Superficial Tale of Two Functional Groups: On the Surface Propensity of Aqueous Carboxylic Acids, Alkyl Amines, and Amino Acids

Abstract

The gas-liquid interface of water is environmentally relevant due to the abundance of aqueous aerosol particles in the atmosphere. Aqueous aerosols often contain a significant fraction of organics. As aerosol particles are small, surface effects are substantial but not yet well understood. One starting point for studying the surface of aerosols is to investigate the surface of aqueous solutions. We review here studies of the surface composition of aqueous solutions using liquid-jet photoelectron spectroscopy in combination with theoretical simulations. Our focus is on model systems containing two functional groups, the carboxylic group and the amine group, which are both common in atmospheric organics. For alkanoic carboxylic acids and alkyl amines, we find that the surface propensity of such amphiphiles can be considered to be a balance between the hydrophilic interactions of the functional group and the hydrophobic interactions of the alkyl chain. For the same chain length, the neutral alkyl amine has a lower surface propensity than the neutral alkanoic carboxylic acid, whereas the surface propensity of the corresponding alkyl ammonium ion is higher than that of the alkanoic carboxylate ion. This different propensity leads to a pH-dependent surface composition which differs from the bulk, with the neutral forms having a much higher surface propensity than the charged ones. In aerosols, alkanoic carboxylic acids and alkyl amines are often found together. For such mixed systems, we find that the oppositely charged molecular ions form ion pairs at the surface. This cooperative behavior leads to a more organic-rich and hydrophobic surface than would be expected in a wide, environmentally relevant pH range. Amino acids contain a carboxylic and an amine group, and amino acids of biological origin are found in aerosols. Depending on the side group, we observe surface propensity ranging from surface-depleted to enriched by a factor of 10. Cysteine contains one more titratable group, which makes it exhibit more complex behavior, with some protonation states found only at the surface and not in the bulk. Moreover, the presence of molecular ions at the surface is seen to affect the distribution of inorganic ions. As the charge of the molecular ions changes with protonation, the effects on the inorganic ions also exhibit a pH dependence. Our results show that for these systems the surface composition differs from the bulk and changes with pH and that the results obtained for single-component solutions may be modified by ion-ion interactions in the case of mixed solutions.

Figure 1

Estimated surface concentration of 50 mM butyric acid in water (solid lines and filled symbols) and 50 mM n-butylamine (dashed lines and empty symbols) from the intensities of the C 1s line of the carboxylic carbon in butyric acid and the N 1s line in butylamine, recorded using surface-sensitive liquid-jet XPS.

Figure 2

Schematic illustration of the surface composition aqueous solutions of a carboxylate ion and an alkylammonium ion, i.e., their environmentally most relevant forms, alone with inorganic counterions and together. The number of molecular ions in the figure schematically illustrates the relative numbers of molecular ions at the solution surface.

Figure 3

Cross section of small aqueous droplets with selected amino acids from MD simulations, showing the distribution between the bulk and surface. Original data from ref (37).

Figure 4

Most relevant species of cysteine in the bulk (bottom) and in the surface (top) as a function of the pH of the aqueous solution. The atoms of the structures are colored dark gray (carbon), red (oxygen), blue (nitrogen), yellow (sulfur), light gray (hydrogen), purple (Na), and green (Cl), where the last two are counterions. The apparent size of each structure is proportional to the relative abundance of the species; see also Table 1.