Page 12 - AN INTRODUCTION TO SURFACE CHEMISTRY By ERIC KEIGHTLEY RIDEAL

ANGLE OF CONTACT 7
a value of 21 dynes per cm. This is the increase of surface energy
owing to adsorption of water by wax. As we shall have occasion to
note a water surface will hold hydroxyl groups of alcohols quite
frmly but will exert little attraction for hydrocarbons, thus we may
assume that the surface of the wax employed by Ablett contained a
certain number of hydroxyl groups to which the water would adhere.-
For liquids which wet solids the angle is very nearly zero and
may be taken as actually equal to zero without serious error. For
ready determination of contact angles it is convenient to use surface
tension measurements by two methods of which, one depends upon,
and the other is independent of the angle of contact. Such a pair
of methods is the capillary rise and the bubble pressure method.
If the surface tensions as calculated by these two methods agree
to within one part in 500 it may be concluded that sin 0 < l Or
0<T' of are. This is true of most pure wetting liquids for which
accurate values of a by two such methods are available. By various
direct measurement methods Magie (Phil. Mag. xxv1. 150, 1888),
Langmuir (Trans. Farad. Soc. xv. 62, 1920), Anderson and Bowen
(Phil. Mag. xxx1. 143, 285, 1916) and Richards and Carver (J.A.0.S.
XLI11. 827, 1921) have likewise concluded that the angle of contact
is zero under these conditions.
A contact angle of zero or 180° implies that the liquid or gaseous
surface is asymptotic to the solid at contact. In a capillary tube
a liquid with zero angle of contact must therefore wet the tube to
some distance above the visible meniscus. It is an experimental
fact that unless care be taken to observe this condition erratic
values for a are obtained, no doubt because then the angle of
contact does not vanish. A liquid with an angle of contact of 180°
should similarly fail to wet a containing tube until well below the
meniscus: the column of liquid will then be shielded by a gas film
from the glass tube until this film is squeezed out by the hydro-
static pressure of the column, A zero contact angle between a given
liquid and solid is equivalent therefore to the spreading of the
liquid over the solid in presence of the gaseous phase present.
In the following pages a brief description of the more important
methods employed for the accurate determination of the surface
tensions of liquids is given.
Seo also N. K. Adam and G. Jessop, J.0.S, cxxvn, 1863, 1925.