Measurement of self-diffusion coefficients in extractant systems using the FTNMR technique

HydrometaUurgy, 19 (1987) 129-134

129

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Technical Note

M e a s u r e m e n t of Self-Diffusion Coefficients in Extractant S y s t e m s using the F T - N M R Technique T. WARNHEIM

Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm (Sweden) and E.Y.O. PAATERO

Laboratory of Industrial Chemistry, fibo Akademi, SF-20500 ~tbo (Finland) (Received January 21, 1987; accepted in revised form March 7, 1987)

ABSTRACT Wiirnheim, T. and Paatero, E.Y.O., 1987. Measurement of self-diffusion coefficients in extractant systems using the F T - N M R technique. HydrometaUurgy, 19: 129-134. The Fourier-transform NMR pulsed field gradient spin-echo method was applied to the measurement of the self-diffusion coefficients of the active components in the commercial extractants LIX65N, LIX63 and Cyanex 272. CC14 and CDC13 were used as solvents. The self-diffusion coefficients in 0.16 M CDC13 solutions are 8.2, 9.2 and 6.7 ><10- lo m 2 s - 1, respectively.

INTRODUCTION

In the modelling and description of liquid extraction processes, a knowledge of the diffusion coefficients of the different species in the system is needed. Conventional techniques for selectively measuring the diffusion coefficients in a multicomponent system are generally laborious and time-consuming; for measuring the self-diffusion coefficient of a component with tracer techniques tedious isotopic labelling is required [1]. N M R techniques are a straightforward alternative for these measurements. During the last few years, the FTN M R pulsed gradient spin-echo method has evolved as a rapid and accurate method for the measurement of self-diffusion coefficients [ 2 ]. In this technical note, the method is briefly discussed and it is applied to the measurement of the self-diffusion coefficients of the following common extractants: 2-hydroxy-5-nonylbenzophenone oxime (abbreviated as HNBPO, the active compound in LIX65N), 5,8-diethyl-7-hydroxy-6-dodecanone oxime ( abbreviated as DEHO, the active compound in LIX63) and bis ( 2,4,4-trimethylpentyl) phosphinic acid (abbreviated as B T M P P A , the active compound in Cyanex 272). 0304-386X/87/$03.50

© 1987 Elsevier Science Publishers B.V.

130 BACKGROUND The use of NMR as a tool for determining self-diffusion coefficients was, in fact, started in the early fifties. The spin-echo experiment [ 3 ], which is a direct method for measuring the nuclear spin-spin relaxation time T2, may also contain information on the diffusion coefficient; the spin-echo attenuation in a sample is a function not only of T2 but also of the magnetic field inhomogeneity and the diffusion coefficient D of the component. The technique was later developed by using pulsed gradients, i.e. gradients set up only during a part of the spin-echo experiment [ 4 ]. The echo attenuation becomes, for a linear gradient g,

A/Ao = e x p ( - 2~/T2)exp [ -~2g2dZD(A-~/3)]

(1)

where A is the amplitude with and Ao without field gradient, y is the magnetogyric ratio and d, A and ~ are time parameters in the experiment, explained in Fig. 1. It is not possible to employ the simple spin-echo experiment in multicomponent systems, since the echo contains contributions from all the species in the system. However, a Fourier transformation of the echo gives a frequency-separated spectrum analogous to normal NMR spectra. This is, of course, the case for the pulsed gradient spin-echo experiment as well. Thus, the self-diffusion coefficients of all fully resolved components of the spectrum can be determined; this was demonstrated for the first time at the beginning of the seventies [ 5 ]. However, the method was not put into practical use until the end of the seventies, when it was demonstrated that a standard high resolution FT-NMR spectrometer can be rather easily modified for the method [6]. In a typical low-viscosity solution with reasonably high concentrations of all components ( > 1% ), the self-diffusion coefficients can be determined in ten minutes with an accuracy of up to one percent [ 2 ]. The present set of measurements has been performed to demonstrate the feasibility of using the method in extractant systems. Low concentration of the component to be measured can in some cases be troublesome, since a large solvent peak may distort the baseline in spite of the fact that the chemical shifts of solvent and solute are widely different. Thus, in dilute systems, the measurements must in some cases be performed in deuterated solvents, which normally are commercially accessible at a reasonably low price. EXPERIMENTAL

Chemicals The pure anti-2-hydroxy-5-nonylbenzophenone oxime (HNBPO) was isolated from LIX65N (Henkel Co. ) as a copper complex by a method described

131

A 2T Fig. 1. The experimental arrangement of a pulsed gradient spin-echo experiment. The pulsed gradient duration is fi and the time between the pulses is A (see eqn. 1 ).

previously [ 7 ]. The free oxime was prepared by dissolving the complex in the solvent in question and stripped with sulfuric acid and washed with water. 5,8Diethyl-7-hydroxy-6-dodecanoneoxime (DEHO) was isolated as a solid from LIX63 (Henkel Co.) following the method of Tammi [8]. Bis(2,4,4-trimethylpentyl)phosphinic acid (BTMPPA) was isolated from Cyanex 272 (American Cyanamid Company) via its copper complex. The purity was controlled by gas chromatography analysis and the concentration of the free phosphinic acid in the solution was determined by potentiometric titration. CDC13 (Fluka, >99.8% D) and CC14 (Merck, >99.5%) were used without further purification.

Di[fusion measurements The self-diffusion coefficients were measured using the pulsed field gradient spin-echo FT-NMR method [2,6,9]. The measurements of the oximes were performed on a Bruker CXP-100 spectrometer, as described in Ref. [9], operating at 90 MHz for 1H, at the ambient probe temperature of 26.5 _+0.5 ° C. In the Cyanex 272 system the measurements were performed on a Jeol FX-100 at 99.96 MHz and 25 ° C. The diffusion coefficients were evaluated by a non-linear least squares curve fitting to the relation of eqn. (1). A was kept constant at 140 ms or 100 ms and 5 was varied between 20 and 95 ms (cf. Fig. 1 ). The gradient was calibrated using values for pure solvent systems [10,11]. A typical standard deviation in an experiment for a self-diffusion coefficient fitted to eqn. (1) is 3%. RESULTS AND DISCUSSION

In Figs. 2 and 3 some measured self-diffusion coefficients of the oximes are shown. Figure 2 displays the self-diffusion coefficients for HNBPO in two solvents, CC14 and CDC13, at varying concentrations of the solute. In Fig. 3 the corresponding data are shown for DEHO. There is in all cases a slight decrease in the self-diffusion coefficients with

132

14 m2 x 16 TM ~12

!0

8(

D 6

~"

~

CC14

4

2

0

i

I

I

0.04

0

I

I

0.08

1

I

0.12

I

0.16 kmol m3

C Fig. 2. ( • ) The concentration dependence of the self-diffusion coefficient of HNBPO dissolved in CDC13 and in CCI4. ( O ) The self-diffusion coefficients calculated using the Wilke and Chang correlation at infinite dilution.

increasing concentration of oxime, in the order of ten percent when increasing the concentration from 0.028 k m o l / m 3 to 0.16 k m o l / m 3. There is a difference between the two species in that H N B P O diffuses slower in both solvents compared on a molar basis. However, the main effects are clearly due to the solvent: 14 x 10-10 m2

121 10

8 {

D 6

CC14

T 4

2

0

i 0

I 0.04

I

I

I

0.08

I 0.12

i

I 0 16 •

kmol m3

C Fig. 3. ( • ) The concentration dependence of the self-diffusion coefficient of DEHO dissolved in CDCl~ and in CCI4. ( O ) The self-diffusion coefficient calculated using the Wilke and Chang correlation at infinite dilution.

133 TABLE 1 Experimental diffusion coefficients for the extractants at a concentration of 0.160 kmol/m3 Solvent

Solute

D (10- ,0 m2/s)

CC14

HNBPO DEHO BTMPPA

3.9 _+0.3 5.O+ O.3 4.5 _+0.2

CDC13

HNBPO DEHO BTMPPA

8.2 __0.3 9.2 + 0.2 6.7 +_0.1

the molecules diffuse 40-50% faster in CDC13 than in CC14. The physical background to this behavior is complicated, but can be phenomenologically ascribed to two factors. Firstly, the oximes remain essentially monomeric in CDC13 while they self-associate in CC14 [ 7 ], suggesting a slower diffusion in the latter solvent. The other factor is of course the properties of the solvents themselves; diffusion of solvent and diffusion of solute molecules are coupled [ 1,12 ]. Similar trends can be found in the viscosity of the systems as well. This has formed the basis for different types of empirical correlations for calculating the diffusivities from macroscopic properties of the solvent and the solute. It is of methodological interest to compare the measured self-diffusion coefficients with those from a conventionally used correlation, such as the one by Wilke and Chang [ 13 ]. This correlation gives the mutual diffusion coefficient which is not directly comparable with the self-diffusion coefficient; however, in the limit of infinite dilution, the two should coincide [ 1 ]. W h e n such an extrapolation is carried out, the values become close (Figs. 2 and 3 ). However, the accuracy of the experimental data in the dilute region does not allow a complete evaluation of the difference. In Table 1 the self-diffusion coefficient of B T M P P A at 0.160 kmol/m 3 is shown. In CC14 the diffusion coefficients decrease in the order D E H O > B T M P P A > H N B P O and thus follow the size of the molecules. In CDC13 phosphinic acid is slowest, which can be attributed to the fact that B T M P P A is in dimeric form even in this solvent [ 14 ], while the oximes occur as monomers. The self-diffusion coefficient of the phosphinic acid could also be determined directly in the unpurified Cyanex 272 in CDC13; in a 5 wt% solution ( [ B T M P P A ] = 0.098 k m o l / m 3 ) D = 7.5 + 0.9 × 10- lO m2/s. CONCLUDING REMARKS The F T - N M R pulsed gradient spin-echo method is a convenient and rapid method for determining self-diffusion coefficients. Provided that well-resolved peaks are obtained in the N M R spectra, a simultaneous determination of the

134 self-diffusion coefficients o f a large n u m b e r o f c o m p o n e n t s can be p e r f o r m e d . T h i s m a k e s it feasible to m e a s u r e t h e self-diffusion coefficients in real multic o m p o n e n t e x t r a c t a n t systems. S u c h efforts, c o u p l e d to a s o u n d d e s c r i p t i o n o f t h e t r a n s p o r t process, s h o u l d i n c r e a s e t h e possibility o f a c c u r a t e modelling of extractant system performance. ACKNOWLEDGEMENT P r o f e s s o r P e t e r Stilbs k i n d l y p e r f o r m e d t h e m e a s u r e m e n t in t h e C y a n e x 272 system.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Tynell, H.J.V. and Harris, K.R., Diffusion in Liquids, Butterworth, London, 1984. Stilbs, P., Progr. NMR Spectrosc., in press. Hanh, E.L., Phys. Rev., 80 (1950) 580. Stejskal, E.O. and Tanner, J.S., J. Chem. Phys., 42 (1965) 288. James, T.L. and MacDonald, J.G., J. Magn. Reson., 11 (1973) 58. Stilbs, P. and Moseley, M.E., Chem. Scr., 15 (1980) 176, 215. Paatero, E.Y.O., Hydrometallurgy, 11 (1983) 135. Tammi, T.T., Hydrometallurgy, 2 (1976/77) 371. W~irnheim, T., Thesis, Royal Institute of Technology, Stockholm, 1986. Mills, R., J. Phys. Chem., 69 (1965) 3116. Mills, R., J. Phys. Chem., 77 (1973) 685. Stilbs, P. and Hermansson, B., J. Chem. Soc., Faraday Trans. 1, 79 (1983) 1351. Wilke, C.R. and Chang, P., AIChE J., 1 (1955) 264. Li, K., Muralidharan, S. and Freiser, H., Solv. Extr. Ion Exch., 3 (1985) 895.