JOURNAL OF COLLOID SCIENCE 11,352-355 (1956)

PARTIAL MOLAL VOLUMES IN ALKYL SULFATE SOLUTIONS

Eric Hutchinson and Carol S. Mosher

Department of Chemistry, Stanford University, Palo Alto, California

Received March 26, 1956

ABSTRACT

Measurement of the partial molal volumes of n-hexyl and n-octyl alcohol in sodium decyl and dodecyl sulfate solutions shows that for these materials dissolution occurs with almost no change in apparent volume. From the partial molal volumes of sodium decyl and dodecyl sulfate it appears that the "hydrocarbon center" of the micetle is a slightly looser structure than in liquid hydrocarbons. Added salts and higher alcohols have very little effect on the partial molal volumes of these alkyl sulfates.

In a recent paper (1) the three-component system composed of sodium alkyl sulfate-water-addit ive has been treated as a two-phase system con- sisting of an aqueous phase containing single electrolyte ions and additive molecules, on the one hand, and a micellar phase composed of alkyl sulfate ions (and gegenions) and additive in a binary mixture on the other. Some reservations must be made about calling an object as small as a micelle a true phase, but it was demonstrated that a general interpretation of these systems, in agreement with the usual laws of solutions, was possible on this basis. Attention was focused on the ideality or otherwise of the binary micellar solutions. This paper deals with the partial molal volumes of the components of the micellar solutions.

Partial molal volumes at 25°C. were evaluated from density data from the following equations.

M__~ _ (noMo + n~M~ + n2M~) ( Op) [l] V1 -~ p p2 ----~ p,T,no,n 2

M__~2 _ (noMo + ~IM1 -~- n2M2) / (gp V2 [2]

p p2 ~ ~ / p,T,nO,n 1

In these equations

no, nl, n~ = mole number of water, alkyl sulfate, and additive.

M0, M1, 1]//2 = corresponding mole weights.

p = density of solution.

V1, !72 = partial molal volume of alkyl sulfate and additive.

P, T = pressure and temperature.

352

PARTIAL MOLAR VOLUMES IN ALKYL SULFATE SOLUTIONS 353

The additive (component 2) consisted in some cases of sodium chloride or sodium sulfate; in others it consisted of n-hexyl or n-octyl alcohol.

The densities of the solutions were measured in pycnometers having the shape given in Fig. 1, holding approximately 20 ml. of solution. ~ The solu- tions were equilibrated in a thermostat at 25°C. -4- 0.005°C. Sodium chloride and sodium sulfate of analytical grade were used. n-Itexyl and n-octyl alcohols were obtained by the catalytic reduction of carefully

FIG. 1

purified methyl esters of hexanoic and octanoic acids. The alkyl suLfates were prepared by a standard procedure (2).

RESULTS

Over the range of solutions studied the densities of the solutions above the C.M.C. were linear functions of the mole numbers. These densities are summarized in the equations below, which were obtained from the straight-line plots computed by the method of least squares. All data refer to 25°C.

p = a + b n l ( o r a Jr bn2) g./ml.

Sodium decyl sulfate in water. a = 0.99737, b = 0.041484, V1 = 216.5 ml. :t: 2.5

Sodium decyl sulfate in 0.09277 molal sodium chloride. a = 1.00134, b = 0.040302, V1 --- 216.9 ml. ~: 2.5

1 Deta i l s of the use of these pycnomete r s as well as the ex tended dens i ty da t a are avai lable in Technica l Repor t 5. ONR Con t rac t N 6 0 N R 25130, S tanford Univers i ty .

3 5 4 E R I C H U T C H I N S O N A N D C A R O L S. M O S H E R

Sodium decyl sulfate in 0.10069 molal sodium sulfate. a = 1.00988, b = 0.038334, V1 = 216.9 ml. -4- 2.6

Sodium decyl sulfate in 0.052458 molal n-octyl alcohol. a = 0.99622, b = 0.039692, V1 = 216.9 ml. d= 2.4

n-Hexyl alcohol in 0.2000 molal sodium decyl sulfate. a = 1.00630, b = -0.021215, V2 = 123 ml. ± 0.7

n-Octyl alcohol in 0.20358 molal sodium decyl sulfate. a = 1.00587, b = -0.27150, V2 = 158.2 ml. ± 0.5

Benzene in 0.2000 molal sodium decyl sulfate. a = 1.0061~, b = -0.009730, V2 = 87.4 ml. =t= 0.1

Sodium dodecyl sulfate in 0.0488s molal n-octyl alcohol. a = 0.99645, b = 0.034569, V1 = 250.3 ml. -4- 2.6

n-Octyl alcohol in 0.19942 molal sodium dodecyl sulfate. a = 1.00405, b - - -0.023584, V2 = 154.8 m] . -4- .3

The partial molal volumes of the various components showed some varia- tion with concentration, as indicated in the data above and as shown in Fig. 2. The average partial molal volumes of n-hexyl and n-octyl a lcohol- - 123.3 ml. and 158.2 ml., respect ively-- in sodium decyl sulfate may be compared with the molal volumes of these compounds at 25°C., viz., 125.9 ml. and 157.8 ml. The difference between the partial molal volume of n-octyl alcohol in sodium decyl and sodium dodecyl sulfate solutions is

G

o 218

E ._~ 216

o "6

E ~ 214

0

I

H20-" ~ ~ O ' l m NaCI

I I I o.I 0.2 0.3

Molality of sodium decyl sulphate

FIG. 2

PARTIAL MOLAL VOLUMES IN ALKYL SULFATE SOLUTIONS 355

outside the error of measurement but is quite small. Thus the solutions of these alcohols in micelles exhibit no marked nonideality as far as volume change on mixing is concerned. Similarly with benzene the partial molal volume in sodium decyl sulfate micelles, 87.4 ml., compares favorably with the molal volume of pure benzene, 89.4 ml.

The effect of additives on the partial molal volume of sodium deey] sulfate is almost negligible. Since the effect of electrolyte additives is thought to be twofold, viz., to increase the size of the micelles and to de- crease their net charge (3), this result is a little surprising. It demonstrates that, if these postulated effects are real, they exert virtually no change on the closeness of packing or organization in the micelle. Although we have not obtained the partial molal volume of sodium dodecyl sulfate in pure water, it seems reasonable to assume that the effects of additives-- either electrolytes or nonelectrolytes--is similarly small. Therefore, taking the partial molal volume of sodium dodecyl sulfate in n-octyl alcohol solutions as being the same as in pure water, the change in partial molal volume in going from decyl sulfate to dodecyl sulfate is 33.4 ml., as com- pared with 32.5 ml. between liquid n-hexyl alcohol and n-octyl alcohol. This difference is small but, we believe, real. I t is to be compared to the corresponding difference of 27.4 ml. in going from decanoic to dodecanoic acid (4) in the solid state. Thus the alkyl sulfate micelle appears to have a slightly looser structure than paraffinic hydrocarbons.

ACKNOWLEDGMENT

This work was carried out as part of a program supported by the Office of Naval Research under Contract N 6 0 N R 25130 with Stanford Universi ty.

~:~EFERENCES

1. HUTCHINSON, E., INABA, A., AND BAILEY, L. G., Z. physik. Chem. (N.F.) 5, 344 (1955).

2. DREGER, E. E., et al., Ind. Eng. Chem. 36, 610 (1944). 3. PHILLIPS, J. N., AND MYSELS, K. J., J. Phys. Chem. 59,325 (1955) ; KLEVENS, H. B.,

Chem. Revs. 47, 1 (1950). 4. GARNER, W. E., ANn RYDER, E. A., J. Chem. Soc. 127, 720 (1925).