Monday, 17 May 2010
The below discussion colloids is quoted from an article by Dr James B. Calvert. The article might be of great interest to any colloidal silver enthusiasts (as well as those interested in colloidal gold and colloidal copper, etc.).
There should be a more exciting name for the phenomena to be discussed here. The dull term colloid that reminds us of glue is, nevertheless, the accepted word. It was coined in the early 19th century by the Father of Physical Chemistry, Thomas Graham (1805-1869), to distinguish those materials in aqueous solution that would not pass through a parchment membrane from those that would. Glue was indeed a material that would not, and the Greek for glue is kolla, from which we also get "protocol" and "collagen." Those that would pass through were things like salt, and other soluble crystalline substances, which Graham called crystalloids. As we shall see, the field is much, much richer than this.
Colloids received little attention until the end of the century, when van't Hoff, Oswald and Nernst founded modern physical chemistry and they, and others, became fascinated by colloid phenomena. There had been famous observations by Tyndall and others in the meantime, but chemists could not get excited over glue. Then, in the 1920's and 1930's, the importance of colloids to industrial processes and biochemistry changed everything. Colloids became a hot field, and soon every elementary textbook said something about them. In writing some of the other articles on the site recently, I became gradually aware of the fascination of colloids, and recognized that my knowledge of them was very deficient. This article is the result, and I hope it will serve as an introduction to what colloids are all about, and demonstrate how interesting and useful they are.
An interesting philosophical point was suggested by this study. Colloids are often called a "fourth state of matter," and I wondered just how meaningful this concept is. We shall find that it is very difficult to encapsulate any concept concerning colloids in a word, though heaven knows chemists have tried, and many words have been coined. It is necessary to name things to think about them efficiently, and one thing scientists have done assiduously is to assign names. Biology comes to mind, with endless terms and names based only on surface appearances, at least until recently. Naming gives the appearance of knowledge, where there is no real knowledge at all. The antithesis to mere naming is mathematical analysis, which gives real conclusions and effective knowledge. The danger of names comes when they are regarded as real things and are used to delimit instead of simply to denote and describe.
It is easy to recognize the three conventional states of matter in ice, water and steam. The names solid, liquid and gas can be attached to certain suites of properties, and makes a useful distinction. In a gas, particles of the substance move freely and have to be stopped by walls. In a liquid, the particles are sometimes associated, sometimes not, but always occupy a certain volume. In a solid, the particles cannot move far relatively, and can only vibrate. Many substances can be classified by these properties, but the terms do not separate matter into three mutually exclusive boxes, and may not be descriptive enough. Where is tar, for example, or jelly, or a substance above its critical point? Colloids will give many examples of substances for which the simple classification into three states is wholly inadequate.
The properties of matter depend almost completely on its structure. All metals are alike, to a good approximation. They are shiny, soft, tough substances that conduct electricity. All ionic crystals are alike (granted differences in crystal symmetry). They are hard, transparent and do not conduct electricity in the solid state. They can usually be crushed into white powders. The variations between metals, or between ionic crystals, are very much less important than their similarities. Saying that a substance is a metal, or an ionic crystal, says much more than simply that it is a solid. Solidity is only a macroscopic appearance, of no fundamental significance, like being green.
I have seen the definition of matter as "that which occupies space." But what about gases? They occupy space, of course, but two or more gases can occupy the same space, as far as appearances go. The important thing is to use terms like solid, liquid, gas only as far as they are useful descriptions, and not consider them as exclusive classifications into which everything must fit. To see that this is not trivial, consider the many sciences (not generally chemistry or physics) in which there are bitter controversies about which named category to assign to some object or process. We should not be limited by the arbitrary names we give our concepts.
Properties of Colloids I recalled that colloids were particles larger than molecules, but smaller than grains of sand. This is true, and colloidal dimensions can be considered to be from about 10 nm up to 1000 nm, or 1 ìm, but mere size is not the important thing about colloids. The overwhelmingly important property of colloids is that they have very large surface area. To some degree, they are all surface and their properties are those of their surfaces. I do not remember appreciating this properly before, but I can assure you of its significance. Incidentally, 1 ìm is about the size of a bacterium. I shall use the word "colloid" to refer to a substance of colloidal dimensions, or to a colloidal system, indifferently.
To see the significance of this observation, consider the cubic centimeter in the diagram at the right. In this form, it has an area of 0.0006 m2. We could say that it is almost all volume. Most of its molecules are safely resident behind its surface, secure from disturbance or attack. Let us now divide it into thin laminae, 10 nm thick, a colloidal distance. The cube becomes a million laminae, with a total surface area of 200 m2. Every molecule is now only a short distance from the cold outdoors, and the material is all surface. We have turned the mass cube into a laminated colloid by this delicate slicing alone.
Continuing, we now slice each of the million laminae into a million fibers, and the surface area doubles. We still have a colloid, of course, with two dimensions colloidal, but have not increased the area greatly, not as we did in the first slicing. We can expect fibrillar colloids as well as laminar ones. Finally, each fiber is chopped into a million bits, giving a corpuscular colloid. This increases the surface area only by 50%, to 600 m2. From the mass to the corpuscle, the surface area has been increased by a factor of a million, which is typical of a colloid. Note that most of this increase came with the first dimension to "go colloidal," so we can call anything with any least dimension of colloidal size to be a colloid. This was another thing that I did not appreciate in my ignorance.
References Most textbooks of Elementary Chemistry will include a chapter on colloids that makes a good introduction to the subject.
R. J. Hartman, Colloid Chemistry (London: Pitman & Sons, 1949). A classic reference with a great deal of description of colloid phenomena of all types.