Antihydrophobic solvent effects: an experimental probe for the hydrophobic contribution to enzyme-inhibitor binding

The hydrophobic component to the binding affinities of one acyclic phosphinate (4) and three macrocyclic phosphonamidate inhibitors (1-3) to the zinc peptidase thermolysin was probed by varying the solvent composition. Increasing the percentage of ethanol in the buffer solution over the range 0-9% increases the inhibition constants, K(i), by up to an order of magnitude. This approach represents an experimental method for distinguishing solvation from conformational or other effects on protein-ligand binding. The size of the "antihydrophobic effect" is correlated with the amount of hydrophobic surface area sequestered from solvent on association of the inhibitor and enzyme, although it is attenuated from that calculated from the surface tension of ethanol-water mixtures. The results are consistent with the Lum-Chandler-Weeks explanation for the size dependence of the hydrophobic effect.     

In situ Wilhelmy balance surface energy determination of poly(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene) during electrochemical doping-dedoping

Changes in the contact angle between conjugated polymers surface poly(3-hexylthiophene) [P3HT] and poly(3,4-ethylenedioxythiophene) (PEDOT) upon electrochemical doping-dedoping in aqueous electrolyte were determined in situ using a Wilhelmy plate tensiometer in an electrochemical cell. The hydrophobic P3HT was less hydrophobic in the oxidized state than in the neutral state; the more hydrophilic PEDOT was less hydrophilic in the oxidized state than when neutral. The tensiometry results were in good agreement with those measured by contact angle goniometry, and further corroborated by the capillary rise upon doping in a fluid cell with two parallel polymer coated plates, another in situ dynamic determination method. The contact angle changes depend on doping potential, electrolyte type, and concentration. We also deconvoluted the surface energy into components of van der Waals and acid-base interactions, using three probe liquids on the polymer surfaces, ex situ the electrochemical cell. The methods and the obtained results are relevant for the science and technology areas of printed electronics and electrochemical devices and for the understanding of surface energy modification by electrochemical doping.     

Ultrasonic evidence of hydrophobic interactions. Effect of ultrasound on benzoin condensation and some other reactions in aqueous ethanol

The kinetics of KCN-catalyzed benzoin condensation of benzaldehyde in water and ethanol-water binary mixtures was investigated both under ultrasound at 22 kHz and without sonication. Thermodynamic activation parameters were calculated from kinetic data obtained at 35, 50, and 65 degrees C. Evidence that ultrasound can retard reactions is reported and hence a direct proof that sonochemical processes occur in the bulk solution. Former results and literature data for ester hydrolyses and tert-butyl chloride solvolysis are involved in the discussion. A quantitative relationship between sonication effects and the hydrophobicity of reagents is presented for the first time. Ultrasound affects hydrophobic interactions with the solvent, which are not manifested in conventional kinetics. When it suppresses the stabilization of the encounter complexes between reagents, sonication hinders the reaction but accelerates it when it perturbs the hydrophobic stabilization of the ground state of a reagent.     

Partial Wetting in Capillary Liquid Absorption by Limestones

We report measurements of the capillary absorption (imbibition) of water, n-heptane, n-decane, n-dodecane, methanol, ethanol, propan-2-ol, and ethanol-water mixtures into a number of dry calcitic limestones. The data are analyzed on the basis of unsaturated flow theory to give values of sorptivity S for water and for each organic liquid at different temperatures. The results show that for the organic liquids S varies as (surface tension/viscosity)(1/2) but that for the water S is anomalously low, indicating partial wetting. The wettability of these limestones is discussed in relation to natural contamination of the pore surface. Copyright 2000 Academic Press.     

Preferential solvation of phenol in binary solvent mixtures. A molecular dynamics study

Molecular dynamics computer simulations were carried out to study the preferential solvation of phenol in equimolar acetonitrile-water and ethanol-water binary mixtures. Two water models were used to investigate the model dependence of preferential solvation. The results are compared to recent intermolecular 1H NOESY experiments reported on the same systems. In the case of acetonitrile-water the local mole fraction obtained from simulations agrees quite well with experiments. In the case of ethanol-water there was a qualitative difference, which was observed for both water models. However, when comparing the degree of preferential solvation of the two cosolvents ethanol and acetonitrile with each of the two water models, the trend obtained from the simulations agrees with experimental data.     

Definition and computation of intermolecular contact in liquids using additively weighted Voronoi tessellation

We present a definition of intermolecular surface contact by applying weighted Voronoi tessellations to configurations of various organic liquids and water obtained from molecular dynamics simulations. This definition of surface contact is used to link the COSMO-RS model and molecular dynamics simulations. We demonstrate that additively weighted tessellation is the superior tessellation type to define intermolecular surface contact. Furthermore, we fit a set of weights for the elements C, H, O, N, F, and S for this tessellation type to obtain optimal agreement between the models. We use these radii to successfully predict contact statistics for compounds that were excluded from the fit and mixtures. The observed agreement between contact statistics from COSMO-RS and molecular dynamics simulations confirms the capability of the presented method to describe intermolecular contact. Furthermore, we observe that increasing polarity of the surfaces of the examined molecules leads to weaker agreement in the contact statistics. This is especially pronounced for pure water.     

High performance liquid chromatography of glycoconjugates

Stationary phases were prepared for the analytical and micropreparative high performance affinity chromatography of carbohydrates and glycoproteins by covalent attachment of lectins and certain biogenic amines to the surface of microparticulate, macroporous diol-silica. Columns packed with immobilized concanavalin A or wheat germ agglutinin afforded rapid separation of both glycoproteins and low molecular weight carbohydrates. The combined use of the two lectins in a mixed bed column or in two columns in series by sequential elution with the appropriate haptenic sugars facilitated the separation of mixtures having a broad range of complex carbohydrates. Chromatographic experiments for the study of the interaction of certain biogenic amines with immobilized glycoproteins used as the stationary phase led to the development of siliceous bonded phases with serotonin, tryptamine or phenylpropanolamine ligates. The sorbents thus obtained were mildly hydrophobic and cationic in contact with eluents of pH 8 or less. They were used for the separation of various carbohydrates; in addition they also proved effective for the separation of protein mixtures under conditions similar to those customarily employed in electrostatic or hydrophobic interaction chromatography.     

Conductance of lithium,sodium and potassium nitrates in acetonitrile-water and ethanol-water isodielectric mixtures at 25°C

Precise conductance measurements of solutions of lithium, sodium and potassium nitrates were made at 25°C in acetonitrile-water and ethanol-water isodielectric mixtures, containing up to 15 mole percent organic-solvent, and the data analyzed by the 1978–80 Fuoss conductance equations. Single ion conductances were determined with the help of transference numbers in the case of ethanol-water mixtures. Ion-pair association constants are discussed in terms of contact and solvent-separated ion-pairs and were found to be almost equal to those for the halides. The anion appears more sensitive than the cations to the solvent structure in mixtures rich in water.     

Small-angle x-ray scattering measurement of a mist of ethanol nanodroplets: an approach to understanding ultrasonic separation of ethanol-water mixtures

Small-angle x-ray scattering measurements using a brilliant x-ray source revealed nanometer sized liquid droplets in a mist formed by ultrasonic atomization. Ultrasonic atomization of ethanol-water mixtures produced a combination of water-rich droplets of micrometer order and ethanol-rich droplets as small as 1 nm, which is 10(-3) times smaller than the predicted size. These sizes were also obtained for mists generated from the pure liquids. These results will help to clarify the mechanism of "ultrasonic ethanol separation," which has the potential to become an alternative to distillation.     

Adsorption of protein-surfactant complexes at the water/oil interface

Interfacial tension measurements have been performed at the water/hexane interface on mixtures of the bovine milk protein β-lactoglobulin and positively charged cationic surfactants (alkytrimethylammonium bromides). The addition of surfactants with different chain lengths leads to the formation of protein-surfactant complexes with different adsorption properties as compared to those of the single protein. In this study, the formation of complexes has been observed clearly for protein-long chain surfactant (TTAB and CTAB) mixtures, which has shown in addition to specific electrostatic interactions the relevance of hydrophobic interactions between surfactant molecules and the protein. The modeling of interfacial tension data by using a mixed adsorption model provides a quantitative understanding of the mixture behavior. Indeed, the value of the adsorption constant of the protein obtained in the presence of surfactants has strongly varied as compared to the single protein. Actually, this parameter which represents the affinity of the molecule for the interface is representative of the hydrophobic character of the compound and so of its surface activity. Even if a more hydrophobic and more surface active protein-surfactant complex has been formed, the replacement of this complex from the interface by surfactants close to their cmc was observed.     

甘氨酸在多元醇-水混合溶剂中的体积性质

利用精密数字密度计测定了甘氨酸分别在不同组成的乙二醇 水和丙三醇 水混合溶剂中的密度,计算了甘氨酸的表观摩尔体积、极限偏摩尔体积和理论水化数.根据结构水合作用模型讨论了迁移偏摩尔体积的变化规律,并与乙醇 水体系作了比较. 结果表明,甘氨酸分子在醇 水混合溶剂中增体积效应的大小与醇分子所含OH基数目的多少有关,但最直接也是最重要的影响因素是其水合壳层的结构形态. 乙醇 水体系中的增体积效应特别显著与该溶剂结构变化上的微观不均匀性和不连续性有关.     

Wettability of Nematic Liquid Crystals

Organic liquids such as nematic liquid crystals should wet solids with high surface energies like mica, yet they generally do not. In the model proposed here, the affinity to wet the solid in form of Hamaker forces is opposed by elastic effects due to nematic order. Results predict correctly that such liquids show small contact angles and the formation of ultrathin liquid films ahead of the bulk drops. Copyright 2001 Academic Press.     

ASPHALTENES ADSORPTION BY QUARTZ AND FELDSPAR

The adsorption of asphaltenes and resins from toluene solutions onto quartz and feldspar and the effect of this process on the properties of the mineral-aqueous solution interface have been investigated. Asphaltenes were adsorbed to a greater extent than resins, and the adsorption of mixtures was at least equivalente to the weighted average of the adsorption of both components separately.

The electrophoretic mobility of quartz or feldspar was not modified by the adsorption of asphaltenes or resins, indicating that the sites responsible for the surface charge of the minerals were unaffected by the presence of the adsorved organic species. The adsorption turned the minerals partially hydrophobic. This effect is more important for asphaltene covered particles which do not immerse in aqueous electrolyte solutions, indicating a contact angle larger than 90a immersion become spontaneous in liquid mixtures (methanol-water, ethanol-water), presenting a surface tension lower than 35 mNm-1.

Ethoxylated and ethoxylated-propoxylated surface active agents prevent the adsorption of asphaltenes and resins on the minerals, and this effect increases as the ethoxylated/propoxylated moiety of the surfactant increases.

The results indicate that fine particles with adsorbed polar fractions of oil play an important role in the stabilization of the water-in-oil emulsions formed in some secondary oil recovery processes and that ethoxylated/propoxylated surfactants and/or solvents may prevent the formation of these emulsions by modifying the wetting behaviour of the mineral particles.     

Hydrophobic solvation of Gay-Berne particles in modified water models

The solvation of large hydrophobic solutes, modeled as repulsive and attractive Gay-Berne oblate ellipsoids, is characterized in several modified water liquids using the SPC/E model as the reference water fluid. We find that small amounts of attraction between the Gay-Berne particle and any model fluid result in wetting of the hydrophobic surface. However, significant differences are found among the modified and SPC/E water models and the critical distances in which they dewet the hydrophobic surfaces of pairs of repulsive Gay-Berne particles. We find that the dewetting trends for repulsive Gay-Berne particles in the various model liquids correlate directly with their surface tensions, the widths of the interfaces they form, and the openness of their network structure. The largest critical separations are found in liquids with the smallest surface tensions and the broadest interfaces as measured by the Egelstaff-Widom length.     

Evaporation of microdroplets of ethanol-water mixtures on gold surfaces modified with self-assembled monolayers

The wetting property and evaporation behavior of ethanol-water mixtures of various concentrations on gold surfaces modified with 1-decanethiolate self-assembled monolayers (SAMs) were studied by digital contact angle analysis. It has been shown that the initial contact angle decreases monotonically with increased concentration of ethanol in the mixture. Evaporation studies revealed a general trend with a preliminary increase in contact angle accompanied with a decrease in contact area, then a constant contact angle accompanied with a slower, linear decrease in contact area. At the very beginning of the evaporation process, the contact angles showed a rapid decrease for the microdroplets of a binary mixture with equal volume fractions (i.e., 50% ethanol). Three distinct stages of the evaporation profile for the ethanol-water mixtures were observed, which differ from the inclusive "pinning" and "shrinking" behavior observed for the pure liquid case. Ultimately, the study makes possible the use of an evaporation profile to monitor the change in concentration of a binary system and allows a better understanding of the interactions between liquid microdroplets with solid substrates.     

Capillary rise of liquids over a microstructured solid surface

The location of the triple line as a function of time has been recorded for a series of organic liquids, with various surface tension to viscosity ratios, wicking upward a rough Cu(6)Sn(5)/Cu intermetallic (IMC) substrate. The complex topographical features of such an IMC rough surface are characterized by surface porosity and surface roughness. A theoretical model for wicking upward a rough surface has been established by treating the rough IMC surface as a two-dimensional porous medium featuring a network of open microtriangular grooves. The model is verified against experimental data. The study confirms that the kinetics of capillary rise of organic liquids in a nonreactive flow regime over a porous surface having arbitrary but uniformly distributed topographical features involves (i) surface topography metrics (i.e., permeability, tortuosity/porosity, and geometry of the microchannel cross section); (ii) wicking features (i.e., contact angle and filling factor); and (iii) physical properties of liquids (i.e., surface tension and viscosity). An excellent agreement between theoretical predictions and experimentally obtained data proves, for a selected filling factor η, validity of the analytically established model. Scaled data sets show that, for a given rough surface topography, (i) wicking kinetics of considered liquids depend on properties of liquids, that is, surface tension to viscosity ratios and contact angles; (ii) the filling factor for all tested liquids is an invariant, offering good prediction within the range of ~0.9-1.0. The distance of the wicking front versus square root of time relationship was well established throughout the whole considered wicking evolution time.     

Control and applications of immiscible liquids in microchannels

Photolithography was used in combination with photocleavable self-assembled monolayers to pattern surface free energies inside microchannels enabling the control of the boundary between immiscible liquids. While aqueous solutions are confined to the hydrophilic pathways by surface forces alone, organic liquids are confined to the hydrophobic region only if the aqueous liquid first occupies the hydrophilic region. In this way, stable liquid boundaries between immiscible liquids are possible as long as the pressures are maintained below critical values. The maximum pressures are determined by the interfacial tension of the aqueous solution and organic liquid, channel depth, and advancing contact angle (theta;(a)). Experimental results on maximum pressures are in good agreement with the analytical values. The ability to confine and position the boundary between immiscible liquids inside microchannels leads to a broad range of applications in microfluidic systems, which is exemplified by fabrication of a semipermeable membrane in a surface-patterned channel via interfacial polymerization.     

Effects of simulated acid rain on the surface tension of selected leaves

In this study the effect of simulated acid rain on the surface tension of leaves of selected crop plants is reported. The contact angle measurements of liquids (water and n-propanol mixtures) on various plant leaves (bean, mustard, lettuce, cress) showed that the acid rain effect can be investigated when analyses of the surface tension of leaves are estimated. Acid rain was found to give rise to a change in the polar surface tension of leaves, while the apolar surface tension was unaffected. This effect was strongest in bean leaves, which suggests that this is related to the polar surface tension of leaves.     

Wettability and topography of phospholipid DPPC multilayers deposited by spin-coating on glass, silicon, and mica slides

The surface free energy of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) layers deposited on glass, silicon, or mica by the spin-coating method was estimated. For this purpose, the advancing and receding contact angles of water, formamide, and diiodomethane were measured, and then two concepts of the interfacial interactions were applied. In the contact angle hysteresis approach, the apparent total surface free energy is calculated from the advancing and receding contact angles of the probe liquids, and in the Lifshitz-van der Waals/acid-base approach, the total surface free energy is calculated from previously determined components of the energy, that is, the apolar Lifshitz-van der Waals and the polar electron-donor and electron-acceptor, which are calculated from the advancing contact angles of the probe liquids alone. Comparison of the results obtained using these two approaches provided more information about changes in the hydrophobic/hydrophilic character of the DPPC layers and, simultaneously, a verification of the approaches. Moreover, the roughness and topography of the investigated layers were also examined by atomic force microscopy measurements. The hydrophilic character of the DPPC layers decreased if up to 0.5 mg of DPPC/mL was used to deposit on the substrates by the spin-coating method. Then it increased and leveled off if up to 2-2.5 mg of DPPC/mL was used. The changes in the energy were correlated with the changes in topography of the surfaces.