Page 84 - Introduction to Colloid and Surface Chemistry

74 Liquid-gas and liquid-liquid interfaces
where p l and p 2 are the densities of the lower and upper phases; a =
1 2
0.7250 and b = 0.090 75 m" s for all rings; c = 0.045 34-1.679 r/R;
and r is the radius of the wire.
Drop-volume and drop-weight methods

Drops of a liquid are allowed to detach themselves slowly from the tip
of a vertically mounted narrow tube (Figure 4.7) and either they are
weighed or their volume is measured. At the point of detachment

y (4.13)
2irr 2irr
where m is the mass of the drop, V is the volume of the drop, p is the
density of the liquid, r is the radius of the tube and <f> is a correction
factor.

Figure 4.7 Detachment of a drop from the tip of a narrow tube

The correction factor <J> is required because on detachment (a) the
drop does not completely leave the tip, (b) the surface tension forces
are seldom exactly vertical and (c) there is a pressure difference
147
m
across the curved liquid surface . $ depends on the ratio r/V .
Values of $ have been determined empirically by Harkins and
148 149 m
Brown ' . It can be seen that values of rlV between about 0.6
and 1.2 are preferable (Figure 4.8).
A suitable tip which has been carefully ground smooth used in
conjunction with a micrometer syringe burette gives a very convenient