Understanding Surface Tension of Liquids: Factors and Measurement Methods

[pivoxa15]

The surface tension of a liquid such as water depends on the medium in which it shares (or the substance that shares with the liquid's boundary)?

If that medium or substance is air than the surface tension measured would be higher than if it was glass.

Correct?

If you try to measure surface tension by only using the method of putting a (glass) capillary in a liquid such as water and see how far it rises, you will only calculate the surface tension of water to glass. It is not possible to calculate the surface tension of water to air this way?

 

[Gokul43201]

To the best of my knowledge, a liquid surface (if not additionally specified) is defined as the liquid-gas interface. A liquid-solid interface does not typically fall under the classification of a surface, unless specifically stated.

The reason for including only the former type of interface is that the liquid-gas intermolecular interactions are typically negligible compared to the liquid-liquid intermolecular interactions, irrespective of the gas (whereas the liquid-solid interactions depend strongly on the nature of both substances). This large difference is what gives the surface different properties than the bulk. And the ability to approximate the liquid-gas interactions as non-existant, makes it relatively easy to calculate these properties.

With a liquid-sold interface, things become more tricky - there could be adhesive forces between the two. In the limit of very weak adhesion between a liquid and its neighboring solid (e.g., Hg and glass, but not mercury and brass nor water and glass), it becomes reasonable to include this interface also as a part of the liquid surface. In the limit of strong adhesive forces, the interface becomes the mathematical opposite of a surface, and can be treated as though the "surface tension" had a negative value there. This adhesion between some liquid-solid pairs (e.g., water and glass) is what leads to capillarity. And the height of the column is a function of the liquid-solid surface tension (as you indicated above). However, the angle of contact at the walls of the mesiscus is a function of both the liquid-solid as well as the liquid-air surface tensions (since the line of contact is the boundary between all three phases). So, by measuring the height and the angle of contact, we can know the value of both surface tensions.