Page 11 - AN INTRODUCTION TO SURFACE CHEMISTRY By ERIC KEIGHTLEY RIDEAL

6 SURFACE TENSION OF LIQUIDS

If az, no, 0 be the respective interfacial surface tensions the
total increase in energy will be
6Aa z +~Ao pe cos a -- Aa0,
where ~A is the increase in area of the solid now in contact with
B instead of with C, but this total energy change is zero, hence
o4¢-1n=ecos ,
Hence the difference between the tensions at the two solid-fluid
interfaces which is the quantity always involved in equations of
equilibrium can be expressed in terms of the fluid-fluid tension and
an angle, called the angle of contact which is plainly susceptible of
direct measurement.
The determination of this angle involves a certain amount of
difficulty and very different results have been obtained by different
observers for the angle of contact between the same pair of fluids at
8 given solid surface. The results have been found to differ according
to whether the fluid has been in previous contact with the solid and
the length of time during which contact has occurred. Thus Quincke
found the angle of contact of mercury-air on a glass plate to be
initially 148° 55' but this value fell after two days to 137° 14. Ablett
investigated the angle of contact between water and a horizontally
revolving cylinder of wax half immersed in the liquid. With the
drum at rest a was found to be 104° 34' ± 5' at 104°C. When the
drum was rotated at speeds of 044 mm. per sec. or greater so as to
bring an unwetted part of the paraffin in contact with the water a
was found to be 113° 9 45', on reversal of the rotation so that the
wet wax moved upwards out of the surface a was 96° 20' ±5, 'The
mean of the two latter angles is 104° 44' which agrees within limits
of error with the value for the cylinder at rest. As the velocity of
rotation became less than 044 mm./sec. the difference between
the extreme angles diminished but the mean remained the same as
before.
If these results are compared with the equation
o¢-cg=opecosa,
it follows that c%as.ate must be greater for wet wax than for dry since
the surface tensions water-air and water-wax remain unaltered. 'The
increase of surface tension of the wax-air interface on wetting is
ope(cos a, -- cos a,). Taking a,o as 743 dynes per cm. we obtain