NOTES Micelle Size of Barium Dinonylnaphthalenesulfonate in Low Polarity Solvents by Vapor Pressure Osmometry
by R. C. Little Surface Chemistry Branch, Chemistry Division, Naval Research Laboratory, Washington, D . C . 2OS90 (Received October SO, 1969)
The solubility behavior of the alkali metal sulfonates has been thoroughly studied in a wide variety of polar and nonpolar so1vents.l The observed trends in the solubility and micellar behavior of these sulfonates have been successfully described through the application of Hildebrand's solubility parameter theory.2 The solubility parameter concept has also been a useful index to soap-solvent behavior for phase change phenomena in a number of carboxylate soap-solvent system^.^ I n addition, the solubility parameter concept has been shown to be of some utility in practical applications for the prediction of lithium grease dropping point^.^ The solubility parameter concept, however, was found to be inapplicable to soap solutions in strongly polar solvents, particularly those which bring about extensive dissociation of the soap molecule into its constituent ions.5 While earlier work treated the effect of solvent on the micelle size of the monovalent alkali salts of dinonylnaphthalenesulfonic acid, no work has yet been reported on divalent salts. Barium dinonylnaphthalenesulfonate in particular was selected as a candidate divalent salt toward this purpose since its benzene and toluene solutions have been studied by several other techniques .6--8
Experimental Section The dinoiiylnaphthalenesulfonic acid used (HDNNS) was a specially prepared research grade supplied by King Organic Chemicals Inc. The preparation of this acid has been previously described.' Barium dinonylnaphthalenesulfonate,Ba(Di\'NS)2, was prepared by neutralizing a 2-propanol-water solution of the acid with an excess of solid barium hydroxide and back titrating to the neutralization end point. The soap was then extracted with petroleum ether and flash evaporated. Benzene solutions of this barium sulfonate were subsequently flash evaporated to remove the bulk amounts of alcohol and petroleum ether. After three flash evaporations with dry benzene the soaps were again taken up in dry benzene and freezedried under vacuum. The barium soap was a nearly white powder. The barium soap structure-as a result of steric and group-directing effects in the alkylation and sulfonation steps-is considered to consist of two highly branched nonyl groups in the 1,4 positions with
1817 the sulfonate group in either the 6 or 7 position of the naphthalene ring. The solvents used were ACS grade or better except foi the polymethylsiloxane dimer, which was a commercial sample from the Dow Corning Corp. All solvents mere passed through molecular sieve materials and Florisil to remove water and polar contaminants and were used as soon as possible after percolation. Benzil, used for calibrating the osmometer, was twice crystallized from anhydrous ethanol, dried at SO", and stored in a desiccator over P206 until use. Vapor pressure lowering data were obtained by means of a commercial thermoelectric device, the lllechrolab Model 301A osmometer. I n all cases a drying agent, Linde molecular sieves, was added to the solvent cup to maintain a water-free solvent atmosphere in the vapor chamber.
Results and Discussion Figure 1 presents number average aggregation numbers for barium sulfonate micelles in low polarity solvents as a function of concentration. As was found in earlier worklJ the aggregation number (monomers per micelle) is again independent of Concentration. This invariance of aggregation number for a specific soap in a specific nonpolar solvent appears to be a definite characteristic of the metal dinonylnaphthalenesulfonates regardless of the cation or the nonpolar solvent chosen [except for those solvents which possess functional groups capable of interacting with a specific cation and provided studies are not made in the region of the critical micelle concentration.]' The invariance of micelle size in a given solvent as the soap concentration is varied is a necessary condition for the application of a theory recently proposed which accounts for the effect of solvent on micelle size.l The relation between micelle size and solvent solubility parameter may be expressed as follows N = 1/K (62 61) 1 where N = the aggregation number; 6* = the solubility parameter of the soap monomer; 61 = the solubility parameter of the solvent; K = a constant (considered t o be a shielding factor). This equation should be of general applicability provided that the
+
(1) R. C. Little and C. R. Singleterry, J . Phys. Chem., 68, 3453 (1964). (2) J. H. Hildebrand and R. L. Scott, "The Solubility of Nonelec-
trolytes," 3rd ed, Van Nostrand-Reinhold Co., Inc., Princeton, N . J., 1950. (3) R. C. Little, J . Colloid Interfae. Sei., 21, 266 (1966). (4) R. N. Bolster and R . C. Little, I n d . Eng. Chem. Prod. Res. Develop., 5, 198 (1966). (5) R . C. Little and C. R. Singleterry, J . Phys. Chem., 68, 2709 (1964). (6) T. F. Ford, S. Kaufman, and 0. Nichols, ibid., 70, 3726 (1966).
(7) S. Kaufman and C. R. Singleterry, J . Colloid Sei., 10, 139 (1955). (8) A . Fryar and S. Kaufman, J . Colloid Innterfae. Sei., 29, 444 (1969).
Volume 74, Number 8 April 16, 1970
1818
-
NOTES
.,-
0
8-
n
P
0
.
SILOXANE DIMER
w
W _I
-a
LY
L
1
0
6 -
c. v
0
v1
An aggregation number containing 18 acid residues appears to be the ultimate micelle size reached in low polarity solvents for both the bivalent and monovalent soaps since the micellar phase begins to separate as a liquid-like phase below a solvent solubility parameter of 6.0.’ An ultimate aggregation number of 18 residues appears to be consistent with the most probable spatial geometry of the dinonylnaphthalenesulfonates. Rotation of representative molecular models in space indicate that an average solid angle of 26” is generated by tangents between the polar head group and the bulkiest section of the hydrocarbon residue. As an exercise, one can construct spheres from cones of various solid angles and thus count the number of cone units which make up the sphere. From simple geometry, however, one may
8 w,,,,,,,. CYCLOHEXANE
E !A
4 -
m
I z 3
z
0
2
0--.--02-
W 0
U
0 .
0
I
I
I
I
0 05
0 IO
0 15
0 20
and a correction for those areas of the sphere not covered by conical curved surfaces is as follows
w
-I
16
L
N = 1.05128 4 8 ? 4
0.
$6I
z
-
I
r
fz .
-
z
0
2 w
8 2 4
I
I
I
I
1
following conditions are met: (1) that the micelles be essentially monodisperse in the given solvent ; ( 2 ) that the micelles be spherical in structure and contain a polar core; (3) that the condensed micellar phase have an amorphous or liquid-like character; and (4) that the micellar concentration be at least an order of magnitude greater than the monomer concentration. Figure 2 summarizes the experimentally observed effect of solvent on barium sulfonate micelle size. Aggregation number in terms of monomers per micelle is plotted against the solubility parameter of the solvent. The values of & = 11.3 and K = 0.690 for the barium soap obtained from the plot compare with 8 2 = 10.5 and K = 0.295 for the alkali sulfonates. The greater solubility parameter value for the barium soap monomer reflects the increased polarity of its ionic head while the -greater K value is primarily a result of the larger degree of shielding per monomer unit by the bdrocarbon residues in the bivalent soap. T h e Journal of Physical Cherni&u
+ 0.487
Substituting 8 = 26” (0.436 radian) into the formula yields N = 18 acid residues for the fully packed micelle--in good keeping with the experimental results obtained in the polymethylsiloxane dimer (6 = 6.5) in which 8 monomer units or 16 acid residues are counted. I n addition, extension of the aggregation number os. solubility parameter plot of Figure 2 t o 6 = 6.0-
O n t h e Liquid Film Remaining in a Draining Circular Cylindrical Vessel by Paul Concus Lawrence Radiation Laboratory, Uniuersity of CaZijorniu, Berkeley, Calijornia 94720 (Received October 2S, 1969)
It is of interest to have a theoretical estimate for the thickness of the film that remains on the wall of a right circular cylindrical vessel as it is being drained of a wetting liquid. Such an estimate, which includes the effects of surface tension and viscosity, has been given by Levichl for the case where the contact angle is zero and the static meniscus away from the wall becomes essentially a horizontal plane. I n this note we
v. G , Levi&, “physicochemical Hydrodynanlic~,”PrenticeHall, Englewood Cliffs, N. J., 1962, p 681, eq 133.26.
