Journal of Structural Chemistry. Vol. 47, No. 6, pp. 1091-1097, 2006 Original Russian Text Copyright © 2006 by A. G. Ramazanova, V. V. Korolev, and E. V. Ivanov
STRUCTURAL EFFECTS OF SOLVATION OF UNSATURATED FATTY ACIDS ɋ18:n IN TETRACHLOROMETHANE FOUND IN INVESTIGATIONS OF THE VOLUMETRIC PROPERTIES OF SOLUTIONS UDC 531.756:547.39
A. G. Ramazanova, V. V. Korolev, and E. V. Ivanov
The limiting partial molar volumes of solutes and excess molar volumes of their mixtures with nonpolar solvents are calculated on the basis of precise (error 510–6 gcm-3) densitometric measurements for dilute solutions of oleic, linoleic, and linolenic acids in tetrachloromethane at 298.15 K. It is found that selfassociation processes of the components of the mixtures play the key role in dissolution effects of fatty acids in CCl4. As the number of C=C double bonds in the acid molecule increases, acid desolvation becomes less pronounced and is accompanied by compression (reduction of volume) of the structural packing of the solvate complex formed due to the strengthening interaction of the acid with tetrachloromethane. Keywords: tetrachloromethane, unsaturated fatty acids (oleic, linoleic, and linolenic), solvation, volumetric characteristics.
INTRODUCTION Unsaturated fatty acid (UFA) molecules are known to be indispensable and active participants of metabolism and biochemical synthesis including pharmacological processes [1, 2]. The UFAs that form homological series of acids with a general structural formula C18:n (n is number of C=C bonds in the cis-9, cis-6, and cis-3 positions relative to the end methyl group), namely, oleic (nC=C = 1), linoleic (nC=C = 2), and linolenic (nC=C = 3) acids, are the major representatives of this class of organic compounds. All of these UFAs possess strong antioxidant properties, the latter two (polyunsaturated acids) belonging to so-called essential fatty acids from the group of vitamin F [2]. However, purposeful study of the properties of biologically active media involving these acids is restrained by their extremely low solubility in water. Meanwhile, a branch of technology is successfully developing now in which these UFAs are used as effective surfactants that provide sedimentation and aggregate stability of magnetic liquids synthesized in tetrachloromethane and other nonpolar aprotic solvents [3-5]. Thus structural and thermodynamic data on CCl4–UFA liquid binary systems are of great importance. However, only data on the enthalpy effects of dissolution of oleic and linoleic acids in tetrachloromethane at 298.15 K [3] are now available. Therefore any additional experimental data on the properties of the studied solutions, including the solvation characteristics of UFAs, will undoubtedly promote rationalization of processes that determine the stability and efficiency of the synthesized magnetite-containing liquid systems.
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo;
[email protected]. Translated from Zhurnal Strukturnoi Khimii, Vol. 47, No. 6, pp. 1102-1109, November-December, 2006. Original article submitted November 25, 2005. 0022-4766/06/4706-1091 © 2006 Springer Science+Business Media, Inc.
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Problems in (and hence the high cost of) the production of pure “synthetic” linoleic and linolenic acids demand an experimental technique which would guarantee reliability of measurements with minimal possible amounts of UFA samples. These requirements are satisfied by the precision vibration densimetry method, providing trustworthy information on volumetric (or packing) characteristics directly related to structural transformations in the solvent under the influence of solute molecules. Moreover, analyzing the volumetric effects of solvation using semiempirical model approximations, one can estimate changes in the parameters of intermolecular interaction in binary liquid systems containing isomeric analogs or compounds from the same homological series as solutes. The present paper describes the results of a densitometric study of tetrachloromethane solutions of oleic, linoleic, and linolenic acids at 298.15 K. The structural effects of solvation of the above-mentioned UFAs on the volumetric characteristics of the liquid binary systems are also discussed.
RESULTS OF EXPERIMENTS AND CALCULATIONS Tetrachloromethane (carbon tetrachloride, extrapure grade) was additionally purified from HCl and CS2 using the technique of [6]. Solvent purity was determined by IR spectroscopy (Specord M80 spectrophotometer, CaF2 cells) and densitometry. The density of tetrachloromethane prepared for experiments (U1 = 1.584995 gcm-3 at 298.15 K) agreed with the results of precision measurements [7] (U1 = 1.58500±0.00005 gcm-3 on the average). Oleic (cis-9-octadecenic) and linoleic (cis-6-cis-9-octadecadienoic) acids (pure grade) were purified according to the recommendations of [8] until the content of the basic substance became at least 99.9 mass %. Linolenic (cis-3-cis-6-cis-9octadecatrienoic, Sigma) acid with more than 99.9 mass % basic substance was used as purchased. The quality of UFA was also estimated spectrometrically (Specord M80) by measuring the integrated intensities of IR absorption bands in thin films on KRS-5 glass plates. Data on the density U2 of acids for 298.15 K are absent in the literature. The values of U2 measured in our work (gcm-3) are 0.895593 (C18:1), 0.902707 (C18:2), and 0.917132 (C18:3). Solutions of acids (2) in CCl4 (1) were prepared by the weight method with an error of less than ±1×10–5 g. The concentration of the solute is expressed in terms of the solvomolality scale.* As shown in [9, 10], using this scale permits one f ). to significantly raise the accuracy of evaluation of the limiting partial (apparent) molar volumes V2f ( { Vij,2
The density U1,2 of solutions was measured (with an error of less than ±5×10–6 gcm-3) with a precise vibrating densimeter of original design. The cell with a volume of less than 2 cm3 was thermostatted to within 2×10–3 K. Details of apparatus design and experimental procedure are described in [9, 11]. The results of measurements of U1,2 are presented in Table 1. Regression analysis showed that the obtained dependences of U1,2 on csm,2 are adequately described by the first order equation U1,2 = a0 + a1csm,2. The corresponding coefficients are also listed in Table 1. The densities U1,2 approximated with the values of ai (in the measured range of csm,2, see Table 1) were used for f calculating the V2f ( { Vij,2 ) of the acids dissolved in CCl4. To evaluate V2f , the rational (not extrapolation) procedure [9, 12]
was applied. In this procedure, the apparent molar volume is stable against errors in determining U1,2 and csm,2 in the region of low concentrations. According to the given procedure, the concentration dependence of the total volume V1,2 = (55.50843M1 + csm,2M1)/ U1,2 of the solution (on the scale of solvomolal concentrations n1 = 55.50843 and n2 = csm,2) was approximated by an equation V1,2 b0 ( { 55.50843V1 )
f 2 b1 ( { V2f { Vij,2 ) ccm,2 b2 ( { BV ) csm,2 .
(1)
*Solvomolality ɫsm,2 is a dimensionless parameter of the composition of a solution expressed as csm,2 = (n2/n1)55.50843, where n2 and n1 are the quantities of the solute and solvent [9]. The normalizing multiplier 55.50843 equals the quantity of the substance (ɇ2Ɉ, mole) in 1000 g of water.
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TABLE 1. Densities (U1,2, gcm-3, 298.15 K) of Tetrachloromethane Solutions of Oleic, Linoleic, and Linolenic Acids and Parameters of Equation U1,2 = a0 + a1csm,2 csm,2u103
csm,2u103 U1,2 Linoleic acid
U1,2 Oleic acid
1.5100 3.0218 3.9185 6.8060 10.479 13.198 32.337 35.309 52.511 65.804 97.398 168.52 260.01 443.27 564.94 669.27 736.78 854.18 999.93
1.584961 1.584828 1.584828 1.584699 1.584553 1.584404 1.583602 1.583620 1.582949 1.582267 1.580732 1.577790 1.574151 1.566149 1.561278 1.556691 1.553804 1.548951 1.543076
1.6725 3.0316 4.0542 5.7027 7.2705 9.2001 29.051 37.296 58.392 59.184 71.481 102.97 190.13 288.90 334.73 513.21 544.55 692.63 883.30 1041.3
csm,2u103 U1,2 Linolenic acid
1.584907 1.584902 1.584811 1.484784 1.584730 1.584322 1.583807 1.583611 1.582629 1.582440 1.582313 1.580442 1.577102 1.573400 1.571554 1.564476 1.563381 1.557271 1.549178 1.542412
1.8426 2.4742 5.0329 7.2010 9.8661 22.884 30.066 41.601 47.379 55.467 70.490 98.226 145.61 224.62 318.94 434.83 555.86 699.95 865.11
1.584849 1.584951 1.584844 1.584652 1.584601 1.584167 1.583914 1.583361 1.583091 1.582896 1.582372 1.581075 1.579666 1.576441 1.573207 1.568455 1.564121 1.558336 1.552267
Acid
a0
–a1
r
Oleic Linoleic Linolenic
1.584991 (3.0710–5) 1.584989 (6.0110–5) 1.584993 (3.2910–5)
0.042137 (7.5110–5) 0.040448 (15.510–5) 0.037826 (10.610–5)
0.99997 0.99986 0.99993
Note. The densities were obtained by averaging the results of several measurements (n = 4-5) for each value of csm,2; x2 = csm,2/(55.50843 + csm,2) for acid solution concentrations expressed in terms of mole fractions and cm,2 = csm,2 M1(H2O)/M1(CCl4) for concentrations expressed in terms of molalities. The standard error of approximation is given in parentheses (r is the correlation coefficient). Here M1 and M2 are the molar masses of the components; V1 = M1/U1 is the molar volume of the solvent; BV is the “slope parameter”; (V1,2 – 55.508443V1)/csm,2 = VM,2(csm,2). Using this technique obviously has certain advantages over the traditional extrapolation procedure because in the region of high dilution, the values of VM,2 are found in a narrow corridor of confidence intervals relative to the line that f crosses the ordinate at a given point specified preliminarily: V1,2
b0 = 55.50843V1 [Eq. (1)]. The results of calculations by
Eq. (1) and the volumetric characteristics of pure UFAs are presented in Table 2.
DISCUSSION OF RESULTS The data of Table 2 show that compactness of molecular packing of UFAs in an individual state (the ratio Vw,2/V2) increases with nC=C. This tendency also takes place for infinitely dilute solutions of acids in tetrachloromethane. The only difference is that Vw,1 / V2f Vw,2 / V2 or V2E,f ! 0 (Table 2), and that the transition from oleic to linolenic acid is 1093
TABLE 2. Volumetric Properties (cm3/mole) of Unsaturated Fatty Acids in the Individual State (Vw,2, V2) and in Infinitely Dilute Solutions in Tetrachloromethane (V2f , V2E,f ) at 298.15 K Acid
Vw,2
V2
Vw,2 / V2
Oleic Linoleic Linolenic
180.3 177.3 174.3
315.40 310.68 303.59
0.571 0.571 0.574
f
V2
321.33 314.32 304.18
Vw,2/ V2f
BV
0.561 0.564 0.573
8.87 8.32 7.47
V2
E,f
V2 –V1
f
5.93 3.64 0.59
224.28 217.27 207.13
Note. Vw,2 = vw,2NA is the van der Waals volume of an acid (NA is the Avogadro constant) estimated by the method suggested in [13] for calculating the vw,2 of “overloaded” organic molecules; V2 = M2/U2 is the molar volume of the acid; V2E,f = V2f – V2 is the excess limiting partial molar volume (or “volume effect of dissolution” [14, 15]) of the acid. The values of 'G*,0 = RT (V2f V1)/V1 for CCl4–UFA systems (in kJ/mole) are 5.73 (C18:1), 5.55 (C18:2), and 5.29 (C18:3) (the molar volume of the tetrachloromethane sample prepared by us is V1 ~ 97.049 cm3/mole).
Fig. 1. Concentration dependence of excess molar volumes of dilute (relative to the acid) solutions of oleic (1), linoleic (2), and linolenic (3) acids in tetrachloromethane at 298.15 K. accompanied by more significant structure condensation in UFA–CCl4 solvate complexes (for nC=C = 3, V2E,f becomes even close to zero, i.e., V2f | V2). Also note that in the given (dilute) region of mixture compositions, the transfer of UFA molecules from their own liquid medium to the closer packed tetrachloromethane medium leads to loosening of the latter, which increases with csm,2 and in the series of acids: C18:3oC18:2oC18:1. This can be seen from the value of density (Table 1) and from the character of variation of the concentration dependence of the excess molar volume calculated by the formula E V1,2 = x2(VM,2 – V2) [16] (Fig. 1).
Taking into account the structural features of organic compounds from the series under consideration (Table 3), one can state that changes in packing during formation of their extremely dilute solutions in CCl4 (Table 2) are directly related to differences in the ability of UFA molecules to be built into the structural matrix of the solvent. The relationship between the energy characteristics of CCl4–CCl4 (1-1) and CCl4–UFA (1-2) interactions is one of the most important problems here as contacts between solute molecules (2-2) are excluded for infinitely dilute solutions. To solve this problem one can use the approach suggested in [17], which relates the molecular affinity parameter 'G*,0 to the volumetric properties of the binary system:
'G *,0 /( kTȡ1* ) V1'G*,0 /( RT )
0 0 G11 G12
V2f V1.
(2)
0 0 Here k is the Boltzmann constant; U1* = NA/V1 is number density; G11 and G12 are group integrals from the limiting
expressions of Kirkwood–Buff theory for molar volume [18, 19]. 1094
TABLE 3. Structure of Planar Molecules of Fatty Acids [1-3] Acid
Structural formula
Oleic (C17H33COOH) Linoleic (C17H31COOH) Linolenic (C17H29COOH)
0 0 It follows from Eq. (2) and from the data of Table 2 that V2f V1 0 and 'G*,0 o 0, i.e., G11 G12 (in absolute
value). Thus we can conclude that the affinity of solvent molecules to one another prevails over the affinity of the latter to UFA molecules. In other words, the volumetric changes found in solutions are caused by very low solvability of fatty acids C18:n in tetrachloromethane. As individual UFAs are inclined to self-association (mainly forming dimers) [20], one can assume that this process dominates in the region of real acid concentrations close to infinite dilution. Indirect proof is VM,2 (parameter BV in Table 2) increasing with UFA concentration in tetrachloromethane. At the same time, the tendency toward reduction of the positive effect of 'G*,0 observed in Table 2 (due to the partial replacement of CCl4–CCl4 bonds by weaker UFA–CCl4 bonds) for increasing nC=C in acid molecules points to increased solvation of these molecules (decomposition of dimers), forming closer packed structures with surrounding solvent molecules. This is supported by increased exothermicity [3] of UFA dissolution in tetrachlormethane during the transition from oleic to linolenic acid. Thus at T = 298.15 K and csm,2 | 8.510–4 the solution enthalpy ' sol H 2c (CCl4 ) is 9.26 kJ/mole for the former and 6.86 kJ/mole for the latter. Taking into account the above changes in the volumetric effects of UFA solution, one can state that ' sol H 2c is within the limits of 4-4.5 kJ/mole for the CCl4–linolenic acid system. Details of structural transformations in the liquid systems can be obtained using model representations allowing one to divide the total (integrated) volumetric characteristics of UFA solvation into separate components. One of these approaches is statistical-mechanical calculation of contributions to the volumetric properties of an extremely dilute solution of a fatty acid in tetrachloromethane based on approximation of component molecules by hard spheres, in particular, within the limits of scale particle theory (SPT). Although this theory [21-23] makes some formal allowances, it is based on the experimentally measured thermodynamic (including volumetric) properties of solvents such as isothermal compressibility ET,1 = (wV1/wp)T. The expression for the latter is an exact solution of the Percus–Yevik equation of state [21]. Moreover, hypothetical replacement of a “real” polyatomic molecule by an equivalent (in volume) hard sphere with a diameter V in the case of CCl4 seems to be quite justified; in the case of the “long-chain” UFAs in question (Table 3), it allows quantitative evaluation of relative changes in packing during solvation of homologs. According to SPT, the expression for the partial molar volume of the solute in infinitely dilute solution is V2f
f Vcav Vintf ET,1RT ,
(3)
f is the term that is due to the formation of a solvate cavity in the liquid solvent and Vintf is the term that is due to where Vcav the intermolecular 1-2 interactions; ET,1RT is variation of the standard state of the system during the gasoliquid transition. f The value of Vcav was obtained using the equation [22] f Vcav
ª y
ET,1RT «
¬1 y
3 yz (1 z ) (1 y ) 2
9 y2z2 º
SV32 N A
(1 y ) ¼
6
3»
.
(4)
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TABLE 4. Parameters of Eq. (3) and Volumetric Characteristics of the Components of Extremely Diluted Solutions of Unsaturated Fatty Acids in Tetrachloromethane at 298.15 K Needed for Their Calculation y
V1u108, cm
V2u108, cm
z
ET,1RT, kJ/mole
f
Acid
Vcav ,
Vint ,
f
cm3/mole
cm3/mole
Oleic Linoleic Linolenic
0.4398 0.4398 0.4298
5.135 5.135 5.135
8.300 8.254 8.207
1.616 1.607 1.598
2.70 2.70 2.70
300.5 296.3 292.1
18.1 15.3 9.4
Note: EɌ,1 | 10.9110–10 Pa–1 [24]. Here y is the packing parameter expressed in terms of the relation ET,1 = V1(1 – y)4/[RT(1 + 2y)2]; z = V2/V1, where V1 and V2 are the hard-sphere diameters of the solvent (CCl4) and solute (UFA) molecules, respectively. The expression y = SV13 N A /(6V1 ) [21] is used to calculate V1, and the formula V2 = [6Vw,2/(SNA)]1/3 [23] is employed for calculating V2.
Table 4 presents the parameters of Eqs. (3) and (4) and the volumetric contributions to V2f calculated from them for f CCl4–UFA liquid systems. Analyzing the data of Table 4 we can draw the following conclusions. First, the value of Vcav
decreases monotonously (by ~4.2 cm3/mole) as the number of C=C double bonds in the UFA molecule increases. Second, the positive volumetric contribution Vintf decreases by a factor of two from oleic to linolenic acid. The difference between the values of Vintf in the stated series of UFA also decreases by a factor of two when linolenic acid is replaced by linoleic acid. These circumstances confirm the above assumption that, on the one hand, the solvation ability of tetrachloromethane relative to UFA molecules is rather weak and that, on the other hand, the tendency toward stronger interactions between CCl4 and UFA molecules becomes especially pronounced if we compare different polyunsaturated fatty acids. It should be borne in mind that planar UFA molecules differ slightly in the size of the hydrocarbon chain and in polarizability D0 (for oleic and linolenic acids, D0 is 33.2×10–24 cm3/mole and 34.7×10–24 cm3/mole at 293.15 K, respectively)*. Thus van der Waals (basically dispersion) intermolecular interactions are not likely to be the reason for the revealed packing (structural) transformations in liquid CCl4 induced by solution of fatty acids C18:n therein. From the viewpoint of the structure of the UFAs, the assumption of [1, 2] about the dominant role of C=C double bonds in structure formation of acid–solvent molecular solvate complexes looks most substantiated. It is not excluded that S–V-interactions with the solute environment lead to compression of the spatially stretched UFA molecule (or change its structural conformation), which increases with nC=C. This is also favored by the electron acceptor ability of CCl4 [26, 27], which is higher than the electron acceptor abilities of other nonpolar aprotic solvents. However, the latter conclusion demands additional justification using other, more structurally informative experimental and model approaches.
CONCLUSIONS Investigation of the volumetric thermodynamic properties of solutions that convey information on macroscopic or supramolecular changes in binary systems certainly fails to give an unequivocal explanation of the nature of structural transformations that occur in the solvent under the influence of a solute at the molecular level. At the same time, our results show that by investigating these characteristics it is possible to carry out not only qualitative, but also quantitative analysis of packing effects induced by dissolution (solvation) of structurally related nonelectrolytes (in this case, of unsaturated fatty acids from the series C18:n) in CCl4. The following major conclusions may be drawn from this work. First, self-association of mixture components are
*The values of D0 were found by the Lorentz–Lorentz formula using the refraction indices and V2 from [25]. 1096
dominant among the dissolution effects of the UFAs in tetrachloromethane. Desolvation of acids becomes less appreciable when C=C double bonds increase in number in their molecules. Second, decreased endothermicity of acid dissolution in the series “oleicolinoleicolinolenic acid” is accompanied by compression (reduction of volume) of the structural packing of the resulting UFA-CCl4 solvate complex. The latter circumstance is of critical importance for modeling adsorption processes (in particular, within the framework of the theory of volume filling of micropores), and also for analyzing the efficiency of UFA selection (as surfactants on the surface of ferroparticles [3, 4]) for creating stable highly technological magnetic liquids in future. This work was supported by RFBR grant No. 03-03-32996 and by Programs of the Presidium of the Russian Academy of Sciences “Fundamental problems of physics and chemistry of nanosystems and nanomaterials” (State Contract 10002-251/P-08/128-134/030603-455).
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