Fluorocarbon-hybrid pulmonary surfactants for replacement therapy--a Langmuir monolayer study

A Sigma1 Receptor Agonist Alters Fluidity and Stability of Lipid Monolayers

Interactions between the sigma1 receptor agonist PRE-084 and various lipid monolayers, including dipalmitoylphosphatidylcholine (DPPC), DPP-ethanolamine (DPPE), DPP-glycerol (DPPG), DPP-serine (DPPS), palmitoylsphingomyelin (PSM), and cholesterol (Ch), were investigated to elucidate the effects of PRE-084 on membrane fluidity and stability. Their interactions with sigma1 receptor agonists have potential implications for neuroprotection, antidepressant, analgesic, and cognitive enhancement effects. In this study, we observed that the presence of PRE-084 in the subphase led to increased fluidity in DPPC and DPPE monolayers, whereas decreasing fluidity was observed in DPPG, DPPS, and PSM monolayers. The interaction of PRE-084 with Ch monolayers was found to be distinct from its interaction with other lipids. Fluorescence microscopy images revealed changes in the size and shape of liquid-condensed domains in the presence of PRE-084, supporting the notion of altered membrane fluidity. Our findings provide new insights into the interaction of PRE-084 with lipid monolayers and its potential implications for biological and membrane science.

Unraveling Complex Hysteresis Phenomenon in 1,2-Dipalmitoyl-sn-Glycero-3-Phosphocholine Monolayer: Insight into Factors Influencing Surface Dynamics

This study explores the hysteresis phenomenon in DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayers, considering several variables, including temperature, compression and expansion rates, residence time, and subphase content. The investigation focuses on analyzing the influence of these variables on key indicators such as the π-A isotherm curve, loop area, and compression modulus. By employing the Langmuir-Blodgett technique, the findings reveal that all the examined factors significantly affect the aforementioned parameters. Notably, the hysteresis loop, representing dissipated energy, provides valuable insights into the monolayer's viscoelasticity, molecular packing, phase transition changes, and resistance during the isocycle process. These findings contribute to a comprehensive understanding of the structural and dynamic properties of DPPC monolayers, offering insights into their behavior under varying conditions. Moreover, the knowledge gained from this study can aid in the development of precise models and strategies for controlling and manipulating monolayer properties, with potential applications in drug delivery systems, surface coatings, as well as further investigation into air penetration into alveoli and the blinking mechanism.

Nondestructive Compression and Fluidization of Phospholipid Monolayers by Gaseous and Aerolized Perfluorocarbons: Promising Substances for Lung Surfactant Treatment

We present a surface-sensitive X-ray scattering study on the influence of gaseous and aerolized perfluorocarbons (FCs) on zwitterionic and anionic phospholipid Langmuir films, which serve as a simplified model system of lung surfactants. It was found that small gaseous FC molecules like F-propane and F-butane penetrate phospholipid monolayers and accumulate between the alkyl chains and form islands. This clustering process can trigger the formation of lipid crystallites at low initial surface pressures. In contrast, the large linear FC F-octyl bromide fluidizes membranes, causing a dissolution of crystalline domains. The bicyclic FC F-decalin accumulates between the alkyl chains of 1,2-dipalmitoyl phosphatidylcholine but cannot penetrate the more densely packed 1,2-dipalmitoyl phosphatidic acid films because of its size. The effects of FCs on lung surfactants are discussed in the framework of currently proposed therapeutic methods for acute respiratory distress syndrome using FC gases, vapor, or aerosol ventilation causing monolayer fluidization effects. This study implies that the highly biocompatible and nontoxic FCs could be beneficial in the treatment of lung diseases with injured nonfunctional lung surfactants in a novel approach for ventilation.

Inverse electron-demand diels-alder reactions of tetrazine and norbornene at the air-water interface

The surface chemistry of the inverse electron-demand Diels-Alder (IEDDA) reaction at the air-water interface is elucidated. Tetrazine (C18-Tz) and norbornene derivatives (C16-NCA) were used as the reactants. Langmuir monolayers of C18-Tz, C16-NCA, and their binary mixtures were prepared on aqueous substrates. The surface properties were analyzed using the surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms, as well as fluorescence microscopy to monitor the progress of the reaction. First, to provide comparison data to evaluate the reaction on the surface, the two components were mixed in stock solutions of organic solvents for the IEDDA reaction. The Langmuir monolayer spread from the reaction solution was characterized as a function of the reaction time. In the subsequent experiments, the Langmuir monolayers were deposited onto the surface of the substrate solutions by spreading from separate stock solutions of C18-Tz and C16-NCA. The variation of the surface behavior of the monolayers with the molecular area, surface composition of the two components, compression speed of the monolayers, and the temperature was studied. We discuss the effects of the air phase in the reaction field on the reaction efficiency by comparing the results obtained from the two methods.

Synthesis and physicochemical evaluation of fluorinated lipopeptide precursors of ligands for microbubble targeting

Ligand-targeted microbubbles are focusing interest for molecular imaging and delivery of chemotherapeutics. Lipid-peptide conjugates (lipopeptides) that feature alternating serine-glycine (SG) n segments rather than classical poly(oxyethylene) linkers between the lipid polar head and a targeting ligand were proposed for the liposome-mediated, selective delivery of anticancer drugs. Here, we report the synthesis of perfluoroalkylated lipopeptides (F-lipopeptides) bearing two hydrophobic chains (C n F2 n +1, n = 6, 7, 8, 1-3) grafted through a lysine moiety on a hydrophilic chain composed of a lysine-serine-serine (KSS) sequence followed by 5 SG sequences. These F-lipopeptides are precursors of targeting lipopeptide conjugates. A hydrocarbon counterpart with a C10H21 chain (4) was synthesized for comparison. The capacity for the F-lipopeptides to spontaneously adsorb at the air/water interface and form monolayers when combined with dipalmitoylphosphatidylcholine (DPPC) was investigated. The F-lipopeptides 1-3 demonstrated a markedly enhanced tendency to form monolayers at the air/water interface, with equilibrium surface pressures reaching ≈7-10 mN m-1 versus less than 1 mN m-1 only for their hydrocarbon analog 4. The F-lipopeptides penetrate in the DPPC monolayers in both liquid expanded (LE) and liquid condensed (LC) phases without interfacial film destabilization. By contrast, 4 provokes delipidation of the interfacial film. The incorporation of the F-lipopeptides 1-3 in microbubbles with a shell of DPPC and dipalmitoylphosphatidylethanolamine-PEG2000 decreased their mean diameter and increased their stability, the best results being obtained for the C8F17-bearing lipopeptide 3. By contrast, the hydrocarbon lipopeptide led to microbubbles with a larger mean diameter and a significantly lower stability.

How Self-Assembled Nanodomains Can Impact the Organization of a Phospholipid Monolayer-Flower-Like Arrays

We found that monolayers of dipalmitoylphosphatidylcholine (DPPC) and semi-fluorinated tetrablock di(F10H16) self-assemble to form a new type of large, complex flower-like patterns on the surface of water and on solid substrates. The hierarchical organization of these unusual self-assemblies was investigated using compression and surface potential isotherms, in situ fluorescence and Brewster angle microscopies, and atomic force microscopy after transfer.

Lateral interaction of cholesterol with a semifluorinated amphiphile at the air-water interface

Lipid rafts consisting mainly of sphingomyelin and cholesterol (Ch) on biomembrane surfaces are deeply related to cellular processes such as protein trafficking and signal transduction. During the processes, the raft microdomains affect the fluidty of biological membranes, which is controlled to large extents by Ch. In this paper, we have investigated the interaction between Ch and a semiflurinated alcohol (F6H9OH) from the aspect of a fluidty control using surface chemistry. The two-component Langmuir monolayer at the air-water interface was characterized by the surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms. The compressibility modulus and excess Gibbs free energy of mixing were calculated from the π-A isotherms. And also the two-dimensional phase diagram was constructed on the basis of phase transition pressures and monolayer collapse pressures. Furthermore, the phase behavior of binary monolayers was visualized with fluorescence microscopy (in situ) and atomic force microscopy (ex situ). The result here indicates a possibility of fluidity control of Ch-related membranes by arranging the fluorination degree of the constituent lipids.

Fluorocarbon Exposure Mode Markedly Affects Phospholipid Monolayer Behavior at the Gas/Liquid Interface: Impact on Size and Stability of Microbubbles

Although most phospholipid-shelled microbubbles (MBs) investigated for medical applications are stabilized by a fluorocarbon (FC) gas, information on the interactions between the phospholipid and FC molecules at the gas/water interface remains scarce. We report that the procedure of introduction of perfluorohexane (F-hexane), that is, either in the gas phase above dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayers, or in the aqueous subphase, radically affects the compression isotherms. When introduced in the gas phase, F-hexane is rapidly incorporated in the interfacial film, but is also readily desorbed upon compression and eventually totally expelled from the phospholipid monolayers. By contrast, when introduced in the aqueous phase, F-hexane remains trapped at the interface. These dissimilar outcomes demonstrate that the phospholipid monolayer acts as a barrier that effectively hinders the transfer of the FC across the interfacial film. F-hexane was also found to significantly accelerate the adsorption kinetics of the phospholipids at the gas/water interface and to lower the interfacial tension, as assessed by bubble profile analysis tensiometry. The extent of these effects is more pronounced when F-hexane is provided from the gas phase. The size and stability characteristics of DMPC- and DPPC-shelled microbubbles were also found to depend on how the FC is introduced. As compared to reference MBs prepared under nitrogen only, introduction of F-hexane always causes a decrease in MB mean radius. However, while for DMPC this decrease depends on the F-hexane introduction procedure, it is independent from the procedure and most pronounced (from ∼2.0 μm to ∼1.0 μm) for DPPC. Introducing the FC in the gas phase has the strongest effect on MB half-life (t_1/2 = ∼1.8 and 6.8 h for DMPC and DPPC, respectively), as compared to when it is delivered through the aqueous phase (∼0.8 and ∼1.7 h). Fluorocarbonless reference DMPC and DPPC bubbles had a half-life of ∼0.5 and 0.8 h, respectively. The effects of F-hexane on MB characteristics are discussed with regard to the interactions between phospholipids and F-hexane and monolayer fluidization effect, as revealed by the Langmuir and tensiometric studies.

Pre-clinical assessment of a water-in-fluorocarbon emulsion for the treatment of pulmonary vascular diseases

Hypoxic pulmonary vasoconstriction (HPV) is a well-characterized vascular response to low oxygen pressures and is involved in life-threatening conditions such as high-altitude pulmonary edema (HAPE) and pulmonary arterial hypertension (PAH). While the efficacy of oral therapies can be affected by drug metabolism, or dose-limiting systemic toxicity, inhaled treatment via pressured metered dose inhalers (pMDI) may be an effective, nontoxic, practical alternative. We hypothesized that a stable water-in-perfluorooctyl bromide (PFOB) emulsion that provides solubility in common pMDI propellants, engineered for intrapulmonary delivery of pulmonary vasodilators, reverses HPV during acute hypoxia (HX). Male Sprague Dawley rats received two 10-min bouts of HX (13% O₂) with 20 min of room air and drug application between exposures. Treatment groups: intrapulmonary delivery (PUL) of (1) saline; (2) ambrisentan in saline (0.1 mg/kg); (3) empty emulsion; (4) emulsion encapsulating ambrisentan or sodium nitrite (NaNO₂) (0.1 and 0.5 mg/kg each); and intravenous (5) ambrisentan (0.1 mg/kg) or (6) NaNO₂ (0.5 mg/kg). Neither PUL of saline or empty emulsion, nor infusions of drugs prevented pulmonary artery pressure (PAP) elevation (32.6 ± 3.2, 31.5 ± 1.2, 29.3 ± 1.8, and 30.2 ± 2.5 mmHg, respectively). In contrast, PUL of aqueous ambrisentan and both drug emulsions reduced PAP by 20–30% during HX, compared to controls. IL6 expression in bronchoalveolar lavage fluid and whole lung 24 h post-PUL did not differ among cohorts. We demonstrate proof-of-concept for delivering pulmonary vasodilators via aerosolized water-in-PFOB emulsion. This concept opens a potentially feasible and effective route of treating pulmonary vascular pathologies via pMDI.

Phase-separated surfactant monolayers: Exploiting immiscibility of fluorocarbons and hydrocarbons to pattern interfaces

The mutual immiscibility of hydrogenated and fluorinated surfactants at interfaces frequently leads to phase-separation, which provides a useful and flexible method for patterning air-water and solid-air interfaces. In this article, we review recent advances in the use of hydrogenated-fluorinated surfactant mixtures to achieve interfacial patterning. For even relatively simple systems comprised of binary mixed monolayers of hydrogenated and perfluorinated fatty acids, a diverse range of film morphologies can be generated at the air-water interface and successfully transferred onto solid substrates. Systematic investigations reported over the past several years have allowed for correlation between the chemical structure of the film constituents with the gross film morphology and underlying crystalline structure of the films. Early thermodynamic models based on the interplay between dipole-dipole repulsion forces between charged headgroups balanced by line tension between phases that were formulated to describe phase-behavior in simple phospholipid monolayer systems have proven highly useful to describe morphologies for the immiscible surfactant blends. Beyond simple binary fatty acid mixtures, highly-structured films have also been reported in mixed phospholipid systems, which have found important application in controlling the physical, compositional and performance properties of lung surfactant mixtures, as well as in semifluorinated alkane monolayers which form unique, hemimicellar structures at both liquid and solid interfaces. We also describe advances in using these approaches to pattern photopolymerizable, luminescent surfactants, which have found extensive use in colorimetric and fluorometric sensing devices. The long-term outlook for this field, with an emphasis on potential applications and future research directions are discussed.

Lateral Diffusion and Molecular Interaction in a Bilayer Membrane Consisting of Partially Fluorinated Phospholipids

Fluorinated lipids and surfactants are attractive biomimetic materials for the extraction and reorganization of membrane proteins because of the biological inertness of fluorocarbons. We investigated the fundamental physical properties of a partially fluorinated phospholipid (F4-DMPC), such as phase transition, area thermal expansion, and lateral lipid diffusion, to evaluate the intermolecular interaction of F4-DMPC in the hydrophobic region quantitatively on the basis of free-volume theory. Fluorescence microscope observation of the supported lipid bilayer (SLB) of F4-DMPC showed that the phase transition between the liquid crystalline and gel phases occurred at 5 °C and that the area thermal expansion coefficient was independent of the temperature near the phase transition temperature. We performed a single particle tracking of the F4-DMPC-SLB on a SiO₂/Si substrate, to measure the diffusion coefficient and its temperature dependence. The apparent activation energy (E'_a) of lateral lipid diffusion, which is an indicator of intermolecular interaction, was 39.1 kJ/mol for F4-DMPC, and 48.2 kJ/mol for a nonfluorinated 1,2-dioleoyl-sn-glycero-3-phosphocholine as a control. The difference of 9 kJ/mol in E'_a was significant compared with the difference due to the acyl chain species among nonfluorinated phosphatidylcholine and also that caused by the addition of cholesterol and alcohol in the bilayer membranes. We quantitatively evaluated the attenuation of intermolecular interaction, which results from the competition between the dipole-induced packing effect and steric effect at the fluorocarbon segment in F4-DMPC.

Interactions of a Tetrazine Derivative with Biomembrane Constituents: A Langmuir Monolayer Study

Tetrazine (Tz) is expected to be used for bioimaging and as an analytical reagent. It is known to react very fast with trans-cyclooctene under water in organic chemistry. Here, to understand the interaction between Tz and biomembrane constituents, we first investigated the interfacial behavior of a newly synthesized Tz derivative comprising a C18-saturated hydrocarbon chain (rTz-C18) using a Langmuir monolayer spread at the air-water interface. Surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms were measured for monolayers of rTz-C18 and biomembrane constituents such as dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), dipalmitoyl phosphatidylethanolamine (DPPE), palmitoyl sphingomyelin (PSM), and cholesterol (Ch). The lateral interaction between rTz-C18 and the lipids was thermodynamically elucidated from the excess Gibbs free energy of mixing and two-dimensional phase diagram. The binary monolayers except for the Ch system indicated high miscibility or affinity. In particular, rTz-C18 was found to interact more strongly with DPPE, which is a major constituent of the inner surface of cell membranes. The phase behavior and morphology upon monolayer compression were investigated by using Brewster angle microscopy (BAM), fluorescence microscopy (FM), and atomic force microscopy (AFM). The BAM and FM images of the DPPC/rTz-C18, DPPG/rTz-C18, and PSM/rTz-C18 systems exhibited a coexistence state of two different liquid-condensed domains derived mainly from monolayers of phospholipids and phospholipids-rTz-C18. From these morphological observations, it is worthy to note that rTz-C18 is possible to interact with a limited amount of the lipids except for DPPE.

Overcoming inactivation of the lung surfactant by serum proteins: a potential role for fluorocarbons?

In many pulmonary conditions serum proteins interfere with the normal adsorption of components of the lung surfactant to the surface of the alveoli, resulting in lung surfactant inactivation, with potentially serious untoward consequences. Here, we review the strategies that have recently been designed in order to counteract the biophysical mechanisms of inactivation of the surfactant. One approach includes protein analogues or peptides that mimic the native proteins responsible for innate resistance to inactivation. Another perspective uses water-soluble additives, such as electrolytes and hydrophilic polymers that are prone to enhance adsorption of phospholipids. An alternative, more recent approach consists of using fluorocarbons, that is, highly hydrophobic inert compounds that were investigated for partial liquid ventilation, that modify interfacial properties and can act as carriers of exogenous lung surfactant. The latter approach that allows fluidisation of phospholipid monolayers while maintaining capacity to reach near-zero surface tension definitely warrants further investigation.

Interfacial behavior of pulmonary surfactant preparations containing egg yolk lecithin

Mammalian lungs are covered with lipid-protein complexes or pulmonary surfactants. In this work, which aimed towards the less expensive production of artificial pulmonary surfactants, we produced surfactants composed of egg yolk lecithin (eggPC), palmitic acid, and hexadecanol (= 0.30/0.35/0.35, mol/mol/mol ) containing different amounts of Hel 13-5 (NH2-KLLKLLLKLWLKLLKLLL-COOH) as a substitute for the proteins in native pulmonary surfactants. Surface pressure (π)-molecular area (A) and surface potential (DV)-A isotherms of the mixtures were measured via the Wilhelmy and ionizing (241)Am electrode methods, respectively. The interactions between the lipid components and Hel 13-5 led to variations in the surface pressure caused by the expulsion of fluid components from the surface. Furthermore, the π-A and DV-A isotherms featured large hysteresis loops for the surfactant that contained a small amount of Hel 13-5 during compression and successive expansion cycling. To elucidate the morphology, the phase behavior was visualized in situ at the air-water interface by means of fluorescence microscopy; the images suggested less effective interactions between Hel 13-5 and the unsaturated PC in eggPC despite the similarity of their monolayer properties.

Investigation of interfacial behavior of glycyrrhizin with a lipid raft model via a Langmuir monolayer study

An interaction of glycyrrhizin (GC) with a lipid raft biomembrane model that consisted of N-palmitoyl-d-erythro-sphingosylphosphorylcholine (PSM), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and cholesterol (CHOL) was systematically studied using the Langmuir monolayer technique. To construct the lipid raft model, the surface pressure (π)–molecular area (A) and surface potential (ΔV)–A isotherms for three-component (PSM/DOPC/CHOL) systems on 0.02 M Tris buffer with 0.13 M NaCl (pH 7.4) were primarily measured by changing their compositions. Thermodynamic and interaction parameters for binary PSM/DOPC and PSM/CHOL systems revealed that PSM interacts more strongly with CHOL than with DOPC. In addition, a morphological analysis performed with Brewster angle microscopy (BAM) and fluorescence microscopy (FM) revealed an optimal ratio of PSM/DOPC/CHOL (1/1/1, by mole) as a model of lipid rafts. Second, the interaction of GC with the ternary PSM/DOPC/CHOL monolayers was investigated on Tris buffer solutions containing different GC concentrations (1, 5, 10, 25, and 50 μM). In BAM and FM images, microdomains were found to become smaller by increasing the GC concentration in the subphase, suggesting that GC regulates the size of raft domains, which provide dynamic scaffolding for numerous cellular processes. More interestingly, the distinctive GC striped regions were formed at the interface at 50 μM, which shows that GC divides the ternary monolayer into pieces. This phenomenon was observed only in the presence of CHOL in the monolayer. These results suggest that CHOL plays an essential role in the interaction with GC, which results in one of the major activities associated with saponins' membrane disruption.

Influence of film composition on the morphology, mechanical properties, and surfactant recovery of phase-separated phospholipid-perfluorinated fatty acid mixed monolayers

Monolayer surfactant films composed of a mixture of phospholipids and perfluorinated (or partially fluorinated) surfactants are of potential utility for applications in pulmonary lung surfactant-based therapies. As a simple, minimal model of such a lung surfactant system, binary mixed monolayer films composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and perfluorooctadecanoic acid (C18F) prepared on a simplified lung fluid mimic subphase (pH 7.4, 150 mM NaCl) have been characterized in terms of mixing thermodynamics and compressibility (measured through π–A compression isotherms), film morphology (via atomic force, fluorescence, and Brewster angle microscopy), as well as spreading rate and hysteresis response to repeated expansion–contraction cycles for a variety of compositions of mixed films. Under all mixing conditions, films and their components were found to be completely immiscible and phase-separated, though there were significant changes in the aforementioned film properties as a function of composition. Of particular note was the existence of a maximum in the extent of immiscibility (characterized by ΔG(ex)(π) values) and enhanced surfactant recovery during hysteresis experiments at χ(C18F) ≥ 0.30. The latter was attributed to the relatively rapid respreading rate of the perfluorinated amphiphile in comparison with DPPC alone at the air–water interface, which enhances the performance of this mixture as a potential pulmonary lung surfactant. Further, monolayer film structure could be tracked dynamically as a function of compression at the air–water interface via Brewster angle microscopy, with the C18F component being preferentially squeezed out of the film with compression, but returning rapidly upon re-expansion. In general, addition of C18F to DPPC monolayers resulted in improvements to mechanical, structural, and respreading properties of the film, indicating the potential value of these compounds as additives to pulmonary lung surfactant formulations.

Surfactants in droplet-based microfluidics

Surfactants are an essential part of the droplet-based microfluidic technology. They are involved in the stabilization of droplet interfaces, in the biocompatibility of the system and in the process of molecular exchange between droplets. The recent progress in the applications of droplet-based microfluidics has been made possible by the development of new molecules and their characterizations. In this review, the role of the surfactant in droplet-based microfluidics is discussed with an emphasis on the new molecules developed specifically to overcome the limitations of 'standard' surfactants. Emulsion properties and interfacial rheology of surfactant-laden layers strongly determine the overall capabilities of the technology. Dynamic properties of droplets, interfaces and emulsions are therefore very important to be characterized, understood and controlled. In this respect, microfluidic systems themselves appear to be very powerful tools for the study of surfactant dynamics at the time- and length-scale relevant to the corresponding microfluidic applications. More generally, microfluidic systems are becoming a new type of experimental platform for the study of the dynamics of interfaces in complex systems.

Effects of perfluorocarbon gases on the size and stability characteristics of phospholipid-coated microbubbles: osmotic effect versus interfacial film stabilization

Micrometer-sized bubbles coated with phospholipids are used as contrast agents for ultrasound imaging and have potential for oxygen, drug, and gene delivery and as therapeutic devices. An internal perfluorocarbon (FC) gas is generally used to stabilize them osmotically. We report here on the effects of three relatively heavy FCs, perfluorohexane (F-hexane), perfluorodiglyme (F-diglyme ), and perfluorotriglyme (F-triglyme), on the size and stability characteristics of microbubbles coated with a soft shell of dimyristoylphosphatidylcholine (DMPC) and on the surface tension and compressibility of DMPC monolayers. Monomodal populations of small bubbles (~1.3 ± 0.2 μm in radius, polydispersivity index ~8%) were prepared by sonication, followed by centrifugal fractionation. The mean microbubble size, size distribution, and stability were determined by acoustical attenuation measurements, static light scattering, and optical microscopy. The half-lives of F-hexane- and F-diglyme-stabilized bubbles (149 ± 8 and 134 ± 3 min, respectively) were about 2 times longer than with the heavier F-triglyme (76 ± 7 min) and 4-5 times longer than with air (34 ± 3 min). Remarkably, the bubbles are smaller than the minimal size values calculated assuming that the bubbles are stabilized osmotically by the insoluble FC gases. Particularly striking is that bubbles 2 orders of magnitude smaller than the calculated collapse radius can be prepared with F-triglyme, while its very low vapor pressure prohibits any osmotic effect. The interface between an aqueous DMPC dispersion and air, or air (or N(2)) saturated with the FCs, was investigated by tensiometry and by Langmuir monolayer compressions. Remarkably, after 3 h, the tensions at the interface between an aqueous DMPC dispersion (0.5 mmol L(-1)) and air were lowered from ~50 ± 1 to ~37 ± 1 mN m(-1) when F-hexane and F-diglyme were present and to ~40 ± 1 mN m(-1) for F-triglyme. Also noteworthy, the adsorption kinetics of DMPC at the interface, as obtained by dynamic tensiometry, were accelerated up to 3-fold when the FC gases were present. The compression isotherms show that all these FC gases significantly increase the surface pressure (from ~0 to ~10 mN m(-1)) at large molecular areas (70 Å(2)), implying their incorporation into the DMPC monolayer. All three FC gases increase the monolayer's collapse pressures significantly (~61 ± 2 mN m(-1)) as compared to air (~54 ± 2 mN m(-1)), providing for interfacial tensions as low as ~11 mN m(-1) (vs ~18 mN m(-1) in their absence). The FC gases increase the compressibility of the DMPC monolayer by 20-50%. These results establish that, besides their osmotic effect, FC gases contribute to bubble stabilization by decreasing the DMPC interfacial tension, hence reducing the Laplace pressure. This contribution, although significant, still does not suffice to explain the large discrepancy observed between calculated and experimental bubble half-lives. The case of F-triglyme, which has no osmotic effect, indicates that its effects on the DMPC shell (increased collapse pressure, decreased interfacial tension, and increased compressibility) contribute to bubble stabilization. F-hexane and F-diglyme provided both the smallest mean bubble sizes and the longest bubble half-lives.