Interactions In Dilute Solutions Research Articles

Monte Carlo Simulation on the Assembly of Nanorods with Anisotropic Interactions

The assembly of nanorods with anisotropic interactions in dilute solution is studied using lattice dynamic Monte Carlo simulation. Each nanorod consists of two different beads with one lattice length apart. The anisotropic interactions are of orientational dependence, and two kinds of nearest-neighbor interactions are considered: (a) side-to-side attraction εxy when two nanorods are parallel with head−head and tail−tail being nearest-neighbors and (b) end-to-end attraction εz when they are in end-to-head alignment. Three structures, wire, sheet, and pillar, are assembled by slowly cooling the system. Nanowires are assembled at large ratio r = εz/εxy, while sheets are assembled at small r. At intermediate values of r, three-dimensional (3D) pillars are formed. The aspect ratio A of the 3D pillar increases with r, which can be expressed as A = 0.238r2.13 ± 0.05. The exponent is different from that of theoretical calculation, which is probably caused by a kinetic effect.

Low-temperature and high-temperature approximations for penetrable-sphere fluids: Comparison with Monte Carlo simulations and integral equation theories

The two-body interaction in dilute solutions of polymer chains in good solvents can be modeled by means of effective bounded potentials, the simplest of which being that of penetrable spheres (PSs). In this paper we construct two simple analytical theories for the structural properties of PS fluids: a low-temperature (LT) approximation, that can be seen as an extension to PSs of the well-known solution of the Percus-Yevick (PY) equation for hard spheres, and a high-temperature (HT) approximation based on the exact asymptotic behavior in the limit of infinite temperature. Monte Carlo simulations for a wide range of temperatures and densities are performed to assess the validity of both theories. It is found that, despite their simplicity, the HT and LT approximations exhibit a fair agreement with the simulation data within their respective domains of applicability, so that they complement each other. A comparison with numerical solutions of the PY and the hypernetted-chain approximations is also carried out, the latter showing a very good performance, except inside the core at low temperatures.

Coarse-graining intramolecular hydrodynamic interaction in dilute solutions of flexible polymers

We present a scheme for coarse-graining hydrodynamic interactions in an isolated flexible homopolymer molecule in solution. In contrast to the conventional bead-spring model that employs spherical beads of fixed radii to represent the hydrodynamic characteristics of coarse-grained segments, we show that our procedure leads naturally to a discrete model of a polymer molecule as a chain of orientable and stretchable Gaussian blobs. This model accounts for both intrablob and interblob hydrodynamic interactions, which depend on the instantaneous shapes of the blobs. In Brownian dynamics simulations of initially stretched chains relaxing under quiescent conditions, the transient evolution of the mean-square end-to-end distance and first normal stress difference obtained with the Gaussian-blob model are found to be less sensitive to the degree of coarse graining, in comparison with the conventional bead-spring model with Rotne-Prager-Yamakawa hydrodynamic interactions.

Acoustic and volumetric studies of intermolecular interactions in dilute solutions of methanol in aromatic amines

Limiting partial compressibilities and volumes of methanol in pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine and 2,6-dimethylpyridine were calculated from the experimental speeds of sounds and densities of dilute solutions at 298.15 K. The limiting functions were found to be linearly correlated with the association energies of isolated 1:1 complexes of the pyridine derivatives with water. Those association energies are close to the energies for similar complexes with methanol. The results evidence that ortho effect enhances the ability of pyridines to hydrogen bonding with methanol in the same way as with water. The effect consist in changes of hydrogen bond energy, unspecific interactions with a steric hindrance, e.g. between hydrocarbon tail of the alcohol molecule and the methyl group in the ring, and changes in resonance interactions. Stronger hydrogen bonds cause smaller partial molar compressibilities and volumes of the solute, as well as greater negative enthalpies of solution. Importantly, single molecule of water or methanol forms one hydrogen bond with the proton-accepting solvent.

2-(2′-Pyridyl)pyrroles: Part II. Spectroscopic investigation of pyridylpyrrole alcohol complexes

A homologous series of 2-(2′-pyridyl)pyrroles were studied as a possible model system for intermolecular hydrogen bonding interactions in molecules that can act as both hydrogen bond donors and acceptors. Steady state spectroscopic methods were used to assess the importance of intermolecular hydrogen bonding interactions in dilute solutions of 2-(2′-pyridyl)pyrroles in the absence and presence of alcohols. The absorption and fluorescence properties of such solutions were investigated to determine the relevance of such hydrogen bonding interactions in the ground and excited state behavior of the 2-(2′-pyridyl)pyrroles. Over the concentration range studied, no evidence was found for the formation of hydrogen-bonded 2-(2′-pyridyl)pyrrole dimers. However, it was determined that 3,5-dimethyl-2-(2′-pyridyl)pyrrole forms weak (K ∼ 5–12 M−1) 1∶1 hydrogen-bonded complexes with methanol and t-butyl alcohol in the ground state, and 3,5-di-tert-butyl-2-(2′-pyridyl)pyrrole forms both 1∶1 and 1∶2 complexes with the same alcohols. Despite the formation of hydrogen-bonded 2-(2′-pyridyl)pyrrole alcohol complexes, no experimental evidence was found for excited state proton transfer in such complexes.

On the nature of the multivalency effect: a thermodynamic model.

A quantitative model is proposed for the analysis of the thermodynamic parameters of multivalent interactions in dilute solutions or with immobilized multimeric receptor. The model takes into account all bound species and describes multivalent binding via two microscopic binding energies corresponding to inter- and intramolecular interactions (Delta G(o)inter and Delta G(o)intra), the relative contributions of which depend on the distribution of complexes with different numbers of occupied binding sites. The third component of the overall free energy, which we call the "avidity entropy" term, is a function of the degeneracy of bound states, Omega(i), which is calculated on the basis of the topology of interaction and the distribution of all bound species. This term grows rapidly with the number of receptor sites and ligand multivalency, it always favors binding, and explains why multivalency can overcome the loss of conformational entropy when ligands displayed at the ends of long tethers are bound. The microscopic parameters and may be determined from the observed binding energies for a set of oligovalent ligands by nonlinear fitting with the theoretical model. Here binding data obtained from two series of oligovalent carbohydrate inhibitors for Shiga-like toxins were used to verify the theory. The decavalent and octavalent inhibitors exhibit subnanomolar activity and are the most active soluble inhibitors yet seen that block Shiga-like toxin binding to its native receptor. The theory developed here in conjunction with our protocol for the optimization of tether length provides a predictive approach to design and maximize the avidity of multivalent ligands.

Characterization and intermolecular interactions of hydroxypropyl guar solutions.

Aqueous solutions of guar galactomannan and hydroxypropyl guars (HPG) with different molar substitution (MS) levels were studied using dilute solution viscometry and gel permeation chromatography. When guar is modified to HPG, the added hydroxypropyl groups sterically block the hydrogen bonding sites on the guar backbone and reduce the hydrogen bonding attractions between guar molecules. The effects of molar substitution on the intermolecular interactions are inferred from measurements of the Huggins coefficients, which measure intermolecular interactions in dilute solution, and molecular volumes, which reflect intrachain associations. The behavior can be divided into three regimes: (1) at low MS levels (0 < MS < approximately 0.4), there is a sharp decrease in intermolecular interactions as a function of MS; (2) in the intermediate range ( approximately 0.4 < MS < approximately 1.0), interactions become independent of MS; (3) at high substitution levels (MS > approximately 1.0), the temperature dependence of inter- and intramolecular hydrophobic interactions produces a temperature dependence in the Huggins coefficient and molecular volumes that is not seen at lower substitutions. By acid hydrolysis, HPG samples with a range of molecular weights and consistent polydispersities were obtained. On the basis of these samples, the Mark-Houwink-Sakurada parameters and "characteristic ratio" C(infinity) were evaluated for HPG (MS approximately 0.6) and compared to the values for guar. The HPG chain stiffens as the degree of substitution increases.

Luminescence Characterization of a Novel, Hydrophobically Modified and Heavy Atom-Functionalized, Water-Soluble Polymer

A hydrophobically modified water-soluble polymer, based upon acrylic acid and styryl derivatives, was synthesized. A proportion (ca. 75 mol%) of the styryl residues in the copolymer contain a bromine substituent as a “heavy-atom” functionality. It has been shown that room-temperature phosphorescence (RTP) can be induced in an acenaphthylene (ACE) label, covalently bound to the polymer chain, through intramacromolecular interactions in dilute solutions of the copolymer. This is the first instance in which RTP has been generated in either label or solubilized guest, in such a fashion. The conformational behavior of the functionalized copolymer, BrSTY/STY/AA, has been studied using RTP, fluorescence lifetime, and time-resolved anisotropy measurements and compared to that of both its unbrominated, styrene-modified analogue, STY/AA, and poly(acrylic acid) PAA itself. The conformation transition which accompanies conversion of each polyacid into the corresponding fully neutralized polysalt is much more dramatic in either hydrophobically modified species than in poly(acrylic acid). Intramacromolecular aggregation leading to the creation of hydrophobic domains within the coils of the macromolecules is enhanced at a low pH and severely impedes segmental motion in the two styrene-modified polyacids. The effect is greater in the bromine-containing polymer, which suggests that more densely packed domains are formed in this species than in STY/AA. In addition to altering the magnitude of the effect that neutralization has upon the molecular dynamics of the polyacid in aqueous media, hydrophobic modification raises the pH range over which the change in conformational behavior of the macromolecule is apparent.

Effect of chain length on rates of diffusion-limited small molecule-polymer and polymer-polymer reactions: Phosphorescence quenching studies

Model interactions have been studied by phosphorescence quenching to obtain a better understanding of the chain length dependence of interpolymeric chain end-chain end reactions such as those involved in the termination step of free radical polymerization. For small molecule-polymer interactions in dilute cyclohexane solution, quenching rate constant (kq) data agree with the Smoluchowski equation prediction that kq scales as polymer molecular weight (MW) to the -½ power, confirming self-diffusion control. For polymer-polymer interactions in dilute solution, the chain length dependence is weaker than that predicted by translational diffusion control, as described by the Smoluchowski equation, but is stronger than that predicted by renormalization group theory. For interactions between 70000 MW benzil-end-labeled polystyrene and varying MWs of anthracene-end-labeled polystyrene at 300 g/L polymer, kq decreases by a factor of 10 in going from MWs of 100 to 1000 g/mol; beyond 1000 g/mol, kq is nearly independent of chain length. Such effects indicate that the importance of oligomeric radical self-diffusion and polymer radical chain-end segmental mobility must be carefully considered in understanding the termination process in free radical polymerization. © 1996 John Wiley & Sons, Inc.

Conductometric, Viscometric and Ultrasonic Studies of Terbium Laurate in Benzene-Methanol Mixture

Abstract The conductometric, viscometric and ultrasonic velocity measurements of the solutions of terbium laurate in benzene-methanol mixture (7:3 v/v) were carried out at 40 ± 0.05°C. The conductivity results showed that the soap behaves as a weak electrolyte in dilute solutions. The viscosity results were explained on the basis of equations proposed by Einstein, Vand, Moulik and Jones-Dole and the values of molar volume evaluated by using Einstein's and Vand's equations were in close agreement. The ultrasonic velocity measurements were used to evaluate various acoustic parameters. The results showed that the soap-soap interactions are weaker than soap-solvent interactions in dilute solutions and the soap molecules do not aggregate below the critical micelle concentration in dilute solutions.

Red edge excitation shifts of crystallins and intact lenses: A study of segmental mobility and inter-protein interactions

The shift that occurs in the fluorescence emission wavelength upon changing the excitation wavelength towards the red edge of the absorption band is termed red edge excitation shift (REES). We have monitored the REES of intrinsic protein fluorescence of freshly isolated intact lenses, of individual crystallins in their native, denatured and photodamaged states and also of crystallin mixtures. The observed REES values for the lenses from different species are different suggesting that the mobilities and packing of the crystallins may vary with the species. Lens photodamage in all the cases resulted in an increase of REES. Denaturation of crystallins in solution reduces REES and renaturation restores it. Mixtures of α- and β-crystallins prepared either by directly mixing equimolar solutions or mixing them in 4 M urea followed by dialysis (reconstituting) gave similar REES values indicating the absence of any specific interactions in dilute solutions. Possible existence of induced alterations facilitating inter-crystallin interactions at high protein concentration is suggested.

Interactions in macromolecular solutions manifested by the mobility of solvent molecules in the low-frequency raman range

The results of investigations on the effect of macromolecular interactions in dilute solutions upon the mobility of solvent molecules, observed in the low frequency Raman range ( 8 cm −1 < δ v 50 cm −1 ), are reported. Both polarized (VV) and depolarized (VH) components of the scattered light were measured in order to detect the joint effect of the correlated anisotropy fluctuations and intermolecular vibrations. Three frequency ranges were observed in the spectra, characterized by different relaxation times attributed to different types of the solvent molecules mobility. The effect of the structure of polymer side groups on the increase in the relaxation time, τ 1, measured in the range 8 < δ v < 30 cm −1 was observed. These interactions are considerably weaker in solutions of polyetherester with a rigid backbone than those occurring in solutions of flexible macromolecules with mesogenic side groups.

Substituent and pH effects on self-association of pyridine derivatives in aqueous solution. An ultraviolet study

Self-association of pyridine, 2-picoline, pyridine monocarboxylic acids, methyl nicotinate, and dipicolinic acid, has been studied in aqueous solution by ultraviolet spectroscopy, as an extension of previous work. The spectra of these compounds, under suitable conditions of pH, show an hypochromic effect with increasing concentration over the range from 10 −6 to 10 −1 M, which is interpreted in terms of self-association. Association constants are calculated from dimerization and polymerization models. The results are discussed in terms of the nature of substituents and the protonation states, and show that interactions in dilute solutions are important in the self-association processes of these compounds in aqueous solution.

Polymer-polymer interactions in dilute solution

Coil-coil interactions in dilute solution for ply-(α-methylstyrene) in sub-θ,θ, marginal, and good solvents were studied using static and dynamic light scattering. The behavior observed across the solvent quality range is well described by a semiempirical fit to the interchain parameter k D φ versus the reduced radius X≡S/R H using a recent theoretical treatment developed by Akcasu and co-workers

Thermodynamic treatment of coulomb interactions in non-stoichiometric oxides

Due to the presence of charged defects, Coulomb interactions play a major role in a number of non-stoichiometric oxides. Various treatments of these interactions have been discussed. It has been shown that even for relatively high values of the dielectric constant, only two varieties of Monte-Carlo treatment, the Ewald summation method and minimum image with dodecahedron cells, provide reliable results. A typical consequence of these interactions is a strong increase of the partial molar enthalpy of oxygen near the stoichiometry due to the globally attractive nature of electrostatic interactions in dilute solutions.

Effects of surfactant concentration on polymer-surfactant interactions in dilute solutions: a computer model

Une simulation par ordinateur est developpee pour modeliser l'agregation des particules de polymere et d'agents de surface dans une solution diluee

Mischbarkeit und Phasenverhalten von Polymerblends

AbstractMeasurements of polymer‐polymer interactions by gas‐chromatography facilitate the quantitative thermodynamic description of polymer blends in terms of Flory‐Huggins and Flory equation‐of‐state interaction parameters. These parameters were found to depend on concentration and molecular weights of the components. The interaction surface including the phase diagram of poly(styrene)‐poly(vinylmethylether) is obtained. Poly(styrene)‐poly(phenyleneoxide) were investigated by gas‐chromatography too. Lower critical solution behaviour is also observed for this system. The results are discussed and compared with theoretical calculations according Flory's equation‐of‐state theory. There is only qualitative correspondence between theoretical calculations and experiments. Additionally polymer‐polymer interactions in dilute solution were investigated. The so‐called light scattering under “optical θ‐condition” make it possible to state whether a polymer pair is compatible or not.

Polyelectrolyte solutions: Counterion condensation and intermolecular interactions

AbstractThis article demonstrates that the neglect of nonlinear effects in the conventional counterion condensation theory for the double layer about a charged cylinder can be significant, especially for phenomena involving intramolecular or intermolecular interactions in dilute solutions. For concentrated solutions the Manning theory derives from a linearized superposition approximation for the potential, in contrast to the cylindrical‐cell model, which explicitly treats interactions within an ordered array of parallel cylinders. A new theory which treats interactions explicitly while permitting disorder in two dimensions is presented, and predictions for the osmotic pressure are compared with those from the Manning and cylindrical‐cell models.

Hydrophobic interactions in dilute solutions of poly(vinyl alcohol)

High-precision density and light-scattering measurements in water and 10–3 mol dm–3 electrolyte solutions of samples of 80 and 90% poly(vinyl alcohol)(PvOH) over an extended range of concentration and temperature have revealed a complex pattern of solution behaviour; this report is restricted to the behaviour of solutions at concentrations below 0.15% w/v.

Use of internal and external references in nuclear magnetic resonance determinations of association constants of weak molecular complexes

For each of three typical weak electron-donor–acceptor interactions in dilute solution in an “inert” solvent, the values of association constants, based on n.m.r. chemical shift measurements, are the same whether an internal or an external reference is used.