Liquid Expanded/liquid Condensed Research Articles

Dexamethasone and Dexamethasone Phosphate: Effect on DMPC Membrane Models

Dexamethasone (Dex) and Dexamethasone phosphate (Dex-P) are synthetic glucocorticoids with high anti-inflammatory and immunosuppressive actions that gained visibility because they reduce the mortality in critical patients with COVID-19 connected to assisted breathing. They have been widely used for the treatment of several diseases and in patients under chronic treatments, thus, it is important to understand their interaction with membranes, the first barrier when these drugs get into the body. Here, the effect of Dex and Dex-P on dimyiristoylphophatidylcholine (DMPC) membranes were studied using Langmuir films and vesicles. Our results indicate that the presence of Dex in DMPC monolayers makes them more compressible and less reflective, induces the appearance of aggregates, and suppresses the Liquid Expanded/Liquid Condensed (LE/LC) phase transition. The phosphorylated drug, Dex-P, also induces the formation of aggregates in DMPC/Dex-P films, but without disturbing the LE/LC phase transition and reflectivity. Insertion experiments demonstrate that Dex induces larger changes in surface pressure than Dex-P, due to its higher hydrophobic character. Both drugs can penetrate membranes at high lipid packings. Vesicle shape fluctuation analysis shows that Dex-P adsorption on GUVs of DMPC decreases membrane deformability. In conclusion, both drugs can penetrate and alter the mechanical properties of DMPC membranes.

Interactions of model airborne particulate matter with dipalmitoyl phosphatidylcholine and a clinical surfactant Calsurf

HypothesisLung surfactant protects lung tissue and reduces the surface tension in the alveoli during respiration. Particulate matter with an aerodynamic diameter of less than 2.5 μm (PM2.5), which invades primely through inhalation, can deposit on and interact with the surfactant layer, leading to changes in the biophysical and morphological properties of the lung surfactant. ExperimentsLangmuir monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and clinical surfactant Calsurf were investigated with a PM2.5 model injected into the water subphase, which were characterized by surface pressure-area isotherms, Brewster angle microscopy, atomic force microscopy, fluorescent microscopy, and x-ray photoelectron spectroscopy. The binding between DPPC/Calsurf and PM2.5 was studied using isothermal titration calorimetry. FindingsPM2.5 induced the expansion of the monolayers at low surface pressure (п) and film condensation at high п. Aggregation of PM2.5 mainly occurred at the interface of liquid expanded/liquid condensed (LE/LC) phases. PM2.5 led to slimmer and ramified LC domains on DPPC and the reduction of nano-sized condensed domains on Calsurf. Both DPPC and Calsurf showed fast binding with PM2.5 through complex binding modes attributed to the heterogeneity and amphiphilic property of PM2.5. This study improves the fundamental understanding of PM2.5-lung surfactant interaction and shows useful implications of the toxicity of PM2.5 through respiration process.

Influence of Stereochemistry on the Monolayer Characteristics of N-alkanoyl-Substituted Threonine and Serine Amphiphiles at the Air-Water Interface.

Thermodynamic and structural properties of the N-alkanoyl-substituted α-amino acids threonine and serine, differing only by one CH3 group in the head group, are determined and compared. Detailed characterization of the influence of stereochemistry proves that all enantiomers form an oblique monolayer lattice structure whereas the corresponding racemates build orthorhombic lattice structures due to dominating heterochiral interactions, except N-C16-dl-serine-ME as first example of dominating homochiral interactions in a racemic mixture of N-alkanoyl-substituted α-amino acids. In all cases, the liquid expanded–liquid condensed (LE/LC) transition pressure of the racemic mixtures is above that of the corresponding enantiomers. Phase diagrams are proposed. Using the program Hardpack to predict tilt angles and cross-sectional area of the alkyl chains shows reasonable agreement with the experimental grazing incidence X-ray diffraction (GIXD) data.

The effect of acyl chain length and saturation on the interactions of pirarubicin with phosphatidylethanolamines in 2D model urothelial cancer cell membranes

The interactions of an anticancer drug pirarubicin (THP) with a series of phosphatidylethanolamine (PE) lipids differing in the length and unsaturation degree of acyl chains were investigated using the Langmuir technique supplemented with Brewster angle microscopy (BAM) and thermodynamic analysis of compression-expansion cycles. The incorporation of the drug from the subphase into the lipid monolayer treated as a simple model of urothelial cancer cell membranes leads to the increase in the areas per molecule at lower surface pressures, while further compression of the monolayer at the air-water interface results in the expulsion of the drug. In the case of 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) undergoing liquid expanded – liquid condensed (LE-LC) phase transition the isotherms in the presence and absence of THP are essentially indiscernible at higher surface pressures and the drug was found to locate in between the domains. For other lipids (1,2-distearoyl-sn-glycero-3-phosphoethanolamine, DSPE, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine, SOPE and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, DOPE) the isotherms recorded in the presence of THP were characterized by areas per molecule smaller than those for the isotherms obtained for lipids on a pure buffer. It shows that the drug, despite squeezing out from the lipid monolayer, is still interacting strongly with lipid molecules. The formation of THP/lipid aggregates takes place as shown by the results of the thermodynamic analysis for the compression-expansion cycles. It may explain a disruptive effect of THP on the lipid packing, which is supported by a decreased value of Cs−1max. The most significant interactions were observed for 1,2-dilinoleoyl-sn-glycero-3-phosphoethanolamine (DLOPE). Due to the tilt of the acyl chains caused by the presence of two double bonds in each chain, a pocket is formed allowing THP to effectively penetrate the hydrophobic part of the DLOPE monolayer even at higher surface pressures without squeezing-out effect. Interestingly, this penetration is not accompanied by any formation of lipid-drug aggregates as shown by thermodynamic analysis of hysteresis. Moreover, THP is also able to effectively incorporate into the preformed DLOPE layers. These results are important for the detailed explanation of the molecular mechanisms of the passive transport of THP through the urothelial cancer cell membranes, which are especially enriched in the longer chain, double unsaturated PE lipids such as DLOPE.

Lipid monolayers and adsorbed polyelectrolytes with different degrees of polymerization.

Polystyrene sulfonate (PSS) of different molecular weight M(w) is adsorbed to oppositely charged DODAB monolayers from dilute solutions (0.01 mmol/L). PSS adsorbs flatly in a lamellar manner, as is shown by X-ray reflectivity and grazing incidence diffraction (exception: PSS with M(w) below 7 kDa adsorbs flatly disordered to the liquid expanded phase). The surface coverage and the separation of the PSS chains are independent of PSS M(w). On monolayer compression, the surface charge density increases by a factor of 2, and the separation of the PSS chains decreases by the same factor. Isotherms show that on increase of PSS M(w) the transition pressure of the LE/LC (liquid expanded/liquid condensed) phase transition decreases. When the contour length exceeds the persistence length (21 nm), the transition pressure is low and constant. For low-M(w) PSS (<7 kDa) the LE/LC transition of the lipids and the disordered/ordered transition of adsorbed PSS occur simultaneously, leading to a maximum in the contour length dependence of the transition enthalpy. These findings show that lipid monolayers at the air/water interface are a suitable model substrate with adjustable surface charge density to study the equilibrium conformation of adsorbed polyelectrolytes as well as their interactions with a model membrane.

Transition from Newtonian to non-Newtonian surface shear viscosity of phospholipid monolayers

The surface shear viscosity of DPPC (dipalmitoylphosphatidylcholine) monolayers on the air/water interface was determined over a wide range of surface concentrations in an annular channel. DPPC is studied widely because it is ubiquitous in biological systems. Brewster angle microscopy (BAM) was found to be capable of measuring the monolayer velocity field, even in the absence of co-existing phase domains. Interfacial velocimetry via cross correlations of BAM images provides accurate and non-invasive measurements, useful for both macro and microrheology. The measured velocity profiles are compared with computed profiles obtained over a range of surface shear conditions using the Boussinesq-Scriven surface model, from which the surface shear viscosity was determined. For monolayers in the liquid expanded (LE) and liquid expanded/liquid condensed (LE/LC) co-existing phases, we observe Newtonian behavior. We also show how the flow departs from the Newtonian regime for monolayers with larger surface concentration, corresponding to LC phase transition to solid phase.

Two-Dimensional Miscibility Behavior of Two Chemically Similar Amide Amphiphiles

The monolayer characteristics of two chemically similar amphiphiles 3-hydroxy-N-tridecyl propanoic acid amide (HTPA) and tetradecanoic acid-(2-hydroxyethyl)amide (TDAHA) and their selected mixtures are studied. Despite the slight structural difference (the position of the two substituents at the acid amide group), the pure components reveal large differences and peculiarities in the surface pressure–area (π-A) isotherms, the monolayer morphologies, and structures. Therefore, their miscibility behavior in monolayers is of special interest. At low temperatures (T ≤ 10 °C), the π-A isotherms of the pure components show a striking second critical point accompanied by an abrupt change of important 2D lattice parameters, indicating the existence of a second phase transition between two condensed phases. This second phase transition is strongly temperature-dependent for TDAHA, nearly temperature-independent for HTPA, and not occurring in the investigated mixtures. The results of Brewster angle microscopy and grazing incidence X-ray diffraction support ideal miscibility of HTPA-TDAHA monolayers already suggested by the linear relationship between the main phase transition pressure and the mole fraction. The lattice parameters of the mixed TDAHA-HTPA monolayers measured for three mole fractions (0.25, 0.5, and 0.75) at 5 °C are compared and discussed. The tilt angle in the mixed TDAHA-HTPA monolayers passes through a minimum, which is connected to the largest cross-sectional area and the smallest entropy change during the main LE/LC (liquid expanded/liquid condensed) transition. An HTPA-TDAHA phase diagram that illustrates the transitions between the different phases is proposed.

Molecular dynamics simulation of phase transitions in model lung surfactant monolayers

To explore the role of individual lung surfactant components in liquid-condensed (LC)/liquid-expanded (LE) phase transitions the MARTINI coarse-grained (CG) model is used to simulate monolayers containing DPPC and additional lipid or peptide components. Our analysis suggests that the LC phase forms from the LE phase via a nucleation and growth mechanism, while the LC–LE transition occurs by melting that originates from defects in the monolayer. On the time scale of our simulations, DPPC monolayers display a substantial hysteresis between the ordering and disordering transitions, which is decreased by the addition of a second component. In binary mixtures of DPPC with lung surfactant peptide fragment SP-B1–25, the ordered side of the hysteresis loop is abolished altogether, suggesting that SP-B1–25 effectively nucleates disorder in the monolayer on heating. SP-B1–25 is observed to perturb the packing of the surrounding lipids leading to local fluidization of the monolayer and to aggregate within the LE phase. In 1:1 DPPC:POPC monolayers, a high concentration of unsaturated phospholipid leads to a substantial decrease in the LC–LE and LE–LC transition temperatures. Adding cholesterol to pure DPPC increases the LC–LE and LE–LC transition temperatures and increases the order on the disordered side of the hysteresis loop leading to a phase of intermediate order, which could be the liquid-disordered (Ld) phase. Cholesterol is also observed to show a preference for LC–LE domain boundaries. The results of our molecular dynamics simulations coincide with many experimental observations and can help provide insight into the physiological roles of individual surfactant components.

Studies of Dipalmitoylphosphatidylcholine (DPPC) Monolayers Embedded with Endohedral Metallofullerene (Dy@C82)

Toxicological effects of carbon nanomaterials have attracted increasing attention. In this work, we studied the interaction between Dy@C(82) and dipalmitoylphosphatidylcholine (DPPC) in a monolayer at the N(2)/Tris buffer interface by thermodynamic analysis of surface pressure-area (pi-A) and surface potential-area (DeltaV-A) isotherms. Dy@C(82) was found to impact considerably more on the physical properties of the monolayers than C(60) because of its elliptical structure and distinctive dipole. The addition of Dy@C(82) essentially closed down the liquid expanded-liquid condensed (LE-LC) phase coexistence region of the mixed monolayers. Furthermore, Dy@C(82) reduced elasticity of the monolayers, as indicated by the decreasing elastic modulus (C(s)(-1)) with increasing molar ratio of Dy@C(82) (X(Dy@C82)). Brewster angle microscopy (BAM) and atomic force microscopy (AFM) revealed that the dispersion of Dy@C(82) depend on the state of the mixed films. Dy@C(82) formed flocs from aggregation of Dy@C(82) towers in the LE and LE-LC coexistence regions, accompanied by gradual falling down of Dy@C(82) from the towers and permeation of the falling metallofullerenes into the LE phase during their compression-induced reorientation process. In the LC and solid phases, the Dy@C(82) flocs were dispersed into isolated towers, accompanied by the partial squeezing out of the embedded metallofullerenes to above the DPPC monolayer. The continuous falling down of Dy@C(82) from the towers resulted in their height decrease but diameter enlargement. When the surface pressure was increased to the kink value (53 mN/m), Dy@C(82) was almost completely extruded from the DPPC monolayers. These findings are believed to be important for understanding the impact of fullerenes, metallofullerenes, and nanomaterials in general on biological membranes.

Topography of dipalmitoyl-phosphatidyl-choline monolayers penetrated by β-casein

In this work we have analyzed the topography by atomic force microscopy (AFM) of dipalmitoyl-phosphatidyl-choline (DPPC) monolayers previously spread at the air–water interface and penetrated by β-casein. AFM images of β-casein–DPPC monolayers were taken from Langmuir–Blodgett films deposited onto hydrophilic mica substrates at different initial surface pressures ( π i) and after the compression of the mixed films. The monolayer topography depends on the initial structure of the phospholipid:liquid expanded (LE) at 3 mN/m, coexistence between LE and liquid condensed (LC) structures at 7 mN/m, at the end of the LE–LC transition at 10 mN/m, and with a LC structure at 15 mN/m. The area occupied by DPPC domains in the mixed film increases with the π i value, especially for DPPC with a LC structure at 15 mN/m. At this surface pressure the thickness of the film is at a maximum. After the film compression at 25 mN/m, which is above the equilibrium spreading pressure of β-casein ( π e β -casein ), this protein is displaced from the interface by DPPC and the topography of the mixed monolayer depends on the initial structure of the DPPC monolayer. A notable feature of the topography of these mixed monolayers is the presence of multilayers of β-casein and DPPC of high thickness (50–70 nm) at the lower π i values. Although the film is dominated by DPPC at the highest surface pressures (at 25 mN/m), β-casein is not displaced totally from the interface and coexists as β-casein collapsed domains within the network of the DPPC structure.

Langmuir films of nitro substituted N-benzylidene hexadecylamine Schiff bases at air/water interface—Phase transitions and molecular dynamics simulation

Formation and packing of Langmuir monolayers of ortho, meta and para substituted N-benzylidene hexadecylamine have been studied using surface pressure–molecular area ( π– A) isotherms over a wide range of temperatures. A cusp with a plateau in the isotherms shows a first-order phase transition between the liquid expanded (LE) and liquid condensed (LC) phase in all the three compounds at air/water interface. Reflection spectra of the Langmuir monolayers in the LC phase and absorbance spectra of the corresponding Langmuir–Blodgett films (LB films) show fairly ordered films. In the ortho derivative the intramolecular H-bonding seems to compete with intermolecular H-bonding leading to a lesser hydration-induced repulsive interactions between the head groups thereby inducing better packing. The changes in enthalpy (Δ H) was used to understand the nature of the packing of the molecules in the LE–LC phase. Molecular dynamics simulation carried out on the three amphiphiles on water using amorphous cells showed that the compound with the –nitro group in the ortho position shows the highest interaction energy suggesting reduced hydration. This correlates well with the ordering in the different Langmuir films.

Temperature and compression rate independent domain shape in Langmuir monolayers of di- n-dodecyl hydrogen phosphate at the air–water interface

Thermodynamic and morphological properties of Langmuir monolayers of di- n-dodecyl hydrogen phosphate (DDP) have been studied by film balance and Brewster angle microscopy (BAM) over a wide range of temperature between 5 and 40 °C. From π – A isotherms, a generalized phase diagram consisting of gas (G), liquid expanded (LE) and liquid condensed (LC) phases is constructed for the DDP monolayers. The BAM images show the formation of gas bubble in the bright background of LE phase during G–LE phase transitions and fingering LC domains during LE–LC phase transitions. The shapes of these domains are independent of temperature, showing a sharp contrast to the temperature-dependent monolayer morphologies of amphiphilic systems where the shape of the LC domains changes either from compact circular to fingering or from irregular or spiral to compact patterns with increasing temperature. In addition, the domains do not show any change in their shapes with decreasing the compression rate. Since the two-alkyl chains are directly attached by covalent bonds to the phosphate group, the rearrangement of the molecules needs to move the whole molecules including the hydration sphere. The difficulty related to such a movement of the molecules causes the fingering domains, which are independent of external variables. Although the domains are formed in a fingering shape, the equilibrium shape can be attained by about 120 min at 15 °C indicating a rather slow relaxation rate.

Molecular behavior of a microbial lipopeptide monolayer at the air–water interface

The monolayer of a microbial lipopeptide has been studied as functions of subphase temperature and pH with a Langmuir–Blodgett (LB) film balance. The surface pressure–area ( π– A) isotherm and the surface compressibility–area ( C s– A) curve indicate that the lipopeptide monolayer exists in four different physical states classified as: liquid-expanded (LE) film, liquid-condensed (LC) film, LE–LC transition and a plateau region. Physical states of the monolayer are greatly influenced by subphase temperature and pH. Significantly, these two factors exert influences on physical states in different ways. The LE film and LE–LC transition are much affected by subphase temperature, while the shapes of the plateau region are mainly influenced by subphase pH. LE film turns more condensed and LE–LC transition becomes clearly evident when temperature increased from 5 to 15 °C. The plateau region becomes less horizontal and begins at lower pressure when subphase pH increased from 2 to 10. The variation of the effects may be related to that subphase temperature alters the mobility of fatty acid chains and subphase pH impacts the ionization of peptide loop. The lipopeptide films transferred from air–water interface onto mica substrate are also investigated with atomic force microscopy (AFM). AFM images provide direct evidence that the lipopeptide molecules take up distinct organizations at different physical states, and the plateau region in π– A isotherm is a phase transition from monolayer to multilayer on acidic subphase.

How many phases and phase transitions do exist in Gibbs adsorption layers at the air–water interface?

Four different phases and four different first-order phase transitions have been shown to exist in Gibbs adsorption layers of mixtures containing n-hexadecyl dihydrogen phosphate ( n-HDP) and l-arginine ( l-arg) at a molar ratio of 1:2. These conclusions have been made from surface pressure–time ( π – t ) adsorption isotherms measured with a film balance and from monolayer morphology observed with a Brewster angle microscopy (BAM). The observed four phases are gas (G), liquid expanded (LE), liquid condensed (LC) and LC ′ phases. Three first-order phase transitions are G–LE, LE–LC and LC–LC ′ . However, the thermodynamically allowed G–LC phase transition in a 1.2 × 10 −4 M mixture at 2 °C, which is below the so-called triple point, is kinetically separated into the G–LE and LE–LC phase transitions. The most interesting observation is that the homogeneous LC phase shows a new first-order phase transition named as LC–LC ′ at 2 or 5 °C. The LE and LC phases represent circular and fractal shaped domains, respectively, whereas the LC ′ phase shows very bright, anisotropic and characteristic shaped domains.

Surface phase behavior of di- n-tetradecyl hydrogen phosphate in Langmuir monolayers at the air–water interface

Surface phase behavior of di- n-tetradecyl hydrogen phosphate, DTP, has been studied by measuring π – A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM) at different temperatures. A generalized phase diagram, which shows a triple point for gas (G), liquid-expanded (LE) and liquid-condensed (LC) phases at about 32 °C, is constructed for the amphiphile. Below the triple point, a first-order G–LC phase transition has been shown to occur, whereas a first-order G–LE phase transition followed by another first-order LE–LC transition has been found to take place at a temperature above the triple point. The amphiphile shows the fingering LC domains with uniform brightness indicating the presence of untilted molecules. The domain shapes are independent of the change in temperature and compression rate. The existence of similar fingering domains over a wide range of temperature is rather uncommon in the monolayer systems and is considered to be due to the restricted movement of the molecules incorporating into the LC phase. Because the two-alkyl chains are directly attached to two covalent bonds of the phosphate head group, the rearrangement of the molecules, which is an essential condition for the circular domain formation, needs the movement of the whole molecules including the hydration sphere. The difficulty related to such a movement of the molecules causes fingering domains, which are independent of external variables.

Effect of temperature on the surface phase behavior of n-hexadecyl dihydrogen phosphate in adsorption layers at the air–water interface

We present the adsorption kinetics and the surface phase behavior of n-hexadecyl dihydrogen phosphate (n-HDP) at the air–water interface by film balance and Brewster angle microscopy (BAM). A phase diagram, which shows a triple point at about 25.8 °C, is constructed by measuring the surface pressure (π)–time (t) adsorption isotherms. Below 25.8 °C, each of the π–t curves shows a plateau at about zero surface pressure indicating the existence of a first-order phase transition. The BAM observation confirms the order of this phase transition by presenting two-surface phases during this plateau. However, the BAM observation also shows clearly another second-order phase transition from an isotropic phase to a mosaic-textured liquid condensed (LC) phase. The initial phase is a gas (G) phase. Considering the peculiarity of the middle phase, we suggest this phase as an intermediate (I) phase. Above the triple point, the π–t curves predict the existence of two-step first-order phase transitions. Similar to the results at lower temperatures, the BAM images show two-surface phases during these first-order phase transitions together with a second-order phase transition from an isotropic phase to an LC phase. These transitions are classified as a first-order G–LE (liquid expanded) phase transition, which is followed by another first-order LE–I phase transition. The second-order phase transition is an I–LC phase transition. Contrary to these results, at 36 °C both the π–t measurements and the BAM observation present only two first-order phase transitions, which are G–LE at zero surface pressure and LE–LC transition at higher surface pressure. The shape of the domains during the main transitions shows a peculiar change from a circular at 20 °C to an elongated at 24 °C and finally to a circular shape at 36 °C. Such a change in the domain shapes has been explained considering the dehydration effect at higher temperatures as well as the nature of phases.

Two-dimensional facets in Langmuir monolayers of 1- O-hexadecyl- rac-glycerol at the air–water interface

We study the surface phase behavior in Langmuir monolayers of 1- O-hexadecyl- rac-glycerol (C16G) by film balance and Brewster angle microscopy over a wide range of temperatures. A cusp point followed by a pronounced plateau region in the pressure–area ( π– A) isotherm indicates a first-order phase transition between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase at the air–water interface. A wide variety of condensed domains are found to form just after the appearance of the cusp point. The observed surface morphology was compared with that of ethylene glycol mono- n-hexadecyl ether (C16E1) that bears an ethylene oxide (EO) unit in the head-group. As usually observed, the domains of C16E1 are found to be circular at lower temperatures and fractal at higher temperatures. Contrary to this usual behavior, the domains of C16G are found to be strip-like structures at lower temperatures, which attain increasingly compact shape as the temperature increases and finally attain faceted structures at ⩾ 25 °C . It is concluded that a higher degree of dehydration around the head-group region of C16G appreciably reduces the hydration-induced repulsive interactions between the head-groups and imparts to the molecules an increase in hydrophobicity, thereby a closer molecular packing. As a result, the molecules form increasingly compact domains as the temperature increases. Since the head-group of C16E1 is much smaller than that of C16G, dehydration effect cannot appreciably increase its hydrophobic character. Rather, increases in subphase temperature result in a decrease in the line tension of the interface giving fractal structures at higher temperatures. In addition, the changes in enthalpy ( Δ H ) and entropy ( Δ S ) values were also calculated to understand the thermodynamic nature of condensation of the molecules in the LE–LC transition region.

Influence of temperature and alkyl chain length on phase behavior in Langmuir monolayers of some oxyethylenated nonionic surfactants

We study the surface phase behavior in Langmuir monolayers of a series of nonionic surfactants of the general formula C nE1 with n = 14 , 16, and 18 by film balance and Brewster angle microscopy (BAM) over a wide range of temperatures. A cusp point followed by a pronounced plateau region in the pressure–area ( π – A ) isotherms indicates a first-order phase transition in the coexisting state between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase at the air–water interface. The formation of bright two-dimensional (2D) LC domains in a dark background visualized by BAM further confirms this observation. In addition to the cusp point at the onset of the LE–LC coexistence state, another cusp point followed by a small plateau is observed for the C14E1 and C18E1 monolayers, indicating a second phase transition between two condensed phases of different compressibility and tilt orientation of the molecules. This unusual two-step phase transition is explained by the Ostwald step rule. The C16E1 and C18E1 monolayers show a kink in their respective isotherms, after which the surface pressure increases steeply with only a little decrease in the molecular area, suggesting that the molecules undergo a transition from a tilted to an almost vertical orientation with respect to the water surface. The thermodynamic parameters for the condensation of the molecules in the LE–LC coexistence state were calculated by employing the 2D Clapeyron equation. The temperature coefficient of the critical surface pressure d π c / d T values shows a decreasing trend from C14E1 to C18E1, suggesting that the condensation process becomes less and less prone to thermal perturbation as the chain length increases. For all the amphiphiles, the Δ H values are found to be negative, suggesting an exothermic nature of condensation. The negative Δ S values obtained from the relation Δ H / T probably come from the restriction on the rotational and translational motion of the molecules constrained in a confined area in the LE–LC transition region.

Phases and phase transitions in Gibbs monolayers of an alkyl phosphate surfactant

We present the adsorption kinetics and the surface phase behavior of water-soluble n-tetradecyl phosphate ( n-TDP) at the air–water interface by film balance and Brewster angle microscopy (BAM). The relaxation of the surface pressure at about zero value in the surface pressure ( π ) –time ( t ) adsorption isotherm is found to occur from 2 to 20 °C with appropriate concentrations of the amphiphile. These plateaus are accompanied by two surface phases, confirming that the relaxation of the surface pressure is caused by a first-order phase transition. Only this phase transition is observed at <6.5 °C and it is considered as a gas (G)–liquid condensed (LC) phase transition. Above 6.5 °C, the phase transition at zero surface pressure is followed by another phase transition, which is indicated by the presence of cusp points in the π– t curves at different temperatures. Each of the cusp points is followed by a plateau, which is accompanied by two surface phases, indicating that the latter transitions are also first-order in nature. At >6.5 °C, the former transition is classified as a first-order G–liquid expanded (LE) phase transition, while the latter transition is grouped into a first-order LE–LC phase transition. The critical surface pressure ( π c) necessary for the G–LC and G–LE phase transitions is zero and remains constant all over the studied temperatures, whereas that for the LE–LC transition increases linearly with increasing temperature. Based on these results, we construct a rather elaborated phase diagram that shows that the triple point for Gibbs monolayers of n-TDP is 6.5 °C. All the results are consistent with the present understanding of the Langmuir monolayers of insoluble amphiphiles at the air–water interface.

Effect of the polar headgroup of phospholipids on their interaction with actin

It is generally admitted that actin filaments are anchored to a membrane by membranar actin-binding-proteins. However, we found that actin may also interact directly with membrane phospholipids. The actin–phospholipid complex has been investigated at the air–water interface using a film balance technique. In order to probe the effect of the phospholipid headgroup on the actin–phospholipid interaction, we focus mainly on phospholipids that have the same acyl chain length but different headgroups. For all the phospholipids, the apparent area per molecule (the total surface divided by the number of lipid molecules) increases after the injection of the protein into the subphase, which suggests an intercalation of actin between the phospholipid molecules. This effect seems to be more important for DMPE and DMPS than for DMPG, suggesting that the headgroup plays an important role in this intercalation. The critical surface pressure associated to the liquid expanded–liquid condensed (LE-LC) phospholipid transition increases with the concentration of G-actin and thus suggests that G-actin acts as an impurity, simply competing as a surfactant at the air–water interface. On the other hand, F-actin affects the LE to LC transition of phospholipids differently. In this case, the LE to LC transition is broader and F-actin slightly decreases the critical surface pressure, which suggests that electrostatic interactions are involved.