August, 1923
I X D U S T R I A L A X D EA-GIA'EERISG CEIEJIISTRY
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T h e Vapor Pressure of Volatile Solvents'" Sdlutions of Benzene, Hexane, and Cyclohexane in Various Types of Lubricating Oils, with Molecular Weight Data By Robert E. Wilson and Edward P. Wylde &IASSACHUSETTS I N S T I T U T E OF
The work described in this paper was designed to determine the vapor pressure exerted by various types of volatile solvents dissolved in nonvolatile oils. The data are of particular importance in connection with (a) the recovery of solvents f r o m gases by scrubbing with absorbent oils, (b) crank-case dilution in internal combustion engines, (c) the removal of traces of solvent from solvent-extracted edible oils. Direct vapor pressure measurements were made on benzene, hexane, and cyclohexane (selected as typical of the three important classes of hydrocarbons), dissolded in severnl proportions in asphaltbase, parufin-base, and California lubricating oils, and in various types of gas-absorbent oils. The apparatus used was similar to that previously employed by Wilson and Barnavd in measuring the effectioe volatility of motor fuels, and whife not extremely precise was quite satisfactory for the pirrpose on hand. According to Raoult's law, the vapor pressure of solutions of similar molecules should be directly proportional to the mol per cent prcscnt. I n order to determine whether or not this law could be relied upon as reasonably accurate for mixtures of hydrocarbon such as those studied in this paper, or to determine the direction and magnitude of the deviations therefrom, the molecular weights of the various oils used were determined by measuring thefreezingpoint lowering of dilute benzene solution. For oils of similar viscosity, the molecular weight increases in the following order: California, a:rphalt-base, parafin-base, and vegetable or animal. The difference between the $rst two is slight. The vapor pressure of most of the combinations check reasonably well with Raoult's law. The most striking exception to the rule is hexane in castor oil, which is the only combination which deoiates in one direction at low concentrations and in the other at high. Benzene in castor oil and also in parafin-base oil gives much lower vapor pressures than those calculated from Raoult's law at low molecular concentrations, but abooe 60 mol per cent the deviations are not large even in these cases. I n all cases increasing the temperature from 25" to 100" C. either had a negligible effect on the relative oapor pressure of the solutions or, more frequently, lowered it slightly. The effect of temperature was most pronounced on those solutions which gave vapor pressures above or near that indicated by Raoult's law, and least on those which gave the lower vapor pressures. Hexanc:, which is of especial interest as representing the parafin hydrocarbons, which constitute most of the present-day motor fuel, when dissoloed in parafin-base oil, checks Raoult's law fairly well, as might be expected f r o m their molecular similarity. I n asphaltbase oil, especially at lower temperatures, it gives abnormally high vapor pressures, indicating a lower solubility. Benzene, probably because of its unsaturated nature, gives evidence of molecular attraction and consequent low uapor pressures, i n castor, asphalt-base, and parafin-base oil. Combining the results in this paper with those on various gas-absorbent oils, it seems that as a general rule benzene goes gradually f r o m slight Presented beiore the Section of Petroleum Chemistry a t the 64th Meeting of t h e American Chemical Society, Pittsburgh, P a , September 4 t o 8, 1922. Published as Contribution No. 66 from the Research Laboratory of Applied Chemistry, IN.I. T.
TECHNOLOGY, C.4MBRIDGE, MASS.
positive deviations for oils of low molecular weight to large negative deviations as the molecular weight of the oil increases. Cyclohexane behaves very much like benzene, though g i v i n g i n general slightly higher curves. As f a r as deviations from Raoult's law at the lower concentrations are concerned, the four kinds of oil may be arranged in the following general order of increasing tendency to absorb, for a given molecular weight: California, asphalt-base, parafin-base, and castor. I t will be noted, however, that this is also precisely the order of increasing molecular weight for a given viscosity, and since if Raouli's law held precisely the oils of higher molecular weight would absorb less solvent, the two effects tend to neutralize one another to a surprising extent, especially for the mineral oils. The same tendericy was observed in the case of the gas-absorbent oils of different molecular weight but similar source-the deviations f r o m Raoult's law almost exactly counterbalanced diserences in molecular weight. Indeed, in some ways the most striking fact brought out by the whole investigation is the comparatively small difference in the per cent by weight or volume of a given solvent absorbed under specified conditions by widely different kinds of hydrocarbon oils. This conclusion does not apply to other nonvolatile oils containing oxygen, such as glycerides, cresols, etc. Castor oil is not completely miscible with hexane at temperatures below 48.5" C. A solubility curve is given for this combination at temperatures down to 20" C. The method of applying the foregoing results to the three practical problems mentioned abooe, and some of the conclusions to be drawn from such application, are briefly discussed.
KKOWLEDGE of the vapor pressure of volatile solvents, such as benzene and hexane, dissolved in various types of relatively nonvolatile oils, is of commercial importance in a considerable number of applications. Of these, the three most important are: (a) the recovery of solvents from gases by scrubbing with cold absorbent oils, as in recovering benzene from coal gas or gasoline from natural gas; (b) the dilution of crank-case oils with fuels in internal combustion engines; and ( c ) the removal of the last traces of volatile solvents from oils and fats obtained by solventextraction processes. Furthermore, in order to be of real value, measurements should be made a t a sufficient variety of different temperatures to make possible accurate inter- or extrapolations over a fairly wide temperature range. I n spite of the commercial importance of such information, very little work has been done along these lines, even a t ordinary temperatures, and no generalizations have been drawn which would make it possible to calculate approximate figures for a specific case. It was the object of the investigation undertaken by this laboratory to make possible such generalizations and approximate calculations for the three general types of volatile hydrocarbons-paraffin, naphthene, and aromatic-dissolved in most of the ordinary types of nonvolatile oils. This paper deals with the general methods employed in measuring the vapor pressures and determining
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
802
the molecular weights, and details the results on various types of lubricating oils, including castor oil. A subsequent paper covers the results on the different kinds of gas-absorbent oils. What few data are available in the literature are discussed under the specific sections to which the data apply.
Vol. 15, No. 8
pure solvent. For the purpose in hand, benzene appeared to be the most suitable solvent; since it readily dissolves practically all hydrocarbons, is easily obtainable in a fairly pure state (it need not be absolutely pure, since the freezingpoint lowering is figured by difference, using the freezing point of the particular sample as the reference point), and its freezing point is very convenient (5.48' C.) for experimental manipulation. It gave excellent results with various gas-absorbent oils (molecular weights 160 t o 260) and lighter lubricating oils, and was fairly satisfactory even for a paraffin-base oil (molecular weight 407). Castor oil (molecular weight 900) gave rather unsatisfactory result^.^ The general method of determining molecular weights by the freezing point-lowering method is described in several textbooks on physical chemistry, and need not be discussed in any detail. The apparatus used was essentially the regular Beckmann arrangement, the solutions being made up by weighing the oil and then the benzene directly into the inside test tube of the apparatus. The tube is then placed inside the larger tube, which is surrounded by the ice bath, and the solution is stirred constantly. The mercury in the Beckmann thermometer falls slowly to a point slightly below the freezing point, because of supercooling, and then rises to a substantially constant level, the freezing point of the solution. This maximum constant temperature is recorded and the crystals of benzene are melted by the heat of the hand and refrozen. The two freezing points should cbeck to 0.005 degree, and are often identical. I n the more concentrated solutions, particularly those of the heavier oils, some yellow colloidal matter slowly coagulates on standing, but does not appear to affect the results, except that it generally promotes supercooling and makes the freezing point somewhat more fugitive. The general procedure in calculating molecular weight.s from data on the freezing-point lowering is to work with extremely dilute solutionsand calculate the results according to the approximate formula:
340
f
98 a
32L
where T o = freezing point of pure solvent T = freezing point when containing solute Fo = 65.6, freezing-point constant for benzene N = number of mols of solute N O= number of mols of solvent M = molecular weight of solute D = TO-T=freezing-point lowering W = grams solute (oil) W O= grams solvent (benzene)
300
,MOLEcu LAR WEIGHTDETERMI NATI oNS In order to compare the observed vapor pressures of the various solutions with those calculated from Raoult's law, it is necessary to determine the average molecular weight of the various oils used, that of the volatile solvents being, of course, well known. Surprisingly little information is available along these lines, and all the determinations had to be made in this laboratory. Vapor density methods are obviously not suitable for the determination of the molecular weights of these oils, on account of their low volatility. The simplest and most suitable method appears to be the measurement of the freezingpoint lowering produced when the oils are dissolved in some
It is the experience of the writers, however, that for work in the average laboratory the errors are diminished by making most of the measurements with concentrations high enough to give freezing-point lowerings from 0.5 to 2" C. For such fairly high concentrations this simple form of the freezing point-lowering equation is far from precise, and 3 When this paper was presented before the Section of Petroleum Chemistry, C F. Mabery stated t h a t he had not found benzene satisfactory for the determinations of molecular weights of hydrocarbons above 300 or 400, and recommended stearic acid for the purpose T h e general procedure would be the same for either solvent It is of course, quite possible t h a t different solvents would give somewhat different results for the molecular weights of certain oils, owing t o varying degrees of association of the heavier molecules, b u t if all the results are calculated in terms of molecular weights as determzned in dzlule benzene solutzon, any deviations due to small amounts of association will merely be lumped with other causes of deviations from Raoult's law
August, 1923
INDUSTRIAL A N D E.NGINEERIlYG CHEMISTRY
803
with that employed by this laboratory in its investigation of the volatility of motor fuelsS5 It is shown diagrammatically in Fig. 5, and consists essentially of a 20-cc. bulb with M a small balancing manometer attached, both of which are N W To- T = F O -N=+ N F oo n W o immersed in a constant temperature bath. In adapting this apparatus and procedure to the problem %+ 7 8 in hand, it was necessary to make certain modifications. Even this equation In working with gasoline the various components were not is not rigidly exact widely different in boiling point, and it was possible to remove up to such high con- the dissolved air by boiling off up to 1 per cent of the liquid centrations, but by without appreciably changing the vapor pressure of the plotting the values residue. In this case, however, as much as 95 per cent of calculated f o r t h e the liquid is frequently a nonvolatile oil, and boiling out the molecular me i g h t air by the previously recommended method was found to against the freezing- change the concentration sufficiently to make a serious error point lowering, as in in some cases. the following figures, It was found, however, that the loss in the volatile conand by extrapolation stituent could be kept negligibly small, and yet almost of this representa- completely remove the air by cooling the flask in an ice tive line back to zero and salt bath, evacuating the air with a good vacuum freezilig-point lower- pump, and then closing the stopcock, shaking, evacuating ing, it is possible to again, and repeating this procedure for five or six times until obtain results of suffi- the sample ceased to give any indication of frothing or of cient accuracy for a rise in pressure on shaking. Even then a trace of air or the purpose in hand, other fixed gas probably remained in the flask, but it did not a t least for hydro- cause appreciable error on account of the method of treating carbon oils below 400 the results, described in the subsequent section. in molecular weight. Another precaution found necessary was in the treatment I n order to check of the nonvolatile oils before the addition of the volatile up the freezing-point constant for the sample of benzene solvent. Vapor pressure measurements were made on all used and the method of calculation a t higher concentrations, the pure oils in the same manner as on the solutions, and a a few determinations were made on carefully purified naph- number of them, especially the vegetable oils, had vapor thalene, the molecular weight thus determined checking the pressures far higher the true value (128) within one unit. than was indicated by In determining the molecular weights of various oils, it their boiling points. It will be noted that practically all the results indicate higher was found that these apparent molecular weights a t the higher concentrations. abnormally high values This is probably due to the tendency of some of the constitu- were generally due to ents in the oil to associate or even to form colloidal particles traces of moisture, and a t the higher concentrations. This might be taken to in- in order to decrease dicate that benzene is not as good a solvent as might be de- the magnitude of the sired, but it is doubtful if any solvent could be found in which blank correction, all the heavier molecules would not associate to some extent. the oils were heated The extrapolation of the line back to zero concentration to 125" C. and a small should, however, eliminate most, if not all, of the error due amount of dry nitroto association. gen or hydrogen was Molecular weight determinations by the foregoing pro- bubbled through becedure have been made on thirteen different kinds of 0iL4 fore they were used in The results on the eight gas-absorbent oils, which check making up the solusurprisingly well with their known properties, are discussed tions. This generally in a subsequent paper, but the curves for the four lubricating reduced the vapor presoils are shown in Figs. 1 to 4, inclusive. Of these, the results sure of the oil itself to with the asphalt-base oil were quite satisfactory, while the a very low figure, exthree heavier oils gave a rather rapid change of apparent cept in gas absorbent molecular weight with concentration. Although the possible oil No. 8, which was error on the castor oil is rather large the figure 900 is quite quite appreciably volreasonable, since pure ricinolein would have a molecular atile in itself. weight of 932 and the impurities would almost certainly The apparatus diflower this figure. The extrapolation to zero concentration fers from the one preof the curves for the other two oils gives results which are viously used only in the FIG.5 probably reliable within 2 or 3 per cent, which is sufficient substitution of a twofor prestnt purposes. way vacuum stopcock for the three-way cock previously used, which gave some trouble on account of leakage. &
