T.1., WARDAND W.S. SINGLETON
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A second consideration in the use of these equations is whether or not the viscosity data have been adjusted to a mean velocity gradient of 500 see.-'. As Conrad and others have S ~ O I V I I , ~the ~ J ~apparent viscosity of 0.5 g./dl. solutions of cellulose in cuprammonium can vary as much as 1 0 0 ~ o in , the case of high D.P. cellulose, depending on the rate of shear existing during the measurement. This rate of shear effect becomes less significant at lower D.P., and in the case of celluloses having an intrinsic viscosity of less than 5-6, may be neglected for many purposes. Thus, the equations presented would be approximately valid for use with unadjusted data when the intrinsic viscosity was below 5-6, but their use with higher D.P. celluloses requires adjustment of the data to the gradient specified. While the velocity gradient effect is of lesser absolute magnitude at lower concentrations, it is of equal significance when the absolute viscosity is converted to reduced viscosity, so that it cannot be obviated by going to smaller concentrations. The standard error of estimate of [ q ] by these equations is about 5%. This error would amount to about 250 glucose units in the case of a cellulose
Vol. 56
of D.P. 5,000, or about 3 glucose units in a cellulose of D.P. 50. Errors of this magnitude are usually permissible in ccllulose investigations. The ratio of the intrinsic viscositics measured in cupri-ethylenediamine to those measured in cuprammonium was shown in a previous investigation6 of a group of seven samples having intrinsic viscosities ranging from 20 or 30 to 2 to be approximately constant and equal to 1.365. For the thirteen celluloses measured in both solvents in this study, which included seven of those examined previously,Bthis ratio was found to be 1.354, in excellent agreement with the earlier value. Thus, if the factor of 260 proposed for conversion of intrinsic viscosities to D.P. for cuprammonium measurements is accepted, over the entire range of D.P. from 50 to 5,000, 190 would seem to be appropriate for use with cupri-ethylenediamine data. Acknowledgment.-Thanks are expressed to Miss Hilda M. Ziifle for the construction of the figures and the statistical calculations of the data, and to Mr. Bernard J. Barrett for some of the viscosity measurements.
PT-IYSTCAL PROPERTIES OF FATTY ACIDS. 11. SOME DILATOMETRIC ANI) THERMAL PROPERTIES OF PALMITIC ACID BY T. L. WARDAND W. S. SINGLETON Southern Regional Research Laboratory,l New Orleans, Louisiana Received JuZv 16, 1961
Pure palmitic acid has been examined by dilatometric and calorimetric methods. The properties investigated and the values found were expansion of the acid in the solid and liquid states, 0.000280 and 0.000968 ml./g./"C., respectively; melting dilation, 0.1806 ml./g.; specific volume over the range of temperature from solid to liquid; specific heat; heat of fusion, 51.2 cal./g. (13.12 kcal. per mole); and entropy. Equations were developed for expressing the specific heat (C,) of palmitic 0.0013t; (liquid state) C, = 0.4624 0.00175t. acid a t any temperature ("C.): (solid state) Cp = 0.3831
+
The expansibilities and the volume changes which accompany melting of the saturated fatty acids have received scant attention. The data of Normann,2and Garner and Ryder3 on a limited number of fatty acids comprise the only sources of information on transition dilation. Garner, Madden and Rushbrooke4 have reported values for the heats of fusion and mean specific heats for some of the fatty acids. The present investigation, extends the previous work of the authors6 to cover the specific volumetemperature relationship, expansibility of the solid and liquid states, and the melting dilation of palmitic acid, and includes data for the specific heat, heat, of fusion and entropy of this acid. Purification of Palmitic Acid.-Commercial grade palmitic acid (85%) was sulfonated and thoroughly washed with water to rcmovc unsaturated impurities. The sat( I ) One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture. Article not copyrighted. (2) W. Normann, Chem. Umachau Fette, ole, Wachae Harze, 38, 17 ( 1 931 ).
J. Chem. Sac., l a T , 720 (1925). W. E. Garner, F. C. Madden and J. E. Rushbrooke, ibid., 2491
(3) W. E. Garner and E. A. Ryder, (4)
(1928). (5) W. S. Singleton,
T. L. Ward and F. G. Dollear, J. Am. Oil Ckemists' SOC.,27, 143 (1050).
+
urated portion was esterified with methanol, the methyl esters fractionally distilled, and the methyl palmitate fraction reconverted to palmitic acid. The free acid was crystallized from atetone 14 times which gave a product having a constant value for its solubility. After drying over phos horus pentoxide, the palmitic acid melted (capillary tubef a t 62.8-63.0'. Its freezing point, determined by modification of the method of Glasgow, el al.,n WFS 62.75 and its absolute density was 0.8414 g./ml. a t 80.0 Palmitic acid has been shown by Francis, et al.,' to exist in two principal polymorphic modifications one of which was obtained by crystallizing the acid on a n X-ray mount from a solution of benzene; the other was obtained by a similar crystallization from a solution of glacial acetic acid. The forms were identified by X-ray diffraction patterns obtained immediately after formation of the crystals. The crystals which separated in the benzene solution were of the thermodynamically unstable B-form, those which formed in the solution of lacial acetic acid weie of.the stable C-form. Thibaud and%aTour,a however, experienced considerable difficulty in obtaining similar patterns by crystallizing thin layers of palmitic acid. These workers found it necessary to carry out the crystallization below room temperature in order to obtain the unstable B-form. At room temperature or above, they obtained the C-form of the acid and single crystals were always obtained in this form. The present authors6 have shown that the temperature
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(6) A. R. Clasgow, Jr.. A. J. Streiff and F. D. Rossini, NalE. Bur. Sfandards, J . Research, 86, 355 (1945). (7) F. Francis, F. J. E. Collins and S. H. Piper, Proc. Rov. Soc. ( L o n d o n ) , A168, 691 (1937). (8) J. Thibaud and F. D. LmTour, J. chim. phys., 29, 153 (1933).
Jim, 1052
DILATOMETRIC A N D THERMAL PROPTCRTIICS OF PALMIT,IC ACID
607
a t which crystallization from cithcr polar or non-polar solvcnts occurs determines the polymorphic form of st,caric acid which will arparate from solution. Tlic critical tcmprratrire is that rtt which the polymorpliic transformation begins. The results of Thibaud and LaTours indicate that the same applies to palmitic Fig. 1.-Diffraction patterns of psliiiitic acid crystallized from glacis1 acetic acid: ( 1 ) acid. and that the transition of C-form:, (21 . , mixture OI IS- and C-tornis; (3) same as (2) after wainiing crystals. the B- to C-form begins in the vicinity of room temperature, thereby accounting for the the range of expansibility covered can readily be inability of the latter workers to obtain single crystals of calculated as the reciprocal of the specific volume. the B-form of this acid from solvents a t room temperature. I n the present work, palmitic acid was crystallized from. TABLEI benzene and from glacial acetic acid a t both 10 and 45', to dctermine whether or not both the B- and the C-forms of the SPECIFIC V o ~ u m Os F PALMITIC ACID AT D I F F E R E X T acid could be obtained from the same solvent by varying TEMPERATURES the temperature as in the case of stearic acid.6 X-Ray Specific Specific diffraction patterns (Fig. 1) of the acid were prepared immevolume, Temp., volume, Temp., O C rnl./g. oc. ml./g. diat.ely following crystallization from glacial acetic acid at different temperatures. The pattern of the acid crystallized 1.0050 0.9917 39.50 6.89 a t the higher temperature indicated only the C-form, where1.0067 ,9928 43.10 - 2.25 as that of the acid crystallized a t the lower temperature was 1.0093 I9949 47.20 5.85 a mixture of both B- and C-forms. Upon warming the latter, the B-form disappeared and only the form identical 53.25 1.0124 12.00 .90135 with that obtained by crystallizing the acid a t 45' was ob1.0138 ,9988 57. GO 17.41 served. GI, GO 1 ,0232 21.55 ,9998 Both the C-form and a mixture of the B- and C-forms of 1.171O 1 ,0000 62,90 24.88 palmitic acid were obtained from benzene, depending on t,he temperature of crystallization. 05.10 1.1770 25.50 1,0009 29. GO 1.0016 72.00 1.1807 Experimental 80,OO 1,1885 33.30 1 ,0029 The dilatometric and calorimetric methods and the calculations employed have been described in previous publiSpecific Heat.-The specific heat of palmitic acid cations.g*lo Palmitic acid, crystallized from glacial acetic in both the solid and liquid states was determined acid, was placed in each of four calibrated dilatometers. and the data applied to developing equations for the The dilatometers were immersed in a controlled-temperature bath and cooled t,o an initial temperature of -26'. Dilata- specific heat in absolute calories per gram as a function measurement#s were made from -26.0' to approxi- tion of temperature in degrees centigrade, as mately 10' above the melting point of the samples. The C p = 0.3831 0.0013t Solid state ( - 7 3 t o 40') expansibility was also measured after filling the dilatometers C, = 0.4624 0.001751 Liquid state (63 to 92") with premelted palmitic acid followed by slow cooling to an initial temperature of -6.89". It was not possible to obtain expansion and transition The observed specific heats of palmitic acid are data for the B-form of the acid by the dilatometric method compared with the calculated values in Table 11. employed. The technique used to fill and seal the dilatonieters is such that the transition of the B- to the C-form may TABLE 11 occur a t room temperature. SPECIFIC HEATO F PALMITIC A C I D For the thermal measurements the sample of palmitic Temperature, Calculat~dC p , Observed Cp, Deviation, acid was placed in the calorimeter in the melt.ed form which I
