Capillarity and Surface Forces (Liquids) - Surface and Colloid Chemistry: Principles and Applications - page 27

Ring

Water

FIGure 2.12 Ring method (see text for details).

ring, which is usually constructed of platinum, is to be used; the ring should be kept
horizontal (a departure of 1° was found to introduce an error of 0.5%, whereas one
of 2.1° introduced an error of 1.6%); and care must be taken to avoid any disturbance
of the surface as the critical point of detachment is approached. The ring is usually
flamed before use to remove surface contaminants such as grease, and it is desirable
to use a container for the liquid that can be overflown so as to ensure the presence of
a clean liquid surface.
A zero or near-zero contact angle is necessary; otherwise, results will be low. This
was found to be the case with surfactant solutions in which adsorption on the ring
changed its wetting characteristics, and in which liquid-liquid interfacial tensions
were measured. In such cases, a Teflon or polyethylene ring may be used. When used
to study monolayers, it may be necessary to know the increase in area at detachment,
and some calculations of this are available (Adamson and Gast, 1997).

2.6.3 W If l h e l m y S l I d e ( o r p l a T e ) m e T h o d
The methods so far discussed have required more or less tabular solutions, or oth-
erwise correction factors to the respective “ ideal ” equations. Further, if continuous
measurements need to be made, then it is not easy to use some of these methods
(such as the capillary rise or bubble method).
The most useful method of measuring surface tension is by the well-known
Wilhelmy plate method. If a plate-shaped metal is dipped in a liquid, the surface ten-
sion forces will be found to produce a tangential force (Figure 2.13). This is because
a new contact phase is created between the plate and the liquid.
The total weight measured, W total , would be

W total = weight of the plate + γ (perimeter) − updrift (2.37)

The surface force will act along the perimeter of the plate (i.e., length [L p ] + width
[W p ]). The plate is often very thin (less than 0.1 mm) and made of platinum, but even
plates made of glass, quartz, mica, and filter paper can be used. The forces acting
on the plate consist of the gravity and surface tension downward, and buoyancy due