Hydrosols behavior

This question may be answered in two manners, as represented by communications to the Ralph K. Her Memorial Symposium. On the one hand, Yates proposes a thermodynamic approach to replace the failing DLVO theory. This approach can explain several experimental facts, but suffers from poor generality. According to Healy, on the other hand, the DLVO theory should give a coherent description of the hydrosol behavior on the condition that all the forces that play a role in the interaction are introduced in the model. This correction leads to a good description of the observed properties and to a better description of the interface structure. 

A. Behavior of Concentrated Nanosize Fumed Silica Hydrosols... 

Before I began writing. I considered the pos.sibility of a general text covering small particle behavior in both gases and liquids. Much of the theory of physical behavior is the same or very similar for both aerosols and hydrosols. almost as much as in the fluid mechanics of air and water. The differences include double layer theory in the case of aqueous solutions and mean free path effects in gases. There are other important, specifically chemical differences. 

For example, it is well known that the silica hydrosols are stable at their point of zero charge (pzc) and that they also coagulate in alkaline solutions, in which their electrical smface charge is high and should therefore increase their stabUity. Such behavior is very unusual indeed, and this question arises immediately Why does the Deijaguin-Landau-Verwey-Overbeek (DLVO) theory seem to be unable to cope with the silica hydrosols while it explains satisfactorily, at least to the best of our knowledge, the behavior of all other colloidal systems ... 

Table I reports the values of the static (Agt) and dispersive (Ajisp) parts of the Hamaker constant of silica in water, calculated from Equation (2). The corresponding values for TiOa, a typical electrocratic colloidal oxide, are also included for comparison, which is probably the key to an explanation of the special behavior of the silica hydrosols. These data show that the Hamaker constant of Si02 is approximatively 35 times smaller than that of Ti02. Thus, the attraction energy between two silica particles is 35 times smaller than that between two Ti02 particles of the same size. This weakness of the attraction energy enhances the role of the afore-mentioned structural forces, which are strongly dominated by the London-van der Waals attraction in the case of Ti02.

Analogous behavior is shown by ten other monoazo dyes. The color differences between the alkaline solutions of the dye and the hydrosol of Mg(OH)2 dye lake are readily seen if the test is made on a spot plate Idn. Limit 0.1 y Mg). ... 

From what has been said it is. evident that the question, whether hydrosols are solutions or suspensions, is purposeless. As a matter of fact they occupy an intermediate position. They exhibit properties that resemble crystalloidal solutions or suspensions depending upon the kind of hydrosol and upon the fineness of division. As Nernst f has often pointed out, the general behavior of most colloidal solutions would place them with the crystalloidal rather than with suspensions. The fact that hydrosols diffuse, and that they possess osmotic pressure would tend to justify this point of view. 

A. UltrafiUration. — By the use of filters that allow electrolytes to pass freely through, but retain the colloidal particles, colloidal stannic acid must have, after filtration, not only its ultramicrons, with their attendant anions, but also an equivalent amount of alkali ion molecules. The excess of the electrolytes, KOH, KaSnOa, etc., that were dissolved in the disperse medium, have passed through. The adsorbed portion of the alkali, regardless of whether it is dissociated or not, is an essential part of the hydrosol for if it is removed the colloid will coagulate. Duclaux, who has studied the behavior of colloidal iron oxide and cupric ferrocyanide in this connection, has proposed the name Micells for the ultramicrons together with their adsorbed molecules... 

Wedekind f has demonstrated that zirconium sol may be made by etching the metal with hydrochloric acid. A powder is thus obtained that goes into colloidal solution on washing with water. This hydrosol gives very peculiar reactions with electrolytes. Most acids precipitate it, but tartaric and picric acids will not. The hydroxides of the alkali metals precipitate it immediately, but ammonia water does so very slowly. Most neutral electrolytes have little or no effect. It is probable that the treatment with HCl forms a protective colloid which causes the unusual behavior. 

The author has observed the peculiar behavior of sulphur hydrosols toward collodion membranes. The latter remove almost all of the sulfur from the liquid. Colloidal sulfur had a pressure against its The Svedberg Koll.-Zeit., 2, 49-54 (1909). 

Protective Effect of Colloidal Stannic Acid. — Contrary to the behavior of silicic acid the freshly prepared hydrosols of stannic acid have a protective effect on colloidal gold, sometimes even in the presence of sodium chloride. If dilute hydrochloric acid is used instead of sodium chloride, a deep red or purple red precipitate is obtained, which is known as the gold purple of Cassius. 

We will briefly describe the filtration behavior of hydrosols in a granular filter bed, but first, it is useful to conceptualize the structure of the granular medium. The bed is assumed to consist of a large number of unit bed elements (UBEs) in series (Tien, 1989). Each UBE has a certain type of flow channel, and the granular medium surface acts as a particle collector, collecting particles from the fluid flowing in the channel. The porous medium in each UBE may be represented in a number of ways. 

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