Consider a system composed of water that contains an organic electrolyte RNaz and an inorganic electrolyte NaCl that both dissociate completely such that:
The Gibbs Adsorption equation in terms of the relative surface excess becomes:
The Relation Between Surface Tension and The Surface Excess Concentration becomes:
where  is the coefficient of the Gibbs adsorption.  Values of  are calculated using the Double layer (interfacial) models of Helmholtz, Gouy, and Stern.
Substances can have different effects on surface tension as shown :
* No effect, for example sugar
* Increase of surface tension, inorganic salts
* Decrease surface tension progressively, alcohols
* Decrease surface tension and, once a minimum is reached, no more effect: surfactants
Therefore, the Gibbs isotherm predicts that inorganic salts have negative surface concentrations. However, this view has been challenged extensively in recent years due to a combination of more precise interfacially sensitive experiments and theoretical models, both of which predict an increase in surface propensity of the halides with increasing size and polarizability. As such, surface tension is not a reliable method for determining the relative propensity of ions toward the air-water interface.
A method for determining surface concentrations is needed in order to prove the validity of the model: two different techniques are normally used: ellipsometry and following the decay of C present in the surfactant molecules.