Solubility of alkali metal carboxylates in hydrocarbons

JOURNAL OF COLLOID SCIJgNCE 17, 857-864 (1962)

SOLUBILITY OF ALKALI METAL CARBOXYLATES IN HYDRO CARBON S Erik Kissa Research and Development Division, Organic Chemicals Department, E. I. du Pont de Nemours and Company, Wilmington, Delaware Received November 28, I961; revised April 24, 1962 ~BSTRACT The solubility of alkali metal, m a i n l y lithium, carboxylates in n-heptane, isooctane, or benzene has been investigated. Regardless of s t r u c t u r e v a r i a t i o n s the solubility of pure aliphatic or Micyclic alkali m e t a l carboxylates was found to be below 10-4 m o l e / h at 27.0°C. I t is shown t h a t erroneously high results can be o b t a i n e d

by (a) the presence of trace amounts of impurities, (b) incomplete separation of the liquid and solid phases, or (c) the instability of the colloidal dispersion. The solubility of alka]i metal naphthenates or arylstearates is related to the fact that they are mixtures. The exceedingly low solubility of pure alkali metal carboxylates is explained by their inability to form stable micelles in hydrocarbon media. INTRODUCTION Solutions of soaps in hydrocarbons have been extensively studied, and the understanding of their nature has been significantly advanced (1-11). However, little information has been available on the factors causing solubility. Whereas in general alkali metal carboxylates are believed to be insoluble in hydrocarbons at room temperature (12-15), alkali metal salts of naphthenie or arylstearie acids have been reported to be soluble (1, 2, 6, 16). Undoubtedly the term solubility has been used in a broad sense. Various methods have been applied for the estimation of the solubilities of alkali metal carboxylates in hydrocarbons. These methods include measurements of the temperature at which a previously heated soap-solvent mixture of known composition becomes anisotropic upon cooling (12, 14, 17-19), and an estimation b y means of an electron microscope (20). The existing perplexity could be avoided by reserving the term solubility for stable solutions only. Therefore, solubility is defined as the equilibrium concentration of a compound which dissolves in one or more compounds forming a single thermodynamically stable phase without a chemical reaction. Tiffs definition includes colloidal solutions which are formed spontaneously and are in true and reversible equilibrium (21). As shown by MeBain (22), association colloids can meet these requirements. 857

858

KISS~ EXPERIMENTAL

Preparation of Carboxylates Each parent acid was purified by either crystallization from a suitable solvent, by fractionM distillation, or by both methods. The purity of each acid was established by determination of the boiling point or the melting point, by determination of the neutralization equivalent, and by C and H analyses. The alkali metal carboxylates were prepared by one of the two methods: (1) titration of the acid with the metal hydroxide past the potentiometric end point and back titration with the acid to the exact end point, or (2) reaction between the acid and the metal in a hydrocarbon solvent. The Mkali metal carboxylates were purified by crystallization from isopropyl alcohol or some other suitable solvents and dried at 105°C. prior to use. The salts were analyzed for the metal content, for uncombined acid, and, in some eases, for C and H. It became apparent, however, that solubility values are more sensitive to impurities than analytical data. Therefore, a salt was considered pure if (a) further purification did not change its solubility or (b) if the solubility of the salt became immeasurably low as a result of purification, and (c) if the concentration of the salt in the hydrocarbon phase was independent of the amount of solid phase present.

Solvents Benzene, n-heptane, and isooctane were dried over metallic sodium, purified by fractional distillation, kept over metallic sodium, and distilled immediately before using.

Determination of Solubilities The anhydrous alkali metal carboxylate and the solvent were transferred to a stoppered centrifuge tube and placed in a constant-temperature bath at 27.0 ° ~ 0.1°C. A motor was used to rock the bar to which eight clamps, each holding one centrifuge tube, were attached. After 24 hours rocking the solution was allowed to stand without further agitation. For sampling the centrifuge tube was quickly placed in a thermostated centrifuge and the solution centrifuged in a gravitational field of 1200 g for 30 minutes. The supernatant liquid was analyzed for the alkali metM content by flame photometry. An accuracy of =t=2.10-5 mole/1, was attained by introducing various refinements of the analytical method. Although the solubilities of some salts were of the same order, a more sensitive quantitative method was not known. Equilibrium was established by periodic sampling or by approaching it from the sides of undersaturation and supersaturation.

SOLUBILITY OF ALKALI METAL CARBOXYLATES IN HYDROCARBONS 859 RESULTS AND DISCUSSION

Solubility of Pure Alkali Metal Carboxylates P u r e alkali m e t a l carbo×ylates can be considered insoluble at room t e m p e r a t u r e in n - h e p t a n e , isooetane, a n d b e n z e n e for all practical purposes. T h e change of solubilities w i t h m o d i f i c a t i o n of the s t r u c t u r e are so small in a b s o l u t e t e r m s t h a t possible t r e n d s are almost invisible. N e l s o n a n d P i n k (3) have s h o w n t h a t the solubilities of h e a v y m e t a l c a r b o x y l a t e s in n o n p o l a r solvents increase w i t h the increasing l e n g t h of t h e c a r b o n chain. T h e solubilities of n o r m M l i t h i u m earboxylates (Table I) in h y d r o c a r b o n s are too low to p e r m i t a similar conclusion. TABLE I

Solubility of Normal Li Carboxylates Compound

Li Li Li Li Li

Solubility (mole/l. X 10-6) at 27.0°C. in

No. of carbon atoms

acetate eaproate eaprate stearate oleate

Benzene

n-Heptane

Isooctane

<1 4 3 2 4

<1 1 1 4 1

<1 1 3 4 1

2 6 10 18 18

TABLE II

Solubility of Branched Chain Li Carboxylates Compound

Li Li Li Li Li Li Li Li Li Li Li Li Li Li Li Li

isobutyrate isovalerate 2-methylbutyrate pivalate 2,2-dimethylbutyrate 2-methyl-2-ethylbutyrate 2-ethylhexanoate 3,5,5-trimethylhexanoate 2,2,4,4-tetramethylpentanoate 2-isobutyl-4-methylpentanoate 2-propylheptanoate 2-isopropyl-5-methythexanoate 4-ethyl-5,5-dimethylhexanoate 2-rnethyldodecanoate 2-hexyldecanoate 2,2-dimethylstearate

No. of carbon atoms 4 5 5 5 6 7 8 9 9 10 10 10 10 13 16 20

Solubility (mole~1. X 10-5) at 27.0°C. in Benzene n-Heptane Isooctane

2 2 1 3 1

<1 <1 <1 1 1

4

1

8

7

1 1 1 1 3 1 4 4 1 8 3 4 3 11 2 11

860

KISSA

The branched chain lithium earboxylates investigated (see Table II) had 4 to 20 carbon atoms, featuring a main chain with 3 to 18 carbon atoms. The alkyl branches with one to six carbon atoms included isopropyl and isobutyl groups. The location of the branching was varied from the a to the 3' position. Regardless of these variations of the structure the solubilities of the branched chain lithium earboxylates were found to be very low in each hydrocarbon solvent used. The solubilities of alieyelic lithium earboxylates (Table III) are of the same order as the solubilities of aliphatie lithium carboxylates. Not enough data were obtained to warrant any general conclusions concerning the effect of functional groups. However, the introduction of a --C1, - - O H , --Nt-I~ or --OCI-I(Ct-I3)2 group did not increase significantly the solubility of some simple lithium earboxylates listed in Table IV. The solubilities of pure sodium and potassium earboxylates, as exemplified by an aliphatic and an alicyelic salt, are so low that the effect of the cation on solubility is not detectable by the analytical method used (Table V). TABLE III Solubility of Alicyclic Li Carboxylates

Compound

Solubility (mole/l. X 10-5) in isooctane at 27.0°C.

Li 2-methyl-2(2', 2'-dimethyl)-propylcycloprop~nec~rboxylate Li cyclopentanec~rboxylate Li 1-methylcyclopent~necarboxyl~te Li campholate Li isofencholate Li cyclohexanecarboxylate Li 1-naphthoate

11

TABLE IV Solubility of Substituted Li Carboxylates

Compound Li pivalate Li 2, 2-dimethyl-3-ehloropropionate Li 2,2-dimethyl-3-hydroxypropionate Li isovalerate Li 2-aminoisovalerate Li isobutyrate Li 2-isopropoxybutyrate

Solubility in isooctane

(mole/l. X 10~5) at 27.0°C.

SOLUBILITY

OF

ALKALI

METAL

TABLE

861

IN HYDROCARBONS

CARBOXYLATES V

Effect of the Cation on Solubility Solubility in isooctane

(mole~1. XIO -5) at 27.0°C,

Metal

Li Na K

Pivalate

Campholate

1 1 3

4 2 2

Effect of Impurities Even trace amounts of impurities can have a pronounced effect upon the solubility of alkali metal carboxylates. An agreement between experimental data of elemental and functional group analyses and the theoretical values is not a sufficient criterion of purity. The solubilities of analytically pure lithium 2-isopropyl-5-methylhexanoate and lithium isofencholate decreased 30 and 50 times after further purification by recrystallization from isopropyl alcohol: Analyses Calc.

Li 2-isopropyl-5methylhexanoate Li isofencholate

C 67.4 H 10.8 Li 3.89 Acid 0.00 C 68.2 H Li

9.72 3.92

Acid 0.00

Found (lst recryst.)

Number of recryst,

Solubility in isooctane at 27°C.

(mole/t. X 10-9

67.7 10.9 3.75 Nil 68.3

1 2 4

1.2 0.5 0.04

1

1.0

9.90 4.06

3

0.02

Nil

This observation led us to investigate the solubility of mixtures of pure carboxylates. Indeed, the solubility of a mixture of carboxylates featuring a branched aliphatic chain or an alkyl substituted alicyclic ring exceeds by far the sum of individual solubilities in the same solvent (23). Thus the relatively high solubility of alkali metal naphthenates (1, 16) in hydrocarbons can now be explained. The naphthenates are salts of a complex acid mixture which is composed of alieyelic, branched chain aliphatic, and normal chain aliphatie acids (24). The solubility data shown above include pure salts of (a) acids which have been identified as components of the naphthenic acid mixture, e.g., cyclopentanoic and cyclohexanoie acids, and (b) salts of acids which are closely related to the known

862

KISSA

components of the naphthenie acid mixture. The exceedingly low solubility of these pure salts indicates that the high solubility of alkali metal naphthenates in hydrocarbons is not caused by a favorable structure alone. Rather the fact that naphthenates are mixtures seems to be responsible for their high solubility in hydrocarbons. The alkali metal salts of arylstearic acids are also appreciably soluble in hydrocarbon solvents (1, 2, 6). In common with naphthenates they are not pure compounds but mixtures of isomers not related to naphthenates by structure. Lithium phenylstearate dissolved in isooetane without agitation forming a solution which was stable over a long period of time: Time

Concentration in isooctane at 27.0°C. after centrifuging (mole/l. X 10~)

2 days 3 days one year

0.24 0.53 2.2

A definite value for the solubilities of alkali metal arylstearates could not be found because the solutions became viscous upon concentration finally approaching a gel.

Unstable Dispersion During the measurement of solubilities it became necessary to distinguish between stable solutions and unstable dispersions of alkali metal carboxylates in hydrocarbons. Both are clear to the naked eye and pass a filter with a maximum pore size of 10 ~. The unstable dispersions do no~ form spontaneously; either heating or stirring or some pretreatment of the solid carboxylate is required: 1. Heating followed by agitation of the cooling carboxylate-hydrocarbon mixture. Using this method clear dispersions were prepared which contained, after filtration, 4.1 X 10-3 moles/1, of Li campholate in benzene. However, upon centrifugation in a gravitational field of 800 g sedimentation occurred, leaving only a trace amount of the solute in solution. 2. A treatment of the carboxylate with a hot solvent in a Blount extractor (25) combines two effects which facilitate dissolution: heat and a relatively high solid-solvent ratio. The following compounds were treated 16 hours with hot isooctane and the resulting dispersions were centrifuged after cooling to room temperature: Compound Li isobutyrate Li 2-methyldodeeanoate Li 2,2-dimethylstearate

Concentration of the dispersed carboxylate (mole~1. X 10~3) 0.04 6.0 255

SOLUBILITY OF ALKALI METAL CAt~BOXYLATES IN HYDROCARBONS ~ 3

The dispersions were not stable. The dispersed carboxylate could be removed by repeated centrifugation after time intervals. 3. Some alkali metal carboxylates become readily dispersible after a treatment with a hot hydrocarbon solvent. Lithium 2-methyldodeeanoate, which had been treated with hot isooctane in an extractor and dried to remove the solvent, could be dispersed by mild agitation at room temperature. Repeated sampling revealed that the dispersion was not stable: Time

(days)

Concentration of the solute at 27°C. after centrifugation (mdefl. X 10-3)

2 4 7 11 25

1.05 1.0O 0.82 0.55 0.23

Apparently the crystal lattice is loosened by the hot solvent. The reoriented surface-structure is dispersed as unstable micelles which are subject to further aggregation in the liquid phase. The dispersions of pure alkali metal earboxylates in hydrocarbons appear to be unstable for one of the two reasons: 1. The aggregates or erystallites in the hydrocarbon phase are too large to remain dispersed. Centrifugation accelerates their sedimentation, which otherwise may be slow. 2. The original mieelles are sufficiently small and do not settle out. However, their growth is not confined by some limiting mechanism. Upon standing further aggregation occurs with consequent segregation. ACKNOWLEDGMENT The author is indebted to Maimu S. Yllo for analyses by flame photometry and to W. J. Balon, A. W. Bauer, W. A. Blanchard, M. R. Kegelman, G. B. Robbins, C. A. Sandy, and M. S. Whelen for the preparation of several compounds. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

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