Adsorption of Octyl Cyanide at the Free Water Surface as Studied by Monte Carlo Simulation

Abstract

Monte Carlo simulations of the adsorption layer of octyl cyanide have been performed on the canonical (N, V, T) ensemble at 300 K. The systems simulated cover the range of octyl cyanide surface densities from 0.27 to 7.83 mumol/m2. The surface density value at which the saturation of the adsorption layer occurs is estimated to be 1.7 mumol/m2. At low surface densities, the main driving force of the adsorption is found to be the formation of hydrogen bonds between the water and octyl cyanide molecules, whereas at higher surface concentrations, the dipole-dipole attraction between the neighboring adsorbed octyl cyanide molecules becomes more important. At low surface concentrations, the water-octyl cyanide hydrogen bonds prefer tilted alignments relative to the interface; however, in the case of the saturated adsorption layer, the number of such hydrogen bonds is maximized, leading to the preference of these bonds for the orientation perpendicular to the interface. Contrary to nonionic surfactants of multiple hydrogen bonding abilities (e.g., 1-octanol, C8E3), the increasing surface concentration of octyl cyanide was not found to lead to considerable competition of the molecules for positions of optimal arrangement. As a consequence, the energy and geometry of the water-octyl cyanide hydrogen bonds are found to be insensitive to the octyl cyanide surface concentration.

Competitive Adsorption of Surfactants and Polymers at the Free Water Surface. A Computer Simulation Study of the Sodium Dodecyl Sulfate−Poly(ethylene oxide) System

“…This point is further investigated in a following subsection of the paper. It should be noted that similar behavior of the mass density profile upon saturation of the adsorption layer was observed previously for various nonionic surfactants. ,, …”

“…Thus, in these systems about 20% and 50% of the DS − ions have a tilt angle smaller than 15° and 30°, respectively, relative to the macroscopic surface normal axis. This change in the orientation of the DS − ions with their increasing surface density is rather similar to what was previously observed in the adsorption layer of various nonionic surfactants, ,, and can be explained by the decreasing surface area available for the individual molecules, namely by the fact that maximizing the number of DS − ions attached to the surface requires minimizing the surface area occupied by the individual surfactant ions, which can be done by adopting the orientation perpendicular to the aqueous surface.…”

“…The adsorption layer of several other, mostly grafted copolymers has also been the target of computer simulation investigations . Further, the adsorption layer of SDS − and other ionic, − zwitterionic, − and nonionic − surfactants has also been simulated in a number of times both at the free water surface ,,,,− ,− and at the liquid−liquid interface between water and various apolar liquids. − ,,,, However, in spite of the wealth of such studies, we are only aware of one single publication concerning computer simulation investigation of the mixed PEO−SDS system in the bulk phase of their aqueous solution, whereas, to the best of our knowledge, the mixed adsorption layer of PEO and SDS has never been investigated yet by means of computer simulation methods.…”

Molecular Dynamics Simulation of 4-N-Hexyl-4’-Cyanobiphenyl Adsorbed at the Air-Water Interface

“…It was reported that the main driving force of the adsorption is the possibility of forming hydrogen bonds between the adsorbed octyl cyanide and interfacial water molecules. [32] Therefore, the calculated populations to characterize the hydrogen bonds formed by water and 6CB molecules are given in Table 4. We define 6CB and water molecules to be hydrogen bonded if their nitrogen-oxygen distances are less than 3.35 Å and simultaneously the nitrogen-hydrogen distance is less than 2.45 Å.…”

Theoretical description of 2D-cluster formation of nonionic surfactants at the air/water interface

“…But, one of the difficulties for such simulations is the adequate definition of effective interactions between the Bgrains.Î nvestigations of the surfactant behavior at different interfaces using the Monte Carlo method are not so numerous. Nevertheless, corresponding studies enable the estimation of the structural features for surfactant films [132,133], chiral effects in monolayers [134], impact of the structural peculiarities of the surfactants on the parameters of the phase transitions in the monolayers [135]. In the framework of the continuum configurational bias Monte Carlo method Howes and Radke [132] got surface tension isotherms of nonionic amphiphiles and calculated the tilt angles of the surfactant molecules with respect to the interface.…”