92168.pdf

Liquid-gas and liquid-liquid interfaces 73

The detachment force is related to the surface or interfacial tension

by the expression

.. ftF

4-rrR

(4.11)

where F is the pull on the ring, R is the mean radius of the ring and

is a correction factor.

Figure 4.6 Measurement of interfacial tension by the ring method

To ensure zero and, hence, a constant contact angle, platinum
rings carefully cleaned in strong acid or by flaming are used. It is
essential that the ring should lie flat in a quiescent surface. For
interfacial work, the lower liquid must wet the ring preferentially
(e.g. for benzene on water, a clean platinum ring is suitable; whereas
for water on carbon tetrachloride, the ring must be made hydro-
phobic).
The correction factor /8 allows for the non-vertical direction of the
tension forces and for the complex shape of the liquid supported by
the ring at the point of detachment; hence, it depends on the
dimensions of the ring and the nature of the interface. Values of 0
have been tabulated by Harkins and Jordan^145 , they can also be
calculated from the equation of Zuidema and Waters^146.

(p~af =

7T^2 R^2 4irR(p{-p 2 )

+ c (4.12)