A method for correcting the contact angle from the θ/2 method

The θ/2 method, a widely used technique on measuring the contact angle of a sessile drop, assumes that the drop profile is part of a sphere. However, the shape profile of a sessile drop is governed by the Young–Laplace equation and is different from a sphere, especially for drops with a large bound number (e.g. large volume or small surface tension). The spherical assumption, therefore, causes errors on evaluating the contact angles. The deviation of contact angle from the θ/2 method is evaluated from a theoretical calculation in this work. A simple means is given for correcting the measurement error. The corrected angle results from the drop volume, surface tension, liquid density and the contact angle from θ/2 method. An algorithm for finding the correct contact angle without knowing the density and surface tension is also given. At the end, two examples of pendant drops are given for the illustration.     

Effect of pressure on the adsorption and micelle formation of aqueous dodecyltrimethylammonium chloride solution-hexane system

The interfacial tension of aqueous dodecyltrimethylammonium chloride (DTAC) solution/hexane interface has been measured as a function of pressure at concentrations around the critical micelle concentration (CMC). The derivative of the interfacial tension with respect to pressure has been observed to change abruptly at the CMC as in the case of aqueous dodecylammonium chloride (DAC) solution-hexane system studied already. The volume changes associated with the adsorption of DTAC from its monomeric and micellar states have been calculated. The results have indicated that the micelle formation of surfactant is treated like the appearance of a new macroscopic phase in the system. It has also been concluded that the micelle solubilizing hexane bears resemblance to the adsorbed film in terms of the volume. The difference in the pressure dependence of the volume of micelle formation ^MW between DTAC and DAC has been attributed to a larger polar group of DTAC. The fact that the value of ^MW is larger than that estimated from the conductivity data has been explained by the solubilization of hexane into the micelle.     

钛酸钾晶须界面性质研究

通过乳液聚合的方法对钛酸钾晶须进行表面改性,获得了表面包裹聚甲基丙烯酸甲酯的钛酸钾晶须. FT-IR、SEM、EDS的表征表明,钛酸钾晶须的表面包裹了聚甲基丙烯酸甲酯膜,形成了以钛酸钾晶须为核,聚甲基丙烯酸甲酯为壳的复合粒子.研究表明,改性后的晶须表面能降低,在有机溶液中的界面张力大大降低,与聚甲基丙烯酸甲酯的界面性质相似,说明改性后的晶须基本被聚甲基丙烯酸甲酯完全包裹,因此晶须由亲水性变为疏水性,在有机溶液中的分散性显著提高.     

Homogeneous crystallization of the Lennard-Jones liquid. Structural analysis based on Delaunay simplices method

The crystallization process of a simple liquid upon slow cooling has been modeled by the Monte-Carlo method. The model contains 10,000 Lennard-Jones atoms in the model box with periodic boundary conditions. The model structure is investigated at different stages of crystallization using Delaunay simplices. The simplex belonging to one or another particular crystal structure was determined by the shape of the given simplex taking into account the shape of its neighboring simplices. Simplices typical of the fcc and hcp crystal structures, as well as of polytetrahedral aggregates, not typical of crystals, were studied. The analysis has shown that the “precursors” of a hcp structure are strongly dominating over the “precursors” of a fcc structure in liquid phase before the beginning of crystallization. When crystallization starts, small embryos of the fcc structure are observed; the simplices peculiar to hcp are present at that in great amount, but they are distributed over the sample more uniformly. As crystallization proceeds, the portion of the fcc phase grows faster than hcp. However, no unified crystal appears in our case of slow cooling of the model. A complex polycrystalline structure containing crystalline regions with multiple twinning, pentagonal prisms and elements of icosahedral structures arises instead.     

Action of α-cyclodextrin on phospholipid assemblies

Interactions of α-cyclodextrin (α-CD) with dimyristoylphosphatidylcholine (DMPC) and Egg phosphatidylcholine (Egg-PC) were studied (i) by analyzing surface pressure-area isotherms and surface tension of phospholipid monolayers formed at the interface between air and α-CD aqueous solutions and (ii) by X-ray diffraction performed on fully hydrated α-CD/phospholipid binary mixtures. The cyclodextrin molecules strongly interact with the two-dimension phospholipid assembly. Their addition into the aqueous sub-phase leads to the removal of part of the phospholipids from the air-water interface: the higher the α-CD concentration, the higher the phospholipid depletion. This should preferentially involve interactions between cyclodextrin and the phosphatidylcholine head group as α-CD is water-soluble and not surface-active. At the three-dimension level, the bilayer packing of the phospholipid lamellar phase appears not affected by the presence of cyclodextrin as shown by X-ray scattering at small angles whereas wide-angle diffraction patterns reveal the formation of a crystalline phase organized in a pseudo-hexagonal lattice usually characteristic of α-CD dimers. These results point out that α-CD should interact with bilayer-forming phospholipid molecules but likely according to a process that would preserve intact at least a part of the multilamellar assembly.     

A simple adsorption model for ionic surfactants

In the past, few theoretical attempts have been made to describe quantitatively the adsorption of ionic surfactants at liquid interfaces. Well-known adsorption isotherms due to Frumkin or Hill–de Boer cannot respond to the specific electrostatic and geometric properties of the surfactant molecules. Our approach is based on a combination of the Gouy–Chapman theory with a modified Frumkin isotherm. The modification implies that the system is free to choose an optimal head group area and an optimal arrangement of the surfactant molecules in the interface as a function of bulk concentration. Interaction energies between neighbouring adsorbed surfactant molecules and between surfactant and water molecules are taken into consideration. The minimum of the Gibbs free energy of the system is equivalent to a minimal interfacial tension. Thus, the thermodynamically stable isotherm can be obtained as the lower envelope of the family of σ versus ln c isotherms resulting from different choices of the model parameters, including the area per molecule. According to the Gibbs equation, the Γ versus ln c adsorption isotherm is obtained as the derivative of this envelope. By variation of the model parameters, the envelope of the calculated adsorption isotherms can be fitted to experimental data of the interfacial tension versus bulk concentration. A computer program is used to calculate the σ versus c and the Γ versus ln c curves as well as to fit the parameters. Received: 28 October 1999/Accepted: 8 February 2000     

Partial Molar Volumes of Amino Acids and Peptides in Aqueous Salt Solutions at 25°C and a Correlation with Stability of Proteins in the Presence of Salts

Partial molar volumes for a homologous series of amino acids and peptides have been measured in aqueous 1M sodium acetate, sodium thiocyanate, and sodium sulfate at 25^°C. These data have been utilized in conjunction with the data in water to deduce partial molar volumes of transfer V _2,m ⁰(tr) from water to these aqueous salt solutions. The volumes of transfer for the amino acids and peptides are found to be positive. The interpretation is that this result arises from the dominant interaction of the sodium salts with the charged centers of amino acids and peptides. Thermal denaturation of the structurally homologous proteins lysozyme and -lactalbumin has been studied in the presence of these salts. Significant thermal stabilization of hen egg-white lysozyme has been observed in the presence of sodium acetate and sodium sulfate. However, the thermal stabilization observed for -lactalbumin is very small in the presence of these salts and sodium thiocyanate leads to a lowering of its thermal denaturation temperature. The rise in the surface tension of aqueous salt solutions with salt concentration has been correlated with the calorimetric and volumetric measurements. The results show that V _2,m ⁰(tr) depends less on the type of electrolyte than on the ionic strength of the solution. The V _2,m ⁰(tr) values correlate very well with the increase in the surface tension of aqueous salt solutions, indicating significant role of surface tension in interactions of amino acids, peptides, or protein with the salts.     

Studies on the properties of ionic liquid EMIInCl4

This paper reports that an ionic liquid (IL) has been prepared by directly mixing InCl₃ and 1-methyl-3-ethylimidazolium chloride (EMIC) with molar ratio 1/1 under dry argon atmosphere. The densities, and surface tension of the pure IL were determined at temperature range of 293.15 to 343.15 ± 0.1 K. The volumetric properties and the properties of surface for ionic liquid based on group III were discussed by Glasser's theory and Yang's interstice model.     

Surface free energies of styrene-acrylic acid copolymer plates prepared in various frames

Styrene (St)-acrylic acid (AA) copolymer plates were synthesized in the frames made of glass, aluminum (Al) and poly(difluoroethylene) (PDFE). Surface properties of the prepared copolymer plates were characterized with regard to critical surface tension ( _C ), chemical components by IR analysis and surface energies (total energy, _S ; dispersion force components, _S ^d ; polar component, _S ^p ; hydrogen bonding component, _S ^h ) and the following results were obtained.The _C values of the copolymer plates increased with AA content and also depended on the sort of the used frame. The increasing order of the _C values of the copolymer plates corresponded to those of the used frames, namely, PDFE frame < Al frame < Glass frame.The prepared copolymer plates with low AA contents (ca. 10 mol%) were enriched in the AA moiety in the surface layer regardless of the kind of the used frame.The total and the individual components of the surface free energies of the copolymer plates were largely affected by the property of the used frame. The glass frame gave the plate with higher _S ^h values suggesting preferential orientation of the polar site of AA component.     

Interfacial activity of a novel family of polymeric surfactants

A novel series of polymeric surfactants based on carboxy methyl cellulose and alkyl poly(etheroxy) acrylate were synthesized by ultrasonic irradiation. These polymeric surfactants have exhibit excellent surface activity due to their unique structure. The influences of salt, alcohol and alkali on the interfacial activity of these polymeric surfactants were studied by interfacial tensiometery, dynamic laser scattering (DLS), UV spectroscope and environmental scanning electrical microscope (ESEM). The surface tension and interfacial tension (IFT) properties change little with NaCl added. The formed micelles shrink, their size becomes smaller. Alcohols cause the IFT to decrease a little because a small amount of free chains present in solution. Under the influence of added alkali, the IFT of the polymeric surfactants, in aqueous solution, decreases so much that sometimes it is less than 10⁻² mN/m. Using data from the equivalent alkane scan, one cannot draw the conclusion that the action of alkali with the acidic components in crude oil leads to the ultra-low IFT. The analyses by UV, DLS and ESEM show that the micelles formed by polymeric surfactants could be disaggregated or destroyed sharply by the action of alkali. So the size of micelles decreases greatly and the number of free chains increases. That more polymeric surfactants molecules move to the interface of oil/water and rearrange at the interface of oil/water is believed to be the main reason of the ultra-low IFT (10⁻³ mN/m) that is obtained.