Properties of free surface of water-methanol mixtures. Analysis of the truly interfacial molecular layer in computer simulation

Molecular dynamics simulations of the vapor-liquid interface of water-methanol mixtures of five different compositions were performed on the canonical (N,V,T) ensemble at 298 K. In addition, the vapor-liquid interface of the two neat systems was simulated, as well. The obtained configurations were analyzed by means of the novel identification of truly interfacial molecules method, which provides a full list of the molecules that are right at the surface (i.e., at the boundary of the two phases). The molecular level roughness of the surface, the adsorption of the methanol molecules at the surface layer, the orientation of the surface molecules, the residence time of the molecules at the surface layer, as well as the surface aggregation of the molecules were analyzed in detail. Both the frequency and the amplitude of the surface roughness were found to become larger with an increasing methanol content. This effect was found to be stronger for the amplitude, which falls in the range of 2-4 A, depending on the composition of the system. Methanol was found to be adsorbed at the surface layer, being preferentially at the humps of the molecularly rough surface. Surface methanol prefers to orient in such a way that the O-CH(3) bond remains perpendicular to the macroscopic plane of the surface, pointing the methyl group to the vapor phase. The main orientational preference of the water molecules is to lie parallel to the surface. Methanol was found to remain considerably longer at the surface layer of the mixed systems than water. Thus, contrary to the fact that the residence times of the two molecules were found to be rather similar to each other at the surface of their neat liquids, the residence time of the methanol molecules was an order of magnitude larger than that of water molecules at the surface of their mixtures. A strong lateral microscopic segregation of the molecules was observed at the surface layer; the minor component of the system (irrespective of whether it was water or methanol) was found to form two-dimensional aggregates, leaving the rest of the surface empty for the major component. The effect of the vicinity of the vapor phase on the properties of the molecules was found to vanish very quickly: the composition of the second layer as well as the properties of the molecules of this layer (e.g., dynamics and orientation) did not differ considerably from those in the bulk liquid phase.     

Molecular dynamics study of the interface between water and 2-nitrophenyl octyl ether

We present results of molecular dynamics simulations of the interface between water and 2-nitrophenyl octyl ether (NPOE). This system is analyzed in detail using a procedure to calculate intrinsic profiles of several important properties (density, radial distribution functions, hydrogen bonds, molecular orientation, self-diffusion). The interface was found to be molecularly sharp but corrugated by thermal fluctuations. Using a method based on capillary wave theory, we have estimated the interfacial tension and obtained good agreement with values calculated from the virial route. The results were compared to simulations of the water/nitrobenzene interface. The presence of an alkyl chain in NPOE introduces an added degree of hydrophobicity, which causes an increase in the interfacial tension. Furthermore, interfacial NPOE molecules are less organized than nitrobenzene and show a distinct dynamic response. These results shed light on the observed differences between these two organic liquids in electrochemical studies.     

Structure of the liquid-vapor interface of water-methanol mixtures as seen from Monte Carlo simulations

Monte Carlo simulation of the vapor-liquid interface of water-methanol mixtures of different compositions, ranging from pure water to pure methanol, have been performed on the canonical (N, V, T) ensemble at 298 K. The analysis of the systems simulated has revealed that the interface is characterized by a double layer structure: methanol is strongly adsorbed at the vapor side of the interface, whereas this adsorption layer is followed at its liquid side by a depletion layer of methanol of lower concentration than in the bulk liquid phase of the system. The dominant feature of the interface has been found to be the adsorption layer in systems of methanol mole fractions below 0.2, and the depletion layer in systems of methanol mole fractions between 0.25 and 0.5. The orientation of the molecules located at the depletion layer is found to be already uncorrelated with the interface, whereas the methanol molecules of the adsorption layer prefer to align perpendicular to the interface, pointing straight toward the vapor phase by their methyl group. Although both the preference of the molecular plane for a perpendicular alignment with the interface and the preference of the methyl group for pointing straight to the vapor phase are found to be rather weak, the preference of the methyl group for pointing as straight toward the vapor phase as possible within the constraint imposed by the orientation of the molecular plane is found to be fairly strong. One of the two preferred orientations of the interfacial water molecules present in the neat system is found to disappear in the presence of methanol, because methanol molecules aligned in their preferred orientation can replace these water molecules in the hydrogen-bonding pattern of the interface.     

Protein adsorption at the electrified air-water interface: implications on foam stability

The surface chemistry of ions, water molecules, and proteins as well as their ability to form stable networks in foams can influence and control macroscopic properties such as taste and texture of dairy products considerably. Despite the significant relevance of protein adsorption at liquid interfaces, a molecular level understanding on the arrangement of proteins at interfaces and their interactions has been elusive. Therefore, we have addressed the adsorption of the model protein bovine serum albumin (BSA) at the air-water interface with vibrational sum-frequency generation (SFG) and ellipsometry. SFG provides specific information on the composition and average orientation of molecules at interfaces, while complementary information on the thickness of the adsorbed layer can be obtained with ellipsometry. Adsorption of charged BSA proteins at the water surface leads to an electrified interface, pH dependent charging, and electric field-induced polar ordering of interfacial H(2)O and BSA. Varying the bulk pH of protein solutions changes the intensities of the protein related vibrational bands substantially, while dramatic changes in vibrational bands of interfacial H(2)O are simultaneously observed. These observations have allowed us to determine the isoelectric point of BSA directly at the electrolyte-air interface for the first time. BSA covered air-water interfaces with a pH near the isoelectric point form an amorphous network of possibly agglomerated BSA proteins. Finally, we provide a direct correlation of the molecular structure of BSA interfaces with foam stability and new information on the link between microscopic properties of BSA at water surfaces and macroscopic properties such as the stability of protein foams.     

Molecular dynamics simulation of the water|nitrobenzene interface

The structure and dynamics of the neat water|nitrobenzene liquid|liquid interface are studied at 300 K using molecular dynamics computer simulations. The water is modeled using the flexible SPC potential, and the nitrobenzene is modeled using an empirically determined nitrobenzene potential energy function. Although nitrobenzene is a polar liquid with a large dielectric constant, the structure of the interface is similar to other water|non-polar organic liquid interfaces. Among the main structural features we describe are an enhancement of interfacial water hydrogen bonds, the specific orientation of water dipoles and nitrobenzene molecules, and a rough surface that is locally sharp. Surface roughness is also characterized dynamically. The dynamics of molecular reorientation are shown to be only mildly modified at the interface. The effect due to the polarizable many-body potential energy functions of both liquids is investigated and is found to affect only mildly the above results.     

Water at a hydrophilic solid surface probed by ab initio molecular dynamics: inhomogeneous thin layers of dense fluid

We present a microscopic model of the interface between liquid water and a hydrophilic, solid surface, as obtained from ab initio molecular dynamics simulations. In particular, we focused on the (100) surface of cubic SiC, a leading semiconductor candidate for biocompatible devices. Our results show that in the liquid in contact with the clean substrate, molecular dissociation occurs in a manner unexpectedly similar to that observed in the gas phase. After full hydroxylation takes place, the formation of a thin (approximately 3 A) interfacial layer is observed, which has higher density than bulk water and forms stable hydrogen bonds with the substrate. The presence of this thin layer points at rather weak effects on the structural properties of water induced by a one-dimensional confinement between approximately 1.3 nm hydrophilic substrates. In addition, our results show that the liquid does not uniformly wet the surface, but molecules preferably bind along directions parallel to the Si dimer rows.     

阴离子Gemini表面活性剂在油/水界面行为的分子动力学模拟

采用分子动力学方法研究了磺酸盐型阴离子Gemini表面活性剂在油/水界面的吸附行为, 考察了不同长度的连接基(Spacer)对表面活性剂在界面的聚集形态及界面性质的影响. 密度分布和微观结构信息显示, Gemini表面活性剂能在油/水界面形成单层膜结构. Gemini表面活性剂能使油/水界面的厚度显著增大, 并使界面形成能降低. 当连接基为6个碳时, 此类磺酸盐型Gemini表面活性剂的界面厚度最大, 形成的界面最稳定. 连接基长度对Gemini表面活性剂单层膜周围的水分子和Na⁺的吸附结构影响不大, 但是能影响水分子的扩散行为.     

Thin films of discotic liquid crystals and their applications

ABSTRACT

Discotic liquid crystals (DLCs) are considered as fascinating systems due to their unique property of self-assembly to yield different columnar structures. DLCs are organic semiconductors and create pathways for the development of numerous optical and electrical devices. The thin films of DLCs can be considered as low dimensional system which can exhibit remarkable optical and physical properties. In this article, we present a review on ultrathin films of some interesting DLC molecules at air–water and air–solid interfaces. The Langmuir monolayer and Langmuir–Blodgett films of DLC molecules are extensively studied. The ultrathin films of DLC molecules can yield highly anisotropic layer wherein the molecular orientation and aggregation can have large impact on the physicochemical properties of the film. Different surface phases with different molecular orientations as function of surface density and temperature can be obtained by forming the Langmuir monolayer of the DLC molecules at the air–water interface. The Langmuir monolayer in a particular phase can be deposited onto the active area of a device layer-by-layer by employing a highly controlled Langmuir–Blodgett technique. Here, we report some interesting results related on molecular orientation of the DLC molecules at different interfaces. Such aggregation of DLC molecules in ultrathin films may find applications in thin film-based electro-optical devices.     

不同稀释剂中N_(1923)的界面性质

研究了伯胺 N192 3在不同稀释剂 /0 .1 mol/L(H,Na) NO3(p H=2 .3 4)体系中的界面性质 ,计算了界面吸附特性参数 cmin和 AI.N192 3在不同稀释剂体系中的界面活性顺序为 :正庚烷 >环己烷 >苯 >甲苯 >四氯化碳 >氯仿 >醋酸异戊酯 >甲基异丁基酮 (MIBK) .这一结果可能与稀释剂和萃取剂、稀释剂和有机相中增溶水及萃取剂和界面层水分子之间的相互作用有关 .对有关结果及其与萃取性能的关系做了分析和讨论     

Structural characterization of interfacial n-octanol and 3-octanol using molecular dynamic simulations

Structurally isomeric octanol interfacial systems, water/vapor, 3-octanol/vapor, n-octanol/vapor, 3-octanol/water, and n-octanol/water are investigated at 298 K using molecular dynamics simulation techniques. The present study is intended to investigate strongly associated liquid/liquid interfaces and probe the atomistic structure of these interfaces. The octanol and water molecules were initially placed randomly into a box and were equilibrated using constant pressure techniques to minimize bias within the initial conditions as well as to fully sample the structural conformations of the interface. An interface formed via phase separation during equilibration and resulted in a slab geometry with a molecularly sharp interface. However, some water molecules remained within the octanol phase with a mole fraction of 0.12 after equilibration. The resulting "wet" octanol interfaces were analyzed using density profiles and orientational order parameters. Our results support the hypothesis of an ordered interface only 1 or 2 molecular layers deep before bulk properties are reached for both the 3-octanol and water systems. However, in contrast to most other interfacial systems studied by molecular dynamics simulations, the n-octanol interface extends for several molecular layers. The octanol hydroxyl groups form a hydrogen-bonding network with water which orders the surface molecules toward a preferred direction and produces a hydrophilic/hydrophobic layering. The ordered n-octanol produces an oscillating low-high density of oxygen atoms out of phase with a high-low density of carbon atoms, consistent with an oscillating dielectric. In contrast, the isomeric 3-octanol has only a single carbon-rich layer directly proximal to the interface, which is a result of the different molecular topology. Both 3-octanol and n-octanol roughen the water interface with respect to the water/vapor interface. The "wet" octanol phases, in the octanol/water systems reach bulk properties in a shorter distance than the "dry" octanol/vapor interfaces.     

烷基苯磺酸盐在油水界面行为的介观模拟

采用耗散颗粒动力学(DPD)方法在介观层次上模拟了表面活性剂烷基苯磺酸盐在油/水界面的排布行为, 考察了分子结构、浓度、盐度、油相等因素对表面活性剂界面密度和界面效率的影响, 并探讨了利用表面活性剂复配协同效应提高界面活性的理论机制. 分子模拟给出的分子水平的微观信息为强化采油技术中配方筛选和表面活性剂的有效应用提供指导.     

十二烷基苯磺酸钠在SiO2表面聚集的分子动力学模拟

采用分子动力学方法研究了阴离子表面活性剂十二烷基苯磺酸钠(SDBS)在无定形SiO2固体表面的吸附. 设置不同的水层厚度, 观察固液界面和气液界面吸附的差异. 模拟发现表面活性剂分子能够在短时间内吸附到SiO2表面, 受碳链和固体表面之间相互作用的影响形成表面活性剂分子层, 并依据吸附量的大小形成不同的聚集结构; 在水层足够厚的情况下, 由于有较多的表面活性剂分子吸附在固体表面,从而形成带有疏水核心的半胶束结构; 计算得到的成对势表明极性头与钠离子或水分子之间的结合或解离与二者之间的能垒有关, 解离能垒远大于结合能垒, 引起更多Na+聚集在极性头周围而只有少数Na+存在于溶液中; 无论气液还是固液界面, 极性头均伸向水相, 与水分子形成不同类型的氢键. 模拟表明, 分子动力学方法可以作为实验的一种补充, 为实验提供必要的微观结构信息.     

Lipids and lipid mixtures in boundary layers: From hydration lubrication to osteoarthritis

The hydration layer surrounding the phosphocholine headgroups of single-component phosphatidylcholine lipids, or of lipid-mixtures, assembled at an interface greatly modifies the interfacial properties and interactions. As water molecules within the hydration layer are held tightly by the headgroup but are nonetheless very fluid on shear, the boundary lipid layers, exposing the highly hydrated headgroup arrays, can provide efficient boundary lubrication when sliding against an opposing surface, at physiologically high contact pressures. In addition, any free lipids in the surrounding liquid can heal defects which may form during sliding on the boundary phosphatidylcholine layer. Similar boundary lipid layers contribute to the lubricating, pressure-bearing, and wear-protection functions of healthy articular joints. This review presents a survey of the relationship between the molecular composition of the interfacial complex and the lubrication behavior of the lipid-based boundary layers, which could be beneficial for designing boundary lubricants for intra-articular injection for the treatment of early osteoarthritis.     

Molecular Dynamics Simulations of Sulfobetaine-Type Zwitterionic Surfactant at the Decane/Water Interface

Automated Topology Builder (ATB) and GAMESS (US) were used to build the model of a new sulfobetaine-type zwitterionic surfactant. And a serious of molecular dynamics simulations of the new sulfobetaine-type zwitterionic surfactant were performed at the decane/water interface by GROMACS, the influence of surfactant structure to the interfacial properties was investigated. The results show that the surfactant molecules can adsorb at the decane/water interface closely and reduce the interfacial tension significantly between decane and water. In another word, the model of the sulfobetaine-type zwitterionic surfactant is correct. The minimum interfacial tension could reach up to 1.6 mN · m^?1 when the number of surfactants was up to 134, which corresponds to the critical micelle concentration and consistent with the experimental values of the system.     

Effects of Charged Surfactants on Interfacial Water Structure and Macroscopic Properties of the Air-Water Interface

Surfactants are used to control the macroscopic properties of the air-water interface. However, the link between the surfactant molecular structure and the macroscopic properties remains unclear. Using sum-frequency generation spectroscopy and molecular dynamics simulations, two ionic surfactants (dodecyl trimethylammonium bromide, DTAB, and sodium dodecyl sulphate, SDS) with the same carbon chain lengths and charge magnitude (but different signs) of head groups interact and reorient interfacial water molecules differently. DTAB forms a thicker but sparser interfacial layer than SDS. It is due to the deep penetration into the adsorption zone of Br⁻ counterions compared to smaller Na⁺ ones, and also due to the flip-flop orientation of water molecules. SDS alters two distinctive interfacial water layers into a layer where H⁺ points to the air, forming strong hydrogen bonding with the sulphate headgroup. In contrast, only weaker dipole-dipole interactions with the DTAB headgroup are formed as they reorient water molecules with H⁺ point down to the aqueous phase. Hence, with more molecules adsorbed at the interface, SDS builds up a higher interfacial pressure than DTAB, producing lower surface tension and higher foam stability at a similar bulk concentration. Our findings offer improved knowledge for understanding various processes in the industry and nature.     

Effect of nanoparticles on the interfacial properties of liquid/liquid and liquid/air surface layers

An investigation is reported on the interfacial properties of nanometric colloidal silica dispersions in the presence of a cationic surfactant. These properties are the result of different phenomena such as the particle attachment at the interface and the surfactant adsorption at the liquid and at the particle interfaces. Since the latter strongly influences the hydrophobicity/lipophilicity of the particle, i.e., the particle affinity for the fluid interfacial environment, all those phenomena are closely correlated. The equilibrium and dynamic interfacial tensions of the liquid/air and liquid/oil interfaces have been measured as a function of the surfactant and particle concentration. The interfacial rheology of the same systems has been also investigated by measuring the dilational viscoelasticity as a function of the area perturbation frequency. These results are then crossed with the values of the surfactant adsorption on the silica particles, indirectly estimated through experiments based on the centrifugation of the dispersions. In this way it has been possible to point out the mechanisms determining the observed kinetic and equilibrium features. In particular, an important role in the mixed particle-surfactant layer reorganization is played by the Brownian transport of particles from the bulk to the interface and by the surfactant redistribution between the particle and fluid interface.     

Solvent Extraction: Structure of the Liquid–Liquid Interface Containing a Diamide Ligand

Knowledge of the (supra)molecular structure of an interface that contains amphiphilic ligand molecules is necessary for a full understanding of ion transfer during solvent extraction. Even if molecular dynamics already yield some insight in the molecular configurations in solution, hardly any experimental data giving access to distributions of both extractant molecules and ions at the liquid–liquid interface exist. Here, the combined application of X‐ray and neutron reflectivity measurements represents a key milestone in the deduction of the interfacial structure and potential with respect to two different lipophilic ligands. Indeed, we show for the first time that hard trivalent cations can be repelled or attracted by the extractant‐enriched interface according to the nature of the ligand.     

Molecular organization in binary mixtures of derivatives of naphthalenebicarboxylic acid and naphthoylenebenzimidazole with a liquid crystal in two-dimensional layers

Langmuir films of some dichroic dyes, namely derivatives of naphthalenebicarboxylic acid and derivatives of naphthoylenebenzimidazole, as well as of their mixtures with the liquid crystals 4-octyl-4′-cyanobiphenyl (8CB) and 4-pentyl-4″-cyano-p-terphenyl (5CT) were prepared. Surface pressure/mean molecular area isotherms were recorded from which some information about the alignment of molecules in a monomolecular layer at an air–water interface could be deduced. It was found that the properties of the monolayer are highly sensitive to the molecular structure of the side groups substituted on the main skeleton of the dye molecule, and to the mixture composition. Moreover, information about the miscibility or the phase separation of the two components in Langmuir films formed from dye/liquid crystal mixtures was obtained by using the excess area criterion and surface pressure rules.