Exploring the Nanostructures Accessible to an Organic Surfactant Atmospheric Aerosol Proxy

The composition of atmospheric aerosols varies with time, season, location, and environment. This affects key aerosol properties such as hygroscopicity and reactivity, influencing the aerosol’s impact on the climate and air quality. The organic fraction of atmospheric aerosol emissions often contains surfactant material, such as fatty acids. These molecules are known to form three-dimensional nanostructures in contact with water. Different nanostructures have marked differences in viscosity and diffusivity that are properties whose understanding is essential when considering an aerosol’s atmospheric impact. We have explored a range of nanostructures accessible to the organic surfactant oleic acid (commonly found in cooking emissions), simulating variation that is likely to happen in the atmosphere. This was achieved by changing the amount of water, aqueous phase salinity and by addition of other commonly coemitted compounds: sugars and stearic acid (the saturated analogue of oleic acid). The nanostructure was observed by both synchrotron and laboratory small/wide angle X-ray scattering (SAXS/WAXS) and found to be sensitive to the proxy composition. Additionally, the spacing between repeat units in these nanostructures was water content dependent (i.e., an increase from 41 to 54 Å in inverse hexagonal phase d-spacing when increasing the water content from 30 to 50 wt %), suggesting incorporation of water within the nanostructure. A significant decrease in mixture viscosity was also observed with increasing water content from ∼104 to ∼102 Pa s when increasing the water content from 30 to 60 wt %. Time-resolved SAXS experiments on levitated droplets of this proxy confirm the phase changes observed in bulk phase mixtures and demonstrate that coexistent nanostructures can form in droplets. Aerosol compositional and subsequent nanostructural changes could affect aerosol processes, leading to an impact on the climate and urban air pollution

Schematic overview and X-ray fibrillar model.

(A) Correlation between nanoscale fibrillar pre-strain and microscale residual compressive strain. Left: schematic of fibrillar pre-strain gradients (this work; Figs 2, 3 and 6) across the AC, CP, and TB in the bovine MCP joint. Right: From independent work on the medial condyle of bovine tibiae [49], residual compressive strain (after loading) visualised using microcomputed tomography (CT) digital volume correlation (DVC). The calcified plate (both CC and SCB) are seen as regions with both high fibrillar tensile pre-strain (in unloaded state) and high compressive strain (energy storage) after loading. (B) Fibre-diffraction simulations showing how increasing fibril radius affects the skewness of the peak and estimated D-period. Left: schematic of meridional SAXS ellipsoidal peaks from a single fibril for different ratios of parallel to axial peak widths wp/wa. Blue: wp/wa = 1; red: wp/wa = 5; green: wp/wa = 10. Middle: Simulated I(q) plots for 3rd order meridional peak for these ratios (with D-period kept constant), showing increasing skewness as wp/wa increases, along with a slight rightward peak shift. Right: effect of increasing wp/wa from 1 to 10 on (top): D-period calculated by moments method and (bottom) peak skewness, showing that the moment method underestimates the D-period as fibril radius decreases (1→2→3).</p

Variation of fibril D-period, degree of orientation and direction of orientation across the bone-cartilage interface.

1D line plots, averaged in the horizontal direction, for the high resolution 2D scans in Figs 6 and 7. (A) The mean D-period (B) mean ρ and (C) mean orientation, averaged for each y-coordinate, displayed with error bars representing standard error of the mean. Horizontal (abscissa)-axis is in microns. Interface between calcified and uncalcified cartilage is on the left, indicated by bone-cartilage interface with left-arrow. In A), it is noted that D-period rises from a local minimum toward the interface between the calcified plate and articular cartilage (on the left); it stabilises at the first vertical dashed line ~100 μm to the right of the interface. The right two vertical dashed lines indicate a proposed demarcation of the calcified plate (CP) into calcified cartilage (CC) and subchondral bone (SCB), at the same points as the dashed rectangle in Fig 7A.</p

SAXS scans of full-length scan.

Colour map of the depth-wise variation in SAXS derived parameters across bovine bone-cartilage core of 5mm length and 2mm diameter for a full-length scan. This single sample was scanned across a greater depth in the trabecular bone but is otherwise similar to the samples imaged in Figs 2 and 3 in the main text. Step size of 20 microns, sample size ~0.38mm width, 5mm length. In this sample we were unable to resolve the thin superficial zone (SZ) at the top. Colour plots display: (A) Regions TZ: transitional zone, DZ: deep zone, CP: calcified plate, and TB: trabecular bone (as in Fig 1A, main text), (B) D-period (nm), reflecting collagen pre-strain, (C) total SAXS intensity (a.u.); areas of high intensity correspond to mineral-dense regions, (D) Total SAXS intensity from the background-corrected meridional collagen peak intensity; here, high intensity is observed in articular cartilage and (E) degree of orientation ρ (a.u.), showing high values in the deep zone (DZ), intermediate values in the calcified plate (CP) and transitional zone (TZ), and low values in trabecular bone (TB). (TIF)</p

Asymmetry in I(q) plots.

(A) Azimuthally integrated I(q) plots from the different tissue zones (SZ, TZ, DZ, CP and TB), corrected for diffuse background. I(q) plots are laterally averaged across sample width at specific vertical depths from cartilage surface; x-axis wavevector q is in nm-1 (B) Plots from (A) normalized to maximum peak intensity, to show the peak shape variations more clearly. (TIF)</p

High spatial resolution map of fibril degree of alignment and orientation at the calcified/uncalcified boundary in the BCU.

(Same sample as in Fig 6) (A) degree of alignment ρ and (B) collagen fibril orientation. From A) the tissue shows a high degree of fibrillar alignment when transitioning from the deep zone of articular cartilage to the calcified plate, and in B) we see fibres predominantly orientated in the angle of 90° (green colour), perpendicular to the articulating surface. In the lower part of the calcified plate (dashed rectangle; ordinal values from 0.15mm to 0.3mm) the ρ values are lower than in the upper part; it is suggested this is subchondral bone as opposed to calcified cartilage. From 0.3 mm downward, the degree of orientation is much lower, and orientation is more random, characteristic of trabecular bone.</p

Fibre diffraction modelling B.

Blue bars: D-period (in nm) for the different I(q) curves in S4 Fig, calculated using the first moment of area method used in the main text. Black bars: True D-period, obtained by correcting for the artificial increase in D-period due to the skew, demonstrated in S5B Fig. (TIF)</p

Fibril orientation and alignment across the BCU.

Colour map (for the sample in Fig 2) for collagen fibril orientation (degrees) and ρ (a.u.) parameter, where the length of the white line and the colour scale corresponds to ρ, and the direction of the white line indicates the fibrillar orientation across BCU. There is a clear increase in the degree of fibrillar alignment at the bone-cartilage interface, with the fibrillar orientation perpendicular to articulating surface. However, there is random orientation with lower degree of alignment in TB.</p

Region classification of samples.

Representation of the region classification into SZ, TZ, DZ, CP and TB, for the 6 samples used in the analysis. Note that Sample 2 does not have an observable superficial (SZ) zone. The colour indicates the type of tissue region across BCU. (TIF)</p

Polar vector map of preferential collagen fibril alignment across the right PK specimen of Patient 1, determined using WAXS.

<p><b>The larger plots (indicative of greater fibril alignment) have been scaled down for montage display by factors indicated in the colour key.</b> Broken circle: limbus. Arrow: tangential fibril alignment along the graft margin. Arrowhead: abnormal inward extension of tangential limbal fibrils into the peripheral cornea, corresponding to regions of prominent corneal opacity. Rectangle: fibril alignment delineates separated wound edges and is disturbed in intervening tissue. Solid line: superior tag. Inset: location of noted features on actual specimen.</p