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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,  ACETIC ACID  /////  A t h e s i s submitted i n p a r t i a l  fulfilment  of the r e q u i r e m e n t s f o r t h e degree o f M a s t e r of A p p l i e d S c i e n c e - • ~• i n Chemical E n g i n e e r i n g - a t the University  of B r i t i s h  Columbia  Alan. N. Mearns  October  At*J  1947  f S Z . / ^ ^ " ^ r  A n V n n w l fidrntifint.  The a u t h o r would l i k e t o acknowledge the k i n d h e l p - a n d g u i d a n c e of P r o 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 o u t .  TABLE OP CONTENTS  I.  Introduction  —  Page 1.  II. " Theory of S o l u b i l i t y  ~  2.  I I I . Calculation of Freezing Points and S o l u b i l i t i e s  3  IV7  Materials Used  6  V.  Experimental Procedure  7  VI. ' Results-.  .-'  •  .  1°  VII. Discussion of Results  15  VIII. P u r i f i c a t i o n by R e e r y s t a l l i z a t i o n  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.  S o l u b i l i t y Curves : Hexacosane i n 2.Ethyl n Butyric Acid and Propionic Acid  Pig. 2.  S o l u b i l i t y of Hexacosane i n Acetic Acid  Pig. 3.  12.  log x vs. l / T  13V 14.  ABSTRACT  The s o l u b i l i t y of a long chain p a r a f f i n hydrocarbon (C 6H(jg) over a temperature range of 75° 2  n a  s been measured  i n acetic, propionic and ethyl butyric acids*  The phase  relationship f o r the acetic acid - hexacosane system d i f f e r s considerably from that of the other two.  The r e s u l t s show  that propionic acid, i s the best solvent to use f o r reeryst a l l i z a t i o n and p u r i f i c a t i o n of a long chain hydrocarbon.  THE SOLUBILITY OF KEXACOSANE i i ?. ETHYL N BUTYRIC ACID, PROPIONIC ACID, AND ACETIC ACID  —  - • ~ Introduction  -During the l a s t ten years several researches have "been undertaken at the University of B r i t i s h Columbia on the mutual s o l u b i l i t i e s of hydrocarbons. This work together with information supplied by other u n i v e r s i t i e s has helped establish the f a c t that the mutual s o l u b i l i t y of hydrocarbons i s a function of t h e i r molecular weights. Use has been Made of this knowledge by the petroleum industry-to i s o l a t e and p u r i f y petroleum fractions-by pointing out the most suitable solvent to be used and i n d i c a t i n g the range i n which any  specific  solvent would be applicable. Most information on s o l u b i l i t i e s has been of a q u a l i t a t i v e nature. As a r e s u l t , there i s much need f o r quanti t a t i v e data which, when applied to e x i s t i n g t h e o r e t i c a l equations,  may  add support to, or help disprove, e x i s t i n g  conceptions of the laws governing s o l u b i l i t y . The need of the petroleum industry f o r such information has arisen from the increased use of solvent extraction  processes i n supplementing  older r e f i n i n g methods. Increased  speeds, pressures, and temperatures  i n automotive and aviation  engines have demanded more stable lubricants, and hence more e f f i c i e n t r e f i n i n g methods to obtain these lubricants. Examples of these solvent extraction processes are to be found i n various patent records. This present research was undertaken  to provide  fundamental data on the s o l u b i l i t y of the hydrocarbon hexacosane, ^26 '5>A» ^ S  tiiTQe  organic acids - 2 ethyl n butyric  acid, propionic acid, and acetic acid. It i s hoped that t h i s data may prove to be of some use i n solvent extraction work. -Theory of  Solubility  Solid-Liquid Systems Equilibrium between s o l i d and l i q u i d i n a two component system may be considered from two points of view., If the l i q u i d mixture i s i n equilibrium with the s o l i d phase i n excess, the solution i s said to be at i t s freezing point, and the curve representing the v a r i a t i o n of t h i s temperature  with  the composition of the system i s known as the freezing point curve, I f , on the other hand, a r e l a t i v e l y  small proportion  of s o l i d phase i s i n equilibrium with the l i q u i d , the system i s said to be a saturated solution and the v a r i a t i o n of temperature with composition i s represented by a s o l u b i l i t y curve. There i s no fundamental difference between freezing point curves and s o l u b i l i t y curves. Common practice i s to refer to freezing point curves when dealing with a system i n which the two components are chemically similar, e.g., when both are  metals, s a l t s , or organic compounds of not very d i f f e r e n t melting point. When the two compounds are chemically d i f f e r e n t or there i s a marked difference i n t h e i r melting points, i t i s possible to make the conventional d i s t i n c t i o n between solvent and solute. In such a casey the condition under which the s o l i d solute separates out i s indicated by a s o l u b i l i t y curve, while the equilibrium between the l i q u i d and s o l i d solvent i s represented by a freezing point curve. More conveniently, the whole curve may he referred to as a s o l u b i l i t y curve. Calculation of Freezing Points and S o l u b i l i t i e s By use of the Clausius-Clapeyron  equation,.Glasstone  derives the following general form of Hildebrande's  equation : 2  l n x = - Lf/Rd/T - l / T ) 0  f o r constant pressure conditions, where T i s the temperature at which the s o l i d present to the extent of x mol f r a c t i o n i n the mixture i s i n equilibrium with the l i q u i d . L f i s the heat of fusion of the s o l i d and T  0  i s its melting point i n the pure  3tate. The derivation of t h i s equation involves no assumption concerning the nature of the s o l i d phase, and s o . i t applies equally to both components of the l i q u i d mixture. In the general form given above, i t i s assumed the mixture behaves i d e a l l y . I t i s thus possible, by means of t h i s equation, to calculate the freezing point T of a solution i n which x i s the mol f r a c t i o n of the solvent; a l t e r n a t e l y , the equation gives the s o l u b i l i t y , i . e . mol f r a c t i o n x, at; the temperature T. In each case L f and T  Q  refer to the component separating  as the s o l i d phase. This equation i s limited i n i t s usefulness  1  "by scarcity of data on latent heats of fusion. Also, deviations are observed "because the systems do not behave i d e a l l y . By observation of the above equation several q u a l i t ative deductions of interest may be made. It i s seen that f o r 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 , i t follows that Q  s o l u b i l i t y must increase with temperature : further, the heat of solution must be equal to the heat of fusion of the solute, provided the system behaves i d e a l l y . Condensed Systems Since three degrees of freedom -  temperature,  pressure, and composition - are possible i n 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 s i m p l i f i e d considerably by a r b i t r a r i l y f i x i n g one of the variables. In s o l i d - l i q u i d systems the effect of pressure i s r e l a t i v e l y small, and measurementsmade under atmospheric pressure would be very l i t t l e d i f f e r e n t from those obtained i f the s o l i d and l i q u i d are i n equilibrium under their own vapor pressure. A system i n which only s o l i d and l i q u i d phases are considered i s c a l l e d 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 i n this research. Liquid-Liquid Systems In the study of the mutual s o l u b i l i t i e s of l i q u i d s 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 i n which there are two l i q u i d phases i n equilibrium the phase rule shows that there are two d e c r e e s of freedom, so i f the pressure i s fixed at one atmosphere, the temperature i s s u f f i c i e n t to define the system completely. In other words, at a given temperature the compositions of the two l i q u i d 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 i n t h i s research, and studying the whole range of mixtures, a complete s o l u b i l i t y curve may be obtained. I f the compositions of the conjugate solutions at each temperature are plotted, most systems give a curve similar to that shown i n 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 i s n e g l i g i b l e i n  amount. This point i s i n effect the composition of one of the two layers. By covering the whole range of concentrations the . x,  complete s o l u b i l i t y curve may,  however, he obtained. The t i e  l i n e s j o i n i n g the two points representing two layers i n equilibrium at a given temperature are, of course, horizontal. It may be seen from the figure that, as the temperature 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 solution temperature"  c  i s known as the  or "cohsolute -temperature".  "critical  Above t h i s  temperature the two l i q u i d s are miscible i n a l l proportions. In f a c t , a l l systems l y i n g outside of the curve consist of one layer only, while those l y i n g inside the curve consist of two layers, the compositions of which are given by points on the curve f o r the same temperature.  For example, a system designated  by point X w i l l consist of two layers % amounts being given by: Amount of 1^ Amount of L g  =  and L  2  , the r e l a t i v e  Distance X L o Distance XL-L  Materials used The hexacosane used i n t h i s research was  synthesized  by the Peterson e l e c t r o l y t i c method from Eastman Kodak myristic a c i d . It was p u r i f i e d by treatment with small quantities of 3  !  concentrated s u l f u r i c acid following the method of Piper et al£ and further p u r i f i e d by repeated c r y s t a l l i z a t i o n from benzene u n t i l a constant melting point of 55.6Pc was  obtained.  In the l i t e r a t u r e , various values f o r the melting point of hexacosane are to be found. Four of these are l i s t e d  i n chronological order : 56.4 , 57 , 56.17, 55.8 . 5  6  8  Garner, van Bibber, and King give the following equation f o r the melting point of hydrocarbons : T = (0.6G85n - 1.75)/(0.001491n / 0.00404) where T i s the absolute temperature  and n i s the number of  carbon atoms i n the hydrocarbon. Using t h i s equation, the melting point of hexacosane i s calculated to be 55.6°C. The melting point of the hexacosane used i n this research i s thus seen to be i n exact agreement with the calculated value and i n very close agreement with l a t e s t observed values. The 2 ethyl n butyric acid was p u r i f i e d i n the laboratory by r e c r y s t a l l i z a t i o n andhad a b o i l i n g point of 190° C. A technical grade of Eastman Kodak propionic acid with a b o i l i n g point of 141,1°G, and Baker's Analyzed C P . g l a c i a l acetic acid with a b o i l i n g point of 118.1°C was used. Experimental  Procedure  A l e x e j e f f s synthetic method f o r the determination f  of s o l u b i l i t y was decided on as being the most p r a c t i c a l method of obtaining the desired r e s u l t s . This method consists of preparing a mixture of c a r e f u l l y determined amounts of solute and solvent i n a sealed bulb to prevent vaporization, and then gradually r a i s i n g the temperature u n t i l the l a s t c r y s t a l of s o l i d disappears, at which point the temperature i s noted. On recooling the bulb and i t s contained l i q u i d a point i s reached at which the s o l i d begins to c r y s t a l l i z e out. The mean of these two points represents the s o l u b i l i t y of one compound i n the other. The mean i s taken i n order to counter-  act the effects of temperature lag and supercooling. Thick walled bulbs . of approximately 2 cm. i n diameter were blown and sealed to stems of 6 mm.  glass tubing, 15 cm. i n  length. The hexacosane was introduced into the bulb by melting and pouring through a long stemmed funnel. The solvents, which were a l l l i q u i d s 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 a f t e r introducing both hexacosane and the solvent. The contents of the bulb were then frozen with dry i c e i n a Dewar f l a s k and sealed to a vacuum l i n e . By alternately evacuating the bulb and sweeping i t out with hydrogen, most of the a i r i n the bulb was thought to be removed and the bulb was then sealed. Thus the s o l i d and l i q u i d i n the sealed bulbs were i n equilibrium under their own vapor pressure. The prepared bulbs were then placed i n a water bath equipped with an e l e c t r i c immersion heater, cooling c o i l s , and an e l e c t r i c variable speed s t i r r e r . By alternately r a i s i n g and lowering the temperature  of the bath, and noting the points at  which the l a s t c r y s t a l of hexacosane disappeared and then reappeared, the c r i t i c a l point f o r 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 t h i s f a c t , a  l i q u i d - l i q u i d system resulted and the temperature had to he raised considerably above the melting point of hexacosane before the point of mutual s o l u b i l i t y was bath was  obtained. A p a r a f f i n  employed i n order to reach the desired temperatures.  The c r i t i c a l point-was defined quite sharply by the d i s appearance of the two layers .on r a i s i n g the temperature and by a sudden t u r b i d i t y on lowering the temperature. This point of t u r b i d i t y was very sharp and reproducible and was  considered  to be the more accurate indication of the mutual s o l u b i l i t y . Because of the low vapor pressures of the l i q u i d s used, at the experimental  temperatures, and the small volume  of the bulbs above the l i q u i d , corrections f o r vaporization of the solvent i n 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 0.0  P.P.. °C < -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 i n 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 i s f e l t that results are accurate only to the nearest tenth.  Tftblg 3. Hexacosane - Propionic Acid Hexacosane. g.  Propionic A c i d  f  g.  Mol % Hexacosane  P.P.«°C  0.0 0.0436  3.6116  0.24  -22 30 . 68  0.0807  2.7604  0.59  37.31  0.2422  0.9891  4.72  43.85  0.4477  0.8019  10.1  46.75  0.5038  0.5992  14.5  47.08  0.6870  0.5329  20 . 65  47.36  1.0159  0.3694  35.7  48.65  1.4838  0.3248  48.0  48.85  1.5628  0.1196  72.6  50.. 45  1.8411  0.0774  82.8  51.82 55.6  100.0 Table 3. Hexacosane - Acetic Acid Acetic A c i d  r  g. Mol ^Hexacosane M i s c i b i l i t v Pt. 0.0  16.6 .  0.1491  1.4658  1.64  180.0  0.4660  0.8473  8.30  186  0.8806  0.4876  22.0  170  1.0788  0.3714  32.2  137  1.6603  0.3601  42.9  109.5  1.. 4019  0.1998  53.5  90  100.0  55.6  Discussion of Results By p l o t t i n g mol concentrations against temperature for  the systems hexacosane - 2 ethyl n butyric acid and  hexacosane propionic acid the curves i n f i g . 1. were obtained. Fig.  2. gives the curve obtained f o r 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 f o r the two s o l i d - l i q u i d systems of f i g . 1. From an observation of f i g s . 1 and 3 i t may be seen • that a f t e r a concentration of about 10 mol percent i s reached the solute would appear to exert i t s influence i n determining the shape of the curves. Also, when a temperature of about 50°C i s reached, there i s another change i n the slope of the curves. This i s seen most s t r i k i n g l y i n f i g . 3. where a change of slope denotes a change i n the latent heat of fusion. This temperature of 50°C corresponds very closely to 50.1°C as reported by Garner et a l . ? to be the t r a n s i t i o n point f o r the change from the <*. to f& form of hexacosane.  I t would appear  obvious from the shape of the curves that the systems are not i d e a l . ThereforeI i n the equation f  In x «  -Lf/Rd/T - l / T ) Q  should be replaced by the d i f f e r e n t i a l 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 d i f f e r e n t i a l heat of solution corresponds very closely to the heat of d i l u t i o n of the solvent. From about 10 mol percent up to the t r a n s i t i o n point, the d i f f e r e n t i a l heat of solution corresponds  16.' more closely to the latent heat of fusion of p  hexacosane,  and the curve approaches the i d e a l , which i s a straight l i n e with slope - L /R. After the t r a n s i t i o n point i s reached, we f  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 i n the curve f o r hexacosane - 2 ethyl n butyric acid i s perhaps due to association of the acid with the hexacosane. This system does not appear to act normally. In f i g . 2. the s o l u b i l i t y curve f o r hexacosane i n acetic acid i s plotted accurately up to about 54 mol percent hexacosane* Pore increased concentrations of hexacosane the surface area between the two l i q u i d s was so small, and the point of t u r b i d i t y so i n d i s t i n c t , that accurate results could not be obtained by v i s u a l methods. We know from c r y s t a l l i z a t i o n work that the melting point of hexacosane i s s l i g h t l y depressed i n the presence of acetic acid. This i s indicated i n 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 s o l u t i o n temperature of the system hexacosane - acetic acid i s 187°C. P u r i f i c a t i o n bv R e c r v s t a l l i z a t i o n In order f o r a solvent to be a satisfactory r e c r y s t a l l i z i n g medium, to be used i n purifying a desired product, i t i s necessary that the s o l u b i l i t y curve have a small temperature gradient i n the desired range. That i s to say, f o r a small decrease i n temperature a comparatively large amount of solute must separate out. It would appear from t h i s research  17 that decreasing molecular weight increases the r e c r y s t a l l i z i n g effect of organic acids i n purifying p a r a f f i n s . Por hexacosane, propionic acid i s seen to be the best r e c r y s t a l l i z i n g medium, inasmuch as i t gives the largest amount of the p a r a f f i n separating out f o r a given decrease i n  temperature.  R e c r y s t a l l i z a t i o n i s most e f f e c t i v e between 10 and 60 mol percent of hexacosane. In the system hexacosane-acetic  acid,  the fact that a l i q u i d - l i q u i d system i s obtained makes the acid unsuitable f o r r e c r y s t a l l i z a t i o n . Conclusions and Recommendations In future s o l u b i l i t y studies.of this type, the author f e e l s that certain refinements i n technique would make the work more accurate and less tedious. In the v i s u a l method as employed i n this research, each bulb required undivided attention during the period of r a i s i n g and lowering the temperature, was usually a long, drawn out operation. The  which  temperatures  reported are the means of a large-number of observations f o r each bulb. It i s 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 l i g h t were directed through the solution i n the bulbs to detect the crystals of solute by r e f l e c t i o n , immediately  on p r e c i p i t a t i o n .  Some method of preventing supercooling would also prove very e f f e c t i v e , as constant a g i t a t i o n appears to have very l i t t l e e f f e c t . This might possibly be done by sealing a piece of inert wire i n 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 p r e c i p i t a t i o n forthe  18.  crystals of solute and would decrease supercooling to a minimum. - Summary 1.  The s o l u b i l i t y curves f o r the systems hexacosane -  2 ethyl n butyric acid, hexacosane - propionic  acid, and  hexacosane - a c e t i c - a c i d have been found. 2.  The c r i t i c a l solution temperature of the system  hexacosane-acetic acid i s seen to be 187°C. 3.  The data obtained adds support to the-belief that  hexacosane exists i n two enaniotropic  forms. The t r a s i t i o n  temperature would appear to be about 50°C.  19.  Bibliography - 1.  Glasstone, S., "Textbook of Physical Chemistry", 1940.  2.  Hildebrand, J 7 H.', " S o l u b i l i t y  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,  5.  Buchler, Ind..Eng. Chem., 19, 723,.(l927).  6.  Hildebrand, J-.-H., J . Am. Chem. S o c , 51,-2487,  7.  Garner, van Bibber, and King, J. Chem. S o c , 111, 1533,  of Non-Electrolytes", 1936.  (1931).  (1929).  (1931 8. Seyer, W. P., Patterson, R. P., and Keays, J.L., J . Am. Chem. S o c , 66, 179, (1944).  

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