The Law of Parsimony and the Negative Charge of the Bubbles

Abstract

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 ΔG = ΔH − TΔS, 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.

Review of High-Frequency Ultrasounds Emulsification Methods and Oil/Water Interfacial Organization in Absence of any Kind of Stabilizer

“…Several simulation studies alone or combined with experimental studies showed that hydroxide ions are adsorbed at the hydrophobic medium/water interface [ 101 , 107 , 111 , 112 , 121 ]. According to quantum mechanical calculations, gaseous carboxylic acids deprotonation at the interface would be inhibited by a kinetic barrier, unless OH − would be present.…”

“…Recent calculation studies of adsorption energies of OH − and H 3 O + at the air/water interface showed that OH − adsorption is energetically favorable, whereas H 3 O + adsorption is unfavorable. HCO 3 − adsorption would be also favorable, but to a lesser extent than OH − [ 112 ].…”

“…Numerous authors thought there is not only OH − , nor only H 3 O + , but both ions at the interface to ensure electroneutrality of the double layer [ 25 , 77 , 98 , 112 , 119 , 121 , 123 ]. Two main hypotheses appeared around this supposition: (i) H 3 O + would be placed in the surface layer, although OH − would be in the diffuse layer ( Figure 9 A), according to simulations and spectroscopic studies [ 28 , 118 ].…”

Commercial‐scale removal of short‐chain PFAS in a batch‐wise adsorptive bubble separation process by dosing with cationic co‐surfactant

“…These hypotheses of Buckley et al (2023) are not supported by experimental evidence, which is particularly important for the second mechanism because the hypothesis is novel. It is noted that air bubbles exhibit a slight negative charge (Karakashev & Grozev, 2020) which would favor the second mechanism over the first. However, the question of which mechanism is dominant for the various PFAS species is largely immaterial in the context of practical interventions to enhance the removal of short‐chain PFAS using adsorptive bubble separation processes.…”

Electrostatic Forces in Control of the Foamability of Nonionic Surfactant

“…For this reason, we prefer to study a simpler system, i.e., the effect of the electrostatic repulsion between the bubbles on the foamability of an aqueous solution of nonionic frother (e.g., MIBC). The intrinsic negative surface potential of the bubbles in water at normal pH value is still not well understood, but it is proved that it depends on the pH value [53]. Their isoelectric point is at pH ≈ 4 [1,54].…”