Mobile phase driving force

A variation of gradient CEC is pressurized-flow CEC or PEC (pressurized flow electrochromatography). A pump forms the gradient and then allows part of this pressurized flow to pump the mobile phase through the packed bed. In this way, one can perform isocratic or gradient CEC with part of the mobile-phase driving force being pumped, part electrophoretic and part electroosmotic flows [1,8,10]. 

Technique Mobile phase Driving force Stationary phase... 

Jost et al. (212) studied the use of TLC as a pilot technique for transferring retention data to column LC (HPLC). TLC is potentially an inexpensive and convenient method for this purpose if essentially identical phases with the same retention mechanisms are used. However, there are inherent procedural differences in TLC and HPLC, which make exact transfer of data questionable. These differences include a capillary mobile phase driving force in TLC, and forced flow with constant and adjustable rates in HPLC formation of mobile phase gradients (solvent demixing) when multicomponent solvents are used in TLC preloading of the stationary phase with components from the gas phase of the TLC solvent and the presence of binder in layers but not columns. 

Forced flow separations overcome the principal deficiencies of capillary flow separations by establishing a constant and optimum mobile phase velocity. Forced flow separations require specially designed developing chambers exploiting either centrifugal or pneumatic forces to drive the mobile phase through the layer. Centrifugal methods are more popular for preparative-scale separations and have been little used for analysis. The preferred approach for analytical separations is to seal the open face of the layer by contact with a flexible membrane, under hydraulic pressure, and deliver the mobile phase to... 

The analysis of macropore diffusion in binary or multicomponent systenis presents no particular problems since the transport properties of one compos nent are not directly affected by changes ini the concentration of the bther components. In an adsorbed phase the situation is more complex since ih addition to any possible direct effect on thei mobility, the driving force for each component (chemical potential gradient is modified, through the multi-component equilibrium isotherm, by the coiicentration levels of all components in the system. The diffusion equations for each component are therefore directly coupled through the equilibrium relationship. Because of the complexity of the problem, diffusion in a mixed adscjrbed phase has been studied tjs only a limited extent. 

It is clear that tire rate of growdr of a reaction product depends upon two principal characteristics. The first of these is the thermodynamic properties of the phases which are involved in the reaction since these determine the driving force for the reaction. The second is the transport properties such as atomic and electron diffusion, as well as thermal conduction, all of which determine the mobilities of particles during the reaction within the product phase. 

Since hydrogen ions are six to twelve times more mobile than other cations, there will be a delay between loss of hydrogen ions from solution and migration of glass cations into the aqueous phase. Presumably, this electrical imbalance results in an electric field which acts as a driving force for the migration of cations. Aluminium and fluoride are almost certainly transported as cationic aluminofluoride complexes, AIF and AIFJ, mentioned above. 

The force driving the mobile phase migration is the decrease in the free energy of the solvent as it enters the porous structure of the layer [6,21,22]. Under these conditions, the velocity at which the solvent front moves is a function of its distance from the solvent entry position. As this distance increases, the velocity declines. Consequently, the mobile phase velocity varies as a function of time and migration distance, and it is established by the system variables. 

Forced-flow development enables the mobile phase velocity to be optimized without regard to the deficiencies of a capillary controlled flow system [34,35). In rotational planar chromatography, centrifugal force, generated by spinning the sorbent layer about a central axis, is used to drive the solvent... 

One of the important operational variables in CEC is the analyte—sorbent interaction. In reversed-phase separations (typical in CEC) the hydrophobicity of the stationary phase determines the selectivity of the separation, and retention can be controlled by adjusting the surface chemistry of the packing, composition of the mobile phase, and temperature. In contrast to HPEC, the CEC column plays a dual role in providing a flow driving force and separation unit at the same time hence electrophoretic and chromatographic processes are operational. The stationary phase chemistry is dealt with in detail in Section III on column technology. 

In the lumped kinetic model, various kinetic equations may describe the relationship between the mobile phase and stationary phase concentrations. The transport-dispersive model, for instance, is a linear film driving force model in which a first-order kinetics is assumed in the following form ... 

All of these models predict that the hydrophobic effect provides a significant driving force for the exclusion of even highly polar, charged peptides from an aqueous environment to the nonpolar environment of the RPC sorbent. According to the solvophobic model, in order to place a peptide into a mobile phase, a cavity of the same molecular dimensions must first be created. The energy required to create this cavity is related to the cohesive energy density or the surface tension of the mobile phase. Conceptually, each solvent-accessible unit... 

As its name suggests, a liquid crystal is a fluid (liquid) with some long-range order (crystal) and therefore has properties of both states mobility as a liquid, self-assembly, anisotropism (refractive index, electric permittivity, magnetic susceptibility, mechanical properties, depend on the direction in which they are measured) as a solid crystal. Therefore, the liquid crystalline phase is an intermediate phase between solid and liquid. In other words, macroscopically the liquid crystalline phase behaves as a liquid, but, microscopically, it resembles the solid phase. Sometimes it may be helpful to see it as an ordered liquid or a disordered solid. The liquid crystal behavior depends on the intermolecular forces, that is, if the latter are too strong or too weak the mesophase is lost. Driving forces for the formation of a mesophase are dipole-dipole, van der Waals interactions, 71—71 stacking and so on. 

---