a proton magnetic resonance study of phase transitions in rubidium and caesium stearates

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A Proton Magnetic Resonance Study of Phase Transitions in Rubidium and Caesium Stearates * BY D. J. Smwt AND B. A. DUNELL Received 19th June, 1961 The proton magnetic resonance absorption in anhydrous rubidium and caesium stearates has been studied over the temperature range 88°K to 440°K. Abrupt changes in line width and second moment were observed at 350°K and 416°K for rubidium stearate and at 342°K and 373°K for caesium stearate. These correspond approximately to known phase transitions and have been confirmed by X-ray measurements. The results suggest that the first of these transitions involves the onset of rotation of the hydro- carbon chains about their long axes and that the chain spacings depend on the radius of the cation. After the second of these transitions, the hydrocarbon chains may be free to move about other axes whilst still maintaining an average position in the crystal lattice. \ Although phase transitions in rubidium and caesium salts of long-chain fatty acids have been less thoroughly studied than those in the corresponding sodium and potassium salts, the existence of mesomorphic phases between the true crystalline solid and the isotropic liquid phase of rubidium and,caesiumstearates and palmitates has been establishedby dilatometric,light transmission, and calorimetrictechniques.1-3 Something of the nature of these phases can be determined from proton magnetic resonance absorption by the material, and this method has been used to study sodium and potassium stearates.4 The extension of this work to rubidium and caesium stearate is described here. EXPERIMENTAL The samples, which were kindly given to us by Dr. I. E. Puddington, of the National Research Council of Canada, were those used in his light transmission and dilatometric work, and their. preparation is described in his papers33 Each stearate was crushed, put into an appropriate sample tube, and fused under vacuum to remove all water. The sample tube was then sealed off while it was still evacuated. The magnetic resonance measurements were made on a Varian V-4250 40 Mc/sec broad-line spectrometer with provision for temperature control which has been described previously,s* 6 and which permitted the temperature of the sample to be either lowered by a stream of vapour from liquid nitrogen, or raised by a stream of heated air. The line width of the n.m.r. spectrum was taken as the separation in gauss of points of maximum and minimum slope obtained from the recorded derivative of the absorption curve. The second moments of the ab- sorption spectra were obtained from the experimental derivative curves by numerical integration,7 and corrected for the effect of modulation depth,8 which varied from 1.0 to 0.25 gauss. The magnetic field sweep was calibrated against motion of the recorder chart by ob- serving the signal from liquid water at 30 milligauss modulation while sweeping the main magnetic field through resonance and superimposing a 10 Kc/sec side band from a Hewlett Packard audio oscillator on the 40 Mc/sec r.f. input. The audio oscillator was in turn calibrated by an electronic frequency counter. * contribution from the Chemistry Department, University of British Columbia, Vancouver 8, Canada. t present address : Research Department, Unilever Ltd., Port Sunlight, Cheshire, England. 132 Published on 01 January 1962. Downloaded by University of Prince Edward Island on 26/10/2014 19:41:48. View Article Online / Journal Homepage / Table of Contents for this issue

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Page 1: A proton magnetic resonance study of phase transitions in rubidium and caesium stearates

A Proton Magnetic Resonance Study of Phase Transitions in Rubidium and Caesium Stearates *

BY D. J. S m w t AND B. A. DUNELL Received 19th June, 1961

The proton magnetic resonance absorption in anhydrous rubidium and caesium stearates has been studied over the temperature range 88°K to 440°K. Abrupt changes in line width and second moment were observed at 350°K and 416°K for rubidium stearate and at 342°K and 373°K for caesium stearate. These correspond approximately to known phase transitions and have been confirmed by X-ray measurements.

The results suggest that the first of these transitions involves the onset of rotation of the hydro- carbon chains about their long axes and that the chain spacings depend on the radius of the cation. After the second of these transitions, the hydrocarbon chains may be free to move about other axes whilst still maintaining an average position in the crystal lattice.

\

Although phase transitions in rubidium and caesium salts of long-chain fatty acids have been less thoroughly studied than those in the corresponding sodium and potassium salts, the existence of mesomorphic phases between the true crystalline solid and the isotropic liquid phase of rubidium and,caesium stearates and palmitates has been established by dilatometric, light transmission, and calorimetric techniques.1-3 Something of the nature of these phases can be determined from proton magnetic resonance absorption by the material, and this method has been used to study sodium and potassium stearates.4 The extension of this work to rubidium and caesium stearate is described here.

EXPERIMENTAL

The samples, which were kindly given to us by Dr. I. E. Puddington, of the National Research Council of Canada, were those used in his light transmission and dilatometric work, and their. preparation is described in his papers33 Each stearate was crushed, put into an appropriate sample tube, and fused under vacuum to remove all water. The sample tube was then sealed off while it was still evacuated. The magnetic resonance measurements were made on a Varian V-4250 40 Mc/sec broad-line spectrometer with provision for temperature control which has been described previously,s* 6 and which permitted the temperature of the sample to be either lowered by a stream of vapour from liquid nitrogen, or raised by a stream of heated air. The line width of the n.m.r. spectrum was taken as the separation in gauss of points of maximum and minimum slope obtained from the recorded derivative of the absorption curve. The second moments of the ab- sorption spectra were obtained from the experimental derivative curves by numerical integration,7 and corrected for the effect of modulation depth,8 which varied from 1.0 to 0.25 gauss.

The magnetic field sweep was calibrated against motion of the recorder chart by ob- serving the signal from liquid water at 30 milligauss modulation while sweeping the main magnetic field through resonance and superimposing a 10 Kc/sec side band from a Hewlett Packard audio oscillator on the 40 Mc/sec r.f. input. The audio oscillator was in turn calibrated by an electronic frequency counter.

* contribution from the Chemistry Department, University of British Columbia, Vancouver 8, Canada. t present address : Research Department, Unilever Ltd., Port Sunlight, Cheshire, England.

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Page 2: A proton magnetic resonance study of phase transitions in rubidium and caesium stearates

D. J. SHAW AND B . A . DUNELL 133

30

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i a C g 10- v)

X-ray powder photographs of the samples were taken with a conventional powder camera using copper Ka radiation, a nickel filter being used to remove the p radiation. Pictures were taken at room temperature and also at 75°C and above. The elevated temperature was obtained by blowing heated air over the sample and the temperature was monitored by a thermocouple and recording potentiometer, the thermocouple being fixed 2-3mm above the sample capillary. The temperature varied over approximately 4°C during an exposure time of up to 12 h.

RESULTS

The variation of line width and second moment with temperature is shown for rubidium stearate in fig. 1 and for caesium stearate in fig. 2. For both substances, the line width and second moment decrease slowly with increasing temperature up to about 330”C, beyond which the decrease becomes progressively more rapid until a transition is completed at approximately 350°K for the rubidium salt and approxi- mately 342°K for the caesium salt. Again in both cases, this is followed by a slower decrease in parameters with increasing temperature up to a second transition at about

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- I I I I I I I I I

100 200 30 0 400

temp., OK

FIG. 1.Variation with temperature of the line width and second moment of rubidium stearate.

416°K for rubidium stearate and fairly sharply at 373°K for caesium stearate. During this second transition, the line width falls from a clearly distinguishable value of some 3 gauss to a value which is not determinable either because it is less than the depth of modulation, 0.5 gauss for some and 0.25 for other spectra, or because it is obscured by a narrow component of the spectrum. This narrow component is first observed several degrees above room temperature and found to increase in intensity with increasing temperature. The change in line shape at the second transition is shown in fig. 3 and 4. The lines shown are direct reproductions of recorded spectra. The rather high noise level in these spectra can be accounted for mainly by two factors. First, the heating (or cooling) apparatus surrounding the sample lies within the “insert tube” of the probe and hence places a severe restriction on efficiency of packing of sample in the probe. Secondly, the spectra were recorded using a response time of 3 sec in the output circuit rather than a longer response time which would

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Page 3: A proton magnetic resonance study of phase transitions in rubidium and caesium stearates

134 PHASE TRANSITIONS I N STEARATES

have reduced the noise level, although not necessarily improved the quality of the traces. The second moments remain finite above the second transition temperature because the intensity of absorption in the wings of the curves remains significantly large.

FIG. temp., "K

2.Variation with temperature of the line width and second moment-of caesium stearate.

FIG. 3.-Change in line shape of rubidium stearate near the second phase transition.

X-ray powder photographs at room temperature and at 358°K were taken of rubidium stearate with a 12 h and a 4.5 h exposure respectively. These are reproduced in fig. 5, from which it cansbe seen that there is a change in crystal structure of this

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Page 4: A proton magnetic resonance study of phase transitions in rubidium and caesium stearates

D. J . SHAW AND B. A . DUNELL 135 salt between 300 and 358OK. The inner reflections, corresponding to long spacings, are only slightly moved in the direction to be expected from thermal expansion. From among the reflections corresponding to shorter spacings, however, some have disappeared at the higher temperature and some appear to have moved toward yet shorter spacings despite overall expansion of the lattice. For caesium stearate, 12 h exposures for each of room temperature, 348", 358" and 385°K are shown in fig. 6.

FIG. 4 .4hange in line shape of caesium stearate at the second phase transition.

Changes in the short spacings between room temperature and 348°K can be dis- tinguished clearly. There is a further small difference between the pictures for 348" and 358"K, and a radical change in the picture taken at 385"K, from which all but one of the sharp, short spacings have disappeared.

DISCUSSION

Although the rigid lattice second moment of these salts cannot be calculated because their crystal structure is unknown, one can probably assume, by analogy wth octadecane 9 and sodium and potassium stearates 4 that the experimental value at liquid nitrogen temperature is only a little less than the rigid lattice second moment. We can then say that the rigid lattice second moments of these two soaps lie between 26 and 29 gauss2, of which 18.9 gauss2 is the intra-molecular contribution, as can be readily calculated if one assumes tetrahedral angles in the hydrocarbon chain and bond lengths C-C = 1.54 A and C-H = 1.094 A.4 The theoretical treatment by Gutowsky and Pake of rotational motion in the solid state 10 allows one to calculate that the second moments of these salts would be between 8.5 and 9.5 gauss2 for rotation about the longitudinal axes of the hydrocarbon chains. It can be seen from fig. 1 that the extent of motion of the hydrocarbon chains in rubidium stearate at the end of the first, or " crystal ", phase transition corresponds to rotation of the chains about their long axes. The small increase in molecular motions between liquid nitrogen temperature and room temperature can be interpreted in terms of a torsional oscillation of the chain of small but increasing amplitude about its long axis. It is not understood why the motion in rubidium stearate up to room temperature should be so much less extensive than that in the other stearates, as indicated by the small percentage decrease in second moment of rubidium stearate up to 300°K compared with the corresponding percentage decrease in any of the other stearates we have studied. From fig. 2 it is evident that torsional motion of the hydrocarbon chains

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Page 5: A proton magnetic resonance study of phase transitions in rubidium and caesium stearates

136 PHASE TRANSITIONS I N STEARATES

in caesium stearate increases more rapidly than in rubidium stearate as the temperature is raised from 100" to 300°K. Comparison of caesium with sodium and potassium stearates 4 shows that the increase in extent of motion of the hydrocarbon chains of these three salts is about the same between liquid nitrogen and room temperatures. As the caesium soap passes through the first transition, at about 342"K, the extent of motion of the hydrocarbon chains increases to a little bit more motion than rotation about the long axis of the chain.

The X-ray evidence given here suggests the existence of a transition between crystalline modifications somewhere in the temperature interval 300" to 350°K for both rubidium and caesium stearates. The two crystalline phases of a corresponding transition in the potassium salt have been characterized by the detailed X-ray study of Lomer,ll and a nuclear magnetic resonance transition was found for this salt at about 330OK.4 Thus, the rearrangement of the hydrocarbon chains involved in the crystal transition of each salt allows much more extensive motion of the chains in the higher temperature form. The increasing extent of this motion in the higher temperature modification as one progresses through the series K, Rb, Cs salt suggests that the organic chains interact less with one another and therefore may be spaced farther apart in the higher temperature phase as the ionic radius of the cation in- creases in the series. Chain spacings in the lower temperature phases of the three salts, however, do not appear from the n.m.r. evidence to vary in this regular fashion.

The second transition in caesium stearate, found at 373"K, involves the onset of very extensive motion of the hydrocarbon chain. The reduction of the line width to less than 0.25 gauss and of the second moment to 1.0 gauss2 indicates reorientation of the chains about many axes, but the finite second moment implies that the chains are constrained to maintain an average position in the lattice,l2 presumably by the polar salt layers. Such behaviour is the same as that shown by potassium stearate at 443°K and by sodium stearate at 387°K and is believed to be characteristic of the so-called curd to waxy (or super-curd to sub-waxy) transition. This interpretation is consistent with the disappearance of a number of sharp X-ray reflections from the caesium stearate picture taken at 385°K. Those reflections are presumably due to diffraction by well-ordered hydrocarbon chains, and they disappear when disordering sets in above the second phase transition. A similar situation exists in the rubidium stearate at about 416"K, except that the second moment is reduced only to some 2.5 gauss2. This indicates that there is less extensive motion of the hydrocarbon chains in the high temperature form of the rubidium salt than in the otherwise cor- responding phase of the caesium salt.

The transition temperatures observed by magnetic resonance in caesium stearate agree very closely with those obtained dilatometrically by Puddington and his co- workers.3 For the rubidium soap, the lower transition temperature agrees with the dilatometric study, but the higher magnetic resonance transition lies between two transition temperatures obtained dilatometrically. Because of this lack of agreement and because of the second moment value of 2.5 gauss2 obtained above 416°K for rubidium stearate, the results were checked against the possibility that thermal equilibrium had not been attained with the rubidium salt at high temperature. At 412"K, no variation in the reported line width or second moment was found in spectra taken after 1, 3, and 7 h at that temperature. At 423"K, a redetermination of the spectrum gave a second moment of 1.7 gauss2 after 1, 2, 3, 5, and 17 h exposures of the sample to that temperature. At temperatures near 460°K, the second moment of rubidium stearate was observed to drop below 1 gauss2.

At many temperatures the intensity of the narrow component was sufficiently great to exceed the range of the recorder. No quantitative estimate could be made

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Page 6: A proton magnetic resonance study of phase transitions in rubidium and caesium stearates

FIG. 5.-X-ray powder photographs of rubidium stearate at room temperature and above the first transition temperature.

FIG. 6.-X-ray powder photographs of caesium stearate at different temperatures.

[To face page

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Page 7: A proton magnetic resonance study of phase transitions in rubidium and caesium stearates

D. J . S H A W AND B. A . DUNELL 133

from these results of the fraction of the protons contributing to the narrow component at each stage in the heating process, and there is the possibility that this component is attributable to a low melting impurity in the sample rather than to the onset of liquid-like motion by the main substance of the sample in the vicinity of defects in the lattice.13

We wish to thank the National Research Council of Canada and the Defence Research Board of Canada for their grants in aid of this research.

1 Vold and Vold, J. Physic. Chem., 1945, 49, 32. 2 Benton, Howe and Puddington, Can. J. Chem., 1955, 33, 1384. 3 Benton, Howe, Farnand and Puddington, Can. J . Chem., 1955, 33, 1798. 4 Grant and Dunell, Can. J. Chem., 1960,38,1951,2395. 5 Bemstein, Schneider and Pople, Proc. Roy. Soc. A , 1956, 236, 515. 6Dunel1, Reeves and Stramme, Trans. Faraduy Soc., 1961, 57, 372. 7 Pake and Purcell, Physic. Reo., 1948, 74, 1184. 8 Andrew, Physic. Rev., 1953,91,425. 9Andrew, J. Chem. Physics, 1950, 18, 607.

10 Gutowsky and Pake, J. G e m . Physics, 1950, 18, 162. 11 Lomer, Actu Cryst., 1952, 5, 11. 12 Andrew, Nuclear Magnetic Resonance (Cambridge University Press, 1959, p. 173. 13 Grant and Dunell, Can. J. Chem., 1960,38,359.

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