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Chemistr y and Technology of Fuels and Oils, Vol. 44, No. 5, 2008

METHODS OF ANALYSIS

DETERMINATION OF THE MACROSCOPIC DIPOLE MOMENT

OF ASSOCIATED SOLVENTS

S. V. Tyumkin

____________________________________________________________________________________________________

Central Volga Research Institute on Oil Refining Co. Translated from Khimiya i Tekhnologiya Topliv i

Masel, No. 5, pp. 45 47, September October, 2008.

0009-3092/08/44050352 2008 Springer Science+Business Media, Inc.

The solubility (crystallization) of n-alkanes and mixtures (paraffins) in unassociated (chloroalkanes,

toluene, hexane) and associated (ketones, alcohols, carboxylic acids) solvents and in binary mixtures

of these solvents was investigated. The values of the macroscopic dipole moment (arbitrary

polarity ) were determined for the fi rst time for acetone and its homologs, up to methyl hexyl ketone,

for C4-C

7aliphatic alcohols, and for carboxylic acids from butyric to caprylic. Using the dependences

of the arbitrary polarity on the molecular weight and molar refraction of the solvents, the polarity

values were calculated for water, methanol, formic acid, and other associated substances. The mechanism

of the increase in the solubility of water, glucose, and glycine with a decrease in the difference between

the polarity of the solvent and the dissolved substance was demonstrated.

The effect of the nature of the solvent on the solubility (crystallization) in the liquid liquid and

liquid solid system is evaluated with empirical polarity parameters [1-10] determined with the spectral,

thermodynamic, kinetic, or polar (dielect ric constant, dipole moments of the molecules, etc.) characteristics of the

substances.

The dipole moments m of molecules of associated solvents with polar functional OH groups alcohols

and water, CO groups ketones and COOH in monocarboxylic acids create the following values of m for

homologs of these classes of compounds: 1.70.1, 2.70.1, 0.20.2 D.*

In the electrostatic theory of the intermolecular interaction, the significant difference in associated solvents

with respect to the dissolving power cannot be explained by the values of m. Modeling of the structure of

water [11, 12] and a number of solvents [13, 14] with calculation of m in the assumption of fo rmation of associates(clusters) with 2 to 100 molecules showed that the value of the dipole moment for the associates can be 2-5 times

*1 D = 3.33566410-30 Cm.

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higher than the value for the molecule. The problem of experimental determination of the dipole moment of an

associated solvent as macroproperties of a substance in the liquid phase has not yet been solved.

We propose a method for determining the polarity of associated solvents in the sense of the dipole

moment and understood in [3] as the global ability of a solvent to react with a dissolved substance with formation

of a solid phase.

The method is based on use of data on the solubility (crystallization) of high-melting n-alkanes and

mixtures of them (paraffins) in three groups of solvents:

in almost unassociated substances low-molecular-weight alkanes, chloroalkanes, nitroalkanes, toluene;

the dipole moment of these substances characterizes their dissolving power [15 16];

in associated substances ketones, aliphatic alcohols, monocarboxylic acids, water;

in binary mixtures of solvents from the first and second groups.

When the solubility of a substance (for t= const) in the listed groups of solvents is the same, the dipole

moment of the solvents in the second and third groups was set equal to the dipole moment of the solvents in the

first group. The dipole moment of a polar associated solvent was calculated with the molar composition of two-

three binary solvents [15, 16]. Its values differed from the average by a maximum of 0.1 D. Binary solvents were

used in the case of formation of a second liquid and/or solid phase in the dissolved substance single-component

associated solvent system and for verifying the rules of additivity.

The results of determining the arbitrary polarity Pa

of associated solvents are reported in Table 1. The

values of Pa

obtained are correlated to a high degree (r2 > 0.99) with the molecular weight M and molar

refractionR for ketones and alcohols and to a lesser degree (r2 = 0.94-0.96) for carboxylic acids in equations of the

type

b/MaPa += (1)

b/RaPa += (2)

where a and b are coefficients.

Table 1

Coefficient of Eq.

(1) (2)Solvent Pa, D

a b a b

Acetone 3.2

Methyl ethyl ketone 2.55

Methyl isobutyl ketone 1.85

Methyl hexyl ketone 1.4

0.064 189.4 0.188 48.89

Butanol 2.5

Hexanol 1.8Octanol 1.4

0.05 189.2 0.113 52.68

Acid

butyric 2.3

valeric 1.8

caproic 1.6

caprylic 1.4

0.064 200.55 0.276 43.25

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Using the coefficients reported in Table 1, the values of the convention polarity Pa

for water,

low-molecular-weight alcohols, and carboxylic acids, in which nonpolar high-melting n-alkanes and their mixtures

(paraffins) are almost insoluble or form a second liquid phase, were calculated with Eqs. 1 and 2 (Table 2).

The values ofPa

calculated with the molecular weight and equations for ketones and alcohols are almost

the same, but are 0.15-0.62 D lower than the values calculated with the equation for acids. The differences

ofPafrom the average value are maximum for water (0.4 D) and methanol (0.2 C) and do not exceed 0.1 D for the

other solvents.

The data from calculating Pa

with the molar refraction vary within wider limits and with greater differences

from the average values, except for the data for alcohols.

Fig. 1. Dielectric constant e for C1-C

8alcohols, C

2-C

8ketones, and water as a function of

arbitrary polarity Pacalculated with the molecular weight (solid curve) and molar refraction

(dashed curve).

Pa

Table 2

Value ofPa (D) calculated with the equation forSolvent

ketones alcohols acids average

Difference

ofPa (D) from

average value

With molecular weight

Water 10.45 10.45 11.07 10.66 0.4

Methanol 5.85 5.85 6.20 5.96 0.2

Ethanol 4.05 4.05 4.29 4.13 0.1

Propanol 3.09 3.10 3.27 3.15 0.1

Acid

formic 4.05 4.06 4.29 4.13 0.1

acetic 3.09 3.10 3.28 3.15 0.1

propionic 2.49 2.50 2.64 2.55 0.1

With molar refractionWater 12.89 13.80 11.51 12.73 1.1

Methanol 6.04 6.41 5.45 5.97 0.5

Ethanol 3.98 4.19 3.63 3.93 0.3

Propanol 2.99 3.13 2.75 2.95 0.2

Acid

formic 5.99 6.37 5.41 5.93 0.5

acetic 3.95 4.17 3.61 3.91 0.3

propionic 2.96 3.10 2.73 2.93 0.1

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The values of Pa

for water (10.5-13.8 D) correspond to the modeling data [11] for stable clusters

of 30-40 molecules and the value of Pa

for methanol (~6 D) correspond to the value for its pentameric

cluster [13, 14].

The dielectric constant e as a function of the arbitrary polarity Pa

for alcohols, ketones (see

Tables 1 and 2), and water is shown in Fig. 1. This curve is linear in determination ofPa

both with M and

withR (in a first approximation), while there is no correlation between and m of the molecules.Figure 2 shows the effect of the dipole moment m of molecules of unassociated and the arbitrary

polarity Pa

of associated solvents on the solubility (crystallization) of substances of different polarity, in particular,

water (average Pa= 11.7 D), glucose (m = 14.1 D), and glycine (m = 20.8 D) with the data in [17-19]. Solubility of

these subs tances higher than 0 .1 mole f rac t ion is observed in solvents wi th polar i ty grea ter

than 3, 10, and 12 D, which is in agreement with the change in the polarity of the dissolved substances.

As the polarity of the solvent approaches the polarity of the dissolved substance, the solubility of the

latter monotonically increases to the maximum, equal at the limit (in the case of solubility in itself) to 1 mole

fraction. The greater the difference between the values of the polarity of the solvent and the dissolved substance,

the lower the solubility of the dissolved substance (at t= const).

Based on the data on the solubility (crystallization) of high-melting n-alkanes and their mixtures (paraffins)in associated and unassociated solvents, the macroscopic dipole moment (arbitrary polarity P

a) of molecules of

water, aliphatic alcohols from methanol to octanol, homologs of acetone, and low-molecular-weight carboxylic

acids was determined. The effect of the arbitrary polarity of the solvents on the solubility of substances of

Solubility,molefraction

Fig. 2. Effect of the dipole moment m of unassociated and arbitrary polarity Pa ofassociated solvents on the solubility of water (P

a= 11.7 D) at 38C (curve 1), glucose

(m = 1.41 D) at 25C (curve 2), and glycine (m = 20.8 D) at 25C (curve 3):

trichloroethylene; o trichloromethane; propanol; VVVVV acetone;

dimethylformamide; ethanol; UUUUU N-methylpyrrolidone; mixture of

ethanol and methanol;SSSSS methanol; isopropanol; mixture of acetone and

water;ddddd water; mixture of ethanol and water.

Pa, D

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different polarity was demonstrated. The maximum solubility of a substance in a solvent (at t= const) is observed

in equality of their polarity values.

The values of the arbitrary polarity obtained for widely used solvents allow estimating their dissolving

power relative to a dissolved substance from the point of view of the electrostatic theory of the intermolecular

interaction in solvents.

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