Solvent movement

The different techniques which have been applied to determine transport in polymer electrolytes are listed in Table 6.1. For a fully dissociated salt all the techniques yield the same values of t (small differences may arise due to second order effects such as long range ion interactions or solvent movement which may influence the different techniques in different ways). In the case of associated electrolytes, any of the techniques within one of the three groups will respond similarly, but the values obtained from different groups will, in general, be different. Space does not permit a detailed discussion of each technique, this is available elsewhere (see Bruce and Vincent (1989) and the references cited therein). However, we will consider one technique from each group to illustrate the differences. A solid polymer electrolyte containing an associated uni-univalent salt is assumed. 

Your laboratory period had only 90 min. for the development of the chromatogram. In order to get better separation of the spots you must allow the solvent front to move much farther than the value you reported on your Report Sheet. Assuming a steady rate of solvent movement, how long of a lab period do you need for the solvent front to move 12.5 cm ... 

Group IIB. As, Sb, and Sn The separation of a mixture of the three elements is a difficult operation. The metals are present as their lower chlorides in dilute (2-4m) hydrochloric acid. The solution is spotted on paper and allowed to dry in air for 15 minutes. The solvent consists of 7-5 ml acetylacetone (b.p. 137°-141°) saturated with water and treated with 0 05 ml concentrated hydrochloric acid and 2-5 ml acetone (sufficient of the last-named to give a clear solution). The separation is allowed to proceed for 1 hour in an atmosphere saturated with respect to a saturated solution of acetylacetone in water the solvent movement is about 15 cm. The complexes formed are very stable, particularly that with tin (Rf = 1). The strip is removed from the extraction vessel, the solvent is allowed to evaporate for several minutes, and the strip is sprayed (before it is completely dry) with a chloroform solution of dithizone (0-005 per cent w/v), and then allowed to dry thoroughly. The tin is found in the solvent front. 

Group IIIB. Ni, Co, Mn, and Zn The metals are present as chlorides in dilute hydrochloric acid. The solvent used is acetone containing 5 per cent (v/v) water and 8 per cent (v/v) hydrochloric acid (d 1.18). The separation is conducted in an atmosphere saturated with respect to the solvent. The strip is dried (after a solvent movement of about 25 cm), exposed to ammonia vapour, and then sprayed with a saturated alcoholic solution of alizarin containing 0-1 per cent rubeanic acid and 1 per cent salicyladoxime. The following results are obtained (cf. also Fig. VI.5b). 

In a mobile solvent, the amount of solvent movement depends upon the relative rates of solvent motion and depopulation of the excited state. For the long-lived E and T, states, there is ample time for the solvation shell to adjust to the equilibrium geometry. The situation in the shorter-lived T2 and Ti states is more problematical. Prompt intersystem crossing is probably too fast for appreciable solvent motion, even in a low viscosity medium. Whether, the same is true in the thexi T2 is uncertain. No reliable estimates of rii are available and the lifetime of the thexi state is unknown. 

The basic technique is quite simple. A sheet of cellulose filter paper, such as Whatman No. 1, serves as the separation medium. For one-dimensional paper chromatography (PC), the paper is cut into strips about 5 cm wide and 20 cm long for two-dimensional PC (below), a 20 X 20-cm sheet is commonly used. The papers come in various porosities (fine, medium, coarse) the porosity determines the rate of movement of the developing solvent. Low-porosity paper gives slow solvent movement but good resolution. Thick papers, which have increased sample capacity, are available for preparative separations. 

The development time should be sufficient to separate the components of interest. The rate of solvent movement depends on factors such as the porosity of the paper, the surface tension, viscosity, and volatility of the solvent, and the ambient temperature. Reasonable Ri values for good resolution are about 0.4-0.8 typical separation times for modern PC papers are in the range of 2-4 hours. 

Quartz crystal microbalance studies have shown that the movement of the solvent molecules associated with ions can be considerable. Using PPy prepared in sodium dodecyl sulfate, a mix of both cation- and anion-driven processes was seen when cycled in NaCl, and the mass changes involved indicated that four water molecules moved per Cl and 15 water molecules per Na" " [11]. The role of solvent water molecules has also been examined for PPy in dodecyl benzene sulfonate (DBS), a very widely studied system, where the insertion of cations accounted for only 20% of the mass change upon polymer reduction, indicating that four water molecules were brought into the film with each Na+ [12]. As the electrolyte concentration was changed from 0.1 M to 6 M, the total inserted mass became smaller and the mechanism moved from pure cation transport to an equal amount of anion transport [13]. These results were said to support an osmotic expansion model, whereby the difference in osmotic pressure between the electrolyte and polymer bulk (greater with more dilute electrolyte solutions) drives solvent movement. 

The free-volume theory of diffusion was developed by Vrentas and Duda. This theory is based on the assumption that movement of a small molecule (e.g., solvent) is accompanied by a movement in the solid matrix to fill the free volume (hole) left by a displaced solvent molecule. Several important conditions must be described to model the process. These include the time scales of solvent movement and the movement of solid matrix (e.g. polymer segments, called jumping units), the size of holes which may fit both solvent molecules and jumping units, and the energy required for the diffusion to occur. 

The volume changes of the polymer may result partly from the changes of the polymer backbone, due to the changes of bond lengths and conformation, and partly from osmotic expansion, which is the volume change associated with the solvent movement, of the polymer phase. The osmotic expansion of PPy-DBS was thermodynamically and experimentally described to investigate the osmotic effect on the... 

In Figure 2.6, the movement of solvent is detected by the movement of a bubble in a very narrow tube attached to the solvent compartment of the osmometer. When the photocell detects movement of the bubble, an external head of solvent is established by the raising of the solvent reservoir. In Figure 2.7, a capacitance device is used to detect solvent movement across the membrane. The measured signal is used to generate an appropriate external solvent head equivalent to the osmotic pressure. Modern instrumentation, once set up properly, can reduce the measurement time to a matter of minutes thus reducing the errors which are found if the membranes are anything less than truly semi-permeable. 

Numerous mobile phases (development solvents) are available for lipid work (see Table 1). They often consist of solvent mixtures that vary in polarity, along with small amounts of salts or acids. Because a mixed solvent system allows for an undefined gradient in solvent composition during movement on the silica gel layer, samples with varying polarity can be developed on a single plate in TLC the velocity of the solvent movement is reduced as the solvent front nears the top of the plate optimal separation is obtained with bands or spots with Revalues between 0.1 and 0.6 (63). 

Matty industrial processes rely on dissolution of raw materials and subsequent removal of solvents by various drying process. The formation of a solution and the subsequent solvent removal depends on a solvent transport phenomena which ate determined by the properties of the solute and the properties of the solvent. Knowledge of the solvent movement within the solid matrix by a diffusion process is essential to design of various products and the technological processes. 

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