表面电荷及外电场对液固界面热阻的影响

采用非平衡分子动力学方法模拟了外电场及固体表面电荷对水与固体间界面热阻的影响. 结果表明,外加电场平行于界面时, 其对界面热阻几乎没有影响, 而垂直于界面时, 界面热阻将随着电场强度的增大而减小. 壁面带正电荷或负电荷都将使得界面热阻减小. 界面热阻与表面电荷密度及电场强度均满足二次函数关系. 模拟结果表明施加外电场和表面电荷是控制液固界面热阻的有效方法.     

Particle interactions in kaolinite suspensions and corresponding aggregate structures

The surface charge densities of the silica face surface and the alumina face surface of kaolinite particles, recently determined from surface force measurements using atomic force microscopy, show a distinct dependence on the pH of the system. The silica face was found to be negatively charged at pH>4, whereas the alumina face surface was found to be positively charged at pH<6, and negatively charged at pH>8. The surface charge densities of the silica face and the alumina face were utilized in this study to determine the interaction energies between different surfaces of kaolinite particles. Results indicate that the silica face-alumina face interaction is dominant for kaolinite particle aggregation at low pH. This face-face association increases the stacking of kaolinite layers, and thereby promotes the edge-face (edge-silica face and edge-alumina face) and face-face (silica face-alumina face) associations with increasing pH, and hence the maximum shear-yield stress at pH 5-5.5. With further increase in pH, the face-face and edge-face association decreases due to increasing surface charge density on the silica face and the edge surfaces, and decreasing surface charge density on the alumina face. At high pH, all kaolinite surfaces become negatively charged, kaolinite particles are dispersed, and the suspension is stabilized. The face-face association at low pH has been confirmed from cryo-SEM images of kaolinite aggregates taken from suspension which show that the particles are mostly organized in a face-face and edge-face manner. At higher pH conditions, the cryo-SEM images of the kaolinite aggregates reveal a lower degree of consolidation and the edge-edge association is evident.     

The adsorption of polyampholytes on negatively and positively charged polystyrene latex

The adsorption of two polyampholytes (a random copolymer of -glutamic acid and -lysine, and a well-defined tetramer of -lysyl- -glutamyl-glycine) onto positively and negatively charged latex was studied as a function of the pH and the ionic strength. The adsorbed amount proved to be almost independent of the salt concentration. The pH dependence was found to follow the same trends on negatively charged and positively charged latex. At low pH, where the polyampholytes are positively charged, a high adsorbed amount was found irrespective of the sign of the surface charge. At high pH, where the macromolecules are negatively charged, no adsorption was measured, not even with the positive latex. This is probably due to the very good solubility of the polyampholytes at this pH. Electrophoretic mobility measurements revealed that already at very low concentrations of polyampholyte charge reversal of the particles occurred.     

Oxide-dependent adsorption of a model membrane phospholipid, dipalmitoylphosphatidylcholine: bulk adsorption isotherms

The importance of substrate chemistry and structure on supported phospholipid bilayer design and functionality is only recently being recognized. Our goal is to investigate systematically the substrate-dependence of phospholipid adsorption with an emphasis on oxide surface chemistry and to determine the dominant controlling forces. We obtained bulk adsorption isotherms at 55 degrees C for dipalmitoylphosphatidylcholine (DPPC) at pH values of 5.0, 7.2, and 9.0 and at two ionic strengths with and without Ca(2+), on quartz (alpha-SiO(2)), rutile (alpha-TiO(2)), and corundum (alpha-Al(2)O(3)), which represent a wide a range of points of zero charge (PZC). Adsorption was strongly oxide- and pH-dependent. At pH 5.0, adsorption increased as quartz < rutile approximately corundum, while at pH 7.2 and 9.0, the trend was quartz approximately rutile < corundum. Adsorption decreased with increasing pH (increasing negative surface charge), although adsorption occurred even at pH > or = PZC of the oxides. These trends indicate that adsorption is controlled by attractive van der Waals forces and further modified by electrostatic interactions of oxide surface sites with the negatively charged phosphate ester (-R(PO(4)-)R'-) portion of the DPPC headgroup. Also, the maximum observed adsorption on negatively charged oxide surfaces corresponded to roughly two bilayers, whereas significantly higher adsorption of up to four bilayers occurred on positively charged surfaces. Calcium ions promote adsorption beyond a second bilayer, regardless of the sign of oxide surface charge. We develop a conceptual model for the structure of the electric double layer to explain these observations.     

INTERFACIAL FREE ENERGIES,ZETA POTENTIALS AND THROMBORESI STANCE IN BOVINE AND OVINE CAROTID ARTERIES

The physical properties of the intimal surfaces of both natural and artificial arteries are important In the prevention of thrombosis and clotting. It Is expected thati a) the interface between the blood plasma and the intima should be highly negatively charged, to repel negatively charged platelets and other blood constituents, and b) the interfacial free energy at the plasma-intima interface should be as low as possible, to prevent adhesion If the charge barrier is breached. Few measurements of either the zeta potential or the interfacial free energy at the intimal surface can be found in the literature. Zeta potentials of natural and cross-linked bovine carotid intima, and of an expanded Teflon prosthesis were determined by a streaming potential method in 0.9% (w/v) NaC1. Interfacial free energies of both bovine and ovine carotid intima, and of an expanded Teflon prosthesis were also determined by a contact-angle method using water immiscible liquid drops on the intimal surface Immersed in 0.9% (w/v) NaC1.     

Selective adsorption of protein molecules on phase-separated sapphire surfaces

Site-selective adsorption of protein molecules was found on sapphire surfaces that exhibit a phase separation into two domains: weakly charged hydrophobic domain and negatively charged hydrophilic one. Ferritin and bovine serum albumin molecules, which are negatively charged in a buffer solution, are adsorbed to the hydrophobic domains. Avidin molecules, which are positively charged, are adsorbed to the other domain. Fibrinogen molecules, which consist of both negative and positive modules, are adsorbed to the whole sapphire surface. Hemoglobin molecules, whose net charge is almost zero, are also adsorbed to the whole surfaces. These results indicate that electrostatic double layer interaction is the primary origin of the observed selectivity. Dependence of protein adsorption or desorption behaviors on the pH value can also be interpreted by the proposed model.     

Electrochemical attachment of motile bacterial cells to gold

Selective attachment of Escherichia coli K-12 bacterial cells to charged gold surfaces was demonstrated. Electrostatic binding of E. coli K-12 bacterial cells to positively charged surfaces was observed starting at +750 mV. The binding of E. coli K-12 cells to positively charged gold surfaces is proposed to occur due to long-range electrostatic interactions between the negatively charged O-chain of lipopolysaccharide (LPS) molecules protruding the bacterial cell body and the electrode surface. Removing LPS alters the cellular surface charge and results in cellular attachment to negatively charged surfaces. Thus, applying an electrical potential allows for the direct, real time detection of live, dead or damaged bacterial cells. The attachment of E. coli K-12 bacterial cells to surfaces with an applied potential substantiates the hypothesis that an electrostatic interaction is responsible for the binding of bacterial cells to positively charged molecular assemblies on surfaces used for building bacterial microarrays.     

Influence of interfacial water layer on surface properties of silver halides: effect of pH on the isoelectric point

The hypothesis that pH dependent charge of interfacial water affects electrokinetic charge and electrokinetic potential of hydrophobic colloids, but not the (inner) surface potential was tested. It was found that isoelectric points of silver chloride, bromide and iodide shift to the higher pAg values in the acidic solutions, but that surface potential did not depend on pH. Isoelectric points of water at inert surfaces lie in the range 2相似文献   

Lectins and/or xyloglucans/alginate layers as supports for immobilization of dengue virus particles

Formation of stable thin films of mixed xyloglucan (XG) and alginate (ALG) onto Si/SiO(2) wafers was achieved under pH 11.6, 50mM CaCl(2), and at 70 degrees C. XG-ALG films presented mean thickness of (16+/-2)nm and globules rich surface, as evidenced by means of ellipsometry and atomic force microscopy (AFM), respectively. The adsorption of two glucose/mannose-binding seed (Canavalia ensiformis and Dioclea altissima) lectins, coded here as ConA and DAlt, onto XG-ALG surfaces took place under pH 5. Under this condition both lectins present positive net charge. ConA and DAlt adsorbed irreversibly onto XG-ALG forming homogenous monolayers approximately (4+/-1)nm thick. Lectins adsorption was mainly driven by electrostatic interaction between lectins positively charged residues and carboxylated (negatively charged) ALG groups. Adhesion of four serotypes of dengue virus, DENV (1-4), particles to XG-ALG surfaces were observed by ellipsometry and AFM. The attachment of dengue particles onto XG-ALG films might be mediated by (i) H bonding between E protein (located at virus particle surface) polar residues and hydroxyl groups present on XG-ALG surfaces and (ii) electrostatic interaction between E protein positively charged residues and ALG carboxylic groups. DENV-4 serotype presented the weakest adsorption onto XG-ALG surfaces, indicating that E protein on DENV-4 surface presents net charge (amino acid sequence) different from E proteins of other serotypes. All four DENV particles serotypes adsorbed similarly onto lectin films adsorbed. Nevertheless, the addition of 0.005mol/L of mannose prevented dengue particles from adsorbing onto lectin films. XG-ALG and lectin layers serve as potential materials for the development of diagnostic methods for dengue.     

Monte Carlo simulations of antibody adsorption and orientation on charged surfaces

Monte Carlo simulations were performed to study the adsorption and orientation of antibodies on charged surfaces based on both colloidal and all-atom models. The colloidal model antibody consists of 12 connected beads representing the 12 domains of an antibody molecule. The structure of the all-atom antibody model was taken from the protein databank. The effects of the surface charge sign and density, the solution pH and ionic strength on the adsorption and orientation of different colloidal model antibodies with different dipole moments were examined. Simulation results show that both the 12-bead and the all-atom models of the antibody, for which the dipole moment points from the Fc to (Fab)2 fragments, tend to have the desired "end-on" orientation on positively charged surfaces and undesired "head-on" orientation on negatively charged surfaces at high surface charge density and low solution ionic strength where electrostatic interactions dominate. At low surface charge density and high solution ionic strength where van der Waals interactions dominate, 12-bead model antibodies tend to have "lying-flat" orientation on surfaces. The orientation of adsorbed antibodies results from the compromise between electrostatic and van der Waals interactions. The dipole moment of an antibody is an important factor for antibody orientation on charged surfaces when electrostatic interactions dominate. This charge-driven protein orientation hypothesis was verified by our simulations results in this work. It was further confirmed by surface plasmon resonance biosensor and time-of-flight secondary ion mass spectrometry experiments reported elsewhere.     

Interaction between charged soft microcapsules and red blood cells: effects of PEGylation of microcapsule membranes upon their surface properties

Four types of hydrophilic gel microcapsules containing water have been prepared by an interfacial polymerization method. Each type of microcapsules has a membrane of different composition. Using three kinds of monomers, N,N-dimethylacrylamide (DMAAm), 4-(aminomethyl)styrene (AmSt), and N,N-dimethylaminopropylacrylamide (DMAPAA), one type of aqueous copolymer having primary and tertiary amino groups was obtained. By the polymerization of three kinds of monomers, DMAAm, AmSt, and 2-[(methacryloyloxy)ethyl] trimethylammoniumchloride (METAC), another type of aqueous copolymer having primary and quaternary ammonium groups was also obtained. Two more types of copolymers were synthesized by copolymerization of -acryloxy-ω-methoxy-poly(ethylene glycol) (a-PEG) with the above two kinds of monomer mixture. These copolymers were polymerized with terephthaloyldichloride at the water/oil interface to prepare four types of microcapsules containing water, i.e., poly(DMAAm-co-DMAPAA-co-AmSt-alt-terephthalic acid) microcapsules, poly(DMAAm-co-DMAPAA-co-AmSt-co-PEG-alt-terephthalic acid) microcapsules, poly (DMAAm-co-METAC-co-AmSt-alt-terephthalic acid) microcapsules, and poly (DMAAm-co-METAC-co-AmSt-co-PEG-alt-terephthalic acid) microcapsules, which will be abbreviated to MC 1, MC 2, MC 3, and MC 4, respectively. It has been predicted that the microcapsule membranes are hydrophilic and soft and have two-sublayer structures from electrophoretic mobility measurements and from the analysis of the data with Ohshima’s electrokinetic theory for soft particles. The outer sublayers of MC 1 and MC 2 are negatively charged and those of MC 3 and 4 are slightly positively charged. Also, the surfaces of MC 1 and MC 2 are harder than those of MC 3 and 4. By PEGylation, the surface charge density in the membranes decreases and the surface becomes softer. It has been found that the membrane of red blood cells (RBC) is also soft and is composed of two-sublayers, the outer sublayer of which is negatively charged and the inner one is positively charged. The interaction of four types of microcapsules with RBC has been studied. It was found that microcapsules with soft surfaces (MC 3 and MC 4) do not interact with RBC, even though the microcapsule surfaces are positively charged and the surface of RBC is negatively charged. On the other hand, microcapsules with negatively charged but harder surfaces (MC 1) interact with RBC to introduce hemolysis. The membrane surface of MC 2, which is obtained by PEGylation of MC 1, becomes softer than that of MC 1 so that the interaction with RBC was weakly suppressed. From these, it was concluded that the dominant factor to control the interaction between synthetic polymer surfaces and biological cell surfaces is not the surface charges carried by the polymer surfaces but the softness of the polymer surfaces.     

Effect of dehydration on the interfacial water structure at a charged polymer surface: negligible χ(3) contribution to sum frequency generation signal

Interfacial water structure at charged surfaces plays a key role in many physical, chemical, biological, environmental, and industrial processes. Understanding the release of interfacial water from the charged solid surfaces during dehydration process may provide insights into the mechanism of protein folding and the nature of weak molecular interactions. In this work, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by quartz crystal microbalance (QCM) measurements, has been applied to study the interfacial water structure at polyelectrolyte covered surfaces. Poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) chains are grafted on solid surfaces to investigate the change of interfacial water structure with varying surface charge density induced by tuning the solution pH. At pH ≤ 7.1, SFG-VS intensity is linear to the loss of mass of interfacial water caused by the dehydration of PDMAEMA chains, and no reorientation of the strongly bonded water molecules is observed in the light of χ(ppp)/χ(ssp) ratio. χ((3)) contribution to SFG signal is deduced based on the combination of SFG and QCM results. It is the first direct experimental evidence to reveal that the χ((3)) has a negligible contribution to SFG signal of the interfacial water at a charged polymer surface.     

表面等离子体子共振技术实时研究蛋白质在修饰表面的静电吸附行为

研究蛋白质在固相表面的静电吸附特性,进而控制蛋白质在修饰表面的静电吸附尤为重要,表面等离子体子共振可以检测金属表面吸附物质厚度和折射率的变化^[1]。这种技术已在研究生物分子相互作用^[2]和考察自组装单层的形成^[3]及蛋白质在固体表面吸附行为^[9-11]等方面得到广泛的应用。对蛋白质在固体表面吸附行为的研究多为考察不同的蛋白质在不同的修饰表面的吸附行为。然而,对蛋白质在修饰表面静电吸附的本质影响因素的研究却少有报道^[4]。本文使用表面等离子体子共振技术实时研究了蛋白质在甲羧基化葡聚糖修饰表面的静电吸附与溶液pH值及离子强度的依赖关系。     

Chitosan based surfactant polymers designed to improve blood compatibility on biomaterials

We developed chitosan based surfactant polymers that could be used to modify the surface of existing biomaterials in order to improve their blood compatibility. These polymers consist of a chitosan backbone, PEG side chains to repel non-specific protein adsorption, and hexanal side chains to facilitate adsorption and proper orientation onto a hydrophobic substrate via hydrophobic interactions. Since chitosan is a polycationic polymer, and it is thrombogenic, the surface charge was altered to determine the role of this charge in the hemocompatibility of chitosan. Charge had a notable effect on platelet adhesion. The platelet adhesion was greatest on the positively charged surface, and decreased by almost 50% with the neutralization of this charge. A chitosan surface containing the negatively charged SO(3)(-) exhibited the fewest number of adherent platelets of all surfaces tested. Coagulation activation was not altered by the neutralization of the positive charge, but a marked increase of approximately 5-6 min in the plasma recalcification time (PRT) was displayed with the addition of the negatively charged species. Polyethylene (PE) surfaces were modified with the chitosan surfactant resulting in a significant improvement in blood compatibility, which correlated to the increasing PEG content within the polymer. Adsorption of the chitosan surfactants onto PE resulted in approximately an 85-96% decrease in the number of adherent platelets. The surfactant polymers also reduced surface induced coagulation activation, which was indicated by the PEG density dependent increase in PRTs. These results indicate that surface modification with our chitosan based surfactant polymers successfully improves blood compatibility. Moreover, the inclusion of either negatively charged SO(3)(-) groups or a high density of large water-soluble PEG side chains produces a surface that may be suitable for cardiovascular applications.     

Stabilization of Liposomes by Polyelectrolytes: Mechanism of Interaction and Role of Experimental Conditions

ζ-potential measurements on LUVs allow to evidence the influence of pH, ionic salt concentration, and polyelectrolyte charge on the interaction between polyelectrolyte (chitosan and hyaluronan) and zwitterionic lipid membrane. First, chitosan adsorption is studied: adsorption is independent on the chitosan molecular weight and corresponds to a maximum degree of decoration of 40% in surface coverage. From the dependence with pH and independence with MW, it is concluded that electrostatic interactions are responsible of chitosan adsorption which occurs flat on the external surface of the liposomes. The vesicles become positively charged in the presence of around two repeat units of chitosan added per lipid accessible polar head in acid medium down to pH = 7.2. Direct optical microscopy observations of GUVs shows a stabilization of the composite liposomes under different external stresses (pH and salt shocks) which confirms the strong electrostatic interaction between the chitosan and the lipid membrane. It is also demonstrated that the liposomes are stabilized by chitosan adsorption in a very wide range of pH (2.0 < pH < 12.0). Then, hyaluronan (HA), a negatively charged polyelectrolyte, is added to vesicles; the vesicles turn rapidly negatively charged in presence of adsorbed HA Finally, we demonstrated that hyaluronan adsorbs on positively charged chitosan-decorated liposomes at pH < 7.0 leading to charge inversion in the liposome decorated by the chitosan-hyaluronan bilayer. Our results demonstrate the adsorption of positive and/or negative polyelectrolyte at the surface of lipidic vesicles as well as their role on vesicle stabilization and charge control.     

Reversible switching of protein uptake and release at polyelectrolyte multilayers detected by ATR-FTIR spectroscopy

The reversible switching of uptake and release of the proteins lysozyme (LYZ, IEP = 11.1) and human serum albumin (HSA, IEP = 4.8) at the surface attached polyelectrolyte multilayer (PEM) consisting of poly(ethylene-imine) (PEI) and poly(acrylic acid) (PAC) is shown. Protein adsorption could be switched by pH setting due to electrostatic interaction. Adsorption of positively charged LYZ at PEM-6 took place at pH = 7.3, where the outermost PAC layer was negatively charged. Complete desorption was obtained at pH = 4, where the outermost PAC layer was neutral. Additionally the charge state of the last adsorbed PAC layer in dependence of the pH of the medium could be determined in the ATR-FTIR difference spectra by the ν(COO⁻) and ν(C=O) band due to carboxylate and carboxylic acid groups. Adsorption of negatively charged HSA at PEM-7 was achieved at pH = 7.3, where the outermost PEI layer was positively charged. Part desorption was obtained at pH = 10, where the outermost PEI layer was neutral. PEM of PEI/PAC may be used for the development of bioactive and bionert materials and protein sensors.     

Density functional study of the charge on Aun clusters (n=1-7) supported on a partially reduced rutile TiO2(110): are all clusters negatively charged?

It is widely believed that small gold clusters supported on an oxide surface and adsorbed at the site of an oxygen vacancy are negatively charged. It has been suggested that this negative charge helps a gold cluster adsorb oxygen and weakens the O-O bond to make oxidation reactions more efficient. Given the fact that an oxygen vacancy is electron rich and that Au is a very electronegative element, the assumption that the Au cluster will take electron density from the vacancy is plausible. However, the density functional calculations presented here show that the situation is more complicated. The authors have used the Bader method to examine the charge redistribution when a Aun cluster (n=1-7) binds next to or at an oxygen vacancy on rutile TiO2(110). For the lowest energy isomers they find that Au1 and Au3 are negatively charged, Au5 and Au7 are positively charged, and Au2, Au4, and Au6 exchange practically no charge. The behavior of the Aun isomers having the second-lowest energy is also unexpected. Au2, Au3, Au5, and Au7 are negatively charged upon adsorption and very little charge is transferred when Au4 and Au6 are adsorbed. These observations can be explained in terms of the overlap between the frontier molecular orbitals of the gold cluster and the eigenstates of the support. Aun with even n becomes negatively charged when the lowest unoccupied molecular orbital has a lobe pointing in the direction of the oxygen vacancy or towards a fivefold coordinated Ti (5c-Ti) located in the surface layer; otherwise it stays neutral. Aun with odd n becomes negatively charged when the singly occupied molecular orbital has a lobe pointing in the direction of a 5c-Ti located at the vacancy site or in the surface layer, otherwise it donates electron density into the conduction band of rutile TiO2(110) becoming positively charged.     

Hofmeister effects on the colloidal stability of an IgG-coated polystyrene latex

The effect of various ions related to the Hofmeister series (HS) on different properties of a cationic latex covered with a protein (IgG) is analyzed in this study. NaNO3, NH4NO3, and Ca(NO3)2 were used to compare the specificity of the cations, and NaCl, NaSCN, NaNO3, and Na2SO4, to compare the specificity of the anions. Two pH values, 4 and 10, were chosen to analyze the behavior of these ions acting as counter- and co-ions. At pH 4, the total surface charge is positive, whereas at pH 10 it is negative. Three different phenomena have been studied in the presence of these Hofmeister ions: (1) colloidal aggregation, (2) electrophoretic mobility, and (3) colloidal restabilization. The specific effect of the ions was clearly observed in all experiments, obtaining ion sequences ordered according to their specificity. The most important parameter for ion ordering was the sign of the charge of the colloidal particle. Positively charged particles displayed an ion order opposite that observed for negatively charged surfaces. Another influential factor was the hydrophobic/hydrophilic character of the particle surface. IgG-latex particle surfaces at pH 10 were more hydrophilic than those at pH 4. The SCN- ion had a peculiar specific effect on the phenomena studied (1)-(3) at pH 10. With respect to the restabilization studies at high ionic strengths, new interesting results were obtained. Whereas it is commonly known that cations may provoke colloidal restabilization in negative particles when they act as counterions, our experiments demonstrated that such restabilization is also possible with positively charged particles. Likewise, restabilization of negative surfaces induced by the specific effect of chaotropic anions (acting as co-ions) was also observed.     

Interconnectivity between Surface Reactivity and Self-Assembly of Kemp Elimination Catalyzing Nanorods

Understanding the fundamental facts behind dynamicity of catalytic processes has been a longstanding quest across disciplines. Herein, we report self-assembly of catalytically active gold nanorods that can be regulated by tuning its reactivity towards a proton transfer reaction at different pH. Unlike substrate-induced templating and co-operativity, the enhanced aggregation rate is due to alteration of catalytic surface charge only during reactivity as negatively charged transition state of reactant (5-nitrobenzisoxazole) is formed on positively charged nanorod while undergoing a concerted E2-pathway. Herein, enhanced diffusivity during catalytic processes might also act as an additional contributing factor. Furthermore, we have also shown that nanosized hydrophobic cavities of clustered nanorods can also efficiently accelerate the rate of an aromatic nucleophilic substitution reaction, which also demonstrates a catalytic phenomenon that can lead to cascading of other reactions where substrates and products of the starting reactions are not directly involved.     

Dynamics of charge transfer states on metal surfaces: the competition between reactivity and quenching

The dynamics of excited states of adsorbates on surfaces caused by charge transfer is studied. Both negative and positive charge transfer processes are possible. In particular we are interested in positive charge transfer from a metal surface to molecular or atomic oxygen adsorbed on the surface. Once the negatively charged oxygen on the surface loses an electron it becomes chemically activated. The ability of this species to react depends on the quenching time or back transfer. The analysis of these processes is based on a set of diabatic potential energy surfaces each representing a different charged oxygen species. The dynamics is followed by solving the multichannel time-dependent Schr?dinger equation or Liouville von Neumann equation. Due to the nonadiabatic character of these reactions large isotope effects are predicted.