Computational Prediction of the Adsorption Equilibrium for Ionic Surfactants at the Electrified Oil/Water Interface

Abstract

In a previous study (Somekawa et al., Langmuir 2019, 35, 11345–11350) a non‐Bornian solvation model was successfully applied to computational prediction of the adsorption equilibrium for nonionic surfactants at the oil (O)/water (W) interface. In this study, the method was extended to the adsorption of “ionic” surfactants at the polarized O/W interface. In this extension, the electrostatic potential of an ionic surfactant in the electric double layer formed at the O/W interface was taken into account together with the chemical potential estimated using the non‐Bornian solvation model. The calculation results clearly showed that a cationic surfactant (hexadecyltrimethylammonium ion) was adsorbed at the nitrobenzene/W interface, when the interfacial potential was negative. This prediction was consistent with the previous experimental observation using electrocapillary measurements.

Computational prediction of the critical micelle concentration (CMC) of surfactants using the non-Bornian solvation model

“…The numerical values of these coefficients are shown in Table 2. As described previously, 11 the first term on the rhs of eqn (1) or ( 2) is proportional to the molecular surface area, P S i , mainly related to the so-called cavity formation energy. The second term on the rhs of each equation Paper PCCP represents the contribution from highly negatively or positively charged ionic surfaces with E i r x_ or E i Z x + (x_ and x + being threshold values).…”

“…Furthermore, the relatively hydrophilic counterions such as Na + and K + would be less likely to form ion pairs at the O/W interface, and thus less likely affect the adsorption equilibrium for ionic surfactants. 11,19 For the cationic surfactants, DFT calculations were likewise performed for hydrophilic chloride or bromide salts, i.e., RNH 3 + Cl À , RN(CH 3 ) 3…”

“…These adsorption energies should be recognized as measures of hydrophobicity for the hydrophobic tail and hydrophilicity for the hydrophilic head. In our previous paper, 10,11 it was reported that these adsorption energies for non-ionic (neutral) and ionic surfactants at the O/W interface can be predicted computationally via density functional theory (DFT) calculation. In this method with the socalled non-Bornian solvation model, [12][13][14][15] the chemical solvation or re-solvation energy for neutral molecules or ions is formulated based on short-range (or vicinal) solute-solvent interactions called cavity formation, Coulomb interactions, polarization, etc.…”

“…Then it was shown that the adsorption energy of nonionic surfactants (alkyl alcohols 10 ) at the O/W interface is successfully estimated by considering the adsorption process as a 'partial' transfer of the surfactant molecule across the interface. It was also shown that the electrostatic effect of the electric double layer formed at the O/W interface can be effectively incorporated in the description of the adsorption equilibrium for ionic surfactants (e.g., hexadecyltrimethylammonium 11 ). This paper shows a successful application of the non-Bornian solvation model to the prediction and understanding of CMC for surfactants.…”

“…It was also shown that the electrostatic effect of the electric double layer formed at the O/W interface can be effectively incorporated in the description of the adsorption equilibrium for ionic surfactants ( e.g. , hexadecyltrimethylammonium 11 ). This paper shows a successful application of the non-Bornian solvation model to the prediction and understanding of CMC for surfactants.…”