X-RAY LONG SPACING OF COLLOIDAL ELECTROLYTES
42 1
REFEREKCES (1) BLODGETT AND LANGMUIR: Phys. Rev. 61, 664 (1937). (2) HABERAKD MOSER:Z. Elektrochem. 11, 593-609 (1905). J. Am. Chem. Soc. 60,1190 (1938). (3) LASGMUIR: (4) PORTER AND WYMAN: J. Am. Chem. SOC.69, 1883 (1937). (5) PORTER AND WYMAN: J. Am. Chem. SOC.69, 2746 (1937).
T H E EFFECT OF ADDED ELECTROLYTES UPOX T H E X-RAY LOXG SPACISG O F SOME COLLOIDAL ELECTROLYTES' 0. A. HOFFZrlASZ Department of Chemistry, Stanford r n i u e r s i l y , California Receit,ed .March 21, 1949 Ih-TRODUCTION
The x-ray investigation of solutions of colloidal electrolytes shows that some give x-ray spacings xhile others do not (10, 11). The most familiar examples giving definite x-ray long Bragg spacings dl are the soaps and alkyl sulfates in concentrations above 5 or 10 per cent ( 5 ) . As was point'ed out and emphasized by Harkins and collaborators (3, 13), in addition to the long spacing there is an intermediate spacing called the d, band corresponding to a double lamella such as was suggested by McBain on other grounds as early as 1923.Since the lamellae are charged, they must be surrounded by a diffuse ionic double layer. According to Hess and collaborators, this results in a parallel arrangement of the McBain micelles at the distance dl. This parallelism has recently been queried (3, 4). The theory of the diffuse double layer, insofar as the effects of electrostatic attraction and repulsion and the Brownian movement are concerned, has now been generally accepted (16). It has been recognized since the original work of Gouy (2) in 1910 that added salts should diminish the thickness of the diffuse double layer. A number of authors have therefore tested the effect of added salts on the long spacing (&). Kiessig and Philippoff (8) reported that sodium hydroxide added to 15 per cent sodium oleate had no effect upon long spacing other than t o make the pattern more diffuse. Hess, Kiessig, and Philippoff (6) report that potassium hydroxide added to 18 per cent potassium laurate causes a decrease in the long spacing. Harkins, Mattoon, and C,orrin (4)have reported adding sodium chloride and potassium chloride to potassium laurate solutions and finding a definite increase in long spacing, accompanied by an increasing diffuseness of the diffraction band. However, with sodium chloride the result is complicated by metathesis. Palmer A part of this study was conducted under Kavy contract a t Stanford University and the Stanford Research Institute under the supervision of Professor J. W . McBain. Present address: Operations Evaluation Group, CKO (Op342E), Pentagon, Washington 2 5 , D. C.
422
0. A. HOFFMAN
and Schmitt (IS) report that total nerve lipides are made up of leaflet-shaped micelles. In water suspensions the water layers between the micelles are decreased in thickness by the addition of foreign ions, calcium ion being much more effective than potassium ion. The experiments reported here include the effects of more dilute solutions of added salts on potassium laurate and the cation-active detergent cetylpyridinium chloride. They show that dilute solutions decrease the spacing in the manner to be expected from the theory of the diffuse double layer, but that further addition reverses the effect and causes a marked increase. TABLE 1 Effect of added potassium chloride u p o n long spacing of 18 per cent potassium laurate solution
m
0.385
0.112
,
0.683 0.770
1
A. 51.4
K O line
51.8
TABLE 2 Effect of added potassium chloride u p o n long spacing of 90 per cent cetylpyridinium chloride soluiion
m
A.
0.030
66.4 65.5 64.9
0.034 0.067
I
m
0.134 0.173 0.392
~
i
1
1
A. 63.4 62.9 72.3
0.430 0.588
i3.3
EXPERIMENTAL
The measurements were made with a General Electric Type XRDl unit with copper target, beryllium windows, and nickel filter, using guarded pinholes 0.01 in diameter, 100 mm. apart, with a sample-to-film distance of 163-250 mm. Some of the samples were mounted in the type of capillaries described by Marsden (12) and some were mounted in a metal sample holder described by Hoffman (7). The numerical data are presented in tables 1 to 4 and in figures 1to 4. In every case the addition of electrolyte caused the long spacing line to become diffuse. Also, the diffraction band was much less intense in the presence of salt than without it except for the potassium laurate to which sodium chloride was added, where the long spacing line becomes both more diffuse and more intense, evidently a result of metathesis. In all cases above about 0.2 m concentration of added salt, the pattern becomes diffuse, and a t relatively high concentration it was found necessary to
TABLE 3 Effect of potassium hydrozide u p o n long spacing of 18 per cent potassium laurate solution SOUPCE
,
oooo O.Oo0
I
0.200
A. 48.1 48.'
50.3 50.3* 49.'
~
Hess (5)
1
1
Hoffman Hess (5)
* These points read from a curve TABLE 4 Effect of sodium chloride u p o n long spacing of 18 per cent potassium laurate solution NaCl
I
NaCl
m
i
A. 50.3
,
O.Oo0 0.079
0.125
j
dtln
A. 49.2 51.2
POTASSIUM LAURATE
I 0
m
9z
.
I
dt(A) 0.0
I
-
70
OF
KCI
I
I
I
I
I
CL2
0.4
46
0.6
1.0
I
I
I
I
I
0
20 % CETYL WRlDlNlUM W O R I D E
FIGURE 2
FIG.1. The long x-ray spacing, d l , in Angstrom units, before and after potassium ohloride is added to a n 18 per cent solution of potrtsaium laurate. FIG.2. The long spacing, d l , of a 20 per cent solution of cetylpyridinium chloride before and after addition of potassium chloride. 423
424
0. A. HOFFMAN
make many measurements of the maximum and minimum diameters of a ring and strike an average. The precision of the me?surements a t salt concentrations below 0.1 m is generally a few tenths of an Angstrom unit, but this becomes progressively less until a t 1.0 m salt the deviation may be several h g s t r o m units. The dl spacing is decreased by the first addition of electrolyte, passes through a minimum, and then increases rapidly.
:\ 51 -
FIGURE M L W R3 ATE
42
I
I
I
OF N&l
-\ 49
I
I
FIGURE
4
18% POTASSIUM LAUPATE
FIG.3. Long Bragg x-ray spacings, d l , for 18 per cent potassium laurste before and after addition of potassium hydroxide. Crosses are present data; circles are data of Hess ( 5 ) . FIR 4. Effect of sodium chloride upon long spacings, d,, of an 18 per cent solution of potassium laurate. DISCUSSIOK
In all four systems studied, the long spacing dl is decreased a t low concentrations of added salt. It then passes through a minimum. Larger concentrations definitely increase the spacing, as was found by Harkins. The reversal appears to occur somewhat above 0.1 N in the case of potassium chloride, but with potassium hydroxide it is not observed up to 0.6 N . It seems rather arbitrary to ascribe this to changes in the size of micelles. The present theory of the double layer is also inadequate to account for changes in opposite directions. The author inclines to the view that the dl spacing observed arises from Hess micelles, and the diffuse character of the spacing after adding salt to repress the ionic double layer lends some support.
X-RAY L O S G SPACING O F COLLOIDAL ELECTROLYTES
425
It is of interest that viscosity curves of soaps with added electrolytes show similar minima. At first the viscosity is reduced by added salt, whereas large additions greatly increase it. The experiments of Goldschmidt and Weissmmn (1) were with potassium hydroxide and potassium chloride added t o solutions of the potassium salt of the fatty acids of palm kernel oil. The concentration of alkali necessary to pass the minimum depended greatly upon the concentration of the soap, but mas not much influenced by temperature. Miss King (9) found similar results with sodium chloride added t o sodium palmitate, as did Hess, Kiessig, and Philippoff (6) on adding potassium hydroxide and potassium chloride to solutions of potassium laurate. Other measurements are recorded by Philippoff (15). SUMMARY
The x-ray long spacings, d l , of solutions of moderately concentrated solutions of potassium laurate and cetylpyridinium chloride are first diminished by the addition of electrolytes, but excess of chloride brings about a strong change in the opposite direction. Existing theory does not account for these two effects in opposite directions. REFEREXCES (1) GOLDSCHMIDT, F., A N D D’EISSMASN, L . : Z . Elektrochem. 18, 382 (1912). , . : J. phys. [41 9, 457 (1910); Ann. phys. 191 7 , 129 (1917). (2) G O U YG (3) HARKIKS, W.D.: J. Am. Chem. Soc. 69, 1428 (1947). (4) HARKINS, W.D., l I a T T O O N , R . \T., A S D CORRIS,SI.L . : J. Am. Chem. SOC. 68, 220 (1946). (5) HESS,I
