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Van Der Waals Forces (van + der_waal_force)
Selected AbstractsMicropatterned Polymer Surfaces and Cellular Response of Dictyostelium,ADVANCED ENGINEERING MATERIALS, Issue 5 2010Magdalena Eder Gecko inspired adhesives are surfaces with many microscale pillars that form Van der Waals forces with other surfaces. They differ from conventional tape in that adhesion is reversible and has the potential for switchability. These properties make gecko adhesives interesting for various biomedical applications. The two objectives of this project were to investigate the formation of biofilms on such surfaces and how the surfaces affect cell development. The developmental stages of the model organism Dictyostelium discoideum were observed by time lapse photography using light and environmental scanning electron microscopy. This study shows that micropatterned surfaces can be used as a biophysical tool to interfere with multicellular tissue formation in multiple ways. [source] Crystal structure of ,-phenoxo bridged dicopper complex: {N-[(2-hydroxylato-5-methyl)benzyl-(2,-hydroxylato-3,,5,-dimethylbenzyl)]ethyl amine dicopper(II)}CRYSTAL RESEARCH AND TECHNOLOGY, Issue 12 2002S. M. Malathy Sony Abstract The title compound crystallizes in monoclinic space group C2/c with cell parameters a = 21.404(2), b = 13.962(1), c = 17.917(1)Å, , = 124.394(2)°, V = 4418.3(6)Å3, Z = 8, Dcal = 1.193Mg/m3 and T = 293 K. The structure was solved by Patterson method and refined by full-matrix least-squares procedures to final R = 0.0882 using 5253 observed reflections. The tetra coordinated copper atom have a slight distorted square planar geometry with the Cu-Cu distance of 2.987(1)Å. The two six membered rings containing copper atom assume distorted sofa conformation. C-H,, and C-H,O type of intermolecular interactions play a role in stabilizing the crystal packing in addition to van der Waals forces. [source] Synthesis, Crystal Structure, and Optical Properties of a New Molecular Complex of C60 with a Covalently Linked (FeIIITPP)2O DimerEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 21 2003Aleksey L. Litvinov Abstract A new molecular complex of C60 with covalently linked ,-oxo dimer (FeIIITPP)2O are (TPP = tetraphenylporphyrin) was obtained. The complex has a neutral ground state and is formed mainly by van der Waals forces. The X-ray analysis of the crystal structure of the complex showed it to have isolated packing of fullerenes in which each fullerene molecule is embraced in a pocket built by porphyrins. Optical properties and EPR behavior of the complex are described. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Non-Covalent Interactions of Organic Halogen Compounds with Aromatic Systems , Analyses of Crystal Structure DataEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 6 2005Dariusz Swierczynski Abstract The Cambridge Structural Database showed in mid 2002 about 20.000 structures containing halogen atoms and aryl rests with distances d between the aryl center and the halogen atom, which would allow both hydrogen bonds with the aromatic hydrogens and/or van der Waalsinteractions with the ,-cloud. The latter are characterized by short distances d and by small angles , between the vector along the aryl centroid,halogen line and the perpendicular vector originating in the aryl center (the plane normal). The cases with d < 3.0 Å for F; and d < 3.5 Å for Cl, Br or I (outliers neglected), and , , 10 ± 5°, indicating predominating van der Waals forces, amount to several hundreds. The majority of fragments exhibit larger d and , values, in line with avoidance of electrostatic repulsion between the negative partial charges of the halogens and the ,-cloud center, and with an increasing electrostatic attraction with the aromatic hydrogen atoms. The corresponding hydrogen bonds are characterized by longer distances d and by angle values of , > 60° (about 40 % of the fragments), with only very few cases close to linear hydrogen bonds (then with , = 90°). Compounds containing metal,halogen bonds were omitted in view of possible strong orientational interferences. The structures were screened with four different halogen binding modes, all of them containing the halogen attached to a carbon atom, but with different hybrizations at the connecting carbon centers. The results show only small differences in the distance distributions, with a slight preference for smaller , values for sp2 frameworks. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Preparation and Characterization of Uniformly Sized Sub-micrometer Spherical Silica/Organic Polymer Hybrid ParticlesADVANCED ENGINEERING MATERIALS, Issue 9 2003X.-s. Xing Hybrid particles with a core-shell structure, consisting of a silica core and a polyvinyl alcohol (PVA) shell, (see Figure for an SEM image of a particle material of this type), were fabricated via a two-step sol-gel process. The PVA molecular chains are probably physically adsorbed onto the surface of silica cores by hydrogen bonds and van der Waals forces. [source] Self-Assembled, Molecularly Aligned Conjugated Polymer Nanowires via DewettingADVANCED FUNCTIONAL MATERIALS, Issue 18 2010Shion Seng Chang High aspect-ratio poly(9,9,-dioctylfluorene) (F8) nanowires are fabricated on top of silicon/amorphous Teflon substrates, by dewetting of F8 thin-films. The conjugated nature of the polymers enables the self-assembly of continuous molecularly-aligned nanowires of several micrometers in length and tens of nanometers width, without using a template. The sizescale of the dewetted morphology can be controlled by varying the thickness of the F8 and amorphous Teflon layers. As predicted by van der Waals theory the characteristic sizescale of the dewetted morphology increases with F8 film thickness. However, the dependence of the characteristic sizescale on amorphous Teflon thickness is not accounted for, even qualitatively, by standard spinodal theory with van der Waals forces as the de-stabilizing force across the F8 film. The Rayleigh instability is strongly suppressed in the F8 nanowires in the late stages of dewetting, compared to isotropic, amorphous polymers. Polarized Raman measurements show a systematic increase in molecular alignment along the axis of the nanowires as their width is reduced below the typical liquid-crystalline domain size in polyfluorene films. Thus the dewetting process aligns the polymers, and the aligned polymers suppress the Rayleigh instability and enable the formation of high aspect-ratio continuous nanowires. [source] Formation and Properties of Responsive Siloxane-Based Polymeric Surfaces with Tunable Surface Reconstruction KineticsADVANCED FUNCTIONAL MATERIALS, Issue 3 2009Julie Ann Crowe-Willoughby Abstract Here, the formation of responsive polymeric materials with tunable response time is reported. These structures are fabricated by chemically modifying poly(vinylmethyl siloxane) (PVMS) networks with alkanethiols bearing a hydrophilic end-group (COOH or OH). The response time is facilitated by the liquid nature of the PVMS backbone and increases with increase in length of the methylene spacer (CH2)n in the alkanethiol pendent group. While for n,=,2 and 6, the surface reconstructs almost instantaneously, specimens with n,=,11 resist reconstruction because of strong van der Waals forces, leading to the formation of semi-crystalline regions. It is demonstrated that the responsive nature of PVMSS(CH2)11OH can be fine-tuned by varying the temperature; it possesses a faster response at temperatures above the melting point of the S(CH2)11OH moiety. [source] Range and strength of intermolecular forces for van der Waals complexes of the type H2Xn -Rg, with X = O, S and n = 1,2INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2010Patrícia R. P. Barreto Abstract This study is intended as a continuation of previous experimental and theoretical works on the systems H2O-Rg, H2S-Rg, H2O2 -Rg, and H2S2 -Rg, where Rg = He, Ne, Ar, Kr, Xe. For the H2O-Rg and H2S-Rg systems, molecular and atomic polarizabilities have been calculated and from them, using phenomenological correlation formulas modeling the dispersion-repulsion (van der Waals) forces, the isotropic interaction parameters have been determined and compared with experimental data from this laboratory. For the H2O2 -Rg and H2S2 -Rg systems, the molecular polarizabilities have been calculated and used in correlation formulas to predict well depths and positions of van der Waals forces and a comparison made with the corresponding potential energy surfaces calculated in previous works. The approach correctly predicts the interaction parameters, except for H2O and H2O2 with the heavier rare gases. The correlation formulas have been then extended to include an attractive induction contribution accounting for the interaction between the permanent molecular dipole moment and the instantaneous induced atomic dipole moment, to improve the predicted parameters for H2O and H2O2 -Ar, Kr and Xe. The agreement with experimental and theoretical data is improved but the predicted data still underestimate the interaction. This is probably due to the presence of a significant non van der Waals contribution to the interaction for the heavier gases, as suggested by analogy with the previously studied water-Rg case. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] An investigation of the factors controlling the adsorption of protein antigens to anionic PLG microparticlesJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 11 2005James Chesko Abstract This work examines physico-chemical properties influencing protein adsorption to anionic PLG microparticles and demonstrates the ability to bind and release vaccine antigens over a range of loads, pH values, and ionic strengths. Poly(lactide-co-glycolide) microparticles were synthesized by a w/o/w emulsification method in the presence of the anionic surfactant DSS (dioctyl sodium sulfosuccinate). Ovalbumin (OVA), carbonic anhydrase (CAN), lysozyme (LYZ), lactic acid dehydrogenase, bovine serum albumin (BSA), an HIV envelope glyocoprotein, and a Neisseria meningitidis B protein were adsorbed to the PLG microparticles, with binding efficiency, initial release and zeta potentials measured. Protein (antigen) binding to PLG microparticles was influenced by both electrostatic interaction and other mechanisms such as van der Waals forces. The protein binding capacity was directly proportional to the available surface area and may have a practical upper limit imposed by the formation of a complete protein monolayer as suggested by AFM images. The protein affinity for the PLG surface depended strongly on the isoelectric point (pI) and electrostatic forces, but also showed contributions from nonCoulombic interactions. Protein antigens were adsorbed on anionic PLG microparticles with varying degrees of efficiency under different conditions such as pH and ionic strength. Observable changes in zeta potentials and morphology suggest the formation of a surface monolayer. Antigen binding and release occur through a combination of electrostatic and van der Waals interactions occurring at the polymer-solution interface. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:2510-2519, 2005 [source] Nonclassical forces: Seemingly insignificant but a powerful tool to control macromolecular structuresJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 14 2008Michiya Fujiki Abstract Strong chemical forces such as covalent and ionic bonds are responsible for building discrete molecules, nature dwells on noncovalent forces weaker by three orders in magnitude, like the hydrophobic effect, hydrogen bonding, and van der Waals forces. Despite being weak, they possess the potential to drive spontaneous folding or unfolding of proteins and nucleic acids and the recognition between complimentary molecular surfaces. The power of these forces lies in the cooperativity with which they act, thereby generating a cumulative effect of many bonding interactions occurring together. Many ongoing research aims to translate the potential of these forces to the synthetic world to create desired structures with specific chemical functions. Achieving this offers unlimited opportunities for designing and synthesizing the most complex structures with specific applications. This highlight aims to reflect the critical role these noncovalent forces play in controlling macromolecular structures, which hold immense untapped potential for applications defying conventions, and briefly touches on the concept of homochirality in nature based on chiral and weak noncovalent interactions in synthetic nonpolar Si-catenated polymers. It sheds some light on the discovery and characterization of Si/F-C interactions in fluoroalkylated polysilanes in chemosensing of fluoride ions and nitroaromatics with a great sensitivity and selectivity. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4637,4650, 2008 [source] Origins and Applications of London Dispersion Forces and Hamaker Constants in CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2000Roger H. French The London dispersion forces, along with the Debye and Keesom forces, constitute the long-range van der Waals forces. London's and Hamaker's work on the point-to-point dispersion interaction and Lifshitz's development of the continuum theory of dispersion are the foundations of our understanding of dispersion forces. Dispersion forces are present for all materials and are intrinsically related to the optical properties and the underlying interband electronic structures of materials. The force law scaling constant of the dispersion force, known as the Hamaker constant, can be determined from spectral or parametric optical properties of materials, combined with knowledge of the configuration of the materials. With recent access to new experimental and ab initio tools for determination of optical properties of materials, dispersion force research has new opportunities for detailed studies. Opportunities include development of improved index approximations and parametric representations of the optical properties for estimation of Hamaker constants. Expanded databases of London dispersion spectra of materials will permit accurate estimation of both nonretarded and retarded dispersion forces in complex configurations. Development of solutions for generalized multilayer configurations of materials are needed for the treatment of more-complex problems, such as graded interfaces. Dispersion forces can play a critical role in materials applications. Typically, they are a component with other forces in a force balance, and it is this balance that dictates the resulting behavior. The ubiquitous nature of the London dispersion forces makes them a factor in a wide spectrum of problems; they have been in evidence since the pioneering work of Young and Laplace on wetting, contact angles, and surface energies. Additional applications include the interparticle forces that can be measured by direct techniques, such as atomic force microscopy. London dispersion forces are important in both adhesion and in sintering, where the detailed shape at the crack tip and at the sintering neck can be controlled by the dispersion forces. Dispersion forces have an important role in the properties of numerous ceramics that contain intergranular films, and here the opportunity exists for the development of an integrated understanding of intergranular films that encompasses dispersion forces, segregation, multilayer adsorption, and structure. The intrinsic length scale at which there is a transition from the continuum perspective (dispersion forces) to the atomistic perspective (encompassing interatomic bonds) is critical in many materials problems, and the relationship of dispersion forces and intergranular films may represent an important opportunity to probe this topic. The London dispersion force is retarded at large separations, where the transit time of the electromagnetic interaction must be considered explicitly. Novel phenomena, such as equilibrium surficial films and bimodal wetting/dewetting, can result in materials systems when the characteristic wavelengths of the interatomic bonds and the physical interlayer thicknesses lead to a change in the sign of the dispersion force. Use of these novel phenomena in future materials applications provides interesting opportunities in materials design. [source] Photocatalytic Coatings for Environmental Applications,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2005Norman S. Allen ABSTRACT A series of nano- and micronparticle-grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photocatalytic activities have been determined in a series of paint films by FTIR, chalking, color, gloss change and weight loss after artificial weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of chargecarrier dynamics in the pigments. Pigment particle size, surface area and properties clearly play an important role in dispersion and any polymer-pigment interactions. Photooxidation studies on several types of paint films show a clear demarcation between nanoparticle- and pigmentary-grade titanium dioxide, with the former being more active because of their greater degree of catalytic surface activity. The photosensitivity of titanium dioxide is considered to arise from localized sites on the crystal surface (i.e. acidic OH), and occupation of these sites by surface treatments inhibits photoreduction of the pigment by ultraviolet radiation; hence, the destructive oxidation of the binder is inhibited. Coatings containing 2,5% by weight alumina or alumina and silica are satisfactory for generalpurpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7,10% weight. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminum phosphates of other metals. For example, the presence of hydrous alumina particles lowers van der Waals forces between pigments particles by several orders of magnitude, decreasing particle-particle attractions. Hydrous aluminum oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilization in various water- and oilbased paint media in comparison with conventional organic stabilizers. Hindered piperidine stabilizers are shown to provide no additional benefits in this regard, often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated in this study. In the former case, surface erosion is shown to be controlled by varying the ratio of admixture of durable pigmentary-grade rutile (heavily coated) and a catalytic-grade anatase nanoparticle. For environmental applications in the development of coatings for destroying atmospheric pollutants such as nitrogen oxide gases (NOX), stable substrates are developed with photocatalytic nanoparticle-grade anatase. In this study, porosity of the coatings through calcium carbonate doping is shown to be crucial in the control of the effective destruction of atmospheric NOx gases. For the development of microbiological substrates for the destruction of harmful bacteria, effective nanoparticle anatase titania is shown to be important, with hydrated high surface area particles giving the greatest activity. [source] Synthesis and structural characteristics of lithocholate triads: steroid-type channels occupied by spacer fragmentsACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2008Urszula Rychlewska Reported in this paper are the syntheses and X-ray investigations of C2 symmetrical molecular A,B,A triads consisting of two steroid units (lithocholic acid or its methyl ester) joined together by linkers derived from bifunctional molecules such as terephthalic acid or N,N,-dicarboxypiperazine. Unlike their monomeric analogues, some of these compounds form inclusion complexes. All steroidal triads form crystals that are highly pseudo-centrosymmetric, in which the constituting molecules are held together either exclusively by van der Waals forces or form lattice inclusion complexes, with guest molecules hydrogen bonded to the host. The presence of carboxyl groups promotes the inclusion of pyridine molecules and the formation of the well known carboxylic acid...pyridine hydrogen bonds. Combined with pairwise face-to-face ,-stacking between pyridine rings, these hydrogen-bond interactions lead to the formation of extended supramolecular tapes, analogous to polymers. The co-crystals of pyridine and a lithocholic acid triad undergo a symmetry-lowering phase transition from a P1 cell with Z = 1 to a P1 cell with Z = 2. The two structures are virtually the same, the two independent molecules in the larger cell being related by pseudo-translation. Changes in the type of spacer between two methyl lithocholate units from planar aromatic (terephthalic acid) to highly puckered aliphatic six-membered ring (N,N,-dicarboxypiperazine) bring about inclusion properties and changes in side-chain conformation in a crystal. Although the efficient packing of these highly elongated molecules is hindered, as indicated by low values of crystal density, ranging from 1.16 to 1.19,g,cm,3, several very short C...O and H...H contacts are present in the crystals. [source] Herringbone array of hydrogen-bonded ribbons in 2-ethoxybenzamide from high-resolution X-ray powder diffractionACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2009Silvina Pagola In 2-ethoxybenzamide, C9H11NO2, the amide substituents are linked into centrosymmetric head-to-head hydrogen-bonded dimers. Additional hydrogen bonds between adjacent dimers give rise to ribbon-like packing motifs, which extend along the c axis and possess a third dimension caused by twisting of the 2-ethoxyphenyl substituent with respect to the hydrogen-bonded amide groups. The ribbons are arranged in a T-shaped herringbone pattern and cohesion between them is achieved by van der Waals forces. [source] A new polytype of orthoboric acid, H3BO3 -3TACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2003Robert R. Shuvalov The crystal structure of H3BO3 -3T, a new trigonal polytype of orthoboric acid, consists of sheets of hydrogen-bonded B(OH)3 molecules similar to those found in the triclinic structure of orthoboric acid, H3BO3 -2A. In each case, van der Waals forces connect the sheets. However, the stacking sequences of the sheets differ between the two polymorphs. In H3BO3 -3T (space group P32), the sheets are stacked in the repeating sequence ABC,, whereas in H3BO3 -2A (space group ), the sheets are stacked in the repeating sequence AB,. [source] Poly[trans -diaquamanganese(II)-,-(3-pyridinecarboxylato- N:O)-,-(3-pyridinecarboxylato- O:N)]ACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2000Xiang Hao The title complex, [MnII(nic)2(H2O)2]n [nic is 3-pyridinecarboxylate (also called nicotinate), C6H4NO2,], has a two-dimensional layer structure with the unique Mn atom on an inversion centre. In each layer, all nicotinate ligands are coordinated to Mn atoms in a bridging/bidentate mode, thus linking the {MnII(nic)2(H2O)2} monomeric units together to form a dative-bond-based layered polymer. The intra-layer hydrogen bonds involving all water molecules and the carboxylate groups may play an auxiliary part in stabilizing the layer. The layers are arranged in an ordered manner along the a axis through van der Waals forces so as to complete the solid-state structure of the crystal. [source] Pharmacokinetic study on the mechanism of interaction of sulfacetamide sodium with bovine serum albumin: a spectroscopic methodBIOPHARMACEUTICS AND DRUG DISPOSITION, Issue 2-3 2010Praveen N. Naik Abstract The binding of sulfacetamide sodium (SAS) to bovine serum albumin (BSA) was investigated by spectroscopic methods, namely fluorescence, FT-IR and UV-vis absorption spectral studies. The binding parameters were evaluated by a fluorescence quenching method. The thermodynamic parameters, ,H0, ,S0and ,G0 were observed to be ,49.03,k,J,mol,1, ,99.9,J,K,1,mol,1 and ,18.96,k,J,mol,1, respectively. These indicated that the hydrogen bonding and weak van der Waals forces played major roles in the interaction. Based on Förster's theory of non-radiation energy transfer, the binding average distance, r, between the donor (BSA) and acceptor (SAS) was evaluated and found to be 3.72,nm. The spectral results showed that binding of SAS to BSA induced conformational changes in BSA. The effect of common ions and some of the polymers used in drug delivery for controlled release were also tested on the binding of SAS to BSA. Copyright © 2010 John Wiley & Sons, Ltd. [source] Methylazacalixpyridines: Remarkable Bridging Nitrogen-Tuned Conformations and Cavities with Unique Recognition PropertiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2006Han-Yuan Gong Abstract Methylazacalix[n]pyridines (n = 4, 8) and methylazacalix[m]arene[n]pyridines (m = n = 2, 4) have been synthesized by a convenient fragment coupling approach starting from 2,6-dibromopyridine, 2,6-diaminopyridine, and benzene-1,3-diamine. Thanks to the intrinsic electronic nature of nitrogen, which can adopt mainly sp2 hybridization, allowing it variously to conjugate, partially conjugate, or not conjugate with the adjacent one or two pyridine rings, the resulting nitrogen-bridged calixpyridine derivatives act as a unique class of macrocyclic host molecules with intriguing conformational structures offering fine-tunable cavities and versatile recognition properties. Whilst in solution it is fluxional, in the solid state methylazacalix[4]pyridine adopts a 1,3-alternate conformation with a C2v symmetry in which every two bridging nitrogen atoms conjugate with one pyridine ring. After protonation, the methylazacalix[4]pyridinium species has a different conjugation system of its four bridging nitrogen atoms, yielding the similar twisted 1,3-alternate conformations with an approximate S4 symmetry. The cavity of each protonated methylazacalix[4]pyridine, however, varies finely to accommodate guest species of different size and geometry, such as planar DMF or HO2CCO2, ion, a twisted HO2CCO2, ion, and a tetrahedral ClO4, ion. As giant macrocyclic hosts, both methylazacalix[8]pyridine and methylazacalix[4]arene[4]pyridine interact efficiently with fullerenes C60 and C70 through van der Waals forces. Their ease of preparation, versatile conformational structures, and recognition properties make these multinitrogen-containing calixarenes or cyclophanes unique and powerful macrocyclic hosts in supramolecular chemistry. [source] | ||||||||||||||||