Contribution of Different Molecules and Moieties to the Surface Tension in Aqueous Surfactant Solutions. II: Role of the Size and Charge Sign of the Counterions

Abstract

Understanding the role of the counterion species in surfactant solutions is a complicated task, made harder by the fact that, experimentally, it is not possible to vary independently bulk and surface quantities. Here, we perform molecular dynamics simulations at constant surface coverage of the liquid/vapor interface of lithium, sodium, potassium, rubidium, and cesium dodecyl sulfate aqueous solutions. We investigate the effect of counterion type and charge sign on the surface tension of the solution, analyzing the contribution of different species and moieties to the lateral pressure profile. The observed trends are qualitatively compatible with the Hofmeister series, with the notable exception of sodium. We point out a possible shortcoming of what is at the moment, in our experience, the most realistic nonpolarizable force field (CHARMM36) that includes the parametrization for the whole series of alkali counterions. In the artificial system where the counterion and surfactant charges are inverted in sign, the counterions become considerably harder. This charge inversion changes considerably the surface tension contributions of the counterions, surfactant headgroups, and water molecules, stressing the key role of the hardness of the counterions in this respect. However, the hydration free energy gain of the counterions, occurring upon charge inversion, is compensated by the concomitant free energy loss of the headgroups and water molecules, leading to a negligible change in the surface tension of the entire system.

Understanding specific ion effects and the Hofmeister series

“…This also provides an additional approach to investigating specific counterion effects and SIEs in mixed salt solutions. 357…”

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…214,215 E-AIM (Extended Aerosol Inorganics Model) [216][217][218] and AIOMFAC 152,153 are two additional thermodynamic models that can be applied to predict surface tension. MD simulations have been used to model surface tension for surfactant solutions containing nonionic surfactants (fatty alcohols, 219 polyethylene glycol, 220 tetraethyleneglycol 221 ), cationic surfactants (dodecyl trimethyl ammonium chloride 219 ), and anionic surfactants (dodecylsulfate 219,222,223 ). For more information about experimental and computational techniques for measuring and modeling surface tension, see the 2021 review by Mott et al 208…”

Computer Simulation of the Surface of Aqueous Ionic and Surfactant Solutions

“…A great advantage of using the pressure route is that the lateral pressure can be distributed among the interacting sites in a simulation, allowing the calculation of the surface tension contribution of the individual sites or molecules through eq . This freely available method has recently been applied to aqueous surfaces containing ionic surfactants, revealing the crucial role of the counterions in this respect, as they can contribute to the surface tension with values, either positive or negative, that are several times higher in magnitude than the surface tension itself. , This huge contribution is largely compensated by the charged headgroups of the surfactants and the interfacial water molecules. Although it is clear that the sign of the counterion charge has a large effect on their surface tension contribution (and, hence, indirectly also on those of the headgroups and water molecules) , and the size and polarity (i.e., “hardness”) of the counterion is also related to the surface tension, , the exact relation between the properties of the counterions and the surface tension contribution they give is far from being well understood.…”

“… 57 It should be emphasized that in the case of aqueous surfactant solutions, where the solute and solvent molecules differ considerably in size, the surface layer as well as the subsequent subsurface layers should be defined in terms of (non-H) atoms rather than of molecules. 57 Additionally, atomistic scale structural and dynamic properties, such as lifetimes, preferred orientations, clustering, or ordering of the surface layers, the total surface tension and the individual contributions coming from different molecules or moieties 58 , 59 can be obtained from such simulations selectively for the surface, subsurface, and bulk-like layers. These results can serve as starting points to refine the aforementioned set of assumptions of the adsorption models.…”

“… 58 , 59 This huge contribution is largely compensated by the charged headgroups of the surfactants and the interfacial water molecules. Although it is clear that the sign of the counterion charge has a large effect on their surface tension contribution (and, hence, indirectly also on those of the headgroups and water molecules) 58 , 59 and the size and polarity (i.e., “hardness”) of the counterion is also related to the surface tension, 59 , 69 the exact relation between the properties of the counterions and the surface tension contribution they give is far from being well understood. Therefore, we expect a number of studies in the near future aiming at clarifying this relation.…”