Article pubs.acs.org/EF
Determining the Hansen Solubility Parameter of Three Corrosion Inhibitors and the Correlation with Mineral Oil Martina Levin* and Per Redelius Nynas AB, Nynäshamn, SE-14982, Sweden ABSTRACT: The Hansen solubility parameters were determined for three commercial organic corrosion inhibitors of surface adsorption type, i.e., benzotriazole, tolyltriazole, and Irgamet 39 (a N-methylamino-substituted triazole). The determination was performed using an indirect method using the solubility of the inhibitors in various solvents. The calculated Hansen solubility parameters of the inhibitors were compared to the Hansen solubility parameters and solubility sphere of refined naphthenic mineral oil. The obtained Hansen solubility parameters of benzotriazole indicate very low solubility in mineral oils, while the solubility of tolyltriazole was slightly higher because of the methyl substitution. For Irgamet 39, full miscibility with refined mineral oil was indicated. The adsorption kinetics of the inhibitors in naphthenic oil were considered and related to the Hansen solubility parameters.
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parameters,22 while no such figures have, to our knowledge, been published for TTA and Irgamet 39. Naphthenic Mineral Oil. Naphthenic mineral oil is derived from naphthenic crude oil using various refining processes, such as acid treatment, solvent extraction, hydrogen treatment, or combinations of these processes. The hydrocarbon components of a mineral oil are composed of paraffinic, naphthenic, and aromatic groups. Naphthenic oil contains a larger proportion of naphthenic (cycloalkane) groups than the more common paraffinic mineral oil. The presence of naphthenic groups gives the oil noticeably different solubility characteristics from those of the paraffinic mineral oil. Besides their hydrocarbon content, mineral oils also contain heterorganic components. This nonhydrocarbon content generally consists of organic molecules containing one or possibly two atoms that are not carbon or hydrogen. The dominant heteroatoms are nitrogen, oxygen, and sulfur. Despite the low content of heterorganic components, their influence on the mineral oil is considerably high, affecting, for example, the solubility.23 Determining the Hansen Solubility Parameters. The solubility parameter is a parameter used for estimation of the degree of interaction between materials. Hansen’s threedimensional solubility parameter (eq 1) is based on the notion that the cohesive intermolecular forces of a material can be expressed as the sum of three interactions: dispersive, polar, and hydrogen bonding24
INTRODUCTION Adsorption Corrosion Inhibitors. Organic corrosion inhibitors are extensively used to protect copper and related alloys in aqueous media,1−3 but they have also found usage in non-aqueous media, such as fuels,4 lubricants,5 insulating oils,6,7 and motor oils.8 Heterocyclic molecules containing one or more heteroatoms, such as benzotriazole (BTA) and tolyltriazole (5-methylbenzotriazole, TTA), are extensively used and known to be highly efficient in inhibiting corrosion on copper.9,10 Because BTA has historically been one of the most used organic corrosion inhibitors for aqueous systems, several analytical surface studies regarding the adsorption of BTA on copper in aqueous systems have been performed.1,11−16 However, significantly fewer studies have examined BTA use in non-aqueous systems.15,17 Few published studies have examined the adsorption of TTA on copper in either aqueous systems or non-aqueous hydrophobic systems.17−19 BTA is highly soluble in water but has limited solubility in hydrocarbon media, such as oil. The solubility of TTA in hydrocarbon media is slightly higher because of the methyl substitution. To improve the solubility of the triazoles in hydrocarbon media, the hydrophobicity needs to be enhanced and incorporation of long hydrocarbon chains in the triazole structure are common. This is the case of Irgamet 39, which is a N-methylamino-substituted triazole. Despite the wide use of such compounds in transformer and lubrication oil applications, we found only two surface adsorption studies of Irgamet 39 on copper in hydrocarbon media17,20 and none correlating the issue of solubility to the adsorption kinetics. The kinetics of film formation and the adsorption of organic corrosion inhibitors on metal are believed to be determined by the solubility of the inhibitor in the media, the mass transport of the inhibitor from the bulk to the surface, and finally, the adsorption mechanism, including monolayer formation.21 Thus, the solubility of the corrosion inhibitor has an impact on the corrosion-mitigating properties. A common way to estimate solubility, especially in complex media, such as mineral oil, relies on the concept of solubility parameters. For BTA, the literature contains one reference to the Hansen solubility © XXXX American Chemical Society
δ = (δd 2 + δp2 + δ h 2)1/2
(1)
where δ is the solubility parameter, δd is the dispersion solubility parameter, δp is the polar solubility parameter, and δh is the hydrogen-bonding solubility parameter. Interactions between induced dipoles (i.e., London forces) are regarded as the dispersive forces, while forces between permanent dipoles (i.e., Keesom forces) and between permanent and induced Received: August 3, 2012 Revised: November 7, 2012
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dipoles (i.e., Debye forces) are included in the polar forces. Hydrogen bonding is the attractive interaction of a hydrogen atom with an electronegative atom from another molecule. The three Hansen solubility parameters may be used as the coordinates of a point in a three-dimensional system known as the Hansen space.24 A substance can be visualized as a point in space with the coordinates (δd, δh, and δp), surrounded by its solubility sphere. The radius of the solubility sphere of the substance, also interaction radius, is denoted as R0. A solvent located on the inside of the solubility sphere is likely to be miscible. The Hansen solubility parameters of a pure liquid can be calculated from its physical and chemical properties (e.g., molar volume, dipole moment, refractive index, and dielectric constant).24 For complex mixtures, such as mineral oils, polymers, and bitumens, this approach is impossible; thus, an alternative indirect method is used instead.24−27 This indirect method uses the idea of a three-dimensional Hansen space, in which the location of each substance is given by its Hansen space coordinates. A test substance of unknown Hansen solubility parameters is mixed with a selection of solvents of which the Hansen solubility parameters are known. The location in the Hansen space of each solvent and the solubility with the test substance are then used to calculate the Hansen solubility parameters of the substance. These calculations are usually performed using software that evaluates the input data using a quality-of-fit function.24−26,28 The software output is the Hansen solubility parameters and the radius of the solubility sphere. A total of 40−50 various solvents are commonly used in the method. This fairly large number is required to obtain good accuracy and to cancel the impact of the molecular volume on solubility, which otherwise is not taken into consideration in the Hansen solubility parameters. A setup for selecting the solvents has been suggested by Hansen,29 but the selection of solvents can be adapted to enhance resolution in the specific case. If an approximate solubility parameter is known, increasing the number of solvents close to the estimated solubility sphere surface generally improves the resolution. For those cases in which an approximate solubility parameter is not known, the solubility parameters of the selected solvents should vary as much as possible. The solubility parameter distance, Ra, can be used to calculate the distance between two materials in the Hansen space. By assuming that the solubility body is symmetric, the solubility parameter distance can be calculated using eq 2
RED =
Ra R0
(3)
where Ra is the distance between one solvent and the substance of unknown solubility parameters and R0 is the radius of the solubility sphere of the substance. The RED parameter is used to calculate whether or not a solvent is located inside the solubility sphere. Good solvents will have a RED value of less than 1.0, meaning that they are located inside the sphere. Consequently, non-solvents will have a RED greater than 1.0, meaning that they are located outside the sphere. Solvents with a RED equal to 1.0 are located on the surface of the solubility sphere, meaning that they are on the rim of being a good solvent. In this study, we calculated the Hansen solubility parameters for three inhibitors using this indirect method and an interactive software program. The obtained Hansen solubility parameters were compared to the Hansen solubility parameters of a naphthenic mineral oil obtained in a previous study.32 In addition, because solubility is believed to be a factor influencing the kinetics of film formation, the adsorption kinetics of the inhibitors in naphthenic oil have been considered and related to the Hansen solubility parameters.
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EXPERIMENTAL SECTION
Materials. The three tested corrosion inhibitors were BTA, TTA, and Irgamet 39, all commercial inhibitors obtained from CIBA Specialty Chemicals (Basel, Switzerland); their structures are presented in Figure 1. The solvents used in the solvent interaction
Figure 1. Organic corrosion inhibitors: (1) BTA, (2) TTA, and (3) Irgamet 39, with R = 2-ethylhexyl. test were common laboratory solvents of high purity obtained from commercial chemical suppliers. The solvents, including their purities, are listed in Table 1. The adsorption kinetic study used a highly refined (i.e., severely hydrotreated and sulfur-free) naphthenic mineral base oil (HRNO). It has properties similar to those of the refined mineral oils commonly used in lubrication applications and as insulating oils in electrical power transformers.7 Some characteristics of the mineral oil are presented in Table 2. Solvent Interaction Test. To test the miscibility between the corrosion inhibitors and the solvents, 0.1 g of inhibitor and 1.0 mL of solvent were added to a 1.5 mL glass vial sealed with a silicon septa screw cap and shaken vigorously. The formation of a one- or twophase system was noted immediately and again after 24 h. If the inhibitor and solvent formed a one-phase system, the solvent was denoted as a good solvent, and if a two-phase system formed, the solvent was denoted as a non-solvent. The data from this miscibility test were then used as input data to calculate the Hansen solubility parameters and the solubility sphere for each corrosion inhibitor. The Hsp3D software, developed jointly by Nynas AB (Nynäshamn, Sweden) and the Western Research Institute (Laramie, WY), was used for the calculation. This software has been described in detail by Redelius.26 The program calculates solubility spheres, with most of the good solvents located inside and most of the non-solvents located outside the spheres. If a solvent ends up on the wrong side of the solubility sphere boundary (e.g., a non-solvent located inside the solubility sphere), it is denoted as an outlier.
Ra 2 = a(δd1 − δd2)2 + b(δp1 − δp2)2 + c(δ h1 − δ h2)2 (2)
where δd1 is the dispersion component for component 1, δd2 is the dispersion component for component 2, and a, b, and c are weighting factors. Setting a = b = c = 1 would generate an elliptically shaped solubility. To convert the solubility body to a spherical volume, Hansen has suggested, on the basis of empirical tests, doubling the dispersive force by setting a = 4 and b = c = 1.24 Note that the use of fixed variables has been questioned30 and that studies have indicated that solubility regions are generally unsymmetrical.31 Even so, the use of the Hansen solubility sphere has found use and acceptance, especially in the area of polymers. By combining the solubility parameter distance and the solubility radius, a parameter called the relative energy difference (RED) can be calculated (eq 3) B
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Table 1. Hansen Solubility Parameters22 and Calculated RED Values (Generated by the Axis-Aligned Ellipsoid Algorithm) of the Various Solvents Used Herea
a
solvents
δd MPa0.5
δp MPa0.5
δh MPa0.5
Sol BTA
Sol TTA
Sol I39
RED BTA
RED TTA
RED I39
diisopropylamine (99%) acetone (99.9%) acetophenone (>99%) benzene (99.7%) 1-butanol (≥99.4%) γ-butyrolactone (>99%) carbon tetrachloride (>99%) chlorobenzene (>99.5%) chloroform (>99%) cyclohexanol (>99%) diacetone alcohol (>99%) o-dichlorobenzene (>99%) dimethyl sulfoxide (>99.5%) ethanolamine (>99%) ethyl acetate (>99.7%) hexane (>95%) isophorone (>97%) methanol (99.9%) methyl ethyl ketone (>99.0%) N-methylpyrrolidone (>99.0%) methylene dichloride (>99.5%) nitroethane (>98%) tetrahydrofuran (99.9%) toluene (>99.8%) o-xylene (>98%) salicylaldehyde (>98%) 1.2-dimethoxybenzene (>99%) aniline (>99%) benzaldehyde (>99%) acetic acid (>99.7%) formic acid (98%) cyclohexene (99%) pentane (99%) heptane (99%) 2-pyrrolidone (99%) propionylchloride (98%) acetaldoxime (99%) ethylene carbonate (99%) ethylene glycol (99.5%) isovaleraldehyde (97%) perfluoro-1,3-(dimethylcyclohexane) (80%) perfluoroheptane (85%) propionic acid (>99%) tetraethyl orthosilicate (98%) lactic acid (85%) o-dibromobenzene (98%) dibutyl sebacate (97%) oleyl alcohol (99%) bromoform (96%) glycerol (>99.5%) triethanolamine (>99%)
14.8 15.5 19.6 18.4 16.0 19.0 17.8 19.0 17.8 17.4 15.8 19.2 18.4 17.0 15.8 14.9 16.6 15.1 16.0 18.0 18.2 16.0 16.8 18.0 17.8 19.4 19.2 19.4 19.4 14.5 14.3 17.2 14.5 15.3 19.4 16.1 16.3 19.4 17.0 14.7 12.4 12.0 14.7 13.9 17.0 20.7 13.9 14.3 21.4 17.4 17.3
1.7 10.4 8.6 0.0 5.7 16.6 0.0 4.3 3.1 4.1 8.2 6.3 16.4 15.5 5.3 0.0 8.2 12.3 9.0 12.3 6.3 15.5 5.7 1.4 1.0 10.7 4.4 5.1 7.4 8.0 11.9 1.0 0.0 0.0 17.4 10.3 4.0 21.7 11.0 9.5 0.0 0.0 5.3 4.3 8.3 6.5 4.5 2.6 4.1 12.1 22.4
3.5 7.0 3.7 2.0 15.8 7.4 0.6 2.0 5.7 13.5 10.8 3.3 10.2 21.2 7.2 0.0 7.4 22.3 5.1 7.2 6.1 4.5 8.0 2.0 3.1 14.7 9.4 10.2 5.3 13.5 16.6 5.0 0.0 0.0 11.3 5.3 20.2 5.1 26.0 5.0 0.0 0.0 12.4 0.6 28.4 5.3 4.1 8.0 6.1 29.3 23.3
0 1 1 0 1 1 0 0 1 1 1 0 1 1 1 0 1 1 1 1 1 1 1 0 0 1 1 1 1 1 1 0 0 0 1 0 1 1 1 1 0 0 1 0 1 0 0 1 1 1 1
0 1 1 0 1 1 0 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 0 0 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1 0 0 1 0 1 1 0 1 1 0 1
1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 0 0 1 0 0 1 1 0 1 1 1 1 1 0
1.309 0.780 0.931 1.313 0.641 0.652 1.383 1.137 0.969 0.645 0.626 0.997 0.448 0.497 0.892 1.539 0.700 0.755 0.860 0.571 0.799 0.874 0.758 1.242 1.205 0.329 0.772 0.719 0.854 0.778 0.736 1.123 1.572 1.510 0.518 0.814 0.792 1.000 0.792 1.000 1.795 1.845 0.865 1.447 1.000 1.000 1.275 1.151 1.132 0.998 0.897
1.207 0.707 0.769 1.165 0.625 0.587 1.229 0.970 0.837 0.578 0.575 0.833 0.428 0.587 0.800 1.412 0.592 0.811 0.751 0.450 0.653 0.805 0.650 1.091 1.063 0.264 0.651 0.600 0.699 0.777 0.784 1.000 1.450 1.377 0.517 0.710 0.784 0.967 0.812 0.920 1.699 1.754 0.839 1.334 0.993 0.855 1.190 1.091 1.000 1.004 1.000
0.857 0.681 0.304 0.585 0.630 0.656 0.659 0.394 0.389 0.412 0.590 0.296 0.653 0.895 0.595 0.959 0.469 1.000 0.603 0.441 0.249 0.827 0.426 0.543 0.542 0.366 0.152 0.111 0.203 0.818 0.954 0.553 1.009 0.911 0.704 0.612 0.742 1.000 0.932 0.806 1.292 1.349 0.775 0.994 1.000 0.292 0.933 0.866 0.402 1.072 1.269
“Sol” stands for solubility, with a good solvent denoted as “1” and a non-solvent denoted as “0”. which the data are fitted according to Hansen, except that variable coefficients are applied to the weighting factors. The rotated ellipsoid algorithm fits the data in a way comparable to that of the axis-aligned ellipsoid algorithm, except that the solubility body is allowed to rotate and tilt to obtain a better fit. The last algorithm, the convex hull algorithm, does not generate a traditional sphere; instead, the smallest convex region that contains all good solvents is generated.
The software provided six algorithms for calculating the solubility parameters, four of which were used here. The algorithms allowed use of both fixed and variable weighting factors to obtain the best fit between the solubility surface and the solubility data. The first of the four algorithms is the Hansen ellipsoid algorithm, which uses the traditional Hansen fit with fixed weighting factors a = 4 and b = c = 1 (eq 2). The second algorithm is the axis-aligned ellipsoid algorithm, in C
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axis-aligned ellipsoid algorithms. The Hansen ellipsoid algorithm produced the fewest outliers and the highest Hfit of data, i.e., above 0.9 for both BTA and Irgamet 39 (Table 3).
Table 2. Some Physical and Chemical Properties of the Mineral Oil characteristics density at 15 °C viscosity at 40 °C viscosity at 100 °C flash point, PM pour point sulfur acid number refractive index copper corrosion hydrocarbon-type analysis CA CN CP color color
unit −3
kg dm mm2 s−1 (cSt) mm2 s−1 (cSt) °C °C wt % mgKOH/goil
ASTM test method D4052 D445 D445 D93 D97 D2622 D974 D1218 IEC 62535 D2140
% % % D1500 D156
HRNO
Table 3. Hansen Solubility Parameters Generated by the Hansen Ellipsoid and the Axis-Aligned Ellipsoid Algorithms
0.879 7.6 2.1 145 −63
