Intermolecular Forces between the n-Alkanes Methane to Butane Adsorbed at the Water/Vapor Interface

Abstract

Experimental data on the dependence of the surface tension of water on the adsorption of n-alkanes (methane to butane) from the vapor phase over a range of temperatures are re-interpreted to obtain improved estimates of the two-dimensional second virial coefficients for comparison with calculations based on the Lennard-Jones (L-J) formalism in two dimensions. The analysis includes the fugacity corrections for the alkane gases and takes the L-J parameters as known for the three-dimensional gases. The new L-J calculations use a closed-form analytical solution of the basic equation for the two-dimensional second virial coefficient. These older published computations are correct and, as expected, are in complete agreement with the results by numerical integration with modern computer software. Together with estimates of the standard entropies of adsorption, the results indicate that no significant structuring of the surface water molecules occurs upon adsorption of the alkanes. The two-dimensional second virial coefficients are in fair accord with the L-J predictions, except for butane. The reduction of the attractive component of the potentials of mean force between physisorbed molecules arising from the frequencydependent interaction with their electrostatic images in the bulk aqueous phase, as described by McLachlan (Mol. Phys. 1964, 7, 381), or from the polarization of the adsorbates by a surface electric field (surface field polarization) is minor for methane, ethane, and propane. For butane, these effects may be significant but possible changes in conformation upon adsorption may weaken the validity of the gas-phase L-J parameters in estimating the two-dimensional virial coefficients for this alkane.

The structure of the water–propane interface investigated by x‐ray reflectivity measurements

“…Gas chromatography was used by Karger et al to investigate liquid-gas-systems [4,5]. The discussion of this experimental data which also does not supply any structural information about the film and its surface can be found in [13][14][15][16]. Ragil et al [7] and Bertrand et al [8] used ellipsometry measurements to investigate pentane and propane-hexane mixture films on water surfaces.…”

Mixed Protein/Hexane Adsorption Layers Formed at the Surface of Protein Solution Drops Surrounded by Hexane Vapor

“…[1][2][3][4][5][6][7][8][9] In all these studies, the adsorption of alkane was shown to result in a significant decrease of the surface tension. [1][2][3][4][5][6][7][8][9] In all these studies, the adsorption of alkane was shown to result in a significant decrease of the surface tension.…”

“…The adsorption of alkane vapor from the gas phase at the surface of pure water drops was extensively studied for alkanes of various chain lengths and for different partial pressures of alkanes in the gas phase. [1][2][3][4][5][6][7][8][9] In all these studies, the adsorption of alkane was shown to result in a significant decrease of the surface tension. In ref.…”

Multilayer Adsorption of Heptane Vapor at Water Drop Surfaces

“…Similarly, the desorption terms are proportional to the factors (Γ j ω j − Γ j+1 ω j+1 ), which means the desorption from the j-th layer can occur on the part of the surface already covered by this layer but not blocked yet from the top by the subsequent layer, because the molecules from the subsequent layer prevent desorption from the previous layer. The kinetic equation for the j-th layer, Equation (2), reduces to the standard Langmuir kinetics equation in the particular case when the molecules of the previous layer cover the whole surface (Γ j−1 ω j−1 = 1) and the subsequent layer has Colloids Interfaces 2017, 1, 8 3 of 14 not begun to form yet (Γ j+1 ω j+1 = 0). Note that, in the formulation above, it is assumed that the molar area of the adsorbed alkane molecule can be different in different layers; this will allow involving the intrinsic compressibility of the adsorbed molecules, as shown below.…”

“…Various experimental and theoretical studies of adsorption of alkanes from the gas phase on a water surface were previously reported [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. The thickness of such adsorbed alkane layers was determined with optical methods being maximum up to 4-5 nm [6].…”