Steric Potential Research Articles

Effect of side chains on the dispersing properties of polycarboxylate-type superplasticisers in cement systems

To study the effect of side chains on the dispersing properties of polycarboxylate-type superplasticisers (PCs) in cement pastes, PCs with different side chains were synthesised and used as a superplasticiser for cement suspensions. The effect of side chains of PC on the zeta potential, adsorption, fluidity of the corresponding cement suspensions was investigated systematically, and total interparticle potential energy between particles was calculated. The results show that the higher the short side chains ratio of PC, the larger the adsorption amount of PC. PC with both short and long side chains presents a better dispersing property than PC with short side chains, while PC with long side chains falls in between. The maintaining dispersing ability of PC for cement particles increases as the short side chains ratio increases. The changing rule of total interparticle potential energy produced by PC is in accordance with the changing rule of dispersing property of PC in cement pastes. This indicates that the calculated results of total interparticle potential energy further support the explanation of dispersing property difference of PC in theory. This paper uses the sum of electrostatic energy and steric hindrance potential as the total interparticle potential energy to evaluate the dispersing property of PC for the first time, which is meaningful for the evaluation of dispersing property of polymer dispersant adsorbed on particle surfaces.

The influence of steric potential on the pressure and interparticle correlations in magnetic fluids

The influence of steric potential on the pressure and interparticle correlations in magnetic fluids

Polymer Grafting to Single‐Walled Carbon Nanotubes: Effect of Chain Length on Solubility, Graft Density and Mechanical Properties of Macroscopic Structures

Single-walled carbon nanotubes are grafted with polystyrene chains employing a graft-to protocol. Thermogravimetric analysis allows calculation of the grafted chain density and average interchain separation on the nanotube surface as a function of molecular weight. The separation scales with molecular weight as a power law with an exponent of ca. 0.588, showing the grafted chains to be in a swollen random walk conformation. This implies that chain packing is controlled by coil size in solution. In addition, the dispersed concentration of functionalized nanotubes scales with the size of the steric potential barrier that prevents aggregation of polymer functionalized nanotubes. It is also shown that the molecular weight of the grafted chains significantly affects the mechanical properties of nanotube films.

From density functional steric analysis and molecular electrostatic potential to the estimation of etherification rate constant

This research extends our more recent work on the application of molecular electrostatic potential as an effective approach in describing the influence of substituent on etherification reaction rate constant of phenol derivatives. Here, in addition to electronic factor, the steric effects have also been considered for our purpose. To analyze steric effects on etherification rate constant, we use the novel energy partition scheme proposed recently by Liu [S. B. Liu,J. Chem. Phys. 2007,126, 244103], where the total electronic energy is decomposed into three independent components: steric, electrostatic, and fermionic quantum. In this scheme, the steric potential has also been introduced. We first derive a relationship on the basis of density functional theory to show that the etherification rate constant should be proportional to the electrostatic potential on the atomic sites. Then, a bilinear function of molecular electrostatic potential and steric energy or steric potential is proposed for estimation of etherification reaction rate constants. Taking the experimental kinetics data of 30 substituted phenols, the validity of the proposed approach has been verified in position and momentum spaces. It is worth noting that the remarkable good performance of the momentum densities, which for the first time used in calculations of steric energy for a reaction, has been observed. Finally, using the relationship between new energy partition scheme and information theory, applicability of the Shannon entropy as one of the information theoretic measures is also tested for our goal and considerable results were obtained. Copyright © 2012 John Wiley & Sons, Ltd.

Quantifying steric effect with experimental electron density

Using experimental electron densities, the recent effort of quantifying steric effect within the framework of density functional theory is continued. In this work, steric potential, steric field, and steric charge distributions are systematically examines for diamond and boron nitride crystals. Bader's zero-flux condition has been employed to discuss the atomic contributions of these quantities. Two new concepts, characteristic radius r(s) of steric field and atomic steric charge q(s), are introduced in this work, which are intrinsic properties of a system and thus can be used to characterize atomic properties in a molecule or crystal. We anticipate that these steric effect related quantities together with the new concepts introduced in this work can be applied to characterize variety categories of the chemical bonds or weak interactions and provide in-depth insights to a wide range of organic, inorganic, and biological systems.

Hydroxamic Acids Analogous Against Breast Cancer Cells: 2D‐QSAR and 3D‐QSAR Studies

AbstractThe cytotoxicity of 19 N‐aryl‐substituted hydroxamic acids has been tested in vitro towards human breast cancer MCF–7 cell lines by MTT assay. The IC50 values were found to be in the range from 61.94 to 337.54 μM. A total of 18 out of 19 molecules had higher inhibitory activities than hydroxyurea against MCF–7 cell. Five compounds with IC50 values in micromolar range were 3‐ to 5‐folds more potent than hydroxyurea (IC50=307.15 μM). By partial least squares (PLS) regression, 2D‐QSAR model reported herein provide interesting insight in understanding hydrophobic, electronic, and structural requirements of antitumor activity among these set of the compounds. The cross‐validated Q2cum values for optimal PLS model of hydroxamic acids is above 0.638 (remarkably higher 0.50), indicating good predictive abilities for log 1/IC50 values of hydroxamic acids (HAs). The k‐nearest neighbor molecular field analysis (kNN‐MFA) approach was used to generate three‐dimensional quantitative structure‐activity relationship (3D‐QSAR) models for these sets of molecules. Statistically stepwise variable selection k‐nearest neighbor molecular field analysis (SW‐kNN‐MFA) model is comparatively better as compared to the other two (i.e. simulated annealing k‐nearest neighbor molecular field analysis, SA‐kNN‐MFA, and genetic algorithm k‐nearest neighbor molecular field analysis, GA‐kNN‐MFA) with respect to both the internal (q2=0.7461) as well as external (pred_r2=0.6107) model validation and correctly predicts activity of ca. 74.61% and ca. 61.07% for the training and test set, respectively. It uses one steric and one electrostatic fields along with its 3k nearest neighbor (k=3) to evaluate the activity of new molecules. The developed SW‐kNN‐MFA model field plot indicated that the positive steric and electric potential are favorable for the increase in the activity and hence more bulky substituent at 5‐position of phenyl ring connected at amide group and less electronegative substituent at 3‐position of phenyl ring connected to carboxyl group are favourable for the increase in the potency of the molecules.

Steric effect: A quantitative description from density functional theory

The concepts of steric energy, steric potential, and steric charge are introduced within the density functional theory framework. The steric energy, representing a hypothetical state with all electrons packed into the lowest orbital and other effects entirely excluded, is a measure of the intrinsic space occupied by an electronic system. It is exclusive, repulsive, and extensive, and it vanishes for homogeneous electron gas. When Bader's zero-flux boundary condition is adopted, atoms in molecules are found to achieve balanced steric repulsion among one another with vanished steric energy density interfaces. A few molecular systems involving conformation changes and chemical reactions have been investigated to examine the relative contribution of the steric and other effects, providing insights for a few controversial topics from a different perspective.

Steric Interactions between Physically Adsorbed Polymer-Coated Colloidal Particles: Soft or Hard?

The steric interaction potential between colloidal particles imparted by adsorbed polymer layers is directly related to their structure. Due to the complexity of these interfacial structures, the steric potential may behave differently at different interparticle separations. In this study, we proposed a combined model of the equivalent hard-sphere model (EHS) and the Hayter-Penfold/Yukawa model (HPY) to describe the steric potential due to adsorbed homopolymers on colloidal particles. The EHS potential describes the dense train/small-loop region and the HPY potential the more diffuse tail/long-loop region. The steric potential was extracted from the structure factors measured by small-angle neutron scattering (SANS). It was found that this combined model gave better agreement with experimental data than either of its component models alone. This study also shows that the adsorbed polymer layer in a good solvent partially collapses when the layers approach one another, which is also supported by an NMR solvent relaxation study.

Theoretical estimation of stability ratios for hexadecane-in-water (H/W) emulsions stabilized with nonylphenol ethoxylated surfactants

The effect of steric interactions on the stability of oil-in-water emulsions is studied here by means of emulsion stability simulations (ESS). For this purpose, a new steric potential based on a modification of the one formerly proposed by Vincent et. al. [Colloids Surf. 18 (1986) 261] is employed. The parameters of the calculation correspond to hexadecane in water emulsions stabilized with nonylphenol ethoxylated surfactants of different chain lengths (NPE m). Stability ratios ( W) were calculated using the half life time of the number of drops per unit volume of these systems. A functional relationship between W and the repulsive potential barrier, (Δ V), similar to the one previously found by Prieve and Ruckenstein for electrostatically stabilized suspensions [J. Colloid Interface Sci. 73 (1980) 539] was obtained. However, according to our simulations there exists a threshold for the stability of emulsions with respect to coalescence which is approximately located around 12.7 k B T .

Steric interaction between spherical colloidal particles

The reliability of the Derjaguin approximation for the calculation of the mixing term between sterically stabilized colloidal particles is studied. For this purpose, the steric potential obtained from the experiment of Doroszkowski and Lambourne [J. Polym. Sci., Part C: Polym. Symp. 34, 253 (1971)] is regarded as an exact result. Several analytical expressions corresponding to the mixing term of the steric potential are tested. Vincent et al. [Colloids Surf. 18, 261 (1986)] obtained four of them using the Derjaguin approximation along with different profiles for the volume fraction of segments in grafted polymer layers. As will be shown, the exact calculation of the volume of interaction between two spheres with adsorbed polymer layers already leads to a considerable improvement of the theoretical prediction for the simplest case of constant spatial distribution of polymer monomers. This equation is also better than the four additional expressions that result from using Bagchi's formalism [J. Colloid Interface Sci. 47, 86 (1974)] with similar segment profiles. The deviations of Bagchi's formalism can be substantially minimized using Flory-Krigbaum theory instead of the Flory-Huggins formalism for the calculation of the free energy of mixing. The equations derived here for the steric potentials were derived for particles of distinct radii.

Small-Angle Neutron Scattering Study of Concentrated Colloidal Dispersions: The Interparticle Interactions between Sterically Stabilized Particles

Small-angle neutron scattering (SANS) was used to investigate the interparticle interactions in concentrated dispersions of colloidal silica stabilized either by steric or by electrostatic repulsive interactions. In 10 mM NaCl, an adsorbed PEO layer is required to prevent flocculation, and particles are stabilized by steric repulsions. The adsorbed layer was made invisible to neutrons by contrast matching with the aqueous continuous phase. Dispersions of the same particles at the same concentrations but in the absence of added salt and adsorbed PEO were also studied. In both cases, the SANS spectra of concentrated dispersions show a peak at low Q, which is due to interparticle interactions: a structure factor. The SANS data can be described rather well by a homogeneous spherical form factor and a structure factor based on the Hayter-Penfold/Yukawa potential model. The steric potential was compared to the electrostatic potential obtained by fitting the SANS data of the bare silica dispersions. The steric potential shows a greater dependence on the particle volume fraction, which we ascribe to the penetration and compression of the adsorbed PEO layer as the particles approach.

Small-Angle Neutron Scattering of Ionic Perfluoropolyether Micellar Solutions: Role of Counterions and Temperature

This paper reports a small-angle neutron scattering (SANS) characterization of perfluoropolyether (PFPE) aqueous micellar solutions with lithium, sodium, cesium and diethanol ammonium salts obtained from a chlorine terminated carboxylic acid and with two perfluoroisopropoxy units in the tail (n(2)). The counterion and temperature effects on the micelle formation and micellar growth extend our previous work on ammonium and potassium salts n(2) micellar solutions. Lithium, sodium, cesium and diethanol ammonium salts are studied at 0.1 and 0.2 M surfactant concentration in the temperature interval 28-67 degrees C. SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multicomponent system reduced to a generalized one component macroions system (GOCM). At 28 degrees C, for all the salts, the micelles are ellipsoidal with an axial ratio that increases from 1.6 to 4.2 as the counterion volume increases. The micellar core short axis is 13 A and the shell thickness 4.0 A for the alkali micelles, and 14 and 5.1 A for the diethanol ammonium micelles. Therefore, the core short axis mainly depends on the surfactant tail length and the shell thickness on the carboxylate polar head. The bulky diethanol ammonium counterion solely influences the shell thickness. Micellar charge and average aggregation number depend on concentration, temperature and counterion. At 28 degrees C, the fractional ionization decreases vs the counterion volume (or molecular weight) increase at constant concentration for both C = 0.1 M and C = 0.2 M. The increase of the counterion volume leads also to more ellipsoidal shapes. At C = 0.2 M, at 67 degrees C, for sodium and cesium micelles the axial ratio changes significantly, leading to spherical micelles with a core radius of 15 A, lower average aggregation number, and larger fractional ionization.

Counterion and Temperature Effects on Aqueous Ionic Perfluoropolyether Micellar Solutions by Small-Angle Neutron Scattering

The effect of concentration and temperature on the microstructure of aqueous micelles of carboxylic perfluoropolyether surfactants, with two perfluoroisopropoxy units in the chain (n2) that is chlorine-terminated, are studied by SANS for the ammonium and potassium counterions. The SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multiion system reduced to an effective one component macroions system (OCM). At 28 °C, in the surfactant concentration range 0.05 to 0.12 M, the micelles display spherical shape with inner core radius of 15 Å for both counterions, and interfacial layer thickness of 4 Å. At higher concentration, both counterions provide ellipsoidal micelles, with axial ratio 2 and a limiting dimension of 13 Å. A sharp increase of temperature up to 80 °C enables the ammonium salt at 0.2 M to rearrange its ellipsoidal micelles into spherical ones, while the micelles of the potassium salt remain ellipsoidal. In all cases, the micellar size distribution is extremely narrow and the average aggregation numbers, as well as the surface charge, are found to slightly differ for the two counterions upon variation of concentration and temperature, driving ionization degrees globally spanning from 0.3 to 0.5. The interfacial hydration, the surface potential and the area per polar head at the micellar surface are discussed too.

Interparticle potentials of aqueous Al2O3suspensions

AbstractOptimum dispersing conditions for 43 vol.-% aqueous Al2O3 suspensions have been studied by assessing rheological behaviour as a function of dispersant concentration. A commercial polyelectrolyte was used as dispersant. The particle pair potentials have been calculated by considering the electrostatic and steric contributions. Variations in the steric potential produce significant changes in the shape and level of the potential energy curves and allow the prediction of the stability of suspensions that should coagulate according to the DLVO equation. Colloidal stability and rheological behaviour are correlated with the green and sintered densities of Al2O3 bodies produced by slip casting.

Steric interactions mediated by multiblock polymers and biopolymers: role of block size and addition of hydrophilic side chains

To a first approximation, the primary structure of many food proteins maybe thought of as a sequence of short hydrophobic and hydrophilic blocks. The influence of this type of structure on the steric-stabilising properties of such proteins has been considered here. In line with previous studies, using Self-Consistent-Field calculations, it has been shown that the presence of such protein molecules can lead to attraction and consequently bridging flocculation of colloidal particles. In the low adsorption energy limit for the hydrophobic groups (∼−1 k B T), it is found that the steric potential is significantly influenced by the changes in the number of adsorbed segments, as two surfaces are brought together. This is in contrast to the well-known results in the literature for the high adsorption limiting cases, where the number of such segments remains constant. In particular, the changes in the number of adsorbed hydrophobic units are observed not to be a monotonic function of the separation distance, but increase or decrease in reasonable accord with the oscillatory nature of the steric interactions, observed for various block sizes. Effects of the addition of a moderately sized hydrophilic side chain to the above molecules have also been studied. It is found that, in principle, such a modification can lead to a purely repulsive steric potential in solutions of these hybrid biopolymers. At the hydrophilic side chain sizes considered here, the surface affinity of the molecules is observed not to be drastically different compared to those of unmodified proteins.

Interparticle Potentials in Nonaqueous Silicon Nitride Suspensions

The rheological properties of nonaqueous silicon nitride suspensions are studied. Suspensions were prepared to volume fractions of solids of 0.21, 0.25, 0.29, and 0.33, and dispersed with phosphate ester in a mixture of solvents (methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone). Expanded viscosity curves were obtained by measuring under controlled rate and stress conditions, and the experimental data were fitted to the Cross model that provides the high shear limiting viscosity (η∞). The evolution of viscosity with volume fraction of solids was fitted to the Krieger‐Dougherty equation, to predict the maximum packing fraction (φm). The electrostatic pair potential was calculated based on the DLVO theory by evaluating the dielectric constant of the three‐component solvent and the Hamaker constant of the Si3N4–solvent system. The surface potential was calculated by measuring the elastic modulus through dynamic rheological measurements. The steric potential was also evaluated from the available models. It has been observed that phosphate ester provides a purely steric stabilization at short separation distances (up to 9 nm), while electrostatic forces dominate at larger separation distances.

Linking CoMFA and protein homology models of enzyme–inhibitor interactions: an application to non-steroidal aromatase inhibitors

An approach to compare quantitatively a ligand-based (CoMFA) model and an enzyme active site model was investigated. The active site of the cytochrome P450 human aromatase was constructed by homology modeling techniques and two structurally different non-steroidal aromatase inhibitors were docked into it. A CoMFA model was then developed on a related series of non-steroidal inhibitors by correlating their inhibitory activity (expressed as −log IC 50 values) versus only 11 steric descriptors (i.e. C sp3–ligand steric interaction energies). The resulting 3D-QSAR coefficients (11) and the steric field values of the aromatase active site calculated at the same points of the CoMFA lattice (i.e. eleven C sp3–protein steric interaction energies) were pair-wise compared. Specifically, when a positive coefficient was associated with a negative or low (<5 kcal/mol) value of the protein steric field or, alternatively, a negative coefficient was associated with a large positive value of the protein steric field we recorded as many matches. When a 3D-QSAR coefficient did not correspond to the protein steric potential in the sense described above we considered that point as a mis-matching point. In our view, in spite of several limitations, such a comparison represents a valuable criterion to evaluate quantitatively how convergent are the results from a 3D-QSAR CoMFA model and a homology-built protein 3D structure.

Steric stabilization in emulsion polymerization using oligomeric nonionic surfactants

Although nonionic surfactants show improved stabilization characteristics in emulsion polymerization as well as superior shear and freeze thaw stability of the final latex, their behaviour is generally not well understood. In the present paper a steric stabilization model is developed for describing particle stabilization in emulsion polymerization systems in the presence of nonionic oligomeric surfactants. The model takes into account the effect of unequal particle sizes on the steric interaction potential and the resulting enhanced heterocoagulation. An additional feature of the model is the incorporation of a possible surfactant lateral migration mechanism, which may be significant in the case of oligomeric nonionic surfactants. The proposed model can simulate reasonably well the experimentally observed conversion profiles and average particle sizes as a function of the surfactant molecular structure (i.e., lengths of stabilizing and adsorbing moieties) and surfactant concentration. As a result of the competing effects of the adsorbed volume fraction and surface coverage on particle stability, an optimum hydrophobicity/hydrophilicity ratio for the oligomeric nonionic surfactants can be derived.

Mechanism of Photoreorientation of Azobenzene Dyes in Molecular Films

Aminoazobenzene photoreorientation is investigated in systems in which hydrophobically substituted chromophores are organized in molecular (Langmuir−Blodgett) films on glass substrates. Whereas the cis absorption of the chromophores in CHCl3 after irradiation into their π→π* absorption band is easily measured, we were not able to detect spectral changes characteristic of isomerization in the solid films during irradiation. The quantum efficiency for trans→cis isomerization in LB films is reduced at least by a factor of ∼10-4 from that in organic solvent. We conclude that aggregation of the chromophores in LB films limits conformational changes after photoexcitation, in distinction from the situation when aminoazobenzene derivatives are embedded in thin polymer or liquid-crystal films. Hence, the subsequent reorientation is probably driven by a mechanism that does not encompass the cis isomer. The temperature dependence of the reorientation indicates thermal activation with an activation energy ΔE = 0.24 eV. The irradiation intensity dependence shows that the reorientation is a collective process. It is distinctly different from what would be expected if photoexcitation resulted in reorientation of individual dye molecules and is simulated in a simple model from which the intensity threshold, Ith = 0.95 ± 0.2 mW/cm2, is retrieved. From the experimental observations we postulate that the vibronic coupling of excited states may open a radiationless deactivation channel in which the electronic excitation energy is thermalized and distributed across neighboring molecules of the excited chromophore. The excited chromophore may hence rotate by an infinitesimal amount within the steric potential of its neighbors. This process ceases only when the chromophores are rotated to an orientation in which they are no longer excited by the irradiating light beam.

Discriminating D1 and D2 agonists with a hydrophobic similarity index

Currently, methods for calculating molecular similarity indices have been developed for comparing steric, charge density, and molecular electrostatic potential (MEP) properties. Much of the existing technology may, however, be applied to the quantitative comparison of molecular hydrophobicities. In this article we present an empirical hydrophobic similarity index. We utilize atomic hydrophobic parameters derived from a quantum mechanical semiempirical wavefunction. Hydrophobicity at points on a grid is computed with a recently introduced “molecular lipophilicity potential”. The overlap of pairs of molecules is calculated with the metric introduced by Carbó. This approach is applied to a case in which steric and electrostatic criteria have already been shown to be inadequate in rationalizing selectivity, namely, requirements for recognition at the dopamine D1 and D2 receptors. We demonstrate that, for a set of dopamine agonists, D1 ligands show higher similarity in this property than D2 analogs. This indicator of similarity is more successful at accounting for D1 selectivity than previous methods.