SOAPS AND SIMILAR LONG-CHAIN DERIVATIVES AS SIMPLE HALF-STRONG ELECTROLYTES I Y DILUTE SOLUTIOK JAMES W. McBAIN Department of Chemistry, Stanford University, California Received October 67, 1988
In the years from 1913 to 1920 we based the existence of colloidal electrolytes upon a comparison of osmotic and conductivity data in solutions of moderate concentration. Very dilute solutions were not studied more especially, on account of the complicating effects of hydrolysis which ordinary soap solutions there display. Our later work, first with Reychler’s cetanesulfonic acid and then with the other more soluble straightchain sulfonic acids, showed these to be wholly similar to ordinary soap solutions. Moreover, dilute solutions could be studied because hydrolysis is impossible (10). Our interpretation of soaps as being simple electrolytes in dilute solution, but aggregating with concentration to become colloidal electrolytes, comprising slightly charged molecular aggregates and small but highly charged, and excellently conducting, ionic micelles has long since become familiar. The actual occurrence of the plate-like neutral micelles has been definitely proven by x-ray examination of clear, transparent, alkaline, isotropic, fluid solutions of sodium oleate, whether a t rest or streaming (4, 9). Likewise the diffusion data of M. E. Laing McBain’ clearly necessitate the existence of two kinds of colloid for their explanation. The formation of ionic micelles was also rendered plausible by the fact that the aggregation of like ions to a larger radial spherical aggregate at once increases the conductivity by several fold and separates these mutually repellent charges on the same ionic micelle to a greater distance apart than they would have occupied as single ions distributed uniformly through the solution, whilst also satisfying the tendency for the paraffin tails of these ions to agglomerate through van der Waals’ forces. Such greatly increased conductivity is required to reconcile the fact that in concentrated solution the total free sodium ions revealed by the osmotic data would account for only a minor fraction of the actual conductivity, leaving a major fraction due to colloidal anions. These colloidal anions could not, as a comparison of freezing point data with conductivity data requires, carry such enhanced conductivity if they were loaded down either with Appearing in Proc. Roy. SOC. (London) 170 (1939). 671
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added molecules or with ions of opposite charge. Thus, Howell and Robinson (5) well point out that if n univalent spherical ions (of like charge) coalesce to form one spherical micelle carrying the n charges, then it follows from Stokes' law that the conductivity of the micelle will be n2I3 times as great as that of the sum of the n individual ions. However, they have evidently fallen into error in concluding that a complex ion containing three sodium ions and four anions, with a total negative charge of only one on the univalent complex, will conduct several times faster instead of several times more slowly than a single anion not so loaded. Recently, however, Hartley (2) and Adam ( l ) , while retaining the concept of colloidal electrolytes as association colloids one of whose conducting constituents is a simple ion, have rejected some of this previous interpretation. They have done this partly by omitting consideration of the osmotic data, that is, the lowering of the freezing point or of vapor pressure, in the restricted region to which the present discussion calls at ten tion. I t is the object of this paper to recall the fundamental validity and inevitability of the comparison of osmotic data with conductivity data in our definitions of crystalloids, electrolytes, and colloidal electrolytes. Hartley has replaced the constitution diagram of McBain and collaborators in the upper half of figure 1, based upon the necessity of harmonizing freezing point and conductivity, with the freer sketch in the lower half of figure 1, based upon conductivity alone and incompatible with the osmotic data for the region here emphasized. Figure 1 shows clearly the difference in the predicted lowering of the freezing point in this region necessitated by McBain's diagram in the upper half of the figure as compared with that requisite for Hartley's diagram in the lower half. Taking, for example, the N/100 solution, Hartley has practically all the oleate and the greater part of the potassium locked up in colloidal form, leaving only 36 per cent of the potassium ion as the sole depressant of the freezing point. Hence the predicted lowering of the freezing point is 0.36 X 0.01 x 1.858'. On the other hand, the McBain diagram rightly or wrongly represents 51 per cent as simple ions with 44 per cent as simple ion-pairs or molecules, requiring a freezing point lowering of (0.51 0.51 0.44) X 0.01 X 1.858", a value four times greater than that of the Hartley diagram. Although Hartley stipulates that his figure is not strictly drawn, it serves to illustrate the point that there is a contradiction between the two diagrams in their predictions or requirements as to osmotic data in this region. For the same conductivity the upper diagram would require a much greater osmotic effect, which could only be satisfied by osmotically active but non-conducting material such as molecules or simple neutral ion-pairs. There are four regions of concentration. In the first or most dilute
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all agree that the soap is practically completely ionized, and this is shown by both conductivity and osmotic data. Instead of assuming only simple ions, the author ronsiders that at least some ionic micelle is already
FIG.1. Constitution diagram for potassium oleate solutions. The upper half of the figure is a diagram of McBain and collaborators. The vertical depth of each field gives the fraction of the oleate ions in the specified form. The lower half of the figure is a diagram (reference 2, page 57, figure 12) drawn by Hartley in accordance with the theory outlined in his book. The vertical depths of the two fields give the fractions of the ions in simple or aggregated form. Line 01 refers to oleate ions and line K t o potassium ions. The depth of the shaded area gives the fraction of oleate as covalent acid. Quantitative representation has not been attempted. The annotations in handwriting are by J . W. M.
being formed. In the second region, that discussed in this paper, just after the “critical concentration for micelles” ha.? been reached, the conductivity is falling sharply but the osmotic effects are not falling with corresponding rapidity. Hence in this second region the osmotic effects
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are much greater than the conductivity effects, and the difference, as will be seen, can only be accounted for in terms of molecules or simple ionpairs forming in appreciable amount as a precursor to colloid. I n the third region conductivity and osmotic effects both fall rapidly, but the more rapid fall of the osmotic data proves the formation of the colloid, termed neutral colloid by McBain, which conducts poorly as compared with the simple ions and is not uncharged but only .relatively poorly charged. In the fourth region both conductivity and osmotic effects, having passed through a minimum, increase strikingly. Now the osmotic effects fall so far below the conductivity that McBain found it necessary to postulate the formation of the highly charged, excellently conducting “ionic micelle” in amount steadily increasing in concentration until its proqerties dominate those of the neutral micelle, which was rather suddenly formed in the second and particularly the third regions. The discussion of osmotic data by Hartley (2, pages 54-56) and Adam (1, page 110) refers only to the third or fourth region and overlooks the second region where colloid is about to form. Hartley does not recognize the molecules or ion-pairs of the second region. He has unfortunately used, until recently; the term “ionic micelle” for a single kind of colloid particle of changing degree of dissociation or of “electrical neutralization” and intermediate between the ionic micelle and the neutral micelle, but, like the latter, not strikingly different from any charged colloidal particle. His micelle is not that shown by x-rays (4,9), but is a spherical liquid mass consisting typically of fifty anions and thirty cations with the other twenty cations being free in the surrounding solution. THE CHARACTERIZATION O F CRYSTALLOIDS AND O F WEAK, STRONG, AND
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ELECTROLYTES BY COMPARISON O F OSMOTIC DATA
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CONDUCTIVITY (a)
A crystalloidal non-electrolyte is defined as a substance whose solution exhibits inappreciable conductivity but completely developed osmotic properties. It is convenient to define the osmotic behavior in terms of Bjerrum’s osmotic coefficient 8, which is, 1 minus Lewis and Randall’s j or h. Still more convenient for the present purpose is to adopt van’t Hoff’s single standard i, which for uni-univalent electrolytes is 2g. For freezing point lowering Bo, i = 0/1.858m, where m is molality or weight normality and 1.858 is the molar freezing point lowering of an ideal nonelectrolyte. Similarly, the conductivity may be indicated by a, either the Arrhenius a, the observed molar conductivity divided by that of two completely dissociated ions a t infinite dilution, or or,, the ratio of the observed molar conductivity to that of the fully dissociated ions corrected for interionic attraction and other effects. Thus a crystalloidal non-electrolyte is defined by a = 0, combined with
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i = 1, within experimental or methodical error. It will be noted that here the presence of simple molecules is recognized by the fact that i = 1 a. Had i been 0.5 instead of 1, we should have postulated double molecules, and had i been a minute fraction of unity, we should have spoken of colloid and not crystalloid. The chief point to be brought out in this note is that for a partially dissociated substance, if i = 1 a,we are bound to recognize the presence of simple molecules or of their equivalent, simple ion-pairs. The remainder of this communication will be confined exclusively to solutions and regions where i = 1 a, RS in acetic acid, dichloroacetic acid, and certain COIIcentrations of soaps and long-chain sulfonic acids where, consequently and necessarily, there is practically no colloid present.
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