Interfacial Molecules Research Articles

Probing Gas–Liquid Interfacial Dynamics by Helium Evaporation from Hydrocarbon Liquids and Jet Fuels

We have monitored the speeds of evaporating helium atoms dissolved in liquid octane, isooctane, 1-methylnaphthalene, dodecane, squalane, ethylene glycol, and two jet fuels. In all cases, the average kinetic energies of the evaporating He atoms exceed the Maxwellian value of 2RT. The energies roughly track solvent surface tensions; this correlation may reflect the tighter packing and attractions of interfacial solvent molecules that restrict the gaps through which He atoms escape. Mixtures of dodecane, squalane, and 1-methylnaphthalene generate He evaporation energies that lie between the pure liquid values. We find, however, that He atoms evaporate from pure 1-methylnaphthalene with kinetic energies lower than expected based on its high surface tension, perhaps because the sideways packing of the aromatic rings provides more direct channels for the escaping He atoms. Additionally, He evaporates from two complex fuel mixtures, Jet A and JP-8, with nearly identical energies, implying that the extra additive...

Effect of Organic Alkalis on Interfacial Tensions of Surfactant/Polymer Solutions against Hydrocarbons

The dynamic interfacial tensions (IFTs) of enhanced oil recovery (EOR) surfactant/polymer/organic alkali systems against a homologous series of alkanes have been investigated by a spinning drop interfacial tensiometer. Two organic alkalis with different molecular sizes, ethanolamine (EA) and diethanolamine (DEA), two surfactants with high interfacial activity, heavy alkyl benzenesulfonate (HABS) and petroleum sulfonate (SLPS), and two kinds of polymers, partly hydrolyzed polyacrylamide (HPAM) and hydrophobically modified polyacrylamide (HMPAM), were employed to study the real mechanisms controlling the IFT behavior. The experimental results show that the addition of organic alkali may affect the dynamic IFTs of surfactant solutions through different mechanisms. First, the possible reaction of organic alkali with the oil-soluble components in industrial surfactants will influence the surface active spices at the interface and enhance the dynamic behavior. Second, the hydrophilic–lipophilic balance (HLB) of the surfactant may vary, and the surfactant may become more water-soluble because of the ionization of oil-soluble components, especially for HABS. Finally, the organic counterion with larger molecular sizes may affect the arrangement of interfacial surfactant molecules. For both HABS and SLPS surfactants, interfacial interactions between the hydrophobic part of surfactants and the hydrophobic blocks of HMPAM will reduce the interfacial concentration of surfactant monomers by forming aggregates and result in an obvious increment of the IFT value. On the other hand, HPAM will obviously enhance the water solubility of the surfactant/organic alkali solutions and reduce the IFT values for the hydrocarbons with lower alkyl carbon number (ACN).

Thermal characterization of static and dynamical properties of the confined molecular systems interacting through dispersion force

We investigated the thermal properties of liquid methylcyclohexane and racemic sec-butylcyclohexane, as representatives of a molecular system with only dispersion-force intermolecular interactions, confined in the pores (thickness/diameter d = 12, 6, 1.1 nm) of silica gels by adiabatic calorimetry. The results imply a heterogeneous picture for molecular aggregate under confinement consisting of an interfacial region and an inner pore one. In the vicinity of a glass-transition temperature Tg,bulk of bulk liquid, two distinguishable relaxation phenomena were observed for the confined systems and their origins were attributed to the devitrification, namely glass transition, processes of (1) a layer of interfacial molecules adjacent to the pore walls and (2) the molecules located in the middle of the pore. A third glass-transition phenomenon was observed at lower temperatures and ascribed to a secondary relaxation process. The glass transition of the interfacial-layer molecules was found to proceed at temperatures rather above Tg,bulk, whereas that of the molecules located in the inner pore region occurred at temperatures below Tg,bulk. We discuss the reason why the molecules located in different places in the pores reveal the respectively different dynamical properties.

An ab initio molecular dynamics study of the liquid-vapor interface of an aqueous NaCl solution: inhomogeneous density, polarity, hydrogen bonds, and frequency fluctuations of interfacial molecules.

We have presented a first principles simulation study of the structural and dynamical properties of a liquid-vapor interfacial system of a concentrated (5.3 M) aqueous NaCl solution. We have used ab initio molecular dynamics to examine the structural and dynamical properties of the bulk and interfacial regions. The structural aspects of the system that have been considered here include the inhomogeneous density profiles of ions and water molecules, hydrogen bond distributions, orientational profiles, and also vibrational frequency distributions in the bulk and interfacial regions. It is found that the sodium ions are mostly located in the interior, while the chloride anions occupy a significant portion of the interface of the slab. The water dipoles at the interface prefer to orient parallel to the surface. The dynamical aspects of the interfaces are investigated in terms of diffusion, orientational relaxation, hydrogen bond dynamics, and vibrational spectral diffusion. The results of the interfacial dynamics are compared with those of the corresponding bulk region. It is observed that the interfacial molecules exhibit faster diffusion and orientational relaxation with respect to the bulk. However, the interfacial molecules are found to have longer hydrogen bond lifetimes than those of the bulk. We have also investigated the correlations of hydrogen bond relaxation with the vibrational frequency fluctuations of interfacial water molecules.

Nanorheology of liquid crystal thin films confined between interfaces with anisotropic molecular orientations

The hydrodynamic properties of confined liquids play a significant role in the transportation of fluids in microfluidics, nanofluidics, and flows in porous media so on. In this paper, the effective viscosities of 4-Cyano-4′-pentyl biphenyl (5CB) liquid crystals (LCs) confined in a slab geometry with interfaces of anisotropically oriented molecules were investigated by a quartz crystal microbalance (QCM). A novel nanocell was fabricated onto the QCM substrate chip resulting in a 520-nm-thick slab geometry for viscosity measurement. Rubbed polyimide layer was spinning coated onto the nanocell interfaces to obtain different combinations of interfacial molecular orientations of the added 5CB LCs. The resulted effective viscosity of 5CB LCs increased by ~14 % on the interface with LC molecules oriented vertically to the shearing direction. However, it decreased on the substrate interface with LC molecules parallel along the shearing direction and decreased up to ~58 % when the LCs was confined in the nanocell with both parallel oriented interfaces. A varying magnitude of ~300 % in LC effective viscosity could be obtained by tailoring the interfacial molecular orientations. We also demonstrated that the shear resistance applied on the interface is predominated by the liquid microstructure within nanometers distance from the wall, but weakly influenced by that far from the local interface. The results were analyzed in terms of LC intermolecular friction and the predominance of the interfacial molecules at nanoscale, which are of the utmost importance for fluid transportation in microfluidics, in porous media, or in boundary lubrications.

Properties of the intrinsic surface of liquid acetone, as seen from computer simulations

Molecular dynamics simulations of the liquid–vapour interface of acetone have been performed in the canonical (N, V, T) ensemble at 298 K with two different molecular models, belonging to the transferable potential for phase equilibria (TraPPE) and Chemistry at Harvard Molecular Mechanics 27 (CHARMM27) force fields, respectively. The first three molecular layers of the liquid phase beneath its surface have been identified by means of the identification of the truly interfacial molecules (ITIM) method. The results obtained reveal that the vicinity of the vapour phase affects only the first molecular layer of the liquid phase, and its effect vanishes in every respect already in the second molecular layer. The surface acetone molecules exhibit a dual orientational preference. In their dominant orientation, the molecules stay perpendicular to the macroscopic plane of the liquid surface, the C˭O bond lying almost parallel, whilst the line connecting the two CH3 groups staying almost perpendicular to this plane, declining only by about 5° from the perfectly parallel and perpendicular alignments, respectively. This finding is also in a clear accordance with results of some of the former experimental studies of this system. Besides the above orientation, another alignment is also found to be preferred, in particular, at the tips of the crests of the molecularly wavy liquid surface. In this orientation, the acetone molecule lays almost parallel with the surface plane, declining by about 10° from it, pointing the O atom slightly toward the liquid, whilst the two CH3 groups slightly toward the vapour phase.

Re-evaluating the surface tension analysis of polyelectrolyte-surfactant mixtures using phase-sensitive sum frequency generation spectroscopy.

Surface tension (ST) has been the most important measure of a molecule's surface activity. However, in many cases the complex behaviors of ST are challenging to interpret. For example, aqueous solutions of sodium docecyl sulfate (SDS) and poly(diallyldimethylammonium chloride) (PDADMAC) show dramatic changes in ST when the concentration of SDS varies. Although surfactants are generally described as "substances that reduce surface tension", new evidence shows that ST may have little changes when a significant amount of SDS is present at the water surface. The decrease of surface entropy resulting from a better ordering of interfacial molecules, such as water, counteracts the decrease of surface enthalpy and is able to keep the ST nearly unchanged. The dramatic ST decrease and recovery of the SDS-PDADMAC mixtures was discovered to be a result of a surface charge reversal. Similar surface charge reversal was also observed in cationic surfactant and anionic polyelectrolyte mixtures.

Hydrogen bonded structure, polarity, molecular motion and frequency fluctuations at liquid-vapor interface of a water-methanol mixture: an ab initio molecular dynamics study.

We have performed ab initio molecular dynamics simulations of a liquid-vapor interfacial system consisting of a mixture of water and methanol molecules. Detailed results are obtained for the structural and dynamical properties of the bulk and interfacial regions of the mixture. Among structural properties, we have looked at the inhomogeneous density profiles of water and methanol molecules, hydrogen bond distributions and also the orientational profiles of bulk and interfacial molecules. The methanol molecules are found to have a higher propensity to be at the interface than water molecules. It is found that the interfacial molecules show preference for specific orientations so as to form water-methanol hydrogen bonds at the interface with the hydrophobic methyl group pointing towards the vapor side. It is also found that for both types of molecules, the dipole moment decreases at the interface. It is also found that the local electric field of water influences the dipole moment of methanol molecules. Among the dynamical properties, we have calculated the diffusion, orientational relaxation, hydrogen bond dynamics, and vibrational frequency fluctuations in bulk and interfacial regions. It is found that the diffusion and orientation relaxation of the interfacial molecules are faster than those of the bulk. However, the hydrogen bond lifetimes are longer at the interface which can be correlated with the time scales found from the decay of frequency time correlations. The slower hydrogen bond dynamics for the interfacial molecules with respect to bulk can be attributed to diminished cooperative effects at the interface due to reduced density and number of hydrogen bonds.

Structure and Dynamics of the Liquid–Liquid Interface of an Aqueous NaCl Solution with Liquid Carbon Tetrachloride from First-Principles Simulations

The present work deals with a first-principles theoretical study of the liquid–liquid interfacial system of an aqueous NaCl solution with the hydrophobic liquid of carbon tetrachloride (CCl4). Various structural and electronic properties of the system such as the inhomogeneous density profiles of ions and water molecules, hydrogen bond distributions, orientational profiles, vibrational frequency distributions, and also dipole moments of water at the bulk and interface are investigated. It is found that the chloride ions have a higher propensity for the interface and the sodium ions have a tendency to reside in the inner side of the aqueous medium. The dynamical aspects of the interfaces are analyzed in terms of the diffusion, orientational relaxation, hydrogen bond dynamics, and vibrational spectral diffusion and are compared with those of the bulk regions. It is found that the interfacial molecules have a faster diffusion and orientation relaxation with respect to the bulk molecules. The interfacial wate...

Organic–inorganic hybrid materials designed by controlled radical polymerization and mediated using commercial dual functional organophosphorous coupling agents

Hybrid materials that are composed of a polymeric material interfaced with an inorganic, metal oxide material have been a rapidly expanding research area in the last few decades. However, interfacial regions remain an important area of focus, and as such hybrid materials do not always possess a very robust or stable interface. Tailor-made interfacial molecules have been successfully reported but material scientists wishing to develop composite interfacial materials would favorably use commercial solutions. Our study shows how we can leverage a commercially available organophosphonic acid group that is coupled with a 2-bromo isobutyrate initiator for surface initiated atom transfer radical polymerization (SI-ATRP) for use as a strong interfacial molecule. We illustrate this mechanism with both nanoparticles of titania and flat titania substrates used as the grafting support and polymerization anchoring points. We demonstrate that the size of the organophosphonic acid initiator, specifically the carbon spacer between the reactive groups, controls the stability of the molecule. The actual covalent linkage between the phosphonic acid group and the titania surface while also leaving the ATRP initiating group able to start the polymerization, is confirmed via31P solid state NMR spectroscopy, liquid 1H NMR spectroscopy XPS, DLS and SEM.

Two-dimensional percolation at the free water surface and its relation with the surface tension anomaly of water.

The percolation temperature of the lateral hydrogen bonding network of the molecules at the free water surface is determined by means of molecular dynamics computer simulation and identification of the truly interfacial molecules analysis for six different water models, including three, four, and five site ones. The results reveal that the lateral percolation temperature coincides with the point where the temperature derivative of the surface tension has a minimum. Hence, the anomalous temperature dependence of the water surface tension is explained by this percolation transition. It is also found that the hydrogen bonding structure of the water surface is largely model-independent at the percolation threshold; the molecules have, on average, 1.90 ± 0.07 hydrogen bonded surface neighbors. The distribution of the molecules according to the number of their hydrogen bonded neighbors at the percolation threshold also agrees very well for all the water models considered. Hydrogen bonding at the water surface can be well described in terms of the random bond percolation model, namely, by the assumptions that (i) every surface water molecule can form up to 3 hydrogen bonds with its lateral neighbors and (ii) the formation of these hydrogen bonds occurs independently from each other.

Biophysical investigations of the structure and function of the tear fluid lipid layer and the effect of ectoine. Part A: Natural meibomian lipid films

The tear fluid lipid layer is the outermost part of the tear film on the ocular surface which protects the eye from inflammations and injuries. We investigated the influence of ectoine on the structural organization of natural meibomian lipid films using surface activity analysis and topographical studies. These films exhibit a continuous pressure–area isotherm without any phase transition. With the addition of ectoine, the isotherm is expanded towards higher area per molecule values suggesting an increased area occupied by the interfacial lipid molecules. The AFM topology scans of natural meibomian lipid films reveal a presence of fiber-like structures. The addition of ectoine causes an appearance of droplet-like structures which are hypothesized to be tri-acyl-glycerols and other hydrophobic components excluded from the lipid film. Further the material properties of the droplet-like structure with respect to the surrounding were determined by using the quantitative imaging mode of the AFM technique. The droplet-like structures were found to be comparatively softer than the surrounding. Based on the observations a preliminary hypothesis is proposed explaining the mechanism of action of ectoine leading to the fluidization of meibomian lipid films. This suggests the possibility of ectoine as a treatment for the dry eye syndrome.

An ab Initio Molecular Dynamics Study of Water-Carbon Tetrachloride Liquid-Liquid Interface: Nature of Interfacial Structure, Hydrogen Bonds and Dynamics

We present a theoretical study of the structure and dynamics of water-carbon tetrachloride liquid-liquid interface by means of ab initio molecular dynamics simulations. We have studied the density profiles, orientational profiles, hydrogen bond distributions, vibrational power spectra, diffusion, orientational relaxation, hydrogen bond dynamics and vibrational spectral diffusion of bulk and interfacial molecules. We have also provided an analysis of vacancies present in the interfacial system using Voronoi polyhedra method. The hydrogen bonding interaction is found to be weakened at the interface compared to that in the bulk phase of water. Weakly hydrogen bonded and non-hydrogen bonded water molecules at the interface give rise to peaks at different positions of the vibrational power spectrum. Diffusion and orientational relaxation of water molecules are also found to be faster at the interface, which can be correlated with the vacancies present in the system. The dynamics of vibrational spectral diffusion is studied by means of frequency-time correlations calculated through a time-series analysis using the wavelet method and the results of spectral diffusion are correlated with the dynamics of hydrogen bond fluctuations and that of non-hydrogen bonded hydroxyl modes in the bulk and interfacial regions.

Theory of third-order spectroscopic methods to extract detailed molecular orientational dynamics for planar surfaces and other uniaxial systems.

Functionalized organic monolayers deposited on planar two-dimensional surfaces are important systems for studying ultrafast orientational motions and structures of interfacial molecules. Several studies have successfully observed the orientational relaxation of functionalized monolayers by fluorescence depolarization experiments and recently by polarization-resolved heterodyne detected vibrational transient grating (HDTG) experiments. In this article we provide a model-independent theory to extract orientational correlation functions unique to interfacial molecules and other uniaxial systems based on polarization-resolved resonant third-order spectroscopies, such as pump-probe spectroscopy, HDTG spectroscopy, and fluorescence depolarization experiment. It will be shown (in the small beam-crossing angle limit) that five measurements are necessary to completely characterize the monolayer's motions: I(∥)(t) and I(⊥)(t) with the incident beams normal to the surface, I(∥)(t) and I(⊥)(t) with a non-zero incident angle, and a time averaged linear dichroism measurement. Once these measurements are performed, two orientational correlation functions corresponding to in-plane and out-of-plane motions are obtained. The procedure is applicable not only for monolayers on flat surfaces, but any samples with uniaxial symmetry such as uniaxial liquid crystals and aligned planar bilayers. The theory is valid regardless of the nature of the actual molecular motions on interface. We then apply the general results to wobbling-in-a-cone model, in which molecular motions are restricted to a limited range of angles. Within the context of the model, the cone angle, the tilt of the cone relative to the surface normal, and the orientational diffusion constant can be determined. The results are extended to describe analysis of experiments where the beams are not crossing in the small angle limit.

What Is Selectivity and What Happens at an Interface Spin-Offs of a Collaboration with R.P. Buck

The corresponding author worked with R.P. Buck for three months in 1985 and initiated experimental work on the impedance spectroscopy and current transport properties of plasticized PVC ISE membranes in the Buck lab. This lecture reviews some of the work done with Dick and then discusses how this topic has developed in the author’s lab up to this date.The original purpose of the collaboration with Dick was to study the interfacial electrode kinetics at plasticized PVC ISEs. The results were exciting but quite different from the original goal: we could explain why the apparently ion-free membranes conducted electricity and we could also detect and explain the large surface impedances [1,2]. Both phenomena could be traced back to contaminations in the membrane components. The results with the membrane bulk impedances encouraged us to revive the DC experiments of the Simon group with the PVC membranes. This work led to a full explanation of this complex phenomenon [3].Later on this work was extended to making the first amperometric and cyclic voltammetric measurements with PVC ISEs [4-6]. This was probably the prelude (this word is used here to remember Dick, the musician) to many novel potentiometric methods developed by colleagues. Another direction that followed from our collaboration was to study the ion exchange properties of ISEs [7,8]. This radiotracer study proved that the membranes were indeed selective ion exchangers and their exchange selectivity correlated well with their potentiometric selectivity. Yet another work, this time with small angle neutron scattering [9] helped to understand the structure of plasticized PVC while surface spectroscopies and AFM allowed a detailed analysis of the membrane composition in the immediate vicinity of the interface [10].The distribution of ions in the first few nanometers of the interfacial layer is essential for understanding potentiometric sensors. Lacking suitable experimental tools for such studies our attention turned to computer simulations. We could indeed simulate the interface and correctly predict emf values in complex situations [11, 12]. At this point we left potentiometry and devoted much effort to discovering the structure of the first molecular layer at liquid-liquid and liquid-air interfaces. We have developed a method to identify the truly interfacial molecules and showed that many interfacial phenomena are concentrated to this single layer [13].A promising area of selective analysis similar to ISE potentiometry has emerged recently with molecularly imprinted polymers. Our experience with ISEs has helped to shed light on the less than ideal selectivity of these materials [14]. On the go we have recognized that selectivity, a fundamental concept of analytical chemistry, is very little understood. We have developed a selectivity concept which appears to be sufficiently general and sheds also new light on potentiometric selectivities.

NMR studies on the temperature-dependent dynamics of confined water.

We use (2)H NMR to study the rotational motion of supercooled water in silica pores of various diameters, specifically, in the MCM-41 materials C10, C12, and C14. Combination of spin-lattice relaxation, line-shape, and stimulated-echo analyses allows us to determine correlation times in very broad time and temperature ranges. For the studied pore diameters, 2.1-2.9 nm, we find two crossovers in the temperature-dependent correlation times of liquid water upon cooling. At 220-230 K, a first kink in the temperature dependence is accompanied by a solidification of a fraction of the confined water, implying that the observed crossover is due to a change from bulk-like to interface-dominated water dynamics, rather than to a liquid-liquid phase transition. Moreover, the results provide evidence that α process-like dynamics is probed above the crossover temperature, whereas β process-like dynamics is observed below. At 180-190 K, we find a second change of the temperature dependence, which resembles that reported for the β process of supercooled liquids during the glass transition, suggesting a value of Tg ≈ 185 K for interface-affected liquid water. In the high-temperature range, T > 225 K, the temperature dependence of water reorientation is weaker in the smaller C10 pores than in the larger C12 and C14 pores, where it is more bulk-like, indicating a significant effect of the silica confinement on the α process of water in the former 2.1 nm confinement. By contrast, the temperature dependence of water reorientation is largely independent of the confinement size and described by an Arrhenius law with an activation energy of Ea ≈ 0.5 eV in the low-temperature range, T < 180 K, revealing that the confinement size plays a minor role for the β process of water.

Self-assembly of discotic liquid crystal decorated ZnO nanoparticles for efficient hybrid solar cells

Efficient hybrid solar cells based on a blend of poly(3-hexylthiophene) (P3HT) and discotic liquid crystal ligands dithiol-functionalized triphenylene (TP-S) modified zinc oxide (ZnO) nanoparticles (TP-S@ZnO) were systematically investigated. The TP-S-modified ZnO nanoparticles possess a well-defined dispersibility, especially after annealing under the liquid-crystalline state (130 °C), originating from the help of the supramolecular self-assembly of the TP-S discs. Discotic liquid crystal ligands improve the compatibility between P3HT polymer and ZnO nanoparticles, which is beneficial for enhanced charge separation and transfer efficiency. On the other hand, the interfacial molecules TP-S can play a great role in the ordering and crystallinity of P3HT chains. X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) studies indicate that the spontaneous self-assembly of the promotes P3HT chains to overall, the power conversion efficiency (PCE) of polymer/ZnO hybrid solar cells increased from 0.46% to 0.95% after ZnO was modified with TP-S under thermal annealing. As expected, DLC molecular interface modification can provide a viable and interesting method to promote the compatibility and a large interfacial area between polymers and nanocrystals, subsequently improving the performance of photovoltaic devices.

Dynamics of Molecular Monolayers with Different Chain Lengths in Air and Solvents Probed by Ultrafast 2D IR Spectroscopy

The ultrafast dynamics of functionalized alkylsilane monolayers with two different alkyl chain lengths, C11 and C3, are studied by 2D IR vibrational echo spectroscopy. Terminal sites of the monolayers are functionalized with an IR probe, tricarbonyl (1,10-phenanthroline) rhenium chloride (RePhen(CO)3Cl), to report on the structural dynamics of monolayer-air and monolayer-solvent interfaces. Frequency–frequency correlation functions (FFCF) of symmetric CO stretching mode were extracted from 2D IR spectra. The FFCF provides information on the time evolution of surface structures that contribute to the inhomogeneously broadened IR absorption spectrum of the CO stretch by quantifying spectral diffusion. To elucidate the detailed structural dynamics with accurate time constants, FFCF decays were monitored to 60 ps, a major improvement over prior experiments. Without the presence of solvents, C3 monolayers have significantly slower spectral diffusion (66 ps) than C11 monolayer (38 ps). This difference supports the previous postulate that spectral diffusion associated with the molecular monolayer in air involves the structural dynamics of the tethering alkyl chains. With the C11 and C3 monolayers immersed in dimethylformamide (DMF), in which RePhen(CO)3Cl is soluble, the FFCFs of both samples display biexponential decays with the time constants of 5.6 and 43 ps for C11 and 5.9 and 63 ps for C3. The slower time constants are in good agreement with the spectral diffusion time constants observed in the absence of solvent, indicating that these slower components still reflect the monolayers’ dynamics, which are not affected by the presence of the solvent. Because the faster time constants were independent of chain length and similar to that of the RePhen(CO)3Cl headgroup in bulk DMF solution, they are attributed to the dynamics of the interfacial DMF molecules. When monolayers were immersed in hexadecane, in which RePhen(CO)3Cl is not soluble, slower dynamics were again observed for the C3 monolayer than the C11 monolayer. The orientational dynamics of the IR probes were studied using polarization selective heterodyne detected transient grating experiments as well.

PH Effects on the Molecular Structure of β-Lactoglobulin Modified Air–Water Interfaces and Its Impact on Foam Rheology

Macroscopic properties of aqueous β-lactoglobulin (BLG) foams and the molecular properties of BLG modified air-water interfaces as their major structural element were investigated with a unique combination of foam rheology measurements and interfacial sensitive methods such as sum-frequency generation and interfacial dilatational rheology. The molecular structure and protein-protein interactions at the air-water interface can be changed substantially with the solution pH and result in major changes in interfacial dilational and foam rheology. At a pH near the interfacial isoelectric point BLG molecules carry zero net charge and disordered multilayers with the highest interfacial dilatational elasticity are formed at the air-water interface. Increasing or decreasing the pH with respect to the isoelectric point leads to the formation of a BLG monolayer with repulsive electrostatic interactions among the adsorbed molecules which decrease the interfacial dilational elasticity. The latter molecular information does explain the behavior of BLG foams in our rheological studies, where in fact the highest apparent yield stresses and storage moduli are established with foams from electrolyte solutions with a pH close to the isoelectric point of BLG. At this pH the gas bubbles of the foam are stabilized by BLG multilayers with attractive intermolecular interactions at the ubiquitous air-water interfaces, while BLG layers with repulsive interactions decrease the apparent yield stress and storage moduli as stabilization of gas bubbles with a monolayer of BLG is less effective.

On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics

A methodology for the determination of the solid-fluid contact angle, to be employed within molecular dynamics (MD) simulations, is developed and systematically applied. The calculation of the contact angle of a fluid drop on a given surface, averaged over an equilibrated MD trajectory, is divided in three main steps: (i) the determination of the fluid molecules that constitute the interface, (ii) the treatment of the interfacial molecules as a point cloud data set to define a geometric surface, using surface meshing techniques to compute the surface normals from the mesh, (iii) the collection and averaging of the interface normals collected from the post-processing of the MD trajectory. The average vector thus found is used to calculate the Cassie contact angle (i.e., the arccosine of the averaged normal z-component). As an example we explore the effect of the size of a drop of water on the observed solid-fluid contact angle. A single coarse-grained bead representing two water molecules and parameterized using the SAFT-γ Mie equation of state (EoS) is employed, meanwhile the solid surfaces are mimicked using integrated potentials. The contact angle is seen to be a strong function of the system size for small nano-droplets. The thermodynamic limit, corresponding to the infinite size (macroscopic) drop is only truly recovered when using an excess of half a million water coarse-grained beads and/or a drop radius of over 26 nm.