Insoluble Monolayers Research Articles

Characterization of the In Situ Structural and Interfacial Properties of the Cationic Hydrophobic Heteropolypeptide, KL4, in Lung Surfactant Bilayer and Monolayer Models at the Air−Water Interface: Implications for Pulmonary Surfactant Delivery

This study examines the various equilibrium in situ secondary structures of the pharmaceutical heteropolypeptide, KL 4, in the solid state, in solution, and in the monolayer state alone and mixed with dipalmitoylphosphatidylcholine (DPPC) and palmitoyloleoylphosphatidylglycerol (POPG). In situ surface circular dichroism spectroscopy, using a method first reported by Damodaran (Damodaran, S. Anal. Bioanal. Chem. 2003, 376, 182-188), of equilibrated KL 4, DPPC/KL 4, POPG/KL 4, and DPPC/POPG/KL 4 monolayers at the air-water interface was used to examine the in situ two-dimensional conformation of KL 4. Gravimetric vapor sorption by solid KL 4 was used to analyze the effects of water molecules on the conformation of KL 4 when confined as a monolayer at the surface of water. Solid-state KL 4 conformation was determined by X-ray powder diffraction (XRPD). The equilibrium interfacial and spreading properties were measured at 25 degrees C, 37 degrees C, and 45 degrees C using the Wilhelmy plate method and Langmuir film balance. Equilibrium phase transition temperatures were measured using differential scanning calorimetry (DSC). It was found that solid-state KL 4, which takes up very little water, exhibits beta-sheet and alpha-helix secondary structures, whereas KL 4 in solution appears to exist only as an alpha-helix. KL 4 forms a stable, insoluble monolayer, exhibiting beta-sheet and aperiodic structures. These structures provide KL 4, when confined in two-dimensions, the structural flexibility to maximize favorable cationic lysine-water interactions and favorable leucine-leucine hydrophobic and van der Waals interactions; while effectively "shielding" the leucine residues away from water. In DPPC/KL 4 monolayers, KL 4 retains its native beta-sheet and aperiodic structures, consistent with phase separation of DPPC and KL 4 in bilayers and monolayers. In POPG/KL 4 monolayers, KL 4 exhibits an increase in aperiodic secondary structures (loss of beta-sheet) to maximize favorable electrostatic interactions, consistent with the observed negative deviations from ideal monolayer mixing.

Hydrogen-Bond-Induced Hysteresis in the Compression/Relaxation of Monolayer Films

Two stereoisomers of surfactants were synthesized in which a diketopiperazine ring was inserted between a hydrocarbon chain (of variable length) and an anionic headgroup. It was found (by HPLC, conductivity, surface tension, and diffusion NMR) that these compounds have low solubilities in water, remarkably high Krafft temperatures, and low critical micelle concentrations. Of particular interest were the pressure/area isotherms of insoluble monolayers of nonionic analogues of the amphiphiles. These isotherms showed an unprecedented hysteresis but only during the first compression/relaxation cycle. Additional cycles were normal in that they lacked the original hysteresis. These results were attributed to diketopiperazine rings that initially lie flat on the water surface at the air/water interface. Compression flips the diketopiperazine rings so that they now reside roughly perpendicular to the interface in a stable hydrogen-bonded orientation where the rings occupy far less space.

Dielectric Properties Tangential to the Interface in Model Insoluble Monolayers: Theoretical Assessment

Studies of insoluble monolayers built of phospholipids and various long-chained fatty acids or their glycerin esters are the major source for what is currently known about the relationship between monolayer composition and physicochemical properties. The surface pressure, dipole moment, dielectric permittivity, polarizability, refractivity, and other electrical and optical features are governed by the surfactant structural specificity and solvent organization at the microscopic level. To provide insight into the atomistic details of the interfacial structure, model monolayers at the air/water interface of two distinctly different in composition and isotherm profile surfactants are investigated by means of molecular dynamics all-atom simulations. Analysis of the computational results allows the estimation of empirically unattainable quantities such as tangential (di)electric properties, their decomposition to surfactant and water contributions, and their relationship with the changes in interfacial molecular organization at different surface concentrations. The employed theoretical approach provides a comprehensive description of interfacial phenomena at the molecular level where the traditional phenomenological investigations are ineffective.

N2O5 Reactive Uptake on Aqueous Sulfuric Acid Solutions Coated with Branched and Straight-Chain Insoluble Organic Surfactants

A flow reactor coupled to a chemical ionization mass spectrometer was used to study the reactive uptake coefficients at 273 K of N2O5 on aqueous 60 wt % sulfuric acid solutions coated with insoluble organic monolayers. Both straight-chain surfactants (1-hexadecanol, 1-octadecanol, and stearic acid) and a branched surfactant (phytanic acid) were studied. The reactive uptake coefficient decreased dramatically for straight-chain surfactants. The decrease ranged from a factor of 17 to a factor of 61 depending on the type of straight-chain surfactant. In contrast to the straight-chain data, the presence of phytanic acid did not have a significant effect on the N2O5 reactive uptake coefficient (the decrease was less than the uncertainty in the data) compared to the uncoated solution. In addition to measuring the reactive uptake coefficients, we also investigated the relationship between properties of the monolayers and the reactive uptake coefficients. The reactive uptake coefficients measured on aqueous sulfuric acid subphases showed a relationship to the surface area occupied by the surfactant molecules. However, data obtained with other subphases did not overlap with this trend.

Equation of State for the Phase Coexistence Region of Insoluble Monolayers under Consideration of the Entropy Nonideality

The equation of state for the monolayer with a fluid (G, LE)/condensed (LC) phase transition derived earlier (Fainerman, V.B.; Vollhardt, D. J. Phys. Chem. B 1999, 103, 145) in the framework of a quasichemical approach is generalized. A term is added that takes into account the entropy nonideality of mixing of the monomers and clusters of amphiphilic molecules. The results calculated from the proposed equations agree well with the experimental Pi-A isotherms obtained for various types of amphiphilic monolayers. The values of molecular areas of the amphiphilic molecules estimated from the fitting of experimental data to the proposed equation are quite similar to the real values. Another equation of state capable of describing the fluid state of insoluble monolayers and based on equations for the chemical potential of the solvent in the bulk phase and in the surface layer (Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. B 2006, 110, 10436) is also generalized to be extended to the fluid/condensed phase transition region (A < A(c)), taking into account entropy nonideality for mixing solvent molecules, monomers, and clusters of amphiphilic molecules. The values calculated on this basis agree also well with the experimental data.

Nanostructured self-assembly materials formed by non-ionic urea amphiphiles

Urea is an extremely versatile molecule that has been widely used for many years in a diverse range of applications. The physicochemical properties of urea have been well studied with respect to the role of hydrogen bonding in biological and chemical systems. Urea-based amphiphiles, in contrast, have received little attention with respect to physicochemical properties. Studies reported in the last few years have demonstrated that specific urea amphiphiles display interesting lyotropic liquid crystalline phase behaviour. These nanostructured self-assembly phases are potential candidates for a number of novel applications. In this work, we review the physicochemical properties of non-ionic urea-based amphiphiles, including those of the solid state, insoluble monolayers residing at the air-water interface, Langmuir-Blodgett films deposited on solid substrates and lyotropic liquid crystalline phases. In addition, we present new data that extends the characterisation of the solid state and lyotropic properties of isoprenoid branched urea surfactants.

Incorporation of ovalbumin within cationic octadecylamine monolayer and a comparative study with zwitterionic DPPC and anionic stearic acid monolayer

In this communication we demonstrated the incorporation of water-soluble surface-active protein OVA within an insoluble cationic ODA monolayer and compared with zwitterionic (DPPC) and anionic (SA) monolayer. The incorporation of OVA is found to be more in ODA as compared to that of DPPC and SA. The kinetics of protein adsorption in lipid monolayer gives the idea that unfolding of OVA is less in case of DPPC than SA and ODA. The pi-A isotherm and compressibility study gives the information about the different states of the protein-lipid mixed monolayer. At higher pressure, OVA tend to squeeze out from the lipids monolayer. High-resolution field emission scanning electron microscope (FE-SEM) images confirm this observation. The surface morphology of DPPC-OVA LB film is far better than ODA-OVA and SA-OVA LB film. OVA forms large irregular aggregates on SA and ODA monolayer. Fluorescence study reveals that protein structure is perturbed more in SA and ODA system compared to that of DPPC. The overall results indicate that DPPC monolayer is better to get protein lipid mixed film than SA and ODA monolayer.

Telechelic Poly(ethylene glycol)−POSS Amphiphiles at the Air/Water Interface

Combining two non-surface-active building blocks, oligomeric poly(ethylene glycol) (PEG) and a completely hydrophobic polyhedral oligomeric silsesquioxane (POSS) cage, creates amphiphilic telechelic polymers (POSS−PEG−POSS), which exhibit surface activity at the air/water (A/W) interface. POSS moieties serve as the hydrophobic groups for hydrophilic PEG chains of different number-average molar mass (1, 2, 3.4, 8, and 10 kg mol-1). For short PEG chains (1, 2, and 3.4 kg mol-1), insoluble monolayers form, whereas POSS end groups were not sufficiently hydrophobic to keep higher molar mass hydrophilic PEG blocks (8 and 10 kg mol-1) at the A/W interface. Thermodynamic analyses of the 1, 2, and 3.4 kg mol-1 POSS−PEG−POSS via surface pressure−area per monomer isotherms indicate that the POSS end groups reside at the A/W interface and that the PEG chains are squeezed into the subphase with increasing surface pressure. This conclusion is supported by X-ray reflectivity studies on Y-type Langmuir−Blodgett multilaye...

Thermodynamic analysis of some electrochemical properties of transition metal complexes in electronically excited states

A theoretical treatment is presented of the relationship between absorption and emission maxima for (0–0) electronic transitions in transition metal complexes such as the ruthenium(II) polypyridyl chromophores used as dye sensitizers in photovoltaic cells, and the corresponding shift in standard redox (or half-wave) potential of these ionic complexes. It is shown that the potential shift is proportional to the average of absorption and emission wavenumbers for oxidized or reduced forms, a result that follows from a detailed analysis of medium relaxation effects, based on the Onsager cavity model, and attendant solvatochromic shifts. It is also shown how additional thermodynamic properties (reaction entropies and enthalpies) pertaining to the half reaction if the reactants are present in solution can be obtained from observation of the variation of spectra with temperature. For the case of reactants present in an insoluble monolayer, a general expression is derived for the Gibbs energy change that results from the promotion of all absorbing species to an excited state, and the changes in Gibbs energy resulting from the processes that return the system to ground-state equilibrium.

Exploratory study of dielectric properties of insoluble monolayers: Molecular models

AbstractStudies of insoluble monolayers built of phospholipids, various long‐chained fatty acids, and their glycerin esters are the major source of our current knowledge about the relationship between monolayer composition and its physicochemical properties. Such are the surface pressure, dipole moment, dielectric permittivity, polarizability, refractivity, and other electrical and optical features governed by the surfactant structural specificity and solvent organization at the microscopic level. Therefore, model monolayers at the air/water interface of two surfactants that are distinctly different in composition and isotherm profile are investigated by means of a Monte Carlo/molecular mechanics computational procedure. The combination of experimental and theoretical results permits estimation of empirically unattainable quantities and a detailed insight in the interfacial molecular organization at different surface concentrations as well as an evaluation of the surfactant/water contributions to the overall monolayer characteristics. The employed theoretical approach provides a comprehensive description of phenomena at the molecular level where the traditional phenomenological investigations are ineffective. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Flow-controlled Phase Boundaries in Langmuir Monolayers

Flow-controlled fingering of the liquid expanded/liquid condensed phase boundary in a 2-d insoluble monolayer is investigated using a laser-induced thermocapillary pump. Spatially periodic perturbations of the initially smooth monolayer phase boundary between a liquid expanded and liquid condensed phase are shown to lead to the development of steady profile of one-dimensional fingers. The steady-state modulation wave vector and the transient growth rate increase with the flow velocity that drives the instability following power scaling laws consistent with a theory of Bruinsma, Rondelez, and Levine (Bruinsma, R.; Rondelez, F.; Levine, A. Eur. Phys. J. E. 2001, 6, 191) on flow rather than diffusion dominated instabilities in monolayers.

Phase behavior of polyelectrolyte-surfactant complexes at planar surfaces

We investigate theoretically the phase diagram of an insoluble charged surfactant monolayer in contact with a semidilute polyelectrolyte solution (of opposite charge). The polyelectrolytes are assumed to have long-range and attractive (electrostatic) interaction with the surfactant molecules. In addition, we introduce a short-range (chemical) interaction which is either attractive or repulsive. The surfactant monolayer can have a lateral phase separation between dilute and condensed phases. Three different regimes of the coupled system are investigated depending on system parameters. A regime where the polyelectrolyte is depleted due to short range repulsion from the surface, and two adsorption regimes, one being dominated by electrostatics, whereas the other by short range chemical attraction (similar to neutral polymers). When the polyelectrolyte is more attracted (or at least less repelled) by the surfactant molecules as compared with the bare water-air interface, it will shift upwards the surfactant critical temperature. For repulsive short-range interactions the effect is opposite. Finally, the addition of salt to the solution is found to increase the critical temperature for attractive surfaces, but does not show any significant effect for repulsive surfaces.

Surface Pressure Isotherm for the Fluid State of Langmuir Monolayers

The equation of state for the monolayer comprised of the molecules of different sizes (water and biopolymers) is modified to describe the fluid (liquid-expanded and gaseous) state of the insoluble molecules monolayer. In contrast to the equation of state derived previously in ref 1, this equation does not involve either the Gibbs' adsorption equation or the differential equation for the chemical potential of the insoluble component, but it is based only on the equations for the chemical potential of the solvent in the bulk and in the surface layer. The results calculated from the proposed equations are in perfect agreement with the experimental Pi-A isotherms of the liquid-expanded state obtained for Langmuir monolayers of various types of amphiphilic compounds. The values of molecular areas of amphiphilic molecules estimated from the fitting of experimental data to the proposed equation are found to be quite similar to those measured in independent grazing incidence X-ray diffraction (GIXD) experiments.

Study of the evaporation of pollutant liquids under the influence of surfactants

AbstractIn the present work, reversed‐flow gas chromatography (RF‐GC) is utilized for the study of the evaporation of volatile liquids. Evaporation rates and the respective diffusion coefficients are determined for the evaporation of ethanol and 1,1,1‐trichloroethane in nitrogen, at 306.2 K. The precision (&gt;99.5%) and accuracy of the method, as well as the uncertainty of the determined parameters, ascertain the potential of the presented methodology. The experimentally obtained evaporation rates have units of velocity, and for high volatility compounds their values are identified with the liquid film mass‐transfer coefficients. The latter compare successfully to available literature values, further ascertaining the validity of the presented methodology. The variation of the evaporation rates of ethanol and 1,1,1‐trichloroethane, in the presence of various amounts of Triton X‐100, indicates that different, but in any case high amounts of surfactant are required for the drastic retardation of the vaporization rate. The reduction of the evaporation rate can be attributed either to the formation of densely packed surface monolayers or to the formation of an insoluble monolayer. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Component separation in spread sodium stearoyl lactylate (SSL) monolayers induced by high surface pressure

Insoluble monolayers of commercially available sodium stearoyl lactylate (SSL) spread at the air/water interface were studied by recording pressure–area ( Π– A) isotherms simultaneously with the morphology via Brewster angle microscopy (BAM). Whereas a strong hysteresis on the Π– A isotherms was observed for the first compression–expansion cycle, the second and further cycles show a strongly reduced hysteresis. The BAM images show significant changes in the surface textures during compression/expansion reflecting 2 D-heterogeneities in the monolayer. The most probable explanation of these phenomena can be given when taken into account the composition of the SSL sample consisting of two components of different interfacial activity—so-called SSL1 and SSL2. The less surface-active component was assumed to be pushed out of the monolayer at higher surface pressures. Additional compression experiments performed separately on the systems enriched by either of these compounds, respectively, confirm the explanation.

Kinetics of slow collapse process: Thermodynamic description of rate constants

Insoluble monolayer formed at the air/water interface compressed at a surface pressure above the equilibrium spreading pressure is unstable. In presented paper the classical theory for homogeneous nucleation is modified adding the activation energy term. It allows quantitative thermodynamic interpretation of the slow collapse phase transformation. The collapse of stearic acid Langmuir films has been carried out by systematic measurements of the area loss–time isobaric dependencies at various temperatures and isothermal dependencies at various pressures. Activation energies (activation enthalpies) and activation entropies have been evaluated for the nucleation ( E α = 1.32 eV) and the growth ( E β = 1.55 eV) processes. The experimental data for various pressures are discussed on the basis of the Gibbs energy analysis.

Brewster Angle Microscope – An Excellent Tool for Nanoscience Researchers

An indigenous development of Brewster Angle Microscope (BAM) is completed at National Chemical Laboratory (NCL), India that is real time imaging instrument capable of imaging the monolayer while compressing the barrier trough simultaneously. The instrument will be useful for the basic research in the area of nanoscience. BAM has proven to be an excellent tool for the characterization of the morphology of insoluble monolayers of the surfactant molecules at air‐water interface, which are known as Langmuir films. BAM technique that was developed more than a decade ago globally, is well‐known optical method for visualizing extremely thin films. BAM can reveal inhomoginities, particularly phase domains in ultra thin layer, without the use of special probes. The reflected light is the basis of the Brewster Angle Microscopy of the ultra‐thin films. Imaging of these monolayers yield to many morphological/phase transition based study of these monolayers. The development set up consist of p‐polarized light, designed optics, frame grabber card, CCD camera as detector and teflon trough with barrier mechanism. Movement of a teflon barrier is controlled with stepper motor. An intelligent module based on single chip microcontroller 89C51 is developed for controlled barrier movement. With BAM, the real time image of a monolayer can be viewed on PC monitor. Stored images can be analyzed further through software package developed under VC++. In house software development has been completed at NCL, for instrument control, image acquisition, and image analysis. Standard APIs available with frame grabber cards can not be made applicable for this real time application. A special algorithm has been developed for finding out the area of the interested domain marked by the user. Other analysis software function includes file management, magnification of the image, contrast and brightness control, etc. A standard monolayer of Stearic acid is used for testing purpose. The instrumentation performance has been tested with an amount of 50 microliter of concentration 1 mg/ml Stearic acid spread on the subphase and found satisfactory. Sub phase used was high purity water (Millipore Milli‐ Q) contained in Teflon trough. The monolayer is compressed using teflon barrier. Simultaneously real time imaging was done to record the first order phase behavior of the monolayer. The instrument testing was done with the help of Nanoscience group at NCL. Recently instrument automation has been updated using LabVIEW platform. With LabVIEW addition, simultaneous viewing of compressed monolayer image and barrier movement control is possible beside other functions.

Quantum Chemical Analysis of the Thermodynamics of 2D Cluster Formation of n-Carboxylic Acids at the Air/Water Interface

Within the framework of PM3 molecular orbital approximation the thermodynamic function characteristics for the formation and geometrical structure of monomers, dimers, trimers, and tetramers of nondissociated n-carboxylic acids C(n)H(2n+1)COOH with n = 5-15 are calculated. It is shown that spontaneous aggregation of homologous fatty acids for the homologues with carbon atoms numbers n > or = 13 at the air/water interface can take place, leading to the formation of infinite plane rectangular clusters, whereas for the homologues with n < 11 spontaneous decomposition of large aggregates is energetically preferable. At the same time, the formation of trimers is more probable for the lower homologues (8 < n < 13). These results agree well both with the experimental data reported by various authors and with thermodynamic models developed earlier for soluble and insoluble monolayers. The slopes of the regressions calculated for the dependencies of the thermodynamic parameters on the alkyl chain length for all the clusters considered are all equal to each other. This fact indicates that the contributions of the CH2 groups to the thermodynamic characteristics of alcohols and acids are the same, and the differences in the formation of clusters by these substances should be attributed only to the differences in the structure and interactions of relevant functional groups. Therefore, it enables one to describe both acids and alcohols within the framework of the developed method, and it makes it possible to extend the proposed approach onto other classes of amphiphilic compounds.

Deformation and breakup of a stretching liquid bridge covered with an insoluble surfactant monolayer

The breakup of surfactant-laden drops and jets is of technological interest and fundamental scientific importance. Surfactants are routinely used to control the breakup of drops and jets in applications ranging from inkjet printing to crop spraying. Accurate computation of breakup of surfactant-laden drops and jets is often the key to the development of new applications and to providing a rational fundamental understanding of both existing and emerging applications. While highly accurate algorithms for studying the breakup of surfactant-free drops and jets are well documented and much is now known about the dynamics in such situations, little is known by contrast about the closely related problem of interface rupture when surfactant effects cannot be neglected. The deformation and breakup of a stretching liquid bridge of an incompressible Newtonian fluid whose surface is covered with an insoluble surfactant monolayer are analyzed here experimentally and computationally. In the experiments, high-speed visualization is used to capture the transient deformation of a bridge. The dynamic shapes of bridges (captive between two rods of 3.15 mm diameter) are captured and analyzed with a time resolution of 1 ms. The bridge lengths are 3.15 mm initially and about 4–7 mm at breakup, which occurs after stretching for about 0.1–0.2 s, depending on the volume and viscosity of the liquid and the surface density of spread monolayers. The dynamics of a surfactant-covered bridge is governed by the Navier-Stokes and convection-diffusion equations. First, these equations are solved with a three-dimensional, but axisymmetric, or two-dimensional (2D), finite element algorithm using elliptic mesh generation. Second, the governing set of 2D equations is reduced to a set of one-dimensional (1D) equations by means of the slender-jet approximation and the resulting set of 1D equations is solved with a 1D finite element algorithm. The presence of surfactant results not only in the lowering of surface tension and the capillary pressure, but also in surface tension gradients and Marangoni stresses, both of which affect the transient dynamics leading to breakup. In particular, the role of Marangoni stresses in delaying bridge breakup and on formation of satellite droplets is investigated as a function of the initial surface density and surface activity of the surfactant, and surface Peclet number that measures the importance of convection relative to diffusion. The predictions of the 2D algorithm are confirmed to be faithful to the physics by demonstrating that the computed results accord well with the experiments and existing scaling theories. In the pinch-off region, the surfactant is swept out of a thinning neck by strong convection. The calculations thus reveal that the scaling behavior in the presence of surfactant parallels that observed in the absence of surfactant, in accordance with recent reports by others. The 2D computations and the experiments are used in tandem to identify regions in the space of governing parameters where the 1D equations can be used with confidence.

Modelling of dilational visco-elasticity of adsorbed layers with multiple kinetic processes

In this work, a generalisation of a method to calculate the dilational visco-elasticity of surfactant adsorbed layers is presented. This theoretical approach allows surface relaxation processes to be taken into account, and the effect of their kinetic aspect on the viscous behaviour of the adsorbed layer is studied. The proposed method is rather general because it requires only the condition of small deviation from the equilibrium of the system, being based on the linear system formalism. Examples of application of this method are presented concerning surfactant changing their molar area and insoluble monolayer with chemical reaction. The expressions of the dilational visco-elasticity versus frequency, found with the present method, are in agreement with the results obtained in previous theoretical works and by other authors.