:LEANING TIMES
Solubility Parameters: Part II by John B. Durkee II, Ph.D., P.E., Creative EnterpriZes
~
n the J a n u a r y column, we began coverage of a topic about which most users don't have a good understanding, b u t of which they should have a strong need - - solubility parameters. Our choice of solvent for a particular application involves m a n y factors including environmental, safety, and health concerns; evaporation rate; surface tension; technical service from the supplier; and selling price. At a minimum, we insist on using a solvent that will be effective in dissolving one material while leaving other materials unaffected. The selection of solvents (today), and solvent blends (tomorrow), to satisfy multiple criteria is a justification for suppliers to present papers, users to be concerned, and consultants to have employment. Selection can be a fine art based on experience, trial and error, and intuition. There are rules of t h u m b as "like dissolves like," "take it off the way it went on," and various definitions of solvent "strength." Although it m a y not be necessary to understand q u a n t u m c h e m i s t r y to remove fingerprints, an organized system is needed that can facilitate the accurate prediction of complex solubility behavior. Formulators of coatings and paints value good systems as they would their first-born. Those serious about solvent cleaning would value good systems as they would a Chicago deep-dish pizza or a '57 Chevy coupe.
MEASURES OF SOLUBILITY (SCALES)
aniline is very soluble in aromatic hydrocarbons, but only slightly soluble in aliphatic hydrocarbons. • H e p t a n e n u m b e r - - based on how much heptane can be added to a solvent/resin solution. • Aromatic c h a r a c t e r ~ This is not a m e a s u r e d parameter. Aromatic character of a solvent is the percent of the molecule, determined by adding up the atomic weights, that is benzene-structured. • Solubility Grade - - This p a r a m e t e r is a characterization of a soil (a polymer), not a solvent. It is another cloud-point test, developed by the National Gallery of Art Research Project. In this test, dilute mixtures of the polymer in n-dodecane are further diluted with v a r y i n g a m o u n t of toluene. The Solubility Grade of the polymer is the minimum amount (%) of toluene needed to give a clear solution. This indicates the s t r e n g t h of the solvent needed to dissolve the polymer.
MEASURES OF SOLUBILITY {PARAMETERS) While the above is interesting, it is often as useful as the forecast for y e s t e r d a y ' s weather. The above scales are r e l a t i v e , n o t a b s o l u t e . They apply to soils few users face. They yield no information about why a solvent is useful, or not. Most important, they don't allow predictions about specific applications. What's wanted is a system of solubility characterization that does yield absolute results, does apply to all soils, does yield information about the character of solvents, and does allow predictions.
There are a confusing assortment of such systems.
In the last column we reviewed the Kauri-Butanol number. It was b a s e d on the idea t h a t solvent "power" can be established on a comparative basis by observing to what extent a soil (resin or polymer) precipitates when the solvent is added to a solution of the soil in a good solvent for t h a t soil. Measurements are made of the turbidity produced after t h a t precipitation. O t h e r s y s t e m s found in product literature and technical reports are based on similar ideas: • Wax number - - based on the amounts of a solvent can be added to a benzene/beeswax solution. • Aniline c l o u d p o i n t m b a s e d on the fact t h a t John B. Durkee is President of Creative EnterpriZes, a consulting firm located in Kerrville, Texas. E-mail:
[email protected]; fax (612)-677-3170 42
E n t e r Dr. H i l d e b r a n d
How do we get from solvents and soils to "cohesive energy density and to multiple solubility parameters"? For t h a t we have to t h a n k Dr. Joel H e n r y Hildebrand I and those who have built on his work. And w h y would we w a n t to do that? Because Dr. Hildebrand's ideas and the work derived from them allow us to get what we said we wanted - - a useful system of solubility characterization. Hildebrand's basic idea was t h a t t h e r e was an energy transaction within a fluid when a solution occurs. Here, the solvent molecules must overcome intermolecular forces in the solute (soil). Solvent molecules must find their w a y between and around the solute molecules. And, solvent molecules themselves must be separated from each other by the molMetal Finishing
DLEANING TIME.c
ecules of the solute. H i l d e b r a n d ' s work has two tenets: • To overcome i n t e r m o l e c u l a r forces, e n e r g y is involved. Hildebrand, and those who built on his ideas, showed t h a t t h e s e e n e r g y r e q u i r e m e n t s were at a minimum if the solute (soil) and solvent exerted the same forces upon one another. Said another way, "like d i s s o l v e s l i k e . " • The energy involved with combining a solute (soil) into a solvent is the same as the energy holding the molecules of the solvent together. And the energy holding molecules together against intermolecular forces is the energy required to separate all the molecules from one another. And t h a t is the h e a t of vaporization! E v e r y i n t e r m o l e c u l a r force m u s t be overcome to vaporize a solvent. So the best estimate of the energy involved with solution of a solute (soil) into a solvent is the heat of vaporization of the solvent. Said a n o t h e r way, the same intermolecular attractive forces have to be overcome to vaporize a liquid as to dissolve something in it. What happens when two liquids are mixed? The molecules of each liquid are physically separated by
the molecules of the other liquid, similar to the separations t h a t happen during vaporization. The same i n t e r m o l e c u l a r forces m u s t be overcome in both cases. The energy transaction is described by the following equation: 2
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C-----( A H V a p
-
/Vn,
RT) /
(1)
/
The term AH is the heat of vaporization with units of energy per mole. The term V m is the solvent volume per mole. And C is called the "cohesive energy density," which has the units of pressure. 3 The cohesive energy density of a liquid is a numerical value t h a t indicates the energy of vaporization and is a direct reflection of the forces holding the molecules of the liquid together. 4 A ONE-DIMENSIONAL SOLUBILITY PARAMETER In 1936 Hildebrand proposed the square root of the cohesive energy density as a numerical value indicating the solvency behavior of a specific solvent. The t e r m "solubility p a r a m e t e r " was proposed for
43
:LEANING
TIMES
this value and the q u a n t i t y r e p r e s e n t e d by a delta (6). Then:
cY ~ ( AH
RT)/V
-
(2)
The units of this p a r a m e t e r are:
• Is not specific to one solvent or soil • Does yield i n f o r m a t i o n , w h e n values are compared, about the character of solvents • Does allow predictions about solubility when compared to values from soils. Said a n o t h e r way, a solvent can dissolve a soil when the 6 values for the solvent and the soil are the same.
(Pressure) ~ in mega-pascals (MPa) or
caloriesl/2/cc 3/2 or in honor of it's creator, the Hildebrand: 1 Hildebrand = 1 MPa = 2.0455 calories 1/2 / cc 3/2 Note t h a t t h e r e is a t e m p e r a t u r e s e n s i t i v i t y to H i l d e b r a n d ' s solubility p a r a m e t e r . Solubility p a r a m e t e r s decline with t e m p e r a t u r e - - the product RT increases slightly versus the declining value of h e a t of vaporization. Also notice t h a t in Equation 2, 6 does indeed meet the needs we expressed above. It: • Yields absolute results (it is not a calibrated or relative result)
EXCELLENT SOLVENT CLEANING RESULTS WHEN: 6 solvent = 6 soil
The difference between 6 for a solvent and 6 for the soil is a m e a s u r e of the difficulty of dissolving (cleaning) the soil with t h a t solvent. POOR SOLVENT CLEANING RESULTS WHEN: 6 solvent ++ (5 soil OR 6 solvent • 6 soil
Choose your cleaning solvent to have the same, or similar, H i l d e b r a n d solubility p a r a m e t e r as does your soil. 6 is described as a one-dimensional solubility p a r a m e t e r because it is based on the total of the intermolecular forces within a solvent.
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CLEANING TIME. c
THINK LIKE A MOLECULE In practice there are multiple types of forces t h a t act between the molecules of a solvent. These attractive and repulsive (they are not desirable or distasteful unless you happen to be an molecule) forces between molecules are called van der Waals forces. 5 These forces exist because molecules are not uncharged round objects t h a t pack a void and act on the micro scale as a continuum. Molecules of any substance have various shapes - - some small and simple (methylene chloride), some large and not simple (glycol ethers). The envelope containing a single molecule is not unitbrmly charged (attractive or repulsive). Sections of the b o u n d a r y s u r r o u n d i n g a halogen atom are rich with electrons. Sections surrounding a hydrogen atom are rich with positive electrostatic force (absence of electrons ). There are three types of inter (not intra) molecular forces: • Polar Interactions ~ These dipole-dipole forces exist on molecules t h a t are slightly charged on each side. 6 In water, the oxygen part of the molecule is negatively charged, and the hydrogen side
of the molecule is positively charged. This force m a k e s groups of like molecules tend to "stick '° together, and makes it difficult for t h a t group to accept (dissolve) a n o t h e r molecule t h a t doesn't exert those electrostatic charges. Hydrogen Bonding F o r c e s ~ These are a special type of dipole-dipole interaction. A hydrogen bond is a dipole-dipole interaction that occurs between any molecule with a bond between a hydrogen atom and any of oxygen/fluorine/ nitrogen. The dipole exists because oxygen, nitrogen, and fluorine are extremely good at a t t r a c t i n g electrons and hydrogen is extremely good at losing them. The extremely positive (hydrogen) side of the molecule will orient itself with the extremely negative (oxygen, nitrogen, or fluorine) side of another molecule. Hydrogen bonding forces, an extreme dipole situation, are exceptionally strong. 7 Hexane, ethers, 8 ketones, 8 HFEs, and HFCs don't have substantial hydrogen-bonding forces; alcohols, glycols, water, acids, and ammonia do. Dispersion F o r c e s - - Q u a n t u m theory helps to u n d e r s t a n d dispersion forces. This teaches t h a t the components of atoms aren't static - aren't
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always, in same place all the time. They move at high speed over very short distances. On average, the envelope containing a single molecule has a constant condition of electrostatic charge. BUT, at a n y moment, e v e r y portion of that envelope will have a changing electrostatic environment. Forces of short duration - - temporary forces - - are produced. They are called dispersion forces, and are of considerably less magnitude than are dipole-based forces. 9 E n t e r Dr. H a n s e n Dr. Charles Medom Hansen 1° writes "...needless to say, without the work of Hildebrand and Scott and others ..... this p o s t u l a t e could never have been m a d e .... ,,11 He was referring to the idea t h a t the total energy of vaporization of a solvent consists of several individual components each of which arises because of an intermolecular force. Chiefly, these are the same three forces described above. Basically, Hansen splits the Hildebrand solubility p a r a m e t e r into three component parameters. Each component is associated with an intermolecular force: dipole-dipole, hydrogen-bonding, and dispersion.
THIIIt-~MBMSIONAL $OLUBIIUTY PARAMk'lr~ H a n s e n Solubility P a r a m e t e r s (HSP) come from equating the total cohesive energy of a solvent to the sum of three cohesive energy terms - - each one representing one of the component forces.
( A H Vap
RT)
--
= E Polar
E Hydrogen- Bonding
+
+
E Disperse
(3)
Dividing each term by the molar volume of the solvent, one gets the defining equation:
52
=
5 2Polar +
52 pote (4)
-I-
0o2Disperse
This "Pythagorean Theorem" of solubility parameters allows decomposition of the solubility parameter developed by Hildebrand into three components - each one representing one of the component forces. Hansen's approach solves a fundamental problem with Hildebrand Solubility Parameters. 12 Some solvents have equivalent values of H i l d e b r a n d
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. . . . . . . . . . .
Solubility Parameters. But there molecular struct u r e s a r e n ' t similar. The i n t e r m o l e c u l a r forces between molecules aren't similar. Their solubility performance isn't the same. Some examples of solvents t h a t have the same H i l d e b r a n d Solubility P a r a m e t e r but don't have similar s t r u c t u r e s are: chloroprene and di-isobutyl ketone, cyclopropane and dimethyl cellosolve, xylene and ethyl acrylate, and toluene and 1,1-Dichloropropane, to name but a few pairs. It is this problem, with its solution, t h a t makes Hildebrand Solubility Parameters of limited practical value and makes Hansen Solubility Parameters of significant value in predicting solution behavior between soils and solvents. 13 VALUES OF SOLUBILITY
PARAMETERS
You can calculate Hildebrand's Solubility Parameter from measurements of the heat of vaporization (per volume of one mole) of your solvent, Equation 2, and a choice of the temperature at which you wish to use the value. This is an independent value - - without relation to any soil. No measurements about solubility are made. Values range approximately between 10
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and 40. Values of Hansen Solubility Parameters are determined via measurements of solubility, and other unrelated approaches. These include: • Initial work by H a n s e n and coworkers used a group of 32 polymers and 90 liquids. 14 This is a trial-and-error process by which values of solubility parameters are assigned until the experimental solubility d a t a can be correctly described. These values are related to specific polymer and solvent solubility experiments where the d a t a taken was simple: whether the polymer either was dissolved, or was not. • C a l c u l a t i o n from t h e o r e t i c a l c o n s i d e r a t i o n s based on a n a l y z i n g the functional groups t h a t populate a solvent molecule. 1~, 16 These values are not related to specific polymer and solvent solubility experiments. Equation 4 is the reference equation by which all three HSPs can be checked for consistency. Said another way, the three HSPs m u s t combine to produce the Hildebrand solubility parameter. SOLUBILITY
PARAMETER
DATA
Please r e m e m b e r there are two sets of solubility
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:LEANING
TIMES
p a r a m e t e r s one for solvents a n d one for soils. The value in cleaning work of one set of t hr e e param eters is zilch! W h a t one wants to know is will this solv e n t dissolve this soil. And it doesn't m a t t e r per se w h e t h e r th e v alu es are large or small. Solubility p a r a m e t e r values only have value for you w hen t hey are compared to values for something else - - another solvent or a soil. H a n s e n solubility p a r a m e t e r d a t a 17 for polymeric soils are given in Table I. D a t a for selected solvents are given in Table II. 6Hildebrand is t h a t c o m p u t e d from E q u a t i o n 4.18 T h er e are n u m e r o u s modifications of Dr. Hansen's work. And th er e are other solubility p a r a m e t e r systems.19 In practice, the H a n s e n and Hildebrand solubility p a r a m e t e r s are the most common ones t h a t are used in critical or metal cleaning applications. COilING ATTRACTIONS In the th ir d p a r t of this series, you can read w h a t these solubility p a r a m e t e r s can do to m a ke yo ur life b e t t e r (well, t h a t which involves parts cleaning, anyway). You will learn w hy some solvents won't work for you. We'll cover how to choose solvents based on solubility p a r a m e t e r s and the p a r t i c u l a r soils you
have to clean; w h a t the major problem is with solubility p a r a m e t e r s ; how to select a solvent to clean multiple soils; and about a useful reference to help you do all this. REFERENCES AND NOTES 1. Professor of Chemistry at the University of California Berkeley, awarded the A.C.S.'s highest recognition, the Priestley Medal in 1962. Elected President of the Sierra Club (1937-1940), appointed manager of the U.S. Olympic Ski Team in 1936, discovered that the solubility of helium was much less than that of nitrogen at a given pressure, which led to use of helium and oxygen mixtures for deep underwater diving. Authored classic books such as The Solubility of Nonelectrolytes and Regular and Related Solutions: the Solubility of Gases, Liquids, and Solids. Joel Henry Hildebrand was apparently a very good guy (1881-1983). 2. Hildebrand, J. and R.L. Scott, Regular Solutions, Prentice-Hall, Englewood Cliffs, NJ.; 1962 3. Please recall that, for ideal gasses, PV = RT. 4. Barton, A., CRC Handbook of Solubility Parameters and Other Cohesion Parameters, 2nd edition; 1991 5 .Johannes van der Waals first described them in 1873. These forces do not bind together quantum particles in
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~LEANING TIMES
Table I. Hansen Solubility Parameters for Polymeric Soils Values in Mpa ~2
Cellulose acetate Chlorinated polypropylene Epoxy Isoprene elastomer Cellulose nitrate Polyamide, thermoplastic Poly(isobutylene) Poly(ethylmethacrylate) Poly(methyl methacrylate) Polystyrene Poly(vinyl acetate) Poly(vinyl butyral) Poly(vinyl cyhloride) Saturated polyester
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6 Dispersion
6 Polar
6 Hydrogen-Bonding
18.6 20.3 20.4 16.6 15.4 17.4 14.5 17.6 18.6 21.3 20.9 18.6 18.2 21.5
12.7 6.3 12 1.4 14.7 -1.9 2.5 9.7 10.5 5.8 11.3 4.4 7.5 14.9
11 5.4 11.5 -0.8 8.8 14.9 4.7 4 7.5 4.3 9.6 13 8.3 12.3
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Table II. Hansen & Hildebran Solubility Parameters for Solvents Values in Mpa I/2
~ Dispersion
Acetone Aromatic 150 Benzene t-Butyl Alcohol Cyclohexane Diethylene Glycol Butyl Ether Acetate Diethylene Glycol Monoethyl Ether Dipropyl Ketone Ethanol Ethyl Lactate Ethylene Glycol Diethyl Ether (Ethyl Glyme) Ethylene Glycol Monoethyl Ether (EE) Exxate 1000 Hexane Isopar L D-Limonene Methanol Methyl Acetate Kethyl Ethyl Ketone Methylene Chloride Naphtha High-Flash Pine Oil n-Propyl Bromide Propylene Glycol Monoethyl Ether (PGEE, PE) Propylene Glycol Monomethyl Ether (PM, PGME, MPM) Soygold 1000 Tetrachloroethylene (PERC) i ,1 ,i-Trichloroethane (TCA) Trichloroethylene (TCE) Water Xylene
April 2004
6 Polar
6 Hydroqen-Bondinq
(5Hildebrand
15.5 18.2 18.4 15.2 16.8 16.0 16.1 15.8 15.8 16.0 15.4 16.2 14.9 14.9 14.9 16.6 15.1 15.5 16.0 18.2 17.9 15.6 16.0 15.7
10.4 1.0 0.0 5.1 0.0 4.1 9.2 5.7 8.8 7.6 5.4 9.2 5.7 0.0 0.0 0.6 12.3 7.2 9.0 6.3 0.7 3.0 5.8 6.5
7.0 3.1 2.0 14.7 0.2 8.2 12.2 4.9 19.4 12.5 5.2 14.3 3.1 0.0 0.0 0.0 22.3 7.6 5.1 6.1 1.8 9.8 4.2 10.5
19.9 18.5 18.5 21.8 16.8 18.4 22.2 17.5 26.5 21.7 17.1 23.5 16.3 14.9 14.9 16.6 29.6 18.7 19.1 20.2 18.0 18.7 17.5 20.0
15.6 16.2 18.3 16.8 18.0 15.5 17.6
6.3 4.9 5.7 4.3 3.1 16.0 1.0
11.6 5.9 0.0 2.0 5.3 42.3 3.1
20.4 17.9 19.2 17.5 19.0 22.7 17.9
49
~ L E A N I N G TliVIEE
6. 7.
8. 9.
10.
atoms. They exist between and among molecules of the same substance. Some thought these forces were small gravitational attractions. Van der Waals forces are actually due to electromagnetic interactions between molecules. A dipole is a pair of magnetic poles, each with opposite charge, separated by a short distance. Perhaps one tenth the strength of a covalent bond. Please see http://www.physlink.com/Education/ AskExperts/ae206.cfm. There is no OH bond in ketones, or ethers. The degree of "polarity" that these temporary dipoles confer on a molecule is related to surface area. Larger molecules have a greater number of temporary dipoles and greater intermolecular attractions. Molecules with straight chains have more surface area, and thus greater dispersion forces, than branched-chain molecules of the same molecular weight. These induced attractions are also called London dispersion forces, or induced dipole-induced dipole forces. Please see h t t p ://www. s t a n f o r d , e d u / b y a u t h / b u r k e / s ol p a r / solpar4.html. A pioneer, from the mid-1960s through today in the quantification of formulation of paints, coatings, inks, etc. His list of technical publications contains more than 100 entries. Dr. Hansen is Senior Scientist at Force Technology, P a r k All~ 345, 2605 Br0nby, Denmark, and can be reached at
[email protected]. And Charles Medom is also a good guy who has been kind enough to review this article.
11. Hansen, C.M., Hansen Solubility Parameters - a User's Handbook, page 3, CRC Press, New York; 2000 12. His initial work was trial and error. Today HSPs are determined solely on experiments or calculations using the data one can find for latent heat, dipole moment, group contributions, etc. 13. T H O U G H THEY ARE OF IMMENSE THEORETICAL VALUE. 14. Hansen, C.M., "The Three Dimensional Solubility P a r a m e t e r - Key to Paint Component Affinities,"
International Journal of Paint Technology, 305(511):104-117,505-510; 1967 15. Blanks, R.F., J.M. Prausnitz, "Thermodynamics of Polymer Solubility in Polar and Nonpolar Systems," Industrial and Engineering Chemistry Fundamentals, 3(1):1-8;1964 16. Hansen, C.M. and K. Skaarup, "The Three Dimensional Solubility P a r a m e t e r - Key to Paint Component Affinities," International Journal of Paint Technology, 305(511):511-514; 1967 17. D a t a and table from http://sul-server2.stanford.edu/byauth/burke/solpar/solpar6.html. 18.Hansen, C.M., Hansen Solubility Parameters - a User's Handbook, pages 168-185, CRC Press, New York; 2000 19. A unique one will be to devise a single characteristic will be able to separate / predict toxic and nontoxic solvents. An interesting and lengthy paper on this topic can be found at http://www.aber.ac.uk/~abcwww/ gjsalter/solv15.htm
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