Chemischer InformationsdienstVolume 4, Issue 17 Preparative Organic Chemistry ChemInform Abstract: DARST. UND PHASENVERH. VON ACETYLMONOGLYCERIDEN J. B. MARTIN, J. B. MARTINSearch for more papers by this authorE. S. LUTTON, E. S. LUTTONSearch for more papers by this author J. B. MARTIN, J. B. MARTINSearch for more papers by this authorE. S. LUTTON, E. S. LUTTONSearch for more papers by this author First published: April 24, 1973 https://doi.org/10.1002/chin.197317224Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume4, Issue17April 24, 1973 RelatedInformation

A minor revision based on new data is reported for the polymorphism of 1-propylene glycol monopalmitate and monostearate. Metastable Form I is now found to be intermediate in melting level between α and stable Form II. Form III as previously reported transforms to α below the α mp. For the 80:20 mix of 1-propylene glycol monostearate-2-propylene glycol monostearate, the approximate equilibrium mixture, it is again observed that Form II is stable, melting a little above α; and Form I transforms to α slightly below the α mp; and Form III is missing.

The triglyceride 1 -behenoyldistearin shows α-2, β-2, β′-2 and β-3 forms in order of increasing stability. The previously unobserved β-2 form of the closely related homolog 1-stearoyldimyristin has now been observed on crystallization from hexane. The triglyceride 2-stearoyldibehenin shows an α-2 form and two modifications of β′-2 type. No β-3 form was observed such as has been previously observed for the related homolog 2-myristoyldistearin.

Paris of C n C(n+1)C n (i.e., C16C17C16 and C18C19C18) and C n C(n-1)C n triglycerides (C 16 C 15 C 16 and C 18 C 17 C 18 ) triglycerides,n even, were prepared to compare with C n C(n+2)C n and C n C(n-2)C n glycerides, respectively, in polymorphic behavior. It was found that C n C(n+1)C n compounds were β′ tending and C n C(n-1)C n compounds β tending in line with their all-even counterparts. The glyceride C11C13C11 is β′ stable, like C10C12C10 and C16C18C16, and seems closely related to them in physical behavior and diffraction characteristics.

A homologous series of 1-monoacetyl-3-monoglycerides in which the fatty acyl group varies from C14 to C22 in even C intervals has been prepared by partial acetylation of the appropriate 1-monoglycerides. Crystallization and silica gel chromatography were used to isolate the diglycerides in high purity. The isomeric monoacetyl monostearins were prepared either by application of a similar procedure to 2-monostearin or by acetylation of 1-tetrahydropyranyl-3-monostearin with subsequent removal of the blocking group to obtain 2-acetyl-1-monostearin. In the homologous series of 1-acetyl-3-monoglycerides from monomyristin through behenin the stable form at room temperature, called form I, is a tilted double chain length form, existing up to the melting point for myristin through stearin but transforming to a stable α form before melting for arachidin and behenin. The α form, low melting and entirely metastable for myristin through stearin, transforms reversibly to sub-α on cooling. Both α and sub-α are perpendicular forms and probably of single chain length structure. An interesting feature is the two-step transformation of sub-α to α on warming; the nature of this stepwise transformation, observed for all but myristin, is discussed. A nearly perpendicular double chain length β-like form occurs for palmitin and stearin and perhaps myristin. This form, which has not been well characterized thermally, though possibly entirely metastable, is conceivably a stable form at low temperatures.

Much of the function of lipids, as in shortenings, confections, biological membranes, etc., depends on the structures they exhibit, i.e., their crystalline and mesomorphic states. Triglycerides may serve as a point of departure for considering the crystal state of lipids. The three principle cross-sectional arrangements of long chains-α, β′ and β-in which there is kinship to hydrocarbons, offer a major basis for classification. Different crystal forms are rheologically different. Complications in the chain length direction lead to stepped configurations in the methyl planes and to double and triple chain length structures. A detailed consideration of the time-honored melting point alternation of homologs sheds light on the lipid crystal picture. A startling feature is the catalysis of isomerization by the solid state of diglycerides, with a consequent shift in equilibrium far toward 100% 1,3-diglyceride. Stepwise reversible transformations, as in hydrocarbons, are documented for high molecular weight 1-monoglycerides. A common feature, the sharp drop with mixture of stable form melting points, to “expose” metastable forms as stable forms for intermediate compositions is strikingly exhibited by fatty alcohols. The two bases for mesomorphism, or liquid crystal formation, among lipids are discussed: (1) dichotomy between molecular moieties in degree of polarity, as in phospholipids; and (2) dichotomy in molecular geometry as in cholesteryl esters. The former type is associated with three main basic arrangements-smectic or lamellar, middle or hexagonal (normal and reversed), and viscous isotropic or cubic (normal and reversed). The latter is associated with two main arrangements-cholesteric (essentially limited to cholesteryl compounds) and smectic. The importance of mesomorphism in consideration of membranes and emulsions is emphasized. Developments in understanding the structures of interfacial lipid states (including monomolecular surface films) and the correspondence of interfacial with bulk states is a matter of great continuing interest.

A structural scheme is proposed forβ phases of the six palmitic-stearic triglycerides, based on known configurations of tricaprin and 2-11-bromoundecanoyl-1,1′-dicaprin. The scheme receives some support from short spacing and binary system data.

Phase behavior of a homologous series of saturated even 1-monoglycerides, starting with monomyristin, has been reviewed and their study extended to monoarachidin and monobehenin. The occurrence of sub α, α, β' and β polymorphs was confirmed for all compounds, except in the case of β' for monomyristin. It has been firmly established that there is a reversible sub α2 ⇄ sub α1 transition, (indicated by Malkin for monostearin) below the reversible sub α (sub α1)⇄α transition, for C18 through C22 compounds; it occurs at about 50 C and is apparently almost independent of chain length. The sub α2 to sub α1 transformation is particularly sensitive to impurity and disappears for 1-monobehenin at about 10% 2-monobehenin as produced by heating at 96 C. Heats of transformation are, for β' and β crystal melting, about 50 cal/g; for α melting, about 35; for sub α → α transition, about 15 and for sub α2 to sub α1 transition about 3, which is several times as large as typical heats of melting of mesomorphic states. Diffraction data confirm the essential identity of all long spacing values and the occurrence of tilted chains for all polymorphs of a given compound. Much similarity is encountered between sub α and β' patterns. Sub α2 and sub α1 are difficult to distinguish by diffraction pattern.

Phase behavior of anhydrous monopalmitates and stearates and of most laurates, myristates and behenates of ethylene glycol, erythritol, xylitol, sorbitol, and mannitol has been studied. The compounds, substantially free of diesters, are mostly not isomerically pure but are predominantly primary esters. Also studied were the (nonisomeric) palmitate and stearate of pentaerythritol (Pe) and the oleate of erythritol. Mesomorphism, absent in anhydrous monoglycerides unless of short chain length and symmetrical, is a prominent feature of the members of the present group of compounds containing at least three unesterified hydroxyls. In general, crystal melting points rise with length of polyol, and mesomorphic melting points rise faster. Crystal melting points rise with acyl chain length while mesomorphic melting points run through a maximum. Heats of fusion are of the order 40 cal/g for crystal melting and 0.5 cal/g for melting of mesomorphic states. In the range explored, mesomorphic values fall with both polyol length and acyl length. At higher acyl chain length, notably with behenates, there are new features of mesomorphic behavior. Crystalline phase, from solvent, melts to a neat or lamellar phase, which in turn melts to liquid with subsequent appearance of higher melting middle or cylindrical phase. Polymorphism is general among the crystal states and tends to be complex. In a brief study of solvent crystallized 50–50 binary mixes of xylitol stearate with other xylitol esters and other stearates, considerable evidence of solid solution in Form I crystals (of xylitol stearate type) was observed with very modest eutectic lowering of crystal melting point. Mesomorphic points were almost linearly intermediate between those of components. Of particular interest were mixtures of xylitol stearate and 1-monostearin (50–50 and 25–75), for which mesomorphic melting points were realized, and for which an extrapolation to 100% monostearin leads to a hypothetical mesomorphic melting point far below any monoglyceride crystalline melting point. A brief examination of an aqueous xylitol palmitate system explored to a maximum temperature of 165 C shows extensive occurrence of aqueous mesomorphic state somewhat after the manner of monoglyceride systems. Mesomorphic melting level rises rapidly from the value for anhydrous ester.

Phase behavior has been studied by thermal and diffraction methods for 1-and 2-palmityl and stearyl ethers of glycerol and for 14 trialkyl glyceryl ethers, dialkyl monoacyl glyceryl ethers and monoalkyl diacyl glyceryl ethers, all of which were saturated trichain substituted glycerol compounds containing one or more of the following chains: palmityl (Py), palmitoyl (P), stearyl (Sy) and stearoyl (S). The monoalkyl glyceryl ethers resemble monoglycerides in crystallization behavior but with significant differences. Isomeric 1- and 2-ethers are very close in melting point. The 1-ethers show, besides a stable form, two other forms which transform reversibly to each other. The 2-ethers are polymorphic but with only one clearly established melting level. All trichain compounds were polymorphic also, most being dimorphic, each exhibiting a metastable α form, typically more stable than that of related triglycerides. Forms other than α were labeled I, II, etc., in order of decreasing melting point and were typically obtained from solvent. Polymorphic behavior showed some rather large departures from that of related triglycerides and appeared generally more sensitive to impurities. The two triethers, PyPyPy and SySySy were dimorphic each with a stable form much resembling metastable α in diffraction pattern, hence presumed to be of a new (more dense) hexagonal type of cross sectional structure. Three dialkyl monoacyl compounds PyPyP, SySyS and SyPyS and also three monoalkyl diacyl compounds PPyP, SSyS and SPyS were dimorphic, with Form I a stable, nontilted, somewhat β′-like form. PySyS and PSyP, which were trimorphic, showed such a β′-like form as a Form II, i.e., a second highest melting point. The stable phase of four compounds, namely PyPP, SySS, PySyS (all trimorphic) and dimorphic PySS (and possibly that of SyPP) could be called a β phase. Presence of an alkyl group on the 2 position of glycerol, in all but the PySyS, prevented β structure. PSyP and SyPP exhibited triple chain length structures, not encountered in saturated mixed triglycerides with less than four carbons difference in the acyl chains. SyPP was exceptional in showing four forms.

The polymorphism of single fatty acid odd triglycerides, C11 through C17 has been reinvestigated, with extension of the study to C9, C19 and C21. With study of the even glycerides C8, C20 and C22 it has been possible to review the whole series (odd and even) C8 through C22. The odd glycerides resemble the even in showing three distinct melting levels. Lowest melting forms are α. Stable forms are β except for C9 and C11 which show a different structure type. Intermediate melting β′ forms of odd glycerides are substantially more stable than their even counterparts as well illustrated by differential thermal analysis. There are no vitreous forms. Alternation in mp (between odd and even) is confirmed for stable phases and nonalternation for α and (within experimental error) for β′. Both β′ and β long spacings show alternation but not α. Alternation is evident in the short spacings of β′ and β forms. While short spacings of β′ forms of even triglycerides are much alike especially for C14 through C22, those of odd glycerides show a fortuitous 4-carbon cycle. This appears to involve no significant structural variation as chain length increases but simply an approximate 4-carbon cycle of variation in diffraction details derived from the presumed unvarying 0 ⊥ type subcell structure.

Discontinuities in slopes of interfacial tension vs. temperature (γ vs. T) are observed for triglyceride oilH2O systems, the discontinuities being seen when the oil contains low to moderate percentages of certain oil-soluble, water-insoluble surface-active components such as propylene glycol monoester or monoglyceride.

Melting-point and limited diffraction data are presented for the 13 hitherto unreported binary systems of the 15 obtainable from the six palmitic-stearic triglycerides. Data are presented for metastable α and stabilized states. These and earlier data on the other two systems show continuous solid solution formation and nearly linear melting-point variation for α states. Stabilized state behavior depends on many factors, such as molecular weight, molecular symmetry, phase tendency. In general, β tending components produce β phase systems and β′ tending components produce β′ systems; β predominates in systems of mixed β and β′ tending components.

A number of binary systems have been studied, each involving a 1-monoglyceride. Included was the binary system of 1-monopalmitin (1P)-1-monostearin (1S). Other systems involved 2-monoglycerides, 1,3-diglycerides and triglycerides as follows: 2P-1P, 2S-1S, PP-1P, SS-1S, PPP-1P, SSS-1S. A general principle running through the observed behavior is that the more chemically similar the components, the greater is the interaction or solid solution formation. Thus 1P-1S showed continuous solid solution in both stable and metastable states with “compound formation” in the stable state. The systems 2P-1P and 2S-1S showed α (sub α) stability in a single solid solution phase for the range 20–80% 1-monoglyceride. The system PP-1P and SS-1S showed extensive solid solution formation in an α state but little or none in the stable state. There was essentially no solid solution formation observed for stable states of PPP-1P and SSS-1S; experimental limitations made it difficult to draw certain conclusions about the metastable state, but here, too, it is believed, solid solution formation was negligible.

Largely by x-ray diffraction six crystalline states, I–VI, in order of increasing melting point, have been identified for cocoa butter. Of these states II, IV, V and VI are pure and identifiable with previously (or presently) identified polymorphs of 2-oleoylpalmitoyl stearin (POS), namelyα-2,β′-2,β-3 (“V”) andβ-3 (“VI”); V and VI representing distinct but very closely related crystalline structures. State I is a definite but fleeting and not readily characterized subα state and may be a phase mixture, as state III may be also.

An exploratory study of the aqueous system of dimethyldodecyl amine oxide (DDAO) revealed it to be an interesting and instructive surfactant system showing the unusual occurrence of at least 5 stable phases at room temperature—crystal (100-about 80% DDAO), neat (about 80-70%), viscous isotropic (70-65%), middle (65-35%) and fluid isotropic or nigre (35-0%) in order of decreasing DDAO content. In many respects the system resembles anionic detergent systems such as that of sodium palmitate, in which, however, viscous isotropic is not observed. The maximum temperatures of existence for crystal, neat and middle are, respectively, 116, 145 and 110C. Neat phase is soft and anisotropic, viscous isotropic is somewhat brittle and isotropic, middle is plastic and anisotropic. The mesomorphic phases are particularly well shown microscopically in “dried-down” samples under circular cover glasses. Middle and neat textures under the microscope are typical. At least four crystalline phases were recognized at various degrees of hydration.

Polymorphism has been explored for the pairs of homologs 1) 2-palmitoyl diolein (OPO) and 2-stearoyl diolein (OSO), and 2) 1-palmitoyl diolein (POO) and 1-stearoyl diolein (SOO). The symmetrical compounds show α, β′-2 and stable β-3 forms; the unsymmetrical compounds show α (presumed) and stable β′-3 forms.

Monoglyceride-H2O systems in the range above about 5% H2O exhibit a varied phase behavior with a number of mesomorphic states reminiscent of those found for soap-H2O systems. There are fluid neat, stiff and “short” viscous isotropic, and plastic or stringy middle states.