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The dynamics of a nitroxide radical in water adsorbed on porous supports studied by ESR
The Dynamics of a Nitroxide Radical in Water Adsorbed on Porous Supports Studied by ESR GIACOMO MARTINI, M. FRANCESCA OTTAVIANI, AND MAURIZIO ROMANELLI Istituto di Chimica Fisica, Universitd di Firenze, Via G. Capponi 9, 50121 Firenze, Italy Received August 2, 1982; accepted October 22, 1982 The ESR of the neutral spin probe 2,2,6,6-tetramethyl-4-hydroxypiperidine-l-oxylin the study of its dynamics of water adsorbed onto silica gels with pore diameters in the range 4 to 100 nm is described. The correlation times for the motion and their activation energies were calculated from the ESR linewidth at different temperatures. The mobility of the spin probe decreases with decreasing pore size, thus reflecting an increase in the average viscosity of the adsorbed water. Different behaviors are shown by water in silica gels with pore diameters 10-100 nm and in silica gels with p.d. 4 nm. The results obtained with the nitroxide are compared with those obtained with Cu(II) and Mn(II) on the same supports, and discussed in terms of the nature of the adsorbed water. INTRODUCTION
It has been widely demonstrated that the study of the electron spin relaxation of paramagnetic probes (transition metal ions and stable organic radicals) in solution provides direct information on the structure and on the dynamics of the probe and of the solvent. Once the electron spin relaxation processes are exactly known, the relaxation experiments can be used to check the motional aspects of the solvent molecules in various environments. However, some questions arise from the use of paramagnetic probes, namely: (i) To what extent does the probe affect the system around itself and to what extent do the interphases that may be present affect the probe motion? (ii) What kind of motion modulates the spin energy levels? (iii) Is our theoretical knowledge of the relaxation processes good enough to allow a correct understanding of the obtained ESR results? All these limitations are to be kept in mind in each ESR experiment in colloid and surface chemistry as well as in pure liquid solutions. Paramagnetic cations such as Mn 2÷ and Cu 2+ have been used as spin probes to characterize
the dynamics of water inside porous and nonporous supports (1, 2) or the environment of the exchangeable cations adsorbed on smectite surfaces (3). These ions exist as solvated complexes in fully hydrated solids and usually give rise to solution-like spectra. In particular it must be considered that Mn(H20) 2+, whose relaxation process is determined by the modulation of the so-called zero-field splitting (ZFS) (4, 5), is observable without large spectral complications only if the overall water coordination in the first sphere is maintained, while undetectable broad spectra are expected from tightly surface-bonded complexes. As a contrast, Cu(II) is easily ESR detectable in almost all symmetries, and its magnetic parameters are not extensively influenced by the solvent dynamics. The main relaxation mechanisms for the Cu(II) spin system are the modulation of the magnetic anisotropies and the spin-rotation (6, 7). Almost the same results were obtained from both probes in similar porous systems (1, 2). For a deeper knowledge of the water-porous solid systems we report in this paper the
105 0021-9797/83 $3.00 Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
Copyright © 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.
106
MARTINI, OTTAVIANI, A N D ROMANELLI
results obtained with water solution of the 2,2,6,6-tetramethyl-4- hydroxypiperidine- 1oxyl (TEMPOL) radical:
HsC CH5
adsorbed on homoporous silica gels. This radical is a neutral one, has a good water solubility, and has been used in many studies in colloid chemistry. Nitroxide radicals were also used in several cases for the study of their interaction with SiO2 and A1203 (8-10) and with smectic surfaces ( 11-13). The relaxation mechanism of nitroxide radicals is quite well known (14). In addition, a low interaction of the neutral TEMPOL with the negative surface of the silica gel and no surface exchange reactions are expected, thus minimizing the limitations outlined above. The experiments to be described here have been carried out in an attempt to: (i) get information on the dynamics of an organic spin probe in water adsorbed on pores of varying size, for which different rheological properties were observed (1, 2); (ii) learn more about the physical nature of water and the structural changes it undergoes at different distances from a solid surface. The second question may be of some relevance, e.g., for the knowledge of molecular interactions occurring in colloidal and biological systems. EXPERIMENTAL
TEMPOL radical was obtained from Sigma, Miinchen, and used without further purification. A 10-4 M water solution of the spin probe was prepared by using twice-distilled water and was carefully outgassed in order to remove dissolved oxygen. The porous supports were homoporous silica gels (Merck adsorbents for chromatography) with pore diameter 4 nm (henceforth called $4), 10 nm (S10), 20 nm ($20), and 100 nm (S100) (data as given by the manufacturer). Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
The samples were prepared following the same procedure as described elsewhere (1, 2). The ESR spectra were registered with a Bruker ESR spectrometer model 200tt, operating in the X band (~9.5 GHz). Modulation amplitude was 0.16 G in order to avoid line broadening due to overmodulation. Power level was 10 db; at this level no signal saturation was observed. Hyperfine coupling constants and g factors were measured by comparison with potassium nitrosyldisulfonate ((g) = 2.0054; (AN) = 13.0 G). Pyrex capillaries of 1 mm i.d. were used for each measurement. Temperature variation was achieved with the Bruker ST 100/700 variable temperature assembly. The accuracy was +0.5°C. ANALYSIS OF T H E ESR SPECTRA
Figure 1 shows the ESR spectra of a 10-4 M aqueous solution of TEMPOL in the temperature range 298 to 328°K. The unpaired electron of the spin probe couples with the nitrogen nucleus (I = 1) and with methyl and ring protons as shown by the partially resolved hyperfine lines. The anisotropies of the g and A tensors modulated by isotropic rotational diffusion of the spin probe give a linewidth dependence upon the nuclear spinquantum number mi so that (15) 2ff/(mN, mH) = A + Bran + C m 2 + B'mH + C'm 2 + DmNmH,
[1]
where the coefficients A, B . . . are expressed in terms of the elements of the hyperfine interaction tensors AN and An and of the gyromagnetic tensor g. Table 1 reports the magnetic parameters of TEMPOL that were used in this work. As shown by Jolicoeur and Friedman (16), when the ( A n ) / ( A N ) ratio is small enough (in our case ( A H ) / ( A N ) ~-~ 0.025), in a first approximation the mH dependence in Eq. [1 ] can be neglected and Eq. [1 ] reduces to AH(mN) = A + BmN + Cm~.
[2]
The term A includes two main contributions
107
NITROXIDE ESR IN ADSORBED W A T E R
C = 1.81 × 106[(AAN)2 + 3(rAN)z]~c [ ×
3 1
8(1
-~-
0)27"C) 2
1 ] 8(1
-~--
2 2 OJeTC)
where . ~
298"
K
1 ZXAN -'- AN,zz -- ~ (AN,x× + AN,yy), 1 ~)AN = ~ (AN, xx -- mN,yy), 1 A g = g= -- ~ (gxx + gyy), 6g =
1
(gxx - gy ),
WN = 8.8 × 106(AN), 518"
K
w~ ---- 5.97 × 10 l° Hz.
i
/
,
/
I
_ _
/
~28" K
/
FIG. 1. ESR spectra of a 10 -4 M water solution of TEMPOL in the temperature range 298 to 328°K. The expanded manifold in each spectrum refers to the mN = 0 hyperfine line.
In the case of Brownian motion, the rotational correlation time ~- can be used to calculate the apparent microviscosity of the medium because r is correlated with ~ by the Debye-Stokes-Einstein equation r = 47r~a3/3kT,
where a is the hydrodynamic radius of the probe. This procedure is only strictly applicable when the size of the probe molecule is TABLE I ESR Parameters of TEMPOL Used for Spectral Simulation and for the Evaluation o f the Correlation Times
deriving from: (i) modulation of the g and AN anisotropies; (ii) terms other than motional (spin-rotation, unresolved superhyperfine splittings, etc.). Term A = ~-/(0, 0), where M-/(0, 0) is the peak-to-peak linewidth of the central superhyperfine line (mH = 0) for the m N = 0 manifold. The terms B and C are functions related to the reorientational correlation time for the motion. For the X band (14, 17) B = O.103we[Ag,~AN + 3(bg)(SAN)]7" B
×
2 2" 1 +4(1 +w~r~)
(g)
g~x gyy g~z (AN), gauss Axe, gauss Ayy, gauss A~, gauss (AH)(a-CH3 eq), gauss (AH)(a-CH3 ax), gauss (AH)(fl-CH2 eq), gauss (AH)(fl-CH2 ax), gauss (AH)(-~-CH), gauss
2.0062 2.0095 2.0064 2.027 17.3 6.8 8.2 36.9 0.02" 0.45 a 0.48" 0.31 a 0.07 a
a Ref. (22). Journal of Colloid and Interface Science, Vol.
94, No. 1, July 1983
108
MARTINI, OTTAVIANI, AND ROMANELLI
larger than that of the solvent molecules, as in the present case. For isotropic motion it expected that ~ = T8 = ~c, while ~B ~ rc is often observed in the anisotropic case. If the latter is the case, different parameters, such as mean jump angle (17, 18) or fluctuating torques (18, 19), are needed to fully analyze the motion. By considering that z2d-/(1, O ) = A + B + C , dxH(0, O) = A, ~ H ( - 1, O ) = A - B + C , the coefficients B and C can be evaluated as B = [AH(I,
~(-
O)
l, 0)]/2,
C : [AH(1, 0) + AH(-1, 0)1/2 - 5aq(0, 0). Computer simulation of each nitrogen man!
ifold (assuming the rnr~ dependence as negligible, as outlined above) allows the determination of B and C and thus of the values for rB and rc. The computer simulation was carried out for each manifold with a doubleprecision Fortran program by summing de. rivatives of lorentzian lines with variable widths, assuming the validity of the Bri~re model (20). The best fit in computing the signal was achieved by using the (An) values given in Table I. Figure 2 shows the procedure for true linewidth evaluation in the case of partially resolved and unresolved splittings, True linewidth in the partially resolved manifold was obtained from the ratio K~/K2, while the computed width as a function of the observed width was used for the unresolved (not lorentzian) lines. It is noteworthy that the computed spectra were always symmetrical while the observed
K2
!
I
.I,5
)
Z -i
2
i g
Og,
_
o~
~z
~
o~
o,~
6,6
d~ TRUE
¢,o
-
~6
o~
¢,2
1.-~-
1',6
LINEWIOTH
FIG. 2, Procedure for the evaluation of the true linewidth (in gauss) in partially resolved spectra (line a; higher scale) and in unresolved spectra (line b; lower scale). For details see text. Journal of Colloid and Interface Science, VoL 94, No, 1, July 1983
NITROXIDE
ESR IN ADSORBED
spectra were the more unsymmetrical the higher was the hydrogen splitting resolution. This unsymmetry of the ESR spectrum was observed by other authors (21, 22). It is not due to instrumental inaccuracies but can arise from the oversimplification of the mH independence of the width. We did not try to consider this effect at this stage. The attempt to taken into account the mH dependence of the linewidth is very complicated in TEMPOL. Five different coupling constants and nuclear quantum number as high as 3 are involved. As suggested by Whisnant et al. (22), this simplification is not expected to contribute significantly to the calculation of "mean" correlation times. RESULTS
AND
109
WATER
J i
S?'"
_J S
_S
f
$20
S
I00
DISCUSSION
Figure 3 shows the ESR spectra at 298°K of TEMPOL solutions adsorbed on $4, S 10, $20, and S100 samples together with the spectrum in unadsorbed water. The linewidth of TEMPOL increases with the decrease of the pore size. Figures 4 and 5 show the TEMPOL lineshape variation as a function of the temperature in the extreme cases of $4 and S 100 samples. Figure 6 reports the temperature dependence of (AN) in pure water and in the porous supports. Within the limit of experimental error, almost the same trend of (AN) is shown by all the systems; thus the spectral simulation was carried out with the (AN) values of the unadsorbed solution. A small difference in the (AH) values was observed by Kreilick (23) in the range 5 to 10°C. Since our relaxation measurements were generally performed above this range, the temperature dependence of (An) has not been taken into account. In the S100 samples the widths are quite similar to those in the unadsorbed solution (Fig. 3). This fact indicates that the spin probe is in almost free motion in the liquid inside the pores and its mobility does not appreciably differ from that in unadsorbed water. In the case of the $4 samples at temperatures below ~ 3 2 0 ° K (Figs. 3, 4, and 7), the
H20
FIG. 3. ESR spectra at 2 9 8 ° K given by a 10 -4 M w a t e r solution of T E M P O L adsorbed on silica gels with different pore size. As a reference, the ESR spectrum o f the unadsorbed solution is also reported.
ESR signal due to the free radical is superimposed on a second signal attributable to a radical in slow motion, i.e., with correlation time in the range 10 - 9 tO 3 × 1 0 - 7 s e c . The intensity of this second signal decreases with increasing temperature. It seems likely that this slow-motion signal is due to the TEMPOL fraction in the water layers near the surface. No significant strong surface adsorption can have occurred, since simply washing the $4 sample with water completely removes any ESR absorption. The above result with the nitroxide radical is well in agreement with the suggestion that some water layers (two or three) (24, 25) near the silica surface behave as strongly immobilized water layers, and the surface effect, which induces the soJournal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
1 10
MARTINI, OTTAVIANI, AND ROMANELLI
298" K
308" K
323" K
338°K
I
f
J
the reciprocal temperature; In z against 1/T for TEMPOL in unadsorbed water is also reported. No differences beyond experimental error were noted for re and rc in unadsorbed water and in the S 10, S20, and S 100 systems; i.e., the motion of the probe can be regarded as effectively isotropic. As expected, the mobility of the probe (which reflects the local viscosity) decreases with decreasing pore diameter. Significantly enough, the correlation times of TEMPOL in wide-pore systems ($20 and S100 samples) are not the same as in the unadsorbed water. By using paramagnetic metal ions (Cu 2+ and Mn2+), the same motional behavior of the probes in these systems was found as in water, and large differences were observed only in nar-
353 ° K i
z,
lOG
FIG. 4. ESR spectra given by a 10-4 M water solution of T E M P O L adsorbed on $4 sample in the temperature range 298 to 353°K.
called secondary interactions, extends up to 2-3 nm from the surface (1, 2, 26, 27). Resuits in line with this proposal were obtained from electron spin relaxation data on Cu(II) and Mn(II) water solutions adsorbed on the same support used in this work (1, 2). Again, the ESR spectra in the range 288 to 228°K (Fig. 7) indicate that a large fraction of the water inside narrow pores stays in the liquid state well below 0°C, thus providing further proof of the existence of unfreezable water (1, 2, 27), because the ESR spectra at low temperature strictly resemble those obtained in systems without a well-defined solidification point (for instance, water-glycerol). Figure 8 shows the correlation times evaluated from the absorption widths with the procedure described above as a function of Journal of Colloid andlnterface Science. Vol. 94, No. 1, July 1983
FIG. 5. ESR spectra from a 10-4 M water solution o f T E M P O L adsorbed on S 100 sample in the temperature range 289 to 353°K.
NITROXIDE ESR IN ADSORBED WATER
v
111
't7
TEMPERATURE , * C
FIG. 6. Nitrogen hyperfine coupling constant of TEMPOL water solution adsorbed on silica gels with different pore size as a function o f temperature: ×, $4 sample; O, S10 sample, [], $20 sample; • S100 sample, A, unadsorbed water.
row pores (1, 2). This discrepancy may arise for a n u m b e r o f reasons. For instance, it is known from computer simulation (5) that the Mn(II) ESR linewidth for a set o f probes relaxing with different correlation times is largely dominated by the narrowest components, so that the partially immobilized metal ions, when in a small fraction, can escape identification under the narrow liquid-like sextet. The measured correlation time proves to be shorter than the actual one. Secondly, Mn(II) and Cu(II) near the surface are expected to give broad and solid-like signals, respectively, and, if the fraction of these ions is low as compared to those in the bulk solution, their absorptions may be masked under the more intense liquid-like signals. All these situations are encountered in wide-pore systems, and all that was observed was the probe fraction in the bulk solution. When using organic radicals, the above complications do not contribute to the observed width and the obtained correlation times really reflect the actual behavior o f water inside pores of varying size. If the multistate model proposed by Resing (28, 29) holds, we must assume a rapid exchange o f the nitroxide spin probes between regions of different viscosity inside the pores, with resulting averaged correlation time. The activation energies for the T E M P O L motion in the various systems confirm the
above considerations (Table II). In all cases except the $4 samples, the AE* values are of the same order of magnitude as that of unadsorbed water, thus indicating that adsorbed water retains its overall dynamic features. Some further comments are required
288" K
2T5" K
263= K
255 e K (
) lOG
228" K
(
2oG
I
FIG. 7. ESR spectra from a 10 .4 M w a t e r solution o f TEMPOL adsorbed on $4 sample in the temperature range 288 to 228°K. Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
112
M A R T I N I , OTTAVIANI, A N D R O M A N E L L I 9: 8. 7 6 5 4 I0 - I °
3
S4
W I--
s
Z 0
4 16"
/ tlJ E
!
1 9 8 7 S
s. z.
0 0
e.
7 6. 5. 4.
I(~ 12 3_
I/r (× lO3) FIG. 8. Correlation times against reciprocal temperature for T E M P O L solution adsorbed on silica gels. Symbols as in Fig. 6.
by the $4 samples. First, it seems that the motion of the radical is not isotropic, since different correlation times are obtained from parameters B and C; second, the activation energies are different for r8 and 7c and strongly deviate from the values obtained in the other cases. Water inside $4 samples really does behave in a different way (1). However, these results must be taken with care because the experimental spectra are not simple spectra, but contain different absorptions due to species with varying mobility, Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
part of them not in rapid-exchange conditions on the ESR time scale. The analysis by Freed and co-workers (17) of anisotropic motion in rapid tumbling is of very difficult application in the present case. The system should be defined by two diffusion tensors, the first being for molecule rotation in the laboratory reference frame, the second one describing the local mode of the surface near which the spin probe is located. In addition, the direction cosines along the principal axis of diffusion and the g and A tensors cannot
NITROXIDE ESR IN ADSORBED WATER TABLE II Correlation Times at 298°K and Activation Energies for the TEMPOL Motion in Silica Gels zkE*
System
r (sec)
Silica gel $4 Silica gel S10 Silica gel $20 Silica gel S100 Unadsorbed water
ZB 1.7 rC 2.2 1.3 6.1 3.5 1.8
X X X × X X
10-j° 10-l° 10 ~o 10-~1 10-u 10 H
(kcal/mol) 1.0
1.2 4.6 4.9 4.2 5.0
be evaluated. The conformational interconversion of the TEMPOL spin probe should have no particular relevance in determining r8 ~ r c . It has in fact been suggested that any ring or bond interconversion contributes to the linewidth parameters of six-membered ring probes only at very short r values in nonviscous solvents (18). We can only suggest that the observed anisotropic motion, if actual, may arise from a distortion of the solvated probe from an almost spherical to a rod-like shape, with consequent non-Brownian motion. The negative surface potential of the silica gel may induce such a shape variation. This suggestion was also proposed by Bullock et al. (30) for low molecular weight PMMA end-labeled with nitroxide radical. ACKNOWLEDGMENT Thanks are due to the Ita!ian Council for Research (CNR) for financial support. REFERENCES 1. Martini, G., J. Colloidlnterface Sci. 80, 39 (1981). 2. Bassetti, V., Burlamacchi, L., and Martini, G., 3". Amer. Chem. Soc. 101, 5471 (1979). 3. Pinnavaia T. J., in "Magnetic Resonance in Colloid and Interface Science" (H. A. Resing, and C. G. Wade, Eds.), p, 94. Amer. Chem. Soc., Washington, D.C., 1976.
1 13
4. Luckhurst, G. R., in "Electron Spin Relaxation in Liquids" (L, T. Muus, and P. W. Atkins, Eds.), p. 330. Plenum, New York, 1972. 5. Burlamacchi, L., Martini, G., Ottaviani, M. F., and Romanelli, M., Adv. Mol. Relax. Processes 12, 145 (1978). 6. Lewis, W. B., and Morgan, L. O., in "Transition Metal Chemistry," Vol. IV (R. L. Carlin, Ed.), p. 33. Dekker, New York, 1968. 7. Poupko, R., and Luz, Z., J. Chem. Phys. 57, 3311 (1971). 8. Lumina, E. V., Selivanovskii, A. K., Golubev, V. B., and Strakhov, B. V., Vestnik Moskov. Univ. Khim. 34, 131 (1979). 9. Sistovaris, N., Riede, W. O., and Sillescu, H., Bet. Bunsenges. Phys. Chem. 79, 882 (1975). 10. McBride, M. B., J. Colloid Interface Sci. 76, 393 (1980). l 1. McBride, M. B., J. Phys. Chem. 80, 196 (1976). 12. McBride, M. B., Clays Clay Miner. 27, 97 (1979). 13. McBride, M. B., Clays ClayMiner. 25, 205 (1977). 14. Schreier, S., Polnaszek, C. F., and Smith, I. C., Biochim. Biophys. Acta 515, 395 (1978). 15. Kivelson, D., J. Chem. Phys. 33, 1094, (1960). 16. Jolicoeur, C., and Friedman, H. L., Ber. Bunsenges. Phys. Chem. 75, 248 (1971). 17. Goldman, S. A., Bruno, G. V., Polnaszek, C. F., and Freed, J. H., J. Chem. Phys. 56, 716 (1972). 18. Hwang, J. S., Mason, R. P., Hwang, L. P,, and Freed, J. H., J. Phys. Chem. 79, 489 (1975). 19. Polnaszek, C. F., and Freed, J. H., J. Phys. Chem. 79, 2283 (1975). 20. Bri~re, R,, Lemaire, H., and Rassat, A., Bull. Soc. Chim. France 11, 3273 (1965). 21. Murakami, K., and Sohma, J., Z Phys. Chem. 82, 2825 (1978). 22. Whisnant, C. C., Ferguson, S., and Chesnut, D. B., J. Phys. Chem. 78, 1410 (1974). 23. Kreilick, R. W., J. Chem. Phys. 46, 4260 (1967). 24. Antoniu, A. A., J. Phys. Chem. 68, 2754 (1964). 25. Frohnsdorf, G. F. C., and Kingston, G. L., Proc. Roy. Soc. London A 274, 469 (1954). 26. Belfort, G., Sherfig, J., and Seevers, D. 0., J. Colloid Interface Sci. 47, 106 (1974). 27. Pearson, R. T., and Derbyshire, J. C., J. Colloid Interface Sci. 46~ 232 (1974). 28. Resing, H. A., Adv. Mol. Relaxation Processes 1, 109 (1967-8). 29. Resing, H. A., J. Chem. Phys. 43, 669 (1965). 30. Bullock, A. T., Cameron, G. G., and Krajewski, V., J. Phys. Chem. 80, 1792 (1976).
Journalof Colloidand InterfaceScience.Vol.94, No. 1, July 1983
It has been widely demonstrated that the study of the electron spin relaxation of paramagnetic probes (transition metal ions and stable organic radicals) in solution provides direct information on the structure and on the dynamics of the probe and of the solvent. Once the electron spin relaxation processes are exactly known, the relaxation experiments can be used to check the motional aspects of the solvent molecules in various environments. However, some questions arise from the use of paramagnetic probes, namely: (i) To what extent does the probe affect the system around itself and to what extent do the interphases that may be present affect the probe motion? (ii) What kind of motion modulates the spin energy levels? (iii) Is our theoretical knowledge of the relaxation processes good enough to allow a correct understanding of the obtained ESR results? All these limitations are to be kept in mind in each ESR experiment in colloid and surface chemistry as well as in pure liquid solutions. Paramagnetic cations such as Mn 2÷ and Cu 2+ have been used as spin probes to characterize
the dynamics of water inside porous and nonporous supports (1, 2) or the environment of the exchangeable cations adsorbed on smectite surfaces (3). These ions exist as solvated complexes in fully hydrated solids and usually give rise to solution-like spectra. In particular it must be considered that Mn(H20) 2+, whose relaxation process is determined by the modulation of the so-called zero-field splitting (ZFS) (4, 5), is observable without large spectral complications only if the overall water coordination in the first sphere is maintained, while undetectable broad spectra are expected from tightly surface-bonded complexes. As a contrast, Cu(II) is easily ESR detectable in almost all symmetries, and its magnetic parameters are not extensively influenced by the solvent dynamics. The main relaxation mechanisms for the Cu(II) spin system are the modulation of the magnetic anisotropies and the spin-rotation (6, 7). Almost the same results were obtained from both probes in similar porous systems (1, 2). For a deeper knowledge of the water-porous solid systems we report in this paper the
105 0021-9797/83 $3.00 Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
Copyright © 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.
106
MARTINI, OTTAVIANI, A N D ROMANELLI
results obtained with water solution of the 2,2,6,6-tetramethyl-4- hydroxypiperidine- 1oxyl (TEMPOL) radical:
HsC CH5
adsorbed on homoporous silica gels. This radical is a neutral one, has a good water solubility, and has been used in many studies in colloid chemistry. Nitroxide radicals were also used in several cases for the study of their interaction with SiO2 and A1203 (8-10) and with smectic surfaces ( 11-13). The relaxation mechanism of nitroxide radicals is quite well known (14). In addition, a low interaction of the neutral TEMPOL with the negative surface of the silica gel and no surface exchange reactions are expected, thus minimizing the limitations outlined above. The experiments to be described here have been carried out in an attempt to: (i) get information on the dynamics of an organic spin probe in water adsorbed on pores of varying size, for which different rheological properties were observed (1, 2); (ii) learn more about the physical nature of water and the structural changes it undergoes at different distances from a solid surface. The second question may be of some relevance, e.g., for the knowledge of molecular interactions occurring in colloidal and biological systems. EXPERIMENTAL
TEMPOL radical was obtained from Sigma, Miinchen, and used without further purification. A 10-4 M water solution of the spin probe was prepared by using twice-distilled water and was carefully outgassed in order to remove dissolved oxygen. The porous supports were homoporous silica gels (Merck adsorbents for chromatography) with pore diameter 4 nm (henceforth called $4), 10 nm (S10), 20 nm ($20), and 100 nm (S100) (data as given by the manufacturer). Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
The samples were prepared following the same procedure as described elsewhere (1, 2). The ESR spectra were registered with a Bruker ESR spectrometer model 200tt, operating in the X band (~9.5 GHz). Modulation amplitude was 0.16 G in order to avoid line broadening due to overmodulation. Power level was 10 db; at this level no signal saturation was observed. Hyperfine coupling constants and g factors were measured by comparison with potassium nitrosyldisulfonate ((g) = 2.0054; (AN) = 13.0 G). Pyrex capillaries of 1 mm i.d. were used for each measurement. Temperature variation was achieved with the Bruker ST 100/700 variable temperature assembly. The accuracy was +0.5°C. ANALYSIS OF T H E ESR SPECTRA
Figure 1 shows the ESR spectra of a 10-4 M aqueous solution of TEMPOL in the temperature range 298 to 328°K. The unpaired electron of the spin probe couples with the nitrogen nucleus (I = 1) and with methyl and ring protons as shown by the partially resolved hyperfine lines. The anisotropies of the g and A tensors modulated by isotropic rotational diffusion of the spin probe give a linewidth dependence upon the nuclear spinquantum number mi so that (15) 2ff/(mN, mH) = A + Bran + C m 2 + B'mH + C'm 2 + DmNmH,
[1]
where the coefficients A, B . . . are expressed in terms of the elements of the hyperfine interaction tensors AN and An and of the gyromagnetic tensor g. Table 1 reports the magnetic parameters of TEMPOL that were used in this work. As shown by Jolicoeur and Friedman (16), when the ( A n ) / ( A N ) ratio is small enough (in our case ( A H ) / ( A N ) ~-~ 0.025), in a first approximation the mH dependence in Eq. [1 ] can be neglected and Eq. [1 ] reduces to AH(mN) = A + BmN + Cm~.
[2]
The term A includes two main contributions
107
NITROXIDE ESR IN ADSORBED W A T E R
C = 1.81 × 106[(AAN)2 + 3(rAN)z]~c [ ×
3 1
8(1
-~-
0)27"C) 2
1 ] 8(1
-~--
2 2 OJeTC)
where . ~
298"
K
1 ZXAN -'- AN,zz -- ~ (AN,x× + AN,yy), 1 ~)AN = ~ (AN, xx -- mN,yy), 1 A g = g= -- ~ (gxx + gyy), 6g =
1
(gxx - gy ),
WN = 8.8 × 106(AN), 518"
K
w~ ---- 5.97 × 10 l° Hz.
i
/
,
/
I
_ _
/
~28" K
/
FIG. 1. ESR spectra of a 10 -4 M water solution of TEMPOL in the temperature range 298 to 328°K. The expanded manifold in each spectrum refers to the mN = 0 hyperfine line.
In the case of Brownian motion, the rotational correlation time ~- can be used to calculate the apparent microviscosity of the medium because r is correlated with ~ by the Debye-Stokes-Einstein equation r = 47r~a3/3kT,
where a is the hydrodynamic radius of the probe. This procedure is only strictly applicable when the size of the probe molecule is TABLE I ESR Parameters of TEMPOL Used for Spectral Simulation and for the Evaluation o f the Correlation Times
deriving from: (i) modulation of the g and AN anisotropies; (ii) terms other than motional (spin-rotation, unresolved superhyperfine splittings, etc.). Term A = ~-/(0, 0), where M-/(0, 0) is the peak-to-peak linewidth of the central superhyperfine line (mH = 0) for the m N = 0 manifold. The terms B and C are functions related to the reorientational correlation time for the motion. For the X band (14, 17) B = O.103we[Ag,~AN + 3(bg)(SAN)]7" B
×
2 2" 1 +4(1 +w~r~)
(g)
g~x gyy g~z (AN), gauss Axe, gauss Ayy, gauss A~, gauss (AH)(a-CH3 eq), gauss (AH)(a-CH3 ax), gauss (AH)(fl-CH2 eq), gauss (AH)(fl-CH2 ax), gauss (AH)(-~-CH), gauss
2.0062 2.0095 2.0064 2.027 17.3 6.8 8.2 36.9 0.02" 0.45 a 0.48" 0.31 a 0.07 a
a Ref. (22). Journal of Colloid and Interface Science, Vol.
94, No. 1, July 1983
108
MARTINI, OTTAVIANI, AND ROMANELLI
larger than that of the solvent molecules, as in the present case. For isotropic motion it expected that ~ = T8 = ~c, while ~B ~ rc is often observed in the anisotropic case. If the latter is the case, different parameters, such as mean jump angle (17, 18) or fluctuating torques (18, 19), are needed to fully analyze the motion. By considering that z2d-/(1, O ) = A + B + C , dxH(0, O) = A, ~ H ( - 1, O ) = A - B + C , the coefficients B and C can be evaluated as B = [AH(I,
~(-
O)
l, 0)]/2,
C : [AH(1, 0) + AH(-1, 0)1/2 - 5aq(0, 0). Computer simulation of each nitrogen man!
ifold (assuming the rnr~ dependence as negligible, as outlined above) allows the determination of B and C and thus of the values for rB and rc. The computer simulation was carried out for each manifold with a doubleprecision Fortran program by summing de. rivatives of lorentzian lines with variable widths, assuming the validity of the Bri~re model (20). The best fit in computing the signal was achieved by using the (An) values given in Table I. Figure 2 shows the procedure for true linewidth evaluation in the case of partially resolved and unresolved splittings, True linewidth in the partially resolved manifold was obtained from the ratio K~/K2, while the computed width as a function of the observed width was used for the unresolved (not lorentzian) lines. It is noteworthy that the computed spectra were always symmetrical while the observed
K2
!
I
.I,5
)
Z -i
2
i g
Og,
_
o~
~z
~
o~
o,~
6,6
d~ TRUE
¢,o
-
~6
o~
¢,2
1.-~-
1',6
LINEWIOTH
FIG. 2, Procedure for the evaluation of the true linewidth (in gauss) in partially resolved spectra (line a; higher scale) and in unresolved spectra (line b; lower scale). For details see text. Journal of Colloid and Interface Science, VoL 94, No, 1, July 1983
NITROXIDE
ESR IN ADSORBED
spectra were the more unsymmetrical the higher was the hydrogen splitting resolution. This unsymmetry of the ESR spectrum was observed by other authors (21, 22). It is not due to instrumental inaccuracies but can arise from the oversimplification of the mH independence of the width. We did not try to consider this effect at this stage. The attempt to taken into account the mH dependence of the linewidth is very complicated in TEMPOL. Five different coupling constants and nuclear quantum number as high as 3 are involved. As suggested by Whisnant et al. (22), this simplification is not expected to contribute significantly to the calculation of "mean" correlation times. RESULTS
AND
109
WATER
J i
S?'"
_J S
_S
f
$20
S
I00
DISCUSSION
Figure 3 shows the ESR spectra at 298°K of TEMPOL solutions adsorbed on $4, S 10, $20, and S100 samples together with the spectrum in unadsorbed water. The linewidth of TEMPOL increases with the decrease of the pore size. Figures 4 and 5 show the TEMPOL lineshape variation as a function of the temperature in the extreme cases of $4 and S 100 samples. Figure 6 reports the temperature dependence of (AN) in pure water and in the porous supports. Within the limit of experimental error, almost the same trend of (AN) is shown by all the systems; thus the spectral simulation was carried out with the (AN) values of the unadsorbed solution. A small difference in the (AH) values was observed by Kreilick (23) in the range 5 to 10°C. Since our relaxation measurements were generally performed above this range, the temperature dependence of (An) has not been taken into account. In the S100 samples the widths are quite similar to those in the unadsorbed solution (Fig. 3). This fact indicates that the spin probe is in almost free motion in the liquid inside the pores and its mobility does not appreciably differ from that in unadsorbed water. In the case of the $4 samples at temperatures below ~ 3 2 0 ° K (Figs. 3, 4, and 7), the
H20
FIG. 3. ESR spectra at 2 9 8 ° K given by a 10 -4 M w a t e r solution of T E M P O L adsorbed on silica gels with different pore size. As a reference, the ESR spectrum o f the unadsorbed solution is also reported.
ESR signal due to the free radical is superimposed on a second signal attributable to a radical in slow motion, i.e., with correlation time in the range 10 - 9 tO 3 × 1 0 - 7 s e c . The intensity of this second signal decreases with increasing temperature. It seems likely that this slow-motion signal is due to the TEMPOL fraction in the water layers near the surface. No significant strong surface adsorption can have occurred, since simply washing the $4 sample with water completely removes any ESR absorption. The above result with the nitroxide radical is well in agreement with the suggestion that some water layers (two or three) (24, 25) near the silica surface behave as strongly immobilized water layers, and the surface effect, which induces the soJournal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
1 10
MARTINI, OTTAVIANI, AND ROMANELLI
298" K
308" K
323" K
338°K
I
f
J
the reciprocal temperature; In z against 1/T for TEMPOL in unadsorbed water is also reported. No differences beyond experimental error were noted for re and rc in unadsorbed water and in the S 10, S20, and S 100 systems; i.e., the motion of the probe can be regarded as effectively isotropic. As expected, the mobility of the probe (which reflects the local viscosity) decreases with decreasing pore diameter. Significantly enough, the correlation times of TEMPOL in wide-pore systems ($20 and S100 samples) are not the same as in the unadsorbed water. By using paramagnetic metal ions (Cu 2+ and Mn2+), the same motional behavior of the probes in these systems was found as in water, and large differences were observed only in nar-
353 ° K i
z,
lOG
FIG. 4. ESR spectra given by a 10-4 M water solution of T E M P O L adsorbed on $4 sample in the temperature range 298 to 353°K.
called secondary interactions, extends up to 2-3 nm from the surface (1, 2, 26, 27). Resuits in line with this proposal were obtained from electron spin relaxation data on Cu(II) and Mn(II) water solutions adsorbed on the same support used in this work (1, 2). Again, the ESR spectra in the range 288 to 228°K (Fig. 7) indicate that a large fraction of the water inside narrow pores stays in the liquid state well below 0°C, thus providing further proof of the existence of unfreezable water (1, 2, 27), because the ESR spectra at low temperature strictly resemble those obtained in systems without a well-defined solidification point (for instance, water-glycerol). Figure 8 shows the correlation times evaluated from the absorption widths with the procedure described above as a function of Journal of Colloid andlnterface Science. Vol. 94, No. 1, July 1983
FIG. 5. ESR spectra from a 10-4 M water solution o f T E M P O L adsorbed on S 100 sample in the temperature range 289 to 353°K.
NITROXIDE ESR IN ADSORBED WATER
v
111
't7
TEMPERATURE , * C
FIG. 6. Nitrogen hyperfine coupling constant of TEMPOL water solution adsorbed on silica gels with different pore size as a function o f temperature: ×, $4 sample; O, S10 sample, [], $20 sample; • S100 sample, A, unadsorbed water.
row pores (1, 2). This discrepancy may arise for a n u m b e r o f reasons. For instance, it is known from computer simulation (5) that the Mn(II) ESR linewidth for a set o f probes relaxing with different correlation times is largely dominated by the narrowest components, so that the partially immobilized metal ions, when in a small fraction, can escape identification under the narrow liquid-like sextet. The measured correlation time proves to be shorter than the actual one. Secondly, Mn(II) and Cu(II) near the surface are expected to give broad and solid-like signals, respectively, and, if the fraction of these ions is low as compared to those in the bulk solution, their absorptions may be masked under the more intense liquid-like signals. All these situations are encountered in wide-pore systems, and all that was observed was the probe fraction in the bulk solution. When using organic radicals, the above complications do not contribute to the observed width and the obtained correlation times really reflect the actual behavior o f water inside pores of varying size. If the multistate model proposed by Resing (28, 29) holds, we must assume a rapid exchange o f the nitroxide spin probes between regions of different viscosity inside the pores, with resulting averaged correlation time. The activation energies for the T E M P O L motion in the various systems confirm the
above considerations (Table II). In all cases except the $4 samples, the AE* values are of the same order of magnitude as that of unadsorbed water, thus indicating that adsorbed water retains its overall dynamic features. Some further comments are required
288" K
2T5" K
263= K
255 e K (
) lOG
228" K
(
2oG
I
FIG. 7. ESR spectra from a 10 .4 M w a t e r solution o f TEMPOL adsorbed on $4 sample in the temperature range 288 to 228°K. Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
112
M A R T I N I , OTTAVIANI, A N D R O M A N E L L I 9: 8. 7 6 5 4 I0 - I °
3
S4
W I--
s
Z 0
4 16"
/ tlJ E
!
1 9 8 7 S
s. z.
0 0
e.
7 6. 5. 4.
I(~ 12 3_
I/r (× lO3) FIG. 8. Correlation times against reciprocal temperature for T E M P O L solution adsorbed on silica gels. Symbols as in Fig. 6.
by the $4 samples. First, it seems that the motion of the radical is not isotropic, since different correlation times are obtained from parameters B and C; second, the activation energies are different for r8 and 7c and strongly deviate from the values obtained in the other cases. Water inside $4 samples really does behave in a different way (1). However, these results must be taken with care because the experimental spectra are not simple spectra, but contain different absorptions due to species with varying mobility, Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
part of them not in rapid-exchange conditions on the ESR time scale. The analysis by Freed and co-workers (17) of anisotropic motion in rapid tumbling is of very difficult application in the present case. The system should be defined by two diffusion tensors, the first being for molecule rotation in the laboratory reference frame, the second one describing the local mode of the surface near which the spin probe is located. In addition, the direction cosines along the principal axis of diffusion and the g and A tensors cannot
NITROXIDE ESR IN ADSORBED WATER TABLE II Correlation Times at 298°K and Activation Energies for the TEMPOL Motion in Silica Gels zkE*
System
r (sec)
Silica gel $4 Silica gel S10 Silica gel $20 Silica gel S100 Unadsorbed water
ZB 1.7 rC 2.2 1.3 6.1 3.5 1.8
X X X × X X
10-j° 10-l° 10 ~o 10-~1 10-u 10 H
(kcal/mol) 1.0
1.2 4.6 4.9 4.2 5.0
be evaluated. The conformational interconversion of the TEMPOL spin probe should have no particular relevance in determining r8 ~ r c . It has in fact been suggested that any ring or bond interconversion contributes to the linewidth parameters of six-membered ring probes only at very short r values in nonviscous solvents (18). We can only suggest that the observed anisotropic motion, if actual, may arise from a distortion of the solvated probe from an almost spherical to a rod-like shape, with consequent non-Brownian motion. The negative surface potential of the silica gel may induce such a shape variation. This suggestion was also proposed by Bullock et al. (30) for low molecular weight PMMA end-labeled with nitroxide radical. ACKNOWLEDGMENT Thanks are due to the Ita!ian Council for Research (CNR) for financial support. REFERENCES 1. Martini, G., J. Colloidlnterface Sci. 80, 39 (1981). 2. Bassetti, V., Burlamacchi, L., and Martini, G., 3". Amer. Chem. Soc. 101, 5471 (1979). 3. Pinnavaia T. J., in "Magnetic Resonance in Colloid and Interface Science" (H. A. Resing, and C. G. Wade, Eds.), p, 94. Amer. Chem. Soc., Washington, D.C., 1976.
1 13
4. Luckhurst, G. R., in "Electron Spin Relaxation in Liquids" (L, T. Muus, and P. W. Atkins, Eds.), p. 330. Plenum, New York, 1972. 5. Burlamacchi, L., Martini, G., Ottaviani, M. F., and Romanelli, M., Adv. Mol. Relax. Processes 12, 145 (1978). 6. Lewis, W. B., and Morgan, L. O., in "Transition Metal Chemistry," Vol. IV (R. L. Carlin, Ed.), p. 33. Dekker, New York, 1968. 7. Poupko, R., and Luz, Z., J. Chem. Phys. 57, 3311 (1971). 8. Lumina, E. V., Selivanovskii, A. K., Golubev, V. B., and Strakhov, B. V., Vestnik Moskov. Univ. Khim. 34, 131 (1979). 9. Sistovaris, N., Riede, W. O., and Sillescu, H., Bet. Bunsenges. Phys. Chem. 79, 882 (1975). 10. McBride, M. B., J. Colloid Interface Sci. 76, 393 (1980). l 1. McBride, M. B., J. Phys. Chem. 80, 196 (1976). 12. McBride, M. B., Clays Clay Miner. 27, 97 (1979). 13. McBride, M. B., Clays ClayMiner. 25, 205 (1977). 14. Schreier, S., Polnaszek, C. F., and Smith, I. C., Biochim. Biophys. Acta 515, 395 (1978). 15. Kivelson, D., J. Chem. Phys. 33, 1094, (1960). 16. Jolicoeur, C., and Friedman, H. L., Ber. Bunsenges. Phys. Chem. 75, 248 (1971). 17. Goldman, S. A., Bruno, G. V., Polnaszek, C. F., and Freed, J. H., J. Chem. Phys. 56, 716 (1972). 18. Hwang, J. S., Mason, R. P., Hwang, L. P,, and Freed, J. H., J. Phys. Chem. 79, 489 (1975). 19. Polnaszek, C. F., and Freed, J. H., J. Phys. Chem. 79, 2283 (1975). 20. Bri~re, R,, Lemaire, H., and Rassat, A., Bull. Soc. Chim. France 11, 3273 (1965). 21. Murakami, K., and Sohma, J., Z Phys. Chem. 82, 2825 (1978). 22. Whisnant, C. C., Ferguson, S., and Chesnut, D. B., J. Phys. Chem. 78, 1410 (1974). 23. Kreilick, R. W., J. Chem. Phys. 46, 4260 (1967). 24. Antoniu, A. A., J. Phys. Chem. 68, 2754 (1964). 25. Frohnsdorf, G. F. C., and Kingston, G. L., Proc. Roy. Soc. London A 274, 469 (1954). 26. Belfort, G., Sherfig, J., and Seevers, D. 0., J. Colloid Interface Sci. 47, 106 (1974). 27. Pearson, R. T., and Derbyshire, J. C., J. Colloid Interface Sci. 46~ 232 (1974). 28. Resing, H. A., Adv. Mol. Relaxation Processes 1, 109 (1967-8). 29. Resing, H. A., J. Chem. Phys. 43, 669 (1965). 30. Bullock, A. T., Cameron, G. G., and Krajewski, V., J. Phys. Chem. 80, 1792 (1976).
Journalof Colloidand InterfaceScience.Vol.94, No. 1, July 1983