EVAPORATION BEHAVIORS OF
MIXED LACQUER SOLVENTS This paper examines critically an argument recently advanced, that the assumption, hitherto made, that a solvent mixture of constant boiling point at the drying temperature of the (lacquer) film is a constant-evaporating mixture, is, in general, false and likely to be grossly misleading" (5). It is pointed out that this statement must have been due to a misinterpretation of experimental data. Experimental evidence is presented to show that evaporation of constant-boiling mixtures follows the process of non-ebullition distillation. Constant-evaporation mixtures of a new type, resulting when binary mixtures are evaporated over adsorbents, are shown to exist.
GEORGE W. BENNETT AND W. ANDREW WRIGHT Grove City College, Grove City, Pa.
The objects of this paper are to show that the Lewis, Squires, and Sanders experimental results are not valid support for the first of these concepts, and to adduce experimental data which will serve to refute the other two concepts.
Evaporation Behaviors in Closed Vessels Lewis, Squires, and Sanders carried on their evaporations in large desiccators, and in order to prevent interference by atmospheric moisture they used 275 cc. of 85 per cent sulfuric acid in the desiccant compartment. In order to test the validity of data so obtained for comparison with evaporation carried on in a current of dry air, the authors made a numbet of evaporations of benzene, of methanol, and of several benzene-methanol mixtures in closed vessels over various desiccants and other substances. In all of these experiments 5 cc. of liquid were weighed to milligrams in covered Petri dishes which were then set in 6inch (15.2-cm.) desiccators, the lids were removed, and the dishes were allowed to remain undisturbed for the times indicated in Table I. At the end of the stated period the dishes were covered and reweighed, and the percentage loss in weight was calculated. In the case of mixtures the liquids were analyzed by the refractometer a t 20" C. immediately before and after each evaporation. The data so obtained are summarized in Table I. The 59.1 mole per cent methanol mixture is the constant-boiling mixture a t 58.9" C. which L e w i s , Squires, and S a n d e r s used in their evaporation experiments. All of the liquids mentioned in this p a p e r were c. P. chemicals further purified by standard methods, dried, and redistilled. The defiiccants were either freshly ignited anhydrous comp o u n d s , unused and unregenerated Drierite, or fused sticks from previ7 7 , ~ IN HOUR^ ously u n o p e n e d FIGURE 1. VARIATION IN REFRACTIVE b o t t l e s . Fresh INDICESOF TOLUENE-ISOPROPANOL portions of desicMIXTURESDURING EVAPORATION AT 20" c. cants of roughly The mole per cent toluene in the mixtures a t the same weight the beginning of evaporation was &s follows: were used for each A = 58 D = 42 B = 50 E = 37 evaporation. c = 44.5 F = 29
I
N THE formulation of a lacquer it is essential that the initial balance between the liquids comprising the volatile portion should not change unfavorably during the evaporation period. The role that constant-evaporating mixtures play in maintaining or altering this initial balance of the liquid components is recognized as important. Constant-evaporating mixtures have been, therefore, the subject of a number of experimental studies ( 1 , W , 6, 7). It has been a generally accepted concept that a constantcomposition evaporating mixture a t a given temperature, will, under suitable pressure, be a constant-composition boiling mixture a t the same temperature (?, 9). This viewpoint has, however, been controverted recently by Lewis, Squires, and Sanders (6) who state that "the assumption, hitherto made, that a solvent mixture of constant boiling point a t the drying temperature of the film is a constant-evaporating mixture is, in general, false and likely to be grossly misleading." In support of this assertion they offer experimental evidence on the evaporation behavior of the benzene-methanol system. Using a mixture richer in methanol than that of the constantboiling solution a t the temperature of vaporization, they observed that during the period of evaporation the liquid became richer in benzene rather than in methanol. From this result they concluded that evaporation of constant-boiling mixtures does not follow the simple process of distillation. The assertion of these investigators and the paper containing it expresses or implies three concepts: (1) Evaporations of binary mixtures in closed vessels follow a special mechanism different from the simple process of distillation; (2) constant-evaporating solutions a t the temperatures a t which lacquer films dry do not follow the simple process of distillation for azeotropic mixtures; and (3) mixtures which are constant-boiling a t one temperature are not constant-evaporating a t the same temperature but under somewhat greater pressures. 646
INDUSTRIAL AND ENGI NEERING CHEMISTRY
JUKE. 1936
fractions for both of the liquids of the binary mixture be identical then the mixture would be a constant-evaporating mixture, but for a different reason from azeotropism. The results obtained with Drierite i n d i c a t e t h a t t h i s situation has teen closely approximated because a mixture of 76.5 mole per cent m e t h a n o l did evaporate with only a slight change in composition. If we could use an adsorbent or imbibent that favored benzene, we might expect to reverse the observations of Lewis, OF ABSORBENTS AND ADSORBENTS ON EVAPO- Squires, and Sancters, if their TABLE I. EFFECT RATION OF BENZENE-METHANOL MIXTURES results were due to a selective Weight % removal of one of the vapors. .ibsorbent Initial Concn. Final Concn. Time Evaporated T h u s we might begin with a Mole % Mole % methanol HI.. mixture poorer in methanol than 85% H&04 100 methanol .. 4.0 100 the constant-boiling mixture a t 100 methanol *. 2.0 47 100 benzene 23.0 40 25O C. and find that during the .. 6.0 24 100 benzene evaporation the mixture became None 100 methanol .. 6.0 13 100 methanol .. 23.5 21 richer in methanol. Such is the .. 6.0 21 100 benzene exDerimen t a l case. Using as 23.0 32 100 benzene
From the results obtained with 85 per cent sulfuric acid it is apparent that methanol is readily absorbed by this reagent whereas benzene is not. In 2 hours, as large a fraction of methanol is absorbed as is the case for benzene in 24 hours. Also, not much more benzene evaporates over 85 per cent sulfuric acid than evaporates in an empty desiccator of the same size. The unexpected results of Lewis, Squires, and Sanders are due, therefore, not to some special mechanism of evaporation, but rather to the selective absorption action of the desiccant for one of the two liquids in its vapor form. The vapor phase, therefore, never attains that equilibrium with the liquid phase which should exist in a closed vessel. Their conclusions that evaporation of constant-boiling mixtures in general is not the simple process of distillation cannot be supported.
I
CaClz ZnCln KOH Me0
cio
CugOa K&Oa Drierite
Rubber
59.1 methanol 59.1 methanol 59.1 methanol 59.1 methanol 59.1 methanol 59.1 methanol 59.1 methanol 59.1 methanol 100 methanol 100 methanol 100 benzene 100 benzene 59.1 methanol 76.5 methanol 59.1 methanol 59.1 methanol 34.0 methanol
.
60:7 4.2 4.2 4.2 15.1 51.0 50.5 59.4
6.0
..
23.5 6.0 23.0 5.0 5.0 5.0 2.0 2.25
.
I
..
:
15 0 72.1 95.4 83.2 60.7
6.0 6.0 6.0 5.75 5.5 6.0 6.0 6.0
34 76 67 65 80 41.5 65.5 35.5 35.5 93.5 40.5 96.0 85.5 87.5 95 67 87
Nevertheless, a more extensive experimentation with the various desiccants would have, apparently, supported their experience with 85 per cent sulfuric acid. The third group of data in Table I shows that all of the drying agents used, except the anhydrous potassium carbonate, altered the composition of the 59.1 mole per cent methanol mixture far over towards pure benzene. These results are due to three causeschemical action, solvent action, and adsorption. If it were true that the desiccants do not influence the alteration in composition of the liquid mixture during the evaporation process, then the final concentration should be the same in the presence of any desiccant. It is obvious, however, that the desiccants are influencing the alterations in liquid compositions during the evaporations.
Adsorption Phenomena and Constant-Evaporating Mixtures The phenomenon of adsorption suggests an interesting possibility when binary mixtures are evaporated over materials capable of adsorbing one or both liquids. If the loss in weight during the evaporation is due to adsorption, then the weight of a liquid adsorbed is some definite fraction of the weight of that liquid in the mixture. From the Freundlich adsorption isotherm concept the weight adsorbed is a function of the molar concentration of the vapors surrounding the adsorbent. The value for the ratio of weight adsorbed to weight in the mixture is larger the smaller the molar concentration of the liquid under consideration. Should the values of such
rIEBi
647
SO-
-
70
40 -
so -
9, p
10
O/ O
MOA P
EO
~
30
90
zLvrWr ~ ~ ~
w
FIGURE 2. C O M P A R I ~ O N OF CONSTANT-BOILING
ber in ten pieces, and beginning TERES with a mixture containing only 34.0 mole per cent methanol, 87 per cent of the 5-cc. sample evaporated in 2.25 hours. At the end of this time the mixture contained 60.7 mole per cent of methanol. This mixture, then, underwent a change in concentration in the reverse of that to be expected from the simple process of distillation. It is apparent therefore that evaporations in closed vessels over many materials may not be compared to ordinary evaporations. With an adsorbent selective for benzene we may further predict that there should be a composition of the binary mixture similar to that for the Drierite adsorbent, which would undergo evaporation without change in composition. There are, therefore, the possibilities of many constant-evaporating mixtures of benzene and methanol if the evaporations are carried out over adsorbents. At present there appear to be no practical applications for such constant-evaporating systems, but they should, perhaps, be investigated. The authors propose to extend the preliminary study here outlined.
Evaporations under Average Conditions The assertions of Lewis, Squires, and Sanders are not confined to the special case of evaporations conducted under such conditions that diffusion might be a dominating factor, but they are categorical, presumably covering evaporations under all conditions. Thus evaporations a t room temperatures in air currents of varying turbulence-that is, the average conditions under which lacquer films dry-would also be included as being “grossly misleading.” It is pertinent, then, to show that constant-evaporating mixtures under these average drying conditions do follow the simple process of distillation for azeotropic mixtures. By the term “simple process of distillation” we mean that alterations in liquid compositions would follow the liquidus curve on the pressure-composition phase diagram. Thus, for example, evaporations of benzene-methanol mixtures in closed vessels over sulfuric acid and over rubber do not follow this simple process because compositions on either side of the maximum
INDUSTRIAL AND ENGINEERING CHEMISTRY
648
pressure point on the phase diagram can be made to pass to a composition on the opposite side of this maximum point. The evaporation behavior of toluene-isopropanol mixtures illustrates a general behavior of constant-evaporating systems. In Figure 1 the variations in composition of the liquid plotted against duration of vaporizations are given for various toluene isopropanol mixtures. Compositions are expressed in terms of refractive index and may be estimated from the following: Mole % Toluene 0.0 7.36 15.16 23.46
A New Water-Soluble Nicotine Insecticide-
NICOTINE HUMATE
Refractive Mole % Refractive Mole % Refractive Toluene Toluene Index Index Index 1.378 1.389 1.400 1.411
32.2 41.7 51.7 62.6
1.423 1.434 1.446 1.458
74.2 82.9 100.0
1.471 1.483 1.496
For toluene-isopropanol mixtures, then, there is one mixture (C, Figure 1) which evaporates without change in composition (refractive index) a t 20” C., while every other mixture richer or poorer in toluene, becomes increasingly richer or poorer as the evaporation proceeds. In no case does a mixture of a given composition on the phase diagram so alter in composition as to be subsequently represented by a point on the opposite side of the maximum point. In terms of Figure 1 such an alteration would necessitate a change in refractive index from one side to the other of horizontal line C (the maximum pressure composition) on the graph. The authors have previously reported the experimental details and data for eight such systems ( B ) , all of which exhibit this general evaporation behavior. It may be said, then, that the second point of the Lewis, Squires, and Sanders assertions is not universally true. In regards to the third point-namely, the identity of the constanbboiling and the constant-evaporating mixtures-the authors are unable to cite a research specifically undertaken to establish such identity. Comparable data for the system toluene-ethanol are, however, available (6, IO) over the range 50” to 75” C. This comparison is shown in Table I1 and graphically in Figure 2. On the other hand, similar concordance cannot be established for benzene-methanol based on the published data (1, 3-6, 8).
WEIQHINGTHE INSECTICIDE FOR EXPERIMENTAL SPRAYING, KEARNEYSVILLE, W. VA.
TABLE 11. COMPARISON OF COMPOSITIONS OF CONSTANT-BOILINQ AND CONSTANT-EVAPORATING MIXTURESOF TOLUENE AND ETHANOL Temp. a
c.
75 70 65 60
ConstantConstantBoiling Evaporating Mole % toluene 18.0 18.5 19.5 20.0
18.0 18.7 19.5 20.0
Temp. o c .
L. N. MARKWOOD Bureau of Entomology and Plant Quarantine. U.S.Department of Agriculture, Washington, D. C.
’
20.5 21.5
..
..
20.7 21.5 26.9 36.8
The three concepts, expressed or implied by Lewis, Squires, and Sanders cannot, therefore, be universally true.
Acknowledgment The authors are indebted to Ernest Robinson for the data represented in Figure 1.
Literature Cited (1) Hofmann, IND. ENG.CHEM.,24,135 (1932). (2) Hofmann and Reid, Ibid., 20,687 (1928). (3) Leoat. “L’azBotropisme,” 1st ed., p. 99,Henri Lamertin, Brussels, 1918. (4) Lee, J . Phys. Chem., 35,3558 (1931). ( 5 ) Lewis, Squires, and Sanders, IND. ENG.CHEM.,27, 1395 (1935). (6) Robinson, Wright, and Bennett, J . Phys. Chem., 36,658 (1932). (7) Roscoe and Dittmars, J . Chem. Soc., 12, 128 (1860). ( 8 ) Schmidt, 2.physik. Chem., 99,71 (1921). (9) Taylor, “A Treatise on PhyRical Chemistry,” 2nd ed., Vol. I, P. 523, New York, D.Van Nostrand Go., 1931. (10) Wright, J . Phys. Chem., 37,233 (1933). R E C ~ I V EFebruary D 13, 1936.
0
Constant- ConstantBoiling Evaporating Mole % toluene
55 50 25 0.5
VOL. 28, NO. 6
N A
PREVIOUS paper1 the author described a process in which nicotine and peat are brought together to form a waterinsoluble product named ‘ ‘nicotine peat ,” The liquid separated from nicotine peat also contains nicotine, not as the free base but in combination with humic acid derived from the peat. This aqueous solution can be treated with alkali and the nicotine recovered by distillation, but since the compound in solution, nicotine humate, may have a usefulness of its own, it has been recovered unchanged by evaporating the water. Wcotine peat and nicotine humate are thus companion products formed in a single reaction. Nicotine humate is a black product, soluble in water, forming what is‘ undoubtedly a colloidal solution. As obtained by evaporation it has a high luster, which gives it the appearance of a black crystalline substance. Actually, however, it is amorphous. Its aqueous solution is slightly acid to litmus. The addition of a strong mineral acid, such as hydrochloric, 1
IND. ENQ CHEM., 28561, (1936)
