Critical solution temperatures of aliphatic compounds

MICROCHEMICAL

JOURNAL

7,

Critical

287-296 (1963)

Solution Aliphatic

I.

Homologous

and

Temperatures

of

Compounds’

Vinylogous

Fatty

Acids

and

Methyl

Esters

HARALD H. 0. SCHMID,~ HELMUT K. MANGOLD, AND W. 0. LUNDBERG liniversity

of Minnesota, The Home1 Institute, Austin, Minnesota Received August 5, 1963 INTRODUCTION

The mutual solubility of two liquids which are not miscible in all proportions is a function of the temperature. Rising temperature usually increases the solubility, possibly reaching a point, i.e., the upper critical solution temperature (CST), at which the two components become miscible in all proportions. The critical solution temperature is characteristic for the two liquids involved. For certain liquid systems, decreasing temperature increases the mutual solubility, possibly reaching a lower critical solution temperature. In some cases, both temperatures can be determined for the same pair of substances. This phenomenon has been thoroughly studied and discussed (1, 2, 4, 10, 12). Determinations of upper and lower critical solution temperatures are used, particularly in petroleum chemistry, for characterizing hydrocarbons, for studying the properties of selective solvents, and for estimating impurities and analyzing multiple mixtures (9). Applications in other areas of organic chemistry are rare. The method most widely used with practical precision is heating both liquids together in a test tube, stirring with a thermometer, and observing the temperature at which the two liquids just mix or just cloud (9). A micro technique described by Fischer (6-8) and adapted by the present authors simplifies rapid and accurate determinations of the upper Institutes 1 Supported by the U. S. Public Health Service, National grants AM-06674 and GM-5817, and by The Hormel Foundation. 2 Fulbright Scholar, 1962-1963. 287

of Health

288

H. H. 0. SCHMID,

H. K. MANGOLD,

AND

W. 0. LUNDBERG

critical solution temperature in a range between 20” and 230” C. The disappearance and reappearance of the meniscus between sample and test substance, which are sealed together in a capillary glass tubing and heated on a hot stage, is observed under a microscope and recorded as CST. The apparatus, amounts of sample, speed and accuracy of the determination are similar to the micro determination of melting points as described by Kofler (13). Minute amounts of sample (0.5-2 ~1) are required for one determination. These amounts can easily be isolated and purified by chromatographic methods. The determination of the CST is especially valuable for a substance which cannot be adequately characterized because of a very low melting point or polymorphism, or because it melts over a wide range. Also, by selecting a suitable test substance, variations in temperature range and sensitivity and accuracy of the method are possible. The CST values of fatty acids and methyl esters as presented here were obtained with ethylene glycol, nitromethane, and 1,3-butanediol. These three test substances permit testing and demonstration of the significance and applicability of the method. Systematic studies on homologous and vinylogous series aid in establishing the determination of the CST as a generally applicable analytical method. MATERIALS

AND

METHODS

Test Substances 1,2-Ethanediol (ethylene glycol) : Fisher Scientific Co., Fairlawn, New Jersey (certified, No. E-178); J. T. Baker Chemical Co., Phillipsburg, New Jersey (reagent, No. 9300); and Fluka A. G., Buchs, S. G., Switzerland (puriss. p.a., No. A 50297), tinzo= 1.4316 and dJ20 = 1.112. All samples exhibited a CST of 188’ C with benzene (J. T. Baker Chemical Co., thiophene free, reagent, No. 9154). Nitromethane: Fisher Scientific Co. (certified, No. N-98) and Matheson, Coleman and Bell, Norwood, Ohio, nnzo = 1.3820 and d420= 1.134. They exhibited a CST of 109’ C with water. 1,3-Butanediol: Matheson, Coleman and Bell, Norwood, Ohio (No. BX 1640), and Fluka A. G., (puriss., No. A 51581). Purified fractions of the former obtained by vacuum distillation; the latter, as received, showed nn20 = 1.4402, and dh20= 1.006. Their CST with benzene (J. T. Baker Chemical Co., reagent, No. 9154) was 78” C.

CST

OF

ALIPHATIC

COMPOUNDS.

I.

289

Samples Most fatty acids and methyl esters were obtained from The Hormel Foundation, Austin, Minnesota. Other preparations from Lachat Chemical Co., Chicago 43, Illinois; and Applied Science Laboratories, State College, Pennsylvania, are indicated as (L) and (A) in the tables. Methyl hexadecadienoate and methyl hexadecatrienoate were prepared from the alga Chlorella pyrenoidosa. Adsorption chromatography on thin layers of Silica Gel G (14) was used to analyze each sample. The same method was applied for purifying unsatisfactory preparations. In addition, reversed phase partition chromatography on siliconized Whatman No. 1 paper (1.5) and gas liquid chromatography (Beckman GC 2) (5, 11) were employed as analytical procedures. Apparatus A Reichert microscope type RCH” equipped with a heating stage as described by Kofler (13) was used. A standard mercury in glass thermometer was graduated in 1” C intervals ranging from 20” to 230’ C. Temperature readings could be estimated within 0.5” C. Capillaries of 0.2-0.3 mm inner diameter were made from glass tubing. A 26 X 38 mm microscope slide was adapted for holding the capillaries on the heating stage. Two glass strips (38 X 4 X 1 mm) were glued on to the microscope slide with water glass leaving a channel of 0.5 mm width. The slide was then placed into the shifting device of the heating stage. The melting points of seven different compounds in capillaries were used to determine the accuracy of the temperature readings of the apparatus over the temperature range used. Techniques The liquid sample and the test substance are introduced into a capillary tubing by dipping the latter consecutively into both liquids. Each liquid may be drawn into the capillary to a height of about 7-10 mm. The ratio of the volumes is not important because only a small volume on either side of the interface is involved in the mixing process (6). The capillary is sealed with a micro burner at both ends to a length of 303 C. Reichert, Optische Werke A. G., Hernalser Hauptstr. 219, Wien 17, Austria; U.S. Representative: William J. Hacker & Co., Inc., P. 0. Box 646, West Caldwell, New Jersey.

290

H. H. 0. SCHMID,

H. K. MANGOLD,

AND

W. 0. LUNDBERG

35 mm. The closed capillary should contain no more than about half its volume of liquids to permit their expansion. Samples which are solid at room temperature are melted on a glass slide over a micro burner. The melt is drawn up into the capillary which already contains the liquid test substance. Because the sample solidifies immediately after it has ascended, thus preventing proper sealing, it is liquified again in the capillary through contact with a hot glass strip. The capillary is then easily sealed. If the two compounds in the capillary are separated by air, they are brought into contact by centrifugation. Before centrifuging, solids may be liquified again by inserting the sealed capillary into a centrifuge glass which contains water and is then heated just above the melting point of the sample. Centrifuging is also employed to separate two liquids which form several segments in the capillary, because a number of interfaces migrating during the heating process could prevent exact observation of the CST. When working with substances that may react with each other, the sealed capillary is inserted with the heavier compound, e.g., ethylene glycol, toward the outside to minimize contact of the two liquids while centrifuging. The capillary tubing is inserted into the channel of the microscope slide and heated on the stage. The interface between the two liquid phases is observed under the microscope. It remains visible as meniscus during the heating process, although the components begin to dissolve in each other. The meniscus disappears at the CST with a characteristic movement, and upon cooling reappears at the same temperature and in the same place in the capillary. Some difficulty in observing the meniscus may occur in certain cases in which the refractive indices of both compounds become identical, making the interface invisible. This is observed, for example, with the pair methyl arachidate/l,3-butanediol at temperatures below the CST. No characteristic movement is observed in a certain temperature range and the meniscus becomes visible again upon further heating. If identity of refractive indices occurs near the CST, exact temperature readings are difficult. Adding a small amount of dye to the test substance may permit visualizing the disappearing of the interface but may also influence the CST. No change in the temperature readings with methyl esters was observed if 0.5% w/‘v of methylene blue was added to the 1,3butanediol. Repeating the heating process with the same sample should yield identi-

CST OF ALIPHATIC

COMPOUNDS.

291

I.

cal temperature readings at the CST; if not, decomposition or reaction between the compounds may be assumed. The amounts of “impurities” formed by reactions between sample and test substance, e.g., fatty acidethylene glycol, can be retarded if the capillary is placed on the pre-

CRITICAL

SOLUTICN

TABLE 1 TEMPERATURES OF FATTY ACIDS WITH AND NITROMETHANE

ETHYLENE

GLYCOL

CST Ethylene glycol

Acid Octanoic (caprylic) Decanoic (capric) Hendecanoic (L) Dodecanoic (lauric) Tridecanoic (L) Tetradecanoic (myristic) Pentadecanoic (L) Hexadecanoic (palmitic) Heptadecanoic (margaric) Octadecanoic (stearic) Eicosanoic (arachidic) Docosanoic (behenic)

49 85 111.5 132.5 149.5 167 181.5 208 -a

(L)

9-Hexadecenoic (palmitoleic) 6-Octadecenoic (petroselinic) cis-9-Octadecenoic (oleic) trans-9-Octadecenoic (elaidic) 1 I-Eicosenoic 13-Docosenoic (erucic) 15-Tetracosenoic (nervonic) Hexadecadienoic (palmitolinoleic) 9,12-Octadecadienoic (linoleic) Hexadecatrienoic (palmitolinolenic) 9,12,15-Octadecatrienoic (linolenic) Eicosatetraenoic (arachidonic) Eicosapentaenoic Docosahexaenoic a Dashes indicate because of reaction.

that reproducible

129 166.5 166.5 169.5 193

-

105 145 88 130 -

temperature

readings

Nitromethane 32.5 58.5 68 76.5 84 91.5 98 104 109 113.5 121 128 84 96 96.5 100 106 116 125 66 80 46 64 64.5 46.5 44.5

could not be obtained

heated stage at a temperature just below the expected CST in order to shorten the heating process. A first test gives, in this case, the approximate CST.

292

H. H. 0. SCHMID,

H. K. MANGOLD,

AND

W. 0. LUNDBERG

RESULTS

CST values of fatty acids with ethylene glycol and with nitromethane are listed in Table 1; those of methyl esters with 1,3-butanediol, ethylene glycol, and nitromethane in Table 2. TABLE 2 CRITICAL SOLUTION TEMPERATURES OF METHYL ESTERS WITH 1,3-BUTANEDIOL, ETHYLENE GLYCOL, AND NITROMETHANE CST Methyl

1,3-Butanediol

ester

Me-Pentanoate (valerate) (L) Me-Hexanoate (caproate) (L) Me-Octanoate (caprylate) Me-Decanoate (caprate) Me-Hendecanoate (L) Me-Dodecanoate (laurate) Me-Tridecanoate (L) Me-Tetradecanoate (myristate) Me-Pentadecanoate (L) Me-Hexadecanoate (palmitate) Me-Heptadecanoate (margarate) Me-Octadecanoate (stearate) Me-Eicosanoate (arachidate) Me-Docosanoate (behenate)

(L)

Me-9-Tetradecenoate (myristoleate) (A) Me-9-Hexadecenoate (palmitoleate) Me-cis-9-Octadecenoate (oleate) Me-cis-II-Octadecenoate Me-cis-12-Octadecenoate Me-cis-6-Octadecenoate (petroselinate) Me-trans-9-Octadecenoate (elaidate) Me-ll-Eicosenoate Me-cis-13-Docosenoate (erucate) Me-15-Tetracosenoate (nervonate) Me-Hexadecadienoate (palmitolinoleate) Me-9,12-Octadecadienoate (linoleate) Me-Hexadecatrienoate (palmitolinolenate) Me-9,12,15-Octadecatrienoate (linolenate) Me-Eicosatetraenoate Me-Eicosapentaenoate Me-Docosahexaenoate

30 44 69 92 102.5 113 122 131.5 140 148.5 156 163

(17’5) (187) 119 135 150 150 150 150 153.5 163.5 175 186 128 141.5 120 132.5 143 136 143.5

Ethylene glycol 135 159 196.5 224 CST Nitromethane 21.5 34 44 53.5 62 78.5 91.5 19 41 41 41 41 46 58S 75 88.5

In Fig. 1, CST values with ethylene glycol and nitromethane are plotted for the homologous series of saturated fatty acids and methyl

CST OF ALIPH.~TIC

COMPOUNDS.

I.

293

esters. Figure 2 shows the CST values of vinylogous series of fatty acids with nitromethane, plotted against the total number of double bonds in each compound. CST PC1

200

0/

IS0

100

-

50

0

/

0/ /O / 8 o/O . /t/L /’ ./ 0’ 0’ / o/ i .” 0

/

D

/ i

6

8

IO

12 TOTAL

14 NUMBER

16

I8

20

22

OF C ATOMS

FIG. 1. Critical solution temperatures of saturated fatty acids and methyl esters with ethylene glycol and nitromethane. (A) Fatty acids with nitromethane; (B) fatty acids with ethylene glycol; (C) methyl esters with ethylene glycol; (D) methyl esters with nitromethane. DISCUSSION

Critical solution temperatures with polar test substances obviously depend on chain length, polarity, and number of double bonds of the

294

H. H. 0. SCHMID,

H. K. MANGOLD,

AND W. 0. LUNDBERG

sample. This is evident from Tables 1 and 2 and Figs. 1 and 2. Unlike the melting points, no alternating behavior between the members of a homologous series is observed; this makes predictions of CST values possible. Cis and tram isomers can be distinguished by their CST. It is also evident that different compounds, e.g., methyl octadecanoate and methyl eicosenoate, may yield similar or identical CST values with ethylene glycol. The application of two different test substances, therefore, can facilitate distinction and identification of a compound. Other

150

100

50

FIG. 2.

-

0

Critical

0

0

I

I

I

I

I

I

I

0

1

2

3

4

5

6

solution

DOUBLE BONDS temperatures of fatty acids with

nitromethane.

methods such as the micro determination of refractive index or melting point (13) can be used as supplementary procedures. After determination of the CST, the sample may be recovered from the capillary for further analysis. Compounds which are isolated after separation on thin layers or by means of gas liquid chromatography can be further characterized by their CST.

CST OF ALIPHATIC

COMPOUNDS.

I.

295

The accuracy of the temperature readings depends on the purity of both components involved. The reliability also decreases with temperatures above 200” C. In the temperature range and with the test substances and samples used in this study, the experimental error does not exceed i 0.5” C. The CST values are well defined and reproducible. Critical solution temperatures of several fatty acids with nitromethane, obtained by means of the synthetic method, have been reported (3) ; they agree to some extent with the CST values listed in Table 1. Other compounds present as impurities in the sample or test substance will influence the CST value. Both higher or lower temperature readings than with the pure compound may be obtained depending on the properties and the amounts of contaminations. Also, reaction might be observed which makes exact CST determination impossible. The CST may yield some information concerning the nature or amount of impurities when it is used for analyzing multiple mixtures (6, 9). It is possible that a mixture of compounds would yield the same CST as one of the pure components, but usually identity and purity of a compound are ascertained if complementary purification techniques have not yielded fractionation and if it yields an accurate critical solution temperature. SUMMARY The upper critical solution temperature (CST) was used as a physical characteristic for assessing the identity and purity of organic compounds, CST values of highly purified fatty acids and methyl esters with ethylene glycol, nitromethane, and I,& butanediol were determined under the microscope. The technique used is described in detail and applications of the method for ascertaining the identity or purity of fatty acids and their methyl esters in amounts of OS-2 ul are discussed. REFERENCES 1.

2. 3.

4. 5.

6.

ATACK, D., AND RICE, 0. K., The interfacial tension and other properties of the cyclohexane + aniline system near the critical solution temperature. Discussions Faraday Sot. 15, 210-218 (1953). BARKER, J. A., AND FOCK, W., Theory of upper and lower critical solution temperatures. Discussions Faraday Sot. 15, 188-195 (1953). BROUGHTON, G., AND JONES, D. C., Critical solution temperatures of some fatty acids with nitromethane. Trans. Faraday Sot. 32, 685-689 (1936). BRUSH, S. G., Statistical thermodynamics of mixtures, Trans. Faraday Sot. 54, 1781-1785 (1958). CRAIG, B. M., AND MURTY, N. L., The separation of saturated and unsaturated fatty acid esters by gas-liquid chromatography, Can. /. Chem. 36, 1297-1301 (1958). FISCHER,

R. W., The analysis of fluid mixtures

and solutions

by means of the

296

7.

8.

9. 10.

11. 12.

13.

14.

15.

H. H. 0. SCHMID,

H. K. MANGOLD,

AND

W. 0. LUNDBERG

critical solution temperature. In “Microchemical Techniques” (N. D. Cheronis, ed.), pp. 977-989. Wiley (Interscience), New York, 1962. FISCHER, R., AND KARASEK, G., Die kritische Mischungstemperatur als Hilfsmittel zur Kennzeichnung und Bestimmung kleinster Fliissigkeitsmengen. Mikrochemie ver. Mikrochim. Acta 33, 316-327 (1947). FISCHER, R., AND MOSER, H., ifber die Mikrobestimmung des Anilinpunktes bei Benzinen und den Ersatz des Anilins durch andere polare Lb;sungsmittel. Erdoel Kohle 9, 377-380 (1956). FRANCIS, A. W., “Critical Solution Temperatures,” 246 pp., No. 31, Advances in Chemistry Series. American Chemical Society, Washington, D. C., 1961. HILDEBRAND, J. H., AND SCOTT, R. L., “Solubility of Nonelectrolytes,” American Chemical Society Monograph Series, Third Edition, 488 pp. Reinhold, New York, 1950. HORNING, E. C., MOSCATELLI, E. A., AND SWEELEY, C. C., Polyester liquid phases in gas-liquid chromatography. Chem. Ind. 1959, 751-752. KIENITZ, H., Bestimmung der Lijslichkeit, C., Liislichkeiten von Fliissigkeiten in Fliissigkeiten. In “Methoden der Organischen Chemie (Houben-Weyl) ,” 4. Auflage (Herausgeber E. Miiller) Vol. 3/l, pp. 234-239. Georg Thieme Verlag, Stuttgart, 1955. KOFLER, L., AND KOFLER, A., “Therm0-Mikro-Methoden zur Kennzeichnung Organischer Stoffe und Stoffgemische,” 608 pp. Universititsverlag Wagner, Innsbruck, 1954. MANGOLD, H. K., Aliphatische Lipide. In “Diinnschicht-Chromatographie, Ein Laboratoriumshandbuch” (Herausgeber E. Stahl), pp. 141-192. Springer Verlag, Berlin, GGttingen, Heidelberg, 1962. MANGOLD, H. K., LAMP, B. G., AND SCHLENK, H., Indicators for the paper chromatography of lipids. J. Am. Chem. Sot. 77, 6070-6072 (1955).