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.     

STUDY OF DEMULSIFICATION OF WATER-IN-CRUDE OIL EMULSION

Demulsification of water-in-crude oil emulsion was studied at two different salinities, 0.5% and 10% sodium chloride, using five different nonionic surfactants. Equilibrium crude oil-water interfacial tension was measured with drop volume method. Low molecular weight surfactants were found to be completely ineffective as demulsifiers. Three surfactants which were effective demulsifiers, exhibited good interfacial activity, surface adsorption and surface pressure. The performance of the demulsifiers changed with change in salinity of aqueous phase. Surfactants effective as demulsifiers reduced surface tension of water by more than 25 dynes-cm^-1. For a given crude oil-water system, the surfactant which developed surface pressure in excess of 15 dynes-cm^-1 was found to be good demulsifier for that system. Based upon these studies, a physical model of demulsification has been proposed     

正负离子混合表面活性剂双水相界面张力的研究

用旋转滴法测定了正负离子混合表面活性剂形成的双水相界面张力, 研究了双水相界面张力与表面活性剂的分子结构、正负离子表面活性剂的摩尔比、总浓度、外加无机盐及温度的关系. 结果表明, 双水相界面张力在一定正、负离子表面活性剂的摩尔比时属于超低界面张力范围. 观察到三种界面张力曲线类型, 第一类为摩尔比1:1 的两边的两条曲线, 界面张力随过剩表面活性剂组分的比例增加而降低; 第二类为一条跨过摩尔比1:1的马鞍型曲线; 第三类为位于摩尔比1:1的一边的一条马鞍型曲线. 界面张力曲线的类型主要取决于表面活性剂的分子结构, 包括亲水基类型、疏水链长度及对称性.     

餐厨废油制备的生物基两性表面活性剂的油水界面性能

以餐厨废油制备了生物基两性表面活性剂,应用界面张力和动态光散射方法,研究了该生物基两性表面活性剂体系的油水界面性能及在溶液中的聚集行为。 在无外加碱条件下,由餐厨废油制备的表面活性剂表现出良好的界面性能,在50~70 ℃以及pH值为7~12的条件下,均可以将油水界面张力降至超低值(<10^-3 mN/m),在不同的油藏模拟地层水中均保持较好的界面活性;分别在50、-20和4 ℃下保存,其界面活性均未受到明显影响。 在水溶液中形成的聚集体的平均流体力学半径为10~30 nm,无机盐离子的加入可使聚集体的粒径上升。 基于其优良的界面性质和可再生来源,由餐厨废油制备的生物基两性表面活性剂在三次采油方面具有重要的应用价值。     

Interfacial characteristics of food emulsifiers (proteins and lipids) at the air-water interface

Factors affecting the interfacial characteristics (structure, stability, interfacial rheology, molecular diffusion, and rate of film formation) of food emulsifiers (polar lipids and proteins) at the air-aqueous phase interface are reviewed. The effect of interfacial and aqueous phase (water, and aqueous solutions of ethanol, glycerol, sugars, electrolytes, and pH) compositions have been analyzed as variables. Many measurement methods—such as tensiometry (Wilhelmy plate and pendant drop methods), and Langmuir- and Wilhelmy-type film balances—have been used in the experiments. The effect of the interfacial, aqueous phase composition, and operational conditions (surface density, surface pressure, and temperature) of food emulsifiers (lipids and proteins) at the air-aqueous phase interface are discussed.     

生物表面活性剂胆汁盐胶束化及相行为

生物表面活性剂胆汁盐类以其在生物体内重要的生理功能及不同于传统表面活性剂独特的结构特点在众多领域引起了广泛的关注。本文对国内外关于胆汁盐类表面活性剂的胶束结构、表面吸附行为及胆汁盐在水溶液中的聚集体行为的研究成果进行了综述。同时,对胆汁盐与传统表面活性剂、双尾表面活性剂及天然脂类表面活性剂的相互作用以及聚集体的形成、胆汁盐诱导聚集体结构转变等方面的研究成果也进行了综述。     

Interfacial tension of the aqueous two-phase systems of cationic-anionic surfactant mixtures

Interfacial tensions of the aqueous two-phase systems formed by cationic-anionic surfactant mixtures were measured using spinning drop method. The effects of surfactant structure, molar ratio of cationic to anionic surfactants, surfactant concentration, salt, and temperature on the interfacial tensions were investigated. It was shown that the values of the interfacial tensions of the aqueous two-phase were in the scale of ultra-low interfacial tensions at certain molar ratios of cationic to anionic surfactants. Three types of interfacial tension curves were observed. The first curve comprised two curves that were located on either side of 1:1 molar ratio, and the interfacial tension decreased with the increase of excessive surfactant components. The second one was a saddle-shaped curve that strode over the 1:1 molar ratio. The third type was a saddle-shaped curve that was located beside the 1:1 molar ratio. The types of interfacial tensions depended on the molecular structure of the surfactants such as the hydrophilic groups and the lengths and symmetry of hydrophobic chains.     

Interactions between Hydrolysable Tannins and Lipid Vesicles from Escherichia coli with Isothermal Titration Calorimetry

Isothermal titration calorimetry (ITC) was used to study the interactions between hydrolysable tannins (HTs) and lipid vesicles prepared from a phospholipid extract of Escherichia coli (E. coli). A group of 24 structurally different HTs was selected, and structural differences affecting their affinities to interact with lipid vesicles in aqueous buffered media were identified. In general, the interactions between HTs and lipid vesicles were exothermic in nature, and ITC as a technique functioned well in the screening of HTs for their affinity for lipids. Most notably, the galloyl moiety, the structural flexibility of the entire tannin structure, the hydrophobicity of the tannin, and higher molecular weight were observed to be important for the stronger interactions with the lipids. The strongest interactions with lipids were observed for rugosins D and G. It was also observed that some HTs with moderate hydrophobicities, such as geraniin, chebulagic acid, and chebulinic acid, did not have any detectable interactions with the lipid vesicles, suggesting that a hydrophobic structure alone does not guarantee an affinity for lipids.     

Photosensitization mechanisms at the air–water interface of aqueous aerosols

Photosensitization reactions are believed to provide a key contribution to the overall oxidation chemistry of the Earth''s atmosphere. Generally, these processes take place on the surface of aqueous aerosols, where organic surfactants accumulate and react, either directly or indirectly, with the activated photosensitizer. However, the mechanisms involved in these important interfacial phenomena are still poorly known. This work sheds light on the reaction mechanisms of the photosensitizer imidazole-2-carboxaldehyde through ab initio (QM/MM) molecular dynamics simulations and high-level ab initio calculations. The nature of the lowest excited states of the system (singlets and triplets) is described in detail for the first time in the gas phase, in bulk water, and at the air–water interface, and possible intersystem crossing mechanisms leading to the reactive triplet state are analyzed. Moreover, the reactive triplet state is shown to be unstable at the air–water surface in a pure water aerosol. The combination of this finding with the results obtained for simple surfactant-photosensitizer models, together with experimental data from the literature, suggests that photosensitization reactions assisted by imidazole-2-carboxaldehyde at the surface of aqueous droplets can only occur in the presence of surfactant species, such as fatty acids, that stabilize the photoactivated triplet at the interface. These findings should help the interpretation of field measurements and the design of new laboratory experiments to better understand atmospheric photosensitization processes.

First-principles molecular dynamics simulations of imidazole-2-carboxaldehyde at the air–water interface highlight the role of surfactants in stabilising the reactive triplet state involved in photosensitisation reactions in aqueous aerosols.     

The array and interfacial activity of sodium dodecyl benzene sulfonate and sodium oleate at the oil/water interface

There is a close correlation between the interfacial activity and the adsorption of the surfactant at the interface, but the detailed molecular standard information was scarce. The interfacial activity of two traditional anionic surfactants sodium dodecyl benzene sulfonate (SDBS) and sodium oleate (OAS) were studied by experimental and computer simulation methods. With the spinning drop method and the suspension drop method, the interfacial tension of oil/aqueous surfactant systems was measured, and the influence of surfactant concentration and salinity on the interfacial tension was investigated. The dissipative particle dynamics (DPD) method was used to simulate the adsorption of SDBS and OAS at the oil/water interface. It was shown that it is beneficial to decrease interfacial tension if the hydrophobic chains of the surfactant and the oil have similar structure. The accession of inorganic salts causes surfactant molecules to form more compact and ordered arrangements and helps to decrease the interfacial tension. There is an osculation relation between interfacial density and interfacial activity. The interfacial density calculated by molecular simulation is an effective parameter to exhibit the interfacial activity.     

Responsive Aqueous Foams

Remarkable properties have emerged recently for aqueous foams, including ultrastability and responsiveness. Responsive aqueous foams refer to foams for which the stability can be switched between stable and unstable states with a change in environment or with external stimuli. Responsive foams have been obtained from various foam stabilizers, such as surfactants, proteins, polymers, and particles, and with various stimuli. Different strategies have been developed to design this type of soft material. We briefly review the two main approaches used to obtain responsive foams. The first approach is based on the responsiveness of the interfacial layer surrounding the gas bubbles, which leads to responsive foams. The second approach is based on modifications that occur in the aqueous phase inside the foam liquid channels to tune the foam stability. We will highlight the most sophisticated approaches, which use light, temperature, and magnetic fields and lead to switchable foam stability.     

Adhesive transitions in Newton black films: a computer simulation study

We report molecular dynamics simulations of Newton black films (NBFs), ultra thin films of aqueous solutions stabilized with two monolayers of ionic surfactants, sodium dodecyl sulfate. We show that at low water content conditions and areas per surfactant corresponding to experimental estimates in NBFs, homogeneous films undergo an adhesion "transition," which results in a very thin adhesive film coexisting with a thicker film. We identify the adhesive film with the equilibrium structure of the Newton black film. We provide here a direct microscopic view of the formation of these important structures, which have been observed in experimental studies of emulsions and foams. We also report a detailed investigation of the structural properties and interfacial fluctuation spectrum of the adhesive film. Our analysis relies on the definition of an "intrinsic surface," which is used to remove the averaging effect that the capillary waves have on the film properties.     

Polyzwitterionic brushes: Extreme lubrication by design

Polymers offer the advantage that they may independently combine desirable supramolecular structure with useful local monomeric properties to yield optimal performance of different tasks. Here we utilise the remarkable lubricating properties both of dense polymer brushes, and of hydration sheaths about charges via the emerging paradigm of hydration lubrication, to design a grafted-from polyzwitterionic brush system, where each of the monomers has a structure similar to the highly-hydrated phosphorylcholine headgroups of phosphatidylcholine lipids. Such polyzwitterions are grown from a macroinitiator coating the substrate (mica) surface using atom transfer radical polymerisation (ATRP) of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) to form exceptionally robust poly(MPC) brushes. We have characterized these brush layers via X-ray reflectometry, X-ray photoelectron spectroscopy, surface forces measurements and atomic force microscopy. Such brushes, designed to optimise their lubrication properties, are indeed found to provide state of the art boundary lubrication, achieving friction coefficients as low as 0.0004 at pressures up to 75 atmospheres over a wide range of sliding velocities. Such low friction is comparable with that of articular cartilage in healthy mammalian joints, which represents nature’s benchmark for boundary lubrication in living organisms, and suggests that hydration lubrication plays a major role in reducing friction in biological systems.     

Enzyme activity and stability control by amphiphilic self-organizing systems in aqueous solutions

The interaction of surfactants with proteins in aqueous solutions has been the subject of many investigations to understand the interactions between membrane proteins and lipids, structurally similar to synthetic surfactants. The effect of surfactant on enzyme structure and activity is the result of chemically selective interactions that may be influenced both by the enzyme structure and by the chemistry of the surfactant. For many years, surfactants have been considered as non-specific denaturants of proteins, even if in the literature several of them are reported to enhance activity and/or stability of some enzymes: the detergent can interact with the enzyme and cause a conformational change to a more active form and/or stabilize its native folded structure. Although the surfactant head group seems to have a determining role, other structural features of the detergent are also important in influencing the catalytic properties of an enzyme, i.e. head group size and its hydrophobic/hydrophilic balance. Up to now it is very difficult to predict the molecular features of the surfactant and an extensive investigation on the relationship between the surfactant chemical structure and the catalytic properties of enzyme is still required.     

Electron density matching as a guide to surfactant design

The effectiveness at reducing interfacial tension between water and different organic solvents was studied, with 14 structurally different dichain sulfosuccinate surfactants. Variations in chemical structure ranged from linear/branched alkyl tail groups, to phenyl-tipped tail units, to partially and fully fluorinated tails. The solvents n-heptane, toluene, and perfluoroheptane were used as example oil phases. Interfacial activity was measured in terms of a reduced interfacial tension scale, R(IFT), based on the value in the presence of surfactants compared to that for the pure solvent-water interface. Overall surfactant chain structure was determined to be the key factor affecting R(IFT). Furthermore, a strong correlation was observed between R(IFT) and the electron density rho(e) of the different surfactants: with any given oil, the most effective surfactants have rho(e) values closest to that for the solvent. For example, phenyl-tipped surfactants were shown to be comparatively more effective at the interface with an aromatic solvent (toluene) than with an aliphatic n-alkane (heptane). Furthermore, fluorination of the tail groups decreased effectiveness at the hydrocarbon/water interface, which was substantially increased at the fluorocarbon/water interface: this too followed the electron density-matching pattern. The importance of chain-tip chemical structure was also noted, with regard to the introduction of phenyl, CF3-, and H-CF2- terminal moieties. For branched alkyl-tailed surfactants, it was found that effectiveness could be linked to an empirical "branching factor". The significance of the electron density matching of organic solvent and surfactant for the prediction of interfacial activities is highlighted, and this concept may prove useful for the future design of new high-efficiency surfactants.     

Surface activity and redox behavior of nonionic surfactants containing an anthraquinone group as the redox-active site

Two new nonionic surfactants, α-anthraquinonyloxyhexyl-ω-hydroxy-oligo(ethylene oxide) (ACPEG) and α-anthraquinonyl-ω-hydroxy-oligo(ethylene oxide) (APEG), were synthesized. The fundamental interfacial behavior of these surfactants at the air/water interface has been investigated by means of surface tensiometry to provide an insight into the relationship between the structure of the hydrophobic moiety and the surfactant properties, with specific emphasis on the anthraquinone linkage. Aggregation numbers of the surfactants have been determined from static light scattering measurements. At a constant hydration of the ethylene oxide chain, the increase in the hydrophobic chain length in ACPEG raises its hydrophobic interaction and results in enhanced aggregation and significant variation in the interfacial and micellization properties compared to APEG. The electrochemical behavior of the surfactants has also been studied in 0.16 M NaCl aqueous solutions and in 0.1 M tetrabutylammonium perchlorate acetonitrile solutions at the interface of a glassy carbon electrode. A difference in the extent of aggregation has a pronounced effect on the cyclic voltammetric behavior of the surfactants in aqueous solution. In organic media, on the other hand, the redox process depends only on the molecular geometry of the monomeric species. A comparison of the electrochemical responses in the two phases has been made to explain the distinctive features of the redox properties of the surfactants. Received: 30 May 1999 Accepted in revised form: 29 June 1999     

Mucin from loach skin mucus and its interfacial behavior on gold surface

Loach skin mucin was isolated from loach skin mucus and found to be similar to mammalian mucins in many aspects, i.e., low amino acid residue content, high molecular weight, presence of hydrophobic blocks and gel-forming characteristics in water. However, loach skin mucin can form a weak gel in water at a much lower concentration （3 mg/mL） than mammalian mucins, indicating its good hydrophilicity. Loach skin mucin can also form a stable adsorption layer on gold surface in aqueous environment, owing to the existence of hydrophobic blocks within mucin. The nature of high hydrophilicity and interfacial behavior give loach skin mucin potential as excellent material for use in solid-water interfaces for antifouling and lubrication, and should be crucial to the versatile functions of loach skin mucus.     

Effect of Aqueous Phase pH on the Dynamic Interfacial Tension of Acidic Crude Oils and Myristic Acid in Dodecane

The time dependence of the interfacial tension between water–acidic crude oil and water–synthetic oil was investigated for aqueous phase pHs ranging from 2 to 9 using the du Noüy ring method at 20°C. Myristic acid in dodecane was selected as a model (synthetic oil) for acidic crude oil containing indigenous surfactants, and the similarities and differences between the dynamic interfacial tension behaviours of the natural and synthetic crude oil systems were compared. The initial interfacial tension and the relaxation of the interfacial tension are sensitive to the aqueous phase pH for both systems. The adsorption kinetics of the indigenous surfactants and myristic acid could be well fitted with the monoexponential model, and the time constants obtained in this manner indicates that reorganization of the indigenous surfactants and myristic acid at the w/o interface are pH dependent. The experimental results also indicate that indigenous surfactants in acidic crude oil and myristic acid in dodecane have similar film formation behaviours at the w/o interface for the range of pHs investigated.     

Ellipsometry studies of nonionic surfactant adsorption at the oil--water interface

In the presented study we have developed and implemented a methodology for ellipsometry measurements at liquid interfaces that makes it possible to determine the amount adsorbed without assumptions of refractive index or thickness of the adsorbed layer. It was demonstrated that this is possible by combined measurements from different aqueous phases, H(2)O and D(2)O, which were shown to have sufficiently different refractive indices. The methodology was tested by studying adsorption of two types of nonionic poly(ethylene glycol) alkyl ether surfactants, C(n)H(2)(n)(+1)(OC(2)H(4))(m)OH or C(n)E(m) at the decane--aqueous interface, where C(12)E(5) was adsorbed from the oil phase and C(18)E(50) from the aqueous phase. The observed plateau values of the adsorbed amounts were 1.38 and 0.93 mg/m(2) for C(12)E(5) and C(18)E(50), respectively, which is in agreement with the corresponding values of 1.49 and 1.15 mg/m(2) obtained from applying the Gibbs equation to interfacial tension data for the same systems. We will briefly discuss the adsorption behavior in relation to the molecular structure of the surfactant and the phase behavior of the oil--surfactant--aqueous systems in relation to our experimental results.