Study of hydration layers near a hydrophilic surface in water through AFM imaging

In this work, a hydrophilic silica plate exposed in air, and immersed in an aqueous solution was studied through atomic force microscopy (AFM) imaging in contact‐ and tapping‐mode operations. It was experimentally found that the tapping‐mode AFM images of the silica surface were different when it was immersed in an aqueous solution from those when it was exposed in air. The former showed fewer topographic features. However, the contact‐mode AFM images of the silica surface were almost uninfluenced by the medium in which the surface was placed. This phenomenon might be attributed to the existence of hydration layers near the silica surface in the aqueous solution. The layers are like a large sheet on the surface that hides the details, so that an AFM tip in the tapping mode can read only the hydration layer and therefore image only the rough outline of the surface. This result might suggest the existence of hydration layers near a hydrophilic surface immersed in water. Copyright © 2006 John Wiley & Sons, Ltd.     

In situ surface reduction of a NiO-YSZ-alumina composite using scanning probe microscopy

In situ surface reductions of NiO-YSZ-Al₂O₃ composites into Ni-YSZ-Al₂O₃ cermets were carried out at 312–525 °C in a controlled atmosphere high-temperature scanning probe microscope (CAHT-SPM) in dry and humidified 9 % H₂ in N₂. The reduction of NiO was followed by contact mode scanning of topography and conductance. A reproducible sequence of events was observed which included a conductance decrease upon hydrogen introduction and a reappearance of conductance after some time. It was found that this incubation time from introduction of hydrogen and until conducting Ni appeared was temperature dependent and followed the Arrhenius equation. For samples reduced in dry hydrogen, the Arrhenius plot showed two regions with different activation energies. Scanning electron microscopy confirmed a difference in microstructure between these temperature regimes. A strong retarding effect of steam (H₂O) on the nucleation time of Ni particles was observed.     

Surface forces and drag coefficients of microspheres near a plane surface measured with optical tweezers

Optical tweezers are widely used to measure molecular forces in biology. Such measurements are often influenced by a nearby surface that can perturb both the calibration of the tweezers as well as the hydrodynamic forces acting on microspheres to which the biomolecules are attached. In this study, we have used a very stable optical tweezers setup employing a recently developed calibration method (Toli?-N?rrelykke, S. F.; Sch?ffer, E.; Howard, J.; Pavone, F. S.; Jülicher, F.; Flyvbjerg, H. Rev. Sci. Instrum. 2006, 77 (10), 103101) to determine how the calibration of the tweezers and the forces on the microspheres depend on the height above the surface. We show that the displacement sensitivity of the tweezers is modulated by a standing light wave between the microsphere and the surface. We measured the dependence of the drag coefficient on height and compared it to exact and closed-form solutions to the Navier-Stokes equations. Also, we measured the surface force gradients in different salt solutions and for different surface blocking methods. For a given blocking method, our data suggest that microspheres can experience attractive and/or repulsive forces close to surfaces. For example, a Teflon layer reduces attractive interactions, and the presence of casein can lead to long-range repulsive interactions. These measurements are a prerequisite for the accurate measurement of normal forces with respect to an interface that occur in biological molecules held between surfaces.     

Direct surface force measurement in water using a nanosize colloidal probe technique

The direct force measurement between colloidal surfaces has been an essential topic in both theories and applications of surface chemistry. As particle size is decreased from micron size down to true nano size (<10 nm), surface forces are increasingly important. Nanoparticles at close proximity or high solids loading are expected to show a different behavior than what can be estimated from continuum and mean field theories. The current tools for directly measuring interaction forces such as a surface force apparatus or atomic force microscopy (AFM) are limited to particles much larger than nanosize. Here a modified colloidal probe technique is suggested using a multiwalled carbon nanotube (MWNT) to overcome this problem. Determination of zero separation in AFM is critical to extract a reliable force-separation curve when MWNT is used as a probe. Hence, a systematic approach to the data collection for a nanosize colloidal probe is proposed and a sample of a direct surface force measurement curve obtained with the MWNT probe is presented.     

Polar, hydrophilic compounds in drinking water produced from surface water. Determination by liquid chromatography-mass spectrometry.

Drinking water produced from surface water may contain many polar, hydrophilic compounds in spite of different treatment steps such as soil filtration, ozone treatment and activated carbon filtration. Little is known about these compounds. The objectives of this work were the detection and identification by means of tandem mass spectrometry (MS-MS) coupled on-line by a thermospray interface with liquid chromatography. Quantification is possible if standard compounds are available. The different compounds in the water extracts were not only separated by means of an analytical column but also using MS-MS after loop injection bypassing the analytical column. Molecular weight information in the loop spectra (overview spectra) and collisionally induced dissociation (CID) made possible the identification of some of these compounds which cannot be eliminated in the drinking water treatment process. Identification was not only done by interpretation of the recorded daughter- and parent-ion spectra but also by comparing them with a laboratory-made daughter-ion library of polar, hydrophilic pollutants. Direct mixture analysis using MS-MS allows the detection and identification of some of the pollutants if they reach the drinking water in the course of the surface water treatment process because of their biochemical and chemical persistence and/or non-sorbability during the soil or activated carbon filtration process. The proposed method for the analysis of water for polar, non-volatile and/or thermolabile organic substances is a quick, specific and powerful technique which makes it possible to detect and identify these substances without any chromatographic separation or derivatization     

Influence of water on the surface of hydrophilic and hydrophobic room-temperature ionic liquids

Vibrational spectroscopy, sum frequency generation, has been used to study the gas-liquid interface of imidazolium-based, room-temperature ionic liquids. The ionic liquids are divided into two categories on the basis of their miscibility with water, as hydrophobic (immiscible) and hydrophilic (miscible). The spectroscopy results indicate that water will reorient the cations of the hydrophobic ionic liquid, while in the hydrophilic ones they remain unaffected. Thus, under low water conditions the plane of the imidazolium ring remains parallel to the surface for both types of ionic liquid. However, at high water content the ring will reorient to along the surface normal for the hydrophobic ionic liquid but remain parallel to the surface for the hydrophilic one. This is a reversible process.     

Mathematical modeling of a hydrophilic cylinder floating on water

In this paper, a hydrostatic model of the surface profile anchored to the upper edge of a vertical cylinder is proposed to explain why coins can float on water surface. The sharp edge of a cylinder is thus modeled as a round smooth surface on which the contact line may be anchored at a position according to the weight of the cylinder. The mathematical model of the surface profile is established based on the hydrostatics and a third order ordinary differential equation is resulted. Numerical solution of the model demonstrates under practical conditions the existence of the surface profiles that provide reasonable uplifting force at the contact line so that the force is available for floating coins on water surface. The proposed model explains the obviously enlarged apparent contact angle and the edge effect in the literature. The numerical simulation is found in very good agreement with the experimental data in the literature.     

Imaging structured water and bound polysaccharide on mica surface at ambient temperature

The presence of a water layer on the surface of muscovite mica under ambient conditions is well established. The water molecules are well ordered and seem to be oriented, leading to an icelike monolayer (probably ferroelectric) in epitaxial relation with the mica surface. We have imaged and characterized the height and contact angle of ordered water layer(s) formed by wetting and de-wetting processes on mica surfaces at different states of hydration by tapping mode atomic force microscopy. Implications that the presence of such an ordered water layer may have for imaging of biological samples are also discussed, with consideration of data for the polysaccharide hyaluronan.     

Effect of surface wettability on liquid density, structure, and diffusion near a solid surface

Molecular dynamics and Langevin dynamics simulations are used to elucidate the behavior of liquid atoms near a solid boundary. Correlations between the surface wettability and spatial variations in liquid density and structure are identified. The self-diffusion coefficient tensor is predicted, revealing highly anisotropic and spatially varying mass transfer phenomena near the solid boundary. This behavior affects self-diffusion at a range of time scales. Near a more-wetting surface, self-diffusion is impeded by strong solid-liquid interactions that induce sharp liquid density gradients and enhanced liquid structure. Conversely, near a less-wetting surface, where solid-liquid interactions are weaker, the liquid density is low, the atoms are disordered, and diffusion is enhanced. These findings suggest that altering the wettability of a micro- or nanochannel may provide a passive means for controlling the diffusion of select targets towards a functionalized surface and controlling the reaction rate in diffusion-limited reactions.     

Morphological and spectroscopic properties of thin films of self-assembling amphiphilic porphyrins on a hydrophilic surface as revealed by scanning near-field optical microscopy

We fabricated porphyrin thin films on mica surfaces from acidic aqueous solutions of the preorganized H-aggregates of amphiphilic porphyrins by the simple spin-coating method. The morphological and spectroscopic properties of the film were investigated by scanning near-field optical microscopy. The results obtained in this study demonstrate that the preorganized structure in solution can be transferred as a thin film with a thickness of the monolayer level without losing their substantial structure and photophysical properties.     

Vertical diffusion of water molecules near the surface of ice

We studied diffusion of water molecules in the direction perpendicular to the surface of an ice film. Amorphous ice films of H(2)O were deposited on Ru(0001) at temperature of 100-140 K for thickness of 1-5 bilayer (BL) in vacuum, and a fractional coverage of D(2)O was added onto the surface. Vertical migration of surface D(2)O molecules to the underlying H(2)O multilayer and the reverse migration of H(2)O resulted in change of their surface concentrations. Temporal variation of the H(2)O and D(2)O surface concentrations was monitored by the technique of Cs(+) reactive ion scattering to reveal kinetics of the vertical diffusion in depth resolution of 1 BL. The first-order rate coefficient for the migration of surface water molecules ranged from k(1)=5.7(+/-0.6) x 10(-4) s(-1) at T=100 K to k(1)=6.7(+/-2.0) x 10(-2) s(-1) at 140 K, with an activation energy of 13.7+/-1.7 kJ mol(-1). The equivalent surface diffusion coefficients were D(s)=7 x 10(-19) cm(2) s(-1) at 100 K and D(s)=8 x 10(-17) cm(2) s(-1) at 140 K. The measured activation energy was close to interstitial migration energy (15 kJ mol(-1)) and was much lower than diffusion activation energy in bulk ice (52-70 kJ mol(-1)). The result suggested that water molecules diffused via the interstitial mechanism near the surface where defect concentrations were very high.     

Reduced coupling of water molecules near the surface of reverse micelles

We report on vibrational dynamics of water near the surface of AOT reverse micelles studied by narrow-band excitation, mid-IR pump-probe spectroscopy. Evidence of OH-stretch frequency splitting into the symmetric and asymmetric modes is clearly observed for the interfacial H(2)O molecules. The polarization memory of interfacial waters is preserved over an exceptionally extended >10 ps timescale which is a factor of 100 longer than in bulk water. These observations point towards negligibly small intermolecular vibrational coupling between the water molecules as well as strongly reduced water rotational mobility within the interfacial water layer.     

Diffusion of water vapor through a hydrophilic polymer film

Multifilm techniques have been used to measure the diffusion coefficient of water vapor in cellophane, and the data have been compared with the integral diffusion coefficient obtained in previous work with single films. The multifilm techniques lead to a much sharper resolution of the effect of concentration on diffusion, and the maximum integral diffusion coefficient. The diffusion coefficient for water vapor in cellophane peaks at a moisture content corresponding to about 70% R. H., which is presumably the “critical concentration” discussed in previous work on the thermodynamics of water sorption by cellophane.     

新的吲哚类同型二聚体荧光探针测定蛋白质的分析研究

采用新的水溶性吲哚类同型二聚体探针Ⅰ,建立了一种新的蛋白质荧光分析体系.在酸性条件下,蛋白质分子能与吲哚探针发生结合作用,体系荧光明显增强并向长波方向发生红移,荧光强度与蛋白质质量浓度呈良好的线性关系,在最优条件下,牛血清白蛋白BSA的线性响应范围0.80～25.00 μg/mL,检出限(3σ/K)为0. μg/mL;用于模拟样品的测定,1.00,2.00和5.00 μg/mL的样品测定回收率为99.0%～102.0%,相对标准偏差1.8%～2.8%;与国家标准方法比较,结果测定偏差为1.0%～4.1%.     

Effect of water drag on diffusion of drifting polarons in DNA

It has been shown, theoretically and experimentally, that a hole or an excess electron on a DNA molecule in solution forms a delocalized wave function, a polaron. For an all-adenine (A) sequence or a mixed sequence of guanines (G's) and A's, calculations taking into account the polarization of the solution give the wave function spread over approximately four bases, which appears to be in agreement with experiment. The polaron may move by hopping or by drift. Drift can take place in a region with all the same bases, for example, A's, by the polaron dropping an A on the trailing edge and picking up an A on the leading edge. For drift that is not too rapid, the necessity of the polarization changing as the polaron moves exerts a drag on the polaron. We calculate the drag by using a model introduced earlier to describe the polaron. We find the drag to be proportional to the velocity of the polaron and to the orientational relaxation time of the water molecules. The drag is also a function of the Coulomb interactions of the fractional charges on the bases constituting the polaron, as modified by the polarization charge induced in the solution. The diffusion rate and mobility for all A polarons, calculated taking into account the drag, are 8 x 10(-5) cm(2)/s and 3 x 10(-3) cm(2)/(V s), respectively. We believe that in the experimental studies that have been carried out on hole propagation in a series of A's it was drift being observed rather than the hopping of a localized hole between adjacent A's, as was assumed to be the case.     

Gas chromatographic, quentum-chemical, and molecular statistical studies of cluster adsorption of water and methanol molecules on hydrophilic surface sites of hydrophobic adsorbents

The comprehensive theoretical and experimental study of the adsorption of water and methanol molecules on active sites (carboxyl and phenol hydroxyl groups) on the graphitized thermal carbon black is performed. It is shown that microclusters formed upon the adsorption of these molecules on such sites are characterized by the cyclic structure comprising 4–5 molecules similar to that whose existence was revealed previously in liquid water and on the surface of silver iodide. The analysis of the studied adsorption clusters demonstrated that the formation of such cycles is governed primarily by the hydrogen bonding; however, a definite role is played also by energy effects associated with the changes in the state of molecular motion during adsorption. It is shown that the generalized Langmuir equation derived from molecular statistical considerations provides for better reproducing of the experimental isotherm within a wider surface coverage range, thus making it possible to consistently interpret data on structural and energetic characteristics of adsorption systems obtained by gas chromatography and quentum-chemical method.     

Protein adsorption on the hydrophilic surface of a glassy polymer: a computer simulation study

Using atomistic computer simulations, we study the adsorption of different globular protein fragments with different secondary structures on the surface of a hydrophilic glassy polymer, poly(vinyl alcohol), or PVA, and compare the results with our earlier calculations on hydrophobic graphite. The simulations were mainly carried out with implicit solvent in an effective dielectric medium by energy minimizations and molecular dynamics at room temperature. We find that on the hydrophilic PVA surface the fragments basically retain their globular shape with an incomplete denaturation, at variance with our earlier results for the same fragments on graphite. Correspondingly, the interaction energy between the fragments and the surface is significantly smaller than on graphite, both because less residues are in contact with the surface, and because they interact more weakly. Moreover, very few hydrogen bonds are formed between the adsorbate and the PVA surface, since both the protein fragments and the polymer chains separately optimize these interactions. Additional molecular dynamics simulations in explicit solvent were also performed to study the hydration of the adsorbed fragments and to estimate the possible solvation effects.     

Small molecule diffusion in a rubbery polymer near Tg: Effects of probe size,shape, and flexibility

A novel experimental approach involving fluorescence nonradiative energy transfer (NRET) is employed to study the Fickian diffusion of small molecules in rubbery poly(isobutyl methacrylate) (PiBMA) films near the glass transition, using a formalism that directly relates the small molecule translational diffusion coefficient, D, to changes in the normalized nonradiative energy transfer efficiency, E_N. Values of D for pyrene, 1,3-bis-(1-pyrene) propane (BPP), 1,3-bis-(1-pyrene) decane (BPD), 9,10-bis-phenyl ethynyl anthracene (BPEA), diphenyl Disperse Red 4 (DPDR4), and decacyclene in PiBMA are measured over temperatures ranging from approximately T_g to T_g + 25°C. Among these chromophores, significant differences in both the magnitude and temperature dependence of D are observed which are attributed to differences in molecule shape and flexibility, as well as molar volume. Other factors being equal, chromophore flexibility was shown both to increase the magnitude of D and to decrease its dependence on temperature, as does an increase in aspect ratio. For BPD, these effects are attributed to the ability of the flexible molecule to diffuse in a piecewise manner, requiring the cooperative mobility of fewer polymer chain segments than a rigid molecule of the same molar volume. For BPEA and DPDR4, this deviation from D being dominated by molar volume effects is attributed the to high aspect ratio of these elongated molecules. © 1996 John Wiley & Sons, Inc.     

Dynamic meniscus growth at a scanning probe tip in contact with a gold substrate

Environmental scanning electron microscopy was used to investigate the dynamic meniscus growth at a cantilever in contact with a substrate. The meniscus was observed to take many minutes to reach an equilibrium state. The observed growth rate is similar to initial patterning rates observed from dip-pen nanolithography and suggest that the meniscus growth may be the rate-limiting step in initial pattering rates.