Dee., 1954 PERMEABILITY OF CELLOPHANE MEMBRANES TO SODIUM DODECYL SULFATE SOLUTIONS 1097 I n a more acid region, a proton is added to one of the nitrogens to give the positive ion (+)
HNH
0 I/
uncharged molecule, being symmetrical, is stabilized more by resonance. From the similarity among all the values of kz and k, (except those for V and VI) and the relative insensitivity to variation of pH, one is led to suspect that all of the reactions removing =NH follow the same mechanism, hydrolysis, either uncatalyzed or catalyzed by
H +.
NH
and the rate of hydrolysis of this ion is faster than that of the symmetrical neutral molecule. This accounts for the increase in rate for the acid region. This assumption is not unreasonable because the
Acknowledgment.-The author wishes to thank Dr. P. W. Vittum for his kind encouragement during the work on this problem, and for his help in the preparation of this manuscript. He also wishes to thank Dr. W. R. R.uby for his helpful discussions.
ON THE PERMEABILITY OF CELLOPHANE MEMBRANES TO SODIUM DODECYL SULFATE SOLUTIONS BY B. S. HARRAP AND I. J. O'DONNELL Biochemistry Unit, Wool Textile Research Laboratory, C.S.I.R.O., Melbourne, AzLstralia Received M a y 3.9,1.964
It is shown that purified sodium dodecyl sulfate, on equilibrium dialysis in the presence of salt, does not distribute itself equally across a Cellophane membrane when the total concentration is above the critical micelle concentration. This is considered to support the hypothesis of Yang and Foster that the solution inside the dialysis bag contains micelles plus single ions whereas that on the other side of the membrane contains single ions only. The size of the micelles decreases with decreasing salt concentration to such an extent that, in the absence of salt, diffusion of micelles through the membrane occurs, resulting in equal concentrations of detergent on both sides of the membrane when equilibrium is reached. It is also noted that certain commercial detergents cannot be completely removed from solution by exhaustive dialysis.
Introduction Foster, it seemed of interest to establish whether there is a non-uniform distribution of detergent The work of Yang and Foster' with the commercial detergents, Santomerse No. 3 and technical ions across a Cellophane membrane when a pure alkyl dimethylbenzylammonium chloride2 has detergent is used. That this is indeed the case, shown that, on equilibrium dialysis, the detergent under certain conditions, is borne out by the results ions do not distribute themselves equ'ally across a which were obtained from both equilibrium dialysis Cellophane membrane, except at concentrations and osmotic pressure measurements. below the critical micelle concentration. They Experimental interpreted this as indicating that on one side of (a) Materials.-Pure sodium dodecyl sulfate was prethe membrane the solution a t equilibrium conby the method of Dreger, Keim, Miles, Shedlovsky tains both single ions and micelles whereas on the pared and Ross.4 The first sample was prepared from carefully other side of the membrane it contains single ions fractionated dodecyl alcohol (b.p. 110.9' a t 2.00 mm.).s only. I n a subsequent discussion of this paper Later preparations were made from Eastman Kodak doMysels3 suggested that this apparently non-uni- decyl alcohol which had not been further purified and this to be equally satisfactory. On exhaustive dialysis form distribution may be an artifact due to the proved each sample of detergent used was shown to diffuse compresence of non-diff usible impurities (such as pletely through the Cellophane bag. This was in contrast aromatic oils) in the commercial detergents used. to solutions of the commercial detergents where, initially, a considerable proportion of the detergent diffused through We had already observed (unpublished work) the Cellophane bag, but as dialysis proceeded the originally that solutions of the commercial detergents clear solution remaining in the bag became progressively Duponol C, Santomerse No. 1, etc., could not be more turbid and diffusion almost ceased. On raising the completely freed from detergent by exhaustive temperature, the solution in the bag cleared and more dedialysis against running water. The work on tergent diffused through, but a state was reached eventually where even a t 60' there was a precipitate in the bag and which we were engaged a t the time required a scarcely any effusion of detergent.6 detergent which was completely diffusible through (4) E. E. Dreger, G. I. Keim, G. D. Miles, L. Shedlovsky and J. Cellophane and a sample of sodium dodecyl sulfate Ind. Eng. Chem., 36, 610 (1944). prepared from carefully fractionated dodecyl alco- Ross, (5) This was kindly supplied b y Mr. K. E. Murray, Division of hol was found to fulfil these requirements. I n view Industrial Chemistry, C.S.I.R.O., Melbourne, Australia. ( 6 ) This phenomenon was attributed to the presence of higher of the uncertainty of the effects of detergent purity homologs in the commercial detergents used. Although the higher on the interpretation of the results of Yang and homologs have Krafft temperatures above room temperature they (1) J. T. Yang and J. F. Foster, THISJOURNAL, 57, 628 (1953). (2) The principal components of these detergents are sodium dodecylbenzene sulfonate and n-dodecyldimethylbenzylamxnonium chloride, (3) K. J. Mysels, TVS JOUSNAL, 07, 633 (1953).
would be initially solubilized b y the lower homologs present. As these latter are removed b y dialysis so the solubility of the higher homologs would decrease and, since the Krafft temperature increases rapidly with chain length, i t is not surprising that precipitates should form which are insoluble even at 60°.
R. S. HARRAP AND I. J. O'DONNELL
1098
VOl. 58
Buffer solutions were made with distilled water and tassium chloride it was found that a large osmotic Analytical Reagent grade salts. ( b ) Analysis.-The concentration of detergent on either pressure (ca. 40 em.) was obtained which only side of the Cellophane membrane was determined by titra- slowly fell off over several days and did not reach zero. tion with cetyltrimethylammonium bromide (C.T.A.B.) according to the method of Epton? using methylene blue as indicator. The C.T.A.B. was purified by recrystallizing I.C.I. Cetavlon from aqueous acetone. (c) Equilibrium Dialysis.-Visking Cellophane tubing was freed from soluble material by repeated boiling with distilled water. Aliquots (10 ml.) of detergent solution were placed in Cellophane bags and equilibrated against an equal volume of buffer a t either 28 or 38" for two days with shaking. The detergent concentrations on either side of the membrane were then determined by titration. , , , . , were , , car, , y O ( d ) Osmotic Pressure Measurements.-These 0 5 10 15 20 25 30 35 40 45 ried out on dialyzed solutions in a Hepp-type osmometer as designed by Scatchard, Brown, Bridgeforth, Weeks and L i, mg./ml. Gee.* The osmometer was immersed in a bath a t 40" so Fig. 2.-Ci as a function of COat constant ionic strength; that the detergent remained in solution over the range of r/2 = 0.06, temp. = 2 8 O , p H 7.1. ionic strengths used. 1Measurements were carried out on a 3 % solution of detergent in water and in M / 2 5 potassium Discussion chloride. The membranes were Cellophane 300PTg and were thoroughly washed before use by repeated soaking in The results summarized in Fig. 1 establish the detergent solution and water to free them from soluble main point of Yang and Foster's argument, viz., material.
Results
(a) Equilibrium Dialysis.-It
was of interest first to determine the effect of ionic strength on the ratio of detergent concentrations (Ci/Co) on the inside (Ci) and outside (C,) of the bag for the same total concentration level of detergent. Figure 1 shows the results obtained using pH 7.1 phosphate buffers of differing ionic strength. Because of the increase in Krafft temperature with ionic strength, all of these solutions were equilibrated at 38", a temperature which is above the Kraff t temperature corresponding to the highest ionic strength investigated.
0
0.05 0.10 0.15 0.20 r/2. Fig. 1.-The ratio Ci/C, as a function of ionic strength; total detergent concentration = 1%, temp. = 38", p H 7.1. 0
Figure 2 shows the concentration of detergent both inside (Ci) and outside (Co) the dialysis bag for a series of solutions of varying detergent concentration but constant ionic strength. The buffer used consisted of 0.02 M KzHPOl and 0.02 M KH2P04,p H 7.1 and ionic strength r/2 = 0.06. The temperature of equilibrium was 28". (b) Osmotic Pressure.-In the case of the 301, solution of sodium dodecyl sulfate in water, dialysis led to equal concentrations of detergent on both sides of the membrane and hence there was no osmotic pressure developed. If an undialyzed solution was put in the osmometer the pressure rose quickly to a maximum and then fell off to zero. When the 3% solution of sodium dodecyl sulfate was dissolved in and dialyzed against M/25 po(7) 9. R. Epton. Trans. Faraday SOC.,44, 226 (1948). (8) G. Scatchard, e6 el., Amer. Scient., 40,61 (1952). (9) Cellophane 300PT was supplied by E. I. du Pont de Neinours & Go.. Ino., Wilmington, Delaware, U.S.A.
that under certain conditions detergent does not distribute itself equally across a Cellophane membrane. However, it must be emphasized that this does not occur a t low ionic strength. Figure 1 shows clearly that in distilled water there is virtually a uniform distribution of detergent on either side of the membrane, although the concentration used (1%) is well above the critical micelle concentration for sodium dodecyl sulfate in distilled water (approximately 0.23%-Goddard, Harva and JoneslO). The molecular weight of the micelle a t low ionic strength is thought to be in the range 10-15,000 (Philippoffl'), and it is well known that Cellophane is permeable to particles of this size. Although stable osmotic pressures could not be obtained a t low ionic strength using Cellophane as a semi-permeable membrane, an approximate value of 15,000 was calculated for the molecular weight of the micelles a t ionic strength I'/2 = 0.04 from the osmotic pressure a t a concentration of 3%. This assumes no concentration dependence of osmotic pressure and is therefore of doubtful accuracy but shows that the micellar size is of an order which would diffuse through Cellophane. The decrease in diffusion across the membrane a t higher ionic strengths shown in Fig. 1 may be attributed to an increase in the size and molecular weight of the micelles. It should be noted that Debye12 showed by the light scattering technique that the molecular weight of cationic detergent micelles increases rapidly with chain length. Figure 2 shows that a t detergent concentrations less than about 0.1% the detergent distributes itself uniformly on either side of the membrane even though the ionic strength ( r / 2 = 0.06) is moderately high. This concentration range is probably below the critical micelle concentration so that only freely diffusible single ions are present in solution. As the detergent concentration increases, the slope Co/Ci decreases, indicating non-uniform distribution, until a plateau parallel to the Ci axis is reached. This is in contrast to the results of (10) E. D. Goddard, 0. Hams and T. G. Jones, ?'Tans. Faraday Soc., 49, 980 (1953).
w.
(11) Philippoff, Disc. Faraday floc., 11, 96 (1951). (12) P.Debye, Ann. N.Y. Acad. Sci.. gq, 575 (1949).
Dec., 1954
TERNARY SYSTEMSOF LIQUIDCARBON DIOXIDE
Yang and Foster who found no plateau but simply observed a curve of decreased slope after the initial region where C o / C i = 1. This is probably accounted for by a distribution of homologs in the material used by later investigators. On the basis of Yang and Foster's discussion, the concentration Co a t which this plateau occurs would bie taken as the critical micelle concentration of the detergent. However, in view of the fact that micelles of sodium dodecyl sulfate are able to diffuse through Cellophane under certain conditions, this assumption is not always justified. For purpose of comparison all results in Fig. 2 were obtained after a standard time of two days. The concentration Co corresponding to the plateau (ca. 0.14%) thus represents not the CMC but the sum of the critical micelle concentration and the concentration of micelles which have diffused in this time. When the concentration of detergent within the dialysis bag exceeds approximately 1.5%, the concentration Co increases with increasing C i until a further plateau is reached at C i approximately 2.0%. The concentration Co a t this plateau is approximately
1099
0.27%. This plateau extends to C i = 4.0%, the highest concentration investigated. It is difficult to understand the significance of this second plateau. The most likely explanation is that it represents the formation of a second type of micelle, this latter being in equilibrium with a different concentration of single ions. (For a comprehensive discussion of the types of micelles postulated from time to time see McBain.13) It is interesting to compare this effect with the results of Ekwall and Passinen.14 These workers studied the solubilization of decanol in sodium oleate and sodium myristyl sulfate and found that the composition of the detergent-alcohol complex was constant as the detergent concentration was increased above the critical micelle concentration until, a t considerably higher concentrations, (ca. 5%), the alcohol/detergent ratio increased, again suggesting that the nature of the micelles changes a t these higher concentrations. (13) J. W. McBain, "Colloid Science," D. C. Heath and Co., Boston, Mass.. 1950, pp. 255-261. (14) P. Ekwall and K. Passinen, Acta Chem. Scand., 7 , 1098 (1953).
TERNARY SYSTEMS OF LIQUID CARBON DIOXIDE1 BY ALFREDW. FRANCIS Contribution from Socony-Vacuum Laboratories, A Division of Socony-Vacuum Oil Co., Inc., Research and Development Department, Paulsboro, New Jersey Received June I, I964
Mutual solubilities of liquid carbon dioxide with each of 261 other substances are reported. Nearly half of these are miscible with carbon dioxide. Some relations t o structure are noted. Density observations show contractions of ten to fifteen per cent on mixing. Triangular graphs are presented for 464 ternary systems involving liquid carbon dioxide. These are of many different types, some of them novel. They include those with three separate binodal curves (and three plait points) and several with a binodal band across two sides of the triangle and a separate binodal curve on the third side. Another system has three plait points although one pair of components is miscible. Carbon dioxide has a strong homogenizing action upon pairs of other liquids a t moderate concentrations, but a precipitating action a t higher concentrations. In contrast to most solvents it has a selectivity against dicyclic hydrocarbons. Cosolvents were found necessary to make these unusual properties effective in solvent extraction of hydrocarbon mixtures. The large collection of unusual graphs provides experimental evidence on methods of merging of binodal curves. External contact of convex curves always occurs a t both plait points.
No ternary systems of liquid carbon dioxide have been published. Miscibility relations of this condensed gas with other liquids have now been studied in an iiivestigation of its possibilities for use in solvent extraction.2 Cosolvents are necessary to make its unusual properties available for that purpose. Several ternary systems with two separate binodal curves were presented in a recent paper.s Graphs with three such curves are suggested in many physical chemistry textbooks, but no actual example is recorded in the chemical literature. For this type two incompletely miscible liquids must become homogenized by addition of a third liquid which is not miscible with either of the other two; and this effect must occur with all three pairs. (1) Presented before the Division of Physical and Inorganic Chemistry of the 126th Meeting of the .41nericrtn Chemical Society. New York, September 15, 19h4. (2) A. W. Francis. U. 6 . Patents 2,463,482 (1949): 2,631,966; 2,632,030; 2,646,387 (1953) ; three other U. S . Patents applied for; I n d . Ens. Chem , in press, 1956. (31 A. W. Francis, J. A m . Chem. Soc., 7 6 , 393 (1054).
This would not normally be e x ~ e c t e d . ~Liquid carbon dioxide yields many such systems. Ternary diagrams were observed for 464 systems involving carbon dioxide a t or near room temperature. Several of these are of novel types including 21 systems with graphs showing three separate binodal curves, and 38 showing a binodal band across two sides of the triangle and a separate binodal curve on the third side. There are also 76 systems with two separate binodal curves, 82 systems with a binodal band so highly concave on its borders as to indicate that it can be considered as a result of a merger of two binodal curves; and 29 systems with three liquid phases. The only type of ternary all-liquid system observed elsewhere but not among the carbon dioxide systems is that of island curves (ternary miscibility gaps not connected with binary ones). The property of liquid carbon dioxide which makes these uncommon diagrams possible may be (4) C. R. Bailey, J . C h e m SOC.,113, 2579 (1923); and several textbooke.
