Mobility of solvent

From diffusion measurements it is known that the translatory mobility of solvent molecules in polymers changes abruptly at the glass temperature. Below T0 the diffusion constants are of the order of 10-12 to 10 14sq. cm. /sec., above Tg of the order of 10 4 to 10 8sq. cm./sec. (13). We have found that even below the glass temperature, plasticizer molecules may perform rather rapid motions, preferably rotatory motion. If the plasticizer has long aliphatic side chains, as with phthalates, rotating motions in the side chains should also result in a narrow... 

The third term of the Hamiltonian, Hss, represents the interactions between such molecules, and the last term, Hus the interactions between M and the n solvent molecules. The coordinates (rM, rs) apply to both electrons and nuclei. Nuclear coordinates have to be explicitly considered, because the mobility of solvent molecules is a very important factor in liquid systems, and changes in their internal geometry, due to the intermolecular interactions, may also play a role. 

Gels of SPS with different solvents have been compared to clathrates. WAXD results using toluene (a good solvent for SPS) and decalin (a relatively poor solvent for SPS) show that the structure of the crystalline junctions of the gels is similar to that of the clathrate a phase. A difference can be found in the width of the (010) reflection, which is relative to the width of the (210) reflection, much broader for the gel than for the clathrate. This is caused by the difference in the mechanism involved in crystal formation in gels and clathrates. Experiments performed on quenched samples of SPS with the monomer benzyl methacrylate show that also for this gel the structure of the crystalline part is similar to that of the clathrate phase. This means that solvent is present in both the crystalline and the amorphous parts of the gel. By solid-state nuclear magnetic resonance (NMR) studies, a clear difference in the mobility of solvent molecules in the crystalline and amorphous parts of the gel has been observed [58]. 

It had long been assumed that the solvent in a polymer solution provides a neutral hydrodynamic background, and that the properties of the solvent in a solution, such as viscosity, are the same as the properties found in the neat solvent. We know now that this simple assumption is incorrect Just as the solvent can alter properties of the polymer, so also do polymers alter the properties of the surrounding solvent Translational and rotational mobilities of solvent molecules may be reduced or increased by the presence of nearby polymer chains. Models for polymer dynamics that assume that the solvent has the same properties as the neat liquid are therefore unlikely to be entirely accurate. 

The presence of surface conductance behind the slip plane alters the relationships between the various electrokinetic phenomena [83, 84] further complications arise in solvent mixtures [85]. Surface conductance can have a profound effect on the streaming current and electrophoretic mobility of polymer latices [86, 87]. In order to obtain an accurate interpretation of the electrostatic properties of a suspension, one must perform more than one type of electrokinetic experiment. One novel approach is to measure electrophoretic mobility and dielectric spectroscopy in a single instrument [88]. 

For LC, temperature is not as important as in GC because volatility is not important. The columns are usually metal, and they are operated at or near ambient temperatures, so the temperature-controlled oven used for GC is unnecessary. An LC mobile phase is a solvent such as water, methanol, or acetonitrile, and, if only a single solvent is used for analysis, the chromatography is said to be isocratic. Alternatively, mixtures of solvents can be employed. In fact, chromatography may start with one single solvent or mixture of solvents and gradually change to a different mix of solvents as analysis proceeds (gradient elution). 

High Performance Liquid Chromatography. Although chiral mobile phase additives have been used in high performance Hquid chromatography (hplc), the large amounts of solvent, thus chiral mobile phase additive, required to pre-equiUbrate the stationary phase renders this approach much less attractive than for dc and is not discussed here. 

The reseai ch has been carried out by the liquid chromatograph Perkin-Elmer (Series 200), which has tandem detectors the diode array (X=210 nm) and the refractometer. The temperature of a column was 30 C, speed of a mobile phase is 1.5 ml/ min. As a mobile phase, mixtures of solvents methanol - water and acetonitrile - water with addition of sodium perchlorate. The columns with the modified silica gel C8 and Cl8 (4.6x220 mm, 5 pm) were used for sepai ation of the AIST and FAS components. In order to make the identification of AIST and FAS components more reliable the ratio of the values of the above-mentioned detectors signals of each substance analyzed. 

The complexity of the system increases with the number of solvents used and, of course, their relative concentrations. The process can be simplified considerably by pre-conditioning the plate with solvent vapor from the mobile phase before the separation is started. Unfortunately, this only partly reduces the adsorption effect, as the equilibrium between the solvent vapor and the adsorbent surface will not be the... 

If, in LC, the mobile phase is a mixture of solvents, the pore contents will not be homogeneous. One solvent component, the one with stronger interactions with the stationary phase, will be preferentially adsorbed on the surface [10] relative to the other. Consequently, although the bulk of the contents the pores, (Vp(i)), will have... 

When the silica surface is in contact with a solvent, the surface is covered with a layer of the solvent molecules. If the mobile phase consists of a mixture of solvents, the solvents compete for the surface and it is partly covered by one solvent and partly by the other. Thus, any solute interacting with the stationary phase may well be presented with two, quite different types of surface with which to interact. The probability that a solute molecule will interact with one particular type of surface will be statistically controlled by the proportion of the total surface area that is covered by that particular solvent. 

Where there are multi-layers of solvent, the most polar is the solvent that interacts directly with the silica surface and, consequently, constitutes part of the first layer the second solvent covering the remainder of the surface. Depending on the concentration of the polar solvent, the next layer may be a second layer of the same polar solvent as in the case of ethyl acetate. If, however, the quantity of polar solvent is limited, then the second layer might consist of the less polar component of the solvent mixture. If the mobile phase consists of a ternary mixture of solvents, then the nature of the surface and the solute interactions with the surface can become very complex indeed. In general, the stronger the forces between the solute and the stationary phase itself, the more likely it is to interact by displacement even to the extent of displacing both layers of solvent (one of the alternative processes that is not depicted in Figure 11). Solutes that exhibit weaker forces with the stationary phase are more likely to interact with the surface by sorption. 

Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

Practically a more convenient way of expressing solute retention in terms of solvent concentration for a binary solvent mixture as the mobile phase is to use the inverse of equation (11), i.e.. 

However, there might be exceptions if the mobile phase consists of a binary mixture of solvents, then a layer of the more polar solvent would be adsorbed on the surface of the silica gel and the mean composition of the solvent in the pores of the silica gel would differ from that of the mobile phase exterior to the pores. Nevertheless, it would still be reasonable to assume that... 

If the sample is relatively insoluble in the mobile phase, then it can be dissolved, as a dilute solution, in a relatively large volume of solvent. A large volume of the solution can then be placed on the column, a procedure that results in volume overload. 

It follows from equation (2) that the sample load will increase as the square of the column radius and thus the column radius is the major factor that controls productivity. Unfortunately, increasing the column radius will also increase the volume flow rate and thus the consumption of solvent. However, both the sample load and the mobile phase flow rate increases as the square of the radius, and so the solvent consumption per unit mass of product will remain the same. 

---