Surface Tension, Surface Activity and Surface-Active Substances of Solutions

Surface Tension, Surface Activity and Surface-Active Substances of Solutions
I. Surface of liquid and surface tension
See previous issue 【Basic Knowledge】 Introduction to Surfactants (I)
Surface tension, surface activity and surface-active substances in solution
Pure liquid has only one molecule, so its surface tension is certain when the temperature and pressure are fixed. The surface tension of the solution will change with the concentration, this change is roughly three kinds of cases

The surface tension of a solution changes with the nature and concentration of the solute.
The first situation is that the surface tension is basically unchanged or slightly increased with the increase of solute concentration, and tends to be roughly close to a straight line (line 1 in the figure). Such solutes are NaCl, Na2SO4 and other inorganic salts. The second situation is that the surface tension decreases with increasing solute concentration. It usually decreases faster at first and then slower (line 2). Solutes that fall into this category include low molecular weight alcohols, aldehydes, ketones, carboxylic acids, esters, ethers, and other polar organic substances. The third case is that the surface tension decreases sharply at first, but almost ceases to change after a certain concentration (line 3). Solutes belonging to this category are organic carboxylates with more than 8 carbons, organic amine salts, sulfonates, benzenesulfonates, etc.

Different types of substances are in different states in solution. When a substance is added to a liquid, its concentration in the surface layer of the liquid is different from that in the interior of the liquid, and this phenomenon of changing concentration is called adsorption.
The effect of making the concentration of the surface layer greater than the internal concentration of the liquid is called positive adsorption, and the opposite is negative adsorption, and it is often customary to refer to positive adsorption as adsorption. The property of decreasing the surface tension of a solution due to adsorption of a solute on the surface (positive adsorption) is known as surface activity, and such substances are called surface-active substances. As can be seen from the diagram, the second and third types of substances that reduce the surface tension of a solution are surface-active and belong to the category of surface-active substances, while the first type of substances are not surface-active and are called non-surface-active substances.
If a substance (A) can lower the surface tension of another substance (B), A is said to be surface active to B. A substance that can significantly reduce the surface tension of a solvent at a very low concentration is called a surfactant. Substances in category 1 above are not surface-active, and substances in category 2 are surface-active but not surfactants. Only substances in category 3 can be called surfactants.

The above definition of surfactant was proposed by Freundlich in 1930, and was considered from the point of view of reducing surface tension. With the continuous development of surfactant science, it has been found that Freundlich’s definition has limitations. Because there is quite a class of substances, although its ability to reduce surface tension is poor, but they are easy to enter the interface (such as oil / water interface), in a very small amount can significantly change the physicochemical properties of the interface, this class of substances is also known as surfactants, such as some water-soluble polymers.
Thus, a surfactant is a substance that adsorbs on a surface (interface) and significantly alters the physicochemical properties of the surface (interface) when added in very small quantities, resulting in a range of application functions.

Translated with DeepL.com (free version)