Condensing Effect of Palmitic Acid on DPPC in Mixed Langmuir Monolayers

The interaction between deuterated dipalmitoylphosphatidylcholine (DPPC-d62) and palmitic acid (PA) in mixed Langmuir monolayers is studied using vibrational sum frequency generation (VSFG) spectroscopy. Palmitic acid is an additive in exogenous lung surfactant preparations such as Survanta and Surfaxin. The effect of PA on the chain conformation and orientation of DPPC in the liquid-expanded and condensed phases is explored. A condensing effect of PA on DPPC is observed with VSFG. At 12 mN/m, DPPC-d62 alone is in the liquid-expanded phase. Adding PA increases the conformational ordering of DPPC chains and causes DPPC to transition from the expanded phase into the condensed phase. At 42 mN/m, DPPC-d62 and PA form a mixed structure in the condensed phase. The presence of PA decreases the chain tilt angle of DPPC, increasing the orientational ordering of DPPC chains. At 42 mN/m, there is also evidence from the frequency red shift of the PO2- symmetric stretch that the carboxyl group of PA forms a hydrogen bond with the phosphate group of DPPC in the condensed phase. From this work the effect of PA on DPPC is 2-fold:  (1) PA increases the chain ordering of DPPC and promotes the LE and TC phase separation and (2) due to the miscibility between DPPC and PA in the condensed phase, PA decreases the collapse pressure

Real-Time Investigation of Lung Surfactant Respreading with Surface Vibrational Spectroscopy

The respreading of a lung surfactant monolayer at the air−water interface is investigated with broad bandwidth sum frequency generation (BBSFG) spectroscopy. The lung surfactant mixture contains chain perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62), palmitoyloleoylphosphatidylglycerol (POPG), palmitic acid (PA), and KL4 (a 21-residue polypeptide analogue to the surfactant protein SP-B). DPPC-d62 serves as a probe molecule for the spectroscopic investigation. The BBSFG spectra of DPPC-d62 in the lung surfactant mixture are obtained in the C−D stretching region in real-time during film compression and expansion in a Langmuir trough. The BBSFG intensity of the CD3 stretch peak from DPPC-d62 terminal methyl groups is used as a measure of the interfacial density of DPPC-d62 after careful consideration of orientation effects. For the first time, the interfacial loss of DPPC in a complex lung surfactant mixture is quantified. Spectroscopic results reveal that there is an 18% DPPC-d62 interfacial loss during film respreading. However, the surface pressure−area isotherm measurements demonstrate that there is a rather large trough area reduction (37%) during film expansion. The relatively small interfacial loss of DPPC-d62 and the rather large trough area reduction indicate that the respreading of DPPC and non-DPPC components in the lung surfactant is not uniform and a surface refinement process exists during film compression and expansion. This refinement process results in a DPPC-enriched monolayer with a significant depletion of non-DPPC components after film respreading. Implication for replacement surfactant design from this work is discussed

Diffuse Reflection Broad Bandwidth Sum Frequency Generation from Particle Surfaces

We report the first vibrational sum frequency generation (VSFG) spectroscopic study from particle surfaces of powdered solids using a modified SFG approach, diffuse reflection broad bandwidth sum frequency generation (DR-BBSFG). The DR-BBSFG spectrum of sodium dodecyl sulfate (SDS, C12H25SO4Na) powdered solids was obtained. Five peaks were resolved by calculated fits. Possible origins of the SFG response from SDS particle surfaces are discussed. Potential applications of DR-BBSFG spectroscopy are addressed

Bisulfate Dehydration at Air/Solution Interfaces Probed by Vibrational Sum Frequency Generation Spectroscopy

The structure and organization of ions at vapor/solution interfaces have great implications for the reactivity and growth of atmospheric aerosols. Considering the ionic components of aqueous aerosols, sulfate species are one of the most prevalent due to high levels of SO2(g) emission to the atmosphere from biofuel burning and volcanic eruptions. Atmospheric SO2(g) can undergo direct gas phase oxidation or experience dissolution and subsequent oxidation to sulfate species within aqueous aerosols, where, depending on the pH level, sulfate may exist as SO42–, HSO4–, or H2SO4. Here we probe the molecular environment experienced by the bisulfate anion (HSO4–) at vapor/solution interfaces for H2SO4, Na2SO4, and MgSO4 solutions via vibrational sum frequency generation (VSFG) spectroscopy. VSFG is an inherently interface specific nonlinear optical spectroscopy and is a powerful tool for the study of interfacial structure and organization. Our VSFG results are compared to bisulfate behavior in bulk aqueous solution observed using Raman and infrared spectroscopies. The presence of Na+ and Mg2+ is observed to perturb HSO4– anion hydration compared to H+ which manifests as a blue shift in the observed SO3 symmetric stretching mode frequency of HSO4–. This perturbation is greatly exaggerated for interfacial HSO4– anions residing within vapor/solution interfaces relative to bulk solution. Mg2+ ions are found to disrupt the net bisulfate population hydration within the vapor/solution interfaces tested, while Na+ ions only influence a subpopulation of the interfacial bisulfate distribution. This difference is attributed to the much greater propensity for aqueous solvation that Mg2+ exhibits compared to Na+. Our results are interpreted with a perspective toward understanding interfacial acid dissociation for the bisulfate anion and the role that this may play for tropospheric acidic aerosols

1-Methyl Naphthalene Reorientation at the Air−Liquid Interface upon Water Saturation Studied by Vibrational Broad Bandwidth Sum Frequency Generation Spectroscopy

Vibrational broad bandwidth sum frequency generation spectroscopy was employed to investigate the surface structure of neat 1-methyl naphthalene (1-MN) and the reorientation of the 1-MN molecules upon saturation of the 1-MN liquid with water. The neat 1-MN liquid molecules have their aromatic rings aligned antiparallel to one another with their methyl groups alternating out of the surface and into the subsurface region from molecule to molecule. With the introduction of relatively few water molecules into the 1-MN liquid (1:336 water/1-MN) a rearrangement of the surface molecules is induced, leading to an increased number density of the methyl groups arranged such that more methyl groups are oriented in the same direction into the air phase at the air−liquid 1-MN interface. Surface tension measurements reveal an increase in the surface tension upon water saturation of the 1-MN liquid, indicating surface activity of the water in the 1-MN solution. It is also clear that the reorientation of the surface 1-MN molecules is reversible

Vibrational Spectroscopic Characterization of Hematite, Maghemite, and Magnetite Thin Films Produced by Vapor Deposition

Thin films of three iron oxide polymorphs, hematite, maghemite, and magnetite, were produced on KBr substrates using a conventional electron beam deposition technique coupled with thermal annealing. This method allowed for iron oxide thin films free from chemical precursor contaminants. The films were characterized using Fourier-transform infrared spectroscopy (FTIR), Raman microspectroscopy, and ellipsometry. These spectroscopic techniques allowed for a clear assignment of the phase of the iron oxide polymorph films produced along with an examination of the degree of crystallinity possessed by the films. The films produced were uniform in phase and exhibited decreasing crystallinity as the thickness increased from 40 to 250 nm

1-Methyl Naphthalene Reorientation at the Air−Liquid Interface upon Water Saturation Studied by Vibrational Broad Bandwidth Sum Frequency Generation Spectroscopy

Vibrational broad bandwidth sum frequency generation spectroscopy was employed to investigate the surface structure of neat 1-methyl naphthalene (1-MN) and the reorientation of the 1-MN molecules upon saturation of the 1-MN liquid with water. The neat 1-MN liquid molecules have their aromatic rings aligned antiparallel to one another with their methyl groups alternating out of the surface and into the subsurface region from molecule to molecule. With the introduction of relatively few water molecules into the 1-MN liquid (1:336 water/1-MN) a rearrangement of the surface molecules is induced, leading to an increased number density of the methyl groups arranged such that more methyl groups are oriented in the same direction into the air phase at the air−liquid 1-MN interface. Surface tension measurements reveal an increase in the surface tension upon water saturation of the 1-MN liquid, indicating surface activity of the water in the 1-MN solution. It is also clear that the reorientation of the surface 1-MN molecules is reversible

Sulfate Adsorption at the Buried Fluorite–Solution Interface Revealed by Vibrational Sum Frequency Generation Spectroscopy

Understanding the structure and energetics of adsorbed ions at buried mineral/solution interfaces has great importance to the geochemical and atmospheric chemistry communities. Vibrational spectroscopy is a powerful tool for the study of mineral/solution interfaces as these techniques can be applied in situ, are sensitive to surface structures, and are generally nondestructive. The use of vibrational sum frequency generation spectroscopy (VSFG), which is inherently interface-specific, is applied here to study the adsorption of sulfate at the buried fluorite (CaF₂)/Na₂SO₄ solution surface at pH 7 and 298 K in the presence of an aqueous background electrolyte, NaCl. The use of VSFG allowed for the resolution of adsorbed sulfate complexes from sulfate molecules which reside in the interfacial electric double layer yet remain fully solvated. The sulfate anion is found to adsorb with a bidentate inner-sphere structure at the fluorite surface with an average surface free energy of adsorption of −31 ± 3 kJ/mol for pH 7 solutions at 298 K

Ionic Binding of Na⁺ versus K⁺ to the Carboxylic Acid Headgroup of Palmitic Acid Monolayers Studied by Vibrational Sum Frequency Generation Spectroscopy

Ionic binding of alkali ions Na+ and K+ to the carboxylic acid headgroups of fatty acid monolayers is studied as a proxy toward understanding the fundamental chemistry in cell biology. In this study, we used broad-bandwidth sum frequency generation (BBSFG) vibrational spectroscopy to investigate the ionic binding event that leads to deprotonation and complex formation of fatty acid headgroups. Palmitic acid (C15H31COOH) exists as a monolayer on aqueous surfaces. Surface vibrational stretch modes of palmitic acid from 1400 cm−1 to 3700 cm−1 were observed (νs-COO−, ν-CO, ν-CH, ν-OH of COOH, free OH). Palmitic acid is mostly protonated at the aqueous surface at neutral pH (∼6). However, various degrees of deprotonation are initiated by the introduction of Na+ and K+ that results in the complexation of K+:COO− and solvent separated Na+:COO−. Evidence in several spectral regions indicates that K+ exhibits stronger ionic binding affinity to the carboxylate anion relative to Na+

Incorporation and Exclusion of Long Chain Alkyl Halides in Fatty Acid Monolayers at the Air−Water Interface

Mixed monolayers of deuterated palmitic acid C15D31COOH (dPA) and deuterated stearic acid C17D35COOH (dSA) with 1-bromoalkanes of different alkyl chain length (C4 to C16) at the air−water interface were investigated. Alkanes and 1-chlorohexadecane ClC16H33 (ClHex) were also studied to compare the effects of the halogen on the mixed monolayers. Surface pressure−area isotherms and Brewster angle microscopy (BAM) were used to obtain the organization and phase behavior, providing a macroscopic view of the mixed monolayers. A molecular-level understanding of the interfacial molecular organization and intermolecular interactions was obtained by using vibrational sum frequency generation (SFG) spectroscopy and infrared reflection−absorption spectroscopy (IRRAS). It was found that from the alkyl halide molecules investigated 1-bromopentadecane, BrC15H31 (BrPent), 1-bromohexadecane, BrC16H33 (BrHex), and ClHex incorporate into the fatty acid monolayers. Alkanes of 15- and 16-carbon chain length do not incorporate into the fatty acid monolayer, which suggests that the halogen is needed for incorporation. Isotherms and spectra suggest that BrHex molecules are squeezed out, or excluded, from the fatty acid monolayer as the surface pressure is increased, while BAM images confirm this. Additionally, SFG spectra reveal that the alkyl chains of both fatty acids (dPA and dSA) retain an all-trans conformation after the incorporation of alkyl halide molecules. BAM images show that at low surface pressures BrHex does not affect the two-dimensional morphology of the dPA and dSA domains and that BrHex is miscible with dPA and dSA. We also present for the first time BAM images of BrHex deposited on a water surface, which reveal the formation of aggregates while the surface pressure remains unchanged from that of neat water