D. R. MERKERAND B. F. DAUBERT
2064
A1(OH)4-
Psite
---f
Pbranoh
+ 2H+
that the free energy for formation of a branch is f19.4 kcal./mole. Similarly, for the reaction a t high pH one may divide the reaction Ah3+
+ 4HsO +
Pend
--+
P
+
Pend
+ 4H+
A F o = -49.84 kcal./mole
into parts and obtain for the association of the polymer Al(OH),-
+
Pend
--+ P
+
Pend
AFO = -11.84 kcal./mole
Acknowledgments. This work was supported by the U. S. Atomic Energy Commission under Research Contract AT(30-1) 2479.
The Molecular Structure in Surface Films of Unsaturated 1-Monoglycerides
on Water as Related to Three-Dimensional States'
by D. R. Merker and B. F. Daubert Department of Chemistry, University of Pittsburgh, Pittsburgh, P e n m y h a n t h (Received November $0,1969)
The relationship of the molecular structure in monolayers of unsaturated 1-monoglycerides to three-dimensional states is discussed. Monolayers of cis-monoglycerides were in the expanded state under all the conditions studied and extremely stable. The physical properties of the monolayers in this state resemble in certain respects the physical properties of three-dimensional liquids. An expanded monolayer of 1-monoelaidin was transformed under pressure to a condensed state with a limiting area of 24 This area was in close agreement with the cross-sectional area calculated for a hypothetical liquid crystal from density and X-ray data. The correlation indicates that the orientation of the molecules In the condensed monolayer of 1-monoelaidin approximates the orientation in the anhydrous three-dimensional crystal.
Introduction The effect of unsaturation in monolayers of fatty acids on water has been studied extensi~ely.~-7 The unsaturated CI8 acids occupy much greater areas per 'molecule than does stearic acid a t the same temperature. Hughes4 has suggested that expansion is caused by the attraction of water for the double bonds. Sneider, et a1.,6 have found that the area of the cis C I ~ acids a t negligible pressure increases as the unsaturation increases. They have suggested that this effect and the stability of the expanded monolayers are caused by a progressively greater curl of the unsymmetrical molecules. T h e Journal of Physical Chemistry
This investigation was undertaken in part to ascertain if either of the above-mentioned theories is applicable to the unsaturated 1-monoglycerides. It was ~~
(1) The authors extend their appreciation to Swift and Co. for a research grant that made this work possible. (2) E. K. Rideal and R.K. Sohofield, Proc. R o y . SOC.(London), 110, 170 (1926). (3) E. K. Rideal and A. Hughes, ibid., 140, 253 (1933). (4) A. Hughes, J . Chern. Soc., 338 (1933). (5) J . Marsden and E. K. Rideal, ibid., 1163 (1938). (6) V. L. Sneider, R. T. Holman, and G , 0. Burr, J , P h y s . Colloid Chem., 53, 1016 (1949). (7) E. D. Goddard and A. E. Alexander, Biochem. J., 47, 331 (1950).
2065
MOLECULAR STRUCTIJRE IN SURFACE FILMSOF ~-MOIONOGLYCERIDES
monolayers were extreiiiely stable and collapsed a t also desired to compare the results with those preand 45-50 dynes/cm. The pressure then viously reported for saturated 1- and 2-monogly~erides.~ 30-34 remained constant and did not fall with time indicating Experimental that the monolayers were in equilibrium with an oil Oleic acid was isolated from olive oil by the method lens of equal spreading pressure. acid was prepared of Brown and S h i n o ~ a r a . Elaidic ~ A monolayer of 1-monoelaidin a t 24.3' and negligible pressure also was expanded and occupied 70 by the isomerization of oleic acid with sodium nitrite. molecule. However, this monolayer transformed Linoleic acid was prepared by the debromination in ether of tetra,broiiiostearic acid, which was obtained to a liquid condensed state at 30 and 42 dynes/cm. by brominating the unsaturated acids isolated from At 4.0' the traiisforiiiatioii occurred at approximately corn oil. Linolenic acid was prepared by the debro16 dynes/cni. The limiting area for the coiideiised state mination in pyridine of hexabromoatearic acid, which a t 4.0" was 24 and the relative coinpressibility exwas obtained by brominating the unsaturated acids isopressed as the reciprocal slope of the isotherm was 0.073 lated from linseed oil cm.3/dyne. The limiting area of 24 agrees well Oxalyl chloride was used to prepare the corresponding with the cross-sectional area calculated for a hypoacid chlorides, which were treated with 1,2-isopropylii- thetical liquid crystal from the X-ray long spacing dine glycerol according to the method of Daubert of the three-dimensional unit cell and the extrapolated and Baldwin. lo Acid hydrolysis yielded 1-monoolein density of the three-diiiieiisioiial liquid. There were (n1.p. 35.0-35.5", iodine value 71.0), 1-nionoelaidin no discontinuities on further coiiiprcssioii and the (n1.p. 57.5-58.5", iodine value 70.3), 1-monolinolein fluid monolayer finally collapsed at 58 dynes/cni. ( m p . 14.5-15.5", iodine value 143.2), and l-monoand 21.5 linolenin (m.p. 12.6-13.8", iodine va,lue 213.9). The densities, determined by the displacement of Discussion of Results isooctane in a pycnometer, of 1-monoolein and 1The molecular orientation in the expanded nioiiomonoelaidin in the solid state a t 0' were 1.043 and layers of nionoglycerides appears to be independent 1.058, respectively. Average cross-sectional areas calof the type or degree of uiisaturatioii. Coniplete culated from these densities and X-ray long spaciikgs solvation of the glyceryl groups probably is rcspoiisible for 1-monoolein of 49.5 A. and 50.5 &ll,lz were 22.9 for the formation of the expanded state aiid its charand 22.4 A. for 1-inonoelaidin. Theoretical average acteristic areas because the inoliolayers of saturated cross-sectional areas for these compounds in a hypoiiioiioglyceridcs also occupy an average cross-sectional thetical liquid crystalline state as calculated from the area of 70-75 k2/molecule in this state.8 The inagniX-ray long spacing data and extrapolated densities of tude of this area indicates that the orientation differs 0.958 for 1-inonoolein and 0.962 foir 1-monoelaidin in radically from that in the anhydrous crystals iii which and 24.4 As2,respeca liquid state at 20' were 25.0 the average cr2ss-sectional area per niolecule is aptively. proxiniately 23 A.2. Force-area data for the monolayers were obtained The extreniely high stability of unsaturated monoas described in a previous publication.8 I n order to glycerides in the expanded state is remarkable and rebard autoxidation 0.12% hydroquinone was added appears to be related to the force of double bond interto the distilled water in the trough of the Langmuir action as well as to the strong hydrophilic nature of the film balance as suggested by Rideal and Hughes. glyceryl groups. It is very significant that the spreadCompression was begun as soon as possible after evapoing pressure remained constant and did not fall with ration of the benzene, which was used as a solvent for time after the monolayers collapsed a t 30-33 i , 2 , spreading the monolayers. Several determinations At this point a two-dimensional monolayer and a threewere made on distilled water, which did not contain dimensional oil lens arc in equilibrium and conceivably an antioxidant, but the results were unchanged. could be alike in molecular orientation. The force-area cuives showed that the monolayers of the cis uiisnturatcd nioiioglycerides were expanded and occupied areas of similar magnitude a t a given (8) D. R. Merker and B. F. Dnubert, J . Am. Chem. Soc., 80, 516 (1958). tcniperature and pressure. The liniitiiig areas a t 24.3'' (9) J. B. Brown and G. Y . Shinowara, ibid., 59, 6 (1937). and negligible pressuie for 1-monoolein, I-ponoliiioleiii, (10) B. F. Daubert and ,4.R. Baldwin, ibid., 6 6 , 997 (1944). and 1-monolinolcnin were 72, 74, and 76 A. 2/mo1ecule, (11) L. J. Filer, S. S. Sidhu, B. F. Daubert, and 11. E. Longenecker, respectively. At 4.0' these critical areas decreased ibid., 66, 1333 (1944). slightly but the curvos were basically uiialtered. The (12) M .G. R. Carter and T. Malkin, J . Chem. Soc., 554 (1947). L&.z
Volume 88, Numbor 8
Auoust, 1964
2066
The force-area curve for a sample of l-monolinolenin, which was aged in contact with air for 1 week, showed that oxidation of the double bonds increased the limiting area in the expanded state to a greater extent than does the introduction of double bonds. Therefore, the packing of the chains could be quite compact even in the expanded state. The densities of the C18 unsaturated monoglycerides in the three-dimensional liquid state were greater at any given temperature than the density of l-monostearine8 The monoglycerides in the liquid state thus become more compact with increasing unsaturation and asymmetry. It is well known that unsaturated nzonoglycerides and fatty acids are more stable as liquids than as solids at ordinary teniperatures and atmospheric pressure but the corresponding saturated compounds under the same conditions have a greater tendency to be in the solid state. The same analogy exists in the monolayers. 1-Monostearin is in the liquid condensed state at 20°,s but l-monoolein remains in the expanded state at all pressures. Therefore, in this respect the expanded monolayers are similar to the three-dimensional liquids. The results are interesting in relation to the theories proposed by Hughes4 and Sneidero for the expanded monolayers of unsaturated compounds. In regard to the theory of Hughes it is noteworthy that essentially the same pressure was required to reduce the area to any given point for the three cis unsaturated monoglycerides. If the attraction of water for the double bonds is instrumental in the formation of these expanded monolayers, one would expect that a much greater pressure would be required to reduce the area
The Journal of Phyeical Chemistry
D. R. MERKERAKD B. F.DAUBERT
to a given point for the more highly unsaturated compounds. The results, likewise, were not entirely compatible with the theory of chain curling. However, the magnitude of the areas within the expanded state are such that rotation of the chains should be possible except at the double bonds. l 8 The condensed monolayer of 1-monoelaidin resembled those of saturated 1-monoglycerides. Apparently the orientation of the molecules in this state is similar to that in the three-dimensional crystal but the monolayer is liquid crystalline because of partial interaction with the substrate. Transformation of state probably does not occur in the case of the cis compounds, because the cohesive forces that would exist if the monolayer were condensed simply are not great enough to appreciably aid the mechanical force in effecting a stable rearrangement of the polar group. The introduction of double bonds into the hydrocarbon chain lowers the stability of the liquid condensed state just as it lowers the melting point of the anhydrous crystals and increases the stability of the liquid. The densities of 1-monoolein and 1-monoelaidin in the solid state were found to be lower than the density of 1-monostearin.* The reverse situation, as mentioned above, exists for the liquids. The instability of the three-dimensional crystals and the liquid condensed monolayers of unsaturated monoglycerides is additional support for the theory* that the molecular structures in these states are similar.
(13) J. G. Kirkwood, Publ. Am. Aesoc. Advan. Sci., No. 2, 16 (1943).
