Solutes can have different effects on surface tension depending on the nature of the surface and the solute:
* Little or no effect, for example sugar at water|air, most organic compounds at oil/air
* Increase surface tension, most inorganic salts at water|air
* Non-monotonic change, most inorganic acids at water|air
* Decrease surface tension progressively, as with most amphiphiles, e.g., alcohols at water|air
* Decrease surface tension until certain critical concentration, and no effect afterwards: surfactants that form micelles
What complicates the effect is that a solute can exist in a different concentration at the surface of a solvent than in its bulk. This difference varies from one solute–solvent combination to another.
Gibbs isotherm states that:
*  is known as surface concentration, it represents excess of solute per unit area of the surface over what would be present if the bulk concentration prevailed all the way to the surface. It has units of mol/m
*  is the concentration of the substance in the bulk solution.
*  is the gas constant and  the temperature
Certain assumptions are taken in its deduction, therefore Gibbs isotherm can only be applied to ideal (very dilute) solutions with two components.