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PASUPATI MUKERJEE AND KAROL J. MYSELS
Vol. 62
The dimer, of course, changes the ionic strength of gives 68,48 and 39 A., respectively for our solutions. the solution in a manner easily computed. This in Dimerization reduces these values slightly. Hence turn may be assumed to modify the conductivity of we are covering the region in which imperfect overNaf according to the Onsager theory. If one lapping becomes significant, and the effect of the plots the deviations of the experimental values dimer should begin to differ from that of a divalent from those calculated in this way, one obtains the ion in accordance with the above results. uppermost curve of Fig. 3. Thus the anomaly has Thus if dimerization is properly taken into acbeen removed and in fact there has been an over- count the conductivity of the Na+ ion in NaLS can correction, the deviation is now too large. Hence be fully accounted for. at high concentrations neither the 1-1 electrolyte The incomplete overlapping of the double layers nor the equilibrium monomer-dimer mixture pre- should also result in a shielding of the two charged dict exactly the actual behavior. The effect of the heads from each other. The resulting reduction dimer on the Na+ ions seems to be intermediate, in electrostatic repulsion should facilitate the forsuggesting that at these higher concentrations the mation of dimers. Hence the equilibrium constant dimer does not effectively have a double charge, but K D should increase with increasing ionic strength. a somewhat lower one. This turns out to be quite Acknowledgment.-It is a pleasure to acreasonable indeed. In the dimer, the repulsion between the charged knowledge the kind hospitality and help of Profesheads serves to keep the charges as far apart as sor A. R. Gordon, in whose laboratory this work possible. The spacing between them in the case of was carried out. I am deeply indebted to J. R. NaLS can be 12-18 8. Strictly speaking, such a Graham for introducing me to the technique of the species acts as a divalent ion only a t infinite dilu- measurements and for his guidance, and to G. S. tion where the two ionic atmospheres overlap com- Kell for incidental assistance. Finally, I am very pletely. As concentration increases the thickness grateful to Professor K. J. Mysels, who first sugof the ionic atmosphere decreases and the extent of gested the problem and gave me help and advice overlap is reduced. Finally, when the ionic at- throughout the work and in the preparation of the mosphere becomes small compared to the separation manuscript. of the heads, the dimer would act like two monoThis work was supported by the Office of Naval valent ions as far as ionic strength effects are con- Research under Project NR 356-254 and was precerned. In the intermediate region an intermedi- sented as part of the 10th technical report of this ate effect is to be expected. According to the De- project. Reproduction in part or in whole for purbye-Huckel theory, the characteristic thickness l / ~poses of the United States Government is peris given by 3.04/+ A. for 1: 1 electrolytes,13which mitted.
DILUTE SOLUTIONS OF AMPHIPATHIC IONS. 111. CONDUCTIVITY OF WEAK SALTS1 BY PASUPATI MUKERJEE AND KAROL J. MYSELS Department of Chemistry, University of Southern California, Los Angeles 7, California Received deptember 16, 1967
The conductivities of silver and of the tetramethyl, -ethyl, and -n- ropy1 salts of the LS- ion have been determined in the high dilution region, While by themselves some of these agree wit[ the Onsager theory for simple 1:1 electrolytes, when taken together with the behavior of the sodium and lithium ion, they show that considerable incomplete ionization is present. The formation of ion pairs, unex ected for such large ions, and their increase in stability with the size of the counterion is ascribed to the lowering of interfacial energy as the long chain of the LS- ion coils about the hydrocarbon surface of the counterion.
In the previous two papers the study of dilute solutions of sodium and of lithium salts of the lauryl (dodecyl) sulfate ion (LS-) was used to show that these salts do not behave as simple 1-1 electrolytes because of reversible dimerization of the LS- anions to LSz-. In connection with another study we have also prepared salts of very different cations, the lower alkyl quaternary ammonium ions. The conductivity of these salts seemed normal when considered by itself but in view of the previous study could be explained only in terms of incomplete ionization, Le., weakness of the salt. Later the silver (1) Based in part on the Ph.D. dissertation of P. Mukerjee, University of Southern California, 1957, and presented a t the Kendall award symposium honoring P. J. W. Debye a t the Miami meeting of the A.C.S.,April, 1957.
salt showed also clear indications of this behavior. This paper will present the experimental evidence for this weakness and interpret it, for the quaternaries, on the same basis as dimerization, ie., in terms of the reduction of interfacial energy as the hydrocarbon portions of the two ions merge. In a subsequent paper confirmatory evidence for the possibility of such binding between these ions will be presented. This is based on the lowered critical micelle concentrations of their solutions. Experimental Apparatus and Procedure.-The only change from the final procedure for measurements at high dilution described in the first paper of this seriesa was that the concentrated solution of the silver salt had to be.waymed to about 30" in the weighing pipet to avoid precipitation.
Nov., 1958
CONDUCTIVITY OF WEAKSALTS
Material.-The NaLS has been described previously? Reagent quality silver nitrate was used without further purification. The tetra-methyl and -ethyl ammonium chlorides and the tetra-n-propylammonium iodide were obtained from the Eastman Kodak Co. The first two were reprecipitated from ethanol with ether. The middle fractions were collected and dried in vacuo. The last one was recrystallized from hot ethanol. Titration of the products against standard silver nitrate using dichlorofluorescein indicator agreed with theory to 0.1%. The ethyl alcohol was of ‘‘100%’’ and ‘‘9570’’ grade and was used without further purification. The “absolute” grades available to us proved to give impurities which affected conductivity irregularly and could be removed by recrystallization where the latter was feasible. Preparation of Silver Lauryl Sulfate.-While this salt frequently has been pre ared previously by double decomposition of NaLS witf AgNOs in water, the literature does not indicate the precautions re uired to obtain a pure product. The complications arise %om the fact that the desired product becomes very soluble above its Krafft point of about 30’, while the starting material is very soluble only above its own Krafft point. This is about 14’ in pure water* but increases markedly in the presence of sodium ions which are necessarily present during the course of the reaction. Hence, in too concentrated solution the reaction never goes to completion and in any c’ase i t is complete only over a narrow range of temperatures, fortunately centered around room temperature. I n addition, the solubility of the silver salt below its Krafft point is still of the order of 0.2% so that precipitation from dilute solution involves considerable losses. We used, therefore, precipitation from about 0.3 M solution a t 23-27’ followed by repeated recrystgllization from water between about 35 and 25”. Preparation of Quaternary Salts.-These were prepared by double decomposition from the corresponding halide and AgLS in alcohol. Difficulty was encountered, however, in the usual procedure which requires purification of the product by recrystallization to remove excess reagents. The solubility of the desired salts was so large and increased so rapidly with the size of the cation that while the tetramethyl salt could be recrystallized from water with great losses, the tetraethyl one could not be recrystallized profitably from any of the 15 solvents and solvent pairs tested. Hence our compounds were repared by reaction of exactly equivalent quantities of the galide of the desired ion and of AgLS in concentrated ethanolic solution, separation of the precipitated silver halide, evaporation of the clear solution and drying of the residue in vacuo. The exact equivalence was obtained by mixing concentrated solutions of nearly stoichiometric amounts, filtering and adding small amounts of dilute solutions of the reagents while observing visually the formation of any turbidity in the clear refiltered solution. This method is easily sensitive to 0.02%. It showed that the relative weights of the two reagents were always within O.l,% of theory, thus confirming their purity. The final adjustment was always conducted to leave an excess of less than 0.03% of the halide, which was a less objectionable impurity than the silver salt. Properties of the Quaternary Compounds.-These are white solids whose crystallinity decreases and solubility increases markedly with cation size so that the n-propyl compound is waxy and extremely deliquescent. That solubility is related to the highly symmetrical shape of the cation is shown by the fact that the lauryltrimethylammonium salt of LS- is insoluble while its number of carbon atoms differs by only one from that of our most soluble symmetrical compound. A probable reason for this high solubility is the lowering of the lattice energy as the distance of closest approach of centers of charge increases. A measure of the purity of the final products is given by their limiting equivalent conductivities. This can be computed from the corresponding ionic values given by Kraus for the cations4 and obtained by us previously* for the LSion. The experimental values are higher than the computed (2) Part I of this series, P. Mukerjee, K. J. Mysels and C. I. Dulin,
THISJOURNAL, 62, 1390 (1958). (3) N. K. Adam and K. G. A. Pankhurst, Trana. Faraday Sac., 42, 523 (1946). (4) H. M . Daggett, E. J. Bair and C. A. Kraus, J. Am. Chsm. Sac., 1 3 , 799 (1951).
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I
I
I
2
1
3
dCZ-iF? Fig. 1.-The equivalent conductivity of AgLS in the high dilution region as compared with the Onsager theory and with lines calculated on the basis of dimerization alone and of dimerization and weakness. 67
66
4 65
~~
1
2
3
dTXX Fig. 2.-The equivalent conductivity of (CH&NLS in the high dilution region as compared with the Onsager theory and with lines calculated on the basis of dimerization alone and of dimerization and weakness.
55
54
0 53 1
1
I
1
2
3
drn. Fig. 3.-The equivalent conductivity of (CsH&NLS in the high dilution region as compared with the Onsager theory and with lines calculated on the basis of dimerization alone and of dimerization and weakness. ones. The difference amounts to 0.6, 1.2 and 1.6%, respectively, for our tetramethyl, ethyl and n-propyl compounds. Because the equivalent weight is high and the conductivity low for these compounds, these differences correspond to less than 0.1% of sodium nitrate, the most likely impurity. Acidic or basic impurities could cause them in even smaller proportions. From the point of view of the present work, concerned with slopes rather than absolute values, the effect of 0.1% of a simple salt is negligible. The same holds true for studies on critical micelle concentrations for which these materials also were used.
PASUPATI MUKERJEE AND KAROL J. MYSELS
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1
I
TABLE IV EQUIVALENT CONDUCTIVITY OF (T~-C~H,)~NLS IN DILUTE SOLUTIONS c
x
’\ ‘0,
I
I
I
1
2
3
di3-m
‘\
‘.
I
0%.
4
A
10’
0.2819 0.7188 1.2937 2.2519 3.4640 5.1049 43
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c
45.44 45.24 45.10 44.85 44.59 44.30
x
104
6.8710 9.0470 12.100 15.124 17.598
A
44.06 43.78 43.48 43.15 42.91
lovsky’s equation6 A0 =
-A
1
- Ad5
(1)
to calculate the equivalent conductivity at infi-
Fig. 4.-The equivalent conductivity of (n-CsH7)4NLS nite dilution Ao from the experimental equivalent in the high dilution region as compared with the Onsager conductivity A at concentration c. A and B are theory and with lines calculated on the basis of dimerization constants whose values are 0.2289 and 60.19, realone and of dimerization and weakness.
spectively, for 1-1 electrolytes at 25” in water.6 This analytical method of fitting the Onsager limiting equation avoids the uncertainty of graphical extrapolation. Shedlovsky’s equation is applicable only in the dilute region (
