Stability of fatty acid monolayers and the relationship between equilibrium spreading pressure, phase transformations, and polymorphic crystal forms

Interfacial Enrichment of Lauric Acid Assisted by Long-Chain Fatty Acids, Acidity and Salinity at Sea Spray Aerosol Surfaces

Surfactant monolayers at sea spray aerosol (SSA) surfaces regulate various atmospheric processes including gas transfer, cloud interactions, and radiative properties. Most experimental studies of SSA employ a simplified surfactant mixture of long-chain fatty acids (LCFAs) as a proxy for the sea surface microlayer or SSA surface. However, medium-chain fatty acids (MCFAs) make up nearly 30% of the FA fraction in nascent SSA. Given that LCFA monolayers are easily disrupted upon the introduction of chemical heterogeneity (such as mixed chain lengths), simple FA proxies are unlikely to represent realistic SSA interfaces. Integrating experimental and computational techniques, we characterize the impact that partially soluble MCFAs have on the properties of atmospherically relevant LCFA mixtures. We explore the extent to which the MCFA lauric acid (LA) is surface stabilized by varying acidity, salinity, and monolayer composition. We also discuss the impacts of pH on LCFA-assisted LA retention, where the presence of LCFAs may shift the surface-adsorption equilibria of laurate─the conjugate base─toward higher surface activities. Molecular dynamic simulations suggest a mechanism for the enhanced surface retention of laurate. We conclude that increased FA heterogeneity at SSA surfaces promotes surface activity of soluble FA species, altering monolayer phase behavior and impacting climate-relevant atmospheric processes.

Measuring the Equilibrium Spreading Pressure-A Tale of Three Amphiphiles

A surfactant's equilibrium spreading pressure (ESP) is the maximum decrease in surface tension achievable at equilibrium below the Krafft point. Difficulties in measuring the ESP have been noted previously but no well-established experimental protocols to overcome them exist. We present a case study of three solid amphiphiles with different propensities to spread on the air-water interface. Starting with the partially water soluble n-dodecanol (C12H25OH), which spreads instantaneously. The strong Marangoni flows associated with the spreading result in the dislocating of the Wilhelmy plate or crystals attaching to it. A temporary mechanical barrier in front of the spreading crystals mitigates the flows disturbing the plate. Presaturating the subphase with the amphiphile prevents the establishment of dynamic steady states, reduces the standard error by a factor of three and causes faster equilibration. The perfluoroalkylated analog of dodecanol (11:1 fluorotelomer alcohol, C11F23CH2OH) is slow spreading. With surfactant crystals on the interface, the surface pressure reaches a pre-equilibrium plateau within an hour, followed by equilibration on day-long timescales. We show that it is better to estimate the ESP by averaging the values of multiple pre-equilibrium plateaus rather than waiting for equilibrium to be established. Finally, the nonspreading amphiphile DPPC exhibits a large barrier for the mass transfer from the DPPC crystal to the aqueous surface. This was overcome by introducing a volatile, water-immiscible solvent deposited on the surface next to the crystals to facilitate the spreading process and leave behind a monolayer.

Unveiling Texture and Topography of Fatty Acid Langmuir Films: Domain Stability and Isotherm Analysis

3D texturing by self-assembly at the air-water interface has recently been proposed. The hypothesis of this work is that, if this is true, such domain formation should be inferable directly from pressure-area isotherms and be thermodynamically stable. Monolayers of branched fatty acid mixtures with straight chain analogues and their stability are thus studied using a combination of pressure-area isotherms, thermodynamic analysis, in situ Brewster angle microscopy, and atomic force microscopy of both LB-deposited and drop-cast films on silicon wafers. Isotherms reflecting the behavior of monodisperse 3D domains are shown to be independent of compression rate and display long-term stability. Gibbs analysis further confirms the thermodynamic rather than kinetic origin of such novel species by revealing that deviations from ideal mixing can be explained only a priori by differences in the topography of the water surface, thus also indirectly confirming the self-assembly deformation of the water interface. The intrinsic self-assembly curvature and miscibility of the two fatty acids is confirmed by drop-casting, which also provides a rapid, tunable thin-film preparation approach. Finally, the longevity of the nanostructured films is extraordinary, the long-range order of the deposited films increases with equilibration time at the water interface, and the integrity of the nanopatterns remains intact on the scale of years.

Revealing the Impacts of Chemical Complexity on Submicrometer Sea Spray Aerosol Morphology

Sea spray aerosol (SSA) ejected through bursting bubbles at the ocean surface is a complex mixture of salts and organic species. Submicrometer SSA particles have long atmospheric lifetimes and play a critical role in the climate system. Composition impacts their ability to form marine clouds, yet their cloud-forming potential is difficult to study due to their small size. Here, we use large-scale molecular dynamics (MD) simulations as a "computational microscope" to provide never-before-seen views of 40 nm model aerosol particles and their molecular morphologies. We investigate how increasing chemical complexity impacts the distribution of organic material throughout individual particles for a range of organic constituents with varying chemical properties. Our simulations show that common organic marine surfactants readily partition between both the surface and interior of the aerosol, indicating that nascent SSA may be more heterogeneous than traditional morphological models suggest. We support our computational observations of SSA surface heterogeneity with Brewster angle microscopy on model interfaces. These observations indicate that increased chemical complexity in submicrometer SSA leads to a reduced surface coverage by marine organics, which may facilitate water uptake in the atmosphere. Our work thus establishes large-scale MD simulations as a novel technique for interrogating aerosols at the single-particle level.

Thin Films of Lanthanide Stearates as Modifiers of the Q-Sense Device Sensor for Studying Insulin Adsorption

This article presents new possibilities of using thin films of lanthanide stearates as sorbent materials. Modification of the Q-sense device resonator with monolayers of lanthanide stearates by the Langmuir-Schaeffer method made it possible to study the process of insulin protein adsorption on the surface of new thin-film sorbents. The resulting films were also characterized by compression isotherms, chemical analysis, scanning electron microscopy, and mass spectrometry. The transition of stearic acid to salt was recorded by IR spectroscopy. Using the LDI MS method, the main component of thin films, lanthanide distearate, was established. The presence of Eu²⁺ in thin films was revealed. In the case of europium stearate, the maximum value of insulin adsorption was obtained, -1.67·10^-10 mole/cm². The findings suggest the possibility of using thin films of lanthanide stearates as a sorption material for the proteomics determination of the quantitative protein content in complex fluid systems by specific adsorption on modified surfaces and isolation of such proteins from complex mixtures.

Ice nucleation by particles containing long-chain fatty acids of relevance to freezing by sea spray aerosols

Heterogeneous ice nucleation in the atmosphere regulates cloud properties, such as phase (ice versus liquid) and lifetime. Aerosol particles of marine origin are relevant ice nucleating particle sources when marine aerosol layers are lifted over mountainous terrain and in higher latitude ocean boundary layers, distant from terrestrial aerosol sources. Among many particle compositions associated with ice nucleation by sea spray aerosols are highly saturated fatty acids. Previous studies have not demonstrated their ability to freeze dilute water droplets. This study investigates ice nucleation by monolayers at the surface of supercooled droplets and as crystalline particles at temperatures exceeding the threshold for homogeneous freezing. Results show the poor efficiency of long chain fatty acid (C16, C18) monolayers in templating freezing of pure water droplets and seawater subphase to temperatures of at least -30 °C, consistent with theory. This contrasts with freezing of fatty alcohols (C22 used here) at nearly 20 °C warmer. Evaporation of μL-sized droplets to promote structural compression of a C19 acid monolayer did not favor warmer ice formation of drops. Heterogeneous ice nucleation occurred for nL-sized droplets condensed on 5 to 100 μm crystalline particles of fatty acid (C12 to C20) at a range of temperatures below -28 °C. These experiments suggest that fatty acids nucleate ice at warmer than -36 °C only when the crystalline phase is present. Rough estimates of ice active site densities are consistent with those of marine aerosols, but require knowledge of the proportion of surface area comprised of fatty acids for application.

Insoluble layer deposition and dilatational rheology at a microscale spherical cap interface

The dilatational properties of insoluble monolayers are important for understanding the mechanics of biological systems and consumer products, but isolating the dilatational response of an interface is challenging due to the difficulties in separating dilatation from shear and other deformation modes. Oscillations of a microscale bubble radius are useful for generating purely dilatational flows, but the current deposition methods for insoluble layers onto fluid interfaces are not easily scaled down. In this paper, we describe a miscible solvent exchange procedure for generating insoluble layers at an air-water interface pinned at the tip of a capillary tens of micrometers in diameter. We show that the amount of surfactant adsorbed at the interface can be controlled by the initial concentration dissolved in isopropanol (the starting solvent) and the volumetric flow rate of solvent exchange. Surface pressure-area isotherms and dilatational moduli are measured concurrently for three insoluble surfactants: palmitic acid (PA), dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). The isotherms measured on the microscale interface compare well with previous experiments performed on a Langmuir trough. However, the magnitudes of the dilatational moduli differ from those measured on either Langmuir trough or pendant drop apparatuses. Several possible reasons for the observed differences are discussed. A comparison of the dilatational modulus with the Gibbs elasticity is used to determine the presence of dilatational extra stresses at the interface. The isotherm and dilatational modulus of the insoluble component of the industrial surfactant Tween 80 are measured using this approach. The methods developed here also open the possibility for future study of the important role of finite size effects on microstructure formation and the resulting interfacial mechanics.

Duolayers at the Air/Water Interface: Improved Lifetime through Ionic Interactions

Ionic interactions to stabilize Langmuir films at the air/water interface have been used to develop improved duolayer films. Two-component mixtures of octadecanoic (stearic) acid and poly(diallyldimethylammonium chloride) (polyDADMAC) with different ratios were prepared and applied to the water surface. Surface pressure isotherm cycles demonstrated a significant improvement in film stability with the inclusion of the polymer. Viscoelastic properties were measured using canal viscometry and oscillating barriers, with both methods showing that the optimum ratio for improved properties was four octadecanoic acid molecules to one DADMAC unit (1:0.25). At this ratio it is expected multiple strong ionic interactions are formed along each polymer chain. Brewster angle microscopy showed decreased domain size with increased ratios of polyDADMAC, indicating that the polymer is interspersed across the surface. This new method to stabilize and increase the viscoelastic properties of charged monolayer films, using a premixed composition, will have application in areas such as water evaporation mitigation, optical devices, and foaming.

Dynamics of fatty acid vesicles in response to pH stimuli

We investigate the dynamics of decanoic acid/decanoate (DA) vesicles in response to pH stimuli. Two types of dynamic processes induced by the micro-injection of NaOH solutions are sequentially observed: deformations and topological transitions. In the deformation stage, DA vesicles show a series of shape deformations, i.e., prolate-oblate-stomatocyte-sphere. In the topological transition stage, spherical DA vesicles follow either of the two pathways, pore formation and vesicle fusion. The pH stimuli modify a critical aggregation concentration of DA molecules, which causes the solubilization of DA molecules in the outer leaflet of the vesicle bilayers. This solubilization decreases the outer surface area of the vesicle, thereby increasing surface tension. A kinetic model based on area difference elasticity theory can accurately describe the dynamics of DA vesicles triggered by pH stimuli.

Nucleation in monolayers

Two-dimensional monolayer material of a large number of amphiphiles is transformed into overgrown three-dimensional structures in a state of metastability and supersaturation. This process is described by a theoretical concept and characterised by sensitive experimental techniques. Two theoretical models compatible with each other are based on nucleation and growth of the formed 3D nuclei: (i) the model for limiting cases of nucleation and overgrowth of 3D nuclei with assumed shapes, and (ii) the generalised model under formation of lenticular nuclei. The latter model allows determination of the nucleation rate constant which renders possible the determination of the critical nucleus size and the free energy for the formation of critical nucleus in dependence on the surface pressure. A theoretical model developed on the basis of the classical nucleation theory is applied for this. An effective double-surface pressure-step method is introduced to distinguish between the nucleation and growth processes. Consequently, a critical surface pressure, the limiting surface pressure for the formation of 3D nuclei, and an equilibrium surface pressure, at which the 3D nuclei cease to grow, can be defined and experimentally determined. Despite the different boundary conditions, the constant surface pressure relaxation data can be theoretically described by these two consistent nucleation-growth models. In the most cases, the limiting case of progressive nucleation occurs but also instantaneous nucleation is observed. Direct evidence has been provided by AFM studies that constant surface pressure relaxation of monolayers is caused by nucleation-growth of 3D nuclei from the monolayer material. The quantitative analysis of growth and distribution of the 3D micrograins by AFM supports the mechanism obtained by application of the nucleation-growth theories. New instrumental techniques such as, BAM, AFM, GIXD and X-ray reflectivity provide detailed information on the characteristics of the 3D structures overgrown on the monolayer during constant surface pressure relaxation on microscopic and molecular scales.

Influence of the chain length of ubiquinones on their interaction with DPPC in mixed monolayers

The thermodynamic behavior of representative short (UQ2), middle (UQ4 and UQ6) and long-chain (UQ10) ubiquinones (UQ) mixed with dipalmitoyl-phosphatidylcholine (DPPC) was studied in monolayers at the air-water interface. The influence of isoprenoid chain-length of UQ on miscibility of both lipids was investigated by analysis of surface pressure-area isotherms and using fluorescence microscopy. Analysis of excess areas (A(ex)) and free energies of mixing (DeltaGm), calculated from compression isotherms in the full range of ubiquinones concentrations, has given evidences for UQ-rich constant-size (UQ6, UQ10) or less growth limited (UQ2, UQ4) microdomains formation within mixed films. Fluorescence microscopy observation revealed that ubiquinones are preferentially soluble in the expanded phase. When lateral pressure increased, concomitant evolutions of A(ex) and DeltaGm parameters, and composition dependence of collapse surface pressures, argue for an evolution towards a total segregation, never reached due to expulsion of ubiquinones from the film. The possible significance of these observations is discussed in relation to ubiquinones organization and similar chain length effects in membranes.

Effect of degree, type, and position of unsaturation on the pKa of long-chain fatty acids

Titration of a series of C(18) fatty acids yields pK(a) values that decrease with an increasing degree of unsaturation in the fatty acid chain. The pK(a) values of stearic, elaidic, oleic, linoleic, and linolenic acids were studied and compared to values of area per molecule in a spread monolayer of these acids. The decrease in pK(a) was found to relate to melting point temperature and area per molecule in the spread fatty acid monolayer. The pK(a) value was determined by first dissolving the fatty acid in a high pH solution (pH>10) and subsequently titrating the solution with HCl to obtain the characteristic S-shaped curves used to calculate the pK(a) values. The pK(a) values of stearic, elaidic, oleic, linoleic, and linolenic acids were found to be 10.15, 9.95, 9.85, 9.24, and 8.28, respectively. These pK(a) values were in the same order as area per molecule values of fatty acids in spread monolayers. This suggests that as area per molecule increases the intermolecular distance increases and pK(a) decreases due to reduced cooperation between adjacent carboxyl groups.

Surface-active substances of the guinea pig tubotympanum: a chemical and physical analysis

An attempt to describe the nature of the surface-active substances of the eustachian tube lining layer that influence normal tubal function was undertaken. Under sterile conditions, guinea pig tubotympanic washings were collected, centrifuged, and pooled. Analysis of the pooled lavages using standard surface chemistry techniques confirmed the presence of significant surface-tension-lowering activity in the mucous lining layer of the eustachian tube, but the surface pressure obtained is neither as great nor displays the same degree of hysteresis as pulmonary surfactant. Following separation into aqueous and lipid fractions, measurable amounts of surface activity can be found in both isolates. The chemical composition and concentration of the lipid fraction, and its relative contribution to the surface activity of the tubotympanic washings, however, is smaller and radically different from the phospholipids found in surfactant. A significantly higher concentration of protein was recovered in comparison with the lipid portion, and it was observed that the surface activity of the total washings and the aqueous phase bore remarkable similarities. Although the surface-tension-lowering properties of the tubal lining layer may be the consequence of a combined synergistic action of the lipid and protein moieties, we believe that the current evidence points toward the proteins as being the primary tubal surface-tension-lowering substances.

Correlations between surface area and the rate of enzymatic desaturation with methyl branched 8, 11, 14-eicosatrienoic acid

Methyl-branched derivatives of methyl 8,11,14-eicosatrienoate form stable liquid-expanded monolayers. Surface areas are expanded by the methyl branch. The expansion effect is a function of surface pressure. At high surface pressure, the greatest expansion occurs with a mid-point methyl branch. At low surface pressure, surface area increases continuously as the methyl group is moved along the carbon chain from carbon 19 to carbon 5. Desaturase activity varies inversely with surface area, and a linear correlation exists between surface area at low surface pressure and the desaturation rate. These data support the concept that lipid structure and its effect on short range forces between molecules is an important factor in desaturase activity.

Images of divalent cations in unstained symmetric and asymmetric lipid bilayers

Divalent cations have been microscopiccally visualized in association with simple lipid bilayers. Symmetric and asymmetric oriented bilayers were constructed from fatty acid monolayers and were cut in thin transverse sections for examination by bright field electron microscopy in the absence of stains, fixatives or embedding materials. It has been found that bilayers formed of lipid molecules having alkaline earth head groups exhibit natural electron contrast. The intrinsic image has been liked to local variations in the bilayer absolute electron density profile determined by X-ray diffraction analysis of the same specimens (McIntosh, T. J., Waldbillig, R. C. and Robertson, J. D. (1976) Biochim. Biophys. Acta 448, 15-33). By combining the microscopic, chemical and X-ray evidence it has been estimated that local increments of about 1 g/cm3 can produce detectable elelcron contrast in 500 A transverse sections of bilayers.