July, 1956
917
HEATCAPACITY OF PALMITIC ACIDAND METHYL PALMITATE
LOW TEMPERATURE HEAT CAPACITY OF PALMITIC ACID AND METHYL PALMITATE BY HENRY E. WIRTH,'JOHNW. DROEGE AND JOHNH. WOOD Contribution from the Cryogenic Laboratory and the Department of Chemistry, The Ohio State University, Columbus, Ohio Received December 17, 1866
Heat capacity measurements were made on methyl palmitate and on palmitic acid in the temperature range 14 to 300°K. The calculated entropies a t 298.16"K. were 118.33 i 0.35 and 108.12 f 0.22 cal. (mole degree)-', respectively.
Introduction The polymorphism of soaps and other long-chain compounds is one of their most distinctive characteristics. As part of a program designed to study the phase relationships of palmitic acid, methyl palmitate and sodium palfitate, it seemed advisable to determine the heat capacities and entropies. This paper presents the results for palmitic acid and methyl palmitate. Materials and Procedure A mixture of methyl esters containing primarily palmitate and stearate was prepared by methylating Procter and Gamble Triple Pressed Stearic Acid. The esters were hydrogenated at 180", using Raney nickel catalyst. The iodine number was found to be less than 0.2. The eaters were fractionated in a four-foot column atterned after the design of Weitkamp and Brunstrum.2 l l o w pressure drop was achieved by the use of Stedman packing. Contamination of the roduct by sto cock grease was minimized. A closely reguLted manostat Kept the pressure nearly constant a t 2 111111. The take-off assembly was designed so that small samples could be collected and characterized. The product used was the best fraction of the third successive distillation. The setting point, the highest tem erature reached upon slow solidification of the melt, was k u n d to be 29.59". The purity of this sample is believed to have been about 99.7%. Sodium palmitate was prepared by sa onification of the previously purified methyl palmitate wit1 an excess of sodium hydroxide. The soap waa extracted with ether and converted to palmitic acid by treatment with 20% H,S04. The palmitic acid was washed until neutral and dried under high vacuum. The purity was tested by a procedure similar to that described by Taylor and .Rossini.* The freezing point was found to be 62.65' and the purity, 99.8%. The results gave 62.68" as the freezing point of pure palmitic acid. The impurity probably was predominantly stearic acid. The apparatus and procedure have been described prev i o u ~ l y . ~Samples were loaded into a copper calorimeter given the laboratory designation Solid Calorimeter No. 5. The weight of the palmitic acid sample was 43.282 g. The sample was obtained by crystallization from the melt and was therefore of the form called phase "C" by Francis and Piper .E X-Ray analysis confirmed the identification of phase. The molecular weight was taken to be 256.42. The methyl palmitate was melted into the calorimeter. Its weight waa 59.822 g. The molecular weight was taken to be 270.44. Electrical energy in terms of International Volts and Ohms was converted to Defined Calories by dividing by 4.1833. All weights were reduced to vacuum.
Results The first series Of measurements on methyl palmitate was made ten days after solidifying the sample. (.I Department ) of Chemistry, Syracuse University, Syracuse, N. Y. (2) A. W. Weitkamp and L. C. Brunstrum, Oil and Soap, 18, 47
(1941). (3) W. J. Taylor and F. D. Rossini, J . Research Null. Bur. Standarda, 82, 197 (1944). (4) (a) H.L. Johnston, F. T.Clarke, E. B. Rifkin and E. C. Kerr, J . A m . Chem. Soc., 73, 3933 (1950); (b) H. L. Johnston and E. C. Kerr, ibid., 72, 4733 (1950). (6) F. Francis and 8. H. Piper, ibid., 61, 577 (1939).
The second series was made 26 days later. The last series, beginning at 28OoK., W a s made after ten more days. Results are given in Table 1. Thermodynamic functions derived on the basis of a smooth curve are shown in Table 11. The extraPolation to 0°K. was carried out by use Of the fmction Cp/T. TABLE I HEATCAPACITY OF METHYLPALMITATE Mean temp., OK.
Heat capacity, cal. (mole deg.) - 1
Mean t$mp
Heat cal. (mole capacity, deg.) - 1
Series I 66.67 33.31 74.54 37.43 82.83 41.17 91.60 44.64 101.27 47.96 111.95 54.80 122,55 54.72 133.31 57.76 144.05 ,60.68 151.86 62.58 164.91 66.26 176.37 69.12 186.77 71.94 196.27 74.63 205.27 77.29 215.17 79.97 224.46 82.91 233.67 86.44 244.82 88.86
Series I1 13.80 2.76 15.49 3.30 18.32 4.64 21.30 6.26 23.53 7.54 25.61 8.77 27.98 10.18 30 99 12.17 34.15 14.29 37.15 16.32 40.52 18.32 44.47 20.80 48.88 23.50 53.54 26.41 59.75 30.08 65.72 32.85 100.50 47.58 107.90 50.04 52.47 115.51
253.56 263.29 272.56 281.31
Series 111 280.36 104.54 107.26 285.86 291.34 110.58 296.66 112.86
93.12 97.39 102.01 105.79
I
The palmitic acid results are shown in Table 111. Thermodynamic functions based on a smooth cuwe are shown in Table IV. The heat capacity results in the liquid hydrogen range may be in error by as much as 2%. I n the range 80 to 200°K. the error is probably about 0.2y0,increasing to about 0.5y0 near room temperature. These uncertainties contribute to the entropy at 2 9 8 , 3 6 0 ~ .an , of about 0.3% in the case of methyl palmitate and about 0.2% for palmitic acid. Our data are about one calorie per degree lower than those of Parks and KelIey.6 Some difficulty was encountered in drawing the curve through the acid pointsin the region (6) G.
8. Parks and K.K. Kelley, ibid., 47, 2089 (1925).
918
HENRYE. WIRTH,JOHN DROEGEAND JOHN H. WOOD
TABLEI1 THERMODYNAMIC FUNCTIONS FOR METHYL PALMITATE CR
T, K. 20 40 60 80 100 120 140 160 180 200 220 240 260 273.16 280 298.16 300 0
!
oal. (mole deg.) - 1
so
5.49 17.99 30.09 39.92 47.56 53.96 59.59 64.93 70.14 75.66 81.74 88.26 96.05 101.80 104.88 113.40 114.27
- SaO,
eal.
(mole deg.) - 1
2.91 10.46 20.11 30.16 39.91 49.14 57.88 66.18 74.12 81.80 89.29 96.67 104.04 108.92 111.47 118.33 119.03
-(Fa H0
-
Hao), T cal. (mole deg.) -1
- Ho",
cal. (mole) -1
1.oo 3.70 7.54 11.94 16.56 21.22 25.84 30'.37 34.79 39.11 43.33 47.47 51.54 54.18 55.55 59.17 59.53
38.2 270.5 754.2 1457.8 2334.6 3349.9 4485.7 5729.4 7080.1 8537.4 10110 11809 13650 14952 15658 17639 17849
TABLEI11 HEATCAPACITY OF PALMITIC ACID Mean team+. ,
Heat capacity, cal. (mole deg.) -1
Series I1 74.83 34.75 79.52 36.99 84.71 39.02 89.39 40.54 94.56 42.00 99.62 43.79 44.24 101.80 106.82 45.67 112.28 47.42 117.79 48.86 124.18 50.43 52.28 130:84 136.99 53.77 137.71 54.04 144.14 55.78 150.65 57.67 157.71 59.59 164.99 61.26 172.22 62.66 179.98 64.71 187.98 66.95 196.09 68.94 196.22 69.16 204.55 71.69 213.00 74.10 221.74 76.34 230.32 79.10 239.12 82.08 247.22 84.87 254.62 87.67 255.16 87.71 263.08 91.53 271.69 95.62 280.19 100.97 287.47 103.77 295.56 108.62 302.12 113.12
Mean temp.,
OK.
Heat capacity, cal. (mole deg.) - 1
Series I11 , 64.27 30.26 68.84 32.19 74.38 34.71 81.58 37.60 89.04 40.23 95.99 42.53 102.42 44.44 108.88 46.53 116.69 48.76 Series IV 14.91 1.94 16.59 2.68 18.59 3.36 21.32 4.56 24.51 6.43 28.03. 8.62 32.09 11.15 36.60 14.04 41.86 17.39 47.37 20.72 52.92 24.00 58.94 27.68 65.57 31.13 Series VI1 264.14 91.78 271.46 95.74 277.64 98.52 283.01 101.12 289.41 104.51 295.80 108.40 301.60 111.89
Vol. 60
TABLEIV THERMODYNAMIC FUNCTIONS FOR PALMITIC ACID CP
T, OK.
20 40 60 80 100 120 140 160 180 200 220 240 260 273.16 280 298.16 300
8
eal. (mole deg.) -1
4.00 16.19 27.97 36.96 43.72 49.39 54.68 59.81 64.82 70.27 75.91 82.27 89.96 96.25 99.78 110.10 111.28
So
-cal.So',
(mole deg.) -1
1.69 8.04 16.89 26.24 35.24 43.73 51.74 59.38 66.71 73.82 80.78 87.64 94.52 99.12 101.54 108.12 108.80
-(Fo
He
- Ho',
oal. (mole) - 1
23.6 221.2 665.6 1319.8 2129.1 3062.0 4102.3 5247.9 6493.6 7843.7 9305 10885 12604 13828 14499 16401 16604
-
Ho''), T cal. (mole deg.) -1
0.51 2.51 5.80 9.74 13.95 18.11 22.44 26.58 30.63 34.60 38.48 42.29 46.04 48.50 49.76 53.11 53.45
140 to 180°K. Several points lie slightly above the smooth curve, which might indicate a very small anomaly. The maximum effect involved is about 12.5 cal./mole. It seemed more likely, however, that this irregularity is to be ascribed to experimental error, and the best smooth curve was drawn through the points. Both curves show upward inflections a t about 170°K. This is very probably due to the gradual onset of molecular rotational freedom. A similar effect has been observed in normal hydrocarbons by Finke, Gross, Waddington and Huffman.' Plots of the specific heat of methyl palmitate and palmitic acid vs. the reduced temperature (actual temperature divided by the melting temperature) are practically identical with the similar plot for n-hexadecane given by these authors. The onset of the increase in slopes of these curves occurs a t a value of the specific heat of about 0.3 cal. deg.-l g.-l, and a t a reduced temperature of 0.7. The lower members of the fatty acid series are known t o form hydrogen bonds. It seems probable that hydrogen bonding occurs also in palmitic acid. Crystal structure data indicate that palmitic acid forms monoclinic crystals, made up of double layers of molecules with the carboxyl groups in juxtaposition.8 The data are not sufficient to show the orientation of the carboxyl groups with respect to one another. It may be that two molecules dimerize through the formation of two hydrogen bonds. If this is the case, there are two possible positions for each dimer with respect to its neighbors. It seems likely that such disorder would persist to the lowest temperature and would contribute R In 2 to the entropy of a mole of dimer. This would represent an entropy of '/z R In 2, or 0.69 cal. degree-' mole-l of palmitic acid in addition to the entropy found experimentally. There are other possible ways in which the molecules may be hydrogen bonded, leading to smaller entropy contributions. (7) H. L. Finke. M. E. Gross, Guy Waddington and H. M. Huffman, J . Am. Chem. SOC.,7 6 , 333 (1964). ( 8 ) A. MUller and G. Shearer, J . Chom. Soo., 128, 3156 (1923).
t
I
July, 1956
DIELECTRIC CONSTANT OF HYDROUS SODIUM PALMITATE
Acknowledgment,-The authors wish to express their sincere gratitude to Dr. Herrick L. Johnston for his full cooperation in the use of facilities of
919
the Cryogenic Laboratory, and to Procter and Gamble Co. for a fellowship which partially supported this work.
DIELECTRIC CONSTANT OF HYDROUS SODIUM PALMITATE BY HENRY E. WIRTH~ AND WILLIAM W. WELLMAN Contribution from the Department of Chemistry, The Ohio State University, Columbus, Ohio Received December 17, 1966
The dielectric constant of hydrous p- and &sodium palmitate is practically constant for water contents between 0 and 3%. Above 3% water the dielectric constant increases rapidly with increasing water content, and becomes frequency dependent. This increase in dielectric constant is far greater than would be ex ected from a simple mixture, and is attributed to presence of mobile sodium ions in the water which is weakly adsorbedy! the soap.
On the basis of vapor pressure and dilatometric studies, McBain, Vold and Johnston2 concluded that curd sodium palmitate and sodium oleate can contain up to nearly one mole of combined water per mole of soap a t 25". McBain and Lee3 found that a t 90" anhydrous sodium palmitate takes up 1-2% of water by physical absorption and then suddenly forms a hemihydrate which again takes up water until another phase forms, Ferguson, Rosevear and Nordsieck4found a sharp break in the vapor pressure over hydrous @-sodiumpalmitate a t 2.5% water content a t 27°.5 Below this water content the vapor pressure decreased to practically zero a t 0.2% water. Since there was no "flat" indicating equilibrium between a hydrate and anhydrous soap, and since the X-ray patterns showed only minor and continuous variation through the range 0.2-2.5% water, it was concluded that in this range water is present in interstitial solid solution in the @-phase. Milligan, Bushey and Draper,B on the basis of dehydration isobars, found that a-sodium palmitate is a hemihydrate in agreement with the results of Buerger,' whereas the beta, delta and omega crystalline phases of sodium palmitate are not definite hydrates.8 Vold, Grandine and Schot t9 state that the nature of the changes in the X-ray diffraction pattern found for water contents below 3% suggest that the water is present in solid solution rather than as a stoichiometric hydrate. The present work was undertaken to see if dielectric studies on sodium palmitate of low moisture content would reveal anything of the nature of the binding of the water present.
Experimental
Materials.-@Sodium palmitate was supplied by the Procter and Gamble Co. It was prepared by cooling a homogeneous neat soap containing 30% HzO to 70°, tempering a t 70' for 24 hr., cooling to room temperature and air drying. This material was extracted with ether to remove non-saponifiable material and excess palmitic acid. The extracted material contained about 3% H20. Palmitic acid regenerated from the sample melted a t 62.38", compared with the calculated melting point (Rossini's method) of 62.68' for pure palmitic acid obtained in this Laboratory.10 The calculated purity is 98%. Samples with water content greater than 3% were prepared by rehydrating the extracted material. Lower moisture contents were obtained by dehydration in a vacuum desiccator, or by extraction with ether over sodium. &Sodium palmitate was obtained by cooling a 4% solution of the sodium palmitate quickly from 100 to 0'. The gel obtained was dehydrated a t room temperature to the desired water content. X-Ray powder patterns of re resentative samples were taken and interpreted for us by %r. F. B. Rosevear of the Procter and Gamble Co. Preparation of Cakes for Dielectric Constant Measurements.-The powdered samples were allowed to equilibrate for a t least a week, and usually for several months, before being pressed into cakes. The powdered sample was placed in a stainless steel die 2 in. in diameter. Prior to introduction of the sample, tin foil (0.0005 in. thick) was attached to the inner face of each piston by means of a thin layer of petrolatum. The die was placed in a hydraulic press, and a pressure of 16,000 p.s.i. was applied. A vacuum of ca. 1 mm. of mercury was maintained on the sample in the die before applying the pressure in order to eliminate air pockets. Cakes of uniform thickness (f0.0025 in.), having firmly adhering tin foil top and bottom surfaces were obtained. The cakes were allowed to stand for a t least a week before measurements were made. Determination of Dielectric Constant.-A Schering bridge (General Radio Type 716-C) was used to make the capacitance measurements. The cell was patterned after one designed by Hartshorn and Ward1' and modified by Venable (1) Department of Chemistry, Syracuse University, Syracuse, and Kinn,l2 and had plates 2 in. in diameter; The cell was New York. The capacimaintained at a temperature of 30.0 f 0.1 (2) J. W. McBain, M. J. Vold and 6. A. Johnston, J . Am. Chem. tance of the system was determined for frequencies between Soc., 63, 1000 (1941). 0.1 and 200 kilocycles with the cake in the cell. The cake (3) J. W. McBain and W. W. Lee, Ind. Eng. Chem., 35, 784 (1942). was removed and the upper plate adjusted by means of a (4) R. H. Ferguson, F. E. Rosevear and H. Nordsieck. J . Am. micrometer to give a plate separation equal to the average Chem. Soc., 69, 141 (1947). thickness of the cake (excluding the tin foil). The capaci(5) I t is now believed that the method used for water analysis tance was then redetermined. From these data, and the (weight loss in 45 min. at 150') gives low results. Private comcalculated capacitance of the empty cell, the dielectric conmunication from F. B. Rosevear. stant of the sample was obtained. The dielectric constants (6) W. 0. Milligan, G. L. Bushey and A . L. Draper, THISJOURNAL, so obtained are in error by less than 1% for cakes containing 66, 44 (1951). less than 3% water, as estimated from the variation in ob-
.
(7) M. J. Buerger, Proc. NatZ. Acad. Sci., U. S.,28, 529 (1942): M. J. Buerger, L. B. Smith, A. DeBretteville, Jr., and F. V. Ryer,
ibid., 28, 526 (1942). (8) The phase designations used by Milligan, and also in this paper, are those of R. H. Ferguson, F. B. Rosevear and R. C. Stillman, Ind. Eng. Chem., 35, 1005 (1943). (9) R. D . Vold, Joseph D. Grandine, Znd, and Hans Sohott, T H i a JOURNAL, 66, 128 (1952).
(10) H. E. Wirth, J. R. Droege and J. H. Wood, (bid., 60, 917 (1956). (11) L. Hartshorn and W. H. Ward, J . Inst. Elect. Eng.,79, 597 (1936). (12) D. Venable and T. P. Kinn, "Industrial Electronics Reference Book, Weatinghouse Electrio Corp.," John Wiley and Sons, New York, N. Y., 1948, pp. 409-410.
