Studies on Solubilization. - The Journal of Physical Chemistry (ACS

10

E. C. MERFiILL, JR., AND J. W. MCBAIN

STUDIES ON SOLUBILIZATION’ R. C. MERRILL,JB., AND J. W. MCBAIN Department of Chemistry, Stanford University, California

Downloaded by FLORIDA ATLANTIC UNIV on September 7, 2015 | http://pubs.acs.org Publication Date: January 1, 1942 | doi: 10.1021/j150415a002

Received Aupal 1,1041

In addition to their properties as wetting, suspending, emulsifying, and colloidstabilizing agents, dilute aqueous and non-aqueous solutions of soaps and other soap-like detergents-non-electrolytic as well as anion- and cation-active materials-possess the remarkable property of being able actually to dissolve otherwise insoluble substances (4, 15, 16, 19, 25). Not only do such solutions form spontaneously, but the resulting colloidal solutions are thermodynamically stable, as shown by the fact that they enter into true reversible equilibria with non-aqueous solutions where the solute exists as simple molecules, such as a water-insoluble dye in toluene. They likewise form true equilibria with solid crystals (14). The formation of such solubilized systems is accompanied by a lowering of free energy. For example, the vapor pressure of a volatile insoluble liquid taken into colloidal solution by a solubilizer is far less than that of the volatile liquid alone (17). The vapor pressure of a solution of a soluble volatile hydrocarbon in water is reduced by adding a detergent, and the solubility is correspondingly increased. Clearly this phenomenon is quite distinct from peptization of preexisting particles or droplets, which do not exist in the cases just cited. The present communication includes experiments which help to clarify detergent processes, and provides systematic measurements of the “solubilization” of dyes. The formation of such thermodynamically stable colloidal solutions,“solubilization,”-must be distinguished from other phenomena such as ( a ) emulsification, where the vapor pressure of the emulsified liquid is equal to that in bulk or even greater due to the curvature of the droplets; (b) change in solvent power due to the addition of a large proportion of a second liquid; ( c ) peptizing, protective, and suspending action of preezisting particles, or those formed by mechanical means or by condensation from a supersaturated solution; (d) “sequestration,” for example, of calcium soaps in water by substances such as Calgon (sodium hexametaphosphate) for the action of which we may tentatively, in the absence of more definitive evidence, accept the explanation of Hall (5) as being due to the formation of complexes with the cation, especially since Reitemeier and Buehrer (22) found no colloidal particles visible in the ultramicroscope; and ( e ) “solutizing” of mercaptans (28),in part due to salt formation, by strongly alkaline concentrated solutions of substances which are not effective in dilute solution and where colloids are probably not involved. In general, the above actions are possessed in varying degrees by most commercial detergents and protective colloids. For example, some excellent pro1 Presented at the Eighteenth Colloid Symposium, which was held a t Cornell University, Ithaca, New York, June 19-21, 1941.


STUDIES ON SOLUBILIZATION

11

tective colloids, such as methylcellulose and gelatin, are practically non-effective as solubilizers for the water-insoluble dyes Yellow AB and Orange OT. Others, such as sodium resinate, a water-soluble “Carbowax,” and some vegetable gums, are only slightly effective, whereas the soaps and most synthetic detergents are excellent solubilizers. Solubilization is in some cases only a minor factor in detergency as compared with such prominent effects as wetting or mechanical removal, whether by scraping or by agitation. Solubilization may be required to remove a final film by dissolving it. Besides its theoretical importance as another of the rapidly increasing examples of stable colloidal system, the phenomenon of solubilization is of importance in medicine, in physiology, in pharmaceutical preparations such as liniments, inhalations, mouth washes, and medicated liquid soaps (e.g., Lysol), in the dyeing of fabrics, and in detergency (1, 2). EXPERIMENTAL

Materials Orange OT (F.D. and C. Orange No. 2; l-o-tolylaz0-2-naphthol),containing not less than 98 per cent of pure dye as determined by titration with titanium trichloride, was recrystallized from boiling alcohol, and only the middle portion of the precipitated crystals wm used after drying in an oven a t 80’C. overnight. The resulting crystals were of fairly small size (but not of colloidal size) and were a bright orange in contrast to the dull orange powder of the commercial material. The dye was insoluble at room temperature, and a t 100°C. gave only a faint orange tinge to the water. Orange OT and Yellow AB (phenylazo-8-naphthylamine) are both insoluble in solutions of ordinary electrolytes. In fact, no inorganic substance tested, including various silicates, sodium hexametaphosphate (Calgon), ammonium sulfamate, and a complex sodium borophosphate, was effective in solubilizing either of the two dyes. The sodium salts of esters of sulfosuccinic acid (Aerosols) were supplied in a pure form by the American Cyanamid and Chemical Corporation, the alkyl sulfates by the du Pont Company, the decyl sulfonate by Professor H. V. Tartar, the lauryl esters of amino acids by the Emulsol Corporation, N,N,N‘,N‘tetramethyl-N ,N‘-di-dodecyl-p-hydroxypropylene diammonium bromide (Damol) by the Alba Pharmaceutical Company, and the decyl and dodecyl benzene sulfonates by the Monsanto Chemical Company. All had been especially purified. The sodium deoxycholate and cholic acid, from which the sodium mlt was made by neutralization with carbon dioxide-free alkali, were research products of the Redel-de Haen Company. Sodium oleate was made by neutralization of oleic acid “Kahlbaum”. The solubility of the lauryl esters of the hydrochlorides of glycine, alanine, and a-aminoisobutyric ecid is between 0.5 and 1.0 g. per 100 cc. of water a t 25OC. The remaining detergents used were commercial products of various degrees of purity, some containing large amounts of electrolyte. The salts used were of C.P. grade, and the solvents were of a good technical grade.


12

R. C. MERFtILL, JR., AND J . W. MCBAIN

Method The solutions in table 1 were made by adding 1.00 g. of the detergent aa obtained commercially to 100 cc. of distilled water, whereas the other solutions were made by weighing the required amount of pure solid and adding distilled water up to a given volume of solution. In order to minimize sorption of the detergent, only a slight excem of recrystallized dye was added. Solutions were contained in 100-cc. glass bottles sealed with water-tight, metal-lined plastic caps, and placed in a thennostated shaker a t 25.0"C. until equilibrium was attained (about 16 hr.). The solutions were then allowed to stand quietly for a t least 2 hr. or until all solid dye crystals had settled. The top layer waa then removed carefully with a pipet, avoiding all agitation, examined in a strong light for suspended particles, and analyzed by means of a Klett-Summerson photoelectric colorimeter, using the No. 54 green filter. Calibration curves obtained by dissolving known amounts of solid dye in Aerosol OT solution, in which the dye is colloidal, coincided with those obtained by dissolving the dye in benzene, in which it is molecularly dispersed. The dye concentration-photelometer reading (logarithmic scale) curves were almost linear for dilute solutions, but in more concentrated solutions departed from linearity. Solutions of Orange OT containing more than 4.75 mg. of dye per 100 cc. had to be diluted using volumetric pipets and flasks. Where possible, water was used, but when precipitation of dye occurred on diluting with water, because of reduction in the proportion of colloid present, acetone waa added instead. The first few cubic centimeters of acetone added to a solution of solubilized dye precipitated the dye from solution, although more concentrated solutions were clear,-again differentiating solubilization from change in solvent power. In some cftses the solubiliser was precipitated, although after it had settled the precipitate was colorless and the solution was clear. All solutions of solubilized dye were, and remained indefinitely, perfectly clear and transparent, and showed no particles visible to tho eye or in the microscope, although extremely minute particles were usually visible in the ultramicroscope. A supersaturated solution became turbid on cooling, but on reaching equilibrium was again clear. Again, this behavior serves to differentiate solubilization from protective or suspending action. A?T.UNMENT OF EQUILIBRIUM

The thermodynamic stability of these colloidal solutions is shown by the fact that the same equilibrium solubility is obtained both from supersaturation (induced by warming) and from undersaturation. However, supersaturated solutions crystallize out very slowly, weeks being required in most cases, although the exact time depends on the amount of supersaturation, the solubiliwer, the amount of excess crystals, shaking, etc. This slowness of attaining equilibrium from supersaturation is in all probability due to the protective action of the solubilizers upon very small dye particles crystallizing out and indicates that these solid macroscopic particks merely coated with protective colloid are not



13

stable in the strict thermodynamic sense. A similar phenomenon has previously been noted and accounts for the erroneously high values for a series of long-chain sodium, calcium, and magnesium sulfonates reported by Reed and Tartar (21)and subsequently corrected by Tartar and Wright (27). In order to save time, most of the data on Orange OT in this investigation were obtained from undersaturation and only in case of doubt was equilibrium approached from both sides. The rate of attaining equilibrium seems to depend on the solubility in pure water of the dye to be solubilized. (All the dyes used have, although “water-insoluble,” of course a finite solubility.) Whereas Hartley (6,7) was able to obtain equilibrium in 6 hr., using the slightly soluble trans-azobenzene, we found that Orange OT, which is insoluble in cold water although slightly soluble in hot, requires 12 to 16 hr. Yellow AB, which is entirely insoluble in hot and cold water, aa expected on the basis of structure, requires about 4 days under comparable conditions. Sudan G (benzeneazo-@-resorcinol)and Sudan I (benzeneazo-8-naphthol) require about the same length of time as Orange OT. This dependence of the rate of attaining equilibrium on the solubility of the “waterinsoluble” substance strongly indicates that the dye (probably like other materials as well) dissolves first as simple molecules, which are then incorporated into the micelle. A slight dependence of rate upon the solubilizer used was also noted. DATA

A comparison of the solubilities of Orange OT and Yellow AB in 1 per cent solutions of pure and commercial anionic, cationic, and non-electrolytic detergents is given in table 1, the data for Yellow AB being taken from a previous publication (16). Three different sulfonated oils are included, as well as four pure bile salts. In all cases except one, the more hydrophobic Yellow AB was solubilized to a greater extent, illustrating the general principle of like to like, although the amount varied from three to ten times as much. Similarly, Hartley (6)finds the mole ratio for azobenzene to be approximately ten times those given here. This shows that the substance to be solubilized, as well as the solubilizer used, &ect the amount dissolved. It is interesting to note that the oxidation of the three hydroxyl groups of the four-ring compound, sodium cholate, to the three ketone groups of sodium dehydrocholate has caused a fairly effective solubilizer to become noneffective. Table 2 gives the solubility of Orange OT in 0.01 N solutions of a number of pure colloidal electrolytes. The data of Lottermoser and Puschel (11) on the sodium alkyl sulfates and of Tartar and Wright (27) on sulfonates indicate that this concentration is within the [‘critical region” for the formation of micelles, so that the difference in effectiveness is probably due mainly to the varying amount of colloid in the solution. The mixture of sulfates is more effective than expected on the basis of their effectiveness alone, whereas the mixture of sulfonate and sulfate is about the same. Unpublished freezing-point measurements made

TABLE 1 The solubilities of Yellow A B and Orange OT i n 1 per cent aqueous solutions of various pure and commercial sotubilizers at W C .

1

SOLUBILZZEB

SOLUBILITY I N MILLIL)BhMS OF DYE PEB 100 CC. OF BOLUTION


I Duponal WA (sodium sulfate of technical lauryl alcohol) 20.8 Aerosol OT (sodium salt of the dioctyl (2-ethylhexyl) e8 sulfosuccinic acid). . . . . , , . , , , , , . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . 12.0 Laurylpyridinium iodide. , . , , , . , . . . . , . . .' . . . . , . . . . . . . , . . . . . . . . , . 31.9 Sodium novenate (a sodium naphthenate). . . . . . . . . . . . . . . . . . . . . . 3.1 Sapamine MS (CI~H~,CONHCZH~N(CH~)(CIH~)~SO~CH~). . ... . . 53.5 Anhydrous Wettal (impure condensation product of coconut oil fatty acids, polyethylene glycol, and SO:NH&;H,OH). . . . . . . 60.2 Aquasol A.R. (75 per cent sulfonated cmtor oil, 4.3 per cent SOa, 29.6 per cent water). . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . 62.0 Monosulph (highly sulfonated castor oil, 30 per cent water). . . . . . 75.0 Morpeltex B (sulfonated castor oil, 32.5 per cent 80s on 100 per cent fat basis, 48 per cent pur 36.8 Sodium deoxycholate. . , . . . . , . , . 6.1 Sodium taurocholate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 5.6 Sodium cholate. . . . , , , . , , , , , . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Sodium dehydrocholate. . . . . . . , . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . 0.00

8.0 1.61 12.4 4.6 8.5 10.4 10.0 18.6 3.80 1.52 1.45 0.29 0.00

TABLE 2 The 8olubility of Orange OT i n 0.010 N solutions of variow pure colloidal electrolytes at 96.0"C.

1

8OLUBIyR

IN W

O

W OF

1w cc. or SOLUTION

BUBSTANCE

MolwuQr weight

Sodium decyl sulfate.. . . . . . . . . . . . . . . . . . . . . , . . . . . . . , . . Sodium dodecyl sulfate.. . . . . . . . . . . . . . , . . . . . . , , . . . . . . . Sodium tetradecyl sulfate, . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium deoyl sulfonate., . . . . , . . , , . . . , , , . , , . . . . . , . , , . . Sodium tetradecyl sulfate (0.005 N ) sodium decyl sulfate (0.005 N ) . ., .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium tetradecyl sulfate (0.005 N ) sodium decyl sulfonate (0.005 N). . ... . . . , . . , . . . . . . . , . . . . . . . . . . . . Sodium salt of the dioctyl (2-ethylhexyl) ester of sulfosuccinic acid. . . . . , , . . . . , . , . . . . . . . . , , . , . . . . . . . . Sodium salt of the dihexyl (methylamyl) ester of sulfosuccinic acid. . . . , . . . . . . . . . . . . , , . . . , , , . , , . . . , . , . Sodium salt of the diamyl (2-methyl-3-methyl) ester o sulfosuccinic acid.. . , . . . . . . . . . . . . . . . . . . . . . , . . . , , , Sodium salt of the diisobutyl ester of sulfosuccinic acid. Lauryl ester of glycine hydrochlori Lauryl ester of alanine hydrochlori Lauryl ester of the hydrochloride of acid. . . , , . . . . . , . , . , . , , , . . , . . . , , ,

+ +

H d - +Brr- -. . . . . . , . . , . . . . . . . . . . , , . . . . . . . . . . . . , . . . . Decyl benzene sodium sulfonate'. . . . , . . . , . . . , , . , . , , , , Dodecyl benzene sodium sulfonate'. , . . . . . .. . , . . . . . , , . Sodium cholate.. . . . . . . , . , . . , . . . , . . . , . . . . , . , . , . . . . , , , , Sodium oleate. . . . . . . . . , , . . . , . . . , , . . , . . . . , . . , , . . . , . , , .

* Mixtures of

branched- and straight-chain radicals. 14

hlubility

-

DYE PEE

Moles of dye per mole of aolubiliaer

260.22 288. 27 316.30 225.22

0.06 0.74 6.88 0.03

288.26 (Av.

4.12

0.0157

270.76 (Av.

3.45

0.013

444.56

0.47

0.0018

388.45

0.17

0.00065

360.40 332.20 279.84 293.86

0.0

0.0 0.0

307.88

1.30 1.17

0.0049 0.0045

644.69 400.42 428.47 431.56 304.27

6.7 0.62 2.80 0.03 4.70

0.0028 0.0024 0.010 o.oO011 0.018

0.0 1.30 1.23

O.ooo23 *

0.0028 0.026 0.00014

0.0049 0.0047

15


by 0. E. A. Bolduan in this laboratory indicate the presence of colloid for the dioctyl and dihexyl esters of the sodium salt of sulfosuccinic acid, whereas the others show no micelle formation a t this concentration. It appears, then, that colloid is necessary for solubilizing action and that this might be taken as an indication of micelle formation both in aqueous and in non-aqueous systems (16).


TABLE 3 Solubility of Orange OT i n Aerosol OT solutions at 96°C.

0.0330 0.0300 0.0250 0.0225 0.0200 0.0150 0.0125 0.0100 0.0075 0.0050 0.0037 0.0020 0.0010

2.48 2.32 1.82 1.55 1.25 0.88 0.64 0.47 0.31 0.14 0.07 0.04 0.02

0.0029 0,0029 0.0028 0.0026 0,0024

0.0022 0.0019 0,0018 0.0016 0.0011 0.00072 0.00038 O.OOO38

0.02

0.03

NORMALITY OF AEROSOL OT

FIG.1. The solubility of Orange OT in Aerosol OT solution at 25.0”C.

The effectiveness of Aerosol OT in solubilizing Orange OT as a function of the concentration is shown in table 3 and figure 1. Although the effectiveness of the solubilizer can probably be taken as a measure of the proportion of micelles in the solution, the curve shows no sharp break a t any concentration which might be termed “critical” for the formation of micelles, but rather shows a gradual increase over approximately a tenfold range of concentration. Any hydrolysis of the ester portion was insignificant, since no odor of octyl alcohol wm apparent in the time of the experiment and even the most dilute solutions did not become turbid.

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R. C. MERRILL, JR., AND J. W. MCBAIN


SOLUBILIZATION O F LIQUIDS

When 2 or 3 CC. of toluene is shaken with solutions of Yellow AB in various solubilizers such as 1 per cent Aerosol OT, a cloudy emulsion forms. When heated the emulsion becomes perfectly clear, owing to reversible solubilization of the toluene a t higher temperatures. However, although clarity of the solutions has been extensively used by Pickering (19), Smith (25), Albert (1, 2), Pink (201,and others as a criterion for distinguishing emulsification from solubilization, J. J. O’Connor in this laboratory has noted that solutions of volatile liquids dissolved from the vapor phase became turbid when concentrated. This casts some doubt upon clarity of solution as an exact criterion for measuring the amount of solubilized liquid. Dilute solutions of soaps and hydrolyzable detergents, such as the lower Aerosols, are turbid, owing to hydrolysis and formation of acid complexes, etc., whereas more concentrated solutions are clear, owing to solubilization. SOLUBILIZATION THROUGH MEMBRANES

When toluene solutions of Yellow AB or Sudan I11 inside cellophane membranes (previously swollen in 64 per cent zinc chloride) are immersed in 1 per cent aqueous Aerosol OT, impure lecithin, or sodium deoxycholate, the dye passes through this membrane, which is permeable only to ions, within a few minutes. Likewise Yellow AB solubilized by an aqueous solution of technical sodium lauryl sulfate is able to pass through a semipermeable membrane into the same solution without dye. Evidence was also obtained that water was solubilized through a membrane into a toluene solution of diethanolamine oleate. This passage of insoluble material through membranes distinguishes solubilization sharply from suspending action, and is undoubtedly of great importmce in biological processes. MECHANISM O F SOLUBILIZATION

Five theories have been advanced by different groups to explain the mechanism of solubilization and are here discussed in connection with the present data. ( 1 ) The first suggestion is that solubilization is just a case of “hydrotropy” or change of solvent-a term introduced by Neuberg in 1916 (18) to designate the effect of large additions of various substances in water in increasing the solubility of other substances. The addition of 25 per cent to 100 per cent of foreign material, necessary to obtain the effect, completely changes the thermod,ynamic environment within the solution; whereas a true solubilizer, effective when present to the extent of 0.1 per cent, produces little change in the macroscopic liquid. This,together with the proven presence of colloid in these solutions, rather eliminates hydrotropy as a possible explanation of true solubilizing action. Hydrotropic action is illustrated by the increased solubility of benzene in water caused by adding 50 per cent of ethyl alcohol. We here find that 5 per cent aqueous solutions of Butyl Cellosolve, Butyl Carbitol, Morpholine, acetone, and isopropyl alcohol are completely ineffective in dissolving Orange OT, and even 5 per cent pyridine dissolves less than 0.05



17

mg. per 100 cc. I t requires large amounts of a second solvent to change the properties of a first, whereas a small amount of diluent spoils a good solvent. ( 2 ) Biochemists ascribe the action of the bile in making all sorts of waterinsoluble substances soluble in water to the formation of a “choleic acid” or molecular compound (26). Phase-rule diagrams of the bile acids and of a large variety of all types of organic compounds probably indicate some form of combination, in spite of many thermodynamic impossibilities in the published diagrams, but there is no definite evidence for the existence in solution of a molecular compound between a bile salt and a water-insoluble substance. Indeed, the close parallelism between micelle formation (as indicated by freezing points (13), conductivity (23), etc.) and solubilizing action, and the absence of any simple integral ratio between the number of molecules of dye dissolved per molecule of bile salt and the great variety of substances that can be solubilized, make this explanation extremely doubtful. (3) In 1918 McBain, Beedle, and Bolam (12) suggested that a water-soluble substance could have its solubility increased by sorption on the exterior of the micelle, and Smith (25), in discussing his results on the solubility of organic liquids in 10 per cent sodium oleate, suggested that they displaced the water of hydration. However, in dealing with water-insoluble substances, sorption on the exterior of the micelle may be a minor factor. (4) Hartley (6, 7) considers the micelle to be a spherical droplet with all polar groups exposed, and solubilization to consist in true solution in the “tails.” Besides the steric difficulties involved in such a conception, since the average soap molecule is only three to five times as long as it is thick, the x-ray diagrams of Kiessig and Philippoff (9), Krishnamurti (lo), and Hess and Gundermann (8) in soap solutions as dilute as 7 per cent show the pattern typical of oriented lamellar crystals or liquid crystals. The solubility of Yellow AB in organic solvents varies widely and does not seem in general closely to approximate the mole ratios obtained for solubilizing solutions. Also, consideration of the extraordinarily diverse nature of all the detergents that solubilize in aqueous or nonaqueous solutions and of the variety of materials which they solubilize, possibly even including substances insoluble in paraffin, makes it almost impossible to accept solution as a general explanation. ( 5 ) The x-ray diagrams of Kiessig and Philippoff (9) for sodium oleate solution containing added benzene and of Palmer and Schmitt (24; see also 3) for aqueous cephalin and lecithin indicate that solubilization sometimes consists in the formation of polymolecular layers within the lamellar micelle. Dr. S. A. Johnston in this laboratory has found that the freezing point of potassium oleate is unchanged by adding enough isooctane to change it markedly if the isooctane remained as separate molecules. Clearly, solubilization results from participation in colloidal particles. These may preexist and merely be added to, or new ones of different character may be formed. The almost invariable presence of extremely minute, ultramicroscopically visible particles in solutions of the water-insoluble dye Yellow AB in aqueous solubilizing solutions, reported by Mrs. McBain in previous papers, has

18

R. C. MERRILL, JR., AND J. W. MCBAIN

been confumed and extended to solutions of another water-insoluble dye, Orange OT,in 1 per cent solutions of sodium lauryl sulfate and dodecyl benzene sodium sulfonate. The more hydrophobic molecules seem to be solubilized relatively more. Thus it seems likely that solubilizers containing a large portion of hydrophobic parts are the best.


SUWRY

1. Solubilization has been defined as and shown to consist in the spontaneous formation of thermodynamically stable colloidal particles of detergent and otherwise insoluble material. It is differentiated from other more or less related phenomena. 2. Data on the solubilization of the two water-insoluble dyes, Yellow AB and Orange OT, by twenty-eight pure and commercial detergents have been presented and discussed. 3. Experiments on the solubilization of liquids and on the passage of solubilized dyes through semipermeable membranes have distinguished this phenomenon from emulsification and from suspending or protective action, respectively. 4. Hydrotropy, the formation of a molecular compound or “choleic acid,” and true solution in the micelle have been shown to be unacceptable as general explanations of the mechanism of solubilization. For insoluble substances, sorption on the exterior of the micelle is possibly only a minor factor. In some cases solubilization consists in the formation of mono- and poly-molecular layers within the lamellar micelle. REFERENCES

(1) ALBERT:Australian J. Pharm. 49, 1160 (1934);J. SOC.Chem. Ind. 68, 196 (1939). (2) ALBERTAND RUBBO:Australian J. Pharm. 47,959 (1932). AND SCHMITT: J. Cellular Comp. Physiol. 17, 355 (1941). (3) BEAR,PALMER, (4) ENQLERAND DIECKROFF:Arch. Pharm. 230, 561 (1892). (6) HALL:U.8. patent 1,956,515(April 24, 1934). (6) HARTLEY:J. Chem. SOC.1936, 1968. (7) HARTLEY:Aqueow Solutions of Parafin Chain Salts. Hermann e t Cie, Paris (1936). (8) HESSAND GUNDERMANN: Ber. ?OB, 1800 (1937). Naturwissenschaften 27, 593 (1939). (9) KIESSIQAND PHILIPPOFF: (10) KRISRNAMURTI: Indian J. Phys. 3, 307 (1919). (11) LOTTERMOSER AND P~SCEEL: Kolloid-Z. 69, 175 (1933). (12) MCBAIN,BEEDLE,AND BOLAM:J. Chem. SOC.118,825 (1918);J. SOC.Chem. Ind. 40, 27T (1921). (13) MCBAINAND JOHNSTON: J. Am. Chem. SOC.,communicated. (14) MCBAINAND MCBAIN:J. Am. Chem. 800.68,2610 (1936). (15) MCBAIN,MERRILL,AND VINOQRAD:J. Am. Chem. SOC.62,2880 (1840). J. Am. Chem. Soc.88,670 (1941). (16) MCBAIN,MERRILL,AND VINOQRAD: (17) MCBAINAND O’CONNOR:J. Am. Chem. SOC.62,2856 (1940). (18) NEUBERQ:Biochem. Z.76,107 (1916). (19) PICEERINQ:J. Chem. SOC.111, 86 (1917). (20) PINK:J. Chem. SOC.1939, 53. J. Am. Chem. SOC.67,670 (1935);68,322(1936). (21) REEDAND TARTAR: (22) REITEMEIER AND BWEHRER: J. Phys. Chem. 44, 535 (1940).

SURFACE AREA OF MICROPOROUS SOLIDS

18


(23) ROEPKEAND MASON:J . Biol. Chem. 133, 103 (1940). AND PALMER: Cold Spring Harbor Symposia Quant. Biol. 8, 94 (1940). (24) SCHMITT J. Phya. Chem. 36, 1401, 1672, 2955 (1932). (25) SMITH: T h e Chemistry o j the Sterids. The Williams & Wilkins Company, Baltimore (26) SOBOTKA: (1938). (27) TARTAR A N D WRIGHT: J . Am. Chem. SOC. 61, 539 (1939). (28) YABHOFF AND WHITE: Ind. Eng. Chem. 32, 950 (1940).

T H E CALCULATIO?; OF THE SURFACE AREA OF 3IICROPOROUS SOLIDS FROM SIE.1SURESlESTS OF HEAT COSDUCTIVITY' S. S . KISTLER N o r t o n C o m p a n y , IVorcestcr, Massachusetts

Received J u l y 14, 1941

In 1934 it was demonstrated ( 5 ) that the avcragc diameter of the pore spaces in a microporous solid, such as an aerogel, can he characterized by a number that represents the mean free path of a gas in the pores at such a low pressure that every impact of each molecule is with the pore walls. It was also shown that this mcan free path characteristic of the particular porous solid can be readily calculated from measurements of the heat conductivity of the solid a t thrce suitably chosen gas pressures. In reality, the method uses the normal mean free path of the gas as a measuring stick for th'e pore spaces. Sincc a t atmospheric pressure this length is very near 10-5 cm. for air, it provides a means for measurement in a region well below the resolving power of the conventional microscope. By using moderate pressures, pore diameters can be measured with accuracy down to lo-' em., provided the heat conductivity of the solid structure is not so large as to mask that due to the gas in the pore spaces. This method of measurement is particularly applicable to the aerogels and xerogels. THE MEAN FREE PhTII OF THE PORE SPACES, I S D ITS SIGSIFICASCE

Consider that the air pressure within the pores is so low that each molecule bounces from wall to wall with a negligible number of intermolecular impacts in the free space. Now, if one should observe n molecules as each traverses a distance dz, it would be discovered that dn of these would impact a wall, and from kinetic theory these quantities are related by the equation :

in which 1 is the mean free path. 1 Presented a t the Eighteenth Colloid Symposium, which was held at Cornell University, Ithaca, New York, June 19-21, 1941.

Studies on Solubilization. - The Journal of Physical Chemistry (ACS

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