Alcohols at the aqueous surface: chain length and isomer effects

Abstract

Surface-active organic molecules at the liquid-vapor interface are of great importance in atmospheric science. Therefore, we studied the surface behavior of alcohol isomers with different chain lengths (C4-C6) in aqueous solution with surfaceand chemically sensitive X-ray photoelectron spectroscopy (XPS), which reveals information about the surface structure on a molecular level. Gibbs free energies of adsorption and surface concentrations are determined from the XPS results using a standard Langmuir adsorption isotherm model. The free energies of adsorption, ranging from around -15 to -19 kJ/mol (C4-C6), scale linearly with the number of carbon atoms within the alcohols with ΔGAds per -CH2-≈ -2 kJ/mol. While for the linear alcohols, surface concentrations lie around 2.4 x 10 14 molecules/cm 2 at the bulk concentrations where monolayers are formed, the studied branched alcohols show lower surface concentrations of around 1.6 x 10 14 molecules/cm 2 , both of which are in line with the molecular structure and their orientation at the interface. Interestingly, we find that there is a maximum in the surface enrichment factor for linear alcohols at low concentrations, which is not observed for the shorter branched alcohols. This is interpreted in terms of a cooperative effect, which we suggest to be the result of more effective van der Waals interactions between the linear alcohol alkyl chains at the aqueous surface, making it energetically even more favorable to reside at the liquid-vapor interface.

A Monolayer Partitioning Scheme for Droplets of Surfactant Solutions

“…Measurements are made at room temperature using different experimental methods, including (i) neutron reflection (Li et al, ), (ii) mass spectrometric probing of molecular clusters, presumably in equilibrium in the headspace above a liquid surface (Raina et al, ), and (iii) analysis of experimental surface tension data based on Gibbs' adsorption equation (equation ; Li et al, ). Together with (iv) X‐ray photoelectron spectroscopy (XPS; Walz et al, ), these methods have been used to probe the composition and thickness of water‐ n ‐butanol surfaces. There are quantitative differences between the results obtained from the different measurements; however, considering the complex nature of the experiments, the qualitative agreement (numerically within about a factor of three) is quite reasonable.…”

“…Toribio et al () reported surface enrichment factors for aqueous n ‐butanol, modeled from a statistical mechanics based surface model (Boyer et al, ; Wexler & Dutcher, ), which are of similar magnitude albeit somewhat smaller, compared to the present monolayer model. Curiously, Toribio et al () find similar discrepancies of about a factor of 3 between their model predictions and experimental XPS‐derived surface concentrations of aqueous alcohols (Walz et al, ). This comparison was however done for n ‐butanol concentrations an order of magnitude higher than in the studies compared in Figure .…”

“…Measurements are made at room temperature using different experimental methods, including (i) neutron reflection (Li et al, 1996), (ii) mass spectrometric probing of molecular clusters, presumably in equilibrium in the headspace above a liquid surface (Raina et al, 2001), and (iii) analysis of experimental surface tension data based on Gibbs' adsorption equation (equation (2); Li et al, 1996). Together with (iv) X-ray photoelectron spectroscopy (XPS; Walz et al, 2016), these methods have been used to probe the composition and thickness of water-n-butanol surfaces.…”

“…Werner et al (2014Werner et al ( , 2016 recently studied surfaces of aqueous SCA with and without added salts, using synchrotron radiation-based XPS in combination with molecular dynamics simulations. Experiments were made with the same setup as used by Walz et al (2015Walz et al ( , 2016, and direct comparison between experimental and modeled results and our monolayer model is affected by the same issues as pointed out by Toribio et al (2018) for water-alcohol mixtures. Nevertheless, surface enrichment factors for SCA, x s SCA ∕x b SCA , predicted here from the monolayer model are close to the lower range of the corresponding C s SCA ∕C b SCA values reported by Werner et al (2014Werner et al ( , 2016.…”

Surface Partitioning in Organic–Inorganic Mixtures Contributes to the Size-Dependence of the Phase-State of Atmospheric Nanoparticles

“…Changes 32 Here, c s,max was estimated by choosing a value that successfully reproduced surface tension data of pure SuccH 2 solutions.…”