Lateral Dynamics of Surfactants at the Free Water Surface: A Computer Simulation Study

Abstract

Molecular dynamics simulations of the adsorption layer of five different surfactant molecules, i.e., pentyl alcohol, octyl alcohol, dodecyl alcohol, sodium dodecyl sulfate, and dodecyl trimethyl ammonium chloride are performed at the free surface of their aqueous solution at two surface densities, namely 1 and 4 μmol/m(2) at 298 K. The results are analyzed in terms of the two-dimensional single molecule dynamics, in particlular, lateral diffusion of the surfactants at the liquid surface, in order to distinguish between two possible adsorption scenarios, namely the assumptions of localized and mobile surfactants. The obtained results, in accordance with the dynamical nature of the liquid phase and liquid surface, clearly support the latter scenario, as the time scale of lateral diffusion of the surfactant molecules is found to be comparable with that of the three-dimensional diffusion of water in the bulk liquid phase. The mechanism of this lateral diffusion is also investigated in detail by calculating binding energy distribution of the water molecules in the first hydration shell of the surfactant headgroups and that of the nonfirst shell surface waters, and by calculating the mean residence time of the water molecules in the first hydration shell of the surfactant headgroups. This time is found to be at least an order of magnitude smaller than the characteristic time of the lateral diffusion of the surfactants, revealing that surfactant molecules move without their first shell hydration water neighbors at the surface.

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

“…Thus, we have shown that the lateral dynamics of these surfactant molecules is very fast; it occurs on a time scale comparable with that of the diffusion of the water molecules [1].…”

“…Thus, questions concerning the time scale of the lateral mobility of the surfactants [1], the immersion depth of their polar part into the aqueous phase [2], the role of the headgroup type (i.e., ionic, non-ionic, zwitterionic, catanionic, etc.) and the number, length, topology and degree of saturation of the apolar tails on the structure and dynamics of the adsorption layer, preferred orientation and conformation of the apolar tails and their dependence on the level of saturation of the adsorption layer have remained unresolved until recently or are still not fully understood.…”

“…With the recent development of both various surface sensitive experimental methods and of fast and easily accessible computers, the molecular level properties of surfactant adsorption layers have been studied with increasing intensity in the past two decades both experimentally [3][4][5][6][7][8][9][10][11][12][13][14] and by computer simulation methods [1,2,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. In fact, computer simulation studies can well complement experimental investigations since, given that a reliable enough model of the real system is used, they can provide a three dimensional insight of atomistic resolution into the structure and dynamics of the system to be studied.…”

“…In fact, computer simulation studies can well complement experimental investigations since, given that a reliable enough model of the real system is used, they can provide a three dimensional insight of atomistic resolution into the structure and dynamics of the system to be studied. Thus, the adsorption layer of various cationic [1,2,18,19,25,30], anionic [1,2,20,21,24,[28][29][30][31], catanionic [25], zwitterionic [15][16][17], and non-ionic surfactants [1,2,[22][23][24]26,30] at the surface of their aqueous solutions have been studied several times. However, there is still a need for systematic comparisons of various structural features, e.g., headgroup type or tail length of the surfactants on the properties of their adsorption layers.…”

“…Instead, an adsorption isotherm based on mobile adsorbates, such as the Volmer isotherm [34,36,37], has to be used for such systems [1]. We have also shown that the headgroups of ionic surfactants are immersed into the aqueous phase several molecular layers deep, while those of the alcoholic surfactants stay in the outmost molecular layer of the aqueous phase.…”