phase. This avoids the phases becoming mixed.
With the ring method the surface or interface is renewed permanently due to the
•
movement of the ring. If the ring is moving with high velocity, but also if solutions of
large molecules or with high viscosities are to be measured, the maximum force is
obtained when the diffusion equilibrium at the surface or interface is still not
reached. The measurement failure caused by this effect does not occur with the
plate method. The plate method is a static measurement, i.e. the plate does not
move after the surface or interface has been detected.
θ
Measuring principles
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