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23Na NMR study of ionic mesophases in molten sodium carboxylates
Volume 75, number 2
CHEMICAL
PHYSICS
LETTERS
15 October
1980
23NaNMRSTUDYOF IONICMESOPHASESINMOLTENSODIUMCARBOXYLATES J. BONEKAMP, T. EGUCHI * and J. JONAS Department of Chemrstty, School of Chemical Sciences, ihversrty Urbntuz. Rlrno~s 61801. USA
of IlImos,
Recetved 9 July 1980
The23Na NMR lureshapes are reported for the ionic mesophase and isotroptc phase of the melts of sodrum n-butyrate and sodium isovalerate The powder pattern for the central transrtion typrcal for the second-order quadrupole effect observed In the mesophase melts is of partrcular interest. Some analogres to 23Na behavior in sodium fl-alumina are pomted
out. Short chain alkah-metal carboxyhtes in their molten states exhrbrt a number of unusual physical propertres [I -61 Two members of thrs class of compounds, sodmm rsovalerate and sodium n-butyrate, form a mesophase at T, = 188°C and TF = 252OC, respectrvely. It has been proposed that thrs mesophase consists of randomly onented smectrc domains [1,2]. At TcL = 280°C and TcL = 324°C respectively, the mesophase undergoes a transition rnto an rsotroprc melt. In addition to bemg possible model mesogens for lametlar lyotroprc hquid crystals such as brologrcal membranes, these compounds exhibit some properties typrcal of two-dimensional fast ran conductors, specrfically sodrum &alumina. Previous
work
in this group
[7]
has shown
aruofi
of the domains of the so&urn isovalerate mesophase to be very rapid (D a:5X 10V7 cm*/s) whrle domain-domain interactions are manifested through macroscopic properties such as the estimated shear-rate dependent vrscosity (= 100 P). More recently, srmilar aruon diffusion behavior has been found III the mesophase of sodium n-butyrate [S] . On the basis of theu thermodynamrc measurements for both fusron and cleanng transitrons and because of the high conductivrty (cr = low3 SL-r cm-l) observed in the mesophase of sodmm isovalerate, Ubbelohde et al [ 1,2] suggested an occurrence of diffusion
wrthin
the layers
* Permanent address. lnstrtute ofChemIstry, General Edmatron. 560, Japan.
360
Osaka Untversrty,
College of Toyonaka. Osaka
separate positional melting of cation and amon lattices. Our present NMR study of sodium m these matenals was motivated by the observation of a rapid amon drffusion rn sodium isovalerate [7] and by the success of *3Na NMR experunents in fast ioruc conductors [9] and m a variety of organic and bioorganic systems IlO] To our best knowledge, we report here the first observation of a second-order quadrupole effect for 23Na in any hquid crystal. Indeed thrs appears to be the first 23Na NMR study in anhydrous bquid crystals (thermotropic). The presence of such strong quadrupole effects coupled with rapid anion and cation diffusion suggests that these mesophases have”sandwrchtype” domain structure [3] stabrhzed by electrostatrc forces (charge planes) rather than a large molecular length-to-width ratio (> 4) normally constdered essential for hquid crystalline phase formation. Sodium isovalerate and sodium n-butyrate samples were prepared in a manner previously described [l l] . The 23Na lmeshapes in each compound were recorded at 47.6 MHz by the Fourier transform method using an Oxford Instrument superconducting magnet operating at a field of 42.2 kG. Typical spectra for the central transrtron (m = - + @rn = +) of 23Na (I = 312) are shown in figs. 1 and 2. In the rsotroplc melt, we observe a near lorentzian lmeshape wluch-indicates that the quadrupole mteraction is averaged out by isotropic random matron whereas in the mesophase, we obtain a typical powder pattern for the second-order quadrupole shift. For sodium isovalerate, the observed mtensity profile for
CHEMKAL PHYSICS LEDTERS
Volume 75 _ number 2
15 October L980
the powder pattern is drfferent from that obtained fcrr the calculated spectrum (fig. 2). This discrepancy varres wrth the thermal history of the sample. The best fit for simulated spectra [12J (see fs- 1 and 2) Were SODIUM N-ElUTYRATE
T PC1 3373
313 5
Fig 1. Central hnesbape of the 23Na szgnai in sodmm n-butyrate (a) m the sotroprc meit and @) the mesophase The theore&al hneshape [for detads see the ted) 1s shown IIIfii lc which curresponds to the usual cw spectrum
n
SODIUMlSOVALERATE -I-(-Cl
KHZ
i-
0
25KH2
285 0
obtained using a value of the asymmetry parameter (q) equal to zero and a Iinc~dth (for the gaussian Iineshape function convoluted wrth the quadrupole interactton) approximately 0.3 of that observed For the Lsotropic melt. The isotropic linewidths are 0.4 kHz for sodium n-butyrate and 18 kHz for sodium rsovalerate. The quadrupole coupling constants (e29Q/J’z) calculated from the first-order splittmgs (satellites) were in agreement with those values necessary to reproduce the central transltion powder patterns. For dlustratton, we give fig_ 3 which shows the satelliee as well as the central transition (m =++m=_f) III the mesophase of sodium n-butyrate. The valuesof the q~drupoie coupting constant at tempe~tures immediately pnor to the clearmg transitions (TCr) are 805 it:5 kHz for sodium n-butyrate and 980 C 5 Ws for sodrum isovalerate. These values increase with decreasmg temperature, changmg by only 29 and 8.S%, respectively, over the temperature range of the mesophase. The magnitude of these quadrupole coupling constants IS closer to that of sodium @alumina (*2 MHz) [13] than to typical vahres of 23Na quadrupole coupling constants observed for lamellar lyotrop?e hquid crystals containing carboxyIate amphiphiBes (-2OkHz) [14J. The dlpolar ~ewIdths t~ou~out the mesophase are as narrow as in the Isotropic melt which implies that cation dtifuaon IS very rapid in the mesophase. Thus is confirmed by our prehinary I‘, measwements SODIUM N-BUNRATE
275.4
Fig 2 Central lineshape of the 23Na sigmtl in sodmm ISOMIcrate (a) in the zsotroprc melt and &) the mesophase. The theoretrcal hneshape (for details see the text) 1s shown 111fig. 2c whtch corresponds to the usual cw spectrum.
Fzg. 3. Central hneshape and satellite (m = - $ ++ m = - $> of 23Na tn sadmm mbutyrate at 312A°C.
Volume 75, number 2
CHEMICAL
PHYSICS
O,= lo-lo
s) as well as by a hrgh conductivity [2]. It is also worth notmg that the motlonal narrowing 1s observable already m the sohd state In other words, the cation diffusion is activated enough to satisfy the conditron 1-y1AH,,, T= < 1 m the sohd. It 1s unportant that m spite of the rapid Na+ motion present in the mesophase we observe a secondorder quadrupole effect for the central transitron. In thus regard, our system resembles sodium p-alumma wmch also has rapid cation &ffusron (r,=5 X 1O-g s at room temperature) [I 51 and non-averaged quadrupole effects [13]. Recently the 23Na NMR data for sodium @alumma has been successfuUy described usmg a twodunenslonal-contmuum drffusron model, where on an NMR tune scale, the cation motion IS hqurd-hke [16,17]. In the framework of thrs model which involves a time-dependent perturbatron treatment, the quadrupole effects on the lmeshape do not average out for the twodunensronal drffusive motion In the fast-motion regune (oL rc 4 1) and for the effective asymmetry parameter equai to zero the expression for the second-order quadrupole shift reduces to the well known expression grven by Cohen and Rerf [18] for the second-order quadrupole shrft III solids for the case,q=O. In this model [16,17] it can be shown that an effectrve axrally symmetric field gradient may be the result of rapid motion In view of the smectic structure and the raprd amon and cation arusotroprc diffusion, a twodrmensronaf hqurd model appears to be a arable descnptron of our experrmental data. A more detailed analysis of the lineshape measurements and sodium spm-lattice relaxation m the mesophase and the soled phases of both these sodium carboxylates will be presented m a forthcommg paper The experrments are presently underway to elucidate size and structure of the domams and to explam the anion behavior vra *H and 2D NMR m both the mesophase and the solid phases
Acknowledgement We express our thanks
362
to Greg Hoffman
for Ins ex-
LETTERS
15 October
1980
pert help with the experiments. Thrs research was partially supported by the National Scrence Foundation under Grant NSF-CHE 78-10-7707621 and the Au Gorce Office of Screntrfic Research under Grant AFORS 773185.
References [l] [2] [3] [4] [ii] [6] 171 [S] [9]
[lo] [II] [12] [13] [ 141 (15
]
[ 16 j [17]
[18]
A R Ubbelohde, H J Mlchels and J J Duruz, Nature 228 (1970) 50 J J Duruz, H J Mlchels and A R Ubbelohde, Proc Roy Sot A322 (1971) 281 J J Duruz and A R. Ubbelohde, Proc. Roy Sot A330 (1972) 1. HJ. hllchels and A R. Ubbelohde, Proc Roy Sot A338 (1974) 447. J J Duruz and A R Ubbelohde, Proc Roy. Sot A342 (1975) 39 J J Duruz and A.R Ubbelohde, Proc. Roy. Sot A347 (1976) 301 M. Wdlfe, J. Bonekamp and J. Jonas, J Chem Phys 70 (1979) 3993. J. Bonekamp and J Jonas, unpubhshed results C Berthler, m- Fast Ion transport III sohds. eds P Vashishta, J N Mundy and C K. Shenoy (North-Holland, Amsterdam, 1979) p 171. P. Laszlo, Angew Chem. 17 (1978) 254 JJ Duruz.HJ Mlcheisand A R Ubbelohde.Chem Ind (1969) 1386 H S Story and D. Khne, QCPE 11 (1970) 154 D. Kbne, H S. Story and W L. Roth, J Chem Phys 57 (1972) 5 180. G. Ltndblom and B Lmdman, Mol Cryst Llquld Cryst 22 (1973) 45 D. Jerome and JP. Bodot, J Phys (Paris) 35 (1974) L129. J.L.. BJorkstam and M. VdIa, Phys Rev , to be pubhshed M Villa and JL BJorkstam, in. Fast ion transport m solds, eds. P. Vashlshta, J.N Mundy and G K. Shenoy (North-Holland, Amsterdam, 1979) p. 289. M.HH.Cohen and F Red, III- Sold state physics. Vol 5, eds F Seltz and D. Turnbull (Academic Press, New York, 1957) p 321.
CHEMICAL
PHYSICS
LETTERS
15 October
1980
23NaNMRSTUDYOF IONICMESOPHASESINMOLTENSODIUMCARBOXYLATES J. BONEKAMP, T. EGUCHI * and J. JONAS Department of Chemrstty, School of Chemical Sciences, ihversrty Urbntuz. Rlrno~s 61801. USA
of IlImos,
Recetved 9 July 1980
The23Na NMR lureshapes are reported for the ionic mesophase and isotroptc phase of the melts of sodrum n-butyrate and sodium isovalerate The powder pattern for the central transrtion typrcal for the second-order quadrupole effect observed In the mesophase melts is of partrcular interest. Some analogres to 23Na behavior in sodium fl-alumina are pomted
out. Short chain alkah-metal carboxyhtes in their molten states exhrbrt a number of unusual physical propertres [I -61 Two members of thrs class of compounds, sodmm rsovalerate and sodium n-butyrate, form a mesophase at T, = 188°C and TF = 252OC, respectrvely. It has been proposed that thrs mesophase consists of randomly onented smectrc domains [1,2]. At TcL = 280°C and TcL = 324°C respectively, the mesophase undergoes a transition rnto an rsotroprc melt. In addition to bemg possible model mesogens for lametlar lyotroprc hquid crystals such as brologrcal membranes, these compounds exhibit some properties typrcal of two-dimensional fast ran conductors, specrfically sodrum &alumina. Previous
work
in this group
[7]
has shown
aruofi
of the domains of the so&urn isovalerate mesophase to be very rapid (D a:5X 10V7 cm*/s) whrle domain-domain interactions are manifested through macroscopic properties such as the estimated shear-rate dependent vrscosity (= 100 P). More recently, srmilar aruon diffusion behavior has been found III the mesophase of sodium n-butyrate [S] . On the basis of theu thermodynamrc measurements for both fusron and cleanng transitrons and because of the high conductivrty (cr = low3 SL-r cm-l) observed in the mesophase of sodmm isovalerate, Ubbelohde et al [ 1,2] suggested an occurrence of diffusion
wrthin
the layers
* Permanent address. lnstrtute ofChemIstry, General Edmatron. 560, Japan.
360
Osaka Untversrty,
College of Toyonaka. Osaka
separate positional melting of cation and amon lattices. Our present NMR study of sodium m these matenals was motivated by the observation of a rapid amon drffusion rn sodium isovalerate [7] and by the success of *3Na NMR experunents in fast ioruc conductors [9] and m a variety of organic and bioorganic systems IlO] To our best knowledge, we report here the first observation of a second-order quadrupole effect for 23Na in any hquid crystal. Indeed thrs appears to be the first 23Na NMR study in anhydrous bquid crystals (thermotropic). The presence of such strong quadrupole effects coupled with rapid anion and cation diffusion suggests that these mesophases have”sandwrchtype” domain structure [3] stabrhzed by electrostatrc forces (charge planes) rather than a large molecular length-to-width ratio (> 4) normally constdered essential for hquid crystalline phase formation. Sodium isovalerate and sodium n-butyrate samples were prepared in a manner previously described [l l] . The 23Na lmeshapes in each compound were recorded at 47.6 MHz by the Fourier transform method using an Oxford Instrument superconducting magnet operating at a field of 42.2 kG. Typical spectra for the central transrtron (m = - + @rn = +) of 23Na (I = 312) are shown in figs. 1 and 2. In the rsotroplc melt, we observe a near lorentzian lmeshape wluch-indicates that the quadrupole mteraction is averaged out by isotropic random matron whereas in the mesophase, we obtain a typical powder pattern for the second-order quadrupole shift. For sodium isovalerate, the observed mtensity profile for
CHEMKAL PHYSICS LEDTERS
Volume 75 _ number 2
15 October L980
the powder pattern is drfferent from that obtained fcrr the calculated spectrum (fig. 2). This discrepancy varres wrth the thermal history of the sample. The best fit for simulated spectra [12J (see fs- 1 and 2) Were SODIUM N-ElUTYRATE
T PC1 3373
313 5
Fig 1. Central hnesbape of the 23Na szgnai in sodmm n-butyrate (a) m the sotroprc meit and @) the mesophase The theore&al hneshape [for detads see the ted) 1s shown IIIfii lc which curresponds to the usual cw spectrum
n
SODIUMlSOVALERATE -I-(-Cl
KHZ
i-
0
25KH2
285 0
obtained using a value of the asymmetry parameter (q) equal to zero and a Iinc~dth (for the gaussian Iineshape function convoluted wrth the quadrupole interactton) approximately 0.3 of that observed For the Lsotropic melt. The isotropic linewidths are 0.4 kHz for sodium n-butyrate and 18 kHz for sodium rsovalerate. The quadrupole coupling constants (e29Q/J’z) calculated from the first-order splittmgs (satellites) were in agreement with those values necessary to reproduce the central transltion powder patterns. For dlustratton, we give fig_ 3 which shows the satelliee as well as the central transition (m =++m=_f) III the mesophase of sodium n-butyrate. The valuesof the q~drupoie coupting constant at tempe~tures immediately pnor to the clearmg transitions (TCr) are 805 it:5 kHz for sodium n-butyrate and 980 C 5 Ws for sodrum isovalerate. These values increase with decreasmg temperature, changmg by only 29 and 8.S%, respectively, over the temperature range of the mesophase. The magnitude of these quadrupole coupling constants IS closer to that of sodium @alumina (*2 MHz) [13] than to typical vahres of 23Na quadrupole coupling constants observed for lamellar lyotrop?e hquid crystals containing carboxyIate amphiphiBes (-2OkHz) [14J. The dlpolar ~ewIdths t~ou~out the mesophase are as narrow as in the Isotropic melt which implies that cation dtifuaon IS very rapid in the mesophase. Thus is confirmed by our prehinary I‘, measwements SODIUM N-BUNRATE
275.4
Fig 2 Central lineshape of the 23Na sigmtl in sodmm ISOMIcrate (a) in the zsotroprc melt and &) the mesophase. The theoretrcal hneshape (for details see the text) 1s shown 111fig. 2c whtch corresponds to the usual cw spectrum.
Fzg. 3. Central hneshape and satellite (m = - $ ++ m = - $> of 23Na tn sadmm mbutyrate at 312A°C.
Volume 75, number 2
CHEMICAL
PHYSICS
O,= lo-lo
s) as well as by a hrgh conductivity [2]. It is also worth notmg that the motlonal narrowing 1s observable already m the sohd state In other words, the cation diffusion is activated enough to satisfy the conditron 1-y1AH,,, T= < 1 m the sohd. It 1s unportant that m spite of the rapid Na+ motion present in the mesophase we observe a secondorder quadrupole effect for the central transitron. In thus regard, our system resembles sodium p-alumma wmch also has rapid cation &ffusron (r,=5 X 1O-g s at room temperature) [I 51 and non-averaged quadrupole effects [13]. Recently the 23Na NMR data for sodium @alumma has been successfuUy described usmg a twodunenslonal-contmuum drffusron model, where on an NMR tune scale, the cation motion IS hqurd-hke [16,17]. In the framework of thrs model which involves a time-dependent perturbatron treatment, the quadrupole effects on the lmeshape do not average out for the twodunensronal drffusive motion In the fast-motion regune (oL rc 4 1) and for the effective asymmetry parameter equai to zero the expression for the second-order quadrupole shift reduces to the well known expression grven by Cohen and Rerf [18] for the second-order quadrupole shrft III solids for the case,q=O. In this model [16,17] it can be shown that an effectrve axrally symmetric field gradient may be the result of rapid motion In view of the smectic structure and the raprd amon and cation arusotroprc diffusion, a twodrmensronaf hqurd model appears to be a arable descnptron of our experrmental data. A more detailed analysis of the lineshape measurements and sodium spm-lattice relaxation m the mesophase and the soled phases of both these sodium carboxylates will be presented m a forthcommg paper The experrments are presently underway to elucidate size and structure of the domams and to explam the anion behavior vra *H and 2D NMR m both the mesophase and the solid phases
Acknowledgement We express our thanks
362
to Greg Hoffman
for Ins ex-
LETTERS
15 October
1980
pert help with the experiments. Thrs research was partially supported by the National Scrence Foundation under Grant NSF-CHE 78-10-7707621 and the Au Gorce Office of Screntrfic Research under Grant AFORS 773185.
References [l] [2] [3] [4] [ii] [6] 171 [S] [9]
[lo] [II] [12] [13] [ 141 (15
]
[ 16 j [17]
[18]
A R Ubbelohde, H J Mlchels and J J Duruz, Nature 228 (1970) 50 J J Duruz, H J Mlchels and A R Ubbelohde, Proc Roy Sot A322 (1971) 281 J J Duruz and A R. Ubbelohde, Proc. Roy Sot A330 (1972) 1. HJ. hllchels and A R. Ubbelohde, Proc Roy Sot A338 (1974) 447. J J Duruz and A R Ubbelohde, Proc Roy. Sot A342 (1975) 39 J J Duruz and A.R Ubbelohde, Proc. Roy. Sot A347 (1976) 301 M. Wdlfe, J. Bonekamp and J. Jonas, J Chem Phys 70 (1979) 3993. J. Bonekamp and J Jonas, unpubhshed results C Berthler, m- Fast Ion transport III sohds. eds P Vashishta, J N Mundy and C K. Shenoy (North-Holland, Amsterdam, 1979) p 171. P. Laszlo, Angew Chem. 17 (1978) 254 JJ Duruz.HJ Mlcheisand A R Ubbelohde.Chem Ind (1969) 1386 H S Story and D. Khne, QCPE 11 (1970) 154 D. Kbne, H S. Story and W L. Roth, J Chem Phys 57 (1972) 5 180. G. Ltndblom and B Lmdman, Mol Cryst Llquld Cryst 22 (1973) 45 D. Jerome and JP. Bodot, J Phys (Paris) 35 (1974) L129. J.L.. BJorkstam and M. VdIa, Phys Rev , to be pubhshed M Villa and JL BJorkstam, in. Fast ion transport m solds, eds. P. Vashlshta, J.N Mundy and G K. Shenoy (North-Holland, Amsterdam, 1979) p. 289. M.HH.Cohen and F Red, III- Sold state physics. Vol 5, eds F Seltz and D. Turnbull (Academic Press, New York, 1957) p 321.