JOURNAL OF CHEMICAL EDUCATION
A N OUTLINE OF A COLLOID CHEMISTRY COURSE FRED HAZEL University of Pennsylvania, Philadelphia, Pennsylvania
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course offered a t the University of Pennsylvania consists of two lectures a week for one semester. It is required for the senior chemists in the Towne Scientific School, but it may be elected by other chemistry majors and premedical students who have had adequate training in physical chemistry. Colloidal systems are defined in the introductory lecture as polyphase systems in which dimensions and shape cause certain properties to be displayed. It is pointed out that the study of colloid chemistry involves consideration of the behavior of matter i n a finely divided form and the phenomena associated with intmfaces. The course is divided into three parts: I. Colloidal Suspensions; 11. Surface Chemistry; 111. Miscellaneous Colloidal Systems. PART I. COLLOIDAL SUSPENSIONS
Particle Size and Associated Properties. The subject
of colloidal suspensions is introduced with emphasis on particle size. The values 1 p to 200 u , are indicated to be flexible limits of the colloidal range. Several characteristics of colloidal suspensions may be associated with particle size. Light scattering (Tyndall phenomenon) is cited as an example. Other illustrations are drawn from the molecular-kinetic properties of suspensions. Brownian movement and the slit ultramicroscope are discussed in this connection. Topics such as sedimentation, molecular weight and colligative properties, dialysis, and the Donnan equilibrium are also included. Approximately four lectures are devoted to the above. CZassijkatian~ofSystems. ~ o l l o i d a lsuspensions are classified into hydrophobic and hydrophilic systems on the basis of measurable properties. Among the latter are viscosity, stability toward'electrolytes, reversibility on evaporation, and magnitude of the critical potential.
JANUARY. 1949
The irregular series in coagulation is used to illustrate the striking effects which may be observed in colloid systems with low concentrations of electrolytes under favorable conditions. The example chosen is the behavior of colloidal iron oxide with electrolytes containing btrongly adsorbable ions of opposite sign, e. g., K&Fe(CN)aand NasPaOm The addition of an electro1yte of the above type to the iron oxide system eyentuAfter 2 4 hours ally results in the occurrence of two zones of stability and two zones of coagulation. The students, who have had no laboratory experience with this phenomenon, are likely to get the idea that the recharging of the stable COT stable cosq, particles consists of two steps, uiz., the precipitation of 3.4 2.5 5 8Ky6(cNk iron oxide and then the peptization of the precipitate. 1.2 1.8 4.3 )6000Nqgp6qB The diagram shown in Figure 1 is used to prevent this conc. : P.M. misconception. Data are included to show that the Fig1 first coagulation zone is narrow (a fact contributing to its being overlooked frequently), and the second staColloidal gold sols and gelatin sols are representatives bility zone is of the order of lo3 times as broad as the of the two classes but the properties of many systems do first stability zone. At concentrations of the phosphate not permit sharp classifications. and ferrocyanide, which are just under the minima It is indicated that the degree of hydration cannot be needed for complete recharging, the iron oxide is parevaluated experimentally and that when measureable tially coagulated. The coagulation is not general, properties are considered the "hydrous" oxides may he however, but is confined in the early stages $0 the liquidtreated under the heading of hydrophobic sols. air interface as indicated in Figure 2. This phenomenon in which the bulk of the system is stabilized with a Hydrophobic Sols negative charge and the surface is coagulated has been Preparation. Emphasis is placed on the preparation observed repeatedly in our laboratory during the reof the hydrous oxides and colloidal salts by hydrolysis charging of iron oxide sols with potassium ferrocyanide and metathesis. The mechanism of particle formation and various phosphates and silicates. It is reported now by condensation polymerization is extended to a dis- for the first time, it is believed. It appears to be an cussion of the formation of silicone polymers. example of negative adsorption in which the surface has The application of synthetic exchange resins to the a lower concentration of polyvalent negative ions than preparation and purification of colloidal systems is the bulk of the system. In the latter case a sufficient included in this section. concentration of the electrolyte is present to stabilize Determination of Particle Size. The various methods the recharged system. are enumerated for the determination of the size of The formulation of mechanisms of coagulation is one colloidal particles. of the aims of this section. Data are presented to show Electrical Properties. This section consists of a dis- that the different behavior of electrolytes of the potascussion of the origin of the electric charge and of sium chloride type is not confined to, their high critical clectrokin~tiephen;;nwnl. The mmsurenlent of elec- mobilities but is shown also in coagulation experiments trophoresis and tllr effect of electrolytr.; on the mobility with sols of different concentrations where a marked are stressed. Coafulation. The subject of coagulation with electrolytes is dealt with a t considerable length. Experimental methods and sample data are presented with the objective of aiding the student in gaining an intelligent ,approach to the problem. It is indicated that there is a close parallel in the ability of a given electrolyte to stable CW. stable cwa. decrease the mobility of the particles and to produce coagulation. It has been found with many systems that it is not necessary to decrease the mobility to zero in order to induce coagulation. The critical mobility, a t +-Lower conc. (coagulated) which coagulation occurs, differs for different sols and is greater for hydrophobic systems than for hydrophilic c H ~ g h e rconc. (stable) ones. The critical mobility for a given system is lower with electrolytes containing polyvalent coagulating ions than with 1: 1type electrolytes which cause coagulaSurface Coaguhhon tion onlv a t relativelv high concentrations.' BRTGGS, D. R.,J. Phys. Chem., 34, 1326 (1930).
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JOURNAL OF CHEMICAL EDUCATION
increase in flocculation values with dilution of the sol is observed. Likewise, data are included for the precipitation of negative sols by mixtures of electrolytes where it may bp seen that potassium chloride, a t concentrations below its flocculation value, sensitizes the. sols toward polyvalent ions. These effects are correlated with the fact that ions of the potassium and chloride type .are poorly adsorbed.= Under such conditions coagulation can occur only a t relatively high concentrations of electrolyte and is due to compression of the electric double layer. A different mechanism applies, however, to coagulation with polyvalent ions. In the latter case the ions are adsorbed and the electric charge is decreased correspondingly. Since this effect occurs a t a low concentration of added electrolyte, the ionic atmosphere is diffuse. Accordingly, the electric mobility must be reduced to a low value before coagulation occurs. Mutual coagulation is discussed hriefly. The section on Hydrophobic Sols, comprising nine or ten lectures, is concluded by pointing out that studies of the above type with iron oxide may have certain implications concerning the corrosion of iron. Hydrophilic Systems
The section on hvdronhilic svstems consists of four lectures and is dividkd into twoUparts. The first treats the general characteristics of the systems such as viscosity,stabilityhehavior,saltingout,thelyotropicseries, and protective action. certain complex carbohydrates, linear polymers are employed in the illusproteins, +rot;nnm ".'a"."L*.,.
~h~ second part is devoted to proteins, ~h~ ,,lassification, structure, and certain physical properties of proteins, e, g,, molecular weight and electrophoretic behavior, are discussed briefly. PART 11. SURFACE CHEMISTRY
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Interfacial Tension, Spreading and Films. The origin, HAZEL, F., J . Phys. Chem., 45,747 (1941).
measurement, and factors affecting the interfacial tension and surface energy are discussed. Capillary rise, maximum bubble pressure, and ring methods of measuring surface tension are considered.. This section is concluded with a discussion of the contact angle, wetting and spreading, and films. The treatment of these subjects follows that given in standard test.3 Approximately three lectures are allotted to the above. Adsorption at the Solid-Liquid and Solid-Gas Interfaces. Three lectures are devoted to a discussion of adsorption a t the solid-gas and solid-liquid interfaces. The experimental determination of adsorption is d e scribed. The students are given an exercise in the evaluation, by the method of least squares, of data taken from the literature. Various adsorbents are considered but the emphasis is placed on charcoal. The effects of several factors on adsorption are listed. The subjects of chemisorption and capillary condensation are discussed briefly. PART 111. MISCELLANEOUS COLLOIDAL SYSTEMS
Soaps and Colloidal Electrolytes. The measurement and factors affecting critical concentrations of colloidal electrolytes are discussed briefly. The relationship of colloid formation to solubilization and detergency is indicated. Current literature is used as the source of the and Foams. The treatment of these Sub. jects that found in standard The importance of adsorption and the principles of surface chemistry as applied to the systems is stressed. Gels. The preparation and moperties of a few inorganic and organic gels are considered. Plastic flow is discussed in this Four lectures are devoted to the material in Part 111. -
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WEISER,H. B., "Colloid Chemistry," John Wiley & Sons, h e . , NewYork, 1939; LEWIS,W. K., L. SQUIRES, AND J. BROUGHTON, "Industrial Chemistry of Colloidal and Amor~housMaterials," The Macmillm Company, New York, 1943; N . K. ADAX, "The Physics and Chemistry of Surfaces,'' Oxford University Press, London, 1942. 'PEEBTON, W. G., J . Phys. and Colloid Chem., 52, 84 (1948).
