On the effect of induced electro-osmosis on a cylindrical particle next to a surface

The effect of induced electro-osmosis on a cylindrical particle positioned next to a planar surface (wall) is studied theoretically both under the thin double layer approximation utilizing the Smoluchowski slip velocity approximation and under thick electric double layer conditions by solving the Poisson-Nernst-Planck (PNP) equations. The imposed, undisturbed electric field is parallel to the planar surface. The induced hydrodynamic and electrostatic forces are calculated as functions of the particle's and the medium's dielectric constants and the distance between the particle and the surface. The resultant force acting on the particle is directed normal to and away from the wall. The presence of such a repulsive force may adversely affect the interactions between macromolecules suspended in solution and wall-immobilized molecules and may be significant to near-wall particle imaging velocimetry (PIV) in electrokinetic flows.     

Apparent slip due to the motion of suspended particles in flows of electrolyte solutions

We consider pressure-driven flows of electrolyte solutions in small channels or capillaries in which tracer particles are used to probe velocity profiles. Under the assumption that the double layer is thin compared to the channel dimensions, we show that the flow-induced streaming electric field can create an apparent slip velocity for the motion of the particles, even if the flow velocity still satisfies the no-slip boundary condition. In this case, tracking of the particles would lead to the wrong conclusion that the no-slip boundary condition is violated. We evaluate the apparent slip length, compare it with experiments, and discuss the implications of these results.     

Flow of solutions in thin capillaries coated with an adsorbed polyelectrolyte lyer

The flow of KCl solutions through thin quartz capillaries coated with an adsorbed layer of a cationic polyelectrolyte (CPE), poly(dimethyldiallylammonium chloride) (molecular mass M = 500000), is studied. It is found that the adsorption layer is soft and its thickness depends on shear stress generated by the liquid flow through the capillary. The hydrodynamic thickness of the CPE adsorption layer is 80–90 nm at low flow rates of a solution, and it decreases to values comparable with the experimental error at high flow rates. The dried adsorption layer appears to be hydrophobic (the advancing contact angle is about 80°); in these capillaries, the flow rate of a KCl solution is increased that can be interpreted as a solution slip on the surface of CPE adsorption layer. The long-term contact of the dried CPE adsorption layer with KCl solution, probably, results in the swelling of the adsorption layer, which is accompanied by a decrease in the contact angle and ζ potential of the adsorption layer surface as calculated from the streaming potential of the same solution.     

Influence of cetyltrimethylammonium bromide aqueous solutions on the surface strength of quartz glass under conditions of pH variation

A mechanism of the combined influence of solutions of electrolytes with various pHs (in 10^?3 N KCl solution) and cetyltrimethylammonium bromide (CTAB) cationic surfactant on the surface strength of quartz glass in a mechanical contact is studied. A similar cation-active medium is shown to embrittle quartz glass in neutral and alkaline pH ranges. The brittle strength of a near-surface layer of a material decreases owing to adsorption and electrocapillary reduction of its surface energy. The surface strength of the glass decreases to the largest extent at the CTAB critical micellization concentration. At low concentrations (10^?6M), a protective effect of forming chemisorbed CTAB layers prevails that is manifested in the hydrophobization of the surface of quartz glass by CTAB molecules and its screening from direct contact with an electrolyte solution. At the CTAB concentrations that are higher than the CMC, a formed micellar structure of CTAB has a lubricating effect, leading to softer boundary conditions of a frictional contact and a decreased probability of the damage of the near-surface layer of quartz glass. In acidic electrolyte solutions, CTAB cations or molecules did not affect the surface strength of quartz glass.     

A method for simultaneous estimation of inhomogeneous zeta potential and slip coefficient in microchannels

In the microchannels made of hydrophobic materials, the fluid velocity is determined by the zeta potential and velocity slip, both of which may be inhomogeneous due to the adsorption of protein to the channel wall. The inhomogeneity of zeta potential and slip coefficient sometimes causes recirculating flows which in turn affect the transport and mixing of solutes through the microchannels. In the present investigation we devise a method for the simultaneous estimation of inhomogeneous zeta potential and inhomogeneous slip coefficient using velocity measurements. A conjugate gradient method supplemented by the adjoint variable method is adopted in the solution of the relevant inverse problem to reduce the computational burden. The present method is found to estimate the inhomogeneous zeta potential and the slip coefficient simultaneously even with noisy velocity measurements. This method is expected to contribute to the optimal design and robust operation of various microfluidic devices, where the flow patterns and the volumetric flow rates are critically influenced by the profiles of inhomogeneous zeta potential and inhomogeneous slip coefficient.     

Effect of skimming layer in an electroosmotically driven viscoelastic fluid flow over charge modulated walls

We report a numerical study on the effect of the skimming layer in an EOF of Oldroyd-B fluid over charge modulated walls. Three types of flow conditions were identified on the basis of the relative thickness of the skimming layer and the electrical double layer. We observe maximum slip velocity magnitude when the skimming layer thickness is very less than the thickness of the electrical double layer. For higher skimming layer thickness compared to the thickness of electrical double layer, slip velocity magnitude attenuates, and the polymeric stress inside the skimming layer becomes zero. Enhanced fluid elasticity generates asymmetric flow structures inside the microchannel, which can also be achieved by imposing an asymmetric surface charge along the channel walls. Our present analysis highlights the complex flow dynamics of the EOF of biofluids/polymeric fluids with a near-wall region depleted of macro-molecules.     

Analysis of equal conformity of differential capacitance curves upon the adsorption of organic molecules with different models of surface layer

The differential capacitance curves are calculated, by using the model of three parallel capacitors, both for the electrical double layer dense part and the surface layer as a whole, at different ratios of the attraction constants in this model: a ₁₁, a ₁₂, and a ₂₂. It is shown by regressive analysis of the curves that these capacitance curves agree well with the model of two parallel capacitors, provided allowance is made for the linear potential dependence of the effective attraction constant in this model. When allowance is made for the electrical double layer diffuse part, the agreement with the model of two parallel capacitors is still improved, not only at the attraction constant critical value a ₁₂ = 2 but also at its smaller values (1.5 and 1.0). However, at a ₁₂ = 0.5 and rather high concentration of organic substances whose molecules adsorb in two different orientations the curves as if get divided into two parts separated with a maximum that reflects the process of the adsorption layer transformation. Under these conditions the model of two parallel capacitors turns to be inapplicable. It follows from the obtained results that the observed good applicability of the model of two parallel capacitors to the experimental data on the adsorption of numerous aliphatic compounds at electrodes by no means implies that the adsorption layer structure in the studied systems actually corresponds to the physical picture on which the model is based.     

Adsorption of interacting oligomers and polymers at an interface

We discuss in a qualitative way the physical background of a recently developed polymer adsorption theory, in which all the possible chain conformations for interacting chain molecules near an adsorbing interface are taken into account. Any conformation is described as a step-weighted random walk in a lattice. Each step is weighted according to a segmental weighting factor that contains the adsorption energy (for segments in contact with the surface), the entropy of mixing, and the attraction or repulsion between segments and solvent molecules. A suitable computing method is used to calculate the contribution of all chain conformations to the concentration profile, to the adsorbed amount, to the fraction of trains, loops and tails, to the layer thickness, etc. The theory is valid for any chain length and any concentration in the solution.Results for various chain lengths are given. Oligomers have a low affinity for the surface, whereas polymer adsorption isotherms are of the well known high affinity type. Three concentration regimes can be distinguished. In (extremely) dilute solutions the molecules on the surface adsorb as isolated chains (the Henry region).     

Influence of surfactants on the adsorption and elektrokinetic properties of the system: guar gum/manganese dioxide

The adsorption of non-ionic polysaccharide—guar gum (GG) in the presence or absence of the surfactants: anionic SDS, cationic CTAB, nonionic TX-100 and their equimolar mixtures SDS/TX-100, CTAB/TX-100 from the electrolyte solutions (NaCl, CaCl₂) on the manganese dioxide surface (MnO₂) was studied. The increase of GG adsorption amount in the presence of surfactants was observed in every measured system. This increase results from formation of complexes between the GG and the surfactant molecules. This observation was confirmed by the determination of the influence of GG on surfactants adsorption on the MnO₂ surface. The increase of GG adsorption on MnO₂ was the largest in the presence of the surfactant mixtures (CTAB/TX-100; SDS/TX-100) which is the evidence of the synergetic effect. The smallest amounts of adsorption were obtained in the presence of TX-100, which results from non-ionic character of this surface active agent. In the case of single surfactant solution CTAB has the best efficiency in increasing the amount of GG adsorption on MnO₂ which results from strong interactions with GG and also with the negatively charged surface of the adsorbent. In order to determine the electrokinetic properties of the system, the surface charge density of MnO₂ and the zeta potential measurements were performed in the presence of the GG macromolecules and the above mentioned surfactants and their mixtures. The obtained data showed that the adsorption of GG or GG/surfactants complexes on the manganese dioxide surface strongly influences the diffused part of the electrical double layer (EDL)—MnO₂/electrolyte solution, but has no influence on the compact part of the electric double layer. This is the evidence that the polymers chains are directly bonded with the surface of the solid and the surfactants molecules are present in the upper part of the EDL.     

Brownian dynamics simulations of polyelectrolyte adsorption in shear flow: effects of solvent quality and charge patterning

We probe the effects of solvent quality and charge patterning on polyelectrolyte adsorption in shear flow using Brownian dynamics simulations with hydrodynamic interaction (HI). The polyelectrolyte is modeled as a freely jointed bead-rod chain, and electrostatic and non-electrostatic interactions are accounted for by using screened Coulombic and Lennard-Jones potentials, respectively. In the absence of flow, the conformation of a polyelectrolyte molecule adsorbed onto a uniformly charged surface changes from flat to globular with an increase in bead-bead attraction (hydrophobicity), consistent with prior experimental observations. In the presence of flow, migration due to bead-wall HI and, as a consequence, desorption decrease with an increase in bead-bead attraction, implying that flow-induced desorption is more difficult under poor-solvent conditions. When bead-bead non-electrostatic attraction is strong, desorption can be enhanced by increasing bead-bead electrostatic repulsion. Analogous to the effect of bead-surface electrostatic attraction, an increase in the strength of bead-surface non-electrostatic attraction reduces desorption. We also study the effect of shear flow on the adsorption of a polyelectrolyte molecule onto surfaces decorated with periodic arrays of charged patches. An increase in patch periodicity increases desorption even when the effective surface charge density is kept the same. The results of this work suggest mechanisms for controlling the desorption of polyelectrolyte molecules in shear flows.     

Structure and mechanism of the deposition of multilayers of polyelectrolytes and nanoparticles

MD simulation of the layer-by-layer assembly of polyelectrolytes (PEs) and nanoparticles (NPs) revealed that the assembly process is electrostatically driven with alternating charge reversal and an overcompensation mechanism. Layers were observed to grow in the lateral direction as well as in a direction normal to the surface. Weakly adsorbed PE molecules were observed to desorb from the flat and NP surfaces. Those molecules are attracted by suspended NPs in solution. PE molecules do not only pull NPs toward the surface but bridge NPs both in solution and on the surface, forming agglomerates and islands. The first double layer differs in structure from the second double layer as a result of strong adsorption of the PE molecules to the rigid surface.     

Hydrogen-bonded macrocluster formation of ethanol on silica surfaces in cyclohexane(1)

Adsorption of ethanol onto silica surfaces from ethanol-cyclohexane binary liquids was investigated by a combination of colloidal probe atomic force microscopy, adsorption excess isotherm measurement, and FTIR spectroscopy using the attenuated total reflection (ATR) mode. An unusually long-range attraction was found between the silica (glass) surfaces in the presence of ethanol in the concentration range of 0.1-1.4 mol % at room temperature. At 0.1 mol % ethanol, the attraction appeared at a distance of 35 +/- 3 nm and turned into a repulsion below 3.5 +/- 1.5 nm upon compression. Half of the attraction range agreed with the adsorption layer thickness estimated from the adsorption excess amount by assuming that the adsorption layer was composed only of ethanol. This indicated that the observed long-range attraction was caused by the contact of opposed adsorption layers of ethanol on the silica surfaces and that the sharp increase of repulsion at shorter distance was caused by the overlap of structured ethanol clusters adjacent to the surface. ATR-FTIR spectra demonstrated that ethanol adsorbed on the silica (silicon oxide) surfaces formed hydrogen-bonded clusters (polymers). Practically no ethanol clusters were formed on the hydrogen-terminated silicon surface. These results indicated that the cluster formation involved hydrogen-bonding interactions between surface silanol groups and ethanol hydroxyl groups in addition to those between ethanol hydroxyl groups. At higher temperatures (30-50 degrees C), the range and the strength of attraction decreased owing to the decrease in the hydrogen-bonded clusters monitored by FTIR spectroscopy, reflecting the nature of hydrogen bonding. The range and the strength of the attraction also changed when the ethanol concentration increased: The long-range attraction started to decrease at 0.6 mol % ethanol at room temperature and disappeared at 1.4 mol % while the adsorption excess amount remained almost constant as did the FTIR peak intensity of the hydrogen-bonded OH group of adsorbed ethanol. In the bulk solution, ethanol clusters appeared at 0.5 mol % ethanol; thus, this change in the attraction could be accounted for in terms of the exchange of ethanol molecules between the surface clusters and bulk clusters. The novel self-assembled structure of alcohol on the surface, found in this study may be called a "surface molecular macrocluster" because the hydrogen-bonded clusters extend to distances of ca. 20 nm longer than the typical sizes of common clusters, 2-4 nm, of alcohol (e.g., ethanol).     

酪蛋白氢氧化钠稀溶液的粘度研究

酪蛋白(casein)的溶液性质由于其重要的工业应用价值吸引了广泛的关注,但是在以往酪蛋白稀溶液的粘度研究中忽略了界面效应对粘度测定的影响[1~8],而界面效应对大分子稀溶液粘度行为的巨大作用已得到证实并日益受到重视[9~12].本文研究了20~45℃之间酪蛋白氢氧化钠稀溶液的粘度     

Effects of pH and ionic strength on the stability of nanobubbles in aqueous solutions of alpha-cyclodextrin

Formation of stable nanobubbles in aqueous solutions of water-soluble organic molecules is a spontaneous process. Using a combination of laser light scattering (LLS) and zeta-potential measurements, we investigated the effects of salt concentration and pH on their stability in alpha-cyclodextrin (alpha-CD) aqueous solutions. Our results reveal that the nanobubbles are unstable in solution with a higher ionic strength, just like colloidal particles in an aqueous dispersion, but become more stable in alkaline solutions. The zeta-potential measurement shows that the nanobubbles are negatively charged with an electric double layer, presumably due to adsorption of negative OH- ions at the gas/water interface. It is this double layer that plays a critical dual role in the formation of stable nanobubbles in aqueous solutions of water-soluble organic molecules, namely, it not only provides a repulsive force to prevent interbubble aggregation and coalescence but also reduces the surface tension at the gas/water interface to decrease the internal pressure inside each bubble.     

Model of the dense part of the double layer in the absence of specific adsorption

A model is suggested for the dense part of the double layer in the absence of specific adsorption, which is based on the following assumptions: (1) There are two kinds of adsorbed solvent particles on the electrode surface: associates freely oriented in the double layer field and separate chemisorbed water dipoles; 92) the total number of solvent molecules on the surface is determined by the geometric dimensions of associates and chemisorbed dipoles; (3) account is taken of the electrostatic interaction of associates and chemisorbed dipoles in the surface layer; (4) a discreteness coefficient is introduced, accounting for the degree of influence of the field due to all the other dipoles and associates in the surface layer on the dipole or associate being considered; (5) the dipole moment of a chemisorbed water molecule exceeds the value of μH₂O in the bulk of the solution; (6) account is taken of the decrease with increasing temperature of the average number of water molecules in surface associate.The calculations on the basis of this model describe satisfactorily the shape of the differential capacity curves of the dense layer and their temperature dependence for the system Hg-aqueous NaF solution.     

γ-Fe2O3的表面润湿性与其悬浮液的稳定性

自吸附与表面润湿性的测量确定,油酸钠自水溶液在γ-Fe₃O₂上的吸附为多层吸附。第一层为化学吸附,油酸钠分子的碳氢链朝外,从而使质点表面憎水化。在高浓度时发生第二层吸附,油酸钠分子依靠碳氢链之间的相互作用吸附在第一吸附层上,使质点表面重新亲水化。质点表面的润湿性与其水相和非水相悬浮液的稳定性有直接的对应关系。     

氧化石墨烯吸附亚甲基蓝的分子动力学模拟

采用分子动力学方法研究了亚甲基蓝在不同氧化度的氧化石墨烯表面的吸附行为及其动力学性质, 从微观角度讨论了亚甲基蓝由体相到氧化石墨烯表面的吸附过程及主要作用机制, 并通过亚甲基蓝分子动力学性质解释了氧化石墨烯的氧化度和含氧官能团类型对吸附行为的影响. 结果表明, 吸附过程中, 亚甲基蓝主要受氧化石墨烯表面含氧官能团的静电作用, 以近似垂直氧化石墨烯表面的方向进入, 并以平行的方式吸附于氧化石墨烯表面; 亚甲基蓝不易脱离高氧化度氧化石墨烯的吸附位点; 吸附平衡过程中, 相对于低氧化度的氧化石墨烯, 高氧化度氧化石墨烯对亚甲基蓝的束缚性更强, 同时与亚甲基蓝间相互作用更强; 含氧官能团中的环氧基与亚甲基蓝间的作用势能更强, 且羟基能够与亚甲基蓝间形成氢键结构, 共同保障了亚甲基蓝吸附层的稳定性.     

Molecular macrocluster formation on silica surfaces in phenol-cyclohexane mixtures

The adsorption of phenol, an aromatic compound with a hydrogen-bonding group, onto a silica surface in cyclohexane was investigated by colloidal probe atomic force microscopy (AFM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and adsorption isotherm measurements. ATR-FTIR measurements on the silica surface indicated the formation of surface macroclusters of phenol through hydrogen bonding. The ATR-FTIR spectra were also measured on the H-terminated silicon surface to observe the effect of the silanol groups on the phenol adsorption. The comparison of the ATR-FTIR spectra for both the silicon oxide and H-terminated silicon surfaces proved that the silanol groups are necessary for the formation of phenol clusters on the surface. The surface force measurement using colloidal probe AFM showed a long-range attraction between the two silica surfaces in phenol-cyclohexane mixtures. This long-range attraction resulted from the contact of the adsorbed phenol layers for the phenol concentrations below 0.6 mol %, at which no significant phenol clusters formed in the bulk solution. The attraction started to decrease at 0.6 mol % phenol due to the exchange of the phenol molecules between the clusters in the bulk phase and on the surface. The surface density of phenol in the adsorbed layer was calculated on the basis of the long-range attraction and found to be much smaller than the liquid phenol density. The plausible structure of the adsorbed phenol layer was drawn by referring to the crystal structure of the bulk phenol and orientation of the phenol molecules on the surface, estimated by the dichroic analysis of ATR-FTIR spectroscopy. The investigation of the phenol adsorption on the silica surface in a nonpolar solvent using this novel approach demonstrated the effect of the aromatic ring on the surface packing density.