Role of the Counterions in the Surface Tension of Aqueous Surfactant Solutions. A Computer Simulation Study of Alkali Dodecyl Sulfate Systems

György Hantal,Marcello Sega,Pál Jedlovszky,George Horvai

doi:10.3390/colloids4020015

Abstract

We have investigated the surface tension contributions of the counterions, surfactant headgroups and tails, and water molecules in aqueous alkali dodecyl sulfate (DS) solutions close to the saturated surface concentration by analyzing the lateral pressure profile contribution of these components using molecular dynamics simulations. For this purpose, we have used the combination of two popular force fields, namely KBFF for the counterions and GROMOS96 for the surfactant, which are both parameterized for the SPC/E water model. Except for the system containing Na+ counterions, the surface tension of the surfactant solutions has turned out to be larger rather than smaller than that of neat water, showing a severe shortcoming of the combination of the two force fields. We have traced back this failure of the potential model combination to the unphysically strong attraction of the KBFF counterions, except for Na+, to the anionic head of the surfactants. Despite this failure of the model, we have observed a clear relation between the soft/hard character (in the sense of the Hofmeister series) and the surface tension contribution of the counterions, which, given the above limitations of the model, can only be regarded as an indicative result. We emphasize that the obtained results, although in a twisted way, clearly stress the crucial role the counterions of ionic surfactants play in determining the surface tension of the aqueous surfactant solutions.

Highlights

Surface tension, the intensive counterpart of the surface area of a given phase, is a physical quantity of key importance both in pure interfacial science [1] and in its numerous industrial applications
Here we report a set of computer simulations of the liquid–vapor interface of the aqueous solutions of alkali dodecyl sulfate at saturated surface density, in which the alkali counterion is systematically varied from Li+ to Cs+
Surface Tension and Radial Distribution Functions The density profiles of the systems simulated along the macroscopic surface normal axis, X, as well as the contributions coming from the water molecules, sulfate headgroups and dodecyl tails of the dodecyl sulfate (DS)− ions, and counterions to these profiles are shown in Figure 2, while the surface tension

Summary

Introduction

The intensive counterpart of the surface area of a given phase, is a physical quantity of key importance both in pure interfacial science [1] and in its numerous industrial applications. This energetic penalty is smaller for softer counterions and might even be overcompensated by the energetic gain coming from the vicinity of the layer of oppositely charged surfactant headgroups This explanation is certainly supported by the fact that Na+ is clearly a harder ion than Cl− [59,60,62]; its verification requires a systematic study in which only the soft/hard character of the counterion is varied, while all other properties of the system are kept unchanged. We intend to investigate the validity of the second assumption, namely that the surface tension contribution of the counterions is related to their soft/hard character For this purpose, here we report a set of computer simulations of the liquid–vapor interface of the aqueous solutions of alkali dodecyl sulfate at saturated surface density, in which the alkali counterion is systematically varied from Li+ to Cs+. Lateral pressure can be treated as if it was an additive property of the individual atoms, and the contribution of an appropriately chosen group of atoms to the lateral pressure and, through Equation (1), the surface tension can be meaningfully discussed

Computational Details

Results and Discussion

Surface Tension and Radial Distribution Functions

Conclusions