280
J. Phys. Chem. 1984, 88, 280-284
and also to the circumference s of the particle. Since the monolayer is far from ideal we also have to introduce the surface activity coefficient of the solvent y2,hence v- = klsyl(l - J?/rm). The transfer from the monolayer to the particle u- is proportional to the concentration in the surface. Here also we have to account for the surface activity coefficient of the surfactant y2, hence U-
= Izzsrz(r/r-)
where kl and k2 are rate constants. The collision parameter is related to the velocity u- through w
= u-NA
NA is Avogadro's number. At the equilibrium pressure v- = ut, hence klyle( 1
-
")r-
= k2y2ere
r-
where reis the adsorption at the ESP and yle, yze are surface activity coefficients at the ESP. For tetradecanol we found k2 7X mol cm-' s-l, hence for the collision parameter w 4 X 10l2molecules cm-' s-l. This value compares with those from collapse experiments. Finally we mention that our theory can be modified to the kinetics for the first-order transition in monolayers as seen in spreading films of myristic acid. In view of the twodimensional phase rule of Defay-Crisp3 at the transition pressure two modifications of myristic acid are present which are immiscible, and a border line between both regions occurs; to this border line a line tension T is attributed T is in units of dynes). From the dependence of the transition pressure on the relative lo-' dyn. compression rate we calculated a line tension of T
-
-
-
Acknowledgment. P.D.K. is indebted to the Belgian I.W.O. N.L. for a grant. Registry No. Tetradecyl alcohol, 112-72-1; hexadecyl alcohol, 36653-82-4; octadecyl alcohol, 112-92-5; lauric acid, 143-07-7; myristic acid, 544-63-8; palmitic acid, 57-10-3; stearic acid, 57-1 1-4.
Diffusion-Concentration Product of Oxygen within Water Pools of Aerosol OT-Heptane Reversed Micelles E. Gandin,* Y. Lion, and A. Van De Vorst Physique Experimentale, Universite de Lidge, 4000 Sart- Tilman par Lidge 1 , Belgium (Received: March 14, 1983; In Final Form: June 15, 1983)
ESR spin-exchange measurements on peroxylaminedisulfonate (PADS) solubilized in reversed micelles of water-Aerosol OT (AOT) in n-heptane were used to determine the product, Do2N02,of the oxygen diffusion coefficient, Do,, and the oxygen concentration, No2. The results indicate a linear relationship between Do2N02and the radius of the aqueous core in the range from 12 to 43 A. This is interpreted as a linear decrease of the water microviscosity as the micelle swells. The measurements of the natural line width of peroxylaminedisulfonatevs. the size of the reversed micelle confirm the change in microviscosity and allow the determination of the absolute microviscosities which range from 5.6 to 0.93 CPas the radius increases from 12 to 43 A. Estimation of the oxygen concentration in the aqueous phase of the aerated system leads to a slightly lower value, 1.8 X M, than in bulk water.
Introduction Surfactant aggregates in apolar solvents, termed reversed or inverted micelles, have been the subject of recent interest.'V2 Their structure is such that the polar head of the amphiphiles constitutes the core of the aggregate while the hydrophobic tails extend into the surrounding solution. They are capable of solubilizing a considerable amount of water which is accommodated in the polar centers of the aggregates where it forms spherical pools. The nature of these aqueous microphases and the determination of their size, which is controlled by the [H20]to [detergent] ratio (R),have received much The nature of water bubbles in reversed micelles was found to be quite different from that of ordinary bulk water.6 So, for reversed micelles of sodium bis(2-ethylhexyl) sulfosuccinate (called Aerosol OT or AOT) in alkane solvents, at low water content ( R < 6), all the water molecules are expected to participate in the solvation shell of the
Na' counterion or to interact with the polar head groupe7 As a consequence, the water in this hydration shell is highly immobilized. Additional water is free but mobility is still slower than in bulk water.* In a study of the reactivity toward oxygen of excited states of dyes solubilized in the water pools it is important to know the diffusion-concentration product of oxygen in the water pools. Previous s t u d i e ~have ~ . ~ shown that O2can diffuse with great ease through the reversed micelles and that the effective oxygej concentration in aerated micelles is greater than in bulk water. We report here ESR studies of the diffusion-concentration product of O2 in reversed micelles of AOT in n-heptane as a function of solubilized-water concentration. We apply the procedure originally described by Povich: 'OJ' the ESR line broadening of a label due to spin exchange with paramagnetic molecular oxygen. Bacher et al.12used this method to follow oxygen consumption in respiring mitochondria whereas Windrem and
(1) Fendler, J. H. Acc. Chem. Res. 1976, 9, 153. (2) Eicke, H. F. In "Topics in Current Chemistry: Micelles"; Springer- Verlag: West Berlin, 1980; Vol. 87. (3) Correll, G. D.; Cheser, R. N.; Nome, F.; Fendler, J. H. J . Am. Chem. SOC.1978, 100, 1254. (4) Day, R. A.; Robinson, B. H.; Clark, J. H. R.; Doherty, J. V. J. Chem. Soc., Faraday Trans. 1 1979, 75, 119. (5) Robinson, B. H.; Steytler, D. C.; Tack, R. D. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 48 1 . (6) Zulauf, M.; Eicke, H. F. J . Phys. Chem. 1979, 83, 480.
(7) Wong, M.; Thomas, J. K.; Gratzel, M. J . Am. Chem. Soc. 1976, 98, 2391. (8) Wong, M.; Thomas, J. K.; Nowak, T. J . Am. Chem. SOC.1977, 99, 4730. (9) Matheson, I. B. C.; Rodgers, M. A. J. J . Phys. Chem. 1982,86, 884. (IO) Povich, M. J. Anal. Chem. 1975, 47, 346. (11) Povich, M. J. J . Phys. Chem. 1975, 79, 1106. (12) Bacher, J. M.; Budker, V. G.; Eremenko, S. I.; Molin, Y . N. Biochim. Biophys. Acta 1977, 460, 152.
0022-3654/84/2088-0280$01.50/0
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 2, 1984 281
O2 within Water Pools of AOT Micelles Plachy13 studied by the same method the diffusion-concentration product of O2 in bilayers. Recently, the same product was investigated also in bilayers by means of a hybrid method involving both saturation recovery and continuous wave ~ a t u r a t i 0 n . l ~ Fremy's salt (K2(S03)2NO)was used as a paramagnetic probe. Its ionic and hydrophilic characteristics allow one to avoid the difficulties arising when the probe location is not well-defined. Moreover, the absence of hydrogen nuclei in the free radical allows one to measure the effect of oxygen on the line width without correction for the inhomogeneous broadening due to the unresolved hyperfine splitting of protons.13 The variations in the Fremy's salt ESR spectrum were interpreted as functions of the physical properties (microviscosity, dimension) of the water pools.
Experimental Section Materials. Sodium diisooctyl sulfosuccinate (purum) was obtained from Fluka A.G. The material was purified as described by Matheson and R o d g e r ~ .Potassium ~ peroxylaminedisulfonate (PADS) (Aldrich Chemical Co.) and n-heptane (Merck p.a.) were used as supplied. Bidistilled water was used through all the M) was prepared experiments. Fremy's salt ''stock" solution ( in K2C03( 5 X M) buffered water to reduce the decomposition rate (1% per day). The concentration of the solution was measured optically by using the molar extinction coefficient15 6 (at 545 nm) = 20.8. The viscosities of the H20-glycerol (Merck, p.a.) mixtures were determined with an Ostwald viscosimeter. Line Broadening and Diffusion Solubility. The oxygen nitroxide spin-exchange frequency is related to the oxygen broadening of the nitroxide absorption line. If the line is Lorentzian, then the relationship is in which wE is the spin-exchange frequency, yE is the electron gyromagnetic ratio, and W, is the broadening (in gauss) of the peak-to-peak width of the first derivative spectrum (due to interaction with 02).The spin-exchange frequency is related to wc, the frequency at which a nitroxide molecule collides with 02,by the expression WE
= Pwc
(2)
in which p is the probability for spin exchange during a collision. The collision frequency is related by the Smoluchowski equation W,
= 4Td(Do, + DN)No,
(3)
to the distance d between O2and nitroxide centers upon collision, the sum of the oxygen (Do,) and nitroxide (DN) diffusion coefficients, and the number No, of oxygen molecules per unit volume of solution. Eliminating wE and w, from eq 1-3, and as DN is usually negligible compared with Do,, we obtain
wo = ADO2NO2
(4)
with
If we neglect the line-width contribution due to oxygen-nitroxide dipolar interaction13and let W, equal the line width in the absence of molecular oxygen, the total line width, W, is given by W = W, + Wo = W, + ADozNo2
(5)
Spectra Recording and Analysis. All the spectra were recorded on a Varian E9 X-band spectrometer. The modulation frequency was 100 kHz, the modulation amplitude was lower than 10%of the unbroadened peak-to-peak line width ( W,), the microwave power was kept lower than 20% of the saturation value, and the
Figure 1. ESR spectra of PADS solubilized in aerated solutions: (a) K2C0, (5 X M) buffered water, (b) AOT-H20 (vol 1%) reversed
micelles. TABLE I: Dependence of the Hyperfine Splitting Constant, PADS on the Amount of Water Solubilized in the AOT-Heptane Solution
a N , of
0.5
13.15 13.13 13.11 13.10 13.09
1
1.5 2 2.5 a Buffer
=5 X
3 3.5 1OOQ 100 (+2.5 M NaCI")
13.09 13.09 13.08 13.12
M K,CO,.
scan time was carefully adjusted to sweep the line in a time greater than 200 times the time constant of the spectrometer. Preliminary experiments had shown that the relative amplitudes of the three lines characteristic of aerated PADS solution were modified with the concentration of H 2 0 in the solution, especially for [ H 2 0 ] < 1%. A parameter m defined as the ratio between the intensities of the MI = -1 and M I = 0 lines has been used to check the variation of the concentration of H 2 0 during N2or O2 bubbling of the solution. Sample Preparation. Two milliliters of AOT (0.07 M) in n-heptane was introduced into a tube connected to an ESR flat cell. The final concentration of H 2 0 was achieved by solubilizing 10 pL of the PADS stock solution (final concentration = 5 X lo-* M) followed by addition of water. The tube was then closed by a septum and the solution was bubbled through a syringe needle. The tube was thermostated during bubbling and the gas was forced through a mixture of H 2 0and n-heptane (1/1) in order to warm it and prevent a vaporization of H 2 0 from the solution. After bubbling (30 min), the micellar solution was transferred into the ESR flat cell by shaking.
Results E S R Spectra of AOT in Reversed AOT Micelles. The line widths (W) of the PADS in aerated and deoxygenated water, respectively 0.26 and 0.16 G, and the hyperfine splitting constant (aN = 13.08 G) were found to be in good agreement with the published v a l ~ e s . ' ~ J ' Figure 1 shows the spectra of PADS solubilized in an aerated AOT-heptane solution containing 1% H 2 0 and in aerated water. The shape of both spectra of PADS has been found to be independent of the probe concentration in the range 10-5-10-4 M; a nitroxide-nitroxide interaction can therefore be ruled out. From the values reported by Eicke and Rehakl* on the average ag-
(13) Windrem, D. A.; Plachy, W. Z . Biochim. Biophys. Acta 1980,600, 655.
(14) Subczynski, W. K.;Hyde, J. S.Biochim. Biophys. Acta 1981, 643, 283. (15) Murib, J. H.; Ritter, D. M. J. A m . Chem. SOC.1952, 74, 3394.
(16) Jones, M. T. J . Chem. Phys. 1963,38, 2892. (17) Jpnes, M. T.; Ahmed, R.; Kastrup, R.; Rapini, V. J . Phys. Chem. 1979, 83, 1327. (18) Eicke, H. F.; Rehak, J. Helu. Chim. Acta, 1976, 59, 2883.
282
The Journal of Physical Chemistry, Vol. 88, No. 2, I984 I
I
1.01
Gandin et ai.
'-1 500 -
0.8 400 -
It i
h
-
EO I Oe4
o.2
i
1
2
H20 %
3
;,
300-
200 -
1OOJ
("/v)
gregation number of AOT molecules as a function of the amount of water solubilized, one can calculate that there is one label molecule for every three pools when R = 28.8 (3.5% HzO). The splittini constants of the PADS in AOT-heptane in the presence of different amounts of water are shown in Table I. The similarity of the aNvalues to that in bulk water indicates that the labels are located in the aqueous cores for all the sizes investigated. Studies on AOT reversed micellar systems using nitroxide probes derived from pyrr~lidine'~ and from piperidineZohave shown larger variations (10%) of the hyperfine coupling constant. This effect was explained by the partition (which is dependent of R) of these labels between two different positions, in the water pool and at the interface of water and heptane. The different behavior of PADS probably originates from its hydrophilic and ionic properties. It has been checked that PADS is soluble neither in heptane nor in a 0.07 M AOT solution without any added water. A small increase of the aNvalue, similar to this one observed with PADS solubilized in the micelles when R decreases, can be obtained in aqueous PADS solution by adding NaCl (Table I). The ratios ( m ) of the amplitude of the line at high field ( M I = -1) to the amplitude of the line at medium field ( M I = 0) of the probe in 0.07 M AOT solution in heptane solubilized with different amounts of water are shown in Figure 2. The m values indicate that the probe tumbling rate decreases when the size of the water pool becomes smaller. The pronounced break observed in the range R = 6.58 (0.8% H 2 0 ) to R = 9.88 (1.2%HzO)can be attributed to the presence or absence of free water. Indeed similar breaks around 1% water were observed in the study of water relaxation properties in AOT aggregates8 and in fluorescence properties of ANS' in the same system. These changes were attributed to the intermolecular forces prevailing within the water pools. As the Na+ (counterions)-HzO interaction energy is very high (25 kcal/mol HzO), it is anticipated that water is extremely firmly bound until the completion of the Na+ hydration shell, Le., [H,O]/[AOT] = 6. Water molecules up to a ratio [HzO]/[AOT] = 12 can still undergo binding by ion dipole forces or via hydrogen bonds to sulfonate or carboxyl groups present in the cavity. Only water in excess of R = 12 can be regarded as free with the properties of a true dispersion medium; so, for R > 12, the spin-label can be considered as immersed in water-free molecules and can rotate nearly freely ( m close to l ) , whereas for R < 12, ~
(19) Menger, F. M.; Saito, G.; Sanzero, G. V.;Dodd, J. R.J . Am. Chem. SOC.1975, 97, 909. (20) Yoshioka, H. J. Colloid Interface Sei. 1981, 83, 214.
2
1
4
Figure 2. Effect of added water (~01'3%)on rn (the ratio of the amplitude of the line at high field ( M I = -1) by the amplitude of the line at medium field (M, = 0)).
~~~~~
0 E
H20
3
%('/v)
Figure 3. Effect of added water (vol '3%) on the line width (W) of PADS solubilized in oxygen-saturated ( O ) , aerated (+), and deoxygenated (m) AOT-heptane solutions. Solid lines correspond to the least-squares fit of the experimental data.
400
1
Figure 4. Plot of the spin-exchange line-width broadening (Wool)of PADS vs. the amount of added water (~01%).Solid line corresponds to the least-squares fit of the experimental data.
free water is absent and the label rotates in a medium with high local microviscosity. Effect of Ozon the Line Width. Figure 3 shows the dependence of the line width W of the PADS spectrum ( M I = 0) vs. the amount of Solubilized water in aerated (wd"),deoxygenated ( Wa), and oxygen-saturated solutions ( p 2 ) . For all the H 2 0 contents examined, the presence of oxygen leads to a broadening of the lines, indicating oxygen diffusion through the aqueous core of the micelle. The ratio between the line broadening due to oxygen spin exchange in aerated solution ( Woair) to that in oxygen-saturated solution ( Woo2) was always found to be around 114.8, which is the theoretically computed value as a function of the number of oxygen molecules. The linewidth under nitrogen ( W,) decreases as R decreases. This narrowing, 35 mG for the solution at 1% compared with that of 3.5%, will be discussed later in relation with the microviscosity of the aqueous core. Finally, the linewidth broadening increases as R increases. The dependence of the broadening due to oxygen as a function of various amounts of water is illustrated in Figure 4. It appears that the broadening
The Journal of Physical Chemistry, Vol. 88, No. 2, 1984 283
O2within Water Pools of AOT Micelles
of the line width due to O2is a linear function of the H20contents of the micelle when it is expressed in percent in the range 1-3.5%. Linear regression gives Woo2 (mG) =
- W, = 122.6(vol % H 2 0 ) - 60.6 (6)
Introducing the radius of the aqueous core calculated by the relation7r (A) = 36.65v/g, which expresses the radius as a function of the percentage (v/v) of water (v) and of the percentage (w/v) of the detergent (g), relation 6 becomes Wool (mG) = 10r
(A) - 60.6
(7)
The introduction of the experimental expression of Woo' (eq 6) into relations 1 and 2 leads to a proportionality between the frequency of collisions (w,) and the percentage of incorporated water. From eq 3 the diffusion-concentration product DO2No2 is therefore proportional to the percentage of solubilized water and consequently (eq 7) to the radius of the aqueous core. Equations 6 and 7 indicate that the Do,No, product is 5.9 times greater in a micelle containing 3.5% water (r = 43 A) compared to a similar micelle containing only 1% water (r = 12 A). Whether this change involves either Do, or No2 separately or both factors simultaneously is discussed below. Dependence of the Diffusion-Concentration Product on the Water Pool Radius. Since the diffusion coefficient of oxygen is given by Do, = kT/(6aqa) in the Stokes-Einstein model, the dependence of Do2N02on the micelle radius can be explained by a variation of q or of No, when T is fixed. Previous investigations have shown that the viscosity pool is strongly dependent on the size of the water bubble especially when it is sma11.7~21~22 The observation of an ESR spectrum characteristic of the immobilized probe confirms its low tumbling rate constant under these conditions ( R C 1%). The lack of variation vs. the radius of the fluorescence lifetime of pyrenesulfonic acid solubilized in aerated AOT reversed micelles in n-heptane seems to indicate that the oxygen concentration in the aqueous core is c o n ~ t a n t .These ~ results suggest that the viscosity should change by a factor of 5.9 between 1% and 3.5% if it was to be at the origin of the change of Do2Noh Freed and co-workers have shown that the natural line width (W,) of the PADS spectrum is dependent on T and q. The proposed equation W , (G) = (2.54 X 10-4)(T/q)
+ 0.097
(8)
where T / q is the ratio between the absolute temperature (in Kelvin) and the viscosity expressed in centipoise (cP), allows one to calculate the narrowing due to an increase in the viscosity if the absolute viscosity is known. As W , for an aqueous solution of PADS and for PADS solubilized in a 3.5% HzO-AOT-heptane system are respectively 162 f 5 and 160 f 5 mG, the value of the microviscosity of the aqueous phase of the reversed micelles must be pratically the same as that of bulk water. If one assumes that, in 3.5% HzO, the viscosity probed by the PADS is equal to 0.93 cP, which is the viscosity of bulk water ( T = 23 0C),24the use of eq 8 allows one to compute a narrowing of 67 mG for 1% HzO. This expected value is greater than the narrowing, 35 mG, observed between 3.5% and 1% of water. As relation 8 was established for PADS solubilized in aqueous solutions at different temperatures (between 10 and 40 "C), only a small variation of the ratio T / q was investigated. In order to know the values of W, for PADS as a function of q at fixed T (our experimental conditions) for a larger range of T / q we have solubilized PADS in H,O-glycerol mixtures. In a system H20-glycerol (43% w/v) ( T = 296 K, q = 4 cP) we have obtained W, = 135 f 5 mG. From this narrowing (27.5 mG for 4.3 increase in q ) , if one computes the narrowing corresponding to an increase of 5.9 in viscosity, one (21) Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. (22) Keh, E.; Valeur, B. J . Colloid Interface Sci. 1981, 79, 465. (23) Eastman, M. P.; Bruno, G . V.; Freed, J. H. J. Chem. Phys. 1970,52, 2511. (24) "Handbook of Chemistry and Physics"; CRC Press: Boca Raton, FL, 1978-1979; p F51.
1
2
3
H20 % ('5'")
Figure 5. Effect of added water (vol W ) on the viscosity of the aqueous core of AOT reversed micelles in n-heptane.
should obtain 37 mG, which is consistent with the 35 mG actually observed. From the broadening due to oxygen in aerated water (100 mG) the line width in aerated H20-glycerol ( q = 4 cP) can be computed by the relation
Wd" = W, + 100(0,93/4)k where W, is the line width in the absence of O2 and k the oxygen solubility ratio between HzO-glycerol and HzO. Since k = 0.6,25 Wd" = 149 mG, which is very close to the observed 152 mG.
Discussion and Conclusions The variation of the line-width broadening which reflects the variation of Do2N02.seems therefore to be taken into account by the change in the microviscosity. Consequently, eq 6, which gives the values of Do2N01,can be used to calculate the change in the microviscosity within the water bubbles when they swell. In Figure 5 are illustrated the values of the water microviscosity probed by PADS. The relative values of viscosity are converted into absolute values by assuming that the viscosity of 3.5% H 2 0 micelles is equal to 0.93 cP. Although stabilizing PADS requires introducing K2CO3 into the aqueous core, the low concentration (from 2.5 X M at 1% HzO to 7 X M at 3.5% HzO) should not modify the viscosity of the aqueous phase. Comparison between the values of q illustrated in Figure 5 with published data is not possible: the only values computed for high water contents were published in a model involving a low-viscosity inner region analogous to bulk water and a high-viscosity boundary layer.21 If we assume that the viscosity change explains the variation of the oxygen diffusion-concentration product, we can estimate the concentration of oxygen in the micelles to be 0.75 times the M.26 This concentration of O2in bulk water which is 2.4 X value, 1.8 X M, of the 0,concentration can be used to calculate the average number of O2molecules present in a water bubble. This number, equal to or lower than 1, depedding on R, indicates that the PADS spin probe is serviced by oxygen molecules essentially coming from the organic phase. The bulk organic phase in which the molecular oxygen solubility is about 1 order of magnitude higher than in water must be considered as the oxygen tank from which the oxygen molecules diffuse through the water bubbles. (25) Schlapfer, P.; Audykowski, T.; Buckowiecki, A. Schweiz. Arch. Angew. Wiss. Tech. 1949, 15, 308. (26) Schlapfer, P.; Buckowiecki, A. "Mitteilungen iiber Kiihl und Frostschutzmittel fiir den Motorfahrzengbetrieb, Zurich, 1949", S.58.
J. Phys. Chem. 1984, 88, 284-288
284
These results are not consistent with the previously reported oxygen concentration value,' 5.5 X lo4 M. However, the authors indicated in their discussion that this value could not be attributed with great certainty to the aqueous phase because the fluorescent probe was probably not located only in the water core. Recently, the oxygen partition between the aqueous and hydrocarbon phases in AOT reversed micelles was m e a ~ u r e d .To ~ explain the small partition coefficient determined, 3.2, the authors, who were unable to calculate the values of the oxygen concentration in the two phases, suggest that the hydrocarbon phase may have some water content and the water phase a hydrocarbon content. The similar values of aN obtained for PADS solubilized in 2.5 M NaCl aqueous solution and in 1% H 2 0 micelles (Table I) must not be interpreted as an indication of a high concentration of free sodium ions in the water bubble. One would expect, in this case, an important decrease of the O2 solubility by a salting-out type effect. Published results on the binding of the Na+ counterions with the micellar surface show that, for 6% water micelles, the fraction of bound Na+ is higher than 0.72.8 As this fraction is
expected to increase for smaller micelles, few Na' are free in the range of investigated micelles (0.5-3.5% H 2 0 ) and consequently the concentration of free Na+ in the proximity of PADS is low. The change in the value of aN (Table I) which results from the variation of the spin density on the nitroxide group can be explained by a modification of the solvent-PADS interaction induced by the sodium ions bound to the micellar surface. A similar effect on nitroxide probes is well-known in aqueous salt solutions.27 Consequently, in the micelles, as the sodium ions do not occupy positions which are allowed to oxygen, only a small salting-out type effect on the O2 solubility is expected. In conclusion, it appears that oxygen can diffuse toward the site of the probe for all the water pool sizes investigated and that the increase of the oxygen diffusion-concentration product when the micelles swell is the consequence of a water viscosity change. Registry No. Oxygen, 7782-44-7; Aerosol-OT, 577- 11-7; potassium peroxylaminedisulfonate, 14293-70-0; heptane, 142-82-5. (27) BriEre, R.; Lemaire, H.; Rassat, A. Bull. SOC.Chim. Fr. 1965, ZZ, 3273.
Dielectric Study of a Binary Aqueous Mixture with a Lower Critical Point U. Kaatze* Drittes Physikalisches Institut, Universitat Gdttingen, 0 - 3 4 0 0 Gottingen, West Germany
and D. Woermann Institut fur Physikalische Chemie, Universitat Koln, 0 - 5 0 0 0 Koln, West Germany (Received: March 24, 1983; In Final Form: June 8, 1983)
Measurements of dielectric spectra (1 MHz I Y I 4 0 GHz) of aqueous solutions of 2,6-lutidine (2,6-dimethylpyridine)have been carried out in a concentration range 1 I c I 4 m ~ l - d m and - ~ a temperature range 2 0 I T I 32.6 O C . This system has a lower critical demixing point: c, = 2.3 m~l-dm-~, T, = 33.61 OC. For comparison measurements on solutions of pyridine in water are also included in this study. The temperature dependence of the complex permittivity of a lutidine solution of critical composition shows no critical contribution near the critical temperature (0.01 I T - T, I 12 K) e x d i n g the experimental error ( i l % ) . A Cole-Cole relaxation function is fitted to the measured dielectric spectra to analyze the data. The relative molal change of the main dielectric relaxation time of this function, Bd, is unexpectedly small (& = 0.1 kgmol-I; pyridine: Bd = 0.19 kgmol-'). This result can be explained by assuming solute association effects to be important even in 2,6-lutidine solutions of low concentration (c < c,).
system approaches this point. From a chemical point of view aqueous solutions of weak electrolytes exhibiting a critical mixing point might be suitable systems to study this phenomenon (e.g., isobutyric acid/H20; substituted pyridine compounds/H2,0). However, little is known about the structure and molecular motions ' situation in these mixture^^,^ except the protolytic r e a ~ t i o n . ~This has prompted us to carry out a dielectric study on 2,6-dimethylpyridine (2,6-lutidine)/water mixtures in order to find information on the microdynamics of the water molecules surrounding a lutidine molecule. The system 2,6-lutidine/water has (1) I. Procaccia and M. Gittermann, Phys. Reo. Lerr., 46, 1163 (1981). (2) H. G. E. Hentschel and I. Procaccia, J. Chem. Phys., 76, 666 (1982). (3) E. Wagner, J. Weidner, and H. W. Zimmermann, Ber. Bunsenges. Phys. Chem., 80, 405 (1976). (4) U. Kaatze and D. Woermann, Ber. Bunsenges. Phys. Chem., 86, 81 (1982). (5) M. Eigen, Angew. Chem., 75, 489 (1964). (6) M. Eigen, W. Kruse, G. Maass, and L. de Maeyer in "Progress in Reaction Kinetics", Vol. 2, G. Porter, Ed., Pergamon Press, New York, 1968, p 285. (7) M. Eigen in "Nobel Symposiums", Vol. 5, S. Claesson, Ed., Wiley, New York, 1968, p 245.
0022-3654/84/2088-0284$01.50/0
Experimental Section Materials. 2,6-Dimethylpyridine (2,6-lutidine) with a purity of 99% (Fluka) was fractionally distilled in a concentric tube column of 75 theoretical plates under reduced pressure. Pyridine (Merck, pro analysi) was used without further purification. Water was deionized by bed ion exchange. The samples were prepared by weighing appropriate amounts of the components into suitable flasks. The critical composition of the system lutidine/water having a lower critical point was determined on the basis of the equal volume criterium of coexisting liquid phases. It has a value of y , = 0.2533 (mass fraction of lutidine) corresponding to a critical concentration of c, = 2.3 m ~ l - d m - ~The . visually determined phase separation temperature of the critical mixture had a value of T, = 33.61 OC in accordance with the value given in the literature.* In addition to the sample of critical composition four solutions with concentrations in the range of 1 5 c 5 4 m ~ l e d m -were ~ prepared. The pyridine solutions also had concentrations in this range. The pK values of pyridine and lutidine (8) J. D.Cox, J . Chem. Soc., 3183 (1954).
0 1984 American Chemical Society
