The Law of Parsimony and the Negative Charge of the Bubbles

• Main Authors: Stoyan I. Karakashev, Nikolay A. Grozev
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Coatings
• Subjects: ion-specific effects; DI water; adsorption of ions; bubbles
• Online Access: https://www.mdpi.com/2079-6412/10/10/1003

Why the bubbles are negatively charged? This is almost 100 years old question, which many scientists have striven and still are striving to answer using the latest developments of the MD simulations and various physical analytical methods. We scrutinize with this paper the basic literature on this topic and conduct our own analysis. Following the philosophical law of parsimony: “Entities should not be multiplied without necessity”, we assume that the simplest explanation is the right one. It is well known that the negative change of the Gibbs free energy is a solid criterion for spontaneous process. Hence, we calculated the energies of adsorption of OH⁻, H₃O⁺ and HCO₃⁻ ions on the air/water interface using the latest theoretical developments on the dispersion interaction of inorganic ions with the air/water interface. Thus, we established that the adsorption of OH⁻ and HCO₃⁻ ions is energetically favorable, while the adsorption of H₃O⁺ is energetically unfavorable. Moreover, we calculated the change of the entropy of these ions upon their transfer from the bulk to the air/water interface. Using the well-known formula Δ<i>G</i> = Δ<i>H</i> − <i>T</i>Δ<i>S</i>, we established that the adsorption of OH⁻ and HCO₃⁻ ions on the air/water interface decreases their Gibbs free energy. On the contrary, the adsorption of H₃O⁺ ions on the air/water interface increases their Gibbs free energy. Thus, we established that both OH^- and HCO₃⁻ ions adsorb on the air/water interface, while the H₃O⁺ ions are repelled by the latter. Therefore, electrical double layer (EDL) is formed at the surface of the bubble–negatively charged adsorption layer of OH⁻ and HCO₃⁻ ions and positively charged diffuse layer of H₃O⁺ ions.