Sample cell for high-precision temperature-dependence NMR experiments

JOURNAL

OF MAGNETIC

RESONANCE

98,92-108

( 1992)

Sample Cell for High-Precision Temperature-Dependence NMR Experiments NEVILLEBODEN,* SIMONA.CORNE,* PETERHALFORD-MAW,* DAVIDFOGARTY,* AND KENNETHW.JOLLEY~ *School

of Chemistry, The University, Leeds LS2 9JT, United Kingdom; and Biochemistry, Massey University, Palmerston North,

and TDepartment New Zealand

of Chemistry

Received August 2 1, 199 1 A simple sample temperature control system is described which minimizes temperature gradients and provides for accurate and precise control of sample temperature. The sample is confined in a standard 5 mm o.d. NMR tube, supported in a standard 10 mm o.d. tube, through which a fluid, whose temperature is regulated in an external thermostat, is rapidly circulated by way of a double-pass liquid-flow arrangement. The device can be readily used in any standard 10 mm NMR probe. When water is used as the cryogenic fluid, sample temperatures in the range 273 to 370 K, with an accuracy, precision, and resolution (settability) of kO.005 K, are accessable. We have extensively used the system to study phase transitions and phase behavior of liquid crystals, although many other applications may be envisaged. Herein, the performance characteristics of the device are demonstrated by ‘H and “‘Cs experiments on the cesium pentadecafluorooctanoate (CsPFO)/water liquid-crystal system. 0 1992 Academic press, IIIC.

Many applications of NMR spectroscopy, embracing, for example, studies of chemical exchange, conformational equilibria, and phase transitions, demand better control of sample temperature than is normally practicable with commercial spectrometers. Both transverse and longitudinal sample temperature gradients are present when conventional gas-flow systems are used. These gradients, which increase rapidly with the difference between sample and ambient temperatures, can present severe problems for the precise observation of NMR phenomena which are strongly dependent on temperature ( 1,2). Systems have been reported with temperature stability of the order of fO.l K (3-5) and some commercially available gas-flow systems can now be set to this precision. Improved gas-flow systems have been reported with stabilities of kO.005 K over the range 263 to 373 K (6) and kO.035 K over the range 108 to 393 K ( 7)) but in neither case was the problem of sample temperature gradients addressed. Temperature gradients of less than 0.01 K, at temperatures close to ambient, have been obtained with (“optimized” gas-flow systems (8) and standard NMR probes, but at temperatures away from ambient, specially engineered probes become necessary (9). An alternative approach is to use a liquid as the heat-transfer fluid ( 10-12). A temperature stability of +O. 1 K and the elimination of temperature gradients in highresolution 13C- { ‘H } decoupling experiments have been achieved using a single-pass fluid-flow system (IO). An alternative single-pass water-flow system with an air gap between an outer water jacket and the sample coil, to permit ‘H studies, had a stability 0022-2364192 $5.00 Copyright 0 1992 by Academic Press, Inc. Ail rights of reproduction in any form reserved.

92

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of kO.02 K (11). Both of these systems, however, require a dedicated sample probe. In this paper, the design and construction of a fluid-flow double-pass system is described. No modification of the spectrometer probe is required and it is easily installed. It consists of a thermostat, a fluid circulation system, and a “plug-in” sample cell, custom designed to the probe/magnet system. The performance of this system is demonstrated by 2H and 133CsNMR experiments on the micellar liquid-crystal cesium pentadecafluorooctanoate (CsPFO)/water: these show that, when water is used as the heatexchange fluid, a temperature stability of kO.005 K is obtained with no measurable temperature gradients in the sample. Water is the preferred heat-transfer fluid: it has a high heat capacity and a low viscosity over most of its liquid range at 10 1 kPa, and it can be used for studies of all nuclei other than ‘H. The advantage of water over air a.sa heat-exchange fluid is seen by comparing the two in terms of the temperature drop associated with a heat loss of, say, 10 J min-‘: in the apparatus described herein, typical water-flow rates are of the order of 1.3 dm3 min -’ so that the temperature drop is 2 mK, while for a gas-flow system with a flow rate of 25 dm3 min-’ , which is relatively high, the corresponding temperature drop would be 340 mK. To improve the performance of a gas-flow system, it is necessary to increase the flow rate and minimize energy losses, by improving the thermal insulation of the sample. Both of these requirements place severe restraints on probe design. TEMPERATURE

CONTROL

SYSTEM

The sample cell. The design objectives were to use standard 5 mm sample tubes; to be able to insert the cell directly into a standard 10 mm multinuclear probe of any commercially available NMR spectrometer; and to achieve an accuracy, precision, and resolution of t-O.005 K over the temperature range 273 to 370 K (here determined by the use of water as the heat-exchange liquid). In the design of the cell particular attention was given to maximizing the fluid-flow rate so as to minimize temperature fluctuations arising from heat losses to the surroundings; providing good thermal contact between the sample and the flowing thermostated liquid to ensure fast response of sample temperature to temperature changes; and providing good thermal insulation of the cell. The cell design adopted for use with the 10 mm probes of either a Jeol GX270 or Bruker MSL300 NMR spectrometer, with a wide-bore (89 mm) magnet, is shown in Fig. 1. The outer wall of the upper part of the cell is fabricated from 30 mm o.d. Pyrex glass tubing and the lower part from a conventional 10 mm o.d. thin-walled NMR tube. A glass partition tube separates the water flow in the inlet pipe from that in the return pipe such that a double wall of water surrounds the 5 mm o.d. sample tube. The water-flow rate through the cell is determined by the annular cross sections of the pipes enclosed by the 10 mm o.d. NMR tube. For a pipe, of annular cross section, the rate of flow is proportional to { rt - ri - (r$ - r:)‘/ln(r2/r1)} (13), where rl and r2 are the radii of the inner and .outer tubes. Restricting ourselves to commercially available ( Wilmad Glass Co., Inc.) thin-walled NMR tubes, a simple calculation shows that the maximum flow rate is achieved by using a 7.5 mm (i.d., 6.48 mm) NMR tube as the partition tube. The length of this flow-limiting section ( d2) was kept to a minimum. To facilitate rapid transfer of water from the cryostat to the sample, the

94

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ET

AL.

--

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f+--SQ28

teflon

in

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out-

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El19

socket

-+

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30

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o.d.

outer

18

mm

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extension

4

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“i”-

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--38

10

mm

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-

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---.

7.5 mm

o.d.

5 mm

o.d.

-

in-walled

rotor

‘nmr’

tube

sleeve ‘nmr’

tube

FIG. 1. Schematic diagram of the sample cell constructed for use with Jeol GX270 and Bruker MSL 300 NMR spectrometers. The Teflon rotor is the standard 10 mm sample spinner supplied by the spectrometer manufacturers. The d, and d2 values for the GX270 (Oxford Instruments SCM 270/89) are respectively 575 and 160 mm, while the corresponding values for the MSL 300 (Oxford Instruments SCM 300/89) are 750 and 160 mm.

radii of the outer wall and partition tubes in the upper part of the cell were chosen so as to give an inner annulus volume about one-half that of the outer annulus. The upper part of the cell was insulated by enclosure in 48 mm o.d. Insul Tube (Kenmore) and the lower part by the probe dewar. The Insul Tube insulation also prevents excessive lateral movement of the cell when locating it in the room-temperature bore of the solenoid magnet. The weight of the cell is borne by a cell holder, mounted into the top of the room-temperature bore of the magnet. The B34 groundglass cone (see Fig. 1) fits into a corresponding socket machined in the cell holder. The vertical position of the cell is adjustable within the range f 1 cm: this enables the

TEMPERATURE-DEPENDENCE

SAMF’LE

CELL

95

“Teflon rotor” on the 10 mm NMR tube to be precisely located in the spectrometer probe. Experimental samples are contained in 5 mm od. NMR tubes (typically 5 cm long), inserted into a phosphor bronze sleeve mounted on the lower end of the extension tube. The upper part of this tube is secured to the PTFE cap by means of a chuck. The cap was machined to fit securely into the B 19 socket and is secured to the cell by means of the QC 28 plastic screw cap. The basic cell design can be readily adapted to a wide range of NMR spectrometers using either solenoid or iron magnets. The only variables are the dimensions d, and d2 and the diameters of the outer wall and separation tubes of the upper part of the cell. For our work with liquid crystals there is no reason to rotate the sample about an axis colinear with the direction of the applied magnetic field and so a samplerotation facility has not been incorporated in the cells used with the solenoid magnets. For high-resolution experiments, we believe that spinning could best be achieved by fitting appropriate bearings and water-driven turbines to the central extension tube (IO). For liquid-crystal studies we have incorporated a sample “spinner” into cells designed for use in the iron magnet systems of the JeolFX60 and the Bruker HX-90; a detailed schematic diagram is shown in Fig. 2. The spinner was driven by a stepping motor. Temperatures were measured by means of a copper/constantan thermocouple (Delristor Ltd.). Over the temperature range 273 to 365 K, the temperature of the fluid leaving the cell was found to be within a few millikelvin of that entering the cell, as indeed were the temperatures throughout the entire cell. In usage, the temperature was monitored by placing the thermocouple 10 cm above the actual sample as shown in Fig. 1. The potential of this thermocouple with respect to a similar reference thermocouple placed in an ICELL MkII (Dehistor Ltd.) was measured using a Keithly Model 18 1 digital nanovoltmeter. Thermocouples were calibrated at 2 K intervals using a quartz oscillator thermometer (Hewlett-Packard 280 I A). The cryothermmtat. The choice of the cryothermostat is crucial to the system performance. The cryostat fluid (water) must have a volume many times greater than that contained in the external flow circuit ( -250 cm3), and the pump must generate a pressure sufficient to provide the requisite flow rate through the cell. It is also desirable that the system respond to changes in set temperature within a few minutes. Fastresponse systems, such as the SODEV Model CT-L circulating thermostat, which is capable of a temperature stability of kO.002 K, are also small-volume systems ( -200 cm3), which are totally unsuited for the present purpose. Large-volume systems, on the other hand, have an inherent large thermal inertia which generates excellent temperature stability but very slow response to temperature change. Thus, a compromise is necessary. We have found two commercial cryothermostats to be suitable for use with the NMR cell. One of these thermostats, the Colora WK3 (Colora Messtechnik GmbH) cryothermostat, has a temperature range of 263 to 383 K, a working temperature stability of ltO.005 K (cf. specifications, +O.OlO K), a bath volume of 5 dm3, and circulation rate of 18 dm3 min-’ for unimpeded flow (water). The flow rate (water) through the cell was 1.5 dm3 min-’ at 300 K. With the Colora, as supplied, temperatures are selected by course and fine analog control knobs and it is difficult to reproduce temperature settings. An external digital control system (see below) has therefore been

96

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shnp for

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designed to circumvent this problem. To implement this control it is necessary to modify the Colora control circuitry as shown in Fig. 3. The other is the Techne TU- 16D (Techne Laboratory Equipment) cryothermostat, which gives a flow rate through the cell of 1 dm3 mm’ at 300 K, has a bath volume of 8.0 dm3, and has a stability of +0.005 K. The temperature response is similar to that for the Colora, but the operating temperature range (233 to 474 K) is much greater. Temperatures can be selected by an analog control knob to a precision of 0.1 K, but in practice this control is overridden by the external control system for which an appropriate input socket is available on the bath as supplied. External temperaturecontrolunit. For external and digital control of temperatures a control circuit was constructed in which the control voltage is provided by a digitalto-analog converter (DAC) as shown in Fig. 4. Input to the 16-bit DAC (AD DAC7 lCSB-V) is by PC or by a manual digital control, which utilizes four thumb-wheel hexadecimal switches. The output from the DAC is fed to two amplifiers: one is

TEMPERATURE-DEPENDENCE

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SWIA

t3/

TAA 761A

I^ FIG. 3. Modifications (bold lines) of the control circuitry for the Colora cryothennostat necessary to facilitate external control of temperature. A two-pole toggle switch, 1SW1,allows selection of internal or external control. In the configuration shown, temperature control is via the external digital temperature control unit shown in Fig. 4. The control voltage (output from LH0044; Fig. 4b) is input at A and replaces the internal control potentiometers. The external reference voltage from the LM3 17T (Fig. 4b) is input at B and replaces the internal Zener diodes. This latter modification results in an improvement in stability of the reference voltage by a factor of two.

configured to provide the requisite control voltage range for the Techne (0 to 1.2 V) and the other the corresponding voltage range for the Colora (0.5 to 2.8 V). The external control unit forms the basis of an automatic control system which has been developed for use with the Bruker MSL 300. The Aspect 3000 spectrometer computer was interfaced to a Dell System 200 PC via an RS 232 serial interface. The PC was fitted with a multidigital input-output card and ,aNational Instruments IEEE 488 for communication with the digital temperature control unit and the Keithly Model 18 1 digital nanovoltmeter, respectively. A Pascal program was written for the Aspect 3000 to supervise temperature control and measurement and to initiate spectral accumulation and data storage. Through this program it is possible to select single temperatures (or a series of single temperatures) or to automatically step the temperature over any range within the operating limits of the cryothermostat. A selected temperature is first transmitted to the PC, where it is converted into the corresponding hexadecimal number (obtained from an initial calibration of the system) for input to the DAC. Following a preset delay (usually 10 minutes so as to allow thermal equilibrium to be attained), the sample thermocouple is read and a signal sent to the Aspect 3000 to initiate spectral accumulation and subsequent data storage. SYSTEM

PERFORMANCE

Background. The performance characteristics of the system are now demonstrated by illustrative experiments on the micellar liquid crystal CsPFO/water ( 14, 15). A partial phase diagram for the heavy water system is given in Fig. 5. The feature of especial significance for these studies is the existence of a diamagnetically positive micellar nematic N& (characterized by long-range orientational ordering of discotic,

98

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ET AL.

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AN AkJE CONVERTER

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BUFFER TO ZOMPUTER I

To COLORA 3.W

+15v

REF-

I -15v u

CONNECTOR

b

II u---

FIG.

4. Block diagram (a) and circuit diagram (b ) for the external digital temperature control unit.

i.e., oblate ellipsoidal, micelles) phase over a wide range of concentration (0.225 to 0.632 weight fraction w of CsPFO) and temperature (285.5 to 351.2 K): this lies between an isotropic micellar solution phase I to higher temperatures/lower concentrations and a discotic micellar lamellar phase Ln to lower temperatures/ higher concentrations. The nematic director n undergoes spontaneous alignment along the direction of the magnetic field of the spectrometer to give macroscopically aligned N& or LD phases. For these uniaxial mesophases, the first-order spectrum for any spin I > 4 will consist of 21 equally spaced lines with separation, A;( I$), referred to as the partially averaged quadrupole splitting, given by (14)

TEMPERATURE-DEPENDENCE

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360

W

FIG. 5. Partial phase diagram ofthe CsPF0/*H20 system. Nomenclature: K, crystal; Lo, discotic lamellar phase; N&, discotic nematic phase with positive diamagnetic susceptibility anisotropy; I, isotropic micellar solution.

3 = 21(2Z - 1) IcizzIs~P,(cos $1,

*8@)

where 9 is the angle between II and the magnetic field B and S is the second-rank orientational order parameter representing the ensemble average of the orientational fluctuations of the micellar axes with respect to n . For “‘3Cs+ ions (I = z), I&zls = (mcos 4)sks”ks, 121 where Xc5 is the ‘33Cs quadrupole coupling constant and fc, the fraction of cesium ions bound to the micelle. The quantity ( P2( cos LI))~ ( = ( { cos’a! - i)s), where CY is the angle between the normal to the surface and the symmetry axis of the micelle and the angular brackets denote an average over the surface, accounts for the diffusive motion of the ions over the surface of the micelle. The ‘33Cs spectrum will, therefore, consist of seven equally spaced lines of relative intensity 7: 12: 15: 16: 15: 12:7 and separation *c(4)

= +A%zls=‘~(cos

d).

[31

For 2H (I = 1) in 2H20, I&Is

=

(~2@s

4)s~D(~*lxw)~~b~oo,

[41

where X0 is the quadrupole coupling constant for a water molecule, xA and xw are, respectively, the mole fractions of amphiphile and water, $, is the number of water molecules bound to each amphiphile molecule, and So,, is an “order parameter” representing the averaging due to the local reorientational motion of these water molecules. The 2H spectrum will consist of a doublet with separation *F(4) = %LlsSpZ(cos $). [51 These simple multiplet spectra make these resonances especially suited to monitoring the subtle changes in structure and ordering on approaching phase transitions ( 14-

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ET AL.

16) and, as such, are particularly sensitive to temperature and can therefore be used to monitor temperature stability and homogeneity within the sample. Figure 6 shows the appearance of the 133Csspectrum for a CsPFO/H20 (w = 0.55) sample in the I phase (S = 0), the macroscopically aligned (4 = 0”) N& phase (S > 0) obtained by cooling the sample in the magnetic field, and, for comparison, an unaligned Ln phase obtained by cooling the sample from the I phase outside the magnetic field. The separation of the Pake singularities in the unaligned spectrum is one-half of that of an aligned sample with Q, = 0” at the same temperature. The temperature dependence of A;( 0” ) , observed on cooling a CsPFO / ‘Hz0 ( w = 0.502 ) sample from the I, through the N&, and into the Lo phase (see Fig. 5), is shown in Fig . 7 . ‘33Cs quadrupole splittings and their temperature dependence are an order of magnitude greater than those for the corresponding ‘H quadrupole splittings at w = 0.502 (15). The various phase transition temperatures are readily identified from the discontinuities in A;( 0” ) . The major factor determining the variation of A;( 0” ) with

T/K

(a)

323.280

(b)

(cl 316.023 I

15

3 8 10I ’ ”

II

5

u ,,a0

4

v/kHz T--x--x

FIG. 6. 133Csspectrum for a CsPFO/H20 (w = 0.55) sample in: (a) the I phase (S = 0), (b) the macroscopically aligned ( I$ = 0” ) N 6 phase (S > 0), and (c) the unaligned Lo phase. The sharp welldefined peaks of the aligned sample enable the quadrupole splitting to be measured with high precision (+O. I Hz) in both the nematic and the lamellar phases. The unaligned sample was obtained by cooling from the isotropic phase outside of the magnet. The Ln phase has an infinite twist viscosity coefficient and reorientation of the director does not occur when the sample is placed in the magnetic field of the spectrometer. The above spectra were obtained on a Jeol GX270 spectrometer at an operating field of 6.3 T.

TEMPERATURE-DEPENDENCE

t 314

II 1111 316

SAMPLE

1

’

318

320

CELL

101

322

T/K

FIG. 7. Temperature dependence of the ‘33Csquadrupole splittings in a CsPFO/*HrO (w = 0.502) sample on cooling from the isotropic phase I, through the nematic phase N 6, and into the lamellar phase Lu (see Fig. 5). The dashed lines indicate the upper and lower limits to the kotropic/nematic coexistence regime [ TrN(32 1.15 ( 1) K) and TM (320X20( 5 ) K), respectively], and the upper and lower limits to the nematic/ lamellar coexistence regime [ TNL (315.628( 5) K) and TLN (31552Ot 5) K), respectively].

temperature is an increase in S, but there are smaller contributions from increases in both ,& and ( P2( cos (Y))~ stemming from the growth in the diameter of the micelle as the temperature is lowered ( 14, 15 ) . The absolute change in A;( 0” ) is large as a consequence of the large value of xcs ( = 300 kHz). The downward turn in A;( 0” ) in the Lo and the N& phases on approaching T,, and T NI, respectively, is indicative of strong pretransitional effects, as is the upturn in the N& phase on approaching TNL. In these regions, the quadrupole splittings are particularly sensitive to temperature and provide a good test of sample temperature control. Test of absolute temperature stability and response @-system to changes in temperature. To test the response of the temperature controller and its stability, a CsPFO/ ‘HZ0 ( w = 0.502) sample, of length 23 mm, was first cooled to 3 19.6 K in the nematic phase. The set temperature was then abruptly increased to 320.6 K, at which sAE/sT is - 1.45 kHz K-l, and the quadrupole splitting monitored as a function of time (Fig. 8). The set temperature is seen to be reached within five minutes, without any overshoot. Over a period of one hour, the quadrupole splitting was found to be within the range 2088 t 7 Hz, which translates to a temperature stability of kO.005 K at 320.6 K. Similar experiments were carried out on nematic phase samples over the temperature range 278 K (W = 0.15) to 350 K (W = 0.63). At all temperatures, within this range, the temperature stability was measured to be kO.005 K. Test of temperature homogeneity. Phase transition temperatures in micellar liquidcrystal systems are conveniently determined from discontinuities in the temperature dependence of the quadrupole splittings (see, for example, Fig. 7). The precise determination of these discontinuities requires temperature homogeneity across the entire sample, in addition to absolute temperature stability. Temperature gradients give rise to a distribution of quadrupole splittings, i.e., line broadening, and, in addition, to

102

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ET AL.

2.4

2.0 0

10

20

30

40

50

60

t/min FIG. 8. Time dependence of 133Csquadrupole splitting for the sample CsPF0/*H20 ( w = 0.502) following a change in the set temperature from 3 19.60 to 320.60 K: 6AS,‘dT at 320.60 K is - 1.45 kHz K-’ .

chemical potential gradients, which cause diffusion of surfactant and water and hence give rise to concentration gradients. Both ‘33Cs ( 15 ) and ‘H ( 14) quadrupole splittings are sensitive functions of surfactant concentration. The presence of a temperature or a concentration gradient (or both) in the sample will “smear out” the discontinuities in the slope of the A”v(0”) vs temperature curve and reduce the precision with which phase transition temperatures can be measured. To illustrate the effects of temperature gradients, the ‘33Cs spectrum of a sample with w = 0.502 and length 23 mm was recorded in the Lo phase at 315.50 K, first using the temperature control system described berein and then using the standard air-flow system of a Jeol GX270 NMR spectrometer with the set temperature adjusted to give similar quadrupole splittings: the two spectra are compared in Fig. 9. The substantially larger linewidths of the satellite peaks in Fig. 9b are a direct indication of the presence of temperature gradients in the case of the gas-flow system. The width of the inner satellite peaks ( m = + 4 ( - i) to m = + $ ( - 3) transitions) at half-maximum amplitude are 100 Hz as compared with 18 Hz for the corresponding peaks in Fig. 9a. At 3 15.50 K, 6AS/6T for these transitions is - 1.20 kHz K-l, so that the additional contribution to the linewidth corresponds to a temperature gradient of =30 mK cm-’ along the long axis of the NMR tube. The broadening of the m = +$ (-3) to m = + 2 ( - i) and the m = + 2 ( - 4) to m = + 5 ( -z) transitions is, respectively, twice and three times that of the m = +i ( - 4) to m = +i ( -$) transitions and simply reflects the larger splittings. The central Zeeman peak (m = - 1 to m = + f ) is unaffected by temperature gradients, because the 133Cschemical shifts are a relatively weak function of temperature. The question that needs to be addressed is whether the 18 Hz linewidths observed for the m = +t (-4) to m = +$ (-1) transitions represent the natural linewidths or whether there is still a contribution from a small temperature gradient in the sample. Line broadening arising from temperature /concentration gradients can be distinguished from the intrinsic contributions by examining the effect of a temperature

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I

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(a) !.1; L

FIG. 9. 13”Csspectra of a CsPFO/*HZO (w = 0.502) sample at 315.50 K as measured using: (a) the waterflow system and (b) the standard air-flow system of a Jeol GX270 spectrometer. The linewidths at halfmaximum amplitude for the m = +i (-4) to m = +{ (-i) transitions are 18 and 100 Hz for a and b, respectively. The line broadening in b is the result of a temperature gradient of ~30 mK cm-’ along the length of the sample.

gradient in the presence of magnetic field gradient along the long axis of the sample tube (z axis in a solenoid magnet). The effect is most often apparent in 2H spectra of heavy water, since their intrinsic linewidths are smaller tlhan those of the corresponding ‘33Cs spectra: 1.5 Hz at 320.50 K for a w = 0.502 sample as compared with 18 Hz for the m = +i ( -i) to m = +i ( -i) transitions of 13’Cs. The effect of a temperature gradient alone can be seen by comparing the 2H spectra in Figs. 10a and lob, which correspond respectively to the ‘33Cs spectra in Figs. 9a and 9b. These spectra were obtained immediately following thorough mixing of the samples in the isotropic phase so as to eliminate concentration gradients as a possible cause of line broadening. When the gas-flow system is used, the *H splittings steadily increase from the bottom (warmer end) to the top (cooler end) of the sample tube as a consequence of a 30 mK cm-’ temperature gradient, and this results in linewidths greater (Fig. lob) than those obtained using the water-flow system (Fig. 10a). The line broadening is not as pronounced as that for the ‘33Cs spectra (Fig. 9b), a consequence of the smaller quadrupole splittings of the 2H spectra.

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(a)

FIG. 10. ‘H spectra of heavy water for the CSPFO/~H~O ( w = 0.502) sample at 315.50 K as measured using: (a) the water-flow system and (b to d) the standard air-flow system of a Jeol GX270 spectrometer. The linewidths in b are 4.0 Hz (cf. I. I Hz in a) as a result of a temperature gradient along the z axis (long axis) of the sample. The asymmetric spectra of c and d are, respectively, caused by the application of a positive and negative magnetic field gradient along the z axis of the sample.

The symmetric spectrum of Fig. lob was obtained after carefully adjusting the zhomogeneity coil, so as to minimize magnetic field gradients along the sample. Figure 1Ocshows the effect of applying a positive field gradient along the z axis (field increasing from bottom to top). This causes a displacement of the components having the larger quadrupole splittings (top of sample) to higher frequencies of those having the smaller splittings (bottom of sample). The net result is a narrowing of the low-frequency component of the doublet and a broadening of the high-frequency component. The opposite effect is observed when a negative field gradient is applied (Fig. 10d). Now, it is the components having the smaller quadrupole splittings which are displaced to high frequencies relative to those having larger splittings and this results in a sharpening of the high-frequency component of the doublet relative to the low-frequency one. This effect requires the presence of concomitant magnetic field and quadrupole splitting gradients. When the water-flow system was used for temperature control, no differential

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105

linewidth changes were observed on application of a magnetic field gradient along the z axis; i.e., adjustment of the z-axis shim field produced the same effect on the linewidths of both components of the spectrum. From the air control results, an estimated temperature gradient of 30 mK cm-’ results in differential linewidth changes of about 4 Hz. Linewidth changes of the order of 0.05 Hz can be readily observed, so the absence of any differential linewidth changes in the spectra obtained using the water-flow system implies that temperature gradients are ~0.4 mK cm-‘. Thus, any gradients of temperature lie well within the limits of absolute temperature stability. A demonstration ofperformance. The combination of absolute temperature stability and, essentially, the absence of temperature gradients is mandatory for studies of phase transitions in lyotropic systems. A particularly demanding test of the system is provided by its use in investigations of magnetic-field-induced pretransitional orientational ordering of the micelles in the isotropic phase of the CsPFO/water system on approaching the isotropic-to-nematic transition. This phenomenon is revealed within only some 80 mK (at the magnetic field of the experiment, 6.34 T) of the transition and its quantitative study requires temperature resolution and control well within this range. Figure 11 shows the sequence of 2H spectra observed on cooling a sample, with w = 0.200, from the isotropic phase, across the isotropic/nematic biphasic region, and into the nematic phase. Actually, for this particular sample, the nematic phase is monotropic, as can be seen from the phase diagram in Fig. 5. At high temperatures, the spectrum is a single line with a width of 1.2 Hz. As the temperature is lowered, the line broadens (a) and then splits into a doublet whose separation increases rapidly in magnitude (b and c). The appearance of the outer doublet (d) shows that the temperature is below that of the upper limit to the nematic phase TIN and the sample is in the isotropic/nematic phase coexistence regime. As the temperature is further lowered, the relative amount of the nematic phase increases until, at the lower boundary to the transition TNI, the system is entirely nematic ( f). The quadrupole splittings from both the isotropic and nematic phases are roughly constant in the biphasic regime. The splitting of the signal in the isotropic phase into a doublet arises from the orientational ordering of the micelles induced by the magnetic field. It has exactly the same origin as field-induced optical birefringence ( 17,18). It arises from the magnetic torque acting on the micelles, a torque which is enhanced by the buildup in the angular correlations of the micelles as the transition is approached. The splitting is given by (19)

where Ax is the anisotropy in the magnetic susceptibility of a micelle and T* is the extrapolated supercooling limit of the isotropic phase. Since both AX and I qzzI s are essentially constant over the temperature range of the experiment, a plot of AC-’ versus temperature should be a straight line which intercepts the Taxis at T*. Moreover, the discontinuities in the temperature dependence of Ai-.’ at TINenable the temperature gap TIN- T* to be precisely determined (Fig. 12). The temperature TN,of the nematic to isotropic phase boundary is similarly determined from the discontinuity in the temperature dependence of the splitting of the nematic phase signal (Fig. 7). The precision with which these small temperature gaps can be determined [i.e., TIN- T*

106

BODEN

283.029

ET AL.

)

283.008

i_

cb)

A

cc)

282.980 &L

(d)

282.9121 I

40

,I, ’

1

,

20

0 v/Hz

8

co I

-20

-40

FIG. I 1. ‘H NMR spectra of ‘Hz0 observed on cooling the sample CsPFO/‘H,O ( w = 0.200) from the isotropic I phase (a-c), through the isotropic/nematic coexistence region I/N& (d and e), and into the nematic phase N& (f). Field-induced ordering is shown by the appearance of a quadrupolar splitting of the isotropic signal. The spectra were obtained on a Jeol GX270 spectrometer at an operating field of 6.3 T.

= 0.013(2) K] is a direct illustration system.

of the performance of the temperature

control

CONCLUSIONS

The sample temperature control system which we have described has a number of attractive features: it is inexpensive, both to set up and in use, it is stable, accurate, and portable, and it is easily adapted for use with a.ny commercial NMR instrument by a simple modification of the dimensions of the glass sample cell. Moreover, it can be readily used without any interference with the spectrometer, making it usable on “service” instruments. Temperature control at the millikelvin level is achieved with an absolute stability and precision of 50.005 K using commercially available water cryostats. This level of control is obtainable with special air-flow systems (6, 8). The advantages of our water-flow system is the absence of temperature gradients across

TEMPERATURE-DEPENDENCE

SAMPLE

CELL

8.0

0.8

6.0

0.6

282.98

283.00

283.02

283.04

107

283.06

T/K FIG. 12. Temperature dependence of the field-induced 2H quadrupole splittings of 2H20 (closed circles) and their inverse (open circles) for the sample CsPFO/‘H20 ( w = 0.200) in the isotropic phase I and the isotropic/nematic biphasic region I/N;. The quadrupole splitting diverges with decreasing temperature, but the divergence is quenched upon entering the mixed phase region. The discontinuity in the temperature dependence of the Ai-’ vs temperature plot clearly identifies Tnu [28X006( I ) K] and the intercept on the temperature axis obtained from extrapolation of A;-’ to zero gives T” [ 282.993 ( 1) K]. The quoted errors in TIN and T* are statistical errors in the slope and intercept of the ~101and cover a 95% confidence interval.

the sample over a wide temperature range. This feature is essential for studies of phase transitions in liquid crystals as described herein. Many of the changes in spectra in the vicinity of phase transitions which have been described and interpreted in this paper would be seen only fleetingly, if at all, using conventional gas-flow systems. The absolute stability and consequently temperature resolution could be further improved by employing more sophisticated cryostats and improving the thermal insulation of the cell- and fluid-transfer hoses. The working temperature range can also be extended by appropriate choice of cryogenic fluid and cryostat. For our experiments on liquid crystals in solenoid magnets, sample spinning was unnecessary. However, for high-resolution experiments on liquids, sample spinning could easily be provided by incorporation of appropriate bearings and fluid-flow-driven turbines ( IO). Finally, we mention that the fluid-flow temperature control system described here can be readily adapted for use with other kinds of experiments. For example, we have used it in electrical conductivity and X-ray studies of liquid crystals ( 20-22). ACKNOWLEDGMENTS We thank Colora Messtechnik GmbH for supplying diagrams ti the control circuitry of their WK3 cryothermostat. We also thank the Science and Engineering Research Council for financial support and for a research studentship to SAC. REFERENCES 1. A. ALLERHAND, H. S. GUTOWSKY, J. JONAS, AND R. A. MEINZEF:, J. Am. Chem. Sot. 88,3 2. R. R. SHOUP, E. D. BECKER, AND M. L. MCNEEL, J. Phys. Chew. 76,71 ( 1972).

185 ( 1966).

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ET AL.

H. VANNI, W. L. EARL, AND A. E. MERBACH, J. Magn. Reson. 29, 11 ( 1978). A. B. CHANCE AND S. J. KOHLER, J. Magn. Reson. 45,352 ( 198 1). N. YOSHIDA, M. KUMANO, AND T. IKENONE, Rev. Sci. Instrum. 52,382 ( 1982). R. L. VOLD AND R. R. VOLD, J. Magn. Reson. 55,78 ( 1983). T. C. FARRAR, E. SIDKY, AND J. D. DECAIIJR, J. Mugrz. Reson. 86,605 ( 1990). S. R. MAPLE AND A. ALLERHAND, J. Magn. Reson. 66, 168 ( 1986). P. J. S~HULE AND V. H. SCHMIDT, Rev. Sci. Instrum. 53, 1724 ( 1982). 10. W. DIETRICH, B. FROHLICH, AND G. BERGMAN, J. Magn. Reson. 40,5 19 ( 1980). 11. D. J. GREENSLADEAND M. JUDD, J. Phys. E6, 101 (1973). 12. D. S. WEBSTER AND L. J. LYNCH, J. Phys. E 1, 1253 (1968). 13. L. D. LANDAU AND E. M. LIFSHITZ, “Course of Theoretical Physics. Vol. 6. Fluid Mechanics,” Pergamon Press, Elmsford, New York, 1959. 14. N. BODEN, S. A. CORNE, AND K. W. JOLLEY, J. Phys. Chem. 91,4092 (1987). 15. N. BODEN, K. W. JOLLEY, AND M. H. SMITH, Liq. Crystals 6,481 ( 1989). 16. N. BODEN, J. CLEMENTS, K. W. JOLLEY, D. PARKER, AND M. H. SMITH, J. Chem. Phys. 93, 9096 3. 4. 5. 6. 7. 8. 9.

(1990). 17. C. ROSENBLATT,

S. KUMAR, AND J. D. LITSTER, Phys. Rev. A 29, 1010 (1984). 18. C. ROSENBLATT, Phys. Rev. A 32, 1924 (1985). 19. K. W. JOLLEY, M. H. SMITH, AND N. BODEN, Chem. Phys. Lett. 162, 152 ( 1989). 20. N. BODEN, S. A. CORNE, AND K. W. JOLLEY, Chem. Phys Lett. 105,99 ( 1984). 21. N. BODEN, S. A. CORNE, M. C. HOLMES, P. H. JACKSON, D. PARKER, AND K. W. JOLLEY, Paris 47,2135 (1986). 22. N. BODEN, D. PARKER, AND K. W. JOLLEY, Mol. Cryst. Liq. Cryst. 152, 12 1 ( 1987 ).

J. Phys.