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The solubility of hexacosane in 2 ethyl N butyric acid, propionic acid, and acetic acid Mearns, Alan Norman 1947

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THE SOLUBILITY OF HEXACOSANE IN 2 ETHYL N BUTYRIC ACID, PROPIONIC ACID, A t h e s i s submitted i n p a r t i a l f u l f i l m e n t of the requirements f o r the degree of Master of App l i e d Science - • ~ • i n Chemical Engineering - at the U n i v e r s i t y of B r i t i s h Columbia ACETIC ACID ///// Alan. N. Mearns October 1947 At*J f S Z . / ^ ^ " ^ r A n V n n w l fidrntifint. The author would l i k e to acknowledge the kin d help-and guidance of Pro f e s s o r W« P. Seyer under whose s u p e r v i s i o n t h i s work was c a r r i e d out. TABLE OP CONTENTS I. Introduction — Page 1. II. " Theory of Solubility ~ 2. III. Calculation of Freezing Points and Solubilities 3 IV7 Materials Used 6 V. Experimental Procedure 7 VI. ' Results-. .-' • . 1° VII. Discussion of Results 15 VIII. Purification by Reerystallization 16 IX. Conclusions and Recommendations 17 X. Summary- 18 XI. Bibliography 19. TABLES AND ILLUSTRATIONS Table 1. Hexacosane-2 Ethyl n Butyric Acid 10. Table 2, Hexacosane-Propionic Acid 11, Table 3 Hexacosane-Acetic Acid 11. —> Pig. 1. Solubility Curves : Hexacosane in 2.Ethyl n Butyric Acid and Propionic Acid 12. Pig. 2. Solubility of Hexacosane in Acetic Acid 13V Pig. 3. log x vs. l/T 14. ABSTRACT The solubility of a long chain paraffin hydrocarbon (C26H(jg) over a temperature range of 75° n a s been measured in acetic, propionic and ethyl butyric acids* The phase relationship for the acetic acid - hexacosane system differs considerably from that of the other two. The results show that propionic acid, i s the best solvent to use for reerys-t a l l i z a t i o n and purification of a long chain hydrocarbon. THE SOLUBILITY OF KEXACOSANE i i ?. ETHYL N BUTYRIC ACID, PROPIONIC ACID, AND  ACETIC ACID — - • ~ Introduction -During the last ten years several researches have "been undertaken at the University of British Columbia on the mutual solubilities of hydrocarbons. This work together with information supplied by other universities has helped establish the fact that the mutual solubility of hydrocarbons is a function of their molecular weights. Use has been Made of this knowledge by the petroleum industry-to isolate and purify petroleum fractions-by pointing out the most suitable solvent to be used and indicating the range in which any specific solvent would be applicable. Most information on solubilities has been of a qualitative nature. As a result, there i s much need for quant-itative data which, when applied to existing theoretical equations, may add support to, or help disprove, existing conceptions of the laws governing solubility. The need of the petroleum industry for such inform-ation has arisen from the increased use of solvent extraction processes in supplementing older refining methods. Increased speeds, pressures, and temperatures in automotive and aviation engines have demanded more stable lubricants, and hence more efficient refining methods to obtain these lubricants. Examples of these solvent extraction processes are to be found in various patent records. This present research was undertaken to provide fundamental data on the solubility of the hydrocarbon hexacosane, ^26S'5>A» ^ tiiTQe organic acids - 2 ethyl n butyric acid, propionic acid, and acetic acid. It i s hoped that this data may prove to be of some use i n solvent extraction work. -Theory of Solubility  Solid-Liquid Systems Equilibrium between solid and liquid in a two component system may be considered from two points of view., If the liquid mixture is in equilibrium with the solid phase in excess, the solution is said to be at i t s freezing point, and the curve representing the variation of this temperature with the composition of the system is known as the freezing point curve, If, on the other hand, a relatively small proportion of solid phase is in equilibrium with the liquid, the system is said to be a saturated solution and the variation of temperature with composition i s represented by a solubility curve. There i s no fundamental difference between freezing point curves and solubility curves. Common practice i s to refer to freezing point curves when dealing with a system in which the two components are chemically similar, e.g., when both are metals, salts, or organic compounds of not very different melting point. When the two compounds are chemically different or there i s a marked difference in their melting points, i t i s possible to make the conventional distinction between solvent and solute. In such a casey the condition under which the solid solute separates out i s indicated by a solubility curve, while the equilibrium between the liquid and solid solvent i s represented by a freezing point curve. More conveniently, the whole curve may he referred to as a solubility curve. Calculation of Freezing Points and Solubilities By use of the Clausius-Clapeyron equation,.Glasstone 1 derives the following general form of Hildebrande's equation 2: ln x = - Lf/Rd/T - l/T 0) for constant pressure conditions, where T i s the temperature at which the solid present to the extent of x mol fraction i n the mixture i s i n equilibrium with the liquid. Lf i s the heat of fusion of the solid and T 0 i s its melting point i n the pure 3tate. The derivation of this equation involves no assumption concerning the nature of the solid phase, and so.it applies equally to both components of the liquid mixture. In the general form given above, i t is assumed the mixture behaves ideally. It i s thus possible, by means of this equation, to calculate the freezing point T of a solution i n which x i s the mol fraction of the solvent; alternately, the equation gives the solubility, i.e. mol fraction x, at; the temperature T. In each case Lf and T Q refer to the component separating as the solid phase. This equation is limited i n i t s usefulness "by scarcity of data on latent heats of fusion. Also, deviations are observed "because the systems do not behave ideally. By observation of the above equation several qualit-ative deductions of interest may be made. It i s seen that for substances having similar molar beats of fusion, those with lower melting points have the higher s o l u b i l i t i e s . Since T must always be less than the melting point T Q, i t follows that solubility must increase with temperature : further, the heat of solution must be equal to the heat of fusion of the solute, provided the system behaves ideally. Condensed Systems Since three degrees of freedom - temperature, pressure, and composition - are possible in a two component system, a three dimensional figure would be required to represent accurately the conditions of equilibrium of such a system. This representation may be simplified considerably by arbitrarily fixing one of the variables. In solid-liquid systems the effect of pressure i s relatively small, and measurementsmade under atmospheric pressure would be very l i t t l e different from those obtained i f the solid and liquid are in equilibrium under their own vapor pressure. A system in which only solid and liquid phases are considered i s called a condensed system. Such a system may be graphically represented by the use of only two variables - temperature and composition. Such a manner of representation was emplyed in this research. Liquid-Liquid Systems In the study of the mutual solubilities of liquids i t i s generally the practice to work, not at the vapor pressure of the system, but at atmospheric pressure, and to determine the compesitions of the two layers at various temperatures. For, a system of two components in which there are two liquid phases in equilibrium the phase rule shows that there are two decrees of freedom, so i f the pressure i s fixed at one atmo-sphere, the temperature i s sufficient to define the system completely. In other words, at a given temperature the comp-ositions of the two liquid layers are fixed and are independent of the amounts of the two phases. By making use of the Alexejeff or synthetic method, as was done in this research, and studying the whole range of mixtures, a complete solubility curve may be obtained. If the compositions of the conjugate solutions at each temperature are plotted, most systems give a curve similar to that shown in the following figure : t_ T t N At any temperature t, the conjugate solutions have compositions as indicated by the points L-L and L 2 . The synthetic method gives only one of the two points representing the composition and temperature at which the second layer is negligible in amount. This point i s in effect the composition of one of the two layers. By covering the whole range of concentrations the . x, complete solubility curve may, however, he obtained. The tie lines joining the two points representing two layers in equilibrium at a given temperature are, of course, horizontal. It may be seen from the figure that, as the temper-ature i s raised, the compesitions of the two layers approach each other u n t i l eventually point C i s reached at temperature tc when the two layers become one; t c i s known as the " c r i t i c a l solution temperature" or "cohsolute -temperature". Above this temperature the two liquids are miscible in a l l proportions. In fact, a l l systems lying outside of the curve consist of one layer only, while those lying inside the curve consist of two layers, the compositions of which are given by points on the curve for the same temperature. For example, a system designated by point X w i l l consist of two layers % and L 2 , the relative amounts being given by: Amount of 1^ = Distance X L o Amount of L g Distance XL-L Materials used The hexacosane used in this research was synthesized by the Peterson electrolytic method from Eastman Kodak myristic acid 3. It was purified by treatment with small quantities of ! concentrated sulfuric acid following the method of Piper et al£ and further purified by repeated crystallization from benzene unt i l a constant melting point of 55.6Pc was obtained. In the literature, various values for the melting point of hexacosane are to be found. Four of these are l i s t e d in chronological order : 56.45, 576, 56.17, 55.88. Garner, van Bibber, and King give the following equation for the melting point of hydrocarbons : T = (0.6G85n - 1.75)/(0.001491n / 0.00404) where T is the absolute temperature and n is the number of carbon atoms in the hydrocarbon. Using this equation, the melting point of hexacosane i s calculated to be 55.6°C. The melting point of the hexacosane used in this research i s thus seen to be in exact agreement with the calculated value and in very close agreement with latest observed values. The 2 ethyl n butyric acid was purified in the laboratory by recrystallization andhad a boiling point of 190° C. A technical grade of Eastman Kodak propionic acid with a boiling point of 141,1°G, and Baker's Analyzed CP. glacial acetic acid with a boiling point of 118.1°C was used. Experimental Procedure Alexejeff fs synthetic method for the determination of solubility was decided on as being the most practical method of obtaining the desired results. This method consists of preparing a mixture of carefully determined amounts of solute and solvent in a sealed bulb to prevent vaporization, and then gradually raising the temperature u n t i l the last crystal of solid disappears, at which point the temperature i s noted. On recooling the bulb and i t s contained liquid a point is reached at which the solid begins to crystallize out. The mean of these two points represents the solubility of one compound in the other. The mean is taken in order to counter-act the effects of temperature lag and supercooling. Thick walled bulbs . of approximately 2 cm. in diameter were blown and sealed to stems of 6 mm. glass tubing, 15 cm. in length. The hexacosane was introduced into the bulb by melting and pouring through a long stemmed funnel. The solvents, which were a l l liquids at room temperature, were merely run through a long stemmed funnel into the bulb. Correct weights of solvent and solute were obtained by weighing the bulb before and after introducing both hexacosane and the solvent. The contents of the bulb were then frozen with dry ice in a Dewar flask and sealed to a vacuum line. By alternately evacuating the bulb and sweeping i t out with hydrogen, most of the a i r in the bulb was thought to be removed and the bulb was then sealed. Thus the solid and liquid in the sealed bulbs were i n equilibrium under their own vapor pressure. The prepared bulbs were then placed in a water bath equipped with an electric immersion heater, cooling coils, and an electric variable speed s t i r r e r . By alternately raising and lowering the temperature of the bath, and noting the points at which the last crystal of hexacosane disappeared and then reappeared, the c r i t i c a l point for each bulb was obtained. This system was used with good results to study the systems hexacosane- 2 ethyl n butyric acid and hexacosane-propionic acid. When the system hexacosane-acetic acid was studied, i t was found that the two components were not mutually soluble below the melting point of hexacosane except at very minute molar concentrations of hexacosane. Because of this fact, a liquid-liquid system resulted and the temperature had to he raised considerably above the melting point of hexacosane before the point of mutual solubility was obtained. A paraffin bath was employed in order to reach the desired temperatures. The c r i t i c a l point-was defined quite sharply by the dis-appearance of the two layers .on raising the temperature and by a sudden turbidity on lowering the temperature. This point of turbidity was very sharp and reproducible and was considered to be the more accurate indication of the mutual solubility. Because of the low vapor pressures of the liquids used, at the experimental temperatures, and the small volume of the bulbs above the liquid, corrections for vaporization of the solvent in the bulb were thought to be unnecessary and were not applied. Results Table 1. Hexacosane - 2 Ethvl n Butyric Acid  Hexacosane. g. Butvric Acid, g. Mol % Hexacosane P.P.. °C • • • • • 0.0 < -15 0.0200 2.5281 0.25 19.5 0.0834 1.9698 1.33 30.88 0.2633 1.4492 5.45 40.32 0.3279 1.1552 8.23 42.98 0.4348 0.7383 15.72 45.82 0.4164 0.6406 17.0 46.02 0.8276 0.7681 25.5 47.38 0.6305 0.5443 26.9 47.71 0.9602 0. 5690 34.9 48.55 1.3078 0.4477 48.0 49.09 1.2273 0.1358 74.2 51.40 0.9958 0.0769 80.4 52.28 100.0 55.6 Temperatures tabulated in these tables are the means of a large number of readings. Although the readings were taken to the nearest hundredth of a degree, and hence the tabulated results are given to the nearest hundredth, i t is f e l t that results are accurate only to the nearest tenth. Tftblg 3. Hexacosane - Propionic Acid  Hexacosane. g. Propionic Acid f g. Mol % Hexacosane P.P.«°C 0.0436 0.0807 0.2422 0.4477 0.5038 0.6870 1.0159 1.4838 1.5628 1.8411 0.1491 0.4660 0.8806 1.0788 1.6603 1.. 4019 0.0 -22 3.6116 0.24 30 . 68 2.7604 0.59 37.31 0.9891 4.72 43.85 0.8019 10.1 46.75 0.5992 14.5 47.08 0.5329 20 . 65 47.36 0.3694 35.7 48.65 0.3248 48.0 48.85 0.1196 72.6 50.. 45 0.0774 82.8 51.82 100.0 55.6 Table 3. Hexacosane - Acetic Acid Acetic Acid r g. Mol ^Hexacosane Misc i b i l i t v Pt. 0.0 16.6 . 1.4658 1.64 180.0 0.8473 8.30 186 0.4876 22.0 170 0.3714 32.2 137 0.3601 42.9 109.5 0.1998 53.5 90 100.0 55.6 Discussion of Results By plotting mol concentrations against temperature for the systems hexacosane - 2 ethyl n butyric acid and hexacosane propionic acid the curves in f i g . 1. were obtained. Fig. 2. gives the curve obtained for the system hexacosane -acetic acid, which was a l i q u i d - r l i q u i d system. In f i g . 3., log x vs. l/T i s plotted for the two solid-liquid systems of f i g . 1. From an observation of f i g s . 1 and 3 i t may be seen • that after a concentration of about 10 mol percent i s reached the solute would appear to exert i t s influence in determining the shape of the curves. Also, when a temperature of about 50°C i s reached, there i s another change in the slope of the curves. This i s seen most strikingly in f i g . 3. where a change of slope denotes a change in the latent heat of fusion. This temperature of 50°C corresponds very closely to 50.1°C as reported by Garner et al.? to be the transition point for the change from the <*. to f& form of hexacosane. It would appear obvious from the shape of the curves that the systems are not ideal. ThereforeI f in the equation In x « -Lf/Rd/T - l/T Q) should be replaced by the differential heat of solution. This fact would help explain the changes i n slope of the curves of f i g . 3. At low mol percentages of hexacosane the differential heat of solution corresponds very closely to the heat of dilution of the solvent. From about 10 mol percent up to the transition point, the differential heat of solution corresponds 16.' more closely to the latent heat of fusion of p hexacosane, and the curve approaches the ideal, which i s a straight line with slope - Lf/R. After the transition point i s reached, we are dealing with the <* form of hexacosane and the slope of the curve again changes, approaching the latent heat of fusion of <A hexacosane. The discontinuity seen in the curve for hexacosane - 2 ethyl n butyric acid is perhaps due to association of the acid with the hexacosane. This system does not appear to act normally. In f i g . 2. the solubility curve for hexacosane in acetic acid i s plotted accurately up to about 54 mol percent hexacosane* Pore increased concentrations of hexacosane the surface area between the two liquids was so small, and the point of turbidity so indistinct, that accurate results could not be obtained by visual methods. We know from crystallization work that the melting point of hexacosane i s slightly depressed in the presence of acetic acid. This i s indicated in f i g . 3. by the dotted curve plotted from Hildebrande•s equation. From the data given, i t i s seen that the c r i t i c a l solution temperature of the system hexacosane - acetic acid i s 187°C. Purification bv Recrvstallization In order for a solvent to be a satisfactory recrystallizing medium, to be used in purifying a desired product, i t i s necessary that the solubility curve have a small temperature gradient i n the desired range. That is to say, for a small decrease in temperature a comparatively large amount of solute must separate out. It would appear from this research 17 that decreasing molecular weight increases the recrystallizing effect of organic acids i n purifying paraffins. Por hexacosane, propionic acid is seen to be the best recrystallizing medium, inasmuch as i t gives the largest amount of the paraffin separating out for a given decrease in temperature. Recrystallization is most effective between 10 and 60 mol percent of hexacosane. In the system hexacosane-acetic acid, the fact that a liquid-liquid system is obtained makes the acid unsuitable for recrystallization. Conclusions and Recommendations In future solubility studies.of this type, the author feels that certain refinements in technique would make the work more accurate and less tedious. In the visual method as employed in this research, each bulb required undivided attention during the period of raising and lowering the temperature, which was usually a long, drawn out operation. The temperatures reported are the means of a large-number of observations for each bulb. It is f e l t that the c r i t i c a l point could be obtained more quickly and more accurately i f a strong beam of light were directed through the solution in the bulbs to detect the crystals of solute by reflection, immediately on precipitation. Some method of preventing supercooling would also prove very effective, as constant agitation appears to have very l i t t l e effect. This might possibly be done by sealing a piece of inert wire in the bulb, and touching the external end of the wire with dry ice when approaching the c r i t i c a l point from above. This would afford a center of precipitation forthe 18. crystals of solute and would decrease supercooling to a minimum. - Summary 1. The solubility curves for the systems hexacosane -2 ethyl n butyric acid, hexacosane - propionic acid, and hexacosane - acetic-acid have been found. 2. The c r i t i c a l solution temperature of the system hexacosane-acetic acid is seen to be 187°C. 3. The data obtained adds support to the-belief that hexacosane exists in two enaniotropic forms. The trasition temperature would appear to be about 50°C. 19. Bibliography - -1. Glasstone, S., "Textbook of Physical Chemistry", 1940. 2. Hildebrand, J 7 H.', "Solubility of Non-Electrolytes", 1936. 3. Keays, J.' L., Masters Thesis, U. B. C., 1941 4. Piper, S . ' H., e t - a l . , - B i o . J., 25, 2072-2094, (1931). 5. Buchler, Ind..Eng. Chem., 19, 723,.(l927). 6. Hildebrand, J-.-H., J. Am. Chem. Soc, 51,-2487, (1929). 7. Garner, van Bibber, and King, J. Chem. Soc, 111, 1533, (1931 8. Seyer, W. P., Patterson, R. P., and Keays, J.L., J. Am. Chem. Soc, 66, 179, (1944). 

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