REYNOLD C. MERRILL Philadelphia Quartz Company, Philadelphia, Pennsylvania
&om forty-five years ago studies on casein, soaps, and Congo Red led outstanding colloidal chemists of that day to the recognition that solutions of certain colloids had characteristics in common with those of ordinary electrolytes. For example, J. Duclaux in 1909 compared some colloids, such as a hydrous iron oxide sol formed by hydrolysis of ferric chloride, to regular electrolytes (9). Independently in 1910, W. B. Hardy published a paper with the title "Electrolytic colloids," and included in that group soaps and proteins (19). Shortly thereafter J. W. McBain defined "colloidal electrolytes" as "salts in which an ion has been replaced by a heavily hydrated polyvalent micelle that carries an equivalent sum-total of electrical charges and conducts electricity just as well or even better than the simple ion it replaces" (68). Colloidal electrolytes constitute one of the two major subgroups of the general class of lyophilic colloids. The other major subgroup comprises lyophilic colloids such as rubber or polyvinyl chloride which are dissolved or peptized readily in a suitable solvent but which are nonelectrolytes. Colloidal electrolytes contain particles, micelles, or aggregates of ions, one or more of whose dimensions is in the "colloidal range" (i. e., between lo-' and lo-' cm.) which when dissolved conduct electricity about as well as solutions of simple ionized salts such as sodium or calcium chloride. Their good, welldefined conductivity is due to ion groups such as carboxylate, sulfonate, or alkyl sulfate which are an integral part of the colloidal particle or micelle. This is in direct contrast to the weak electrical conductivity of lyophobic sols due to impurities or to sorbed ions. The definition of colloids in terms of size is arbitrary. Such systems are perhaps better defined in terms of the appearance of certain properties characteristic of this range of particle sizes. These properties include non-diffusibility or a very slow rate of diffusion through parchment or collodion membranes, scattering of light in the well-known Tyndall effect, and the great importance of surface interaction or sorption on the behavior of the particles. Because of the relatively large size of colloidal particles as compared to simple ions, osmotic or colligative effects, which depend on the number of particles in solution, are small. The distinguishing feature of colloidal electrolytes is the combination of low osmotic or colligative effects with good electrical conductivity. Two general types of colloidal electrolytes may be distinguished. In the first the particle is a single molecule held together by primary covalent chemical
bonds which is of such a size that it comes within the colloidal range and exhibits the characteristic properties. The size of the particle is negligibly affected by changes in temperature or concentration or by the addition of salts. The colloidal particle does not diffuse through parchment or collodion membranes. This is a large and important class. It includes practically all proteins (except when a t their isoelectric points), and such ionizable carbohydrate colloids as alginic, pectinic, and polyuronic acids, carboxyrnethylcellulose, gum arabic, and their soluble salts. Solutions of cellulose xanthate in alkali used in the production of viscose rayon are industrially important members of this class as are the lignin sulfonic acids and their salts obtained in the sulfite pulping of wood. Synthetic polymeric products such as the polyacrylic acid and polyvinylpyridonium salts which Fuoss calls "polyelectrolytes" (18) belong in this first group of colloidal electrolytes. Also included are the sodium and potassium silicates which are simple dissociated electrolytes when their silica to alkali (NazO) molecular ratio is one or less, as in the meta- or sesquisilicate, but show colloidal properties when this ratio is greater than about three (60). Application of modern techniques and theory to inorganic colloidal electrolytes such as the siliceous soluble silicates and the polyphosphates should greatly increase our meager knowledge of their constitution. The first, single molecule, type of colloidal electrolytes can be designated as the "molecular" type. The second type of colloidal electrolyte, with which this paper is primarily concerned, can be designated as the "micellar," "associated," or "aggregated" type. In extreme dilution solutions of members of this group show osmotic or colligative effects equal to those of simple ionized salts such as sodium chloride, and the variation of electrical conductivity with concentration expected solely on the basis of electrostatic interaction of the ions. With increasing concentration the osmotic or colligative properties become smaller than would be obtained even for a nonelectrolyte and yet the solution is still a good conductor of electricity showing that ionized particles are still present. The decrease in the osmotic coefficient is due to aggregation of the ions into a particle or micelle of colloidal dimensions. In contrast with the first type the size of the particle is greatly affected by change in temperature, concentration, or the addition of salts. Such changes are readily reversible. Since the ions in the micelle are in equilibrium with free unaggregated ions, which are sufficiently small to pass through small pores, these colloidal
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electrolytes diffuse slowly through membranes. MemThe "micellar" or aggregated class of colloidal elecbers of this group are the soaps of saturated, unsatu- trolytes may be divided into an anion active group rated, branched or straight chain, cyclic or aromatic whose long chain or polycyclic micelle-forming ion has acids containing more than about eight carbon atoms a negative charge as in sodium palmitate, and a cation which have been neutralized with either alkali metal active group whose aggregating ions carry a positive hydroxides or organic bases such as triethanolamine. charge as in lauryl pyridinium chloride. Colloidal Long chain sulfoliic, nucleic, or sulfuric acids, amines, electrolytes have been prepared which contain long quaternary ammonium, phosphonium or arsonium chain groups in both cation and anion. Most of these bases, and their salts are also included. Aromatic are water insoluble hut octyl trimethyl ammonium ring compounds such as alkyl substituted pbenanthrene octyl sulfonate is sufficiently soluble a t ordinary temand naphthalene sulfonates, cholesterol sulfates, and peratures to behave as a colloidal electrolyte (76). bile salts belong in this group as do the lecithins and Both long chain cations and anions probably form numerous dyes such as Congo Red. Formulas of some these micelles. A wide variety of nonelectrolytic detergents has becolloidal electrolytes in this second group are given in come commercially available in recent years, most of Table I. TABLE I Soma Typical "Aggmgated" or "Mieellac" Colloidal Electrolytes 0 Sodium palmitate
CH~CH~CH~CHSCH~CH~CH~CH~CH~CH~CH~CH~CH~CH~CH~&-N~+ 0
// Lau~ylsulfaic acid CH3CH3CHsCH&H2CH~CH1CH1CH~CH1CH1CHtS-sOH+
B
Decyl benzene sodium sulfomte
0
\CH&H1CHnCH1CH2CH1CH1CH1CH1CH3 Lauryl pyridinium chloride
Sodium deoxycholaie
0
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which are made by condensing ethylene oxide with long chain or cyclic alcohols, mercaptans, ethers, phenols, and acids. These molecules aggregate to form micelles in aqueous solution and in many of their properties resemble the soaps and synthetic detergents. Since solutions of pure nonelectrolytic detergents do not conduct electricity, these cannot be considered colloidal electrolytes. They can be regarded as belonging to the nonelectrolyte class of lyophilic colloids and as comprising the "aggregating" or "micelle forming" group of this class in contrast with the high polymer or molecular group whose molecules constitute the colloidal particle. Micellar colloidal electrolytes have great biological, industrial, and theoretical importance. The bile salts are essential for the proper absorption of fats and other water-insoluble foods and medicinals in the intestines of living mammals. Cationic detergents are becoming widely used as germicides and bactericides. The contribution of soaps and the modern synthetic detergents to human welfare, health, and happiness by fighting filth and disease and improving morale is well recognized. The present production of sodium soaps in the United States is around three billion pounds per year and an additional 500 million pounds of synthetic organic detergents and about 50 million pounds of potash soaps are being manufactured. Research on micellar colloidal electrolytes has been and still is of considerable importance in the development of the modern theories and understanding of colloids. Thirty years ago it was believed that colloids had unknown and variable structures and properties that were at best only qualitatively reproducible. The work on soaps and synthetic detergents demonstrated that these substances of known, relatively simple structure form colloidal particles whose properties are completely and quantitatively reproducible. These colloidal micelles are in true reversible, thermodynamic equilibrium with each other and with simple ions. Systems of soaps, bile salts, and synthetic detergents conform to pbysicochemical laws such as the phase rule. However, they also show new and unexpected properties which necessitate the extension and modification of previous physicochemical laws and theories and the development of entirely new ones unique to the colloidal electrolytes. EARLY WORK ON SOAPS
McBain's definition of colloidal electrolytes was based on careful studies (beginning in 1908) of the physicochemical properties of solutions of pure sodium and potassium soaps. When this work was started soap was believed to be just an ordinary colloid. This was largely due to the observations of Krafft who in 1895 reported that soap solutions boil at practically the same temperature as pure water and of Smits who stated in 1902 that their vapor pressure was also not significantly different. Kahlenberg and Schreiner had shown in 1898 that dilute soap solutions conduct elec-
tricity but this was generally attributed to the alkali formed by hydrolysis and to impurities. McBain first showed that soap solutions over a wide concentration range had electrical conductivities comparable to those of ordinary electrolytes such as sodium chloride and acetate. He next demonstrated by means of potential measurements with the hydrogen electrode and by the rate of the alkali-catalyzed decomposition of nitrosotriacetonamine that the hydrolysis alkalinity of soaps was very small and could not account for their observed conductivity. For example, the per cent hydrolysis of sodium palmitate in water a t 90" was 0.2 a t 1.0 N soap and 6.6 a t 0.01 N corresponding to hydroxyl ion concentrations of the order of 0.001 N. The sodium hydroxide formed by hydrolysis could a t the most account for about 26 per cent of the conductivity a t 90°C. of a 0.01 N sodium palmitate solution and only 1.2 per cent a t 1.0 N. At least part of the soap was therefore dissociated into ions. On reinvestigating the colligative effectsof soap solutions, McBain foundthat the measurements of Krafft and of Smits were in error due to the presence of relatively large amounts of dissolved or occluded air. Measurementsof thedew-pointlowerings of soapso1utionsat90"C. by techniques which eliminated this error showed they had a small but significant osmotic effect. Some of these measurements are summarized in Table 11. They e
TABLE I1 Dew-Point Lowering ('C.) of Sodium Soap Solutions at 90°C. (58) . .
Soap
O.BN
0.6N
l 0 N
1.5N
Caprylate Laurate Palmitste Stearate (Any nonelectrolyte-theory) (Uni-univalent saletheory) (100% ionized)
0.17 0.15 0.13 0.11 0.10 0.20
0.37 0.28 0.20 0.18 0.24 0.48
0.62 0.34 0.25 0.23 0.48 0.96
0:33 22 0.18 0.72 1 .44
n ~
can be regarded as essentially equivalent to a rise in boiling point under reduced pressure a t 90°C. In all cases the osmotic effectof the soap solutions is less than would be obtained if they were completely dissociated into simple ions. In concentrated solutions of the higher molecular weight soaps the observed osmotic effect is actually considerably smaller than would be produced by a nonelectrolyte or if no dissociation occurred. However, the high electrical conductivity of these solutions shows they are extensively dissociated. McBain attributed this apparent discrepancy to the association or aggregation of the long chain soap ions into colloidal micelles which did not exert an appreciable osmotic effect. The number of simple sodium and other ions as determined by the dew-point lowering was not nearly sufficient to account for this observed good electrical conductivity. Therefore a t least part of the colloid conducted electricity and was ionic. This "ionic" micelle is in reversible equilibrium with other constituents of the solution. The extent of
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micelle formation increases with concentration and chain length of the soap ion, and decreases with increasing temperature. During the past 30 years considerable evidence of all types for the validity of this explanation has accumulated and today it is generally accepted as correct.
Od
PHYSICAL CHEMICAL PROPERTIES
A correct understanding of the nature of solutions of colloidal electrolytes can only be obtained from a comparison of many properties over a wide range of concentrations for a variety of types of representative compounds. Experience has shown that explanations or deductions based only on a few properties or over a limited concentration range are apt to be misleading. Colligative Properties. Osmotic or colligative properties are those which depend on the numhkr of
