Water proton magnetic relaxation in silver iodide sols

Water Proton Magnetic Relaxation in Silver Iodide Sols In view of the importance o f AgI in studies of (a) ice nucleation and (b) colloid stability, there is considerable interest attached to the determination of the properties of liquid water adjacent to a silver iodide surface. There is early evidence (1, 2) to support the idea that a structured layer of water exists at the A g l / a q u e o u s solution interface, especially near the point of zero charge, and that it m a y " m e l t " around 50°C. In the first c o m m u n i c a t i o n from our laboratory (3) a small increase in water proton magnetic relaxation rates in AgI sols was attributed to s u c h a layer. H o w e v e r , the data referred only to room temperature and no attempt was made to derive quantitative information. We have n o w extended the relaxation m e a s u r e m e n t s to c o v e r the range 3-60°C and have obtained results which suggest that less than a m o n o l a y e r of water molecules associated with the surface of the particles is responsible for the effect. No evidence was found for a phase transition around 50°C. Silver iodide sols were prepared by the addition o f 0.1 M silver nitrate solution to 0.11 M p o t a s s i u m iodide solution followed by dialysis against frequently ren e w e d triply distilled water for 10 days in a dark room. This procedure reduces the e x c e s s iodide concentration to about 10 -~ M (pl 5) or below. Proton relaxation times (TI and Tz) were m e a s u r e d with a pulsed s p e c t r o m e t e r operating at 45 M H z (4). The temperature o f the sample probe was controlled by m e a n s of a gas flow s y s t e m and monitored by a thermocouple. At least 2 0 - 3 0 min were allowed for the sample to equilibrate after a change of temperature. Longitudinal relaxation times (T1) were obtained in the standard w a y from 180°-T-90 ° pulse s e q u e n c e s while transverse relaxation times (T2) were obtained from C a r r - P u r c e l l - M e i b o o m - G i l l trains with a 180° pulse period of 2 m s e c (5). Electron micrography of the samples was carried out using the freeze-fracture technique (6). The relaxation data are presented in Fig. 1. In contradiction with the earlier findings we did not observe any increase in the longitudinal relaxation rate in the sol but we did confirm the existence of a transverse relaxation rate e n h a n c e m e n t . T h e difference b e t w e e n T~ and T2 for the dialyzing m e d i u m is due to s p i n - s p i n splitting of the proton r e s o n a n c e by 170 which is only partially averaged out by proton e x c h a n g e (7). In considering the t r a n s v e r s e relaxation of water nuclei in h e t e r o g e n e o u s s y s t e m s Glasel and Lee (8) have argued that differences in magnetic

susceptibilities can produce large magnetic field inhomogeneities which dominate t r a n s v e r s e relaxation near to surfaces. Other a u t h o r s (9) have concluded that while line widths m a y be d o m i n a t e d by this effect, m e a s u r e m e n t s with a C P M G s e q u e n c e are unaffected. To test w h e t h e r the T2 relaxation rate was sensitive to the surface state of the Agl particles a sol at p l 3 was made by adding 1 cm a of 10 - 2 M KI to 9 cm 3 o f dialyzed 0.1 M AgI sol and this was c o m p a r e d with a sol m a d e from 1 cm a water + 9 cm 3 0.1 M AgI. The T2 values were 1.82 and 1.66 sec, respectively, which is in a g r e e m e n t with the observation that the structuring effect o f the AgI reduces as the sol m o v e s away from the point of zero charge (pl 10.5) (2). It appears likely therefore that the relaxation data do relate to the layer of water influenced by the surface and, as a basis for interpretation, we will adopt a simple model in which s u r f a c e - b o u n d water m o l e c u l e s are in d y n a m i c equilibrium with the bulk phase. In general w h e n rapid e x c h a n g e o f nuclei occurs b e t w e e n two states an average relaxation rate is o b s e r v e d (10). It is clear from the data that this requires the transverse relaxation time for the surface water protons (Tzs) to be very m u c h smaller than the bulk value. For water molecules associated with AgI particles long e n o u g h to s e n s e their motion, the magnetic dipolar interaction between the protons will be modulated at a frequency determined by the tumbling rate of the particles. B e c a u s e o f the colloidal d i m e n s i o n s of the particles their correlation times for reorientation (%) will be s u c h that too% "> 1 (where tOois the r e s o n a n c e frequency) for the protons w h e n T2s b e c o m e s very short and Tls b e c o m e s very long (ll). H e n c e a relaxation rate e n h a n c e m e n t is o b s e r v e d for T2 but not for T1. In a situation where a small fraction of molecules f are present in a b o u n d state having a m u c h shorter T2 than in the bulk, the relaxation rate e n h a n c e m e n t is given by (1/T2)sol - (1/T2)medtum= f / ( Z 2 s + Ts)

[1]

where Ts is the average residence time o f a molecule in the b o u n d state (12). M e a s u r e m e n t s of I/T2 in diluted sols at 20°C showed the linear d e p e n d e n c e of the relaxation rate on concentration which is predicted by the above equation (inset to Fig. 1). Binding of water molecules to AgI particles for periods comparable to the particle reorientation time implies an exothermic process and on raising the temperature f would be

592 0021-9797/78/0653-0592502.00/0 Copyright © 1978by AcademicPress, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 65, No. 3, July 1978

NOTES

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t/°C FIG. 1. Proton magnetic relaxation rates for a 0.1 M AgI sol and for m e d i u m against which sol was dialyzed. Filled circles refer to sol, open circles to the dialyzing m e d i u m . Inset: T r a n s v e r s e relaxation rates at 20°C.

expected to decrease. Figure 1 s h o w s h o w e v e r that the relaxation rate e n h a n c e m e n t remains c o n s t a n t o v e r the whole temperature range covered. To c o m p e n s a t e for a reduction in f there m u s t be a corresponding decrease in (T~s + rs) and, since Tzs and rs have opposite temperature coefficients, the former increasing and the latter decreasing with increasing temperature, we can deduce that rs >> T2s. The temperature d e p e n d e n c e o f rs will be governed by an activation enthalpy for desorption AH~ s u c h that t s t~ e x p ( A H $ / R T ) while the temperature d e p e n d e n c e o f f will be determined by &d/°, the standard enthalpy difference b e t w e e n b o u n d and free water, according to the relation f ct exp(-AHe/RT). If the activation energy for adsorption onto the surface is negligible, then A/ar e = - A H $ and f and r~ will decrease in parallel with increasing temperature, leading to a c o n s t a n t Tz relaxation rate e n h a n c e m e n t as is observed. An approximate estimate o f f can be obtained by considering the magnitude o f r s which is n e c e s s a r y for it to be large in c o m p a r i s o n with T2s which in turn can be estimated from the rotational correlation time o f the particles. Electron micrographs gave an average particle diameter of 46 rim, which can be

used to estimate the rotational correlation time from the Debye equation re = ~V/kT, valid for a sphere o f volume V undergoing Brownian rotational diffusion in a c o n t i n u o u s m e d i u m o f viscosity "0. Substituting in values for the viscosity of water and the particle volume which is taken to be spherical, we obtain re = 2 - 0.5 x 10 -5 sec over the range 3-60°C. B o u n d water protons experiencing magnetic field fluctuations with this order of correlation time will have relaxation times corresponding to the o n s e t of rigid lattice behavior (13) which is characterized by T2s --- ~'~. To be large in c o m p a r i s o n w i t h T2s therefore, rs m u s t be at least o f the order 10 -~ sec which m e a n s that f ~> 10 -'~ to give an e n h a n c e m e n t in I/T2 o f 0.1 s e c - L An upper limit o f 10 -'~ sec can be put on rs since this is the average lifetime o f a proton in a given water molecule at r o o m temperature (7). T h e fraction of water molecules b o u n d in the 0.1 M AgI sol m u s t therefore lie in the range 10 - 5 - 1 0 -~. Taking the density of the particles to be 5.7 g cm -a and the area of an adsorbed water molecule to be 0.2 n m 2 this c o r r e s p o n d s to a coverage o f 0 . 1 - 1 monolayer. This result is in a g r e e m e n t with the conclusion of Lyklema, based on double layer m e a s u r e m e n t s and stability studies, that there are no thick stagnant Journal of Colloidand Interface Science, Vol. 65. No. 3, July 1978

594

NOTES

water layers at the AgI interface (14). It also agrees with the notion, founded on vapor adsorption studies, that AgI particles nucleate ice crystals by adsorbing small patches of suitably oriented water molecules (15-17). ACKNOWLEDGMENT C.K.R. thanks the Science Research Council for a postgraduate studentship. REFERENCES 1. Lyklema, J., Disc. Faraday Sac., 42, 81 (1966). 2. Vincent, B., and Lyklema, J., Spec. Disc. Faraday Soc. 1, 148 (1970). 3. Fawcett, A. S., Partitt, G. D., and Smith, A. L., Nature (London) 204, 775 (1964). 4. Eley, D. D., Hey, M. J., and Ward, A. J. I., Proc. Roy. Soc. A331, 457 (1973). 5. Farrer, T. C., and Becker, E. D., in "'Pulse and Fourier Transform NMR," Chap. 2, pp. 18-34. Academic Press, New York, 1971. 6. Menold, R., L0ttge, B., and Kaiser, W., Advan. Coll. Interface Sci. 5, 281 (1976). 7. Meiboom, S., J. Chem. Phys. 34, 375 (1961). 8. Glasel, J. A., and Lee, K. H., J. Amer. Chem. Soc. 96, 970 (t974). 9. Bull, T. E., and Tiddy, G. J. T., J. Amer. Chem. Soc. 97, 236 (1975).

Journalof Colloidand Interface Science, Vol.65. No. 3, July 1978

10. Zimmerman, J. R., and Brittin, W. E., J. Phys. Chem. 61, 1328 (1957). 11. Carrington, A., and McLachlan, A. D., in "Introduction to Magnetic Resonance," Chap. 11, pp. 176-203. Harper and Row, New York, 1967. 12. Woessner, D. E., and Zimmerman, J. R., J. Phys. Chem. 67, 1590 (1963). 13. Barnaal, D. E., and Lowe, I. J., J. Chem. Phys. 46, 4800 (1967). 14. Lyklema, J., J. Colloid Interface Sci. 58, 242 (1977). 15. Tcheurekdjian, N., Zettlemoyer, A. C., and Chessick, J. J., J. Phys. Chem. 68, 773 (1964). 16. Bassett, D. R., Boucher, E. A., and Zettlemoyer, A. C., J. Colloid Interface Sci. 34, 436 (1970). 17. Barchet, W. R., and Corrin, M. L.,J. Phys. Chem. 76, 2280 (1972). D. D. ELEY M. J. HEY C. K. Rlx Department o f Chemistry The University Nottingham, NG7 2RD U.K Received November 18, 1977; accepted January 20, 1978