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Determination of Three-Dimensional Solubility Parameters and Solubility Spheres for Naphthenic Mineral Oils Martina Levin* and Per Redelius Nynas AB, Nyna¨shamn, SE-149 82, Sweden ReceiVed April 14, 2008. ReVised Manuscript ReceiVed June 9, 2008
The Hansen solubility parameters were determined for three hydrocarbon fluids, equivalent to naphthenic mineral oil of different degrees of refinement. This was done by an indirect method based on the solubility of the hydrocarbon fluids in a system of solvents with known Hansen solubility parameters. It was concluded that the method may serve as a tool to estimate the approximate solubility parameters of naphthenic oil. However, in this study, the accuracy was insufficient to separate naphthenic oil of different degrees of refinement.
Introduction Mineral Oil. Naphthenic mineral oil is often used as process oil in the manufacture of rubbers,1,2 adhesives,3,4 and inks.5 Preferred process oils have a certain degree of miscibility or solvency in the polymers. The solubility of naphthenic mineral oil depends upon its origin and the degree of refining. Crude oil, from which mineral oil is produced, contains thousands of different types of hydrocarbon molecules of various sizes and lengths. An average oil molecule may contain several different structures, e.g., straight- and branched-chain saturated hydrocarbons (paraffins), closed-ring saturated hydrocarbons (naphthenes), and unsaturated, aromatic hydrocarbons. Naphthenic mineral oil is derived from naphthenic crude oil by various refining methods, such as acid treatment, solvent extraction, hydrogen treatment, or combinations of these processes. A naphthenic crude oil contains a large proportion of closed-ring saturated groups that gives the oil noticeably different characteristics compared to the more common paraffinic mineral oil, which contains a higher proportion of straightchain saturated hydrocarbons. Even though hydrocarbons are the dominant group of compounds present, crude oil also contains heteroatoms, such as nitrogen, oxygen, and sulfur. Most heteroatom compounds are removed from the oil during refining, e.g., during solvent extraction or hydrotreatment, but some are retained to enhance various properties in the finished product. The heteroatom compounds are believed to be located in the more aromatic parts * To whom correspondence should be addressed: Nynas AB, Nyna¨shamn, SE-14982, Sweden. Telephone: +48-8-520-652-43. Fax: +46-8-520-20743. E-mail:
[email protected]. (1) Kurz, S. S.; Sweely, J. S.; Strout, W. J. Plasticizers for rubber and related polymers. In Plasticizer Technology; Bruins, P. F., Ed.; Reinhold Publishing Corporation: New York, 1965; Vol. 1. (2) Corman, B. G.; Deviney, M. L., Jr.; Whittington, L. E. Migration of extender oil in natural and synthetic rubber-IV. Rubber Chem. Technol. 1970, 43 (6), 1349–1358. (3) Neau, A. Improved adhesives. Polym. Paint Colour J. 2005, 195 (4492), 50–52. (4) Gala´n, C.; Sierra, C. A.; Go´mez Fatou, J. M.; Delgado, J. A. A hotmelt pressure-sensitive adhesive based on styrene-butadiene-styrene rubber. The effect of adhesive composition on the properties. J. Appl. Polym. Sci. 1996, 62 (8), 1263–1275. (5) Schmidt, U. Precipitation temperature as a characteristic value in the quality control of printing ink resins. Am. Ink Maker 1994, 72 (5), 42– 43, 46, and 71.
of the oil. Freshly refined solvent extracted oil may contain oxygen, whereas virtually no oxygen is present in freshly refined hydrotreated oil. However, the oil may oxidize upon storage, creating different oxygen-containing compounds. The presence of heterocompounds influences the solubility of the process oil by introducing polarity to the otherwise nonpolar hydrocarbon oil structure. Because the number of heterocompounds is small compared to the amount of nonpolar hydrocarbons, the oil is still considered to be nonpolar. Solubility Test Methods Used for Naphthenic Oils. Solubility is an experimental parameter used to monitor the compatibility of different materials. The solubility of a process oil largely depends upon the structure of the hydrocarbon chain, e.g., paraffinic content and degree of aromaticity. There are a variety of ways to estimate solubility of naphthenic process oils, such as the viscosity gravity constant (ASTM method D 2501), aniline point (ASTM method D 611), and Kauri-butanol number (ASTM method D1133). The viscosity gravity constant (VGC) is a dimensionless constant based on mathematical processing of viscosity and density values, whereas the aniline point is based on the measurement of the temperature at which aniline dissolves in the oil (aniline is very soluble in aromatic hydrocarbons but only slightly soluble in aliphatics). The Kauributanol number of an oil represents the maximum amount of oil that can be added to a stock solution of kauri resin in n-butanol without causing cloudiness. Most of these methods are very empirical and difficult to correlate to methods used by rubber, adhesive, and ink producers, hence, the need to search for a better tool to describe solubility properties. One alternative way of stating solubility is by using solubility parameters. This method has found usage especially in the area of polymers. If solubility parameters could be used to predict solubility, this would facilitate predictions of interactions between the process oil and different polymer mixtures. However, naphthenic process oil is a complex combination of a variety of molecules, and because most solubility parameters are developed for single-component materials, it is not completely evident that solubility parameters are appropriate for describing the solubility and compatibility of the oil. Solubility Parameters. The term “solubility parameter” was first described in the book “Regular Solution Theory” by Hildebrand and Scott, but prior work performed by Scatchard
10.1021/ef800256u CCC: $40.75 2008 American Chemical Society Published on Web 07/19/2008
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was tributary to this evolution.6–8 The solubility parameter is sometimes also called the cohesion energy parameter because it is derived from the energy required to vaporize 1 mol of the liquid involved. Cohesive energy describes the intermolecular forces and can be defined as the square root of the cohesive energy density. The relationship between the solubility parameter and cohesive energy density is shown in the following expression: δ ) √c )
-
E ) Vm
∆H - RT Vm
(1)
where δ is the solubility parameter, c is the cohesion parameter, E is the latent heat of evaporation, Vm is the molar volume, ∆H is the heat of vaporization, R is the gas constant, and T is the temperature. However, several different interpretations of cohesive energy are available in the literature.8–11 The approximation made in eq 1 by substituting the molar cohesive energy with the energy of vaporization might generate small deviations in the solubility parameter.12 The Hildebrand solubility parameter was originally intended for nonpolar, non-associating systems, but the concept has been further expanded for improved fit to polar systems. Arkel, Small, and Prausnitz, respectively, divided the solubility parameter into one nonpolar and one polar component.13 The concept of Hansen’s three-dimensional solubility parameter (HSP) is based on the notion that the cohesive intermolecular forces of a substance can be expressed as the sum of three components: dispersive interactions, polar interactions, and hydrogen-bonding interactions (eq 2).14 The dispersive forces consist of interactions between induced dipoles (London forces), while forces between permanent dipoles (Keesom forces) and forces between permanent and induced dipoles (Debye forces) are included in the polar forces. The last interaction contributing to the cohesive energy is hydrogenbonding interactions δ ) (δd2 + δp2 + δh2)1/2
known HSP can be used.15 Variations of this method have successfully been used to calculate the HSP of bitumens and polymers.13,16 The HSP for a process oil can be visualized as a point in space, with the coordinates (δd, δh, and δp), surrounded by its solubility body. The center coordinates in the solubility body represent the partial solubility parameters for the tested process oil. Usually 40-50 test liquids are used, and each liquid is noted as being either a good solvent or a nonsolvent for the oil. This reasonably large number is required for accuracy and cancelation of the effect that molecular volume has on the solvent power, which is otherwise not taken into consideration in the HSP. If an approximate solubility parameter is known, the selection of solvents can be designed to enhance the resolution. Otherwise, the selection of solvents should vary in HSP as much as possible. A high number of solvents located close to the estimated solubility sphere surface (as described below) generally improves the resolution. When the miscibility of the 40-50 test liquids is known, the solubility body of the oil can be calculated using eq 3 if the solubility body is assumed symmetric Ra2 ) a(δd1 - δd2)2 + b(δp1 - δp2)2 + c(δh1 - δh2)2
where Ra is the distance between one solvent and the process oil, δd1 is the dispersion component for the solvent, δd2 is the dispersion component for the process oil, and a, b, and c are weighting factors. Hansen has suggested on the basis of empirical tests setting a ) 4 and b )c ) 1 because a doubling of the dispersive force converts the otherwise elliptic shaped volume of the solubility body to an almost spherical volume.17 However, the use of fixed variables for complex mixtures has been questioned.18 Other experiments have showed that, in general, solubility regions are unsymmetrical.19 If Ra is divided by the radius of the solubility sphere, R0, a useful parameter called relative energy difference (RED) is obtained (eq 4)
(2)
where, δd is the dispersion component, δp is the polar component, and δh is the hydrogen-bonding component. Even though the total HSP corresponds to the Hildebrand solubility parameter, the two quantities should not be expected to be identical. Determination of the Hansen Solubility Parameter. The HSP for a pure liquid can be calculated from its physical and chemical properties, but this is not possible for complex mixtures, such as naphthenic oils. Instead, an alternative indirect method based on the miscibility of the mixture in solvents of (6) Hildebrand, J.; Scott, R. L. The Solubility of Nonelectrolytes, 3rd ed.; Reinhold: New York, 1950. (7) Hildebrand, J.; Scott, R. L. Regular Solutions; Prentice-Hall. Inc.: Englewood Cliffs, NJ, 1962. (8) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press, Inc.: Boca Raton, FL, 1983. (9) Maloney, D. P.; Prausnitz, J. M. Solubility of ethylene in liquid, low-density polyethylene at industrial-separation pressures. Ind. Eng. Chem. Process Des. DeV. 1976, 15 (1), 216–220. (10) Lyckman, E. W.; Eckert, C. A.; Prausnitz, J. M. Generalized liquid volumes and solubility parameters for regular solution application. Chem. Eng. Sci. 1965, 20, 703–706. (11) Hoy, K. L. New values of the solubility parameters from vapor pressure data. J. Paint Technol. 1970, 42 (541), 76–118. (12) Verdier, S.; Andersen, S. I. Internal pressure and solubility parameter as a function of pressure. Fluid Phase Equilib. 2005, 231 (2), 125–137. (13) Gharagheizi, F.; Sattari, M.; Angaji, M. T. Effect of calculation method on values of Hansen solubility parameters of polymers. Polym. Bull. 2006, 57 (3), 377–384, and references therein. (14) Hansen, C. M. Hansen Solubility Parameters, A User’s Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007.
(3)
RED )
Ra R0
(4)
A solvent with identical partial solubility parameters to the process oil will have a RED equal to 0; good solvents will have a RED value of less than 1.0; and for nonsolvents, the RED is larger than 1.0. Solvents with an RED equal to 1.0 are located on the surface of the solubility sphere. Using solvents with known solubility parameters and an interactive computer program, the HSP parameter and the solubility volume radius for different naphthenic oils were calculated. Turbidimetric Titration. Turbidimetric titration is a process in which a solvent is added incrementally to a highly dilute polymer (or bitumen) solution and the intensity of light scattered by or the turbidity because of the formation of a two-phase system is measured as a function of the amount of solvent added. Titration is often used to determine the solubility of polymers, (15) Hansen, C. M. Hansen Solubility Parameters, A User’s Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007; Chapters 1 and 9. (16) Redelius, P. Bitumen solubility model using hansen solubility parameter. Energy Fuels 2004, 18 (4), 1087–1092. (17) Hansen, C. M. Hansen Solubility Parameters, A User’s Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007; Chapter 1. (18) Zellers, E. T. Three-dimensional solubility parameters and chemical protective clothing permeation. I. Modelling the solubility of organize solvents in viton gloves. J. Appl. Polym. Sci. 1993, 50 (3), 513–530. (19) Wisniewski, R.; Smieszek, E.; Kaminska, E. Three-dimensional solubility parameters: Simple and effective determination of compatibility regions. Progress Org. Coat. 1995, 26 (2-4), 265–274.
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Table 1. Characteristic Data of the Test Fluids characteristic data color density at 15 °C (kg/m3) refractive index, 20 °C viscosity, 40 °C (mm2/s) (cSt) viscosity, 100 °C (mm2/s) (cSt) sulfur (wt %) nitrogen (ppm) hydrocarbon-type analysis, Ca/Cn/Cp (%)
N-oil
D-oil
The Hansen ellipsoid algorithm (eq 6) uses the well-known Hansen fit with fixed variables.
method ASTM
PAO
D1500 D4052
50.0 6.7 1.3 7.0 21 12.4 1.0 3.5 >50.0 4.5
also the case for PAO, because being a synthetic iso-alkane, it does not contain any aromatic compounds. The moderately high values for dispersive interactions and the low polar interactions (close to zero) obtained in this study were therefore anticipated. The difference seen in δP between the axis-aligned ellipsoid and the Hansen ellipsoid algorithm is due to the use of variable coefficients instead of constants. The axis-aligned ellipsoid algorithm, which uses variable coefficients, generated a slightly higher value on polar interaction than the Hansen ellipsoid algorithm that uses constants. The Hansen results are more in line with expectations. The values for hydrogen interactions were unexpectedly high,
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Table 5. HSP Values Obtained Using Either Hansen Ellipsoid Algorithm or Axis-Aligned Ellipsoid Algorithm algorithm used Hansen ellipsoid algorithm PAO N-oil D-oil axis-aligned ellipsoid algorithm PAO N-oil D-oil
δD (MPa0.5)
δP (MPa0.5)
δH (MPa0.5)
Ra (mm)
Hfit
number of outliers
15.7 15.6 16.6
0.0 0.0 0.0
5.2 4.1 5.0
8.38 9.94 9.76
0.876 0.854 0.899
14 12 6
15.5 17.2 17.3
1.7 3.0 3.3
5.5 5.3 5.3
8.03 7.28 7.47
0.902 0.881 0.908
17 13 9
between 4.1 and 5.5 MPa0.5. Previous studies using the same interactive computer program have generated values of the hydrogen-bonding contribution in line with anticipations, for example, a δH ) 4.4 MPa0.5 for naphthenic bitumen and a δH ) 0.6 MPa0.5 for naphthenic crude oil.16 The cause for the higher hydrogen interaction values obtained in this study is not fully elucidated. One possible explanation is that the high values may be mathematical artifacts. From the plot of the solubility sphere for the N-oil (Figure 1), it is evident that there is a lack of nonsolvents with low hydrogen-bonding interactions. This suggests that the computer program had insufficient data to estimate the true shape of the sphere at low hydrogen bonding, thus generating the high values on hydrogen interactions. The higher fit obtained by using the axis-aligned ellipsoid algorithm compared to the Hansen ellipsoid algorithm is probably due to the use of variable coefficients instead of the Hansen fixed coefficients. When using all 108 solvents, the coefficients calculated by the axis-aligned ellipsoid algorithm are a ) 3.83, b ) 8.70, and c ) 11.60 for the N-oil; a ) 5.30, b ) 8.41, and c ) 11.60 for the PAO; and a ) 4.13, b ) 8.70, c ) 11.60 for the D-oil. These are all notably higher than those suggested by Hansen: a ) 4 and b ) c ) 1. One possible reason for these large differences could be the uneven distribution of nonsolvents throughout the solubility space, which meant that there was insufficient information to make a precise estimate of the true shape of the pseudo-sphere. Of the 108 solvents used, 6-17 outliers were detected. An outlier is here defined as a solvent or a nonsolvent ending up on the wrong side of the interface of the best-fitted spheroid.
Figure 1. Plot of the solubility sphere for N-oil using the Hansen ellipsoid algorithm and the locations of tested solvents. Filled triangles correspond to nonsolvents, and unfilled triangles correspond to good solvents.
The number of outliers depends upon the test fluid and the algorithm chosen for making the fit. When using the axis-aligned ellipsoid algorithm, 13 outliers were found for N-oil, 17 outliers were found for PAO, and 9 outliers were found for the D-oil. When using the Hansen ellipsoid algorithm, 12 outliers were found for N-oil, 14 outliers were found for PAO, and 6 outliers were found for the D-oil. Generally, D-oil had the lowest number of outliers, and PAO had the highest number of outliers. This is probably due to the higher solubility of the D-oil. Ideally, the number of outliers should be as low as possible, but when outliers have RED values close to 1.0, then they are located near the interface. The presence of this kind of outlier is expected, and further optimization of solubility sphere calculation will probably decrease their number. In addition, some of the solvents were hygroscopic, and even though precautions were taken, some water may still have been present, resulting in a change in the solubility of the solvent. A few outliers were located far away from the interface, such as aniline, diacetone alcohol, and 1,2-dimethoxy benzene, which were tested as nonsolvents but their solubility parameters were located inside the sphere, and salicylaldehyde, which was tested as a good solvent (for N- and D-oil) but its solubility parameters fell outside the sphere. The calculated solubility parameters used as coordinates for the solvents in the solubility space were collected from the literature.23 Some of these calculated solubility parameters are also confirmed by experimental data. However, the values are not precise, especially not for substances with a high HSP.23 The solubility character of a substance may be altered when the substance is introduced to different surroundings; for example, the behavior of a substance in water is different from its behavior in a hydrocarbon liquid. Some of the solvents tested can undergo internal hydrogen bonding, which also influences their electron distribution and hence their solubility parameter (Figure 2). These shifts in solubility parameter may to some extent explain the presence of outliers. Nevertheless, the existence of outliers located far from the interface of the sphere indicates that there are some aspects of the solubility of the test fluids that are not fully covered by the Hansen approach. Variations between Different Hydrocarbon Test Fluids. In this study, only small variations in HSP and solubility spheres were observed between the three test fluids (Figure 3). This is due to the small number of solvents exhibiting different solubility in the three test fluids. Solvents that act as nonsolvents in one oil and as good solvents in another are crucial to obtain good estimations of the difference in solubility properties known
Figure 2. Intramolecular hydrogen bonding in salicylaldehyde, one of the solvents used.
Naphthenic Mineral Oils
Figure 3. Solubility spheres for PAO (blue), N-oil (gray), and D-oil (red) calculated using the Hansen ellipsoid algorithm.
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solubility sphere of N-oil obtained in this study, some interesting results are seen. The HSP of nitrile rubber24 (δD ) 19.8, δP ) 13.3, and δH ) 2.2) is located a small distance from the solubility sphere of the N-oil. The HSP of neoprene rubber24 (δD ) 16.0, δP ) 8.8, and δH ) 4.0) is close to the surface of the solubility sphere, and the HSP of butyl rubber24 (δD ) 17, δP ) 1.5, and δH ) 0.0) is located inside the solubility sphere. This corresponds to the general understanding that nitrile rubber and naphthenic oil are not miscible, while it is possible to add some naphthenic oil to neoprene (usually up to 10%). There should not be a problem with compatibility when adding naphthenic oil to butyl rubber. The small difference between the solubility spheres of PAO and N-oil indicate that PAO would be miscible in neoprene. However, this is not the case. Titration Results. To increase the precision of the solubility spheres of the three test fluids, titration experiments were conducted. In the first titration experiment, N-oil was titrated with nonsolvents and the experiment demonstrated that naphthenic oils differ from polymers and bitumens in that they have virtually no semisolvents (Table 3). In most cases, the addition of one drop (0.01 mL) of nonsolvent titrant to the N-oil was enough for creation of a two-phase system. The titration method was then changed; this time, the N-oil was mixed with a nonsolvent giving a two-phase system that was titrated back to a one-phase system by using a good solvent as the titrant. The titration results from the second titration were used in recalculation of the solubility spheres. This could be performed because the hsp3D program permits the user to define points located exactly on the surface of the solubility spheroid (RED ) 1). The recalculated spheroids were considerably different in shape from the original spheres. When benzaldehyde was used as a titrant, the sphere became smaller but predominantly kept its shape and location. When o-xylene was used as a titrant, the sphere volume was strikingly reduced, whereas in the cases of MEK and dichloromethane, the resulting spheres were similar in size and shape to the sphere obtained from benzaldehyde, with the exception that they were slightly shifted toward higher dispersive forces. Conclusions
Figure 4. Differences in size of solubility spheres of one bitumen (blue), one crude oil (red), and the N-oil (gray) using the Hansen ellipsoid algorithm.
to exist. Despite the unusually large number of tested solvents, only two solvents, acetone and 1.2-dimethoxybenzene, proved to be nonsolvents for the N-oil but good solvents for the D-oil. This low number contrasts with the pronounced differences in solubility properties of the two oils seen in other tests. Between the N-oil and PAO, the difference was larger, with 11 good solvents for N-oil being nonsolvents for PAO. This is visible by the smaller solubility sphere of PAO. Different bitumens and crudes may have large variations in solubility depending upon their origin. One naphthenic bitumen and one naphthenic crude oil have been chosen here to illustrate the differences in solubility compared to naphthenic oil. The solubility spheres of the three materials are plotted on the same graph (Figure 4), and the evidently larger solubility sphere of naphthenic oil is visualized. Naphthenic oils are often used as process oils in polymer blends. If the HSP of common industrial rubbers, e.g., nitrile, neoprene, and butyl rubber, are compared to the HSP and
The HSP and the solubility sphere for three hydrocarbon test fluids were determined by calculations based on the solubility of the fluids in 108 solvents with known solubility parameters. The test fluids, which corresponded to naphthenic oils of different degrees of refinement, proved to be soluble in a large number of different solvents compared to earlier tests on bitumen and naphthenic crude oil.16 In this study, the lack of nonsolvents with low hydrogen-bonding interactions limited the ability of the computer program to estimate the true solubility shape of the sphere. Variations of this method have been used successfully to calculate the HSP of bitumens and polymers.13,16 It may also serve as a tool to estimate the approximate solubility parameter of naphthenic oil. However, in this study, the accuracy was insufficient to distinguish between different degrees of refinement of naphthenic oil. EF800256U (23) Hansen, C. M. Hansen Solubility Parameters, A User’s Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007; Appendix A. (24) Hansen, C. M. Hansen Solubility Parameters, A User’s Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007; Chapter 12.
