Hydrophobic mechanism

Organic modifier Solvent strength Solvent strength generally increases with the volume percent of organic modifier. Its effect is most important when hydrophobic mechanisms contribute significantly to retention. In this case, changing... 

In a sediment system, the hydrolysis rate constant of an organic contaminant is affected by its retention and release with the sohd phase. Wolfe (1989) proposed the hydrolysis mechanism shown in Fig. 13.4, where P is the organic compound, S is the sediment, P S is the compound in the sorbed phase, k and k" are the sorption and desorption rate constants, respectively, and k and k are the hydrolysis rate constants. In this proposed model, sorption of the compound to the sediment organic carbon is by a hydrophobic mechanism, described by a partition coefficient. The organic matrix can be a reactive or nonreactive sink, as a function of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe 1983, 1985 Burkhard and Guth 1981), using different organic compounds, show that hydrolysis is retarded in the sohd-associated phase, while alkaline and neutral hydrolysis is unaffected and acid hydrolysis is accelerated. 

As viewed from the Coulombic versus solvophobic dichotomy, the mean-field-like behavior is quite unexpected. Thermodynamic properties of the salt-free systems clearly point toward a hydrophobic mechanism for phase... 

Whilst sulfate appears to be fundamental to haze formation, other wine components such as phenolic compounds remain as candidate haze modulators. One possibility is that white wine phenolic compounds affect the particle size of denatured aggregated proteins, possibly through crosslinking. Several researchers (Oh et al. 1980 Siebert et al. 1996b) have suggested a hydrophobic mechanism for the interaction between phenolic compounds and proteins, in which the protein has a fixed number of phenolic binding sites. More of these sites are exposed when the protein is denatured. 

Hydrophobic interaction chromatography (HIC) is a mode of separation in which molecules in a high-salt environment interact hydrophobically with a nonpolar bonded phase. HIC has been predominantly used to analyze proteins, nucleic acids, and other biological macromolecules by a hydrophobic mechanism when maintenanee of the three-dimensional structure is a primary eoneern [1-4]. The main applications of HIC have been in the area of protein purification because the reeovery is frequently quantitative in terms of both mass and biological activity. 

Supports for SEC of proteins are designed to be neutral and very hydrophilic to avoid disruption of protein structure and interaction of the solutes with the support by ionic or hydrophobic mechanisms. The base matrix can be either silica or polymer efforts are made to totally mask its properties with a carbohydratelike stationary phase. The pore structure is critical to successful SEC. Not only must the total pore volume (F,) be adequate for separation, the pore diameter must be consistent and nearly homogeneous for attainment of maximum resolution between molecules with relatively small differences in molecular size (radius of gyration or molecular weight). A twofold difference in size is usually required for separation by SEC. Pore homogeneity can be assessed from the slope of the calibration curve of the logarithm of the molecular weight versus the retention time or the partition coefficient (Kd) = (F - Fq)/F , where F is... 

Unhke the hydrophobic mechanism that forms Afi fibrils, metal-induced A precipitation proceeds through two pathways- 1. reversible, ionically-mediated oligomerization, 2. Cu-mediated A oxidation and cross-finking. High affinity chelators both inhibit and reverse Afi precipitation induced by metal ions, and dissolve deposits from post-mortem human brain tissue [ 143,183] (Table 2). A also recruits the contaminating metal ions in ordinary laboratory buffers to form the micronuclei that seed the precipitation of peptide solutions into fibrils. Therefore, even the formation of fibrils in the... 

Reversed-phase solid-phase extraction (SPE) involves the partitioning of organic solutes from a polar mobile phase, such as water, into a nonpolar solid phase, such as the C-18 sorbent (Fig. 4.1). Partitioning involves the interaction of the solute within the chains of the stationary phase, which may be a C-18 hydrocarbon, C-8 hydrocarbon, or the polymeric sorbents (such as styrene-divinylbenzene). The word hydrophobic mechanism is commonly... 

Figure 2. Schematic representation of the effect of surfactant, heat, and electrolyte on polymer cross-linking by a hydrophobic mechanism.

Fundamental Properties of Polymers, Metals and Ceramics (e.g., strength in compression, tension and bending elasticity/plasticity failure mechanisms phase diagrams transition temperatures surface roughness hydrophobicity) Mechanical Properties of Biological Tissues (e.g., elastic viscoelastic, hysteresis, creep, stress relaxation)... 

The techniques presented here allow to produce ultralight materials with anisotropic properties. Optical absorption and emission, hydrophobicity mechanical strength and index of refraction can be tuned within the same monolith. Conceivable applications of our technique include fabrication of photonic devices, membranes, radiation collimators, fuel cell electrodes as well as hierarchically structured materials that combine mechanical strength with the acoustic and thermal insulation properties of conventional aerogels. 

The thermodynamic parameters for hydrophobic interactions have been studied extensively both theoretically [1-4] and experimentally. They are diagnostic of this special aqueous-solution interaction which occurs for small molecules as well as for macromolecules. [5-13] Most data used to assess the contribution of hydrophobic interactions to protein folding have been derived from solubility studies which yield free energies of transfer. Little work has been done on the direct measurement of the thermodynamic parameters for association by hydrophobic interactions. We report in this paper our investigations on the use of heats of dilution as measured with a flow microcalorimeter as a probe for the detection and measurement of solute-solute interactions with particular emphasis on association by the hydrophobic mechanism. 

The thermodynamic parameters for the association of 1,1,3,3-tetramethyl urea in aqueous solution, AH° positive, AS° positive and ACp negative, are all consistent with a hydrophobic mechanism [2]. 

Heat of dilution measurements on small molecules provide a method of assessing solute-solute association mechanisms. A contribution from hydrophobic mechanisms is suggested if heats of dilution are negative in water. For 1,1,3,3-tetramethyl urea, which was analyzed most completely, all the thermodynamic parameters are consistent with a hydrophobic interaction mechanism for association in aqueous solution. 

The authors further defined a sequence of importance of each of the five parameters on the individual selectivity of reversed phases for the solution of given separation problems. Although the retention is mainly governed by the hydrophobic mechanism (H parameter), this parameter contributes least to the individual selectivities of the various reversed phases. It is by no means a new perception, but the comparison of the parameter lists for different phases confirms that selectivity in RP-H P LC is mainly based on the so-called secondary interactions. According to the empirical LFER approach, the importance of the phase parameters for selectivity control increases in the following sequence ... 

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