Structure of the adsorption layer of various ionic and non-ionic surfactants at the free water surface, as seen from computer simulation and ITIM analysis

Abstract

Abstract:Molecular dynamics simulations of the adsorption layer of five different surfactant molecules, namely pentanol, octanol, dodecanol, dodecyl trimethyl ammonium chloride, and sodium dodecyl sulphate have been performed at the free surface of water at two different surface densities, namely 1 mol/m 2 (corresponding to unsaturated adsorption layer), and 4 mol/m 2 (corresponding to saturated adsorption layer), on the canonical ensemble at room temperature. The surfactants have been chosen in such a way that the effect of their headgroup charge as well as alkyl tail length on the properties of the adsorption layer can be separately investigated. The results are analysed in terms of the molecular level structure of the adsorption layer; organisation of the different groups and molecules along the macroscopic surface normal axis as well as conformation and orientation of the apolar tail is investigated in detail. In addition, the roughness of the surface of the aqueous phase is also analysed, using the ITIM method for accurately locating the real, capillary wave corrugated surface of the aqueous phase.

Experimental Depth Profiles of Surfactants, Ions, and Solvent at the Angstrom Scale: Studies of Cationic and Anionic Surfactants and Their Salting Out

“…These ionic compounds display a remarkable property when dissolved in water: the surface-active organic ions segregate to the surface, dragging with them their nominally surface inactive inorganic counterions. This interfacial enrichment in inorganic ions is expected on the basis of local charge attraction between the two ions, and it has been confirmed by experiments and simulations. − We ask in this study how both surfactant and inorganic ions distribute themselves in the interfacial region on the angstrom scale and how these depth distributions are altered when the surfactant ion pair is “salted out” toward the interfacial region by addition of an alkali halide. , The chosen systems are the cationic surfactant tetrahexylammonium bromide (THA + /Br – ) and the anionic surfactant sodium dodecyl sulfate (Na + /DS – ) in the absence and presence of Na + /Br – salt in liquid glycerol (Figure a).…”

Contribution of Different Molecules and Moieties to the Surface Tension in Aqueous Surfactant Solutions

“…The temperature of the systems was controlled using the Nosé–Hoover thermostat , with a time constant of 1.0 ps. To set up the starting configuration of the simulations, we started from previously equilibrated configurations, which were doubled in the two lateral directions, resulting in a surface area four times as large as in our previous studies, ,, and inserted an additional, 10 Å wide water slab in the middle of the aqueous phase, to push the two surfaces farther from each other. After a 20 ns long equilibration period, a 20 ns long production run per system was performed, during which equilibrium configurations were saved after every 0.5 ps for the calculation of density and lateral pressure profiles in a postprocessing step.…”

“…These questions seem to be particularly interesting in the light of our recent results, namely (i) that in neat molecular liquids, the largest part (i.e., at least 80%) of the surface tension comes from the first molecular layer, whereas the rest comes from the second layer , and (ii) that the immersion depth of the surfactant molecules into the aqueous phase, in terms of molecular layers, depends sensitively on the type of the headgroup and can be as large as 6–8 molecular layers, at least for ionic surfactants . In spite of the large number of both experimental − and computer simulation studies ,− targeting a number of properties of aqueous surfactant solutions, this problem has, to the best of our knowledge, not been addressed yet.…”

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

“…A particularly interesting system in this respect is the liquid–vapor interface of aqueous surfactant solutions. Although such systems have been intensively studied in the past decades by various experimental, − computer simulation, − and theoretical methods, − the issue of the contribution of different molecules to the surface tension has not been addressed until recently. , Clearly, in such systems, the presence of the surfactant triggers a marked decrease of the surface tension. By analyzing the contribution of the different molecules and moieties to this residual surface tension, we obtained the rather surprising result that, in the case of ionic surfactants, the counterions play an unexpectedly important role .…”