J. Phys. Chem. 1992, 96, 8578-8581
8578
Dielectrlc Properties of 1-AlcohoVHydrated Copper Nitratenetraethyl Orthosilicate Solutions John Sjiiblom,* Tore Skodvin, Merete H. Selle, Jens Olav Saeten, Department of Chemistry, University of Bergen, N-5007 Bergen, Norway
and Stig E. Friberg Centerfor Advanced Materials Processing, Clarkson University, Potsdam, New York 13699- 5814 (Received: April IO, 1992; In Final Form: June 30, 1992)
Dielectric properties of mixtures of 1-alcohols(1-butanol and 1-hexanol), tetraethyl orthosilicate (TEOS), and hydrated copper nitrate ( C U ( N O ~ ) ~ . ~ ' / ~were H~O determined ) by means of the time domain method (TDS) in the interval 20 MHz / ~ H ~ion O pairs in the to 2.5 GHz. A Cole-Cole model function was fitted to the experimental points. C U ( N O ~ ) ~ - ~ ' forms alcohols studied. With TEOS present the hydration water seems to initiate the hydrolysis. After a critical molar ratio of 2 between hydration water and TEOS, the effects in static permittivity and relaxation times can be linearized as a function of TEOS concentration.
Introduction The exceptional properties of organosilicon compounds to form siloxane polymers have been the basis for the sol/gel technique.' This method includes hydrolysis and condensation of tetraethyl or tetramethyl orthosilicate, Le., TEOS or TMOS, in an alcohol environment. In summary Si(OC2HJ4 + 4H20 Si(OH)4 4C2HSOH (1) Si(OH)4
-
SiOz
+
+ 2H20
(2)
where reaction 1 is much faster than 2. The entire process involves reactions 1 and 2 and also a number of nonequilibrium transformations such as drying and sintering. Hence, investigations to clarify the reactions leading to the gel state as well as its internal structure have been numerous,"" and the main features have been systematically reviewed. Recently an extension has been made to incorporate metal salts into the gel by utilizing water-in-oil microemulsi~ns.'~-~~ The microdroplets of water could be changed to very high concentrations with metal salts (5096 by weight) giving a hydrocarbon phase with solubilized metal salts. The gelation of TEOS took place without phase separation, and the gelation gave a colloidal dispersion of metal salts in a silica/surfactant matrix. The presence of surfactants in the gel and the xerogel is a disadvantage, and other options would be desirable to prepare silica matrixes with fmely dispersed metal salts. One such option was found in the results from earlier which have demonstrated a high solubility of metal salt hydrates in alcohols, and we found the potential appealing to prepare siloxane gels with colloidally or molecularly dispersed metal salts without the presence of surfactants. In the present study we have focused on the interaction between tetraethyl orthosilicate, hydrated copper nitrate, and 1-alcohols as studied by means of dielectric spectroscopy. The study is an extension of our previous work on binary hydrated metal salt/ 1-alcohol and 1-alcohol/TEOS Dielectric spectroscopy offers the possibility of following changes in alcohol structure and ion-pair geometry during the hydrolysis stage in 1.
Experimental Section Materials. 1-Butanol (99.5%) and l-hexanol(98%) were both from Riedel-de HaBn, C U ( N O ~ ) , - ~ ~ /was ~ Hfrom ~ O Fisher Co., and the tetraethyl orthosilicate (Si(OEt),) was from Htils AG (98%). The chemicals were all used without further purification. heparation of Solutions. Samples containing 5, 10, 20, 30, 50, and 70 wt % of C U ( N O ~ ) ~ * ~in~ the /~H two~ alcohols O were weighed into glass vials. 50 and 70% C I I ( N O ~ ) ~ . ~ ' / , Hin~ O H~O I-butanol and 30, 50, and 70% C U ( N O ~ ) ~ . ~ ' /in~ 1-hexanol were insoluble within the time of these experiments.
To solutions with the same ratio between alcohol and Cu(N03)2*21/zH20 were added 5, 10,20,30, and 50 wt 96 of silicon tetraethoxide (TEOS). DleloctricMeawremeata The dielectric spectra were measured by the time domain spectroscopy technique. The method where the pulse is reflected against a sample placed at the end of an open coaxial line was used. The influence of the dielectric sample on the shape of a reflected step pulse is, via a Fourier transform, used to derive the dielectric spectrum:
= &(a)- it"(w)
t*(~)
(3)
The measurements were made relative to a cell containing I-butanol or 1-hexanol, using a cell with a pin length of 5.2 mm. The pulse shapes were observed with time windows of 40 or 50 ns. The pulse shapes were Fourier transformed at 45 frequencies between 25 MHz and 2.5 GHz (for time window of 40 ns) and 20 MHz and 2 GHz (50 ns). The total permittivity ~ * ~ , , ~ ( wcalculated ), from the Fourier transform of pulse shapes, will in addition to the dielectric part e*(@) include a contribution from the dc conductivity u, i.e. = t*(w) - iu/wt,
(4)
Here E,, is the vacuum permittivity. In time domain spectroscopy u can be obtained from the final levels of the pulses reflected from
the sample and the reference liquid, respectively. A Cole-Cole model function
was fitted to the experimental points. Here T is the mean dielectric relaxation time and a is a distribution parameter. t, and are the static permittivity and the permittivity at very high frequencies, respectively. Re3ultS The results are summarized in five figures. Figure 1 gives a model spectrum of l-butan0l/Cu(NO~)~.2'/~H~O (95/5 by weight) with 1.93 and 8.66 mol % TEOS. In Figure 2 the influence of C I I ( N O J ~ . ~ ~ / ~onH the ~ O dielectric parameters of 1-butanol and 1-hexanol are displayed. The static permittivity (4) shows an initial rise and levels out at higher salt concentrations. The dielectric relaxation time ( 7 ) shows a slow increase to start with but ends up at saturation with a doubling of the relaxation time of the pure 1-alcohol. Figure 3 gives a summary of the influence of tetraethyl orthosilicate (TEOS) on 1-butanol solutions containing 5%
0022-3654/92/2096-8578$03.00/00 1992 American Chemical Society
Dielectric Properties of Orthosilicate Solutions
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8579 Static Permittivity
Permittivity
20
I
Relaxation time [nsl
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1
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0.4
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Mole % TEOS
Frequency IHzl
Figure 1. Permittivity spectra of 1 -b~tanol/Cu(N0~)~.2~/~H~O (95/5 Static Permittivity by weight) with 1.93 mol 95 (+) and 8.66 mol 95 (m) TEOS of the total. 20 Static Permittivity
,
0
Relaxation time Ins1
Relaxation time [nsl
1
20,
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Figure 2. Influence. of C U ( N O ~ ) ~ . ~ ~on / ~the H ~dielectric O parameters, c, (*) and 7 (m) in (a, top) 1-butanol and in (b, bottom) 1-hexanol.
C U ( N O , ) ~ . ~ ~ / ~(Figure H ~ O 3a), 10% C U ( N O , ) ~ . ~ ' / ~ H(Figure ~O 3b), and 20% C U ( N O , ) ~ * ~ ' / ~(Figure H ~ O 3c). In Figure 3a all the effects can be more or less linearized over the whole TEOS interval. This accounts for the changes in static permittivity as well as in the relaxation time. With higher initial contents of C U ( N O , ) ~ * ~ ~ present, / ~ H ~ Othe deviation from ideality is more /~H~O and more conspicuous. With 10% C U ( N O ~ ) ? S ~ ~present, the nonideality covers about 10% of TEOS, whde F w r e 3c shows a nonideality for concentrations up to 20% of TEOS when 1butanol contains 20% C U ( N O ~ ) ~ - ~ ' / ~Figure H ~ O .4 gives corresponding data for the system TEOS/ 1-hexanol/Cu(NO,),21/2H20. The trend in nonideality follows Figure 3b,c. The
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Mole 36 TEOS Figure 3. Influence. of tetraethyl orthosilicate (TEOS) on solutions of 1-butanol and C U ( N O , ) ~ . ~ ~ / ~atHdifferent ~O ratios: (a, top) 95/5, (b, middle) 90/10, and (c, bottom) 80/20 (by weight). c, (a) and 7 (a). R = 2 is the molar ratio between hydration water and TEOS.
conductivities are summarized in Figure 5.
Discussion The total system investigated is with necessity rather complicated, and discussion benefits from an outline of the conditions in the simpler subsystems. Hydrogen-Bonded Structures in Neat Alcohols. Dielectric spectroscopy has been widely used to investigate structures in alcohol solutions. The molecular picture emerging from these investigations is that the liquid self-associated state, stabilized by hydrogen bonds, consists of linear polymers and cyclic aggreg a t e ~ . ~The ~ -probability ~~ of finding cyclic aggregates decreases with increasing molecular weight of the alcohol. In general one may conclude that linear complexes might predominate over the cyclic ones.
8580 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 Static Permittivity
Relaxation time [nsl -14
"K 12
Sjbblom et al. lo3
Conductivity LSlml x I&"
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Mole % TEOS Static Permittivity
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Influence of TEOS on solutions of 1-hexanol and Cu(N03)2G1/2H20at different ratios, (a, top) 95/5 and (b, bottom) 90/10. c, (S) and T (B). R = 2 is the molar ratio between hydration water and TEOS.
Figure 4.
The dielectric spectrum of 1-alcohol is normally deconvoluted into three different relaxation times. Of these the low-frequency relaxation (7,)has been correlated with the self-associated state of the alcohols. This relaxation time for 1-alcohols shows a gradual increase with the molecular weight for the linear homologs. The second relaxation time (72) is generally considered to reflect the motions of the monomeric alcohol species and usually falls in the range 25-50 ps for 1-alcohols. The fastest relaxation time T~ (-3 ps) is a hydroxylic group relaxation mode, and hence 73 is independent of molecular size.27 Influence of TEOS on Alcohol Structure. Tetraethyl orthosilicate is by itself nonpolar as a total entity, and one should predict modifications of the alcohol association structure in accordance with those obtained when adding a nonpolar hydrocarbon. In a detailed study of n-heptane/ 1-alcoholsolutions it was found that the hydrocarbon ruptured the self-associated 1-alcohol state. As a consequence the content of alcohol monomers was found to increase, most rapidly so for 1-heptane content in excess of equimolar ratios of 1-hexanol and n-heptane>8*29 In a recent the study the same qualitative trend was found for TEOS/ 1-alcohol solutions.2' After the first 2-3 mol % of TEOS the dielectric parameters, Le., static permittivity as well as relaxation time, could be linearized over the whole concentration interval up to 20-25 mol %, which was the upper limit of the investigation. This concentration dependence reveals that no strong nor specific interactions take place upon TEOS addition to 1-alcohols. This is in obvious contrast to the influence of polar additives, such as The linear decrease in t, and T at higher TEOS concentrations is a combination of two overlapping effects. t, reflects the increasing volume fraction of TEOS in the mixture, and the decrease should be linear without specific interaction. The decrease in T upon mixing reflects the rupturing of hydrogenbonded complexes and formation of less associated alcohol
0
1
2
3
4
5
6
7
8
Mole % Cu(N03)2 2 M H 2 0
Figure 5. Influence of C U ( N O ~ ) ~ - ~ ' /on ~H the~ conductivity. O (a, top) The system without TEOS, 1-butanol ( X ) and 1-hexanol (+). (b, bottom) The system with TEOS. The ratios between 1-butanol and Cu(N03)2.21/2H20are 95/5 (e),90/10 (+), and 80/20 (*) and the ratios between 1-hexanol and C U ( N O ~ ) ~ . ~ ' / ~are H 95/5 ~ O (W) and 90/10 (X).
molecules. Both these are obvious parallels to the influence of n-heptane on l - h e ~ a n o l . ~ ~ In summary, no change of TEOS structure occurred, and the molecule obviously retained its symmetry when being combined with the alcohols. Hence, no significant exchange took place between the ethoxy groups of the TEOS molecule and the alcohol molecules. Such changes within the TEOS molecule would create a dipole and the whole dilution behavior would not be linear. A combination of dipolar TEOS and 1-alcoholswould have given rise to results more reminiscent of those earlier obtained for mixtures of water (diols) and alcohols or even dipolar ion pairs and 1-alcohols. This was, however, not the case judged from the experimental curves which confirm results from other investigations.' Inlluenceof&ddytes 011 the A l d o l Strudwe. The influence of the copper nitrate on the TDS spectra (Figures 2 and 5 ) at first appears complex but is in fact, well in accordance with earlier findings. The electrolyte affects the association structure of the solvent to a varying degree highly depending on its own nature. Dielectric properties of solutions consisting of simple 1:1 electrolytes in H 2 0 or CH30H have been extensively studied. The main feature is a decreasing static permittivity, while T can increase or decrease depending on the medium. The reduction in t, can be explained by the formation of a hydration sheath where the water molecules are 'irrotationally" bound to the ions, hence giving a different dielectric response than the bulk molecules. The reduction in t, can be quantified as hydration numbers for the single ions. When this model is adopted for alcohol systems, the result is unreasonably
Dielectric Properties of Orthosilicate Solutions
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8581
high alcohol solvation numbers. To correct for this fact, the Hubbard-Onsager kinetic depolarization effect has been put f ~ r w a r d . ~This ~ J ~model predicts per se a reduction in the static permittivity e, given by A~ = -24 x 1 0 9 4 ( ~-, C . , , ) / t s 1 7 ~
(6)
The resulting decrease in e, is hence a sum of the hydration/ solvation effect and the dielectric depolarization effect due to ionic mobility. It has been shown that hydrated multivalent electrolytes deviate from the picture above. For instance, when higher hydrated salts are added to 1-propanol, there is an initial increase in static permittivity (4) in contrast to the behavior of 1:l electrolytes. The mean relaxation time ( 7 ) is in this case increased as in LiCl solutions. The different behavior in e, can be attributed to the formation of ion pairs and to a structure effect, an increased g factor in 1-propanol. The latter most likely has its origin in the interaction between alcohol and the hydrated cation, Le., [Me(H20),]m'.19 A recent study of Ca(N03)2.4H20in the isodielectric solvents acetone and 1-propanol revealed a distinct indication of an ion pair formation in acetone with 7 100 ps.18 Such an effect can only partly account for the increase in 4 and for associated liquids such as alcohols the Kirkwood-FrGhlich equation
The linear decrease of both 7 and e,, shown in Figures 3 and 4 support such an interpretation. At high R values the evaluation becomes even more difficult, because the amount of water of hydration is now in excess of the amount needed for reactions 1 and 2 to take place. Reaction 1 consumes water that is partially released in reaction 2, and the kinetics of dehydration/rehydration of the metal salt combined with the unknown influence on e, and 7 by ion pairs or doublet pairs with less than 2.5 water of hydration becomes extremely difficult to analyze. Acknowledgment. M.H.S. and J.O.S.acknowledges the Norwegian Research Council for Science and Humanities (NAVF) for research grants (dr. scient) and S.E.F. thanks the New York State Commission for Science and Technology through its CAMP program at Clarkson University. Registry No. CU(NO,)~.~'/~H~O, 19004-19-4; 1-butanol, 71-36-3; I-hexanol, 111-27-3; tetraethyl orthosilicate, 78-10-4.
References and Notes
(1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing, Academic Press: New York, 1990. (2) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S. J . NonCryst. Solids 1982, 48, 47. ( 3 ) Klein, L. C. Annu. Rev. Mafer.Sci. 1985, 15, 227. (4) Yoldas, B. E. J. Non-Cryst. Solids 1986, 82, 11. (5) Brinker, C. J.; Scherer, G.W.; Roth, E. P. J . Non-Cryst. Solids 1985, 72, 345. (E, - t,)(2cs + e m ) =- NA (6) Kelts, I. W.; Efinger, N. J.; Melpolder, S. M. J . Non-Crysf. Solids (7) €,(em 2)2 9kTVeogp2 1986, 83, 353. (7) Orcel, G.; Hench, L. J . Non-Cryst. Solids 1986, 79, 177. (8) Pouxviel, J. C.; Boilot, J. P.; Beloeil, J. C.; Lallemand, J. Y. J. Noncan be applied. k is the Boltzmann constant, T i s the absolute Crysf.Solids 1987, 89, 345. temperature, Vis the molar volume, and p is the gas-phase dipole (9) Sanchez, C.; Livage, J.; Henry, M.; Babonneau, H. J . Non-Crysf. moment. Solids 1988, 100, 65. The g factor is a measure of the dipolar correlation. In a (10) Cabane, B; Dubois, M.; Duplessix, R. J . Phys. 1987, 48, 2131. (1 1) Cabane, B.; Dubois, M.; Lefaucheux, F.; Robert, M. C. J . Non-Cryst. nonassociated liquid, g = 1. The high permittivity of 1-alcohols Solids 1990, I 1 9, 121. can be understood from a high dipolar correlation, i.e., a high g (12) Friberg, S. E.; Yang, C. C. Innovations in Materials Processing Using factor. Qualitatively an increase in c, can be taken as an indication Aqueous, Colloid and Surface Chemistry; Doyle, F. M., Raghaven, S,Soof an augmented dipolar correlation. masundaran, p., Warren, G.w., eds.; The Minerals, Metals and Materials In this respect a hydrated cation such as Ca or AI seems to have Society: Wanendale, PA, 1988; pp 181-191. (13) Friberg, S. E.; Yang, C. C.; Sjoblom, J. Lungmuir 1992, 8, 372. a much stronger structuring effect than a simple Li ion. This (14) Friberg, S. E.; Jones, S. M.; Yang, C. C. J. Dispersion Sci. Technol. structuring can be mediated through the hydrated water molecules 1992, 13, 45. extending to several layers of alcohol molecules by their chainlike (15) Friberg, S. E.; Jones, S. M.; Yang, C. C. J. Dispersion Sci. Technol, association. This picture also agrees with the increase of the mean 1992, 13, 65. Fardedal, .; Skodvin, T.; Sjoblom, J.; Amran, A.; (16) Saeten, J. 0.; relaxation time (T ) and a broadening of the dispersion, i.e., higher Friberg, S.E. J . Colloid Interface Sci., in press. a values. (17) Friberg, S. E.; Yang, C. C.; Selle, M. H.; Sjoblom, J. Prog. Colloid Figure 2 summarizes the effect of C U ( N O ~ ) ~ . ~ ~on/ ~1-H ~ OPolym. Sci., in press. butanol and 1-hexanol structures. All the qualitative trends in (18) Gestblom, B.; Sjoblom, S. Chem. Phys. Lett. 1985, 122, 553. (19) Gestblom, B.; Mehrotra, S.C.; Sjoblom, J. J. Solution Chem. 1986, the interaction described above are identified. Most likely the
+
15, 55. copper nitrate is in an ion pair state in pure 1-butanol and 1(20) Gestblom, B.; Sjoblom, J. J . Solution Chem. 1986, 15, 259. hexanol. (21) Saeten, J. 0.;Selle, M. H.; Sjoblom, J.; Friberg, S. E.; Gestblom, 8. Iateractiom in Tetraethyl Orthosilicate/Hydrated Copper NiJ . Solution Chem. 1991, 20, 1149. (22) Sjoblom, J.; Dyhr, H. Acta Chem. Scand. 1981, A35, 219. trate/Alcobol Solutions. Figures 3 and 4 clearly show that the (23) Dannhauser, W. J . Chem. Phys. 1968, 48, 1918. ideal characteristic for the TEOS/ 1-alcohol mixtures does not (24) DAprano, A.; Donato, D.; Caponetti, E. J . Solution Chem. 1979,8, obtain. When combined with C U ( N O ~ ) ~ . ~ ~and / ~ 1-alcohol, H~O 135. initial additions of TEOS resulted in a varied behavior, which changed to a linear reduction of both 7 and 4 for higher amounts of TEOS. The discussion so far shows that this behavior should be traced to the TEOS/copper nitrate interaction. The species (27) Garg, S.K.; Smyth, C. P. J . Phys. Chem. 1965, 69, 1294. to interact with the TEOS is the hydration water on C U ( N O ~ ) ~ . (28) Merhrotra, S.C.; Bestblom, B.; Sjoblom, J. Finn. Chem. Lett. 1985, 34. This water can partially hydrolyze the TEOS and form Si(OH)i, (29) Noreland, E.; Gestblom, B.; Sjoblom, J. J . Solution Chem. 1989, 18, and the break at the molar ratio (R) of 2 between water and TEOS 303. certainly supports such a conclusion. (30) Gestblom, B.; Sjoblom, J. Acta Chem. Scand. 1984, A38, 47. The interpretation is complicated by the fact that conditions (31) Gestblom, B.; Sjoblom, J. Acta Chem. Scand. 1984, A38, 575. (32) Mashimo, S.; Kuwabara, S.;Yagihara, S.; Higasi, K. J . Chem. Phys. of small R values favor alcohol producing condensation and ethanol 1989, 90, 3292. is released. The present results show that the small amounts of (33) Hubbard, J.; Colonomos, P.; Wolynes, P. G.J . Chem. Phys. 1979, ethanol released do not contribute sufficiently to c, to compensate 71, 2652. (34) Hubbard, J.; Onsager, L. J. Chem. Phys. 1977, 67, 4850. for the reduction caused by the presence of the unreacted TEOS.
