“…It has been well-documented that, at low surface pressure, the methyl ester headgroup is E -configured for expanded fatty acid methyl ester monolayers, where a substantial part of the oxo-methyl group is pointing out of the water (Figure S2 of the Supporting Information). The E isomer of fatty acid methyl esters permits the hydration of the carbonyl group and impedes the interactions between the headgroup and cation, whereas at higher surface pressure, the headgroup is forced into Z configuration, with the oxo-methyl group pointing to the subphase for more orderly packed compressed states. , In addition, several studies have clarified the expulsion of water molecules with an increasing surface pressure. ,, When the fatty acid methyl ester monolayers are Z -configured, the shielding of the oxo-methyl component protects the carbonyl group from being hydrogen-bonded by water molecules. The expelled water from fatty acid methyl ester films could impact the structure of the Z isomer, allowing the cations into the monolayer (Figure S3 of the Supporting Information).…”
Section: Results
and Discussionmentioning
confidence: 99%
“…The E isomer of fatty acid methyl esters permits the hydration of the carbonyl group and impedes the interactions between the headgroup and cation, whereas at higher surface pressure, the headgroup is forced into Z configuration, with the oxo-methyl group pointing to the subphase for more orderly packed compressed states. 33,55 In addition, several studies have clarified the expulsion of water molecules with an increasing surface pressure. 30,31,33 When the fatty acid methyl ester monolayers are Z-configured, the shielding of the oxo-methyl component protects the carbonyl group from being hydrogen-bonded by water molecules.…”
The structure of sea spray aerosols (SSAs) has been described as a saline core coated by organic surfactants. The presence of surface-active compounds at the air-water interface can have a large impact on physical, chemical and optical properties of SSAs. The surfactant molecules chosen for this study, palmitic acid (PA), stearic acid (SA), arachidic acid (AA), methyl palmitate (MP), methyl stearate (MS) and methyl arachidate (MA), were used to investigate the effect of alkyl chain-length, head-groups and sea salts on the surface properties of these monolayers. A Langmuir trough was used for measuring surface pressure−area (π−A) isotherms to reveal macroscopic phase behavior of the surface films at the air-water interface. Infrared reflection absorption spectroscopy (IRRAS) was employed to have a molecular-level understanding of the interfacial molecular organization. The π−A isotherms indicated that sea salts, present in the subphase, exert a strong condensing effect on fatty acid monolayers, while exerting expanding effect on fatty acid methyl ester monolayers, which was confirmed by results from IRRAS experiments. IRRAS further revealed that the alkyl chains were in an all-trans conformation, which can be evidenced by the relatively low νa(CH2) and νs(CH2) stretching frequencies. The conformational order changes in the alkyl chains of different film-forming species (C16 < C18 < C20) were directly revealed by analyzing the relative intensity of the νa(CH2) and νs(CH2) peaks in the C-H stretching region. Thus, all the three factors alter the phase behavior and molecular packing of the monolayers at the air-aqueous interface.
“…It has been well-documented that, at low surface pressure, the methyl ester headgroup is E -configured for expanded fatty acid methyl ester monolayers, where a substantial part of the oxo-methyl group is pointing out of the water (Figure S2 of the Supporting Information). The E isomer of fatty acid methyl esters permits the hydration of the carbonyl group and impedes the interactions between the headgroup and cation, whereas at higher surface pressure, the headgroup is forced into Z configuration, with the oxo-methyl group pointing to the subphase for more orderly packed compressed states. , In addition, several studies have clarified the expulsion of water molecules with an increasing surface pressure. ,, When the fatty acid methyl ester monolayers are Z -configured, the shielding of the oxo-methyl component protects the carbonyl group from being hydrogen-bonded by water molecules. The expelled water from fatty acid methyl ester films could impact the structure of the Z isomer, allowing the cations into the monolayer (Figure S3 of the Supporting Information).…”
Section: Results
and Discussionmentioning
confidence: 99%
“…The E isomer of fatty acid methyl esters permits the hydration of the carbonyl group and impedes the interactions between the headgroup and cation, whereas at higher surface pressure, the headgroup is forced into Z configuration, with the oxo-methyl group pointing to the subphase for more orderly packed compressed states. 33,55 In addition, several studies have clarified the expulsion of water molecules with an increasing surface pressure. 30,31,33 When the fatty acid methyl ester monolayers are Z-configured, the shielding of the oxo-methyl component protects the carbonyl group from being hydrogen-bonded by water molecules.…”
The structure of sea spray aerosols (SSAs) has been described as a saline core coated by organic surfactants. The presence of surface-active compounds at the air-water interface can have a large impact on physical, chemical and optical properties of SSAs. The surfactant molecules chosen for this study, palmitic acid (PA), stearic acid (SA), arachidic acid (AA), methyl palmitate (MP), methyl stearate (MS) and methyl arachidate (MA), were used to investigate the effect of alkyl chain-length, head-groups and sea salts on the surface properties of these monolayers. A Langmuir trough was used for measuring surface pressure−area (π−A) isotherms to reveal macroscopic phase behavior of the surface films at the air-water interface. Infrared reflection absorption spectroscopy (IRRAS) was employed to have a molecular-level understanding of the interfacial molecular organization. The π−A isotherms indicated that sea salts, present in the subphase, exert a strong condensing effect on fatty acid monolayers, while exerting expanding effect on fatty acid methyl ester monolayers, which was confirmed by results from IRRAS experiments. IRRAS further revealed that the alkyl chains were in an all-trans conformation, which can be evidenced by the relatively low νa(CH2) and νs(CH2) stretching frequencies. The conformational order changes in the alkyl chains of different film-forming species (C16 < C18 < C20) were directly revealed by analyzing the relative intensity of the νa(CH2) and νs(CH2) peaks in the C-H stretching region. Thus, all the three factors alter the phase behavior and molecular packing of the monolayers at the air-aqueous interface.
“…Thus, DHO molecules can form a 2D hydrogen-bonded network in which each molecule can interact with four different molecules at best. Wang et al have conformed the formation of intermolecular hydrogen bonding for DHO molecules on the basis of FT-IR spectroscopy measurements,8d while for β-fluorohydrin and α-fluorohydrin there are only three and two molecules at best, respectively. So, the strength of the intermolecular hydrogen bond has the following sequence: DHO > β-fluorohydrin ≈ α-fluorohydrin.…”
Section: Resultsmentioning
confidence: 97%
“…This is demonstrated by the BAM images that show at A ≤ 0.26 nm 2 /molecule exclusively bright 3D domains (Figure a). It is interesting to note that the π− A isotherm of the monolayer of methyl 2,3-dihydroxyoctadecanoate (DHO), 8b,d which has two vicinal hydroxyl groups in 2,3-position, shows a clear 2D fluid-condensed phase transition state. Correspondingly, DHO forms 2D domains in the fluid-condensed phase coexistence region as shown by BAM measurement. 8b,d 4 Typical BAM images for (a) β-fluorohydrin, π = 35.4 mN/m and A = 0.22 nm 2 /molecule; and (b) α-fluorohydrin, π = 37.1 mN/m and A = 0.20 nm 2 /molecule.…”
Section: Resultsmentioning
confidence: 99%
“…The effect of regiochemistry on the thermodynamic properties and phase behavior of hydroxylated fatty acid monolayers has been extensively studied . Previous investigations on methyl dihydroxyoctadecanoates at the air−water interface have shown differences in the phase behavior depending on the position and the stereochemistry of the two vicinal hydroxyl groups. 8a-d The phase behavior of these amphiphiles at the air−water interface has been changed dramatically when replacing the hydroxyl groups with other substitutents in methyl octadecanoate, for example, OCH 3 or fluorine. 8e-g…”
The phase behavior of 2,3-disubstituted methyl octadecanoate monolayers at the air-water interface is studied by film balance and a Brewster angle microscope (BAM). The comparison of the surface pressure-molecular area (pi-A) isotherms with the corresponding BAM images provides information on the phase behavior of the monolayers. Variations in the phase behavior of different 2,3-disubstituted methyl octadecanoate monolayers can be correlated with the size of the headgroups, the interactions between the polar molecular moieties and the subphase, and the intermolecular interactions. The enlarging of the headgroups makes forming a condensed monolayer difficult for the molecules, even after introduction of substituents giving rise to the formation of hydrogen bonds between the molecules, which may balance the steric repulsion and stabilize the monolayers. Model calculations of the two-dimensional lattice structure of the 2,3-disubstituted methyl octadecanoates on basis of the pg and p1 space group are performed and correspond well with the experimental results.
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