Liquid Expanded/liquid Condensed Research Articles

Surface phase behavior in Langmuir monolayers of diethylene glycol mono-n-hexadecyl ether at the air-water interface

The surface phase behavior of condensed-phase domains formed during a first-order phase transition in Langmuir monolayers of diethylene glycol mono-n-hexadecyl ether at the air-water interface has been investigated by Brewster angle microscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). A variety of two-dimensional (2D) structures are observed just after the appearance of the phase transition at different temperatures. At 10 and 15 degrees C, the domains are found to be small nuclei of irregular structures. Spiral structures are observed at 20 and 22 degrees C, while striplike structures at 24 degrees C. The spiral domains attain increasingly compact shape with increasing temperature, and finally become circular at >or=26 degrees C. Increases in temperature result in dehydration in the ethylene oxide chain, which increases the hydrophobicity, and impart to the molecules a longer-chain-like character. As a result line tension increases with increasing temperature, which probably outweighs the dipole-dipole repulsions showing circular domains at higher temperatures. The PM-IRRAS measurement reveals that the nu(as)(CH(2)) mode moves to lower wave numbers indicating that the LE-LC (liquid expanded-liquid condensed) phase transition during the compression of the monolayer involves changes in the conformational order of the molecules with a preferential increase in the planner trans zigzag conformation of the hydrocarbon chains. The nu(as)(CH(2)) mode in the LC region of the isotherm shows a constant value around 2917.8 cm(-1) indicating a stable state of the monolayer with an almost all-trans conformation of the hydrocarbon chains. The downward band at 1124 cm(-1) assigned to the nu(as)(C-O-C) mode indicates that the corresponding transition dipole moment is oriented perpendicular to the water surface.

The influence of inorganic ions on the properties of nonionic Langmuir monolayers

This paper deals with the characteristics of ethyl palmitate (EP) Langmuir monolayers spread on water and aqueous solutions containing Na + and Ca 2+. No relevant differences due to the presence of ions were observed in the surface pressure–area ( π–A) isotherms at 15, 20 and 25 °C. Brewster angle microscopy (BAM) was applied for a direct visualization of monolayers morphology. Circular and compact domains of liquid expanded (LE)–liquid condensed (LC) transition become dendritic shaped upon temperature increase. On ion-containing subphases, the temperature governs the average LC domain size along the LE–LC transition. During the monolayer collapse, numerous microdomains of high refractive index appear, indicating the transformation of the monolayer into three-dimensional (3D) structures through a nucleation process. For monolayers spread on subphases containing Ca 2+, independently on temperature, these nuclei are formed at π below the collapse pressure ( π c), on domains fusion. The resistance of monolayer to molecular aggregation depends both on the presence of ions and temperature.

Monolayers of mono- and bipolar palmitic acid derivatives

Monopolar and bipolar derivatives of hexadecanoic acid (HA), 16-hydroxyhexadecanoic acid (HHA), methyl hexadecanoate (MH) and methyl 16-hydroxyhexadecanoate (MHH) have been investigated on pure water and NaCl solutions with different ion concentrations (1, 2 and 3 mol l −1). Surface pressure area isotherms show that HA forms a fully condensed monolayer on pure water at 20 °C [E. Teer, C.M. Knobler, S. Siegel, D. Vollhardt, G. Brezesinski, J. Phys. Chem., B104, 43, 2000, pp. 10053–10058] whereas in the case of the corresponding bipolar HHA the hydroxy group as a second polar moiety leads to a destabilization of the monolayer. The presence of two relatively strong hydrophilic polar groups at opposite ends of the chain prevents the formation of condensed films. The esterification of the carboxyl group (MH) changes the phase sequence from L 2–Ov–LS for HA to L 2–LS. Inserting a hydroxy group at the end of the chain (MHH) shifts the liquid expanded/liquid condensed (LE/LC) phase transition to higher surface pressures but does not change the phase sequence, however it increases the chain tilt. The pressure of the first-order phase transition LE/LC is strongly temperature dependent for MH, while the transition pressure of MHH is almost temperature independent. The phase behavior of MHH and MH on pure water was further studied by surface potential, Brewster angle microscopy (BAM), fluorescence microscopy and grazing incidence X-ray diffraction (GIXD) measurements. The LC domains of MHH on pure water are so small that no inner texture can be observed by BAM in contrast to the LC domains of MH. 3M NaCl in the subphase does not change the MH textures, while it increases the size of the LC domains of MHH. The influence of the hydroxy group on the monolayer behavior is discussed in terms of the formation of hydrogen bonds. The presence of NaCl in the subphase expands the monolayers. The results obtained are explained by changes in monolayer–monolayer and monolayer–subphase interactions.

Model membrane/substrate interactions: ethanol and procaine interactions

The ability of ethanol to lower the surface tension of water plays a major role in its ability to affect membrane lipids. Typical lipids will show little expansion on exposure to ethanol substrates and may even show condensations. At the same time, the overall stability of the lipid phase is significantly reduced. Previously, we reported experimental and theoretical studies of the stearic acid (SA)/procaine (PR) system. PR substrate concentrations were examined in the 10 −4–10 −2 M range as Gibbs monolayers, in order to establish the surface activity of both charged and uncharged species, and to estimate the orientation of the PR species at the air/water interface. SA interactions with PR substrates were studied by compressing films of the former and recording the surface pressure/area per molecule isotherms at both pH 2 and 8, so that the SA was in an uncharged and charged state, respectively. More recently, we have carried out similar studies with l-α-dipalmitoyl phosphatidylcholine (DPPC), maintaining the substrate pH at between 5 and 6, at PR concentrations of 10 −6–10 −2M. We also carried out studies of the DPPC/PR system using fluorescence microscopy in order to examine the effects of PR on the biphasic liquid expanded/liquid condensed (LE/LC) transition region. In the absence of any lipid film, PR species appear to be horizontally oriented at the air/water interface, while the surface activity of PR species increases in the order PRH 2+<PRH +<PR, with the neutral species being the most surface active. “Penetration” of a lipid film appears to be predominately due to the insertion of the lipophilic portion of the PR species, though some deeper penetration may occur. Penetration also appears to be greater when the film is charged, thus, SA films show greater expansion at pH 8 than they do at pH 2. For both SA and DPPC the penetration of PR species may be reversed by increasing surface pressure and the penetration goes through a maximum value which is dependent on both the substrate PR concentration and the nature of the lipid film. This phenomenon may be related to that of pressure anesthesia reversal. For DPPC, the maximum penetration occurs where the film is undergoing a LE/LC transition and it appears that PRH + penetrates DPPC more easily than SA. Fluorescence microscopy indicates that the presence of PR species creates much smaller LC domains, while maintaining the total amount of that phase. This is interpreted in terms of a reduction of the line tension between the LE and LC phases due to a line activity of the PR species. Thus, PR will preferentially locate at interfaces within the membrane and this observation should also apply to the lipid–protein interface.

Pressure-Dependent Changes in the Infrared C—H Vibrations of Monolayer Films at the Air/Water Interface Revealed by Two-Dimensional Infrared Correlation Spectroscopy

Two-dimensional infrared correlation analysis (2D-IR) was applied to a set of surface pressure-dependent unpolarized IR spectra of a monolayer film of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) at the air/water (A/W) interface. The experimentally measured asynchronous 2D-IR spectra were compared with synthetic spectra calculated by using an “overlapped peaks” model vs. a “frequency shifting” model. The results presented here show that when the experimentally observed monolayer IR spectra are acquired as a function of surface pressure, one model cannot be used exclusively for spectral interpretation. In this study, the monolayer IR spectra were divided into a low-pressure region subset (&lt;11 mN/m) and a high-pressure region subset (&gt;11 mN/m). When the monolayer IR spectra acquired as a function of surface pressure are analyzed by 2D correlation methods, the results strongly support the following conclusions: (1) the low-pressure subset, which encompasses both the liquid expanded (LE) and the liquid expanded/liquid condensed (LE/LC) regions of the DPPC monolayer isotherm, is best modeled by two overlapped peaks correlated with ordered and disordered conformational states of the monolayer film; and (2) the high-pressure subset, which reflects solely the liquid condensed (LC) phase of the monolayer isotherm, is best modeled by a single peak, which undergoes a minor frequency shift, and which may be primarily correlated with gradual packing of the liquid condensed structure. This interpretation of the 2D-IR correlation spectra is in agreement with the interpretation of sub-bands seen in polarized monolayer IR spectra previously reported by our laboratory.

Light scattering microscopy from monolayers and nanoparticles at the air/water interface

Light scattering microscopy (LSM) is introduced here as a versatile technique for the study of interfacial films at the air/water interface. Laser light scattered from the interface is collected by a microscope objective and imaged onto a CCD camera. LSM allows determination of the spatial distribution of submicron particles, phase transitions in two-dimensions, and defects. The power of LSM, especially if conducted simultaneously with fluorescence microscopy or Brewster angle microscopy (BAM), is illustrated in three examples. (a) Visualization of the spatial distribution of nanoscale particles with respect to monolayer phases: calcium oxalate crystals were grown underneath dipalmitoyl phosphatidyl choline (DPPC) and dimyristoyl phosphatidyl serine (DMPS) monolayers in the liquid expanded/liquid condensed (LE/LC) phase coexistence. The density of light scatterers was considerably higher in the respective LE than the LC phase, and a gradual migration of particles was observed towards the domain boundaries. A similar behavior was observed for nanocrystals injected underneath lipid monolayers. (b) Probing two-dimensional (2D) protein crystallization processes underneath functionalized lipid monolayers and the spatial distribution of crystal defects: streptavidin was crystallized as a model protein in 2D through coordination of exposed histidines on the protein surface to the monolayer-anchored metal chelator, Cu-DO-IDA. Vacancies were formed within the 2D protein crystals after injection of the soluble metal chelator EDTA, and the spatial distribution of vacancies was probed by LSM. (c) Detection of monolayer topographic transitions, corrugation and nanoscale budding: The phases of DPPC monolayers were studied under isothermal compression. New nanoscale topographic transitions are observed by LSM if DPPC is compressed into the liquid condensed (LC) state far below the collapse pressure.

Ellipsometric model for two-dimensional phase transition in Langmuir monolayers

A microscopic ellipsometric model of dioctadecyldimethylammonium bromide monolayer compression has been proposed. The suggested model explains the dependence of the ellipsometric parameters on the surface density throughout the entire range of compression, comprising the liquid expanded (LE) and liquid condensed (LC) phases, as well as their coexistence region. The model uses unique microscopic parameters, such as molecular polarizabilities, Lorentz factors and geometric molecular dimensions. Molecules are treated as rods whose cross-section decreases linearly during the LE–LC phase transition. During compression the rods change their orientation from parallel to the interface to almost perpendicular. It was shown that the volume density of the LE phase is 10–15% lower than that of the LC phase.

On the Nature of The Liquid Expanded/Liquid Condensed Phase Transition in Monolayers of Polar Molecules

During the liquid expanded/liquid condensed (LE/LC) phase transition, the long range repulsive interactions among fatty acid or phospholipid molecules induce the formation of a dispersion of LC islands (of almost the same size) as a superlattice in a continuous phase of LE molecules. Assuming that the transformation occurs at thermodynamic equilibrium, one demonstrates that, as the surface fraction of LC phase increases, the surface pressure π increases. Consequently, the LE/LC transition can have characteristics different from those of the conventional first order transitions.

Methylation effects on the microdomain structures of phosphatidylethanolamine monolayers

Increasing methylation of the headgroup in DPPE results in an increase of minimum area per molecule in highly compressed monolayers at the air-water interface. The shape of solid domains, as observed by epifluorescence microscopy, also exhibits marked changes upon increasing headgroup methylation. Branching domains are observed in DPPE and DP(Me)PE, whereas U-shaped or round domains are observed in DP(Me) 2PE and DPPC under our experimental conditions. The domain shape is determined more by the headgroup methylation than by the corresponding shift in critical temperatures, as shown by the study of PCs of different acyl chain moieties. In mixed lipid monolayers, PC (phosphatidylcholine) and PE (phosphatidylethanolamine) do not mix ideally, as indicated by the non-linear variation of the average area per molecule with composition, and by distinct domain shapes in LE/LC (liquid expanded/liquid condensed) coexisting phases representing PE-enriched or PC-enriched domains in those mixed monolayers.

Structural and stability analysis of monolayers of some bile acids at the air—aqueous solution interface

Experimental isotherms have been obtained for the compression of monomolecular films formed at the air—aqueous interface by lithocholic, chenodeoxycholic, deoxycholic and cholic bile acids under various pH and temperature conditions. To determine the different surface phases of the films the compressibility modulus values, at different surface pressures, were calculated for each isotherm. The results show that at the initial stages of compression all the films are in the liquid expanded (LE) state except for those of cholic acid. Under certain conditions tested the lithocholic, chenodeoxycholic and deoxycholic acid monolayers underwent a phase transition from the LE to the liquid condensed (LC) state. This transition is interpreted as being due to the emergence of the carboxyl group from the aqueous phase. The compression-work values calculated for the insoluble monolayers suggest that the LE—LC phase transition involves the same type of molecular reorientation. A study of the stability of these monolayers in terms of the hydroxyl group number for the four bile acids has been included.

Electron microscopic observation of [formula omitted] phase transition in dipalmitoyl phosphatidylcholine monolayers

The monolayer structure of l-α-dipalmitoyl phosphatidylcholine (DPPC) at the air/water interface was examined using improved electron microscopic techniques. The DPPC monolayer is homogeneous in both the liquid-expanded (LE) and liquid-condensed (LC) states. In the intermediate LE LC region, however, the monolayer is nonhomogeneous and biphasic. Our results of two coexisting phases are consistent with the interpretation of a first-order phase transition occurring between the LE and LC states in monomolecular films.