July, 1962
1359
NOTES
crease in hydrostatic pressure should generally inhibit the formation of micelles and so raise the critical micelle concentration (c.m.c.). To test this prediction, some direct measurements have been made of the influence of pressure on the c.m.c. of solutions of sodium dodecyl sulfate.
T i m e minutes.
Fig. 3.-Oxidation of 0.01 M phosphorous acid in 0.1 borate buffer a t pH 8.6. Initial concentration of iodine 0.015 M ; temp, 22". R (as defined in eq. 2) is plotted vs. time, for different values of 6. The filled dots are for deuterated phosphorous acid, D.PO(OH)z, under the same experimenial conditions.
acid D.PO(OH),. In Fig. 3 the results of runs carried out with deuterated phosphorous acid are plotted together with the data for the normal form. The kinetic isotope effect kH/kn N 3.6. Some runs were carried out in acetate buffer at pH 4.4 and 5.3, the runs at pH 4.4 being slower than those at pH 5.3 and both sets being appreciably slower than those at pH 8.6. The decrease may be attributed to the decrease in concentration of the dianion, and the much lower oxidation rate of the monoanion. I n principle it should be possible to derive a value for the bimolecular constant for the oxidation of the monoanion. However, such a derivation depends on an accurate knorvledge of the dissociation constants of H3P03at the variety of ionic strengths obtaining under the experimental conditions. Such data are not, available but a rough estimate gave an upper limit of -10 mole-l min.-l, and it is believed that the reaction of the monoanion is intermediate between that of the undissociated acid and the dianion. Investigations were supported in part by a research grant (RC 5842) from the Division of Research Grants, U.S. Public Health Service. B. S. is the recipient of the nilax and Rebecca Schrire Medical Research Grant.
THE INFLUENCE OF PRESSURE ON THE FORMATIOK OF MICELLES IN AQUEOUS SO1,fJTIONS OF SODIUM DODECYL SULFATE BY
8. D.
HAMhNN
Divssion of Physical Chemistry Australean Commonwealth Seientzfic and industrzal Research Organ&zntzon, Fishermen's Bend, Melbourne, Australia Receaved December l a s 1Q6i
Density measurements1-6 have shown that the partial molar volumes of long-chain aliphatic sa'fts, dissolved in mater, are greater in the micellar state than in the free ionic state. It follows that an in-
Experimental The c.m.c. was determined by measuring the specific conductivity of the solutions as a function of the molality of sodium dodecyl sulfate. At each pressure, the points lay on two straight lines whose intersection was taken Go be the c.m.c.7 (see Fig. I). The conductivity measurements were carried out in a Teflon cell8,Qfitted with platinized platinum electrodes and mounted in a conventional steel pressure vessel.1° The and temperature of the cell was controlled to within 3~0.01~ the pressure to within l t l 0 atm. The conductances of the solutions were measured by a Wayne-Kerr B221 Universal Transformer Bridge and were converted to specific conductivities by subtracting the measured conductance of water at the appropriate pressures and by applying cell constants corrected for the linear contraction of Teflon under pressure.9,11 Two different samples of purified sodium dodecyl sulfate gave identical results.
Results Conductivity measurements were made at eIeven concentrations between 2 X and 4 X lo-, molal, at 25") and at pressures of 1, 500, 1000, 1500, and 2000 atni. Figure 1shows the results of some of the measurements. For the sake of clarity in the diagram, the conductivity data at 500 and 1500 atm. have been omitted. The inset in Fig. 1 shows the variation of the c.m.c. with pressure. The measured value at 1 atm. (0.00827 mole/kg.) agrees well with values reported earlier by Goddard and Benson' (0.0084 niole/kg.) and by Flockhart and Ubbelohdel2 (0.0080 mole/kg.). Discussion The results show that the c.m.c. initially increases with increasing pressure in accordance with the general prediction made in the introduction. On the basis of Stainsby and Alexander's13 quasithermodynamic treatment of micelle formation, and its later refinement by Phillips,14 we should expect the influence of pressure on the c.m.c. to be given by the relationship
(a In (c.m.c.)) ap
T,m =
AB
1.8 RT
(1)
where R is the gas constant, T is the3bsolute temperature, P is the pressure, and AV denotes the change of partial molar volume when the salt passes (1) D. G. Davies and C. R. Bury, J . Chem. SOC..2263 (1930). (2) C . R. Eury a n d G. 8.Parry, ibid., 626 (1935). (3) R. G. Paquette, E. C. Lingafelter, a n d H. V. Tartar, J . Bm. Cham. Sac., 65, 686 (1043). (4) R. J. Tetter, J. Phys. Chem., 51, 262 (1947). (5) K. Hess, W. Philippoff, and H. Kiessig, Kolloid Z., 88, 40 (1939). (6) K. A. Wright and H. V. T a r t a r , -7. A m . Chem. Soc., 61, 544 (1939). (7) E. D. Goddard a n d G. C . Benson, Can. J . Chem., 36,986 (1951). (8) J. C. Jamieson, J . Chem. Phys., 21, 1385 (1953). (9) S. D. Hamann and W. S t r a w s , Trans. Faraday Soc., 61, 1684 (1955). (10) J, Buchanan a n d S.D. Ramann. ibid., 49, 1425 (1963). (11) C. E. Weir, J . Reseaich Natl. Bur. Standards, 53, 245 (1954). (12) B. D. Flockhart a n d A. R. Ubbelohde. J . Colloid Sci., 8 , 428 (1953). (13) G. Stainsby and A. E. Alexander, Trans. Faraday Soe., 46, 587 (1 950). (14) J. N. Phillips, ibid., 51, 561 (1955).
Vol. 66
;?;OTSi;
,.
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Pressure (utm)
0
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1
5
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/5
20
25
M O m q
Pig. 1.-The
x
1 30
I 35
I 40
4
/os,
specific conductivity of sodium dodecyl sulfa.te in aqueous solut,ion at 25' and at 1, 1000, and 2000 atm. Inset: The variation of the c.m.c. with pressure.
from the free ionic state into the micellar state. The factor 1.8allows for the influence of counterions associated with the mi~e1les.l~ Applying formula 1 to the present results we find that the initial rate of change of the c.m.c. with pressure (shown by the-dotted line in Fig. 1) corresponds to a value: AT' = +11 cm.a/mole. This agrees well with the value, +10 A 3 mole, given by Kushner, Duncan, and Hoffman'slS density measurements on aqueous solutions of sodium dodecyl sulfate a t 23'. The reason for the expansion is not, definitely known. Goddard, Hoeve, and BensonlGhave suggested that it might arise from the release of water molecules which had been tightly held as "icebergs" around the hydrocarbon chains of the free ions. To the author, it seems more likely that bound water is released from the solvation shells of the electrically charged terminal groups when .the charges become partially neutralized by counterions associated with the micelles. A closely analogous (15) L. M. Kushner, B. C. Duncan, and J. I. Hoffman, J . Research Natl. Bur. Standards, 49, 85 (1952). (The author is grateful to the referee for drawing his attention to this paper.) (16) E. D. Goddard, C. A. J. Hoeve, and G. C. Benson, J . Phys. Chem., 61, 593 (1957).
expansion occurs when inorganic ions associate into ion pairs in aqueous so1ution.l7 Considering the results at higher pressures, we see from Fig. 1 (inset) that the initial positive trend of the c.m.c. with increasing pressure reverses a t about 1000 atm. If we accep_t the validity of eq. 1,l*the reversal implies that AV changes sign, which is a most unusual effect in a simple chemical system. A change of this kind might occur if the interiors of the micelles became partially solidified under pressure. Bridgmanlg has shown that, a t 2 6 O , the normal hydrocarbons C16H34, CDHLV,and CIOHZZ freeze a t 410, 1650, and 2950 atm., respectively, and the present author has found that 1-dodecanol freezes at 700 atm. I n each case the solidification (17) Some unpublished experiments by Dr. F. H. Fisher have shown that the association
Mg2+ f S042-
-
(Mg+zS042-) ion pairs
in water, a t 25", involves an expansion of $7 cm.s/mole. (18) The validity of eq. 1 is questionable bcaause the a.m.0.. defined aa the point of intersection of the two linear sections of the conductivity/molality plots (Fig. If, may have no real thermodynamic meaning. As Goddsrd and Benson' remarked, there is actually a "smooth transition between the linear portions rather than the abrupt change in Slope indicated." (19) P. W. Bridgman, Proc. Amsr. Acad., 77, 138 (1948-1949).
July, 1962
XOTES
involves a contraction of about 10% (that is, 20 om.~/molefor dodecane) and this contraction would clearly outweigh the expansion which accompanies the formation of micelles of sodium dodecyl sulfate a t low pressures (11 ~m.~/rnole). If this hypothesis is correct it should be possible to vary the freezing pressure, and hence the c.m.c. inversion pressure, by altering the temperature and the hydrocarbon chain length, and by dissolving short chain alcohols in the micelles. It is hoped to study the effects of some of these changes in future work.
l1
1361
t
10 L 30 Fig. 1.-log
BATE OF SPREADING AND EQUILIBRIUM SPREADING PRESSURE OF THE MONOLAYERS OF n-FATTY ALCOHOLS AND n-ALICOXY ETHAKOLS BY A. V. DEO,S. B. KULKARNI, M. I(. GHARPUREY, AND A. B. BISWAS Contribotion No. 486 from the Nattonal Chemical Laboratory, Poona-O, Indza Received December 19, 1961
In recent communications,'V2 we reported the superior water evaporation retarding power of the alkoxy ethanols (glycolmonoalkyl ethers) to those of the commonly used cetyl and stearyl alcohols. With a view to understand this evaporation retardation behavior we have been studying their physico-chemical properties; and in what follows, preliminary results on the rate of spreading from the solid onto a clean water surface and on equilibrium spreading pressure a t 25' are reported. The Rate of Spreading.-For measuring this rate, the alcohol or the alkoxy ethanol sample was prepared by gently withdrawing a glass rod of uniform diameter irom the melt of the substance kept 10' above its melting point. The rod then was left overnight a t room temperature (-25'). The perimeter of the coated rod was measured with the help of a traveling microscope. It then was half-immersed vertically in a known area of clean water surface in a thermostated Langmuir trough fitted with a horizontal film pressure balance. The time required for the film pressure to rise to the low value of X dyne/cm. was noted. From such data and from the previously determined values of the area/molecule of the various substances a t 1 dyne/cm., the rate of spreading in terms of the number of molecules entering the water surface from 1 cm. of the triple interface perimeter in 1 sec. was calculated. The values a t 25' are given in Table I. From the above it may be noticed that a modification of the terminal -OH group to -OCH2CH20H has considerably decreased the melting point and increased the rate of spreading. Again, in s homologous series the rate of spreading is decreased as the melting point is increased along with the chain length. It is interesting to compare the log dN/dt us. melting point curves for the two series as given in Fig. 1. (1) A. V. Deo, N. R. S a n j m a . S. B. Kulkarni, M. K. Gharpurey, a n d A. B. Biswas, Nature, 187, 870 (1960). (2) A. V. Deo, S. B. Xulkami, M. K. Gharpurey, a n d A. B. Biswas. ibid.. 191, 378 (1961).
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50 60 70 Melting point, "C. dN/dt vs. melting point: 0, alcohols; alkoxy-ethanols.
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60 90 120 Time, min. Fig, 2.-Surface pressure vs. time: 0, cetyl alcoholA = 308.5 cm.2; T = 25.3'; p = 2.12 cm. A, octadecoxyethanol-A = 308.5 cm.2; T = 24.7'; p = 2.15 Cm. 0
30
TABLE I MELTINGPOINT,RATEOF SPREADING (dN/dt, NUMBER OF MOLECULES/CM./SEC.), AND EQUILIBRIUM SPREADING PRESSURE (DYNES/CM.) -n-Fatty M.P., OC.
Cic Ci6 CIS Czo Czz 0
alcoholsdN/dt
11.
7 - n - A l k o x y ethanolsM.P., oc. dN/dt
39.5 2.1 X 10" 46.5 35.0 49.5 2 . 8 X 10l3 39.6 43.5 59.4 1.1 X 10l2 35.2 51.7 64.5 7.6 X loll 32.6 60.5" 71.0 6.0 X 10" 27.6 65.6 Compound not extremely pure.
5.2 X 2.3 X 1.8 X 1.2 X 1.5 X
10" 10" 10" 10'' 1012
IIe
48.6 50.4 48.9 49.0 47.2
I n the intermediate range the curves run roughly parallel and here the rate of spreading of the nalkoxy ethanol is about ten times higher than that of the n-alcohol. This may be attributed to the higher escaping tendency of the former compounds from the solid together with the enhanced interaction of the -OCH&H20H group with the water subphase compared to the -OH. The film pressure vs. time curves for the Ciaalcohol and the Cle-alkoxy ethanol a t 2.5' are shown in Fig. 2. The points of discontinuity in the curves naturally correspond to those in the 11-A curves and are a consequence of the sudden change in the film compressibility, and do not signify a sudden change in the rate of spreading at the corresponding pressure.
