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J. Phys. Chem. 1992,96,7107-7 109 (8) Vand, V.; Morley, W. M.; Lomer, T. R. Acra Crysrallogr. 1951,4,324. (9)Malta, V.;Celotto, G.; Zanetti, R.; Martelli, A. F. J . Chem. SOC.B 1971,548. (10)Kaneko, F.; Kobayashi, M.; Kitagawa, Y.; Matsuura, Y. Acta Crystallogr. 1990,C46, 1490. (11) Holland, R. F.; Nielsen, J. R. J . Mol. Specrrosc. 1962,9, 436. (12)Holland, R. F.;Nielsen, J. R. Acta Crystallogr. 1963,16,902. (13)Bunn, C. W.Tram. Faraday SOC.1939,35,482. (14)Strobl, G.;Ewen, B.; Fischer, E. W.; Piesczek, W. J. Chem. Phys. 1974,61,5257. (15) Ungar, G.;Keller, A. Colloid Polym. Sci. 1979,257, 95. (16) Zerbi, G.; Piazza, R.; Holland-Moritz, K. Polymer 1982,23,1921.
(17) Zerbi, G.;Magni, R.; Gussoni, M.; Moriz, K. H.; Bigotto, A.; Dirlikov, S.J . Chem. Phys. 1981,75, 3175. (18) Organ, S.J.; Keller, A. J . Polym. Sci. Polym. Phys. 1987,25, 2405. (19) FrBlich, H. Proc. Phys. SOC.London 1942,54,422. (20)Mansfield, M.; Boyd, R. H. J. Polym. Sci., Polym. Phys. Ed. 1978,
16, 1227. (21)Soto, K. Jpn. J. Appl. Phys. 1979,18, 1019. (22)Sato, K.Jpn. J. Appl. Phys. 1980,19, 1257. (23)Boistelle, R.In Crystallization and Polymorphism of Furs and Furry Acids; Garti, N., Sato, K., Eds.; Marcel Dekker: New York, 1988. (24)Inaoka, K.; Kobayashi, M.; Okada, M.; Sato, K. J . Crysr. Growrh 1988,87,243.
Ion Pairing of Ammonium Nitrate in Methanol? Ruzhong Chen and Gordon R. Freeman* Chemistry Department, University of Alberta, Edmonton, Canada T6G 2G2 (Received: March 20,1992; In Final Form: May 19,1992)
The recent suggestion (J. Phys. Chem. 1991,95,897)that the ion-pair association constant of ammonium nitrate in methanol is K2 >> 10 m3/mol ( lo4 M-I) at about 293 K would have interesting implications for our embryonic understanding of solvent effects,if it were correct. Electrical conductance measurements of solutions at concentrations from 1 to 995 mol/" demonstrate that ammonium nitrate behaves as a normal, strong electrolyte in methanol. The conductances are fitted by the Onsager-Fuoss equation, the second term of which contributes the greatest reduction in conductance and corresponds to the ion-atmosphere (long-range interactions) model of Milner, Debye, and Hackel. A new attack on the theory of concentrated electrolyte solutions should be made by computer simulation.
Introduction In a recent report' of the reactivities of lithium and ammonium nitrates in liquid methanol with solvated electrons, and with electrons prior to solvation, it was suggested that the association constant of ammonium ions with nitrate ions was much greater than that of lithium with nitrate Lis+
+ (NO3-),
(NH4+), + (NO,-),
TABLE I: Conduct" C (mol m-7
1.08 10.0 38.0 61.5 123 199 398 597 796 995
(LiN03)s (NH4N03)s
(2)
with K I S 0.0205 m3/mol (20.5 M-I) and Kz >> 10 m3/mol (lo4 M-I) a t room temperature. The salt concentrations used were within the range 100-1000 mol/m3 (0.1-1.0 M) in both cases. The enormous value of K2compared to K Iseemed surprising to us, but relatively little is known about behavior in concentrated solutions of salts in alcohols. An enormous value of K2in methanol would have several interesting implications for our embryonic understanding of solvation effects in different solvents, so we checked it by electrical conductance measurements.
Experimental Section Methanol (Aldrich, 99.98, Spectrophotometric grade, freshly opened bottle) and ammonium nitrate (Aldrich, 99.999%, opened bottle kept in a vacuum desiccator containing Pz05)were used without further purification. Solutions of 995, 123, and 38.0 mol/m3 were made by weighing nitrate into 100- or 25-mL volumetric flasks and adding appropriate amounts of methanol, with intermittent shaking of the flasks. Aliquots 12.7 mL, measured to 5 1%, of these solutions were diluted to obtain solutions 110.0 mol/m3, and the last was diluted further to obtain 1.08 m0i/m3. The conductance cells were Yellow Springs Instrument Co. Model YSI3403, calibrated a t 298.15 K using the secondary standard solution YSI3161. Temperature was controlled to fO.O1 K, and conductances were measured when the specified temperature had been constant for 30 min. ~~
*To whom correspondence should be addressed.
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Der Mole
A
8.71 7.57 6.39 5.63 4.90 4.52 3.71 3.23 2.89 2.59
of Ammonium Nitrate in Methanol S m2 mol-I)"
10.8 9.10 1.59 6.68 5.83 5.47 4.51 3.95 3.54 3.16
12.4 10.7 8.94 7.84 6.84 6.29 5.22 4.59 4.12 3.71
14.4 12.3 10.2 8.93 7.76 7.22 6.00 5.29 4.75 4.29
'T (K) for columns 2-5 were respectively 283.28,298.29,313.53, and 328.21. The impedance bridge was General Radio Co. Model 1608-A. The measurements were made at 1 kHz.
Results and Discussion The conductance data are listed in Table I. The conductances measured at 283.28-328.21 K vary with the square root of the concentration (Figure 1) in the manner expected for strong electrolytes. The DebyeHiickel-Onsager equation2 for the variation of conductivity A (Sm2 mol-') with concentrations C (mol m-3) below about 100 mol/m3 is
A = A'
- ( A + BAo)C'/2+ DC
(3) where Ao is the conductivity per mole a t infinite dilution, D is an empirical constant, and, for a 1,l-electrolyte (4)
and
where e is the protonic charge, F the Faraday constant, 7 the shear 0 1992 American Chemical Society
7108 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 16 L
n
8
I
.-. 71 cn ' I
I
0
A
A
A 0
4
A A 0
A A
A
6
0
2I
10
0
30
20
I 40
(mol m-3)1/2 Figure 1. Conductivity A of ammonium nitrate in methanol plotted against (concentration)','. T (K): H, 283.28; 0, 298.29; A, 313.53; A, 328.21. The full curves were calculated from eq 3 and the empirical constants in Table 11. cl/2
TABLE II: Parameter Values for Curves in Figure 1 283.28 298.29 313.53 T (K) 7 (io4
Pa
I
A
0
106 s-') with the monomer surfactant is evident. At [H20]/[EO(4)NP] = 0.6, the core viscoSity is at its maximum value and the exchange rate decreases below the limit of slow exchange (vex< lo6 s-I). Upon further increasing the water content, the core becomes more polar and less viscous, with these parameters approaching their values in bulk water. The effect of Cu(I1) ion (in an oil-soluble complex) on the spectra of radical II-confined to the water-pool-has been interpreted by using Leigh's theory, and the 'distance of closest approach" between these two paramagnetic species has been evaluated. This distance is considered to represent a measure of the penetrability of the micellar shell. Radical IV yielded anisotropic spectra in the aggregates with slow exchange. From their ESR parameters the order degree of the surfactant chains in the micellar shell has been evaluated. All results are consistent, indicating reduced penetrability and increased order with increasing dimensions of the water-pool.
Introduction Several surfactants are able to aggregate when dissolved in nonaqueous solvents to yield reversed micelles.'~2Formation of reversed micelles requires traces of water? with their polar core being able to solubilize significant amounts of water. In recent years considerable attention has been paid to these reversed systems both for their resemblance to biomembranes and for their peculiar behavior in catalysis of polar molecules.' The association mechanism of these surfactants in nonpolar solvents and, related to this, the mechanism of the uncommon highly catalytic activity of the system are poorly understood! Therefore, information on the association behavior of the surfactant, on the properties of formed aggregates, and on the water-pool entrapped therein is indispensable for the understanding of any of the two mechanisms. The ESR of spin probes in micellar systems has been proven as a powerful technique for studying the aggregation behavior of different surfactants, the properties (microviscosities and local polarities) of the environment around the probe in the micelles, the effect of different solubilizates on these properties, and the dynamics of the micellization processes.' However, only very few papers reported spin probe studies on reversed micellar systems, 'Romanian Academy, Institute of Physical Chemistry. 'Polytechnic Institute. f Research Institute for Plastic Materials. Romanian Academy, Center of Organic Chemistry.
all of them concerning ionic surfactants, either sodium bis( 2ethylhexyl) sulfosuccinate (AOT)5-8 or dodecylammonium propi~nate.~ The present work reports the detailed information obtained by the spin probe technique on the aggregation behavior of a nonionic surfactant (polyoxyethylene(4)nonylphenol) in a nonaqueous solvent (cyclohexane) in the presence of increasing amounts of water. This system has been formerly studied by Kitahara,lo who found by the light scattering method that the micellar weight increases with the added amount of water up to a [H20]/[surfactant] molar ratio of 1.65. In the present work hydrophilic spin probes have been chosen with the aim of focusing the investigation on the aqueous part of the aggregate. The location of the spin probe has been established using appropriate paramagnetic Cu(I1) compounds which preferentially dissolved in hydrophilic or in the hydrophobic part of the system. In the latter case of a Cu(I1) compound soluble only in cyclohexane, one observes that the ESR signal of the spin probe-confined to the water-pool-loses its intensity without apparent broadening. This experimental fact has been treated within the frame of Leigh's theoretical approach and led to information about the level of cyclohexane penetration through the micellar shell as a function of the amount of water in the aggregate. The use of 5-doxylstearic acid as a spin probe provided valuable information both on the structural order in the shell of the studied reversed micelles and on its dependence on the added amount of water.
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