Immiscibility Ionic strength

The most frequendy used technique to shift the equiUbrium toward peptide synthesis is based on differences in solubiUty of starting materials and products. Introduction of suitable apolar protective groups or increase of ionic strength decreases the product solubiUty to an extent that often allows neady quantitative conversions. Another solubiUty-controUed technique is based on introduction of a water-immiscible solvent to give a two-phase system. Products preferentially partition away from the reaction medium thereby shifting the equiUbrium toward peptide synthesis. 

When two immiscible solvents are placed in contact with each other and a non-ionizable compound is dissolved in one of the solvents, the compound distributes itself between the two solvents. This distribution is referred to as partitioning. The ratio of the concentrations of the compound in each phase is a constant for a specific set of solvents, pH, buffers, buffer concentrations, ionic strength and temperature. This ratio is referred to as a partition coefficient or distribution coefficient and is equal to the ratio of the solubilities in the two solvents. When the compound is a weak acid or base, the distribution of the compound can be shown to be given by the following equation for a monoprotic compound ... 

Electrophoresis — Movement of charged particles (e.g., ions, colloidal particles, dispersions of suspended solid particles, emulsions of suspended immiscible liquid droplets) in an electric field. The speed depends on the size of the particle, as well as the -> viscosity, -> dielectric permittivity, and the -> ionic strength of the solution, and it is directly proportional to the applied electric field. In analytical as well as in synthetic chemistry electrophoresis has been employed to separate species based on different speeds attained in an experimental setup. In a typical setup the sample is put onto a mobile phase (dilute electrolyte solution) filled, e.g., into a capillary or soaked into a paper strip. At the ends of the strip connectors to an electrical power supply (providing voltages up to several hundred volts) are placed. Depending on their polarity and mobility the charged particles move to one of the electrodes, according to the attained speed they are sorted and separated. (See also - Tiselius, - electrophoretic effect, - zetapotential). 

Normally, sufficiently concentrated solutions of biopolymers differing in chemical composition, conformation and affinity for a solvent are immiscible. Biopolymers are usually incompatible at a sufficiently high ionic strength and at pH values above the protein s lEP, where the biopolymers are charged macro-ions. These conditions are typical of most food systems. When the bulk concentration of the biopolymers is below the cosolubility threshold (or the phase separation threshold) the mixed solution of the biopolymers is stable. However, when the bulk biopolymer concentration is increased above this critical level, the mixed solution breaks down into two liquid phases. 

The effective interfacial area a " is increased by increases in ionic strength, ion valence number, or viscosity, by the presence of a solid or immiscible liquid, and by a decrease in liquid surface tension. Thus it is nearly impossible to predict a priori the interfacial area. However, scale-up is practicable from experiments carried out with the actual gas-liquid system in a small agitated contactor (D = 10-20 cm). The experimental work of Sharma et al. (M12, S23) shows that a scale-up basis of equal ndf,/ /D or n - n dJwD (when djD = 0.4-0.5) can be used with a fair degree of confidence (respectively, 10 and 16% average deviations) for agitated vessels with diameters up to 60 cm. 

IPA and 50% water, the alcohol can be separated out as a separate phase if enough NaCl is added to almost saturate the system. This is a direct influence of ionic strength in an extreme case. The fact that polar solvents can be separated as an immiscible phase opens up new sample preparation opportunities. For example, Loconto and co-workers recently demonstrated that the homologous series of polar 2-aminoethanols could be efficiently partitioned into IPA from an aqueous sample of interest to wood chemists (5). The sample was saturated with NaCl, then extracted using IPA. 

Step 5 interference elution step This step selectively removes vmdesired compovmds from the sorbent without eluting the analytes. Generally, the solvent used (1—2 ml for 100 mg of sorbent) is miscible with the sample matrix. For aqueous samples, the ionic strength and pH should be maintained to prevent analyte losses. The flow rate should be adjusted so that the solvent is in contact with the sorbent for 1—2 min. If the elution solvent is water-immiscible, drying of the colvunn is required. 

In this equation, [analyte]aqueous and [analytejorganic are the analyte concentrations in the immiscible phases. The partitioning coefficient of a certain compound is influenced by temperature, pH, ionic strength, and nature of the... 

A nonpolar liquid like heptane (C7H16) has intermolecular bonds with relatively weak London dispersion forces. Heptane is immiscible in water because the attraction that water molecules have for each other via hydrogen bonding is too strong. Unlike Na+ and CP ions, heptane molecules cannot break these bonds. Because bonds of similar strength must be broken and formed for solvation to occur, nonpolar substances tend to be soluble in nonpolar solvents, and ionic and polar substances are soluble in polar solvents like water. Polar molecules are often called hydrophilic and non-polar molecules are called hydrophobic. This observation is often stated as like dissolves like. Network solids (e.g., diamond) are soluble in neither polar nor nonpolar solvents because the covalent bonds within the solid are too strong for these solvents to break. 

Introduction of ionic groups or proteins into polymers (forming ionomers) leads to physical associations at the temperature of use [110, 111]. For example, Surlyn (DuPont) is a copolymer of ethylene and methacrylic add that shows enhanced zero-shear viscosity and elastomeric green strength. Viscoelastic characteristics are also enhanced due to loss of associations at the appropriate dissociation temperature. Ion pair associations are exploited to obtain misdbUity in otherwise immiscible polymers [112],... 

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