Binary Solvents for Zein J
RALPH H. MANLEY’
AND
CYRIL D. EVANS
Northern Regional Research Laboratory, U. S. Department of Agriculture, Peoria, 111. over the complete range of solvent composition in which protein was found to be dispersible. Swallen ( I S ) more recently reported the critical peptization temperatures of zein in binary mixtures of water with n-butyl, isopropyl, and denatured ethyl alcohol (Shellacol), and has also given similar data for the critical peptization temperatures of zein in binary mixtures of anhydrous Shellacol with benzene, ethylene dichloride, nitropropane, diethylene glycol, and methyl lactate. Coleman (2)has been granted a series of patents covering dispersions of zein in binary solvents such as the glycols, benzyl alcohol, and diacetone alcohol with not more than 5 per cent of water. The authors have (9) reported on the solubility characteristics of zein in concentrated and absolute ethyl alcohol. The present study is chiefly concerned with the solubility characteristics of zein in binary solvent systems in which the lower aliphatic alcohols and a number of ketones and glycols are the primary components, and in which water, aromatic hydrocarbons, chlorinated hydrocarbons, nitroparaffins, aldehydes, and cyclic ethers are secondary components. The zein studied was from a special commercial batch used as a reference standard by the Corn Products Refining Company and Prolamine Products. Inc. I n all instances the zein was dried to constant weight in vacuum a t 55’ C. before use. The method used to measure its “critical peptization temperature” in organic solvents, or the critical temperature above which the protein is soluble in all proportions and below which it is soluble only to the extent of 2 or 3 per cent, has been previously described (5, 4).
The objectives of this study were (a)to determine the general types of molecular components required to make good binary organic solvents for zein, and ( b ) to provide commercial users of zein with data on its solubility characteristics in a number of organic solvents, covering a wide range of viscosity, boiling point, and evaporation rate. The work shows that the critical peptization temperatures of zein in many primary organic solvents can be lowered by the addition of active hydrogen compounds. The effectiveness of such substances as secondary solvents for zein is related to the degree of activity of their active hydrogens. Critical peptization temperature curves for zein in a number of binary solvent systems are included.
N A PREVIOUS article (4) the authors showed that zein can be dispersed in a considerable number of single organic solvents, which may be either acids, acid amides, amines, or hydroxy compounds. Thus, all single solvents for zein are amphoteric in the sense that they are capable of forming hydrogen bridges through their capacity both to donate and to accept electrons. Substances of the type represented by chloroform, which are capable of forming hydrogen bonds only through their capacities as electron acceptors, and likewise compounds such as diethyl ether or ethyl acetate, whose role in forming hydrogen bridges is almost exclusively that of electron donors, were all ineffective as single solvents for zein. The earliest observations on binary solvents for zein were made by Osborne ( l a ) in 1891, by Chittenden and Osborne ( I ) in 1892, and by Osborne (21) in 1897; they noted the ready solubility of zein in aqueous methyl, ethyl, and propyl alcohols. I n 1927 a thorough study was made by Dill (5) of the solubility and critical KETONE, % BY WT peptization temperatures of zein in A . (1) tert-Butyl alcohol (2) ethyl alcahol, (3) sec-butyl alcohbl, (4) n-butyl aqueous mixtures of methyl, ethyl, alcohol n-propyl, and tert-butyl alcohols C. (1) Acetonylacetone, (2) acetone
I
Present address, General Mills Ino., Minneapolis, Minn. 1
OXIDE, % BY WT.
(1) n-Prop 1 alcohol (2) isopro yl alcohbl, (3) methyfalcohol, (4) isobutyl)alcohol B
D.
(1) Dioxolane, (2) dioxane
Figure 1. Critical Peptization Temperature Curves for Commercial Zein i n Binary Organic Solvents with Water as One Component
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
662 A
(1) Acetonylacetdne, (2) ace,tone, (3) mesityl oxide, (4) methyl ethyl ketone
B.
(1) Furfural, (2) acetaldehyde. (3)crotonaldshyde, (4) 2ethyl 3 propylacrolein, ( 5 ) acetaldol
-
-
C. (1)Dioxolane. (2) dioxane, (3) Propylene oxide, (4) 1 9 0 phorone, ( 5 ) phorone
D. (1) Chloroform, (2) sym-dichlorodiethyl ether, propylene dichloride, (4) tetrachloroethylene
(3)
E. (1) Methyl acetoacetate, (2) ethyl acetoacetate, ethyl malonate, ethyl formate ( 5 ) butyl Lactate '95% (technical)
%
F. (1) sym-Tetrao h l o r o e t h a n e , (2) pentachloroethane. (?) ethylene dichloride (4) carbon tetraehldride, ( 5 ) dichloropentane
G.
(1) Benzene, (2) toluene. (3) rn-xylene
I. (1) p-Trichloroethane, (2) methylene chloride, (3) trichloroethylene, (4) symdichloroisoproPYl ether
K.
(1) Nitromethane, (2) nitroethane, (3) 1-nitropropane. (4) 2-nitropropane, ( 5 ) 2-nitrobutane
L . (1) Ethylene glycol (2) diethylene gl/col, (3) propylene glycol
@Figure2. Critiea1 Peptization 'Temperature *Curvesfor Commercial Zein in Binary Organic Solvents with Methyl Alcohol as One Component
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June 1943
Ib
20
30
40
do
60
663
A. (1) sum-Tetrachloroethane, (2) methylene chloride (3) pentachloroethane, (4) ' 8-trichloroethane, (5) propylene diohloride
710
B. (1) Dioxolane (2) dioxane. (3) acetonyiacetone
C. (1) Butyl lactate 95%. (2) methyl acetoacetate
D.
40
'LO
I
I
I
i0
60
7b
I
I
1
Benzene and hexane
LO
I
E. (1) Propionic acid, (2) acetic acid
~ ~ t r ~ l eitromethane ) t(3) h ~1-nitroproe, (2) pane, (4) 2-nitropropane, ( 5 ) 2-nitrobutane
G. (1) Ethylene lycol (2) diethylene glyeol, pdpylene glycol
5)
H.
(1) Methyl Cellosolve, (2) Cellosolve methyl Carbitol, (4) Crrbitol
(s)
I. (1) Furfural, (2) acetaldol, (3) crotonaldehyde
Figure 3.
Critical Peptization Temperature Curves for Commercial Zein in Binary Organic Solvents with Ethyl Alcohol a s One Component
INDUSTRIAL AND ENGINEERING CHEMISTRY
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80
20 0 40 '40
1 50
1
1
60
70
80
90 d0
ISOPROPYL ALCOHOL, X B Y WT.
-60d A. B.
10
,b
20 30 40 50 ISOPROPYL ALCOHOL, % B Y WT.
(1) Acetaldol, (2) furfural (1) Ethylene glycol, (2) diethylene glycol, (3) propylene glycol
Figure 4.
70
20 I S O P R O P Y L ALCOHOL, 9.BY WT.
C. D.
(1) sym-Tetrachloroethane, (2) 8-trichloroethane, (3) pentachloroethane (1) Chloroform (2) ethylene dichloride, (3) dichloroethyl ether
ISOPROPYL ALCOHOL,
X B Y WT.
E.
(1) Nitromethane, (2) nitroethane F (1) Methyl Cellosolve (2) Cellosdlve, (3) methyl Carbitol,'(4) Carbitol
Crj tical Peptization Temperature Curves for Commercial Zein in Binary Organic Solvents with Isopropyl Alcohol as One Component
tion. terl-Butyl, isopropyl, and ethyl alcohols are all good solvents for zein but are slightly inferior to n-propyl alcohol in these respects. %-Butyl, sec-butyl, and methyl alcohols are unsatisfactory as components of aqueous alcoholic binary solvents for zein either in respect t o breadth of solvent composition or to low minimum critical peptization temperature. Aqueous binary systems containing acetone, acetonylacetone, dioxane, and dioxolane are good solvents for zein; they are generally comparable t o the better aqueous alcohols except that the acetone-water systems are distinguished by the very low viscosity of their zein dispersions as compared with the viscosity of similar concentrations of zein in the other binary aqueous systems. This sharp difference in viscosity is exemplified by a solution of 20 grams of zein in 100 ml. of 70 per cent (by weight) aqueous acetone, d i i c h has a viscosity of 20 centipoises a t 30" C., while a similar dispersion of zein in 70 per cent aqueous isopropyl alcohol has a viscosity of 83 centipoises a t the same temperature. This difference may be of commercial interest as a means of controlling the rate PEPTIZATION TEMPERATURES OF ZEIN FROM SEVERAL TABLEI. of penetration of the protein into paper and other surfaces EXTRACT ANTS during coating operations. Peptization Temp., The nonaqueous binary solvents for zein constitute a from 85 Wt. % Ethanol, C. Source of Zein much larger group than the aqueous systems and are markedly + 1 1 . 5 t o 12 1938 commercia! product different in character. In general, the nonaqueous zein sys1940 commercial product + 1 1 . 5 t o 12 + 1 1 . 5 t o 12 1941 commercial product tems are less prone to gel under the influence of time and 1942 commercia! product + 1 1 . 5 t o 12 heat, are stable toward such common precipitating agents as Extd. from whole round corn with: - 2 to-4 65 wt. % e t h a u 3 a t 8' C . picric acid, trichloroacetic acid, tannins, and the salts of the 75 wt. o/a ethanol a t 8' C. + 1 to 2 85 wt. yo ethanol a t 38' C. - 1 to -2 heavy metals, and do not tend t o foam as do dispersions Extd. from undried crude corn glucen with 85 containing water (7, 8, 11). wt. Yo ethanol a t 3.5' C. 0 to-1 Extd. from dried crude corn gluten (under reflux) Binary mixtures of the lower aliphatic alcohols with the with 50% abs. ethanol and: t o 22 $0 wt. To nitromethane +20 lower aliphatic, aromatic, and heterocyclic aldehydes are a0 wt. yo mixed nitroparaffins t o 13 +12 good zein solvents. Furfural, for example, yields a wide to 17 50 wt. % methyl Ceilosolve +16 choice of solvent compositions with methyl alcohol, ranging
S ILLUSTRATED in Table I, the critical peptization of several commercial zein samples prepared over a four-year period by extraction with hot 85 per cent (by volume) isopropyl alcohol were very similar. However, the critical peptization temperatures of zein extracted a t varying temperatures and with a variety of solvents differed widely. The peptization curves of such zein samples in any given binary solvent are similar in general contour (Figures 1 to 4). Aqueous solutions of the lower aliphatic alcohols, which constitute the most widely used solvent systems for zein, are characterized by their low viscosities, high evaporation rates, low cost, and the unfortunate tendency of zein to gel from their solutions. Among the alcohols through C4, n-propyl is the best in respect to low minimum critical peptization temperature and broad range of effective solvent composi-
A temperatures
June, 1943
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
from 10 to 90 per cent of either component. Furfural also gives a wide but somewhat lesser range of solvent compositions with ethyl alcohol. The isopropyl alcohol-furfural system follows the same pattern except that its range is narrower and its minimum critical peptization temperature somewhat higher. T H E PARTIALLY chlorinated hydrocarbons are nonsolvents for zein, but in binary mixture with the lower aliphatic alcohols they make excellent dispersing agents for it. Methyl alcohol shows the widest range of solvent composition and the lowest critical peptization temperatures. Ethyl and isopropyl alcohols follow in order in the same manner as in the alcohol-aldehyde systems. As second components with the alcohols, the chlorinated hydrocarbons also follow a definite order of effectiveness as solvents. Among the compounds tested, 1,1,2,2-tetrachloroethanegives the lowest critical peptization temperatures and the widest range of solvent composition; chloroform, 1,2,2trichloroethane, dichloromethane, pentachloroethane, and 1,2-dichloroethane follow in that order. Among the chlorinated methanes, chloroform is most effective, followed by dichloromethane and monochloromethane. Carbon tetrachloride has a slight solvent effect at room temperature with methyl alcohol, but is effective with ethyl and isopropyl alcohols only at temperatures above the boiling range of the solvent system. The nitroparaffins are good solvents for zein when used in binary systems containing the lower aliphatic alcohols; like the chlorinated aliphatic compounds, they are ineffective as single solvents. Also like the chloroparaffins, their solvent power follows a definite pattern; nitromethane is the most effective, and nitroethane, the nitropropanes, and the nitrobutanes follow in that order. Data of Marvel and co-workers (IO) show that the heats of solution of a number of the chloroparaffins with donor solvents decrease in the same order as the effectiveness of these compounds as second components in binary alcoholic solvent systems for zein. Likewise, the data of Ewell and Welch on maximum-boiling mixtures of chloroparaffins (6) and of nitroparaffins (6)with donor liquids indicate that the effectivenessof both the chloroparaffinsand the nitroparaffins as secondary zein solvents with the aliphatic alcohols is a direct function of the capacity of their active hydrogens to form hydrogen bonds. Binary mixtures of unsubstituted ethyl and isopropyl ethers are nonsolvents while sym-dichloroethyl and symdichloroisopropyl ethers with methyl alcohol form excellent binary solvents for zein. sym-Dichloroethyl ether also forms binary solvent systems for zein with ethyl alcohol, but its minimum critical peptization temperatures are considerably higher than similar binary systems containing methyl alcohol. Isopropyl alcohol forms a binary solvent for zein with symdichloroethyl ether but not with sym-dichloroisopropylether a t temperatures below the boiling ranges of the mixtures a t atmospheric pressure. Binary solvent systems of organic esters with the lower aliphatic alcohols are limited to those esters contain'ng one or more active hydrogens, such as the lactates, glycol monoesters, acetoacetates, and formates. Ethyl acetate and esters of higher acids and alcohols act only as diluents.
665
As previously mentioned, the aliphatic ethers do not form binary solvent systems for zein. However, the cyclic ethers, ethylene oxide, propylene oxide, dioxane, and the lower chloroaliphatic ethers are good zein solvents when used in binary systems containing the lower aliphatic alcohols. Acetone, acetonylacetone, mesityl oxide, phorone, isophorone, and diacetone alcohol are good solvents for zein when used as components of binary solvents containing the lower aliphatic alcohols, and diacetone alcohol also forms solvent systems for zein in binary mixtures with acetone. However, none of these ketones are effective in binary systems with chlorinated paraffins, nitroparaffins, aldehydes, or the other compounds which are such good secondary solvents with the lower aliphatic alcohols. Methyl ethyl and methyl isopropyl ketones have solvent properties for zein similar to those of acetone except that the minimum critical peptization temperatures of such systems are higher than for acetone solutions. The ketones containing no methyl groups have not been found to be solvents for zein in binary systems up to their boiling points at atmospheric pressure. I n addition to the investigation of binary solvents containing the lower aliphatic alcohols, a study was made of a number of binary solvent systems in which the first compound is an extremely good single solvent for zein, such as methyl and ethyl Cellosolves, the Carbitols, the glycols, and benzyl alcohol. The addition to them of chloroform, nitromethane, or other compounds to form binary systems does not improve their solvent power to the same extent as these additives improve the solvent power of the lower aliphatic alcohols. However, the extent to which such compounds as chloroform and nitromethane can be added to the good primary zein solvents without causing precipitation of the protein may be looked upon as a measure of their effectiveness as diluents in such binary systems. 1,1,2,2-Tetrachloroethane is much more effective as a secondary solvent in lowering the critical peptization temperature of zein in binary mixtures containing ethyl alcohol than is 1,2-dichloroethane; similarly 1,1,2,2-tetrachloroethanecan be added to methyl Cellosolve in much larger amounts than can 1,2-dichloroethane without impairing the effectiveness of the Cellosolve as a zein solvent. I n general, any compound containing a sufficiently active hydrogen is effective as a second component of binary solvent systems for zein when added to the lower aliphatic alcohols or to such good primary zein solvents as methyl and ethyl Cellosolve, the Carbitols, the glycols, or benzyl alcohol. The effectiveness of the second component in contributing to the solvent power of any given binary system is proportional t o the degree of activity of its active hydrogens. LITERATURE CITED
Chittenden, R. H., and Osborne, T. B., A m . Chem. J., 14, 20-44 (1892).
Coleman, R. E., U. S. Patent 2,185,110-24 (1939); 2,246,779 (1941); 2,270,508 (1942).
Dill, D. B., J. Biol. Chem., 72, 239-47 (1927). Evans, C. D., and Manley, R. H., IND. ENQ.CHEM.,33,1416-17 (1941).
Ewell, R. H., and Welch, L. M., Div. of Phys. and Inorganio Chem., A. C. S. Meeting, St. Louis, Mo. Ewell, R. H., and Welch, L. M., J . A m . Chem. Soc., 63,2475-8 (1941).
ONE of the hydrocarbons are primary solvents for zein, and the members of the paraffin series are also nonsolvent when used as components of binary solvent systems. However, benzene, toluene, xylene, naphthalene, and anthracene are zein solvents when used as components of binary systems containing the lower aliphatic alcohols, their effectiveness decreasing in the order named.
Loiseleur, J., Bull. SOC. chim. biol., 14, 1088-1100 (1932). Loiseleur, J., Compt. rend., 191, 1391-3 (1930). Manley, R. H., and Evans, C. D., J . Biol. Chem., 143, 701-2 (1942).
Marvel, C. S., Copley, M. J., and Ginsberg, Emanuel, J. Am. Chem. SOC.,62, 3109-12 (1940). Osborne, T.B., Ibid., 19, 525-32 (1897). Osborne, T. E., U. S. Patent 456,773 (July 28, 1891). Swallen, L. C., IND.ENQ.CHEM.,33, 394-8 (1941).
