SOME EXPERIMENTS in COLLOID CHEMISTRY HERBERT L. DAVIS1 Appleton, Wisconsin
T
HE increasing application of the colloid point of view to numerous materials, processes, and phenomena brings as a by-product suggestions of experiments which have proved of interest and value. Certain of these experiments have produced methods, modifications of apparatus, or results which may. be of general interest. In most cases they were designed to emphasize colloidal aspects of materials and processes of especial interest in the pulp and paper industry, but the principles applied are universal. FLOCCULATION PHENOMENA
Along with several experiments taken directly from the popular laboratory manuals giving general introductions to colloidal systems and phenomena, is one which demonstrates clearly that'colloidal chemistry is really diierent. The usual ferric oxide sol is prepared by adding ferric chloride to boiling water, and portions of this sol are treated with a few drops of dilute (possibly 0.01 N) solutions of sodium hydroxide, sulfuric acid, and sodium sulfate. It is no surprise to see the iron precipitated by the base, but complete and rapid precipitation by the acid and by the salt provides an excellent opportunity to begin the study of colloidal flocculation and to minimize the significance of stoichiometry in such reactions. The importance of particle size and of zeta or electrokinetic potential becomes obvious. Various other experiments may involve the flocculation of rosin sizes, of alkaline protein dispersions such as that of milk casein or soy-bean protein (determination of the pH of the isoelectric point), and of clay and other dispersions. Of considerable interest is the flocculation of a special wax emulsion (4). This consists of ~araffinextremelv finelv divided and hicrhlv 'Present address: 294 Easton Avenue, New Brunswick, u
New Jersey.
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stabilized by alkaline soy-bean protein and soap. Small amounts of aluminum sulfate or aluminum chloride precipitate the emulsion completely, while larger amounts, added quickly, leave it apparently unchanged. Indeed, these acid systems contain positively charged emulsion particles which are about as stably dispersed as they were in the original alkaline systems. This is a good example of the irregular series (5) so often met, and is used to emphasize the danger of excessive amounts of such..prScipitants and d pH values which goLtoolow. For many industries, studies of the mutual eiTect of colloids on each other are instructive. The usual positively charged ferric oxide hydrosol and the negatively charged arsenic trisulfide hydroso! may be prepared, suitably diluted, and then mixed in possibly eight proportions, with the sols themselves a t each end of the series. The ferric oxide sols and those containing increasing amounts of arsenic sulfide will require decreasing volumes of dilute sodium sulfate solution for their flocculation, while the stability of the arsenic trisulfide side of the series may-be indicated by dilute barium chloride solution. In the series will be one or moresystems requiring no salt for precipitation (11). The sensitization of ferric oxide by small amounts, and the protective action of larger amounts of gelatin may be similarly shown. PARTICLE-SIZE DISTRIBUTION
The pedagogical usefulness of the sedimentation balance for the determination of particle-size distribution justifies some efforts to improve it. The excellent outline of Holmes (3) serves as a basis. A chainomatic balance serves effectively if the left-hand pan be relaced bv a Dan desimed to catch the sedimentinn solid which has been s;spended in a high form beaker. With a flat pan, such as is often used, the recovery of
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typical fillers usually runs from twenty-five to sixty per cent. of the amount of solid calculated to he in a cylinder of suspension over the pan. In such cases, the drawing of conclusions from the data on that portion of the solid which happened to stick to the pan seems hardly justifiable. It appeared that the loss was due to solid being camed over the edge of the flat pan by the descending liquid currents, and this is supported by the observation that often the edge of the pan would he bare after a run. A pan with sides (a cup suspended in its center) was prepared, and in several cases over ninety-eight per cent. of the theoretical weight of clay was caught. It must he remembered that a gram of solid in the cylinder of suspension above the pan will not cause an increased weight of one gram even if it is all caught; correction must he made for the buoyant effect of the liquid on each of the weights obtained during the run. On commercial clays the addition of sodium silicate produces an increasing dispersion of the clays and consequent depression of the curves. Size values so obtained are fictitious unless similar dispersive effects are obtained in the actual use of the clays. The swinging of a sensitive balance is effectually damped by surface tension of the liquid acting on the rod supporting the pan. This may be minimized by small amounts of wetting agents on the surface (9),although excessive amounts are to he avoided-apparently a low surface tension or a contact angle of 90" would be ideal. Very satisfactory also is the practice of a thin film of kerosene on the rod or over the suspension, where i t can also reduce the evaporation rate for long runs.
of starch permits the calculation of the soap adsorbed a t each coucentratiou. This method fails utterly for cellulose since many fonns of pulp, cotton, alpha cellulose, and so forth, suspended in distilled water lower the surface tension by as much as twenty dynes. The materials responsible have not yet been identified, but i t seems possible that such a surface tension lowering may come to be a significant property of pulp or other cellulose forms. In the measurement of iuterfacial tensions between two liquids, the need for mutual saturation should be emphasized. VISCOSITY EXPERIMENTS
In the use of the capillary viscosimeters the work may be directed to the demonstration of the isoelectric point as the pH of minimum viscosity for gelatin sols. An additional experiment will emphasize the oftenneglected fact (6) that the presence of strongly adsorbed ions may have considerableefiect on the pH of minimum viscosity. While this is usually found a t pH 4.7-4.8, for a high-grade gelatin, the minimum point is shifted to lower pH values by strongly adsorbed negative ions, and to higher pH by similar positive ions. In a series of one per cent. gelatin sols, containing added materials and adjusted by HCl or NaOH to various pH values, the pH of minimum viscosity was found to he as follows. TABLE 1
.vegotiac ions
Pariliw iarr 2 pez cent. Aniline
pH 5.0
0.01 per cent. Night Blue (hadc dye)
pH 5.4
0.1 wr eent.
Duprmol (wetting
pH 4.4
0.1 per cent.
dm $an; S-det (add d ~ )
pH 4.2
..
.,
SURFACE TENSIONS
The du Nouy types of ring method tensiometers permit rapid operation to obtain significant results. Dilution-surface tension runs using various soaps and commercial wetting agents are instructive. The beginning of salting-out action can bC detected if one compares the surface tensions of two solutions containing the same concentration of soap, one in water and one in a sodium chloride solution. The salt forces the soap to the surface and thus produces a lower surface tension than is shown without the salt (which by itself raises surface tension slightly). Such coucentratiou-surface tension curves are true adsorption isotherms (2, 10) in which adsorption a t the air-solution interface may he studied. In some cases the plot of the logarithm of concentration against the logarithm of the depression of the surface tension seems to consist of two straight lines, as though there were a true solution of soap (or an acid soap) a t low concentrations and a colloidal surface active form a t higher concentrations. Adsorption isotherms of surface-active materials on solid surfaces may also be studied by surface tension measurements. The addition of uncooked starch granules to a soap solution causes the surface tension to rise as soap is withdrawn from the air interface to go to the solid. The relative positions of concentration-surface tension curves without and in the presence
In the study of the starches and their modifications, the MacMichael viscosimeter may he used to show such effects as those of temperature and duration of cook, the action of acids, bases,, salts, or enzymes. This apparatus will also throw somelight on the mechanism of the flocculation and deflocculation of clays. Clay slips treated with small amounts of sodium silicate promptly drop their readings from about 120' M. to practically zero, then a small addition of alum will restore the original viscosity. In deflocculation, the materials used ranked: sodiumsilicate, pyrophosphate, hydroxide, tetraborate, (tri)phosphate, with triethanolamine the least effective. In all cases alum additions were able to restore the original high viscosity or even to exceed that of the original clay-water systems. By means of silicate-alum additions the viscosity of a clay slip passed through two complete cycles, thus suggesting a method of commercial control. ELECTRICAL EFFECTS
Titratiou in the presence of a glass electrode suggests itself as an indication of adsorption and of the isoelectric point (7, 8). A curve obtained by titrating hydrochloric acid with sodium hydroxide serves as a hlank. In the presence of some clay the observed pH values in titration are higher than the hlank (sorption of
hydrogen ion) until a t pH 3.3 the curves crossed, and toward the negatively charged cup and cylinder which above that hydroxyl ions are sorbed selectively. If become covered with a film of water (1). If the opalum be used instead of HCl, two similar curves are posite electrical connection is made, dehydrated clay obtained crossing a t pH 5.4, again emphasizing the builds up on the positively charged cup and cylinder, need of stating all the circumstances in giving an iso- and the viscosimeter reading rises far above the original electric point. With milk casein in the acid-base ti- value. If the current is left on very long in this directration, there is an excellent separation of curves (strong tion, the slip around the copper ring becomes quite adsorption) which crossed a t pH 4.8, again agreeing dilute, and the reading drops, because the clay deposit on the rotating cup is unable to twist the similar with the usual values. The MacMichael viscosimeter furnishes an unusual deposit on the cylinder through the low-viscosity in^ method to demonstrate electroendosmose. The ro- tervening layer. Since these experiments have been largely exploratory tating cup and the cylinder suspended on the torsion wire serve as one electrode, while a copper ring sus- rnns on commercial materials, few results have been pended in the clay slip midway between the cup and included except to indicate the magnitude of effects to cylinder serves as the other. In the usual clay slip the be expected. The sole purpose has been to outline a clay particles are negatively charged and the water few methods of investigation that may prove of interest positive. Closing a direct current circuit makes the and value to others. The cooperation of a number of reading drop nearly to zero as the water is attracted students is gratefully acknowledged. LITERATURE CITED
(1) CURTIS,Tram. Ekctrochem. Soc., 73, 503-10 (1938).
(2) FREUNDLICE, "Kapillarchemie." Aksdemische Verlaggesellschaft, Leipzig, 1909, p. 165. (3) H o ~ m s "Laboratory , manual of colloid chemistry," 3rd 1934p' 4' ed'' John Sons' In'.' New (4)Knsss AND JOHNSON, U.S. Pat. 2,058,086(Oct. 20, 1936). "Colloids," 2nd d., John Wiley (5) Knurr AND VAN KLOOSTER, & Sons, Inc., New York City, 1930,p. 89. (6) KRUYT AND TRNOELOO, J. PhW. Chem.. 29, 1303 (1925).
(7) MCLEANAND WOOTEN.Jnd. Eng. C h m ~ . .31, 1138-43 (1939). (8) MEYER,I. Research Natl. Bur. Standardr, 13, 245-58 (1934). (9) ROBERTS, Ind. Eng. C h m . , Anal. Ed.,10, 518-19 (1938). (10) WEISER,"Colloid chemistry," John Wiley & Sons. Inc.. New Yark City, 1939, p. 16. (11) WE~SERAND CEAPMAN. I. Pkys. Chcm.. 36, 713-21 (1932)
