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

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.

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

“…This is in clear contrast with our earlier finding in the case of the KBFF model of counterions, resulting in an increase rather than decrease of the surface tension due to the onset of precipitation, clearly signaled by the presence of a first peak in the cation−O s rdf that is an order of magnitude larger than the second one. 54 Further, in the present case, the coordination number corresponding to the second, solvent separated peak of the obtained rdfs is 5−9 times larger than that of the first peak for all counterions (for Li + this ratio being 6.2). These findings confirm that, unlike in the case of other force fields considered, the ionic surfactant stays, as expected, in solution at the surface, instead of precipitating.…”

“…Therefore, we chose the second model combination, and repeated all simulations corresponding to the surface density of 4 μmol/m 2 with the KBFF model of the other alkali cations, i.e., Li + , K + , Rb + , and Cs + , in combination with SPC/E water. 54 Unfortunately, it turned out that, unlike in the case of Na + , the simulations performed with the other cations resulted in higher rather than considerably lower surface tension values than that of neat SPC/E water. The reason for this unphysical behavior of the model was traced back to the far too strong contact pair formation between the surfactant headgroups and counterions, leading to the precipitation of the surfactant salt at the liquid surface.…”

“…The reason for this unphysical behavior of the model was traced back to the far too strong contact pair formation between the surfactant headgroups and counterions, leading to the precipitation of the surfactant salt at the liquid surface. 54 Thus, to study the effect of the softness of the counterions on the relative contribution of the different particles and moieties to the surface tension, we had to find a cation force field that is (i) complete, i.e., includes all alkali ions from Li + to Cs + , and (ii) does not lead to unphysical result with any of the cations when simulating the liquid−vapor interface of the corresponding aqueous alkali DS − solution (i.e., leads to a decrease rather than an increase of the surface tension of neat water in every case). For this purpose, we have tried to use the GROMOS96 model of the DS − ions together with the alkali ion models proposed by Åqvist, 83 Klasczyk and Knecht (KK), 84 and Joung and Cheatham (JC), 85 together with the SPC (in the first two cases) and SPC/E (in the last case) water models.…”

“…However, although the results were compatible with this hypothesis, they could only be regarded as indicative due to the severe failure of the model combination used. 54 In this paper, we present a detailed investigation of the validity of both hypotheses. To address the first hypothesis, we present computer simulations of the liquid−vapor interface of alkali dodecyl sulfate solutions, considering all alkali ions from Li + to Cs + as counterions.…”

Hyperbranched polyglycerols derivatives as cetyltrimethylammonium bromide nanocarriers on enhanced oil recovery processes

“…The number of publications focused on EOR by chemical processes using computer simulation methods, such as classical molecular dynamics (MD) and coarse‐grained modeling, has increased in past decades. In this regard, particular attention has been given to the role played by surfactants in a variety of interfaces at the molecular level 29–34 . The interaction between surfactants may be affected by the medium.…”