Chemical force spectroscopy in heterogeneous systems: intermolecular interactions involving epoxy polymer,mixed monolayers,and polar solvents

We used chemical force microscopy (CFM) to study adhesive forces between surfaces of epoxy resin and self-assembled monolayers (SAMs) capable of hydrogen bonding to different extents. The influence of the liquid medium in which the experiments were carried out was also examined systematically. The molecular character of the tip, polymer, and liquid all influenced the adhesion. Complementary macroscopic contact angle measurements were used to assist in the quantitative interpretation of the CFM data. A direct correlation between surface free energy and adhesion forces was observed in mixed alcohol-water solvents. An increase in surface energy from 2 to 50 mJ/m(2) resulted in an increase in adhesion from 4-8 nN to 150-300 nN for tips with radii of 50-150 nm. The interfacial surface energy for identical nonpolar surface groups of SAMs was found not to exceed 2 mJ/m(2). An analysis of adhesion data suggests that the solvent was fully excluded from the zone of contact between functional groups on the tip and sample. With a nonpolar SAM, the force of adhesion increased monotonically in mixed solvents of higher water content; whereas, with a polar SAM (one having a hydrogen bonding component), higher water content led to decreased adhesion. The intermolecular force components theory was used for the interpretation of adhesion force measurements in polar solvents. Competition between hydrogen bonding within the solvent and hydrogen bonding of surface groups and the solvent was shown to provide the main contribution to adhesion forces. We demonstrate how the trends in the magnitude of the adhesion forces for chemically heterogeneous systems (solvents and surfaces) measured with CFM can be quantitatively rationalized using the surface tension components approach. For epoxy polymer, inelastic deformations also contributed heavily to measured adhesion forces.     

低温等离子体处理聚酯(PET)表面润湿性与表面结构的研究

研究了O₂、N₂、He、Ar、H₂和CH₄气体低温等离子体改性聚酯(PET)的表面润湿性与表面结构的关系.用已知表面张力的液体测定接触角,作Zisman曲线,求得试样的临界表面张力γ_c;并按扩展的Fowkes式计算试样的表面张力γ_s及其三组分值γ_s^a(色散力)、γ_s^c(偶极矩力)和γ(氢键力),发现经O₂、N₂、He和Ar等离子体短时间处理的聚酯表面自由能显著增大,表面润湿性增强,主要是聚酯表面张力的氢键力成分的贡献,X射线光电子能谱分析表明,这是由于聚酯表面含氧或含氮极性基团增加所致.     

Preparation and evaluation of a novel bonded imidazolium ionic liquid as stationary phase for gas chromatography

1‐Butyl‐3‐[(3‐trimethoxysilyl)propyl]imidazolium chloride ionic liquid was synthesized and chemically modified onto the inner wall of a fused capillary column as a stationary phase for gas chromatography. The 1‐butyl‐3‐[(3‐trimethoxysilyl)propyl]imidazolium chloride ionic liquid bonded capillary column was evaluated in detail. The results revealed that the ionic liquid bonded capillary column exhibited high column efficiency of 1.08 × 10⁴ plates/m, and good chromatographic separation selectivity (α ) for polar and non‐polar substances, and a good thermal stability between room temperature and 400°C. Moreover, the determination of thermodynamic parameters and the linear solvation energy relationship were further carried out. The results indicated that the chromatographic retention of each probe molecule on the ionic liquid bonded stationary phase was an enthalpy‐driven process, and the system constants of the linear solvation energy relationship signified that the dispersion interaction, the hydrogen bonding acidity and hydrogen bonding basicity were dominant interactions between probes and stationary phase among five interactions during the chromatographic separation. However, the contribution of each specific interaction for the stationary phase is ranked as the dispersion interaction > the hydrogen bonding basicity > the hydrogen bonding acidity.     

Polar networks control oligomeric assembly in membranes

Polar interactions have a profound influence on membrane stability and structure. A membrane-solubilized GCN4 peptide, MS-1, is used to study the impact of polar networks. Amide functionalities from amino acid side chains have been shown to promote peptide oligomerization, but lacked specificity. Herein, the hydrogen bonding interactions of an Asn side chain are coupled with the hydroxyl of Ser or Thr to generate a polar network. Analytical ultracentrifugation and fluorescence resonance energy transfer studies indicate that a trimer assembly is established where each membrane-embedded hydrogen bond contributes 1 kcal mol-1.     

Effect of solvent on absorption and fluorescence spectra of a typical fluorinated azo dye for its acidic and basic structures

The effect of 15 polar solvents on absorption and fluorescence energies of a typical fluorinated azo dye, 4-(2,3,5,6-tetrafluoro-pyridin-4-yl azo)-phenol, was reported for its acidic, MH, and basic, M, structures.For MH, the absorption energy is described on the basis of multi-linear equation with Taft's π* (solvent polarity) and β (hydrogen bond acceptor) parameters while the fluorescence energy varies rectilinearly with free energy of transferring the proton to the surrounding solvent, ΔG_t°.For M, the hydrogen bonding donor ability of protic solvent, α, is a predominant factor which affects the absorption energy while in aprotic solvents, the absorption energy correlates linearly with Kirkwood function. As the ability of the solvent for hydrogen bonding increases, the absorption band width will increase in parallel with the transition energy.     

Hydrophobic hydration in an orientational lattice model

To shed light on the microscopic mechanism of hydrophobic hydration, we study a simplified lattice model for water solutions in which the orientational nature of hydrogen bonding as well as the degeneracy related to proton distribution are taken into account. Miscibility properties of the model are looked at for both polar (hydrogen bonding) and nonpolar (non-hydrogen bonding) solutes. A quasichemical solution for the pure system is reviewed and extended to include the different kinds of solute. A Monte Carlo study of our model yields a novel feature for the local structure of the hydration layer: energy correlation relaxation times for solvation water are larger than the corresponding relaxation times for bulk water. This result suggests the presence of ordering of water particles in the first hydration shell. A nonassociating model solvent, represented by a lattice gas, presents opposite behavior, indicating that this effect is a result of the directionality of the interaction. In presence of polar solutes, we find an ordered mixed pseudophase at low temperatures, indicating the possibility of closed loops of immiscibility.     

Solvation parameters. Part 5: physicochemical interpretation of experimental solvent values for stationary phases of gas-liquid chromatography

It has been demonstrated for a long time that in the particular case of gas-liquid chromatography (GLC), a linear free energy relationship (LFER) of five terms can be established, each term including a parameter of solute and a parameter of solvent. The nature of some of these parameters has been quite clearly identified, even if not always well predicted from the molecular structure. First of all, the five solute parameters: two involved in the hydrogen bonding and three in the Van der Waals forces; secondly, the two solvent parameters involved in hydrogen bonding. It was remaining an uncertainty concerning the nature of the solvent parameters named D, W and E, respectively associated with the solute parameters of dispersion, orientation and induction/polarizability. This uncertainty has been solved using experimental chromatographic data of McReynolds (56 phases) and of the Kováts group (11 phases). The parameter W appears as of polar nature strictly speaking. The parameters D and E can be expressed by two opposite bilinear functions of 1/V (inverse of molecular volume) and PSA/V (ratio of the polar surface area over the molecular volume). These results are in agreement with previous studies limited to alkanes by the Kováts group.     

Molecular recognition. Design of new receptors for complexation and catalysis

Abstract

In recent years there has been intense activity in the design of synthetic molecules capable of enzyme-like recognition and binding of small substrates.¹ Two fundamental approaches have been taken. The first has generally involved non-directional binding forces (such as solvophobic, π-stacking and dispersion interactions) in water-soluble cyclophane frameworks.² This approach led to extremely important quantitative insights into the hydrophobic effect and the enthalpic and entropic contributions of solvent reorganization to binding.³ However, the weakly oriented nature of the binding interactions has resulted in only moderate substrate selectivity beyond the shape recognition permitted by the cavity. In nature such selectivity is a prerequisite for the chiral recognition and catalytic activity of enzymes and is achieved by hydrogen bonding and electrostatic interactions. The second major approach to artificial receptors makes use of these more directional interactions by incorporating several hydrogen bonding groups into a cleft or cavity of defined geometry.⁴ The resulting hosts form strong and selective complexes to those substrates with complementary shape and hydrogen bonding characteristics.⁵ In these cases, however, the binding free energy is solvent dependent, diminishing to zero as the polarity of the medium increases, due to the strong solvation of the hydrogen bonding sites. A central goal in contemporary molecular recognition research must be to develop receptors that effectively use directed hydrogen bonding interactions in competitive solvents. Success will probably require combining strong (possibly charged) hydrogen bonding groups with hydrophobic sites capable not only of effective apolar association with the substrate but also of protecting the polar sites from full solvation.     

Effect of intermolecular hydrogen bonding on low-surface-energy material of poly(vinylphenol)

We discovered that poly(vinylphenol) (PVPh) possesses an extremely low surface energy (15.7 mJ/m2) after a simple thermal treatment procedure, even lower than that of poly(tetrafluoroethylene) (22.0 mJ/m2) calculated on the basis of the two-liquid geometric method. Infrared analyses indicate that the intermolecular hydrogen bonding of PVPh decreases by converting the hydroxyl group into a free hydroxyl and increasing intramolecular hydrogen bonding after thermal treatment. PVPh results in a lower surface energy because of the decrease of intermolecular hydrogen bonding between hydroxyl groups. In addition, we also compared surface energies of PVPh-co-PS (polystyrene) copolymers (random and block) and their corresponding blends. Again, these random copolymers possess a lower fraction of intermolecular hydrogen bonding and surface energy than the corresponding block copolymers or blends after similar thermal treatment. This finding provides a unique and easy method to prepare a low-surface-energy material through a simple thermal treatment procedure without using fluoro polymers or silicones.     

Classification of structurally related compounds fromAstragalus extract by correlation of the logk w andS

Summary The linear relationship between logk _w andS derived from molecular interactions and statistical thermodynamics was investigated by four series of different polar probe solutes. For each series of similar polar solutes, structurally related compounds with similar dipolarity/polarizability and hydrogen bonding energy, a linear relationship between logk _w andS was obtained. The more similar the solutes, the greater were the regression coefficients obtained. For two series of solutes with different strong polar groups resulting in different dipolarity/polarizability and hydrogen bonding energy, two parallel lines were observed in the logk _w-S plots. Each line represented one series of compounds and the distance between them indicated the difference in the dipolarity/polarizability and hydrogen bonding energy. Based on the parallel lines implying information on structurally related compounds in the logk _w-S plot, a method of classification of structurally related compounds was put forward and the linear logk _w-S correlation for unknown components in nonaqueous RP-HPLC analysis ofAstragalus extract with isopropanol-methanol mobile phase was studied. Two nearly parallel lines were obtained in the logk _w-S plot and two series of structurally related compounds were classified in this way.     

Hydrogen fluoride — a suitable solvent for the formation of micelles

Relatively little attention has been paid to nonaqueous polar solvents as media for micellization. The properties of liquid hydrogen fluoride should favor the formation of micelles. The prerequisites for micellization in this solvent and the unique nature of HF are described. Some of the influencing factors like hydrogen bonding, cohesive energy density and Kirkwood-factor are discussed in detail. The CMC-data of selected surfactants in HF determined by solubilization measurements with hydrocarbons are given and discussed.Micelles in hydrogen fluoride. VII, for part VI see [1]     

A theoretical model for the critical micelle concentration of bile salts

Critical micelle concentration (CMC) is a fundamental parameter in the evaluation of the biological activity of natural and synthetic bile salts. The CMC is logarithmically related to the free energy of solute micellization in aqueous solution. Hydrophobic and hydrogen bonding interaction energies were identified as the primary contributors to this free energy and the logarithm of the CMC was modeled as a linear function of relevant chemical group contributions to the solvent accessible molecular surface area of the solute. The structures (three-dimensional atomic coordinates) of 23 mono-, di-, and tri-hydroxyl bile acids were generated and optimized by energy minimization. The accessible surface area for each structure was computed and partitioned according to calculated charge distribution and polar group orientation. Experimental CMC values were fitted to these computed quantities by least squares multiple linear regression. Two regression equations, based on slightly different surface area partition schemes, were derived and compared. Their significance in explaining the aggregation process and in predicting the CMC of new bile salts is discussed.     

Understanding the recognition mechanism of protein-RNA complexes using energy based approach

Protein-RNA interactions perform diverse functions within the cell. Understanding the recognition mechanism of protein-RNA complexes is a challenging task in molecular and computational biology. In this work, we have developed an energy based approach for identifying the binding sites and important residues for binding in protein-RNA complexes. The new approach considers the repulsive interactions as well as the effect of distance between the atoms in protein and RNA in terms of interaction energy, which are not considered in traditional distance based methods to identify the binding sites. We found that the positively charged, polar and aromatic residues are important for binding. These residues influence to form electrostatic, hydrogen bonding and stacking interactions. Our observation has been verified with the experimental binding specificity of protein-RNA complexes and found good agreement with experiments. Further, the propensities of residues/nucleotides in the binding sites of proteins/RNA and their atomic contributions have been derived. Based on these results we have proposed a novel mechanism for the recognition of protein-RNA complexes: the charged and polar residues in proteins initiate recognition with RNA by making electrostatic and hydrogen bonding interactions between them; the aromatic side chains tend to form aromatic-aromatic interactions and the hydrophobic residues aid to stabilize the complex.     

A structural approach to calculate physical properties of pure organic substances: The critical temperature,critical volume and related properties

A new approach based on computation of the molecular surface interactions (MSI) to estimate several physical properties of pure organic substances is described. MSI are derived from molecular structural data and consist of total molecular surface area, electrostatic molecular surface interactions, and a hydrogen bonding term. This new approach estimates the critical temperature and the molar critical volume of pure organic substances with molecular weights in the range of 40–500 a.u‥ In addition, the following properties can be calculated: the critical pressure, the boiling temperature, the molar volume in liquid state at normal pressure and temperature. The method can be used to predict physical properties of compounds having flexible or rigid, symmetric or asymmetric, polar or nonpolar molecular structures, and compounds with or without hydrogen bonding groups.     

Transport of a liquid water and methanol mixture through carbon nanotubes under a chemical potential gradient

In this work, we report a dual-control-volume grand canonical molecular dynamics simulation study of the transport of a water and methanol mixture under a fixed concentration gradient through nanotubes of various diameters and surface chemistries. Methanol and water are selected as fluid molecules since water represents a strongly polar molecule while methanol is intermediate between nonpolar and strongly polar molecules. Carboxyl acid (-COOH) groups are anchored onto the inner wall of a carbon nanotube to alter the hydrophobic surface into a hydrophilic one. Results show that the transport of the mixture through hydrophilic tubes is faster than through hydrophobic nanotubes although the diffusion of the mixture is slower inside hydrophilic than hydrophobic pores due to a hydrogen network. Thus, the transport of the liquid mixture through the nanotubes is controlled by the pore entrance effect for which hydrogen bonding plays an important role.     

Structure of phosphatidyl serine (PS) involved in interbilayer salt bridging and hydrogen bonding

For understanding the experimental results indicating salt bridging and hydrogen bonding between opposite polar surfaces in planar multibilayer structures of phosphatidyl serine (PS), (1) we carried out molecular modeling of the interacting surface layers. The interacting structures in the planar multibilayers are stabilized by salt bridge arrays of the phosphates with their counterions and by hydrogen bonds of ammonia from one polar surface with carbonyls in the opposite one. In multishell liposomes, where the distances between phosphates on the facing each other surfaces are not equal and they are bound to get out of register, the interbilayer interaction cannot extend over a large enough area to form stable structures, except if the salt bridges are strong enough to break down the curved surfaces and form planar multibilayers.     

Role of Surface Molecular Architecture and Energetics of Hydrogen Bonding Sites in Adsorption of Polymers and Surfactants

Hydrogen bonding is generally thought to be an ubiquitous adsorption mechanism, which often foils selective adsorption schemes. Through investigation of hydrogen bonding energy and its dependence on surface molecular architecture, it may be possible to develop new methodologies to control the adsorption of surfactants and polymeric flocculants, depressants, and dispersants used in particulate processing industries. A model system using St?ber silica spheres and polyethylene oxide, a polymer known for its ability to form hydrogen bonds, was examined. The effect of two different surface treatments of the silica particles, calcination and rehydroxylation, upon the adsorption of two polymer molecular weights was studied. The adsorption behavior was then linked to the respective surface structures via characterization of the surfaces using FTIR, NMR, and Raman techniques. In this paper role of hydrogen bonding sites and surface architecture on adsorption is discussed. Copyright 2000 Academic Press.