Adsorption and micellar properties of a mixed system of nonionic-nonionic surfactants

We study the surface adsorption and bulk micellization of a mixed system of two nonionic surfactants, namely, ethylene glycol mono-n-dodecyl ether (C12E1) and tetraethylene glycol mono-n-tetradecyl ether (C14E4), at different mixing ratios at 15 degrees C. The pure C14E4 monolayer cannot show any indicative features of phase transition because of both hydration-induced and dipolar repulsive interactions between the bulky head groups. On the other hand, the monolayers of pure C12E1 and its mixture with C14E4 undergo a first-order phase transition, showing a variety of surface patterns in the coexistence region between the liquid expanded (LE) and liquid condensed (LC) phases under the same experimental conditions. For pure C12E1, the domains are of a fingering pattern while those for the C12E1/C14E4 mixed system are found to be compact circular and small irregular structures at 2:1 and 1:1 molar ratios, respectively. The critical micelle concentration (cmc) values of both the pure and the mixed systems were measured to understand the micellar behavior of the surfactants in the mixture. The cmc values of the mixed system were also calculated assuming ideal behavior of the surfactants in the mixture. The experimental and calculated values are found to be very close to each other, suggesting an almost ideal nature of mixing. The interaction parameters for mixed monolayer and micelle formation were calculated to understand the mutual behavior of the surfactants in the mixture. It is observed that the interaction parameters for mixed monolayer formation are more negative than those of micelle formation, indicating a stronger interaction between the surfactants during monolayer formation. It is concluded that since both the surfactants bear EO units in their head groups, structural parity and hydrogen bonding between the surfactants allow them to be closely packed during monolayer and micelle formation.     

Surface phase behavior in Gibbs monolayers of bis(ethylene glycol) mono-n-tetradecyl ether at the air-water interface

We present the adsorption kinetics and surface morphology of the adsorbed monolayers of bis(ethylene glycol) mono-n-tetradecyl ether (C14E2) by film balance and Brewster angle microscopy. A cusp point followed by a plateau region in the pressure (pi)-time (t) adsorption isotherm indicates a first-order phase transition in the coexistence region between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase. A variety of condensed phase domains surrounded by the homogeneous LE phase are observed just after the appearance of the phase transition. The domains are of a spiral or striplike structure at lower temperatures. This characteristic shape of the domains is because of strong dipole-dipole repulsion between the molecules. At 18 degrees C, the domains are found to be quadrant structures. A slight increase in subphase temperature (around 1 degrees C) brings about a quadrant-to-circular shape transition in the domains. The circular domains return to quadrant structures as the subphase temperature is lowered. The domains completely disappear when the temperature is increased beyond 19 degrees C, suggesting that the critical temperature for the condensed domain formation is 19 degrees C. Above this temperature, the hypothetical surface pressure necessary for the phase transition exceeds the actual surface pressure attainable from a solution of concentration greater than or equal to the critical micelle concentration. An increase in molecular motion with increasing temperature results in a higher degree of chain flexibility. As a result, the molecules cannot accumulate in the condensed phase form when the subphase temperature is above 19 degrees C.     

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 CnE1 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 (pi-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 dpi(c)/dT 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 DeltaH values are found to be negative, suggesting an exothermic nature of condensation. The negative DeltaS values obtained from the relation DeltaH/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.     

Faceted structures in Langmuir monolayers of diethylene glycol mono-n-octadecyl ether at the air--water interface

We have concurrently studied the surface pressure (pi) versus area (A) isotherms and microscopic surface morphological features of Langmuir monolayers of diethylene glycol mono-n-octadecyl ether (C18E2) by film balance and Brewster angle microscopy (BAM) over a wide range of temperature. At temperatures < or =10 degrees C, the monolayers exist in the form of condensed phase even just after the evaporation of the spreading solvent, suggesting that the melting point of the condensed phase is above this temperature. At > or =15 degrees C, the monolayers can exist as gas (G), liquid expanded (LE), and liquid condensed (LC) phases and undergo a pressure-induced first-order phase transition between LE and LC phases showing a sharp cusp point followed by a plateau region in the pi-A isotherms. A variety of 2-D structures, depending on the subphase temperature, are observed by BAM just after the appearance of the cusp point. It is interesting to note here that the domains attain increasingly large and compact shape as the subphase temperature increases and finally give faceted structures with sharp edges and corners at > or =30 degrees C. The BAM observations were coupled with polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to gain better understanding regarding the conformational order and subcell packing of the molecules. The constancy of the methylene stretching modes over the studied temperature range suggests that the hydrocarbon chains do not undergo any conformational changes upon compression of the monolayer. However, the full width at half-maximum (fwhm) values of the asymmetric methylene stretching mode (nu(as)(CH(2))) are found to respond differently with changes in temperature. It is concluded that even though the trans/gauche ratio of the hydrocarbon chains remains virtually constant, the LE-LC phase transition upon compression of the monolayer is accompanied by a loss of the rotational freedom of the molecules.     

Interaction of an organic cation with Gibbs monolayers of n-hexadecyl phosphate

Surface phase behavior of n-hexadecyl phosphate (n-HDP) and its mixture with L-arginine (L-arg), which behaves as L-argininium cation (L-arg(+)) in aqueous solution, at a molar ratio 2:3 in Gibbs adsorption layers has been studied by film balance, Brewster angle microscopy (BAM) and surface tensiometry at 20 degrees C. The monolayers of n-HDP show three phases that are gas (G), intermediate (I) and liquid condensed (LC), and two phase transitions. A first-order G-I phase transition that is followed by a second-order I-LC phase transition is found in these monolayers. Although the monolayers of the mixtures containing n-HDP and L-arg show three phases, the nature of the middle phase is different from that of the n-HDP monolayers. The three phases observed for the mixed systems are G, liquid expanded (LE) and LC phases. A first-order G-LE phase transition is found at a low surface pressure at > or =10 degrees C. This transition is followed by another first-order LE-LC phase transition at a certain higher surface pressure. The first-order nature of the phase transitions for both the systems is confirmed by the presence of plateaus in the pi-t curves, which are accompanied by two surface phases. A second-order phase transition in the monolayers of n-HDP is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in the surface pressure. The domains formed during the first-order phase transition in the adsorption layers of n-HDP are circular and remain unaffected by changing the temperature. Although the domains of an LE phase are circular, those of an LC phase at the latter transition are fractal in the mixed system. A further branching of the arms of the fractal domains is found to occur by an increase in the temperature. All the results are explained by considering salt formation between anion from n-HDP and L-arg(+).     

Influence of temperature and headgroup size on condensed-phase patterns in langmuir monolayers of some oxyethylenated nonionic surfactants

The surface phase behavior in Langmuir monolayers of some oxyethylenated nonionic surfactants of the general formula C16En, with n = 1, 2, 3, and 4, at the air-water interface has been studied by film balance and Brewster angle microscopy (BAM) over a wide range of temperatures. The C16E4 monolayers cannot show any indicative features of phase transition because of strong dipolar as well as hydration-induced repulsive interactions between the bulky headgroups. On the other hand, the monolayers of C16E1, C16E2, and C16E3 show a sharp cusp point followed by a pronounced plateau region in their respective isotherms with subsequent formation of a variety of structures in the two-phase coexistence region between the liquid expanded (LE) and liquid condensed (LC) phases at different temperatures. As usually observed, the domains of C16E1, which bears only one ethylene oxide (EO) unit in the headgroup, are circular at lower temperatures while fractal at higher temperatures. On the other hand, those for C16E2 and C16E3 are initially found to be irregular structures, which attain increasingly compact shape with increasing temperature, and finally become circular when the subphase temperature is 26 and 15 degrees C for C16E2 and C16E3, respectively. It is concluded that a higher degree of dehydration around the headgroup region appreciably reduces the headgroup size, which imparts to the molecules an increase in hydrophobicity, thereby a closer molecular packing. Consequently, the line tension of the interface increases, showing compact structures at higher temperatures. Since C16E1 bears only one EO unit in its headgroup, the dehydration effect cannot appreciably raise its hydrophobicity to overcome the increases in thermal motion and chain flexibility of the molecules. Rather, increases in subphase temperature result in a decrease in the line tension of the interface, giving fractal structures at higher temperatures.     

Brewster angle microscopy: A preferential method for mesoscopic characterization of monolayers at the air/water interface

Knowledge of the mesoscopic morphology of condensed phase domains formed after the main phase transition in the two-phase coexistence region of Langmuir monolayers progressed rapidly with the development of the highly-sensitive imaging techniques, particularly by Brewster angle microscopy (BAM). Latest developments of commercial BAM instruments have been developed to a high technical level and allow upgrading to imaging ellipsometers which combine optical microscopy and ellipsometry and make the assessment of small layered structures or patterned thin films possible. A large variety of condensed phase domains different in mesoscopic sizes and shapes as well as their textural features has been observed which depend sensitively on the chemical structure of the amphiphilic monolayer and the system conditions, such as surface pressure and temperature. This unsuspected morphological variety of condensed phase domains has been proven not only in Langmuir monolayers but also in adsorbed monolayers (Gibbs monolayers), in Langmuir monolayers penetrated by dissolved surfactants or in adequate molecular recognition systems. The inner textures of domains can be explained on the basis of their geometry and the two-dimensional lattice in dependence of the tilt angle of the alkyl chains and gave rise to the development of a geometric concept on the basis of the molecular packing. New knowledge has been gained about non-equilibrium structures and their transition kinetics into the equilibrium state. Combined results obtained recently by BAM have enhanced the understanding of molecular organization in phase diagrams and binary mixtures. Recent advances in model studies about chiral discrimination effects and of the highly specific structural changes of host-monolayers by recognition of non-surface active guest-components have made progress. Semi-empirical quantum chemical methods have been used to gain insight into the role of different types of interactions involved in the main characteristics of mesoscopic length scale aggregates of mimetic systems.     

Effect of hydroxyl group position and system parameters on the features of hydroxystearic Acid monolayers

The characteristic features of hydroxystearic acid monolayers OH-substituted in the mid position of the alkyl chain deviate considerably from those of the usual nonsubstituted stearic acid. The phase behavior, domain morphology, and two-dimensional lattice structure of 9-, 11-, and 12-hydroxystearic acids are studied, using pi-A isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD), to obtain detailed information on the effect of the exact position of the OH-substitution. The pi-A isotherms of all three hydroxyoctadecanoic acids have an extended flat plateau region, the extension of which only slightly decreases with the increase of temperature. At the same temperature, the extension of the plateau region increases and the plateau pressure decreases from 9-hydroxyoctadecanoic acid to 12-hydroxyoctadecanoic acid. The absolute -DeltaH and -DeltaS values for the phase transition increase slightly from 9-hydroxyoctadecanoic acid to 12- hydroxyoctadecanoic acid and indicate differences in the ordering of the condensed phase under consideration of the special reorientation mechanism of these bipolar amphiphiles at the fluid/condensed phase transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region is specific for the position of the OH-substitution of the alkyl chain, just as the lattice structures of the condensed monolayer phase. 11-hydroxyoctadecanoic acid monolayers form centered rectangular lattices with the chain tilt toward the NNN (next nearest neighbor) direction, and 12-hydroxyoctadecanoic acid monolayers have an oblique lattice over the entire pressure range. A special feature of 9-hydroxystearic acid monolayers is the phase transition between two condensed phases observed in the pi-A isotherm of 5 degrees C at approximately 18 mN/m, where the centered rectangular lattice shows a NNN/NN transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region, just as the lattice structures of the condensed monolayer phase, reveal the high specifity of the monolayer feature of the bipolar hydroxystearic acids OH-substituted in the mid position.     

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 (pi-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 > or = 25 degrees 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 (deltaH) and entropy (deltaS) values were also calculated to understand the thermodynamic nature of condensation of the molecules 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 (pi)-time (t) adsorption isotherm is found to occur from 2 to 20 degrees 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 degrees C and it is considered as a gas (G)-liquid condensed (LC) phase transition. Above 6.5 degrees 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 pi-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 degrees 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 (pi(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 degrees C. All the results are consistent with the present understanding of the Langmuir monolayers of insoluble amphiphiles at the air-water interface.     

Interactions of L-arginine with Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air-water interface

The surface phase behavior of di-n-dodecyl hydrogen phosphate (DDP) in Langmuir monolayer and its interactions with L-arginine (L-arg) have been investigated by measuring pi-A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). The DDP monolayers on pure water show a first-order liquid expanded-liquid condensed (LE-LC) phase transition and form fingering LC domains having uniform brightness at different temperatures. At 15 degrees C, the pi-A isotherms on pure water and on different concentration solutions of L-arg show a limiting molecular area at approximately 0.50 nm(2)/molecule. With increasing the subphase concentration of L-arg up to 4.0 x 10(-4)M, the LE and the LE-LC coexistence regions shift to larger molecular areas and higher surface pressures, respectively. With a further increase in the concentration of L-arg beyond this critical concentration, these isotherms show little or no more expansion. These results have been explained by considering the fact that the L-arg undergoes complexation with the DDP to form L-arg-DDP that remains in equilibrium with the components at the air-water interface. As the concentration of L-arg in the subphase increases, the equilibrium shifts towards the complex. At a concentration of L-arg > or =4.0 x 10(-4)M, the DDP monolayers get saturated and show the characteristics of the new amphiphile, L-arg-DDP. BAM is applied to confirm the above results. When the concentration of the L-arg is <4.0 x 10(-4)M, domains always start forming at an area of approximately 0.64 nm(2)/molecule, which is the critical molecular area for the phase transition in the DDP monolayers on pure water. In contrast, when the monolayers are formed on a solution containing > or =4.0 x 10(-4)M L-arg, comparatively smaller size domains are formed after the appearance of a new cusp point at approximately 0.55 nm(2)/molecule. With an increase in the concentration of L-arg in the subphase, the size of the domains decreases indicating that the fraction of the DDP gradually decreases, whereas the fraction of the complex gradually increases. In addition, a very simple procedure for determination of the stability constant, which is 2.6 x 10(4)M(-1) at 15 degrees C, has been suggested.     

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 degrees C, is constructed by measuring the surface pressure (pi)-time (t) adsorption isotherms. Below 25.8 degrees C, each of the pi-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 pi-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 degrees C both the pi-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 degrees C to an elongated at 24 degrees C and finally to a circular shape at 36 degrees 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.     

Fatty-acid monolayers at the nematic/water interface: phases and liquid-crystal alignment

The two-dimensional (2D) phases of fatty-acid monolayers (hexadecanoic, octadecanoic, eicosanoic, and docosanoic acids) have been studied at the interface of a nematic liquid crystal (LC) and water. When observed between crossed polarizers, the LC responds to monolayer structure owing to mesoscopic alignment of the LC by the adsorbed molecules. Similar to Langmuir monolayers at the air/water interface, the adsorbed monolayer at the nematic/water interface displays distinct thermodynamic phases. Observed are a 2D gas, isotropic liquid, and two condensed mesophases, each with a characteristic anchoring of the LC zenithal tilt and azimuth. By varying the monolayer temperature and surface concentration we observe reversible first-order phase transitions from vapor to liquid and from liquid to condensed. A temperature-dependent transition between two condensed phases appears to be a reversible swiveling transition in the tilt azimuth of the monolayer. Similar to monolayers at the air/water interface, the temperature of the gas/liquid/condensed triple-point temperature increased by about 10 degrees C for a two methylene group increase in chain length. However, the absolute value of the triple-point temperatures are depressed by about 40 degrees C compared to those of analogous monolayers at the air/water interface. We also observe a direct influence by the LC layer on the mesoscopic and macroscopic structure of the monolayer by analyzing the shapes and internal textures of gas domains in coexistence with a 2D liquid. An effective anisotropic line tension arises from elastic forces owing to deformation of the nematic director across phase boundaries. This results in the deformation of the domain from circular to elongated, with a distinct singularity. The LC elastic energy also gives rise to transition zones displaying mesoscopic realignment of the director tilt or azimuth between adjacent regions with a sudden change in anchoring.     

Adsorption of amyloid beta (1-40) peptide to phosphatidylethanolamine monolayers.

The aggregation of soluble, nontoxic amyloid beta (Abeta) peptide to beta-sheet containing fibrils is assumed to be a major step in the development of Alzheimer's disease. Interactions of Abeta with neuronal membranes could play a key role in the pathogenesis of the disease. Herein, we study the adsorption of synthetic Abeta peptide to DPPE and DMPE monolayers (dipalmitoyl- and dimyristoylphosphatidylethanolamine). Both lipids exhibit a condensed monolayer state at 20 degrees C and form a similar lattice. However, at low packing densities (at large area per molecule), the length of the acyl chains determines the phase behavior, therefore DPPE is fully condensed whereas DMPE exhibits a liquid-expanded state with a phase transition at approximately 5-6 mNm(-1). Adsorption of Abeta to DPPE and DMPE monolayers at low surface pressure leads to an increase of the surface pressure to approximately 17 mNm(-1). The same was observed during adsorption of the peptide to a pure air-water interface. Grazing incidence X-ray diffraction (GIXD) experiments show no influence of Abeta on the lipid structure. The adsorption kinetics of Abeta to a DMPE monolayer followed by IRRAS (infrared reflection absorption spectroscopy) reveals the phase transition of DMPE molecules from liquid-expanded to condensed states at the same surface pressure as for DMPE on pure water. These facts indicate no specific interactions of the peptide with either lipid. In addition, no adsorption or penetration of the peptide into the lipid monolayers was observed at surface pressures above 30 mNm(-1). IRRAS allows the measurement of the conformation and orientation of the peptide adsorbed to the air-water interface and to a lipid monolayer. In both cases, with lipids at surface pressures below 20 mNm(-1) and at the air-water interface, adsorbed Abeta has a beta-sheet conformation and these beta-sheets are oriented parallel to the interface.     

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 L₂–Ov–LS for HA to L₂–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.     

Microphase separation in mixed monolayers of DPPG with a double hydrophilic block copolymer at the air-water interface: a BAM, LSCFM, and AFM study

Phase separation and interactions in mixed monolayers of dipalmitoylphosphatidylglycerol (DPPG) with the rhodamine B end-labeled double-hydrophilic block copolymer (DHBC), poly(N,N-dimethylacrylamide)-block-poly(N,N-diethylacrylamide) (RhB-PDMA(207)-b-PDEA(177)), was studied at the air-water interface. Surface pressure versus area isotherms indicate that both components behave almost independently. Brewster angle microscopy (BAM) images show a random distribution of liquid condensed (LC) domains of DPPG in an apparent homogeneous matrix of DHBC, excluding the macroscopic phase separation. The laser scanning confocal fluorescence microscopy (LSCFM) of the rhodamine dye at the end of the PDMA chain showed how the DHBC is distributed in Langmuir-Blodgett (LB) mixed monolayers. The high spatial resolution of atomic force microscopy (AFM) combined with the LCSFM images indicates that DHBC incorporates in the expanded phase of DPPG to form mixed domains, being excluded from the condensed regions. Upon compression, nanosized LC domains of DPPG nucleate inside the mixed domains corralled in the nanopatterning of pure DHBC. The negatively charged polar group of DPPG inhibits rhodamine aggregation, while the long polymer chains promote the formation of corralled nanodomains of DPPG in two dimensions.     

Temperature dependent dendritic domain shapes in Langmuir monolayers of tetradecanoyl N-ethanolamide at the air-water interface

The effect of temperature on the surface phase behavior of tetradecanoyl N-ethanolamide (NHEA-14) in Langmuir monolayers at the air-water interface has been investigated by film balance and Brewster angle microscopy (BAM). It has been observed that dendritic domains are formed in the coexistence region between liquid-expanded (LE) and liquid-condensed (LC) phases at different temperatures. At 10 and 15°C, the domains are four-armed dendrites having wide arms which have a tendency to be fractal while growing in size. At 20°C, five-armed dendritic domains are formed. At a temperature higher than 20°C, the domains are mainly six-armed dendrites having very narrow and sharp arms. The formation of dendritic domains should be due to the presence of interfacial hydrogen bonding among the head groups of the amphiphile. Increased dehydration of the head groups with an increase in the temperature should be responsible for the temperature dependency of the dendritic domain shapes in the monolayers of NHEA-14.     

Influence of hydrophobic alkylated gold nanoparticles on the phase behavior of monolayers of DPPC and clinical lung surfactant

The effect of hydrophobic alkylated gold nanoparticles (Au NPs) on the phase behavior and structure of Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC) and Survanta, a naturally derived commercial pulmonary surfactant that contains DPPC as the main lipid component and hydrophobic surfactant proteins SP-B and SP-C, has been investigated in connection with the potential implication of inorganic NPs in pulmonary surfactant dysfunction. Hexadecanethiolate-capped Au NPs (C(16)SAu NPs) with an average core diameter of 2 nm have been incorporated into DPPC monolayers in concentrations ranging from 0.1 to 0.5 mol %. Concentrations of up to 0.2 mol % in DPPC and 16 wt % in Survanta do not affect the monolayer phase behavior at 20 °C, as evidenced by surface pressure-area (π-A) and ellipsometric isotherms. The monolayer structure at the air/water interface was imaged as a function of the surface pressure by Brewster angle microscopy (BAM). In the liquid-expanded/liquid-condensed phase coexistence region of DPPC, the presence of 0.2 mol % C(16)SAu NPs causes the formation of many small, circular, condensed lipid domains, in contrast to the characteristic larger multilobes formed by pure lipid. Condensed domains of similar size and shape to those of DPPC with 0.2 mol % C(16)SAu NPs are formed by compressing Survanta, and these are not affected by the C(16)SAu NPs. Atomic force microscopy images of Langmuir-Schaefer-deposited films support the BAM observations and reveal, moreover, that at high surface pressures (i.e., 35 and 45 mN m(-1)) the C(16)SAu NPs form honeycomb-like aggregates around the polygonal condensed DPPC domains. In the Survanta monolayers, the C(16)SAu NPs were found to accumulate together with the proteins in the liquid-expanded phase around the circular condensed lipid domains. In conclusion, the presence of hydrophobic C(16)SAu NPs in amounts that do not influence the π-A isotherm alters the nucleation, growth, and morphology of the condensed domains in monolayers of DPPC but not of those of Survanta. Systematic investigations of the effect of the interaction of chemically defined NPs with the lipid and protein components of lung surfactant on the physicochemical properties of surfactant films are pertinent to understanding how inhaled NPs impact pulmonary function.     

Mixing behavior of a poly(ethylene glycol)-grafted phospholipid in monolayers at the air/water interface

Mixed phospholipid monolayers hosting a poly(ethylene glycol) (PEG)-grafted distearoylphosphatidylethanolamine with a PEG molecular weight of 5000 (DSPE-PEG5000) spread at the air/water interface were used as model systems to study the effect of PEG-phospholipids on the lateral structure of PEG-grafted membrane-mimetic surfaces. DSPE-PEG5000 has been found to mix readily with distearoylphosphoethanolamine-succinyl (DSPE-succynil), a phospholipid whose structure resembles closely that of the phospholipid part of the DSPE-PEG5000 molecule. However, properties of mixed monolayers such as morphology and stability varied significantly with DSPE-PEG5000 content. In particular, our surface pressure, epifluorescence microscopy (EFM), and Brewster angle microscopy (BAM) studies have shown that mixtures containing 1-9 mol % of DSPE-PEG5000 form stable condensed monolayers with no sign of microscopic phase separation at surface pressures above approximately 25 mN/m. Yet, at 1 mol % of DSPE-PEG5000 in mixed monolayers, the two components have been found to behave nearly immiscibly at surface pressures below approximately 25 mN/m. For monolayers containing 18-75 mol % of DSPE-PEG5000, a high-pressure transition has been observed in the low-compressibility region of their isotherms, which has been identified on the basis of continuous BAM imaging of monolayer morphology, as reminiscent of the collapse nucleation in a pure DSPE-PEG5000 monolayer. Thus, the comparative analysis of our surface pressure, EFM, and BAM data has revealed that there exists a rather narrow range of mixture compositions with DSPE-PEG5000 content between 3 and 9 mol %, where somewhat homogeneous distribution of DSPE-PEG5000 molecules and high pressure stability can be achieved. This finding can be useful to "navigating" through possible mixture compositions while developing guidelines to the rational design of membrane-mimetic surfaces with highly controlled bio-nonfouling properties.