248
E. S. LUTTON
Vol. 70
[CONTRIBUTION FROM THE PROCTBR AND
GAMBLE CO.]
Tiiple Chain-Length Structures of Saturated Triglycerides’ BY E. S. LUTTON The nature of the polymorphism of the glyceride series, tristearin, tripalmitin, etc., has been made clear by the work of Clarkson and Malkin2 and o t h e r ~ . ~ !The ~ J “triple-melting’’ phenomena of these glycerides are due to three crystalline forms-a, p’ and 8. These forms are of doublechain-length (DCL) .structure, with their molecules presumably in Malkin’s ‘‘tuningfork” type of configuration. The arrangement is indicated schematically in Fig. 1 along with other DCL structures, one for a fatty acid and another for a mixed triglyceride.
2-oleyl d i ~ t e a r i n . ~In * ~ the saturated case, as in the unsaturated case, the TCL structures are presumed to arise from the sorting of chains, short chains from long chains instead of unsaturated from saturated chains. This process of sorting apparently gives rise to both tuning fork structures and a newly proposed “chair” type of molecular arrangement (see Fig. 2).
-
stearyl
5Sj-’
rtraryl
(a) Simple triglyceride
, - - -
(a) 2-Oleyldistearin
(b) Symmetrical
(c) Unsyrnmetrical or “chair” arrangement
--
4 (c) Fatty acid Fig. 1.-Double-chain-length structures.
According to Malkin, et aLI6and many mixed saturated triglycerides are analogous to tristearin in their polymorphic forms. But these authors have shown that there may be serious departures from the tristearin plan, notably with regard to long spacings and DCL structure. No comprehensive scheme has been advanced which treats in any clear-cut way these departures from DCL structure. The presentation of such a scheme is attempted in this paper. It is shown that most, but not all, departures from normal long spacing values can be explained, by crystallization of the mixed glycerides in triple-chainlength (TCL) structures similar to those proposed for the mixed saturated-unsaturated triglyceride (1) Presented in part to the American Society for X-Ray and Electron Diffraction, December 5, 1946. (2) Clarkson and Malkin, J . Ckcm. Soc., 666 (1934). (3) Bailey, e l at., Oil b Soap, BB, 10 (1945). (4) Lutton, THISJOURNAL, 67, 524 (1945). (5) Filer, et al., i b i d . , 68, 168 (1946). (6) (a) Malkin and Meara, J . Chcm. Soc., 103 (1939); (b) Carter and Malkin, ibid., 577 (1939); (c) Malkin and Meara. ibid., 1141 (1939); (d) Carter and Malkin, i b i d . , 1518 (1939). (7) Filer, e1 al., Tals JOURNAL, 67, 2085 (1945). (8) Sidhu nnd Daubert, ibid., 69, 1451 (1947).
Fig. 2.-Triple-chain-length structures.
The quadruple-chain-length (QCL) structures which are discussed later are regarded as doubled DCL structures. A simple geometric basis is proposed for classifying the various reported triglyceride forms as DCL, TCL or QCL structures. In developing this basis the following assumptions are made : 1. With regard to cross-sectional structures, as characterized by short spacings, there are three structure types which may be identified according to Table I. TABLE I CROSS-SECTIONAL STRUCTURE TYPES Type
a ,S’ ,S
Identification
A single strong line correspondingto approx. 4.15 A. Usually two (but occasionally more) strong lines corresponding tcrapprox. 4.2 and 3.8 A. A strong (usually strongest) line corresponding to approx. 4.6 R.
8. With regard to longitudinal structure, as revealed by long spacings, there are three structure types which are distinguished as indicated in Table 11. Each of these conceivably may be, and one of them actually is, associated with all three cross-sectional types. (9) Lutton, ibid.. 68, 676 (19461.
TRIPLECHAIN-LENGTH STRUCTURES OF SATURATED TRIGLYCERIDES
Jan., 1948
+ + +
(LB)T = 3 CB = 6.0 1.138 N (L#’)T = 3 Cg’ = 7.5 1.163 N ( L ~ )= T 3 Co = 8.4 1.250 N
TABLEI1 LONGITUDINAL STRUCTURE TYPES Type
Identification
DCL TCL QCL
Long spacings of apprgx. normal length Long spacings approx. 50% longer than normal Long spacings approx. twice normal
There is, in theory, no exclusion of the possi bility of other cross-sectional and longitudinal structure types.
Calculation of Long Spacings If i t be assumed that for any specified. crosssectional type the angle of tilt and consequently the contribution to long spacing per chain is approximately the same as in single fatty acid triglycerides, a simple calculatioq for long spacings can be postulated. Contributions per chain are indicated by (I) values shown in Table 111.
249 (7) (8) (9)
with corresponding relations for DCL and QCL structures. By these equations, all calculated long spacing values were derived. Geometrically, TCL structures may be represented as in Fig. 2 (in projection) or Fig. 3 (with the tilt indicated). It is not possible to represent DCL structures neatly for mixed glycerides. In Fig. 1 such a structure is represented with long spacing value designated as (L)wto distinguish it from the shorter (L)Dvalue defined as in equation (4). QCL structures are regarded as essentially doubled DCL structures, the manner and basis for doubling not being understood except that they are ir. some way associated with chain length difference in the glycerides.
TABLEI11 CONTRIBUTIONS OF SINGLE CHAINS Long spacings of simple triglycetidw ?I Lo‘ Lgb Lg” 10 (30.6)O (28.2)“ 26.8 12 35.6 32.85 31.2 37.65 35.8 14 41.2 42.3 40.6 16 45.6 46.8 45 18 50.6
Triglyceride Tricaprin Trilourin Trimyristin Tripalmitin Tristearin
a Extrapolated. ref. 4.
Long spacing per chain ( l d n (l@’)n (I@)n 15.3 14.1 13.4 17.8 16.4 15.6 20.6 18.8 17.9 22.8 21.15 20.3 25.3 23.4 22.5
(a) Double
Malkin, et al., ref. 2. =Lutton,
Obviously (Io) = l/&, etc. Now it follows from the linear variation of (1) values with number of carbons that the following simple expressions can be given
+ 1.138-n + 1.163 n + 1.250 n
(la). = 2.0 ( 2 , ~ ) . = 2.5 (lo),, = 2.8
(1) (2) (3)
Arithmetically the long spacing calculation for mixed glycerides is simply a matter of obtaining the average chain length contribution, C, and multiplying by the right “multiplicity factor,” i. e., 2 for DCL, as indicated in Table IV. TABLEIV CALCULATED LONGSPACING VALUES IN TERMSOF (2) VALUES j3 Cross-sectional type Long spactype DCL
( L ~ ) D2 X
-
TCL
(L@)T 3 X
QCL
(L@a
-
4X
+ (WW+ Isha cs 3 + 3 + (l@)n, (lB)ni t (tB)nz + (tD)na I
(l@)ni
(l@)nz
I
+ ( l @ ) n t + (t8)na
([@)ai
3
Additional quantities are defined to express the departure of experimental from calculated values ( A ~ D= (AB)T =
(4)
-
4cs
LP - WB)D
(10)
--
Lg - (LB)T (11) (A@)P L@ - (Li3)Q (12) where Lo is the experimental value, (LB) values are calculated a~ previously described and (As) values represent the differences.
c I8
Calculated value (I@)ni
Fig. 3.-Assumed constancy of tilt angle for given “form.”
c IO
(a) 6 ’ - 3 , C ~
3 C@ (5) (6)
Similar values are postulated for beta prime and alpha cross-sectional ty s. Now, from relations (2) and (3), (L) values may be simply derived from the totat number of acf1 carbons, k.g.: letting (nl ns na) =
E,
+ +
Fig. 4.-Alternative
triple structures for Cis. C,,, Clo.
E. s. L m o N
250
Similar ( A ) values are defined for alpha and beta prime forms. In Tables V and VI, the experimental L and derived ( A ) values are recorded for all. mixed saturated glycerides for which data have been published. Fortunately only one ( A ) value needs to be calculated for a given form. It is generally evident
Vol. 70
by inspection if, for instance, ( A)T is preferable to ( A I D or ( A > @ The L values of Tables V and VI are plotted in Figs. 5 and 6 along with lines which express the calculated (L)values. It is these plots which most strikingly reveal that all the available long spacing data with one exception correspond with the concept of either DCL, TCL or QCL structure.
TABLE V DIFFERENCES BETWEEN EXPERIMENTAL6"'b'C'd'e AND CALC~LATED LONGSPACINGS Recording of ( A)D,( A)T or ( A)Q value signifies,respectively, double, triple, or quadruple (doubled double) chain length structure. Triglyceride
N
c 1 o c 1 2 c 1 2
34
ClzClrClr CirClEC16 ClaHiaC1s
40
---AlphaLa
(A) D
46 43.9 -0.2 .l 52 48.8 c 1 2 c 1 o c 1 2 34 .7 CirCizCir 40 39.6 ClEClkCl6 46 44.4 .3 CiaCi~Cla 52 50.5 +1.6 ClzCloCio 32 ClkcIZclZ 38 ClsC14Cl4 44 42.8 $0.5 ClSc16Cl6 50 47.8 .5 CioCizCio 32 C~ZCI~CIZ 38 C14ClEClr 44 45.0 $2.7 Clsclllcis 50 50.2 +2.9 CioCirCir 38 c14ciEcis 44 43.4 +1.1 CirCiaCis 50 48.5 4-1.2 C14CioCir 38 Ci~C&ia 44 44.6 +2.3 Ci&Cls 50 49.5 $2.2 cl4cloClo 34 clsc~Pclt 40 CiaCirCir 46 46.4 +2.3 CloClrClo 34 CIZCIECIZ 40 46 44.0 -0.1 CirC1sCl4 CioCisCts 42 +1.8* C I X I ~ C I ( 48 47.4 Cl6CloCl6 42 39.0 -1.6 CiaCizC1s 48 47.1 +1.5 Cl6ClOClO 36 CisCinCiz 42 (41.7)" (4-1.1)'
-
+ + +
LP' 30.4 35.3 40.3 44.7
Diacid triglycerides Beta prime (AID (A)T
36.7 42.4 47.5
(NQ
-
-1.0 -0.8 .4 .7
-
+ + .6
+1.7 +2.1
34.5 39.5 43.9 29" 33.6" 38.1' 43.2" 33.8 38.5 43.4 33.7 77.0 44.7 31.3 36.2 41.7 30.3 35.5" 40.0" 74.1 42.8 74.0 42.4
+ .3 + .1
-0.1
38.7, (38.0, 55.0)'
+1.1
-
-
WT
d
.7 .4 .9 -0.7 X 2
.9 .1 .1 +1.0 -1.1 -0.6 - .7
-
+
+ .5 + .I
(A)D
+1.9 +2.1 +2.6 +3.0 +0.1 +0.3 +O.l +0.7 +0.1 +0.1 +0.2 +0.5
d d
.8
-
LP 31.8 36.5 41.5 48.5 30.0 34.7 39.0 44.2 28.4 33.0 37.7 42.5 d
.9 -1.0 -1.1 -0.6
-
-Beta-
35.2 39.8 45.0 52.5 59.0 65.8 47.5 54.6 61.4 46.5
+2.3 +2.5 +3.1 3.2 2.9 2.9 2.8 3.0 3.1 1.8
d d
-
.55x2
-
.6X2
56.5 63.7 49.7 57.0 (57.2)e 49.5 56.8
(0.4)', (-1.4)' CioCinCio 36 Ci~CisCi? 42 40.8 +0.2 37.5 - .1 CloCr8C18 46 73.7* 60.V -1.1 CieCioCia 46 76.3 -2.55X2 61.2 CISCIOCIO38 51.0 -0.8 52.6 C d h C i o 38 51.6 Malkin's listing changed from beta to beta prime on the basis of short spacing data. (&)T Data of E. S. L. -7.3 X 2. c Blank, no data. d Probably no form exists.
2.7 3.7 2.7 3.2 (3.416 2.5 3.0 2.9 3.3 2.3 = 7.8, (Ao)o =
TRIPLE CHAIN-LENGTH STRUCTURES OF SATURATED TRIGLYCERIDES
Jan., 1948
251
TABLE VI DIFFERENCES BETWEEN EXPERIMENTAL”?AND CALCULATED LONGSPACINGS TRIACID TRIGLYCERIDES Beta prime Triglyceride
N
ClSClOclS ClaClzCls ClsClrCls ClSClOCl4 Cl8CIZCl4 Cl8cl6Cl4 CI8ClOC12 ClsClrClr C18C16C12 claclzClo
44 46 48 42 44 48 40 44 46 40
Cl8C14ClO
42
CISCISCIO 44
0
LP’
41.5
--Beta-
(A)D
(A)D
60.0 61.2 63.4 57.3 59.8 40.5
-0.8
3.9 2.9 2.7 3.5 3.7 0.0
3.3 3.5 4.0
54.9 40.1 43.3 35.6OSb 53.0 54.3, 56.P‘ 38.50,’ 59.0
-2.2 -0.8 .5
-
(A)T
59.6 62.4
- 1.1 (8’-3, C18) -2.1 (8’-3, CIS) -0.3 (p’-3, Cio)
.7
-0.3 (8‘-3, Clo) 42 35.7 Cl6Cl4ClZ .4 36 33.4 CI4C12CIO Higher melting form. b Filer’s listing changed from 8 t o 8’ on the basis of short spacing data.
Agreement of Calculation and Experiment Alpha Form (None reported for triacid triglycerides) Among the cases for which data are reported for alpha forms, there are fourteen examples of agreement within 1.6 A. between experiment and calculated DCL structure. Experimental error is
34 38 42 46 50 N = total no. acyl carbons per triglyceride. Fig. 5.+xperimental long spacings (B), O-experimental long spacings (a); solid lines-calculated long spacings for 8-2 and 8-3, dashed line--calculated long spacings for a-2.
-
.2
usually of the order 0.5 A. (but in individual cases it may be greater). I n the,untilted hydrocarbon chain each carbon corresponds to 1.27 A.of length, wherefore it might be said that the correspondence between calculation and experiment is, in these cases, within about 1 carbon atom. The six cases of rather poor agreement (1.6 < (A), absolute value
