Colloidal solutions factors affecting

Rejection of the solute (or dispersed colloid) is, together with permeate flux, one of the two key performance parameters of any ultrafiltration membrane. The values of rejection coefficients are of crucial Importance in many applications of ultrafiltration. The objective of this contribution is to consider and analyze the individual factors affecting rejection of polymer solutes by ultrafiltration membranes. The factors that will be considered include, sterlc rejection (sieving), solute velocity lag and solute-membrane Interaction. 

It should be noted, however, that this table is arranged with regard to solutions that Bechhold employed and does not necessarily represent the correct relations of the size of particles in colloidal solutions in general. In fact Prussian blue, colloidal iron oxide, and many other colloids may be prepared with particles varying so in size that the relation to hemoglobin would be quite different from that indicated by the table. Finally ultrafiltration is affected not only by the size of the particles, but also by other factors, such as adsorption, electric charge, etc. 

Electrode kinetics, to be considered in the next chapter, are profoundly influenced by the structure of the double layer at an electrode-solution interface and it is with such systems that we shall be primarily concerned. However, double layer theory, as developed for electrode-electrolyte solution interfaces, leads on to the proper interpretation of electrokinetic phenomena, an understanding of the factors affecting colloid stability, and to the elucidation of cell membrane and ion-exchange processes. 

The study of the interfacial liquid-liquid phase however is complicated by several factors, of which the chief is the mutual solubility of the liquids. No two liquids are completely immiscible even in such extreme cases as water and mercury or water and petroleum the interfacial energy between two pure liquids will thus be affected by such inter-solution of the two homogeneous phases. In cases of complete intersolubility there is evidently no boundary interface and consequently no interfacial energy. On addition of a solute to one of the liquids a partition of the solute between all three phases, the two liquids and the interfacial phase, takes place. Thus we obtain an apparent interfacial concentration of the added solute. The most varied possibilities, such as positive or negative adsorption from both liquids or positive adsorption from one and negative adsorption from the other, are evidently open to us. In spite of the complexity of such systems it is necessary that information on such points should be available, since one of the most important colloidal systems, the emulsions, consisting of liquids dispersed in liquids, owe their properties and peculiarities to an extended interfacial phase of this character. 

There have been a few experiments related to the effect of illumination of the growth of CdS films. Simple heating of the deposition bath by absorption of the radiation is one obvious factor that can affect the deposition [68]. However, even in this case, other effects occur, since the color of the bath was reported to darken if UV (sunlight) illnmination was employed. Based on previous studies of illuminated CdS colloids when elemental Cd was formed, both as a film and in solution [69], as well as the known tendency of ZnS to undergo reduction to metallic Zn under UV illumination, this darkening may be assumed to be caused by elemental Cd. There are several possible mechanisms that may explain snch an effect re-dnction of the CdS by photogenerated electrons is one possibility. 

The factors that determine crystal size, a topic of particular relevance to this chapter, have been discussed to some extent in Section 3.4. There are two main factors that generally affect crystal size for any particular material the deposition mechanism and the deposition temperature. The hydroxide cluster mechanism is expected to give a crystal size similar to that of the original hydroxide cluster (with some growth possible as deposition proceeds). That size depends mainly on temperature, both because higher temperatures allow more grain growth and, possibly more important, lower temperatures kinetically stabilize very small nuclei in solution that are thermodynamically unstable. For example, in the hydroxide cluster mechanism, where crystal size is believed to be controlled mainly by the size of the Cd(OH)2 colloids, the relevant equilibria are... 

The use of membranes for separating particles of colloidal dimensions is termed dialysis. The most commonly used membranes are prepared from regenerated cellulose products such as collodion (a partially evaporated solution of cellulose nitrate in alcohol plus ether), Cellophane and Visking. Membranes with various, approximately known, pore sizes can be obtained commercially (usually in the form of sausage skins or thimbles ). However, particle size and pore size cannot be properly correlated, since the permeability of a membrane is also affected by factors such as electrical repulsion when. the membrane and particles are of like charge, and particle adsorption on the filter which can lead to a blocking of the pores. 

Nanostructures primarily result from polyelectrolyte or interpolyelectrolyte complexes (PEC). The PEC (also referred to as symplex [23]) is formed by the electrostatic interaction of oppositely charged polyelectrolytes (PE) in solution. The formation of PEC is governed by physical and chemical characteristics of the precursors, the environment where they react, and the technique used to introduce the reactants. Thus, the strength and location of ionic sites, polymer chain rigidity and precursor geometries, pH, temperature, solvent type, ionic strength, mixing intensity and other controllable factors will affect the PEC product. Three different types of PEC have been prepared in water [40] (1) soluble PEC (2) colloidal PEC systems, and (3) two-phase systems of supernatant liquid and phase-separated PEC. These three systems are respectively characterized as ... 

Osmotic pressure is often expressed by Equation 2.10, known as the Van t Hoff relation, but this is justified only in the limit of dilute ideal solutions. As we have already indicated, an ideal solution has ideal solutes dissolved in an ideal solvent. The first equality in Equation 2.9 assumes that yw is unity, so the subsequently derived expression (Eq. 2.10) strictly applies only when water acts as an ideal solvent (yw = 1.00). To emphasize that we are neglecting any factors that cause yw to deviate from 1 and thereby affect the measured osmotic pressure (such as the interaction between water and colloids that we will discuss later), riy instead of n has been used in Equation 2.10, and we will follow this convention throughout the book. The increase in osmotic pressure with solute concentration described by Equation 2.10 is... 

Other factors also affect the stability of eolloidal solutions, in particular the pH of the environment. The surface molecules of the nucleus of the micelles of different colloidal systems can have acid, basic, or amphoteric properties. Colloids characterized by this feature are called, respectively, acidosid, basoids, and ampholitoids (Marchenko, 1965). It has been established that the coagulation threshold of acidoids (sols of weak acids) increases and that of basoids decreases when the pH is increased. 

Soil pH measurements can be ambiguous. Two factors that affect soil pH measurements are the soil-solution ratio and the salt concentration. Increasing either factor normally decreases the measured soil pH because H and A1 cations on or near soil colloid surfaces can be displaced by exchange with soluble cations. Once displaced into solution, the A1 ions can hydrolyze (Eq. 10.2) and further lower the pH. Preferential retention of hydroxy aluminium polymers by soil colloids drives the hydrolysis reactions further toward completion and leads to lower pH. Increasing the neutral salt concentration to 0.1 or 1 M can lower the measured soil pH as much as 0.5 to 1.5 units, compared to soil pH measured in distilled water suspensions. 

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