Motion of a Colloidal Sphere Covered by a Layer of Adsorbed Polymers Normal to a Plane Surface

A combined analytical–numerical study is presented for the slow motion of a spherical particle coated with a layer of adsorbed polymers perpendicular to an infinite plane, which can be either a solid wall or a free surface. The Reynolds number is assumed to be vanishingly small, and the thickness of the surface polymer layer is assumed to be much smaller than the particle radius and the spacing between the particle and the plane boundary. A method of matched asymptotic expansions in a small parameter λ incorporated with a boundary collocation technique is used to solve the creeping flow equations inside and outside the adsorbed polymer layer, where λ is the ratio of the characteristic thickness of the polymer layer to the particle radius. The results for the hydrodynamic force exerted on the particle in a resistance problem and for the particle velocity in a mobility problem are expressed in terms of the effective hydrodynamic thickness (L) of the polymer layer, which is accurate to O(λ²). The O(λ) term forLnormalized by its value in the absence of the plane boundary is found to be independent of the polymer segment distribution and the volume fraction of the segments. The O(λ²) term forL, however, is a sensitive function of the polymer segment distribution and the volume fraction of the segments. In general, the boundary effects on the motion of a polymer-coated particle can be quite significant.     

Effects of polymer solutions on colloid stability

Some theoretical properties of a semidilute polymer solution (in a good solvent) near a repulsive wall (no adsorption) are discussed. (i) Near a single wall, the existence of a depletion layer (where the concentration is strongly reduced) with thickness equal to the correlation length ξ(c) is predicted. Scaling laws are obtained for the concentration c_s at the first layer as a function of the bulk concentration c, and for the concentration profile c(z) at distance z from the wall. (ii) When two parallel walls are separated by a slab of polymer solution (thickness d), there is an attractive interaction between the plates. The range of the attraction is ca. 3ξ(c) and can be varied by suitable choice of concentration. It may lead to destabilization of certain colloidal systems.     

Strength of interaction and penetration of infrared radiation for polymer films in internal reflection spectroscopy

The strength of interaction of infrared radiation with relatively thick polymer films was systematically studied by internal reflection spectroscopy. The so-called “effective thickness” was used as a measure of the strength of interaction. The experimental and theoretical effective thickness were compared, and it was found that the theoretically predicted effective thickness can be approached or achieved experimentally with film samples routinely encountered in the polymer industry when moderate attention is paid to surface preparation and clamping force on the film in the internal reflection sample holder. A series of curves for various types of film samples was developed and found to be useful in predicting the spectral contrast for KRS-5 and germanium internal reflection elements. These data were collected for infrared radiation polarized parallel and perpendicular to the plane of incidence and for unpolarized radiation. The data for unpolarized radiation was checked for the effects of the inherent polarization of the Fourier-Transform Infrared Spectrometer used by comparing it to the arithmetic mean of the data for parallel and perpendicular polarization. These data were in good agreement and this indicated that the inherent polarization of the instrument introduced only minor errors into the data collected with unpolarized radiation. The concept of the “sampling depth” was established where the sampling depth d_s was defined as the depth normal to the surface sampled. The sampling depth was found to be on the order of three times the value of d_p, the depth of penetration, at 45° for polypropylene and polystyrene on KRS-5 and was observed to exceed the effective thickness values at the same conditions. The value of the electric field amplitude was observed to fall to 5% of its value at the surface at d_s.     

Dispersion and polar contributions to surface tension of poly(methylene oxide) and Na-treated polytetrafluoroethylene

Average values for dispersion γ_s^d and polar γ_s^d contributions of the solid surface tension γ_s γ_s^d + γ_s^p for poly(methylene oxide) (PMO) and Na-treated polytetrafluoroethylene (PTFE) are determined by a new computational analysis of wettability data. PMO displays γ_s^d equals; 21.8 ± 0.9 and γ_s^p = 11.5 ± 1.5 dyn/cm while Na-treated PTFE displays γ_s^d = 36.1 ± 3.0 and γ_s^p = 14.5 ± 2.9 dyne/cm. These surfaces present the highest fractional surface polarity p_s = γ_s^p/γ_s = 0.29-0.35 yet encountered for organic polymers or oriented monolayers. These unusual surface tension properties are correlated with surface chemistry and adhesion phenomena.     

Adsorption of cationic polyelectrolyte onto a model carboxylic latex and the influence of adsorbed polycation on the charge regulation at the latex surface

 Adsorption of a well-characterized cationic polyacrylamide (CPAM) onto the surface of a model colloid (monodisperse polystyrene latex with carboxylic functional groups) was studied over a wide range of pH (4–9) and KCl concentration (c _s =10^-3–0.3 M). The surface charge density of the latex particles with and without adsorbed CPAM was also measured over the same range of electrolyte compositions. The adsorbed amount of CPAM increases with increase in c _s and pH. The polyelectrolyte adsorption alters substantially the surface charge density of the latex particles as compared to the polymer-free case. A large overcompensation of the surface charge by the adsorbed polyelectrolyte is established at high c _s and low pH. A qualitative explanation of the observed features is put forward. Received: 3 December 1996 Accepted: 20 January 1997     

Determination of ZnO(0001) surface termination by x‐ray photoelectron spectroscopy at photoemission angles of 0° and 70°

We present an XPS method to determine the termination of the ZnO(0001) surface. By measuring O 1s and Zn 2p_3/2 core‐level x‐ray photoelectron spectra at photoemission angles of 0° and 70° and comparing the intensity ratio (I_O1s/I_Zn2p3)_θ=0/(I_O1s/I_Zn2p3)_θ=70, the Zn and O termination can be distinguished. Calculations show that these two terminations have intensity ratios differing by ～17%. This difference is not affected by a contamination layer provided that the contamination layer thickness is the same for these two differently terminated surfaces. Although this determination method prefers a clean ZnO(0001) surface (in situ measurement), it seems also feasible for surfaces with known contamination layer thickness (ex situ measurement). We have measured ex situ ZnO(0001)‐Zn, ZnO(000&1macr;)‐O single crystals and an epitaxial ZnO film deposited on Al₂O₃(0001). The measured intensity ratios of the first two samples agree with the calculated values for a 0.2 and 0.26 nm contamination layer, respectively. The intensity ratio and the O 1s contamination component intensity of the epitaxial ZnO film are close to those of the ZnO(0001)‐Zn single crystal thus pointing at Zn termination of the film. Copyright © 2004 John Wiley & Sons, Ltd.     

The microstructure and magnetic properties of Ni<Subscript>0.4</Subscript>Zn<Subscript>0.6</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> films prepared by spin-coating method

Nickel zinc ferrite (Ni_0.4Zn_0.6Fe₂O₄) films on Si (100) substrate were synthesized using a spin-coating method. The crystallinity of the Ni_0.4Zn_0.6Fe₂O₄ films with the thickness of about 386 nm became better as the annealing temperature increased. The films have smooth surface, relatively good packing density and uniform thickness. The volatilization of Zn is serious at 900 °C. With the increase of annealing temperature, the saturation magnetization M _s increases in the temperature ranging from 400 to 700 °C, however, decreases above 700 °C, and the coercivity H _c increases in the temperature range 400–800 °C, decreases above 800 °C. After annealed at 700 °C for 2 h in air with the heating rate 2 °C/min, the film shows a maximum saturation magnetization M _s of 349 emu/cc and low coercivity H _c of 66 Oe. The M _s is higher than others which prepared by this method, however, the H _c is lower. The M _s of Ni_0.4Zn_0.6Fe₂O₄ films annealed at 700 °C increases with increasing annealing time and the H _c changes slightly.     

Topological studies on IRC paths of X+H2→XH+H reactions

Topological properties of potential energy and electronic density distribution on five reaction paths X+H₂→XH+H (X=H, N, HN, H₂C, NC) are investigated at the level of UMP2/6–311G(d,p). It has been found that in the region of the reaction paths studied, where B(r_c)|_s>0 [B(r_c)|_s is the product of ρ(r_c) and ∇²ρ(r_c) at the point of reaction process, i.e., B(r_c)|_s=ρ(r_c)∇² ρ(r_c)] is basically the same as the region of V′(s)<0[V′(s) is the second derivative of potential energy with respect to the reaction coordinate, i.e., V′(s)=d²V/ds²], and the point with maximum B(r_c)|_s is almost coincident with the point of minimum V′(s). It can be concluded from the calculated results that there is a good correlation between the topological properties of potential energy and electronic density distribution along the reaction path. The structure transition state of such collinear reactions may be determined by topological analysis of electronic density. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1167–1174     

Characterisation of the surface of a cellulosic multi-purpose office paper by inverse gas chromatography

The surface of multi-purpose cellulosic office paper has been analysed by inverse gas chromatography (IGC). The parameters determined were the dispersive component of the surface free energy, the enthalpy of adsorption and the entropy of adsorption of polar and apolar probes, the Lewis acidity constant, K _a, and the Lewis basicity constant, K _b. It can be concluded that the dispersive component of the surface free energy, _s ^d decreases with temperature, in the range 50–90°C. The temperature coefficient of _s ^d, d_s ^d/dT, is –0.35 mJm ^–2K^–1. The values of K _a and K _b were determined to be 0.11±0.011 and 0.94±0.211, respectively. The predominant surface basicity agrees with expectation, bearing in mind the presence of calcium carbonate, and of a styrene-acrylic copolymer, in the surface sizing formulation. It is thought that during the drying stages following the surface sizing treatment, the starch used as the binder migrates to the interior of the surface sizing layer and then to the paper bulk itself. This migration contributes to a decrease in the hydrophilicity of the surface, and also results in the surface showing only slight Lewis.     

The structural,electronic, and magnetic properties of the stoichiometric (001) surface of double perovskite Sr2FeMoO6

The structural, electronic, and magnetic properties of the stoichiometric (001) surface of double perovskite Sr₂FeMoO₆ have been studied by using a 10‐layer FeMoO₄ and SrO terminated (001)‐oriented slab model and the first‐principles projector augmented wave potential within the generalized gradient approximation as well as taking into account the on‐site Coulomb repulsive (U = 2.0 eV for Fe and 1.0 eV for Mo). An outwards relaxation is observed for several layers near surface, and the accompanying layer rumpling has a decrease tend from surface layer to inner layer. Along Fe–O–Mo–O–Fe or Mo–O–Fe–O–Mo chains, the oxygen atom is closer to the adjacent Mo atom than to the adjacent Fe atom. In FeO₆ or MoO₆ octahedra, the two axial TM?O bonds are not equal, and especially, the surface dangling bond makes the remaining one axial TM?O bond slightly shorter than four equally equatorial TM?O bonds. The half‐metallic nature and a complete (100%) spin polarization character ensure the FeMoO₄ and SrO terminated (001)‐oriented slab of double perovskite Sr₂FeMoO₆ a potential application in spintronics devices. The Fe⁺³ and Mo⁺⁵ ions are still in the (3d⁵, S = 5/2) and (4d¹, S = 1/2) states with positive and negative magnetic moments respectively and thus antiferromagnetic coupling via oxygen between them. There is no direct interaction between two nearest Fe–Fe or Mo–Mo pairs, whereas the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Mo 4d, 5s and 5p orbitals are fairly significant. Copyright © 2016 John Wiley & Sons, Ltd.     

Features of the potential energy surface for the reaction of HO2 radical with acetone

The mechanism of the reaction of acetone with HO₂ has been studied by quantum chemical computations. Different stationary points on the potential energy surface (PES) of the reaction have been characterized. These stationary points are the reactants, products, molecular complexes, and transition states. Three pathways have been studied: two H‐abstraction channels and one HO₂‐addition channel. The MP2 level of theory with the 6‐311G(d,p) basis set was employed for geometry optimization. The electronic energies was obtained at the PMP2, PMP4, and CCSD(T) level of theory with the 6‐311G(d,p) basis set on the computed geometries. The addition pathway is clearly the more favorable, contrary to the acetone + OH system. The pre‐reactive hydrogen‐bonded complexes have been characterized and show a large red shift between the O? H stretching frequency in the HO₂ radical and the one in the HO₂ fragment of intermolecular complexes. Our addition rate constant k₊ at T = 298 K (3.49 × 10^?16 cm³ s^?1) is consistent with previous experimental results (giving an upper limit of the rate constant of 6 × 10^?16 cm³ s^?1 at 298 K). © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Cyclic voltammetric generation and electrochemical quartz crystal microbalance characterization of passive layer of nickel in a weakly acid medium

The electrochemical quartz crystal microbalance results show that nickel electrodissolution and nickel passivation occur simultaneously. Besides, the anodic transferred charge decreases and the passive layer thickness increases with the number of successive voltammetric cycles. Mass balances across the metal/passive layer/solution have been done from the instantaneous F(dm/dQ) function. A dynamic process for generation of an inner NiO and a Ni(OH)₂ outer passive layers is suggested.     

Critical surface tension of poly(vinyl butyral) and poly(vinyl benzal) multilayers and its relation to their surface orientation. I. Effect of surface pressures during deposition

Critical surface tensions γ_c of poly(vinyl butyral) and poly(vinyl benzal) multilayers built up by the Langmuir-Blodgett method were measured with polyhydric alcohols and n-alkanes. The γ_c values of both polymer multilayers increased with increasing pressures of the piston oils used to control pressures of polymers on the water surface during deposition. The γ_c value of poly(vinyl butyral) multilayers built up to lower pressure of the piston oil was approximately consistent with a crystalline hydrocarbon surface, while the γ_c value of the multilayer built up to higher pressure of the piston oil was approximately consistent with a—CH₃ rather than a ? CH₂ ? CH₂? surface. All results for γ_c values of poly(vinyl benzal) multilayers were very close to the γ_c value of benzene ringrich surface. The γ_c value of the multilayer built up to lower pressure of the piston oil almost coincided with the γ_c value for amorphous polystyrene, while the γ_c value for the multilayer built up to higher pressure of the piston oil was in fair agreement with γ_c for an aromatic ring edge in the crystalline state. Values of γ_s^d, the dispersion force contribution to the surface free energy of multilayers calculated by Fowkes' relation, were in fair agreement with the corresponding observed γ_c values, respectively. It is concluded from these measurements that orientations and surface structures in both polymer multilayers are affected by pressure change of piston oils. The properties on monolayers of two polymers at a air-water interface and on barium stearate multilayers are also presented.     

Modeling of interaction properties of surfaces of phyllosilicates: A theoretical forecast of adsorption isotherms of noble gases at the talc surface

The electrostatic potential and the intensity of the electric field above the isolated layer of the phyllosilicate mineral talc (layer group symmetry C2/m) were computed using the semiempirical INDO /2 method. The electrostatic potential V_c and the intensity of electric field (OVERBAR)E_c, above the surface of semi-infinite crystal were obtained as the sum of the contributions of the infinite number of individual layers. The interaction energy U_int between a noble gas atom and the talc crystal was computed as (a) a pure Lennard-Jones energy U_LJ and (b) the sum of U_LJ and interaction energy with induced dipole moment: U_ind = α|(OVERBAR)E|², where α is the dipole polarizability of the noble gas atom. The one-particle configurational integral was calculated within the classical mechanics limit. Both the free and adsorbed gas phases were assumed to be ideal. The obtained results demonstrate that in the case of uncharged and nonpolar adsorbate and an uncharged surface the electrostatic part of the interaction energy should be included in the total interaction energy. © 1996 John Wiley & Sons, Inc.     

Two-Layer Model of Colloidal Gold Bioconjugates and Its Application to the Optimization of Nanosensors

An optical model of the conjugates of colloidal gold nanoparticles with biopolymers is analyzed in terms of two-layer spherical particle with the gold core and dielectric coating. The Mie theory was used to study the dependence of variations (caused by the adsorption of a biopolymer on the particle surface) in the extinction and light scattering (at 90°) spectra on the gold core diameter (d = 5–200 nm), the shell refractive index and thickness s (ratio s/d = 0–1). Some theoretical results by Templeton et al. (J. Phys. Chem. B, 2000, vol. 104, pp. 564–570) on the two-layer dipole model were corrected. It is shown that the dependence of spectral shifts of the extinction and scattering peaks on the conjugate structure is adequately described by the dipole approximation. In particular, we found the universal dependence of the normalized spectral shift of extinction maximum on the s/d ratio. Having in mind the optimization of conjugate–nanosensors, we studied the problem of what particle size is optimal for the transformation of biopolymer adsorption event into the variations in the spectral parameters of extinction and light scattering. Based on the calculations of extinction maximum values and positions, as well as on calculated differential extinction spectra, we concluded that a maximal conjugate efficiency corresponds to the core diameters of 40–80 nm. We also discussed the principles of conjugate–nanosensors optimization for the polymer shell structure.     

Static dipole and quadrupole polarizabilities of alkaline earth atoms

Static dipole and quadrupole polarizability (α_d and α_q, respectively) values of coupled Hartree–Fock quality have been calculated for the alkaline earth atoms Be(2s), Mg(3s), Ca(4s), and Sr(5s) using a method based on the many-body perturbation theory. The present values of α_d and α_q for Be and α_d for Mg are in excellent agreement with the other available results of similar accuracy. The CHF calculations of α_d for Sr and α_q for Mg, Ca, and Sr are being reported for the first time.     

Adsorption of water soluble ionic/hydrophobic diblock copolymer on a hydrophobic surface

Summary We consider the adsorption of aA-B diblock copolymer on a planar hydrophobic surface in aqueous solution. The hydrophobic anchor (A) block is envisioned to avoid water and adsorbs on the solid-liquid interface in a collapsed state. The buoy block (B) is a polyelectrolyte which expands in solution and forms a brush whose structure depends strongly on the ionic strength of the solution. The minimization of the grand canonical free energy of the system gives access to the surface density (), the thickness of the collapsed layer (L _A ) and the thickness of the external polyelectrolyte layer (L _B ). These three parametersL _B ,L _A and are functions of the molecular weight of the anchored block (N _A ), the molecular weight of the buoy block (N _B ), the charge of the polymer (Z) and the ionic strength of the aqueous solution ( _s ).     

Wetting properties of bis(trifluoromethyl)carbinyl acrylate polymers

A series of bis(trifluoromethyl)carbinyl acrylate monomers [Y-C(CF₃)₂ O? CO? CH?CH₂] in which Y is CH₃, CH₃CH₂, CH₃CH₂CH₂, CH₃CH₂CH₂CH₂, C₆H₅, H, F, CF₃, N₃, CN, and CH₃OCH₂CH₂O, was prepared. Polymers were easily prepared from all of these monomers except where Y = CN, wherein a variety of initiation methods failed to produce high molecular polymer. Wettabilities of the polymer films were examined by means of contact angle measurements by using n-alkane test liquids and water. Values of the dispersion force contribution (γs^d) and the polar force contribution (γs^p) to the solid surface energy were calculated by employing both geometric and harmonic mean approximations. Values of γs^d calculated by either method agreed well with γ_c (critical surface tension) values determined graphically from contact angle data employing n-alkane test liquids, confirming the suggestion that γ_c is an approximate measure of the dispersion force contribution to solid surface energy. Values of γs_d ranged from 15 dyne/cm (Y = F or CF₃) to 25 dyne/cm (Y = C₆H₅). Values of the polar force contribution to solid surface energy (γs^p) varied from 0.6 dyne/cm (Y = CH₃CH₂CH₂CH₂) to 3.4 dyne/cm (Y = CH₃OCH₂CH₂O) when calculated by the geometric mean equation. The values of γs^p obtained from the harmonic mean equation followed the same trend upon varying substituents, but were larger in value, ranging from 2.9 dyne/cm (Y = CH₃CH₂CH₂CH₂) to 7.5 dyne/cm (Y γ CH₃OCH₂CH₂O).     

Effect of end group chemistry on surface molecular motion of monodisperse polystyrene films

The surface molecular motion of monodisperse polystyrene (PS) with various chain end groups was investigated on the basis of temperature‐dependent scanning viscoelasticity microscope (TDSVM). The surface glass transition temperatures, T_g^ss for the proton‐terminated PS (PS‐H) films with number‐average molecular weight, M_n of 4.9k–1,450k measured by TDSVM measurement were smaller than those for the bulk one, with corresponding M_ns, and the T_g^ss for M_n smaller than ca. 50k were lower than room temperature (293 K). In the case of M_n = ca. 50k, the T_g^ss for the α,ω‐diamino‐terminated PS (α,ω‐PS(NH₂)₂) and α,ω‐dicarboxy‐terminated PS (α,ω‐PS(COOH)₂) films were higher than that of the PS‐H film. On the other hand, the T_g^s for the α,ω‐perfluoroalkylsilyl‐terminated PS (α,ω‐PS(SiC₂C^F₆)₂) film with the same M_n was much lower than those for the PS films with all other chain ends. The change of T_g^s for the PS film with various chain end groups can be explained in terms of the depth distribution of chain end groups at the surface region.     

Effect of deuterium substitution on the surface interactions in binary polymer mixtures

We have examined the effect of deuterium labeling on surface interactions in mixtures of random olefinic copolymers [C₄H₈]_1−x[C₂H₃(C₂H₅)]_x. Based on surface segregation data we have determined a surface energy difference χ_s between pure blend constituents. In each binary mixture components have different fractions x₁, x₂ of the group C₂H₃(C₂H₅), and one component is labeled by deuterium (dx) while the other is hydrogenous (hx). The mixtures are grouped in four pairs of structurally identical blends with swapped labeled constituent (dx₁/hx₂, hx₁/dx₂). For each pair the surface energy parameter χ_s increases when the component with higher fraction x is deuterated, i.e., χ_s(dx₁/hx₂) > χ_s(hx₁/dx₂) for x₁ > x₂. A similar pattern has been found previously for the bulk interaction parameter χ. This is explained by the solubility parameter formalism aided by the lattice theory relating the surface excess to missing-neighbor effect. χ_s has also an additional contribution, insensitive to deuterium swapping effect, and related to entropically driven surface enrichment in a more stiff blend component with a lower fraction x. Both enthalpic and entropic contributions to χ_s seem to depend on the extent of chemical mismatch between blend components. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2691–2702, 1998