Surface behavior of amphiphiles in aqueous solution: a comparison between different pentanol isomers

Abstract

Position isomerism is ubiquitous in atmospheric oxidation reactions. Therefore, we have compared surface-active oxygenated amphiphilic isomers (1-and 3-pentanol) at the aqueous surface with surface-and chemically sensitive X-ray photoelectron spectroscopy (XPS), which reveals information about the surface structure on a molecular level. The experimental data is complemented with molecular dynamics (MD) simulations. A concentration-dependent orientation and solvation of the amphiphiles at the aqueous surface is observed. At bulk concentrations as low as around 100 mM, a monolayer starts to form for both isomers, with the hydroxyl groups pointing towards the bulk water and the alkyl chains pointing towards the vacuum. The monolayer (ML) packing density of 3-pentanol is approx. 70 % of the one observed for 1-pentanol, with a molar surface concentration that is approx. 90 times higher than the bulk concentration for both molecules. The molecular area at ML coverage (≈ 100 mM) was calculated to be around 32 ± 2 Å 2 /molecule for 1-pentanol and around 46 ± 2 Å 2 /molecule for 3-pentanol, which results in a higher surface concentration (molecules/cm 2 )for the linear isomer. In general we conclude therefore that isomers -with comparable surface activities -that have smaller molecular areas will be more abundant at the interface in comparison to isomers with larger molecular areas, which might be of crucial importance for the understanding of key properties of aerosols, such as evaporation and uptake capabilities as well as their reactivity.

Alcohols at the aqueous surface: chain length and isomer effects

“…A tot saturates. As discussed earlier, 35 the trend of the curve can be interpreted as a result of the adsorption of alcohols at the aqueous surface with increasing bulk concentration and, finally, the formation of a monolayer-like structure (ML) in the surface region. Here, the term monolayer refers to a closely packed layer of amphiphilic molecules at the liquid-vapor interface that has approximately a thickness of a single molecular length, i.e.…”

“…All acquired spectra were evaluated regarding the total area A tot of the liquid phase C 1s photoelectron (PE) signal (A tot = A(C C ) + A(C OH )) and the PE intensity ratio R between the two liquid phase C 1s peak areas (R = A(C C ) / A(C OH )). As discussed in more detail before, 35 the PE signal A tot can be used as a measure for the amount of solutes at the solution interface, while the PE intensity ratio R between the peak areas of C C and C OH reveals information about the orientation of these molecules in the surface region. The binding energy is sensitive to their local chemical and physical environment, i.e.…”

“…At surface coverage close to one ML, the amphiphilic molecules have a preferential orientation such that the hydrophilic hydroxyl groups are immersed into the aqueous phase maintaining hydrogen bonds with water, while the hydrophobic alkyl chains are pointing out of the solution phase being partially dehydrated which was confirmed by MD simulations earlier. 35 This preferential orientation of the amphiphilic molecules is proposed to be driven by hydrophobic / hydrophilic interactions of the amphiphiles with the solvent water and by van der Waals interactions between the alkyl chains to oppose the loss of entropy. In general, the linear alcohols have a higher ratio (i.e.…”

“…In order to calculate a signal, which is directly proportional to the surface concentration N S , from the recorded photoelectron signal A tot (see Fig. 2 a), the signal A tot is divided by the number of carbon atoms in the corresponding molecule and a bulk signal (approximated by means of a reference measurement on sodium formate, see above) 8,35 is subtracted. Linking the surface signal directly to the surface concentration has certain limitations for longer alkyl chain lengths as the photoemission signal is then more attenuated upon a change in orientation of the molecules.…”

Determination of Surface Potential and Electrical Double-Layer Structure at the Aqueous Electrolyte-Nanoparticle Interface

“…Because XPS is traditionally performed in ultrahigh vacuum, measurements with liquid samples have been limited to a few instruments worldwide [20][21][22][23][24]. Recently, however, we demonstrated that aqueous suspensions of nanoparticles (colloids) can be passed within a liquid microjet through the x-ray beam, enabling the unique analytical capabilities of XPS to be extended to aqueous NP dispersions [25].…”