Does oxidation make the organic aerosol coatings more hydrophilic? Insight from molecular dynamics study of oxidized surfactant monolayers

Organic compounds are ubiquitous in atmospheric aerosols. The morphology and structure of the organic phase affect the optical properties of the aerosols, their heterogeneous reactivity as well as their ability to nucleate cloud droplets and ice particles. It is commonly assumed that atmospheric oxidative ageing of the organic material, leading to the formation of polar groups such as carbonyl (=O), hydroxyl (-OH) and carboxylic acid (-COOH), will render the aerosol particle surfaces increasingly more hydrophilic, hence, able to take up more water. Field measurements have shown that a large fraction of the organic material found in aerosols are surface active compounds, such as fatty acids and lipids(Tervahattu, 2002 and 2005). An inverted micelle structure, with an aqueous core surrounded by an organic surfactant layer, has thus been proposed for aqueous aerosols, both marine and continental (Donaldson, 2006). While recent experiments suggest the existence of more complex structures, such as organic inclusions and surfactant lenses (Dennis-Smither, 2012), a monolayer (ML) of surface active organics on an aqueous subphase (the so called Langmuir monolayers) represents the basic model system used in laboratory studies aimed at elucidating the effect of oxidative processes on structural properties of organic coatings on aerosol particles. In our previous work, we used molecular dynamics computer simulations to study the structure and stability of oxidized phospholipid MLs (Khabiri, 2012). In this contribution, we employed the molecular dynamics simulation technique to investigate – with atomistic resolution – structural changes occuring in a fatty acid ML upon moderate degree of oxidation

The opposing effect of butanol and butyric acid on the abundance of bromide and iodide at the aqueous solution-air interface

The efficient oxidation of iodide and bromide at the aqueous solution–air interface of the ocean or of sea spray aerosol particles had been suggested to be related to their surface propensity. The ubiquitous presence of organic material at the ocean surface calls for an assessment of the impact of often surface-active organic compounds on the interfacial density of halide ions. We used in situ X-ray photoelectron spectroscopy with a liquid micro-jet to obtain chemical composition information at aqueous solution–vapor interfaces from mixed aqueous solutions containing bromide or iodide and 1-butanol or butyric acid as organic surfactants. Core level spectra of Br 3d, Na 2s, C 1s and O 1s at ca. 160 eV kinetic energy and core level spectra of I 4d and O 1s at ca. 400 eV kinetic energy are compared for solutions with 1-butanol and butyric acid as a function of organic concentration. A simple model was developed to account for the attenuation of photoelectrons by the aliphatic carbon layer of the surfactants and for changing local density of bromide and iodide in response to the presence of the surfactants. We observed that 1-butanol increases the interfacial density of bromide by 25%, while butyric acid reduces it by 40%, both in comparison to the pure aqueous halide solution. Qualitatively similar behavior was observed for the case of iodide. Classical molecular dynamics simulations failed to reproduce the details of the response of the halide ions to the presence of the two organics. This is attributed to the lack of correct monovalent ion parameters at low concentration possibly leading to an overestimation of the halide ion concentration at the interface in absence of organics. The results clearly demonstrate that organic surfactants change the electrostatic interactions near the interface with headgroup specific effects. This has implications for halogen activation processes specifically when oxidants interact with halide ions at the aqueous solution–air interfaces of the ocean surface or sea spray aerosol particles.ISSN:1463-9084ISSN:1463-907

Interactions of gaseous HNO3 and water with individual and mixed alkyl self-assembled monolayers at room temperature.

The major removal processes for gaseous nitric acid (HNO3) in the atmosphere are dry and wet deposition onto various surfaces. The surface in the boundary layer is often covered with organic films, but the interaction of gaseous HNO3 with them is not well understood. To better understand the factors controlling the uptake of gaseous nitric acid and its dissociation in organic films, studies were carried out using single component and mixtures of C8 and C18 alkyl self-assembled monolayers (SAMs) attached to a germanium (Ge) attenuated total reflectance (ATR) crystal upon which a thin layer of SiOx had been deposited. For comparison, diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) studies were also carried out using a C18 SAM attached to the native oxide layer on the surface of silicon powder. These studies show that the alkyl chain length and order/disorder of the SAMs does not significantly affect the uptake or dissociation/recombination of molecular HNO3. Thus, independent of the nature of the SAM, molecular HNO3 is observed up to 70-90% relative humidity. After dissociation, molecular HNO3 is regenerated on all SAM surfaces when water is removed. Results of molecular dynamics simulations are consistent with experiments and show that defects and pores on the surfaces control the uptake, dissociation and recombination of molecular HNO3. Organic films on surfaces in the boundary layer will certainly be more irregular and less ordered than SAMs studied here, therefore undissociated HNO3 may be present on surfaces in the boundary layer to a greater extent than previously thought. The combination of this observation with the results of recent studies showing enhanced photolysis of nitric acid on surfaces suggests that renoxification of deposited nitric acid may need to be taken into account in atmospheric models

Interactions of gaseous HNO3 and water with individual and mixed alkyl self-assembled monolayers at room temperature.

The major removal processes for gaseous nitric acid (HNO3) in the atmosphere are dry and wet deposition onto various surfaces. The surface in the boundary layer is often covered with organic films, but the interaction of gaseous HNO3 with them is not well understood. To better understand the factors controlling the uptake of gaseous nitric acid and its dissociation in organic films, studies were carried out using single component and mixtures of C8 and C18 alkyl self-assembled monolayers (SAMs) attached to a germanium (Ge) attenuated total reflectance (ATR) crystal upon which a thin layer of SiOx had been deposited. For comparison, diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) studies were also carried out using a C18 SAM attached to the native oxide layer on the surface of silicon powder. These studies show that the alkyl chain length and order/disorder of the SAMs does not significantly affect the uptake or dissociation/recombination of molecular HNO3. Thus, independent of the nature of the SAM, molecular HNO3 is observed up to 70-90% relative humidity. After dissociation, molecular HNO3 is regenerated on all SAM surfaces when water is removed. Results of molecular dynamics simulations are consistent with experiments and show that defects and pores on the surfaces control the uptake, dissociation and recombination of molecular HNO3. Organic films on surfaces in the boundary layer will certainly be more irregular and less ordered than SAMs studied here, therefore undissociated HNO3 may be present on surfaces in the boundary layer to a greater extent than previously thought. The combination of this observation with the results of recent studies showing enhanced photolysis of nitric acid on surfaces suggests that renoxification of deposited nitric acid may need to be taken into account in atmospheric models

Effect of the supplementation linseed oil, inulin and horse chestnut into a high fat diet on the fatty acid profile of pigs

ABSTRACT: In the present study it was evaluated the effect of the addition of linseed oil, inulin and horse chestnut added to a high fat (HF) diet on the content of fatty acids (FAs) in musculuss longissimus dorsi (MLD) of pigs. A 5d with adaptation period was followed by a 70 d experimental period, during which the pigs were fed with a HF basal diet. The HF basal diet which served as a control (group CG) was supplemented either with linseed oil (group LG) or with inulin and horse chestnut (group IG). All of the pigs were slaughtered at the end of the experiment and samples of MLD were taken for FA analysis. The concentration of α-linolenic acid in MLD of the LG group was 58 % and 61 % higher (P˂0.05) compared to CG and IG groups, respectively. The content of eicosapentaenoic acid (EPA) was 0.03 and docosahexaenoic acid (DHA) 0.07 in LG treatment. These FAs were not detected in CG and IG. The ratio of MUFA and PUFA n-6/n-3 in the MLD was the lowest (P˂0.05) in the LG (8.84) compared to CG (14.07) or IG (14.74) groups, representing a difference of 31.2%. The n-3/saturated FA ratio was highest (P˂0.05) in LG group (0.04) when compared to CG and IG groups (0.02). The supplementation of linseed oil, into the HF diet resulted in a higher concentration of α-linolenic acid, EPA, DHA and lower ratio of n-6/n-3 FA in MLD. Inulin and horse chestnut had no effect on FA profile of MLD