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Solubilization of lithium carboxylates with carboxylic acids in hydrocarbon solvents
JOURNAL OF COLLOID SCIENCE 18, 147-156 (1963)
SOLUBILIZATION OF LITHIUM CARBOXYLATES WITH CARBOXYLIC ACIDS IN HYDROCARBON SOLVENTS
Erik Kissa Research and Development Division, Organic Chemicals Department, E. I. Du Pont de Nemours and Company, Wilmington, Delaware Received May 4, 196~; revised September 19, 196P ABSTRACT The extent of solubilization of lithium carboxylates with carboxylic acids in hydrocarbon solvents depends greatly upon the structure of the anion, and, to a lesser degree, upon the structure of the hydrocarbon solvent. Branching of the aliphatic carbon chain or the alicyclie carbon ring of the carboxylate and the carboxylic acid molecules is essential for solubilization. Noncorresponding acids or mixtures of acids solubilize a relatively larger amount of a carboxylate than the corresponding acid. Appreciable amounts of normal carboxylates are soluble only when the acid is a substantial fraction of the liquid phase. The mechanism of the solubilization of lithium carboxylates with earboxylic acids involves the cooperative formation of association mieelles governed by the law of chemical equilibrium. The solutions are thermodynamically stable. INTRODUCTION
Although it has been generally known that carboxylic acids increase the solubility of their alkali metal salts in hydrocarbon solvents, the scope of this phenomenon and the stability of the solutions obtained have not been studied. Several investigators (1-8) working with normal sodium and potassium carboxylates have reported an enhancement of solubility in hydrocarbon solvents by carboxylic acids. McBain and Eaton (1) found that potassium laurate became soluble in benzene upon addition of a large portion of laurie acid. By cooling a homogeneous mixture of benzene, laurie acid, and potassium laurate they obtained crystals having the composition CuH~aCOOK. CuH~3COOH and attributed the solubility increase to the formation of the acid soap. Lawrence (6, 7) found that soaps dispersed in oil are readily peptized by small amounts of polar substances, including fatty acids. Sodium stearate appeared to form a complex with one molecule of stearic acid. Sat5 (8) reported that various sodium and calcium soaps associate with polar substances having OH or COOH groups and form stable gels in spindle oil. More recently Singlcterry and coworkers (9-13) studied the effect of acid upon micelles of acylstearate soaps in hydrocarbon solvents and concluded that acid was taken up by 147
148
K~SSA
the micelles. From vapor concentration measurements Bascom and Singleterry (12) concluded that acetic acid is held by sodium dinonylnaphthalene sulfonate, magnesium dinonylnaphthalene sulfonate, or magnesium phenylstearate micelles in benzene by a coordination mechanism. For the bonding of the acid molecules a definite number of coordination sites are available. The excess acid is held by considerably weaker forces. It was shown in a previous publication (14) that the solubilities of pure alkali metal carboxylates in hydrocarbon solvents are less than 10-4 M at 27.0°C. The solubilization of these salts by carboxylic acids has now been studied. It was found quite unexpectedly that considerable amounts of alkali metal carboxylates are solubilized only when both the salt and the acid have a branched carbon chain. Furthermore, the size of the metal ion of the carboxylate was found to exert a strong influence upon solubilization by acids. Our attention was devoted first to lithium carboxylates (15) because they required the smallest relative amount of acid for their solubilization. EXPERIMENTAL The preparation of the pure carboxylic acids, their salts and the solvents, and the method for the determination of solubilities have been described in a previous publication (14). The total concentration of acid in the liquid phase was determined by a microtitration with potassium hydroxide in isopropyl alcohol. The solubility determinations were carried out at 27.0°C. Equilibrium was approached (a) from the side of undersaturation by adding salt to a solution of acid in the hydrocarbon solvent or (b) from the side of supersaturation by diluting a concentrated solution and seeding with a small amount of the carboxylate. Precautions were taken to exclude water. Considering the long equilibration time (up to one year) and manipulation during sampling, presence of traces of moisture cannot be ruled out. The effect of small amounts of water is being investigated and the results will be published at a later date. RESULTS AND DISCUSSION
Nature of the Solutions The solubilization of lithium carboxylates with carboxylie acids involves a cooperative formation of micelles. The association of salt and acid molecules is spontaneous. Agitation is not necessary although it increases the rate of dissolution. The saturated solutions are in true and reversible equilibrium and essentially the same solubility values are obtained regardless of whether the equilibrium is approached from the side of supersaturation or undersaturation. As a further indication of stability no precipitation occurred during four years of storage. Centrifugation in a gravitational field of 1200 g does not cause sedimentation, at least not during 1 hour.
SOLUBILIZATION OF LITHIUM CARBOXYLATES
149
Considering the existence of a reversible equilibrium the solubilization of lithium carboxylates by carboxylic acids can be described by the following over-all scheme: (RCOOH)2
(RCOOLi)m• (RCOOH),
ooY RC00Li. RC00H ~.............
....................
RC00Li .RC00H
'
lI )li
RC00Li
+
RC00H
Jl .....................
RCOOLi
Particularly at higher acid concentrations the solid phase consists, at least partially, of the acid salt. The acid salt RCOOLi. RCOOH as such is only slightly soluble in the hydrocarbon solvent. If the acid salt were soluble in the undissociated or unassociated state, the molar salt-acid ratio in the liquid phase would approach unity at low concentrations below the CMC. This was not found to be the case with any of the salts studied. Actually, a lithium carboxylate which forms a very stable acid salt is precipitated by the equivalent amount of acid, e.g., when 0.076 mole/1, of Li acetate was added to 0.172 mole/l, of acetic acid in isooctane, 0.091 mole/l, of acetic acid and only 0.00008 mole/1, of Li acetate remained in solution. The liquid phase always contains a small amount of acid not associated with the salt. The presence of acid in the intermicellar liquid introduces difficulties in obtaining accurate measurements of the mieellar weight. For the application of colligative methods the concentration Cz of the acid not present in micelles and the effect of excess acid upon the mieellar weight have to be known. On the basis of cryoseopie and ebullioseopic measurements it can be estimated that the micellar weights of lithium 2-ethylhexanoate and lithium campholate solubilized by their corresponding acids in benzene were above 1000. Mieellar weight measurements based on light scattering were complicated in addition to the possible effect of acid by a relatively low difference between the refractive indices of the micelles and the solvent. The results indicate that the micellar weights of lithium 2-ethylhexanoste and lithium 2-hexyldecanoate solubilized by their corresponding acids in isooctane are below 10,000. Undoubtedly the mieelles are relatively small.
150
I~ISSA
Effect of the Structure of Aliphatic Anions T h e e x t e n t of solubilization of l i t h i u m carboxylates b y carboxylic acids d e p e n d s greatly u p o n the s t r u c t u r e of the anion. B r a n c h i n g of t h e c a r b o n c h a i n of aliphatic alkali m e t a l c a r b o x y l a t e s a n d of t h e i r p a r e n t acids a p p e a r s to be essential for t h e solubilization m e c h a n i s m to become operative. F r o m the solubility d a t a several conclusions emerge, S u b s t a n t i a l a m o u n t s of n o r m a l c h a i n carboxylates are solubilized only a t high acid c o n c e n t r a t i o n s (Table I ) . U n s a t u r a t i o n i n t h e n o r m a l c a r b o n c h a i n e n h a n c e s t h e TABLE I
Solubilization of Li Carboxylates at 27.0°C. by the Corresponding Acid Salt
Normal Li Acetate
Conc. of Acid M X 103 C8
Solub.of Salt M :K 10a C.
Cs Ca
91 43 2040 21 37a
0.07 9.9 0.04 84 0.03 0.01
0.0008 0.0044 0.001 0.041 0.001 0.0003
Isooctane
18 950
0.27 20
0.015 0.03
Benzene n-Heptane ~sooctane
3.4 3.8 4,2 28 680 98 760
Solvent
Isooctane
2220 Li caproate
Isooctane
Li stearate
Isooctane
Normal, unsaturated Li oleate Branched Li 2-Ethylhexanoateb
Li pivalateo
Isooetane
Alicyelie Li eyelopent anecarboxylate
Isooctane
Li eyclohexaneearboxylate
Isooetane
Li campholate~
Isooctane
n-Heptane Benzene
86 1760 35.0 890 7.0 58 6.4 56 7.6
14 14 14 19 520 66 579
0.24 0.27 0.30 0.68 0.77 0.67 0.76
0.62 447 0.38 0.95 9.0 76 8.9 76 12.4
0.07 0.25 0.011 0.011 1.29 1.32 1.39 1.36 1.6
- The solubility of stearic acid in isooetane at 27.0°C. is 0.043 M. b See Figs. 1 and 2. c See Fig. 2. d See Fig. 3.
SOLUBILIZATION OF LITHIUM CARBOXYLATES
151
solubilization of the salt only slightly. Introduction of branching into the carbon chain increases the amount of solubilized salt at a given acid concentration up to a thousand times. The dependence of solubilization of branched chain carboxylates upon their structure is complex, involving the number, the shape, the chain length, and the position of the alkyl branches. Although the possible structural effects have not been studied, at least one conclusion can be drawn. When the length of alkyl branches increases the compound approaches the solubilization characteristics of a normal chain carboxylate. At a concentration of 4 X 10-2 M 2-hexyldecyleicosanoic acid failed to solubilize 10-~ mole of its lithium salt.
The Lithium 2-Ethylhexanoate2-EthylhexanoicAcid-Isooctane System The relationship between the solubility of lithium 2-ethylhexanoate in isooctane and the concentration of 2-ethylhexanoic acid is almost linear at medium and higher concentrations, as shown in Fig. 1. However, at very low acid concentrations a curvature becomes evident corresponding to the critical micelle concentration, or perhaps more properly, corresponding to the concentration range, below which the association to micelles becomes negligibly small (16, 17). In equilibrium with the solid phase the concentration of the extramicellar salt in the liquid phase is constant. Therefore, K-
C~ C~,
[11
where K --- equilibrium constant or, in analogy to complexes in general, the stability constant of the micelle; C¢ = concentration of the micellar species; C] = concentration of the acid not associated with the salt; n = number of acid molecules in the micelle. The relative concentration of micellar species is shown in Fig. 2 by plotting the molar salt-acid ratio in the liquid phase versus the concentration of acid. Neglecting the exceedingly small solubility of the salt not associated with acid, it follows from Eq. [1] that
Ca _ m ( C ~ - C f ) _ m ( 1 Ca
n
Ca
n
Cf)
- ~:
[2]
where C~ = Ca = Ca - C] = m =
solubility of the salt (moles/liter) ; concentration of acid in the liquid phase (moles/liter); concentration of acid present in micelles (moles/liter); and number of salt molecules in the micelle.
152
KZSS•
/
0.20-
0.1 SALT SOLUBIL M
0.10 M SALT / 0.05
0.01
,,
/
y 0.005
~--.~I ' ~ I I 0.01 0.02 0.03 M ACID
• e/ I
0.10
I
0.20 M ACID
I
0.30
FIG. 1. Solubilization of Li 2-ethylhexanoate with the corresponding acid in isooctane at 27.0°C. Equilibrium approached from the side of: O supersaturation; • undersaturation.
The system has been assumed to be nearly ideal and the activity coefficients close to unity. There is no direct indication whether the ratio of the number of salt and acid molecules in a mieelle is independent of the total acid concentration in the liquid phase. However, the shape of the curve obtained by plotting the ratio C~/Caas a function of the concentration of acid Ca (Fig. 2) suggests that the ratio of salt and acid molecules in the micelle is constant, at least over most of the concentration (Ca) range studied. In the case of the lithium 2-ethylhexanoate-2-ethylhexanoic-isooctane system the micelles appear to contain four molecules salt per five molecules acid. It follows from Eq. [2] that the curve in Fig. 2 shows also the relative amount (Cs/Ca) of acid not incorporated in micelles. The amount of
153
SOLUBILIZATION OF LITHIUM CARBOXYLATES
0.8
~ ,
°
....
.
0.5 Cs/Ca
0.5
I
.
L0 Ca
I
2.0 (a)
CflC o
!
3.0
FIG. 2. Solubilization of b r a n c h e d chain aliphatic carboxylates with t h e i r corres p o n d i n g acids in isooctane a t 27.0°C. Li 2 - e t h y l h e x a n o a t e : O from s u p e r s a t u r a t i o n ; • from u n d e r s a t u r a t i o n ; • Li p i v a l a t e ; ~ Li 2-hexyldecanoate.
extramicellar acid (Cs) is almost negligible at total acid (C~) concentrations exceeding 0.5 M. The solubilization characteristics of lithium pivalate and lithium 2hexyldecanoate are similar to those of lithium 2-ethylhexanoate (Fig. 2).
A licyclic Carboxylates For the solubilization of alicyclic lithium carboxylates alkyl substitution is essential. Only slight amounts of lithium cyclopentanecarboxylate or lithium cyclohexanecarboxylate are solubilized by their corresponding acid (Table I). However, lithium salts of alkyl substituted cyclopentanecarboxylic acids, such as lithium campholate and lithium isofencholate, are readily solubilized by their corresponding acids. The lithium campholate-campholic acid-isooetane system (Fig. 3) differs from the lithium 2-ethylhexanoate-2-ethylhexanoic acid-isooctane system in several ways. The lithium campholate-campholic acid micelles contain more salt than acid, the salt-acid molar ratio being 4:3 or 3:2. Although the equilibrium constants were not determined, the intermolecular forces in the lithium campholate-campholic acid micelles appear to be stronger than in the lithium 2-ethylhexanoate-2-ethylhexanoic acid micelles. This is indicated by a comparison of the curves presenting the molar ratio Cs/Ca as a function of the acid concentration. The amount of extramicellar campholic acid is negligible except at total acid (Ca) concentrations below 0.05 M. The amount of lithium campholate solubilizable in isooctane by increasing the concentration of campholic acid appears to be limited. Upon concentration of a solution containing 0.0696 M lithium campholate and 0.0588 M campholic acid in isooctane the acid salt CgH17COOLi. CgHITCOOH separated. The concentrate contained 0.536 M lithium
154
K~SSA 1.5 ~.,-o
. . . . .,o
o
o
1.0
CslCo
0,'~
I
0.05
I
0.10
I
Ca (M)
0.20
FIG. 3. Solubilization of Li campholate with campholic acid in isooctane at 27.0°C. campholate and 0.361 M campholic acid, corresponding to a molar saltacid ratio of 1.49. The values were not reproducible, however, indicating either a failure to attain equilibrium or a presence of traces of moisture. This phenomenon is being investigated further.
Effect of Solvent Branched lithium carboxylates could be solubilized with carboxylic acids in all hydrocarbon solvents tested. The solvents used included benzene, n-heptane, isooctane, diisobutylene, cyclohexane, and kerosene. Not enough quantitative data are available to permit a firm conclusion on the effect of the solvent structure. An aliphatic hydrocarbon is a better solvent than an aromatic one for lithium 2-ethylhexanoate, solubilized with its corresponding acid. An aromatic hydrocarbon appears to be a better solvent than an aliphatic hydrocarbon for lithium campholate, solubilized by campholic acid. The amount of the carboxylate solubilized is larger when the structures of the anion and of the solvent molecules are related, because the anionic hydrocarbon tails form the exterior of the micelle, determining therefore its characteristics as the solute (13).
Solubilization by Noncorresponding Acids For the solubilization of a lithium carboxylate by a noncorresponding acid it is essential that both the carboxylate and the acid have a branched carbon skeleton. Normal carboxylic acids do not solubilize appreciable amounts of noncorresponding normal or branched chain carboxylates
SOLUBILIZATION OF LITHIUM CARBOXYLATES
155
TABLE I I Li Carboxylates Solubilized by Non-corresponding Acids in Isooctane at 27.0°C. Salt
Acid
Normal-branched Li acetate Li caproate Li pivalate Li pivalate Li pivalate
Pivalic Pivalic Acetic Caproic Stearic
Conc. of acid
Conc. of salt
C,
M X 103
M X 10~
Ca
20 50 50 48 42
0.01 3.2 0.04 0.8 0.71
0.0005 0.06 0.0008 0.02 0.02
1,5
o /
o
( E + P ) + E A + PA
4 2 DAYS
1.0 o. E + P + EA + PA
Cs/% o ~ O " " - - - ' -
0.5
/
____-----o~ (m') {E+P) + EA
f ~
o
o
~o--=~.
'~'
I 5
I I0
(IV')
o ~ ( I:I:} E + E A + PA OE+EA (I) • P -I,- PA
I 15 TIME
I 20
(DAYS)
FIG. 4. Rate of solution of mixtures of acids and salts (Ca = 0.014 M). EA--2ethylhexanoic acid; PA--pivalic acid; E--Li 2-ethylhexanoate; P--Li pivalate; (E + P)--neutralized equimolar pivalic + 2-ethylhexanoic acid mixture. (Table I I ) . Accordingly, branched chain carboxylic acids do not solubilize appreciable amounts of normal carboxylates. A small increase of solubility may be caused by the presumably slow (17) reaction RCOOLi normal
R ' C O O H -_, R C O O H
+ branched ~ normal
R'COOLi
+ branched
which m a y produce a pair of soluble species (underlined). At high acid concentrations the increased polarity of the solvent system m a y be also responsible for the increase of solubility. A branched chain carboxylate is solubilized to a greater extent by a noncorresponding branched chain carboxylic acid than by its own corre-
156
KISSA
sponding acid. However, the measurement of solubilities in such systems is complicated by difficulties in establishing equilibrium. As shown in Fig. 4, the amount of salt solubilized in a system derived from pivMic and 2-ethylhexanoic acids depends upon the mode of mixing of the components. The flat parts of curves (II-V), with the probable exception of curve (III), run almost parallel to each other, indicating that a common and therefore true solubility value is not attainable in a reasonable time. As another complication, the concentration of each soluble species at equilibrium cannot be determined readily. It should be noted that in hydrocarbon systems containing two or more branched chain carboxylates the solubility of these carboxylates can no longer be assumed to be negligible (14). The increased solubility of mixtures of carboxylates in the absence of acids will be discussed in a forthcoming publication. ACKNOWLEDGMENT The author is indebted to A. F. Benning for his stimulating advice and criticism, to J. B. Nichols for light scattering measurements, and to Maimu S. Yllo for lithium analyses by flame photometry. I:~EFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
McBAIN, J. W., AND EATON, M., J. Chem. Soc. 2166 (1928). DA FANO, E., Industria olii minerell e grassi 9, 105 (1929). DA FANO, E., Giorn. chim. ind. ed appl. 11, 199 (1929). LEvi, T. G., Gazz. chim. ital. 62,709 (1932). McBAIN, J. W., ANn FIELD, M. C., J. Phys. Chem. 37,675 (1933). LAWRENCE,A. S. C., Trans. Faraday Soc. 35, 702 (1939). LAWRENCE,A. S. C., J. Phys. & Colloid Chem. 52, 1504 (1948). SAT5, K., Yushi, Kagaku KyOkaishi 1, 119 (1952); Chem. Abstr. 47, 2016. HONIG, J. G., AND SINGLETERRY, C. R., J. Phys. Chem. 58,201 (1954). SINGL~T~RRa', C. R., J. Am. Oil Chemists Soc. 32, 446 (1955). HONIG, J. G., AND SINGLETERRY, C. R., J. Phys. Chem. 60, 1108 (1956). BASCOM,W. D., AND SINGLETERRY, C. R., J. Colloid Sci. 13, 569 (1958). BASCOM, W. D., KAUFMAN, S., AND SINGLETERRY, C. R., Proc. 5th Petroleum Congr., Section VI, p. 277 (1959). KlSSA, E., J. Colloid Sci. 17, 857 (1962). KissA, E., U. S. 3,013,869 (1961). JONES, E. R., ANn :BURY, C. R., Phil. Mag. 4, 841 (1927). MURRAY, R. C., ANn HARTLEY, G. S., Trans Faraday Soc. 31, 183 (1935). LAWRENCE, A. S. C., Trans. Faraday Soc. 35, 702 (1939).
SOLUBILIZATION OF LITHIUM CARBOXYLATES WITH CARBOXYLIC ACIDS IN HYDROCARBON SOLVENTS
Erik Kissa Research and Development Division, Organic Chemicals Department, E. I. Du Pont de Nemours and Company, Wilmington, Delaware Received May 4, 196~; revised September 19, 196P ABSTRACT The extent of solubilization of lithium carboxylates with carboxylic acids in hydrocarbon solvents depends greatly upon the structure of the anion, and, to a lesser degree, upon the structure of the hydrocarbon solvent. Branching of the aliphatic carbon chain or the alicyclie carbon ring of the carboxylate and the carboxylic acid molecules is essential for solubilization. Noncorresponding acids or mixtures of acids solubilize a relatively larger amount of a carboxylate than the corresponding acid. Appreciable amounts of normal carboxylates are soluble only when the acid is a substantial fraction of the liquid phase. The mechanism of the solubilization of lithium carboxylates with earboxylic acids involves the cooperative formation of association mieelles governed by the law of chemical equilibrium. The solutions are thermodynamically stable. INTRODUCTION
Although it has been generally known that carboxylic acids increase the solubility of their alkali metal salts in hydrocarbon solvents, the scope of this phenomenon and the stability of the solutions obtained have not been studied. Several investigators (1-8) working with normal sodium and potassium carboxylates have reported an enhancement of solubility in hydrocarbon solvents by carboxylic acids. McBain and Eaton (1) found that potassium laurate became soluble in benzene upon addition of a large portion of laurie acid. By cooling a homogeneous mixture of benzene, laurie acid, and potassium laurate they obtained crystals having the composition CuH~aCOOK. CuH~3COOH and attributed the solubility increase to the formation of the acid soap. Lawrence (6, 7) found that soaps dispersed in oil are readily peptized by small amounts of polar substances, including fatty acids. Sodium stearate appeared to form a complex with one molecule of stearic acid. Sat5 (8) reported that various sodium and calcium soaps associate with polar substances having OH or COOH groups and form stable gels in spindle oil. More recently Singlcterry and coworkers (9-13) studied the effect of acid upon micelles of acylstearate soaps in hydrocarbon solvents and concluded that acid was taken up by 147
148
K~SSA
the micelles. From vapor concentration measurements Bascom and Singleterry (12) concluded that acetic acid is held by sodium dinonylnaphthalene sulfonate, magnesium dinonylnaphthalene sulfonate, or magnesium phenylstearate micelles in benzene by a coordination mechanism. For the bonding of the acid molecules a definite number of coordination sites are available. The excess acid is held by considerably weaker forces. It was shown in a previous publication (14) that the solubilities of pure alkali metal carboxylates in hydrocarbon solvents are less than 10-4 M at 27.0°C. The solubilization of these salts by carboxylic acids has now been studied. It was found quite unexpectedly that considerable amounts of alkali metal carboxylates are solubilized only when both the salt and the acid have a branched carbon chain. Furthermore, the size of the metal ion of the carboxylate was found to exert a strong influence upon solubilization by acids. Our attention was devoted first to lithium carboxylates (15) because they required the smallest relative amount of acid for their solubilization. EXPERIMENTAL The preparation of the pure carboxylic acids, their salts and the solvents, and the method for the determination of solubilities have been described in a previous publication (14). The total concentration of acid in the liquid phase was determined by a microtitration with potassium hydroxide in isopropyl alcohol. The solubility determinations were carried out at 27.0°C. Equilibrium was approached (a) from the side of undersaturation by adding salt to a solution of acid in the hydrocarbon solvent or (b) from the side of supersaturation by diluting a concentrated solution and seeding with a small amount of the carboxylate. Precautions were taken to exclude water. Considering the long equilibration time (up to one year) and manipulation during sampling, presence of traces of moisture cannot be ruled out. The effect of small amounts of water is being investigated and the results will be published at a later date. RESULTS AND DISCUSSION
Nature of the Solutions The solubilization of lithium carboxylates with carboxylie acids involves a cooperative formation of micelles. The association of salt and acid molecules is spontaneous. Agitation is not necessary although it increases the rate of dissolution. The saturated solutions are in true and reversible equilibrium and essentially the same solubility values are obtained regardless of whether the equilibrium is approached from the side of supersaturation or undersaturation. As a further indication of stability no precipitation occurred during four years of storage. Centrifugation in a gravitational field of 1200 g does not cause sedimentation, at least not during 1 hour.
SOLUBILIZATION OF LITHIUM CARBOXYLATES
149
Considering the existence of a reversible equilibrium the solubilization of lithium carboxylates by carboxylic acids can be described by the following over-all scheme: (RCOOH)2
(RCOOLi)m• (RCOOH),
ooY RC00Li. RC00H ~.............
....................
RC00Li .RC00H
'
lI )li
RC00Li
+
RC00H
Jl .....................
RCOOLi
Particularly at higher acid concentrations the solid phase consists, at least partially, of the acid salt. The acid salt RCOOLi. RCOOH as such is only slightly soluble in the hydrocarbon solvent. If the acid salt were soluble in the undissociated or unassociated state, the molar salt-acid ratio in the liquid phase would approach unity at low concentrations below the CMC. This was not found to be the case with any of the salts studied. Actually, a lithium carboxylate which forms a very stable acid salt is precipitated by the equivalent amount of acid, e.g., when 0.076 mole/1, of Li acetate was added to 0.172 mole/l, of acetic acid in isooctane, 0.091 mole/l, of acetic acid and only 0.00008 mole/1, of Li acetate remained in solution. The liquid phase always contains a small amount of acid not associated with the salt. The presence of acid in the intermicellar liquid introduces difficulties in obtaining accurate measurements of the mieellar weight. For the application of colligative methods the concentration Cz of the acid not present in micelles and the effect of excess acid upon the mieellar weight have to be known. On the basis of cryoseopie and ebullioseopic measurements it can be estimated that the micellar weights of lithium 2-ethylhexanoate and lithium campholate solubilized by their corresponding acids in benzene were above 1000. Mieellar weight measurements based on light scattering were complicated in addition to the possible effect of acid by a relatively low difference between the refractive indices of the micelles and the solvent. The results indicate that the micellar weights of lithium 2-ethylhexanoste and lithium 2-hexyldecanoate solubilized by their corresponding acids in isooctane are below 10,000. Undoubtedly the mieelles are relatively small.
150
I~ISSA
Effect of the Structure of Aliphatic Anions T h e e x t e n t of solubilization of l i t h i u m carboxylates b y carboxylic acids d e p e n d s greatly u p o n the s t r u c t u r e of the anion. B r a n c h i n g of t h e c a r b o n c h a i n of aliphatic alkali m e t a l c a r b o x y l a t e s a n d of t h e i r p a r e n t acids a p p e a r s to be essential for t h e solubilization m e c h a n i s m to become operative. F r o m the solubility d a t a several conclusions emerge, S u b s t a n t i a l a m o u n t s of n o r m a l c h a i n carboxylates are solubilized only a t high acid c o n c e n t r a t i o n s (Table I ) . U n s a t u r a t i o n i n t h e n o r m a l c a r b o n c h a i n e n h a n c e s t h e TABLE I
Solubilization of Li Carboxylates at 27.0°C. by the Corresponding Acid Salt
Normal Li Acetate
Conc. of Acid M X 103 C8
Solub.of Salt M :K 10a C.
Cs Ca
91 43 2040 21 37a
0.07 9.9 0.04 84 0.03 0.01
0.0008 0.0044 0.001 0.041 0.001 0.0003
Isooctane
18 950
0.27 20
0.015 0.03
Benzene n-Heptane ~sooctane
3.4 3.8 4,2 28 680 98 760
Solvent
Isooctane
2220 Li caproate
Isooctane
Li stearate
Isooctane
Normal, unsaturated Li oleate Branched Li 2-Ethylhexanoateb
Li pivalateo
Isooetane
Alicyelie Li eyelopent anecarboxylate
Isooctane
Li eyclohexaneearboxylate
Isooetane
Li campholate~
Isooctane
n-Heptane Benzene
86 1760 35.0 890 7.0 58 6.4 56 7.6
14 14 14 19 520 66 579
0.24 0.27 0.30 0.68 0.77 0.67 0.76
0.62 447 0.38 0.95 9.0 76 8.9 76 12.4
0.07 0.25 0.011 0.011 1.29 1.32 1.39 1.36 1.6
- The solubility of stearic acid in isooetane at 27.0°C. is 0.043 M. b See Figs. 1 and 2. c See Fig. 2. d See Fig. 3.
SOLUBILIZATION OF LITHIUM CARBOXYLATES
151
solubilization of the salt only slightly. Introduction of branching into the carbon chain increases the amount of solubilized salt at a given acid concentration up to a thousand times. The dependence of solubilization of branched chain carboxylates upon their structure is complex, involving the number, the shape, the chain length, and the position of the alkyl branches. Although the possible structural effects have not been studied, at least one conclusion can be drawn. When the length of alkyl branches increases the compound approaches the solubilization characteristics of a normal chain carboxylate. At a concentration of 4 X 10-2 M 2-hexyldecyleicosanoic acid failed to solubilize 10-~ mole of its lithium salt.
The Lithium 2-Ethylhexanoate2-EthylhexanoicAcid-Isooctane System The relationship between the solubility of lithium 2-ethylhexanoate in isooctane and the concentration of 2-ethylhexanoic acid is almost linear at medium and higher concentrations, as shown in Fig. 1. However, at very low acid concentrations a curvature becomes evident corresponding to the critical micelle concentration, or perhaps more properly, corresponding to the concentration range, below which the association to micelles becomes negligibly small (16, 17). In equilibrium with the solid phase the concentration of the extramicellar salt in the liquid phase is constant. Therefore, K-
C~ C~,
[11
where K --- equilibrium constant or, in analogy to complexes in general, the stability constant of the micelle; C¢ = concentration of the micellar species; C] = concentration of the acid not associated with the salt; n = number of acid molecules in the micelle. The relative concentration of micellar species is shown in Fig. 2 by plotting the molar salt-acid ratio in the liquid phase versus the concentration of acid. Neglecting the exceedingly small solubility of the salt not associated with acid, it follows from Eq. [1] that
Ca _ m ( C ~ - C f ) _ m ( 1 Ca
n
Ca
n
Cf)
- ~:
[2]
where C~ = Ca = Ca - C] = m =
solubility of the salt (moles/liter) ; concentration of acid in the liquid phase (moles/liter); concentration of acid present in micelles (moles/liter); and number of salt molecules in the micelle.
152
KZSS•
/
0.20-
0.1 SALT SOLUBIL M
0.10 M SALT / 0.05
0.01
,,
/
y 0.005
~--.~I ' ~ I I 0.01 0.02 0.03 M ACID
• e/ I
0.10
I
0.20 M ACID
I
0.30
FIG. 1. Solubilization of Li 2-ethylhexanoate with the corresponding acid in isooctane at 27.0°C. Equilibrium approached from the side of: O supersaturation; • undersaturation.
The system has been assumed to be nearly ideal and the activity coefficients close to unity. There is no direct indication whether the ratio of the number of salt and acid molecules in a mieelle is independent of the total acid concentration in the liquid phase. However, the shape of the curve obtained by plotting the ratio C~/Caas a function of the concentration of acid Ca (Fig. 2) suggests that the ratio of salt and acid molecules in the micelle is constant, at least over most of the concentration (Ca) range studied. In the case of the lithium 2-ethylhexanoate-2-ethylhexanoic-isooctane system the micelles appear to contain four molecules salt per five molecules acid. It follows from Eq. [2] that the curve in Fig. 2 shows also the relative amount (Cs/Ca) of acid not incorporated in micelles. The amount of
153
SOLUBILIZATION OF LITHIUM CARBOXYLATES
0.8
~ ,
°
....
.
0.5 Cs/Ca
0.5
I
.
L0 Ca
I
2.0 (a)
CflC o
!
3.0
FIG. 2. Solubilization of b r a n c h e d chain aliphatic carboxylates with t h e i r corres p o n d i n g acids in isooctane a t 27.0°C. Li 2 - e t h y l h e x a n o a t e : O from s u p e r s a t u r a t i o n ; • from u n d e r s a t u r a t i o n ; • Li p i v a l a t e ; ~ Li 2-hexyldecanoate.
extramicellar acid (Cs) is almost negligible at total acid (C~) concentrations exceeding 0.5 M. The solubilization characteristics of lithium pivalate and lithium 2hexyldecanoate are similar to those of lithium 2-ethylhexanoate (Fig. 2).
A licyclic Carboxylates For the solubilization of alicyclic lithium carboxylates alkyl substitution is essential. Only slight amounts of lithium cyclopentanecarboxylate or lithium cyclohexanecarboxylate are solubilized by their corresponding acid (Table I). However, lithium salts of alkyl substituted cyclopentanecarboxylic acids, such as lithium campholate and lithium isofencholate, are readily solubilized by their corresponding acids. The lithium campholate-campholic acid-isooetane system (Fig. 3) differs from the lithium 2-ethylhexanoate-2-ethylhexanoic acid-isooctane system in several ways. The lithium campholate-campholic acid micelles contain more salt than acid, the salt-acid molar ratio being 4:3 or 3:2. Although the equilibrium constants were not determined, the intermolecular forces in the lithium campholate-campholic acid micelles appear to be stronger than in the lithium 2-ethylhexanoate-2-ethylhexanoic acid micelles. This is indicated by a comparison of the curves presenting the molar ratio Cs/Ca as a function of the acid concentration. The amount of extramicellar campholic acid is negligible except at total acid (Ca) concentrations below 0.05 M. The amount of lithium campholate solubilizable in isooctane by increasing the concentration of campholic acid appears to be limited. Upon concentration of a solution containing 0.0696 M lithium campholate and 0.0588 M campholic acid in isooctane the acid salt CgH17COOLi. CgHITCOOH separated. The concentrate contained 0.536 M lithium
154
K~SSA 1.5 ~.,-o
. . . . .,o
o
o
1.0
CslCo
0,'~
I
0.05
I
0.10
I
Ca (M)
0.20
FIG. 3. Solubilization of Li campholate with campholic acid in isooctane at 27.0°C. campholate and 0.361 M campholic acid, corresponding to a molar saltacid ratio of 1.49. The values were not reproducible, however, indicating either a failure to attain equilibrium or a presence of traces of moisture. This phenomenon is being investigated further.
Effect of Solvent Branched lithium carboxylates could be solubilized with carboxylic acids in all hydrocarbon solvents tested. The solvents used included benzene, n-heptane, isooctane, diisobutylene, cyclohexane, and kerosene. Not enough quantitative data are available to permit a firm conclusion on the effect of the solvent structure. An aliphatic hydrocarbon is a better solvent than an aromatic one for lithium 2-ethylhexanoate, solubilized with its corresponding acid. An aromatic hydrocarbon appears to be a better solvent than an aliphatic hydrocarbon for lithium campholate, solubilized by campholic acid. The amount of the carboxylate solubilized is larger when the structures of the anion and of the solvent molecules are related, because the anionic hydrocarbon tails form the exterior of the micelle, determining therefore its characteristics as the solute (13).
Solubilization by Noncorresponding Acids For the solubilization of a lithium carboxylate by a noncorresponding acid it is essential that both the carboxylate and the acid have a branched carbon skeleton. Normal carboxylic acids do not solubilize appreciable amounts of noncorresponding normal or branched chain carboxylates
SOLUBILIZATION OF LITHIUM CARBOXYLATES
155
TABLE I I Li Carboxylates Solubilized by Non-corresponding Acids in Isooctane at 27.0°C. Salt
Acid
Normal-branched Li acetate Li caproate Li pivalate Li pivalate Li pivalate
Pivalic Pivalic Acetic Caproic Stearic
Conc. of acid
Conc. of salt
C,
M X 103
M X 10~
Ca
20 50 50 48 42
0.01 3.2 0.04 0.8 0.71
0.0005 0.06 0.0008 0.02 0.02
1,5
o /
o
( E + P ) + E A + PA
4 2 DAYS
1.0 o. E + P + EA + PA
Cs/% o ~ O " " - - - ' -
0.5
/
____-----o~ (m') {E+P) + EA
f ~
o
o
~o--=~.
'~'
I 5
I I0
(IV')
o ~ ( I:I:} E + E A + PA OE+EA (I) • P -I,- PA
I 15 TIME
I 20
(DAYS)
FIG. 4. Rate of solution of mixtures of acids and salts (Ca = 0.014 M). EA--2ethylhexanoic acid; PA--pivalic acid; E--Li 2-ethylhexanoate; P--Li pivalate; (E + P)--neutralized equimolar pivalic + 2-ethylhexanoic acid mixture. (Table I I ) . Accordingly, branched chain carboxylic acids do not solubilize appreciable amounts of normal carboxylates. A small increase of solubility may be caused by the presumably slow (17) reaction RCOOLi normal
R ' C O O H -_, R C O O H
+ branched ~ normal
R'COOLi
+ branched
which m a y produce a pair of soluble species (underlined). At high acid concentrations the increased polarity of the solvent system m a y be also responsible for the increase of solubility. A branched chain carboxylate is solubilized to a greater extent by a noncorresponding branched chain carboxylic acid than by its own corre-
156
KISSA
sponding acid. However, the measurement of solubilities in such systems is complicated by difficulties in establishing equilibrium. As shown in Fig. 4, the amount of salt solubilized in a system derived from pivMic and 2-ethylhexanoic acids depends upon the mode of mixing of the components. The flat parts of curves (II-V), with the probable exception of curve (III), run almost parallel to each other, indicating that a common and therefore true solubility value is not attainable in a reasonable time. As another complication, the concentration of each soluble species at equilibrium cannot be determined readily. It should be noted that in hydrocarbon systems containing two or more branched chain carboxylates the solubility of these carboxylates can no longer be assumed to be negligible (14). The increased solubility of mixtures of carboxylates in the absence of acids will be discussed in a forthcoming publication. ACKNOWLEDGMENT The author is indebted to A. F. Benning for his stimulating advice and criticism, to J. B. Nichols for light scattering measurements, and to Maimu S. Yllo for lithium analyses by flame photometry. I:~EFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
McBAIN, J. W., AND EATON, M., J. Chem. Soc. 2166 (1928). DA FANO, E., Industria olii minerell e grassi 9, 105 (1929). DA FANO, E., Giorn. chim. ind. ed appl. 11, 199 (1929). LEvi, T. G., Gazz. chim. ital. 62,709 (1932). McBAIN, J. W., ANn FIELD, M. C., J. Phys. Chem. 37,675 (1933). LAWRENCE,A. S. C., Trans. Faraday Soc. 35, 702 (1939). LAWRENCE,A. S. C., J. Phys. & Colloid Chem. 52, 1504 (1948). SAT5, K., Yushi, Kagaku KyOkaishi 1, 119 (1952); Chem. Abstr. 47, 2016. HONIG, J. G., AND SINGLETERRY, C. R., J. Phys. Chem. 58,201 (1954). SINGL~T~RRa', C. R., J. Am. Oil Chemists Soc. 32, 446 (1955). HONIG, J. G., AND SINGLETERRY, C. R., J. Phys. Chem. 60, 1108 (1956). BASCOM,W. D., AND SINGLETERRY, C. R., J. Colloid Sci. 13, 569 (1958). BASCOM, W. D., KAUFMAN, S., AND SINGLETERRY, C. R., Proc. 5th Petroleum Congr., Section VI, p. 277 (1959). KlSSA, E., J. Colloid Sci. 17, 857 (1962). KissA, E., U. S. 3,013,869 (1961). JONES, E. R., ANn :BURY, C. R., Phil. Mag. 4, 841 (1927). MURRAY, R. C., ANn HARTLEY, G. S., Trans Faraday Soc. 31, 183 (1935). LAWRENCE, A. S. C., Trans. Faraday Soc. 35, 702 (1939).