COLLOIDAL IODINE’ BY WALLACE L. CHANDLER AND ELROY J. MILLER
The literature on the chemistry of iodine contains but few references to researches dealing with the colloidal state or colloidal behavior of this element. I t is replete, however, with reports on studies pertaining to iodine combined with or adsorbed by substances such as starches, proteins, tannins, and charcoal. These combinations are sometimes termed colloidal iodine but they are in reality either true iodine compounds or adsorbents carrying adsorbed iodine. Among the researches dealing with iodine in a truly colloidal state are those of Amann.2 He observed that fresh alcoholic iodine solutions show the Tyndall effect. He also observed with the aid of the ultramicroscope the formation of solutions of iodine in carbon disulfide, chloroform, carbon tetrachloride, toluene, xylene, and other solvents. In carbon disulphide, carbon tetrachloride, and chloroform the iodine was in true solution as shown by the absence of ultramicropscopic particles. In toluene and xylene, however, there appeared suddenly clouds of particles in lively Brownian movement. In water the formation of micellae preceded the formation of crystals. Neither the organosols nor the hydrosols were stable. Hydrosols of iodine were also prepared by Harrison3 by the interaction of hydriodic and iodic acids. The blue sols formed in this manner were unstable, turning gray and flocculating almost instantly. Holmes and Williams‘ in a study of iodine as an emulsifying agent conclude that iodine functions as an emulsifying agent by virtue of the formation of colloidal aggregates a t the liquid-liquid interface. Bordier and RoyKpresented evidence to show that iodine in water at oo is in aggregates made up of more than two atoms. By the addition of a concentrated alcoholic solution of iodine to 0.4% gelatine solution they obtained a colloidal solution that contained 0.4% iodine. In 1 9 2 5 one of use announced the production of a fairly stable suspensoid of iodine by the interaction of hydriodic acid and a hypohalous acid in dilute solution a t oo in the presence of gum arabic. I t is the purpose of this paper to describe methods for the preparation of relatively stable aqueous colloidal suspensions of iodine. Precipitation of Iodine from Solutions Formatzon of unstable suspenszons. In the rapid precipitation of iodine from solutions in organic solvents or aqueous salt solutions there is formed a brick red suspension. This suspension is extremely unstable and changes Contribution from the Bacteriolo ical and Chemical laboratories of the Michigan Agricultural Experiment Station. Pubfished by permission of the Director of the Experiment Station as journal article No. 40. Kolloid-Z., 6, 235 (1910); 7, 67 (1910); Kolloidchem. Beihefte, 3, 337 (1912). * Kolloid-Z., 9,j ( ~ 9 1 1 ) . “Colloid Symposium Monograph”, 2, 138 (1925). Compt. rend., 163,567 (1916). e Chandler: Proceedings Twenty-ninth Annual Meeting of the U. S. Live Stock Sanitary Association ( I 92 5 ) .
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almost instantly into a darkprecipitate that’ quicklysettlesout. Thisphenomenon may be brought about in a number of ways, as, for example, by the acidification with a strong acid of a dilute solution of a mixture of iodide and iodate, by the precipitation of iodine from solution in absolute alcohol or glacial acetic acid through the rapid addition of these solutions to cold water, or, in general, by producing suddenly a highly supersaturated solution of iodine in water. Formation of stable suspensions. The possibility of stabilizing the brick red suspension of iodine was suggested. While the probability of stabilizing this suspension successfully seemed rather reniot e because of the exceedingly rapid growth of the small particles into coarse crystals a number of the more common substances known to possess protective properties were, nevertheless, tried out. Gum arabic proved to be fairly satisfactory so that under certain conditions relatively stable suspensions were formed. I n working with these suspensions it became apparent that certain conditions must obtain in order to produce the most satisfactory stabilization. It was necessary (in accordance with von LYeimarn’s principle) that the production of the supersaturation be exceedingly rapid in order to insure the formation of the smallest particles. I t also seemed necessary to keep the temperature in the neighborhood of o’, possibly in order to delay the growth of the particles and permit time for the formation of the protective coating around them. I n this manner a purplish suspension of fairly small crystals was formed when dilute solutions of sodium iodide and sodium iodate in the proportions of five to one were acidified rapidly in the cold in the presence of gumarabic. zHC1
+ NaI + x a I 0 3 +zSaCl + H I + HIO3 jHI + HIO, 312 + 3HzO -F
The oxidation of the hydriodic acid by t,he iodic acid with the formation of free iodine was apparently rapid enough t o produce minute crystals but it was not sufficiently rapid to produce a large amount of very fine particles for the precipitated iodine remained in suspension only a few minutes. Microscopic examination showed the precipitate to be largely in the form of small crystals but a small portion consisted of very minute globular particles. These will be described in detail later. A suspension of much greater stability was formed under similar conditions by employing the oxidation of hydriodic acid by hypoiodous acid for the rapid production of the high supersaturations. liaJ(O1) zHCl--+ 2SaCI HI HOI
+ -+ + + HOI + + Hd.3
HI
1 2
Sodium iodohypoiodite is easily prepared for this purpose by rapidly treating a one to two per cent solution of sodium hydroxide with iodine until the strawcolored liquid, which forms immediately, just begins to turn red and then pouring this solution from the remaining iodine crystals. 2NaOH
+ 11 -+
SazI(OI)
+ HZO
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This dilute solution of sodium iodohypoiodite appears to be stable for several hours provided it is removed without delay from the excess of iodine. In time, however, the hypoiodite changes into the iodide and iodate. 3Na(OI) 4' ah'a1 Na(I03) When a more concentrated solution of sodium hydroxide is treated with iodine the iodate is formed immediately. If this concentrated solution is heated until the iodate crystals all dissolve and at once diluted with water the hypoiodite is again produced, for the above reaction is reversible. The rapid acidificntion with concentrated hydrochloric acid of a dilute solution of sodium ioau,,ypoiodite in the presence of gum arabic at oo produces a relatively very stable suspension. Its color varies somewhat with the conditions of formation. It may be brick red, light brown, or groundchocolate. When prepared under favorable conditions it remains in suspension for several weeks without showing any appreciable settling. On standing for some months however, there is a tendency for the suspension to settlo and a clear strawcolored supernatant solution begins to appear. Specimens of preparations that have stood from eighteen to twenty one months show considerable settling, but their structure remains apparently unchanged. K h e n viewed through the microscope they were found, like the fresh preparations, to be made up of the small globules in rapid Brownian motion. The material was easily dispersed again by shaking once or twice.
+
Nature of the Suspension When examined under the microscope the suspension was found to be made up almost exclusively of minute globules in rapid Brownian movement. The largest globules were approximately 0 . 2 micron in diameter and surprisingly uniform in size. There were, however, many smaller globules and possibly some were of ultramicroscopic dimensions for the largest ones were just resolvable under the highest power of the microscope (1500times). The larger globules were unmistakably spherical in shape and the smaller ones were presumably so. This suggested that the suspension might be an emulsoid with liquid iodine the disperse phase and a saturated solution of iodine in water the dispersion medium. Further evidence of the emulsoid nature of the system is found when the process of formation of the particlesisobserved under the microscope. The procedure may be conveniently carried out in the following manner: X drop of dilute solution containing a mixture of a metal iodide and iodate in the proportions stated above is placed under a coverslip on a glass slide. The solution is then brought approximately in focus and a drop of hydrochloric acid is placed at the edge of the coverslip. As the acid diffuses into the solution under the coverslip there appears a yellow area (by transmitted light) where the iodine is formed. Immediately behind this yellow area appears a swarm of iridescent globulesin rapid Brownian movement and directly behind these globules there are formed small diamondshaped crystals of iodine. With a movable stage the progress of the diffusion of the acid can readily be followed. When the progress of the diffusion is rapid, as when the acid is first touched to the edge of the coverslip, the swarm
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WALLACE L. CHAI’DLER AND ELROY J. MILLER
of globules always appears just behind the yellow area and the crystals always appear behind the globules. But, when the progress of the diffusion becomes slower the globules cease to appear and the crystals form immediately behind the yellow area. This is in keeping with the fact that it is essential that a rapid mixing take place for the production of the brick-red suspension. The mechanism of the action is probably as follows: The yellow area represents a highly supersaturated solution of iodine. The iodine in excess of the amount necessary for saturation is thrown out in the form of globules of liquid iodine since there is not time for molecular orientation necessary for the formation of crystal structure. These globules of liquid iodine are in equilibrium with a saturated aqueous solution of iodine. From the saturated iodine solution crystallization begins and since the vapor pressure of the liquid is undoubtedly higher than that of the crystals, the globules go into solution as fast, or nearly so, as the crystallization removes iodine from solution. When the higher supersaturation is not reached, as is the case when the diffusion of the acid into the salt solution is slow, the iodine is liberated slowly enough so that the crystallization is sufficient to remove the excess of iodine without the formation of the liquid globules. The presence of gum arabic in the salt solution before acidification changes radically the picture just presented. The yellow area appears as does also the swarm of globules but no crystals are formed. The globules are stabilized and remain in suspension.’ This is the brick red to brownish suspension obtained in quantity as above described. When the protective colloid is absent, the brick red appearance quickly changes to purplish due to the formation of the minute crystals of iodine. Thus the evidence obtained microscopically substantiates and amplifies the conclusions arrived at from the work with the solutions in larger quantities. It might be mentioned in passing that a macroscopic observation of the diffusion of the acid into the salt solution under the coverslip shows in the absence of the gum arabic a steel blue area where the acid has penetrated. In the presence of gum arabic this area is brick red or brown. There is, therefore, considerable evidence that we are dealing with an emulsoid system in which the globules of liquid iodine are suspend-d in a saturated aqueous solution of iodine.
Preparation of Concentrated Emulsions As previously stated, the particles making up the emulsoid are for the most part small and of the order of 0 . 2 micron or less in diameter. They readily pass through filter paper and through Berkefeld filters. After the first few seconds of filtration through a Chamberlain filter, however, they itre held and the emulsion may be concentrated somewhat in this manner by forced filtration. The concentrating of the emulsion does not seem to affect markedly its stability. Attempts at concentration by centrifuging were not successful. Standing for a number of weeks or months results in a partial 1 This appears to be experimental verification of the theoretical considerations stated by Wo. Ostwald. See Ostwald-Fischer: “Handbook of Colloid Chemistry”, Second Edition, page 63.
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concentration in the lower part of the containing vessel and the saturated aqueous iodine solution may be drawn off. Evaporation obviously results in the loss of iodine since the dispersion medium is a saturated solution of iodine in water. The volatility of the iodine thus prevents the successful concentration of the emulsion by this method. A method was found, however, whereby the emulsions could be greatly concentrated and even dried. This method consisted essentially of the formation of a thick syrup by the addition of dextrose or gum arabic to the emulsion. From this syrup the water could readily be evaporated with but little or no loss of iodine. A film or membrane forms on the surface of the syrup. This film permits the evaporation or loss of water molecules, but not of the iodine molecules. The evaporation may be allowed to continue until a hard brittle mass femains. Drying does not cause the particles of iodine to fuse together or coalesce and the particles readily disperse again to form an emulsion when the solid is shaken up in water. The syrup may be spread out in thin layers and quickly dried after which it may be powdered. When it is powdered a small amount of iodine is given off, probably due to the fracturing of the protective coating of some of the particles with the consequent exposure of iodine. If the exposed iodine is driven off by the application of gentle heat the remainder of the iodine is perfectly stable, so stable in fact that the powder has little or no odor of iodine and may be kept in paper containers without staining the paper. Additional evidence of the structure of this colloidal iodine system is obtained from the behavior of this powder or the concentrated emulsion when added to water and alcohol. When either the emulsion or the powder is added t o water the iodine goes in solution quickly and forms a saturated solution of iodine in water. A saturated aqueous solution of iodine ordinarily requires several days for formation from ordinary crystalline iodine owing to the ~ ~2 5 ' ) . With the emulsoid, however, the slight solubility of iodine ( 0 . 0 3 4 at production of a saturated solution is almost instantaneous due undoubtedly to the fact that such small particles have a higher vapor pressure or solution pressure and a lower surface tension than the same material in a coarser state. Since iodine is readily soluble in alcohol it might a t first thought be expected that the addition of the emulsoid or the powder would likewise result in the rapid formation of a concentrated solution of iodine in alcohol. Such, however, is not the case. The colloidal iodine goes in solution more slowly in alcohol than in water and is accompanied by the flocculation and settling out of the gum arabic. This would seem to be explainable on the basis that the gum arabic functioning as a protective colloid and forming a coating around the iodine must first be flocculated before the alcohol can come in contact with the iodine. I n keeping with this explanation chloroform does not extract iodine from the dry stabilized powder, that is, the powder that has been heated to drive off the iodine exposed in grinding. If, however, the powder is first wetted with water, then the chloroform extracts the iodine from the aqueous solution. At the same time more of the colloidal iodine dissolves to maintain a saturated aqueous solution. This process continues until practically all the iodine has passed into the chloroform.
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Value of Colloidal Iodine as an Antiseptic and Disinfectant. Brief mention of a few of the properties of colloidal iodine as an antiseptic and disinfectant may not be out of place a t this point. Unlike alcoholic solutions of iodine, the aqueous suspension of iodine is practically nonirritating when applied to fresh wounds and it is at the same time free from injurious effects of alcohol on living tissue. When applied to the skin it produces a stain but does not blister unless the application is unnecessarily heavy and prolonged. The intensity of the stain is easily controlled, for whatever the quantity of iodine in suspension the solution in contact with the skin is only the 0.034% solution of iodine in water. This concentration of iodine in water is amply sufficient for all germicidal purposes and yet not sufficient to corrode the skin or tissues as does the alcoholic solution. Furthermore, the 0.034% solution is maintained practically constant as long as there is any of the colloidal iodine in suspension for the iodine removed from the solution by the skin or tissues is immediately replaced by the dissolving of some of the emulsoid iodine. The stain, unlike that produced by alcoholic iodine, disappears rapidly and completely within a few hours. Since water offers the most convenient medium for bringing disinfectants in contact with microorganisms, the advantages of the emulsoid in the powdered form for convenience in carrying are obvious. I t is only necessary to add the powder to water and even water is not absolutely essential, for the powder may be dusted directly on wounds. The water from the serum is sufficient to liberate iodine from the powder. Still another way in which the colloidal iodine may be conveniently used is in the form of applicators. These applicators may easily be prepared by dipping one end of wooden, glass, or even metal sticks or rods in the above mentioned thick syrup and drying. Summary Methods have been described for the preparation of colloidal iodine. Evidence has been presented to show that iodine may be brought into 2. the form of an emulsion in water. This emulsion is capable of existing only momentarily. 3. By means of protective colloids the emulsion may be rendered stable. 4. hlethods of concentrating this emulsion have been described. 5 . The concentration of this emulsion may be carried out to the extent of bringing it into a dry condition without impairing its ability to redisperse in water. 6. A few of the possible uses and advantages of colloidal iodine as an antiseptic have been pointed out. I.
East Lansang, Mzchagan.
