Infrared Reflection–Absorption Spectroscopy of α-Hydroxyacids at the Water–Air Interface

Abstract

Organic molecules, including α-hydroxyacids, are ubiquitous in the natural environment. Often found at water−air interfaces, organic molecules can alter the structure of the interface or participate in interfacial chemistry. Despite their prevalence in the environment, the structure and ordering of α-hydroxyacids have not been widely investigated at water−air interfaces, and the impact of the hydrophobic tail length on structure has not been explored. Here, for the first time, we use infrared reflection−absorption spectroscopy to assess the vibrational structure of α-hydroxyacids at a water surface as a function of surface partitioning and surface coverage. We study lactic acid, 2-hydroxyoctanoic acid, and 2-hydroxystearic acid, which have 1 carbon, 6 carbon, and 16 carbon tails, respectively. Vibrational features compared across the set of α-hydroxyacids studied here are used to determine the interaction of the polar headgroup with the water subphase and the ordering of the hydrophobic tail. We find that the carbonyl and α-hydroxyl groups participate in a complex hydrogen-bonding motif at the water−air interface that can be affected by the hydrophobic tail length and places the polar headgroup in or below the water−air interface. Furthermore, molecular ordering increases with the tail length or the surface coverage. The presence of the α-hydroxyl group causes the α-hydroxyacids to maintain a tilted orientation with respect to the surface normal even at high surface coverages. A combination of polar headgroup and hydrophobic tail effects dictates the overall orientation of α-hydroxyacids and can thus affect their ability to participate in chemistry and alter organic coatings on water surfaces.

Infrared Reflection–Absorption Spectroscopy of α-Keto Acids at the Air–Water Interface: Effects of Chain Length and Headgroup on Environmentally Relevant Surfactant Films

“…IR reflection–absorption spectra of α-keto acids at the air–water interface were obtained using a home-built IR-RAS setup and methodology as described previously. ,, Briefly, a PTFE trough from KSV-NIMA (14.5 cm × 7 cm × 0.5 cm) was optionally equipped with a Wilhelmy microbalance and Delrin barriers. The microbalance and barriers were used when studying insoluble molecules and were removed when studying soluble molecules.…”

“…Air–water interfaces are ubiquitous in natural environments, both on the surface of oceans and on atmospheric aerosols. Recent studies have shown that the unique properties of these interfaces lead to morphological and chemical changes that are not fully understood. − These interfaces concentrate and align organic material, and even small, soluble organic molecules partition preferentially to the water surface. − ,− The resulting organic films at aqueous interfaces can control molecular transfer between the vapor and condensed phases, altering their optical properties and thereby affecting the earth’s radiative balance and other atmospheric processes. − ,− Recent reports in the literature have repeatedly demonstrated enhanced reactivity and increased reaction rates in confined microenvironments (e.g., microdroplets, nanoemulsions, and aerosol), − ,− highlighting the critical question of the role that the interface may play in these observed effects. In order to better understand how such aqueous interfaces may provide auspicious reaction environments, fundamental studies that explore the detailed surface behavior of model systems of organics are needed.…”

“…Atmospheric aerosol has diameters ranging from submicrometer to micrometers in size, while molecular scales are on the order of angstroms; therefore, from the molecule’s perspective, the aerosol surface is effectively flat. IR-RAS has previously been used to assess the structure of simple model surfactants with varying degrees of surface activity at the air–water interface, including carboxylic acids ,− and α-hydroxyacids, , demonstrating that both the hydrophobic tail length and the polar headgroup structure can affect the overall orientation of fatty acids at the water surface. Recently, IR-RAS has also been used to elucidate the structure of 2-oxooctanoic acid, but the structures of a range of α-keto acids with varying degrees of surface activity at water surfaces have not yet been investigated.…”

“…In this study, we will investigate a series of α-keto acids (shown in Figure ) with varying hydrophobic tail lengths and therefore varying degrees of surface partitioning at air–water interfaces. We use both Langmuir compression isotherms and IR-RAS, and we make direct comparisons between α-keto acids and other simple surfactants, including unsubstituted carboxylic acids and α-hydroxyacids. ,,− Langmuir isotherms are used to show how the polar headgroup can affect the monolayer phase behavior, and detailed IR-RAS experiments further demonstrate how the tail length and polar headgroup functionality can affect the structure and orientation of organic acids at the water surface.…”

Advances in Atmospheric Chemical and Physical Processes

“…Reactions on surfaces are examined, including a kinetic and mechanistic study of gas–solid and gas-phase ozonolysis of nitrogen-containing alkenes, and the formation of organic nitrates in the heterogeneous reaction of α-pinene with mineral surfaces . Laboratory studies of the air–water interface demonstrate surface adsorption of H 2 O 2 , as well as determining the structure of hydroxy organic acids of varying size at the interface . Additional work addresses adsorption–desorption of semivolatile VOCs on mineral surfaces, and reactive uptake of HgO and a recently discovered dimethyl sulfide oxidation product (hydroperoxymethyl thioformate) on aerosols of various compositions. , The effects of aerosol properties on ice nucleation and on the partitioning of semivolatiles, , the effects of pH on organosulfate structures in the condensed phase, and the accessibility of various nanostructures for surfactants on atmospheric aerosol proxies are also reported.…”