Discogen-DNA complex films at air-water and air-solid interfaces

We have studied films of an ionic discogenic (discotic mesogenic) molecule (pyridinium salt tethered with hexaalkoxytriphenylene (PyTp)) and DNA complex at air-water (A-W) and air-solid interfaces. We have formed an PyTp monolayer on an aqueous subphase containing a small amount of DNA to obtain a PyTp-DNA complex at the A-W interface. Compared to the pure PyTp monolayer, the PyTp-DNA complex monolayer exhibits a higher collapse pressure and lower limiting area, indicating condensation and better stability. A Brewster angle microscope was used for in situ observation of the morphology of the film at the A-W interface. The PyTp-DNA complex films on silicon wafers were prepared using the Langmuir-Blodgett (LB) technique. We find that several tens of layers of the PyTp-DNA complex monolayer can be transferred with good efficiency. Fourier transform infrared spectroscopy studies confirm the presence of DNA in the LB films of the PyTp-DNA complex. Nanoindentation measurements using atomic force microscope reveal that the PyTp-DNA complex films are about two times harder as compared to the pure PyTp films.     

Phase behavior of amphiphilic lipid molecules at air-water interfaces: an off-lattice self-consistent-field modeling

We introduce an extended application of the off-lattice self-consistent-field theory (SCFT) to model lipid monolayers at air-water interfaces. The off-lattice SCFT is used without a priori symmetry assumptions on equilibrium morphologies. This enables us to capture asymmetric lipid membranes at air-water interfaces which are otherwise unattainable with a conventional SCF model. Equilibrium morphologies in systems containing lipid molecules, fractions of air, and water are studied as a function of the relative amount of lipid molecules. The corresponding Langmuir isotherms are analyzed to reveal possible phase transitions. We consider both saturated and unsaturated lipid molecules with a branched structure. For saturated lipids, we find two distinct morphological phases, i.e., micellar and lamellar, showing a pronounced first-order phase transition with a well-defined region of phase coexistence. This region is sensitive to the hydrophilicity of lipid molecules and the miscibility of air with water molecules. The phase coexistence is also influenced by the size of hydrophilic and hydrophobic parts of lipid molecules. In contrast, membranes of unsaturated lipids have developed a continuous range of smooth structural transformations from a circular to an ellipsoidal micellar morphology and eventually to a lamellar structure. The shape of the lamella changes from a slightly undulated to a vigorously curved. Unlike saturated lipid membranes, there is no apparent first-order phase transition or a region of phase coexistence for unsaturated lipid membranes. We interpret this as a result of a higher flexibility of unsaturated lipid membranes which enables them to adopt a wider range of conformations in comparison with saturated lipid membranes.     

Partitioning of clay colloids at air-water interfaces

Colloid sorption onto air-water interfaces in a variety of natural environments has been previously recognized, but better quantification and understanding is still needed. Affinities of clay colloids for the air-water interface were measured using a bubble-column method and reported as partition coefficients (K). Four types of dilute clay suspensions were measured in NaCl solutions under varying pH and ionic strength conditions: kaolinite KGa-1, illite IMt-2, montmorillonite SWy-2, and bentonite. The K values of three types of polystyrene latex particles with different surface-charge properties were also measured for comparison. Kaolinite exhibited extremely high affinity to the air-water interface at pH values below 7. Illite has lower affinity to air-water interfaces than kaolinite, but has similar pH dependence. Na-montmorillonite and bentonite clay were found excluded from the air-water interface at any given pH and ionic strength. Positively and negatively charged latex particles exhibited sorption and exclusion, respectively, at the air-water interface. These results show the importance of electrostatic interactions between the air-water interface and colloids, especially the influence of pH-dependent edge charges, and influence of particle shape.     

Disproportionation of clustered protein-stabilized bubbles at planar air-water interfaces

The rate of shrinkage of air bubbles, of initial radius from 50 to 200 microm, injected beneath a planar air-water interface has been measured. Bubbles were stabilized in solutions of 0.05 wt% gelatin or pure beta-lactoglobulin. It has been observed that small size differences between two closely spaced or touching bubbles result in markedly divergent rates of shrinkage for the two. By studying a number of different initial bubble configurations, it is demonstrated that the overall change in bubble size distribution is strongly dependent on local, interbubble gas diffusion. In this respect, the strong tendency for the gelatin-stabilized bubbles to aggregate and shrink, while remaining in contact, produced patterns of disproportionation significantly different from those observed with beta-lactoglobulin. In beta-lactoglobulin solutions, it was usually found that bubbles initially in contact shrank away from each other with time, becoming increasingly isolated as a result. A theoretical approach that can exactly incorporate the perturbation of local diffusion fluxes due to the proximity of two bubbles is presented. This enables one to map a "stability diagram" that delineates regions where the larger bubble of a pair will either shrink or grow, according to the relative size of the bubbles and their separation. Theoretical calculations show that it is possible for a bubble to exhibit more complex shrinkage behavior than is predicted by a mean field approach or the Lifshitz, Slyozov, and Wagner (LSW) theory of Ostwald ripening for dilute systems. The inclusion of dilatational elasticity in the theoretical model introduces additional complications, which are also briefly discussed.     

Interfacial dynamics and rheology of polymer-grafted nanoparticles at air-water and xylene-water interfaces

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.     

Absolute orientation of molecules at interfaces

A method to determine the absolute orientation of molecules at liquid interfaces by sum frequency generation (SFG) is reported. It is based on measurements of the orientations of two nonparallel vibrationally active chromophores in the molecule of interest combined with a rotation matrix formulation to obtain the absolute molecular orientation. We chose m-tolunitrile, a planar molecule adsorbed to the air/water interface, as a proof-of-method experiment. Quantitative analysis of different polarization sum frequency intensities facilitate unique peak assignments of the methyl and nitrile groups of m-tolunitrile. The SFG analysis of the measurement yields a nitrile group tilting at 53 degrees to the surface normal, and the C3 axis of the methyl group is almost upright at 23 degrees with respect to the surface normal. Using a rotation matrix formulation, we found that the angle between the surface plane and the m-tolunitrile molecular plane is 70 degrees.     

Surface dilational moduli of polymer and blended polymer monolayers spread at air-water interfaces

Surface dilational moduli of polymer monolayers, blended polymer monolayers, and polymer particle monolayers spread at air–water interfaces are reviewed, focusing on measurements using surface pressure isotherm, surface pressure relaxation, and oscillating barrier methods. Differences between the surface dilational moduli of condensed polymer monolayers and expanded polymer monolayers are explored. Moreover, the features of the surface dilational moduli in blended polymer monolayers are discussed in terms of their miscibility.     

Asymmetric orientation of toluene molecules at oil-silica interfaces

The interfacial structure of heptane and toluene at oil-silica interfaces has previously been studied by sum frequency generation [Z. Yang et al., J. Phys. Chem. C. 113, 20355 (2009)]. It was found that the toluene molecule is almost perpendicular to the silica surface with a tilt angle of about 25°. Here, we have investigated the structural properties of toluene and heptane at oil-silica interfaces using molecular dynamics simulations for two different surfaces: the oxygen-bridging (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces of quartz (silica). Based on the density profile, it was found that both heptane and toluene oscillate on silica surfaces, with heptane showing more oscillation peaks. Furthermore, the toluene molecules of the first layer were found to have an asymmetric distribution of orientations, with more CH(3) groups pointed away from the silica surface than towards the silica surface. These findings are generally consistent with previous experiments, and reveal enhanced molecular structures of liquids at oil-silica interfaces.     

Orientation of xanthene adsorbate molecules at dielectric interfaces

The polarization dependence of the second harmonic signal from monolayers of xanthene dyes allows us to develop a model for the orientation of the adsorbates and to locate the group responsible for adsorption. Possible extension of this method to other dyes of biological and medical interest is discussed.     

Effect of humidity on the adsorption kinetics of lung surfactant at air-water interfaces

The in vitro adsorption kinetics of lung surfactant at air-water interfaces is affected by both the composition of the surfactant preparations and the conditions under which the assessment is conducted. Relevant experimental conditions are surfactant concentration, temperature, subphase pH, electrolyte concentration, humidity, and gas composition of the atmosphere exposed to the interface. The effect of humidity on the adsorption kinetics of a therapeutic lung surfactant preparation, bovine lipid extract surfactant (BLES), was studied by measuring the dynamic surface tension (DST). Axisymmetric drop shape analysis (ADSA) was used in conjunction with three different experimental methodologies, i.e., captive bubble (CB), pendant drop (PD), and constrained sessile drop (CSD), to measure the DST. The experimental results obtained from these three methodologies show that for 100% relative humidity (RH) at 37 degrees C the rate of adsorption of BLES at an air-water interface is substantially slower than for low humidity. It is also found that there is a difference in the rate of surface tension decrease measured from the PD and CB/CSD methods. These experimental results agree well with an adsorption model that considers the combined effects of entropic force, electrostatic interaction, and gravity. These findings have implications for the development and evaluation of new formulations for surfactant replacement therapy.     

Sum frequency vibrational spectroscopy of leucine molecules adsorbed at air-water interface

Sum frequency vibrational spectroscopy was used to study adsorption of leucine molecules at air-water interface from solutions with different concentrations and pH values. The surface density and the orientation of the isopropyl head group of the adsorbed leucine molecules could be deduced from the measurements. It was found that the orientation depends on the surface density, but only weakly on bulk pH value at the saturated surface density. The vibrational spectra of the interfacial water molecules appeared to be strongly affected by the charge state of the adsorbed leucine molecules. Enhancement and inversion of polar orientation of interfacial water molecules by surface charges or field controllable by the bulk pH value were observed.     

Toroid morphology by ABC-type amphiphilic rod-coil molecules at the air-water interface

The interfacial and aggregation behavior of the ABC-type amphiphilic molecules with semirigid dumbbell-shaped core and variable length of hydrophobic branched tails (R=(CH2)nCH3 with n=5 (1), 9 (2), 13 (3)) were investigated. At low surface pressure, smooth, uniform monolayers were formed at the air-water interface by molecules 1 and 2, whereas for molecule 3 unique 2D toroid aggregates have been formed. These aggregates were relatively stable within a range of surface pressure and spreading solution concentration. Upon compression, the 2D toroid aggregates collapsed into large, round 3D aggregates. Finally, the choice of spreading solvent has a great influence on aggregation formation into 2D or 3D micelles as a result of the variable balance of the hydrophobic interactions of branched tails and the pi-pi stacking interaction between aromatic segments.     

Detachment of deposited colloids by advancing and receding air-water interfaces

Moving air-water interfaces can detach colloidal particles from stationary surfaces. The objective of this study was to quantify the effects of advancing and receding air-water interfaces on colloid detachment as a function of interface velocity. We deposited fluorescent, negatively charged, carboxylate-modified polystyrene colloids (diameter of 1 μm) into a cylindrical glass channel. The colloids were hydrophilic with an advancing air-water contact angle of 60° and a receding contact angle of 40°. After colloid deposition, two air bubbles were sequentially introduced into the glass channel and passed through the channel at different velocities (0.5, 7.7, 72, 982, and 10,800 cm/h). The passage of the bubbles represented a sequence of receding and advancing air-water interfaces. Colloids remaining in the glass channel after each interface passage were visualized with confocal microscopy and quantified by image analysis. The advancing air-water interface was significantly more effective in detaching colloids from the glass surface than the receding interface. Most of the colloids were detached during the first passage of the advancing air-water interface, while the subsequent interface passages did not remove significant amounts of colloids. Forces acting on the colloids calculated from theory corroborate our experimental results, and confirm that the detachment forces (surface tension forces) during the advancing air-water interface movement were stronger than during the receding movement. Theory indicates that, for hydrophilic colloids, the advancing interface movement generally exerts a stronger detachment force than the receding, except when the hysteresis of the colloid-air-water contact angle is small and that of the channel-air-water contact angle is large.     

Dipoles-dipole association of mesogenic molecules in solution

The dielectric polarization has been used to study dipolar association of 4-n-pentyl-4'-cyanobiphenyl in benzene solution. The results have been interpreted with the assumption of a monomer-dimer equilibrium. To explain the relatively high effective dipole moment of the dimers, a new structure has been proposed for them.     

Block copolymer micelles with enzyme molecules at the interfaces

This research provides an efficient method for the fabrication of hybrid micelles with enzyme molecules at the interfaces. Amphiphilic block copolymer is synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization, and thiol‐modified porcine pancreatic lipase (PPL‐SH) is obtained by treatment of native PPL with Traut's reagent. PPL‐SH is conjugated to the block copolymer chains by thiol‐disulfide exchange reaction. In phosphate buffered saline, the bioconjugate self‐assembles into micelles with enzyme molecules at the interfaces between hydrophobic cores and hydrophilic coronae. The bioactivity of the enzyme molecules on the micelles are compared with the native enzyme. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2047–2052     

A polarizable continuum model for molecules at diffuse interfaces

In this work we illustrate an extension of the polarizable continuum model to describe solvation effects on molecules at the interface between two fluid phases (liquid/liquid, liquid/vapor). This extension goes beyond the naive picture of the interface as a plane dividing two distinct dielectrics, commonly employed in continuum models. The main feature of the model is the use of a diffuse interface with an electric permittivity depending on the position. This characteristic clearly allows the study of simple interfaces as well as more complex membrane or multilayer structures. Moreover the smooth variation of the permittivity in the diffuse interface, in contrast to the sharp boundary between two regions, overcomes the numerical divergences due to charges placed at the boundary. The implementation of the model relies on the integral equation formalism, which allows one to calculate the reaction field acting on a molecule immersed in a dielectric environment once the proper Green's function is known. In the present case, such a Green's function is obtained numerically, allowing a large flexibility in the choice of the dielectric permittivity profile. The applications have been selected with the aim of illustrating the capabilities of the model; its present limitations are also discussed.     

Optical properties and orientational order of deoxyribonucleic acid molecules at interfaces

A high degree of planar orientational order of DNA molecules in thin films has been discovered by the oblique-polarized-beam method. The coefficients of surface birefringence of films with different thicknesses are measured. The thickness of optically anisotropic surface layers, which are characterized by macroscopic values higher than the diameter of DNA molecules by three orders of magnitude in terms of thickness, is determined. A difference in the main polarizabilities of the monomer unit of DNA, which is estimated for the films, is compared with the corresponding value for the solution measured by flow birefringence. The effect of temperature on the coefficient of birefringence of the films is examined. A comparison of the data on the native and denatured DNA shows that the degree of orientational order for the single-helix structure is smaller than that for the double helix.     

Refractometric studies of mesogenic molecules in isotropic solutions

The Lorenz-Lorentz equation for binary isotropic mixtures, consisting of anisotropic molecules, has been derived using the point-dipole approximation taking into account only pair molecular correlations. This permits the calculation of the effect of molecular correlations on the refractive index. Special attention has been paid to the case of infinite dilution in the solvents consisting of isotropic molecules, where an experimental check of the equations is possible. The specific refraction of some solutes with different molecular polarizability anisotropy and polarity has been studied in various solvents. It has been shown, that the main theoretically predicted features are observed in these experiments, but for a quantitative comparison information on the two-particle distribution function is needed.