On the Nature of the Air/Water Interface

Abstract

One of the obscure properties of the water is the negative charge of its boundary with the air. It remained unexplained for almost 100 years despite the efforts of many scientists to reveal this mystery. The development of many experimental techniques and computers helped the researchers to look deeper into the nature of the air/water interface, but the problem appeared to be a nutshell. Hence, basic literature on this topic was scrutinized and an analysis was conducted by means of calculating the change of the Gibbs energy of adsorption \(\Delta\)G on the air/water interface of the ions, which are present in the pure water. So, if for a certain type of ion \(\Delta\)G < 0, this is a solid criterion that it adsorbs on the air/water interface, thus charging the latter. Hence, the energies of adsorption of OH^-, H₃O⁺ and HCO₃^- ions on the air/water interface were calculated at the intrinsic ionic strength of the water using the latest theory. Therefore, it was established that the adsorption of OH^- and HCO₃^- ions is energetically favorable, while the adsorption of H₃O⁺ is energetically unfavorable. As a next step, the change of the entropy of these ions upon their transfer from the bulk to the air/water interface was calculated. Using the well-known formula \(\Delta\)G = \(\Delta\)H - T\(\Delta\)S, it was established that the adsorption of OH^- and HCO₃^- ions on the air/water interface is spontaneous, while the adsorption of H₃O⁺ ions is forced. Hence, both OH^- and HCO₃^- ions adsorb on the air/water interface, thus charging negatively the latter one, while the H₃O⁺ ions are repelled, thus forming an electrical double layer (EDL). The surface potential, which was calculated is \(\Psi\)_s \(\approx\) -4 mV, which is far below the experimental data.