J . Phys. Chem. 1988, 92, 4127-4134 role. In addition, the amounts and identities of coadsorbates have very strong effects on the formation and decomposition of the various species. The crowded surface exhibits very different reaction chemistry from that shown on less crowded surfaces. Such balances are undoubtedly at play at the conditions of catalytic reactions such as the Fischer-Tropsch synthesis and govern, among other things, the probability of higher hydrocarbon formation. Although the reactions seen here are not easily related to the conditions of catalytic reactions, this system can serve as a paradigm of the variety of effects that influence these reactions and can also give semiquantitative information about the reactions of surface species which are catalytically relevant. In detail, the results of this study can be summarized as follows: (1) There are two main channels of ketene chemistry on Ru(001). The first involves hydrogenation of ketene to oxygenated species, v2 C H 3 C H 0 and v2 C H 3 C 0 , while the second involves reactions of methylene groups from dissociated ketene including CCH, formation.
4127
(2) SSIMS results are consistent with both molecular and dissociative adsorption of ketene at 105 K. (3) Only HZ, CO, and C 0 2 are observed in TPD of adsorbed ketene. No hydrocarbons are desorbed. (4) Coadsorption with H2 favors the hydrogenation of ketene to v2 C H 3 C H 0 but does not result in hydrocarbon desorption. (5) During dosing at 350 K, ethylidyne and carbon monoxide accumulate and significant amounts of hydrogen desorb. (6) During dosing at 400 K both C O and H2 desorb, significantly higher quantities of ketene decompose, and no ethylidyne accumulates. (7) The selectivity for ethylidyne is related to the stabilization of CH, and CCH3 by CO.
Acknowledgment. J.M.W. gratefully acknowledges the support of the National Science Foundation under Grant CHE-8505413. Registry No. Ru, 7440-18-8;H2,1333-74-0;ketene, 463-51-4.
Ultrasonic Relaxation Studies of Ternary and Quaternary Systems Containing 2-Butoxyethanol, Water, Decane, and Cetyltrimethylammonium Bromide S . Kato,? D. Jobe, N. P. Rao,t C. H. H O , and ~ Ronald E. Verrall* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0 WO (Received: August 25, 1987; In Final Form: December 14, 1987)
Ultrasonic absorption measurements have been made in ternary microemulsions composed of oil (decane, DEC)-cosurfactant (2-butoxyethanol, BE)-water and quaternary microemulsions of oil (DEC)-cosurfactant (BE)-surfactant (cetyltrimethylammonium bromide, C,,TAB)-water in order to study the exchange processes of BE between the bulk phase and the large aggregates present in these systems. The studies were done in the frequency range 5-210 MHz and at temperatures between 25 and 45 "C. The effect of salt (sodium chloride and calcium chloride) on the ultrasonic absorption parameters was also studied in the quaternary systems. The results of these studies show that there are two relaxation processes which are located at 10 and -60 MHz for the majority of systems investigated. In the ternary systems, both relaxation frequencies are observed to be independent of decane concentration up to a specific mole percent and then they gradually decrease with increasing concentration of oil until phase separation occurs. In the quaternary systems, the higher relaxation frequency decreases while the lower one is invariant with increasing concentration of decane. The salts do not appear to affect, significantly, the ultrasonic parameters for the quaternary systems. The temperature dependence of the relaxation frequencies in ternary systems was analyzed to give an estimate of the activation enthalpies for the relaxation processes. Values of 15-25 and 13-17 kJ mol-' were obtained for the low- and high-frequency relaxation processes, respectively. Photon correlation spectroscopy (PCS) measurements were made in an attempt to obtain additional information on the nature and size of the aggregates present.
-
Introduction A number of thermodynamic studies suggest that the formation and stability of microemulsions may, in part, result from their dynamic character enhanced by the presence of cosurfactant (alcohol). However, only a few studies have been reported concerning the nature of the dynamical properties of cosurfactants. While the dynamics of cosurfactant molecules in micellar and microemulsion systems have been clarified somewhat by means of ultrasonic'" and other chemical relaxation studies,'-I0 most of these studies have focused primarily on dilute aqueous micellar systems and less on conditions of relatively higher concentrations of cosurfactant which are likely to be used in tertiary oil recovery methods. Therefore, studies of these systems covering a wider range of concentrations of cosurfactant, especially in the highconcentration region, are required to obtain a more complete understanding of the dynamical nature of the cosurfactant. 'On leave from the Department of Chemical Engineering, Faculty of
En ineering, Nagoya University, Nagoya
464, Japan.
!On leave from the Department of Physics, Sri Venkateswara University
Post Graduate Centre, Kavali 524202, India.
Research Chemistry Branch, Atomic Energy of Canada Limited, Pinawa, Manitoba ROE 1LO.
0022-3654/88/2092-4127$01.50/0
In a previous paper," the results of a detailed ultrasonic absorption study of the binary systems BE-water and ternary systems BE-water-C16TAB have been reported. This study was made with a view to assessing the kinetics of the exchange process of cosurfactant with large aggregates present in ternary systems containing high cosurfactant concentrations. Since microemulsions (1) Yasunaga, T.; Fujii, S.; Miura, M. J . Colloid Interface Sci. 1969, 30, 399. ( 2 ) Hall, D.; Jobling, P. L.; Wyn-Jones, E.; Rassing, J. E. J . Chem. SOC., Faraday Trans. 2 1977, 73, 1582. ( 3 ) Yiv, S.; Zana, R. J . Colloid Interface Sci. 1978, 65, 286. (4) Gettings, J.; Hall, D.; Jobling, P. L.; Rassing, J. E.; Wyn-Jones, E. J . Chem. SOC.,Faraday Trans. 2 1978, 74, 1957. (5) Lang, J.; Djavanbakht, A.; Zana, R. J . Phys. Chem. 1980,84, 1541. (6) Rao, N. P.; Verrall, R. E. J . Phys. Chem. 1982, 86, 4171. (7) Herrmann, U.; Kahlweit, M. Eer. Bunsen-Ges. Phys. Chem. 1973, 77, 1 1 19. (8) Aniansson, E. A. G.; Wall, S . N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J . Phys. Chem. 1976, 80, 90.5. (9) Zana, R.; Yiv, S . ; Strazielle, C.; Lianos, P. J . Colloid Inferface Sci. 1981, 80, 208. (10) Yiv, S.; Zana, R.; Ulbricht, H.; Hoffmann, H. J . Colloid Inferface Sci. 1981, 80, 224. (11) Kato, S.; Jobe, D.; Rao, N. P.; Ho, C. H.; Verrall, R. E. J . Phys. Chem. 1986, 90,4167.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 14, 1988
are multicomponent systems known to contain oil, surfactant, cosurfactant, etc., it is important that ultrasonic relaxation studies of the exchange processes of cosurfactant molecules in ternary systems consisting of oil, cosurfactant, and water also be made in order to better understand the dynamics of the cosurfactant molecules in the quaternary systems. In this paper attention is focused on studies of ternary systems composed of oil (decane)-cosurfactant (BE)-water and quaternary systems consisting of surfactant (C16TAB)-cosurfactant (BE)-oil (decane)-water, all having high concentrations of alcohol. Temperature-dependent studies also were carried out to provide an estimate of the activation enthalpies associated with the relaxation processes. The present studies were carried out with the following objectives: (1) to measure ultrasonic absorption as a function of frequency and to determine the number of relaxation processes which characterize the ternary and quaternary systems investigated; (2) to compare the relaxation times and activation enthalpies in ternary and quaternary systems described above with those obtained from our previous study" in order to observe whether any trends or changes at these different conditions would provide additional information for the assignment of the observed relaxation frequencies; and (3) to determine the effect of inorganic salts on the ultrasonic relaxation processes from the viewpoint of establishing, in a qualitative manner, their impact on the dynamics of the aggregates. The results of this study and those of our previous work" in ternary BE-C16TAB-H20 systems suggest similar processes are responsible for the observed relaxation phenomena. Although previous studies5 of quatemary systems containing sodium dodecyl sulfate (SDS) as the surfactant showed two ultrasonic relaxation processes, with the low relaxation process being attributed to surfactant exchange between the aggregates and the bulk phase, it seems unlikely that this would be the case for the quaternary systems reported here. First, we observe no ultrasonic processes in the binary H20-C16TAB systems in the range of ultrasonic relaxation frequencies investigated. Second, if the surfactant exchange process were to be shifted to higher frequencies in the presence of BE, as was found in the case of SDS in the presence of butanol, then, because of the lower relaxation frequency due to the longer alkyl chain in C16TAB,it is unlikely that the frequency would shift into the experimental range studied in this work. Therefore, based on this assumption, the results are interpreted in terms of the dynamical processes of the cosurfactant (BE). Experimental Section
Chemicals. 2-Butoxyethanol (BE) (Aldrich, Lot NO. 00406 KM), decane (Aldrich), and cetyltrimethylammonium bromide (C16TAB)(Sigma) were used in these experiments. C16TABwas recrystallized twice from ethanol while BE was distilled under reduced pressure (22 mmHg), and only the middle fractions were used. Decane was used without further purification. Solutions were prepared on a mass basis with deionized water obtained from a Millipore Super-Q system. Sound Absorption and Velocity Measurements. A previously described'' apparatus based on the pulse method was used for ultrasonic absorption measurements in the frequency range 5-2 10 MHz. The sound absorption coefficient was measured by varying the sound path length and observing the resulting attenuation in the intensity of the ultrasonic wave by monitoring the first echo. The accuracy in the measurement of sound absorption is &3%. Ultrasonic velocity measurements were carried out using an interferometer technique operating at a frequency of 4.00 MHz. All measurements were made under constant-temperature conditions maintained to hO.1 OC of the reported values. Photon Correlation Spectroscopy Measurements. A Malvern 4600 photon correlation spectrometer was used to estimate the diffusion coefficients from the fluctuation time constant of the diffraction pattern scattered by the small droplets. An argon ion ( 1 2 ) Verrall, R. E.; Nomura, H. J . Solution Chem. 1977, 6, 541.
Kat0 et al. TABLE I: Composition of Ternary Systems (Cosurfactant-Oil-Water) Investigated 2-butoxy-
ethanol mol %
system
wt %
T6 T7 T8 T9 T10 T11 T12 TI3 T14 T15 T26 T27 T28 T29 T30 T3 1 T32 T33
73.8 67.5 65.5 62.7 61.4 41.5 60.0 59.0 56.4 64.0 83.0 67.0 55.3 98.0 73.5 48.7
21.1 7.8
30.0 29.2 28.8 28.3 28.1 10.0 18.6 18.5 18.3 36.0 42.7 39.7 35.2 88.2 70.6 50.2 23.7 9.1
water wt%
26.2 26.2 23.3 22.3 21.9 57.0 40.0 39.3 37.6 14.6 17.0 13.0 11.3 2.0 1.5 1.0 0.4 0.2
mol % 70.0 67.7 67.1 66.1 65.5 89.7 81.4 81.0 30.1 54.0 57.3 50.5 47.2 11.8 9.5 6.8 3.0 1.5
decane mol 5%
wt % 8.6 11.2 15.0 16.7 1.6
3.1 4.1 5.6 6.4 0.3
1.7 6.1 21.4
0.4 1.6 10.0
20.0 33.3
9.8 17.6
25.0 50.3 78.5 92.0
19.9 43.0 73.3 89.4
laser (100 mW) was used as the light source. The viscosities required to estimate droplet sizes were measured by the capillary flow method using an Ostwald viscometer. Details of the technique and the estimation of the size of the aggregates are given elsewhere." Data Analysis
The ultrasonic absorption, a/f,was measured as a function of frequency,$ The analysis of the absorption data was carried out using the following equations and assuming that the results indicate either one or two discrete ultrasonic relaxation processes are present
P' = (a/f- B)fu
(2)
where f represents the measured frequency,f, is the ith relaxation frequency, A, is the relaxation amplitude, B is the contribution to sound absorption from any other processes that may be occurring at higher frequencies beyond the frequency range of measurement, u is the ultrasonic velocity, and p' is the absorption per wavelength. In the case of double-relaxation processes, suffixes 1 and 2 refer to the low- and high-frequency processes, respectively. The data analysis has been described in greater detail elsewhere.'] A double-relaxation equation was required to fit the sound absorption data in all systems investigated. In most cases the error of fit was less than 2.8%, and in the extreme case it rose to 3.6%. Values of the enthalpies of activation were obtained by means of the Eyring rate expression
fi,= kT/2~h{exp(-AH,*/R7') exp(AS,*/R)]
(3)
where AH,'and AS,' are the enthalpy and entropy of activation, respectively, with respect to the ith relaxation process and T, R, k, and h all have their usual meaning. One can estimate the values of AH,' from the slope of a plot of In (27rfJT) vs 1/T. Experimental a/f data for all systems studied are found in the supplementary material (see the paragraph at the end of text regarding supplementary material). Results Ternary (BE-Decane- Water) Systems. Table I shows the composition of the ternary systems consisting of cosurfactant (BE), oil (decane), and water. The mole ratio of BE to water was kept constant while increasing the composition of oil in these systems. The ternary solutions investigated here have the same "mixed solvent" mole ratio BE/water as those reported" for the C16TAB-BE-water systems; CI6TABhas been replaced by decane, and one can directly compare the effect of these two solutes on
Ultrasonic Relaxation of Ternary and Quaternary Systems
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4129
TABLE II: Ultrasonic Relaxation Parameters and Sound Velocity Data for Ternary Systems (DEC-BE-H20) at 25 OC' A , x 1017, frp A2 X loL7, fr, B x 1017, system cm-I s2 MHz cm-I s2 MHz cm-' s2 m4s-l T6 158 13 85 140 117 1399 1.44 T7 237 13 89 140 94 1353 2.08 T8 302 13 84 140 94 1343 2.68 T9 650 7.0 100 100 100 1328 3.02 T10 600 6.0 170 34 130 1328 2.39 Tllb T12 610 1.6 172 89 158 1430 3.31 172 88 1414 T13 654 7.6 136 3.51 180 86 T14 1233 7.6 127 1381 6.47 T15 205 9.0 65 39 105 1305 1.20 65 150 65 1374 T26 60 15 0.62 T27 92 13 40 110 80 1304 0.78 81 1252 1.56 T28 416 6 103 40
X";s$
,"yj3 8.32 8.43 7.90 6.64 3.84 10.9 11.8 10.7 1.65 6.70 2.87 2.58
For T29-T33, there was no relaxation observed. bNear phase separation region. 7001
\
I
I
\o
VI
II
\\\Q,
,OOt 250F
-
500 400
asi
4 5 'C
300 2 00
100 ,
01 2
--
I 1 1 1 1 1 1 1
3
5
7
10
I
I I I IIIII
20 30 f
50 70 100
3
5
f
200
MHz
Figure 1. Ultrasonic absorption as a function of frequency for ternary systems composed of decane-BE-water at 25 OC. Solid lines represent the calculated relaxation spectra from eq 1, and arrows show the location
20 30
7 10
50 70 100
200
MHr
Figure 2. Ultrasonic absorption as a function of frequency for ternary system T15 at 25, 35, and 45 OC.
of relaxation frequencies.
700
the ultrasonic relaxation spectra arising from the BE-H20 solvent system. The compositions of systems T9, T10, T11, and T28 are near the phase separation region where critical solution phenomena can occur. Sound absorption measurements also were carried out in ternary systems rich in oil (T30-T33) in order to determine whether any ultrasonic relaxation processes can be observed in the oil-rich regions of these ternary systems, but none were observed. The temperature dependence of the ultrasonic absorption of systems T15 and T26-T28 was carefully measured to obtain estimates of the apparent activation enthalpies of the relaxation processes in these solutions. Figure 1 shows a typical result of the sound absorption measurements and data analysis for the ternary systems. The solid lines in the figure represent the calculated ultrasonic relaxation spectra from eq 1, and the arrows show the location of the relaxation frequencies. It is seen from this figure that a double relaxation is required to adequately fit the experimental data. On the contrary, for systems T29-T33 (Figure 1) no ultrasonic absorption is observed in the accessible frequency range, and one can conclude that there are no chemically-related relaxation processes of the type obviously present in the other systems. Systems T9 and T10, whose compositions are in the vicinity of the phase separation region, show a large ultrasonic absorption at low frequencies. Table XI lists the values of the estimated parameters from eq 1 for the ternary systems, as well as the peak sound absorption per wavelength, pmax,,which is defined as pmax,= (Afr,u)/2 (i = 1, 2) (4) Figure 2 shows a typical example (system T15)of the temperature variation of ultrasonic absorption. Table 111 summarizes
600
t x k 3
Nv)
7
500
E, 400
0 300 al",
200
__
100
I 3
I I I1111 5 7 10
1 20
I 30
I I I Ill1 50 70 100
200
f MHz
Figure 3. Ultrasonic absorption as a function of frequency for quaternary systems Q1 to Q3 composed of C,,TAB-BE-water-decane. For comparison purposes data for the ternary system T41 are also plotted.
the values of the relaxation parameters derived from these temperature studies. Quaternary (C16TAB-BE-Decane- Water) Systems in the Presence and Absence of Added Salts. Table IV lists the compositions of the quaternary systems investigated. The majority of the ultrasonic absorption measurements were carried out under the conditions that the relative proportions of cosurfactant (BE), surfactant (CI6TAB),and water were kept constant while that of the oil was increased. Typical results of the sound absorption measurements and their analysis for quaternary systems are shown in Figure 3. The result for the ternary system T41 is also shown for comparison. A double relaxation is sufficient to adequately fit the experimental data.
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The Journal of Physical Chemistry, Vol. 92, No. 14, 1988
Kat0 et al.
TABLE III: Temperature Dependence of Ultrasonic Relaxation Parameters and Sound Velocity Data for Ternary Systems T15 and T26-T28 (DEC-BE-H20) temp, A , X loL7, A,, A2 X fry B x 1017, system O C cm-l s2 MHz cm-l s2 MHz cm-I s2 m 11,s-l T15 25 205 9 65 39 105 1305 1.20 1.65 35 114 13 70 49 76 1272 0.94 2.18 82 18 45 55 60 63 (1 239)” 0.91 2.04 77 12 62 110 T26 15 115 1404 0.65 4.79 60 15 25 65 150 65 1374 0.62 6.70 55 170 40 23 45 35 1342 0.62 6.27 164 10 71 90 T27 15 95 1337 1.09 4.27 92 13 25 40 110 80 1304 0.78 2.87 40 20 27 150 35 60 1273 1.14 2.58 416 T28 25 6.0 103 40 81 1288 1.61 2.65 236 35 8.0 50 66 68 1252 1.18 2.13 221 60 45 9.5 57 47 (1220)“ 1.28 2.09
X”Y3
pia;;,
” Extrapolated. TABLE I V Composition of Quaternary Systems Investigated system T4 1 Q1 42 43 T42 44 Q5 46 Q7 Q8 Q9 QlO
2- butoxyethanol wt % mol ’% 51.2 18.3 47.1 17.8 17.4 43.5 40.6 17.0 60.9 29.2 56.1 28.2 51.8 27.3 29.7 21.5 53.5 18.2 54.5 37.9 47.4 34.9 33.4 16.3
water wt ’% 34.2 31.4 29.0 27.0 21.7 19.9 18.4 9.7 35.7 9.3 8.1 22.0
mol ’% 80.0 78.0 76.0 74.2 68.1 65.8 63.6 46.0 79.6 42.4 39.0 70.3
Wt % 14.6 13.5 12.4 11.6 17.4 16.0 14.8 10.6 5.14 3.5 3.0 18.6
C16TAB mol % 1.7 1.7 1.6 1.6 2.7 2.6 2.5 2.5 0.6 0.8 0.7 2.9
wt % 0 8.0 15.0 20.8 0 8.0 15.0 50.0 5.8 32.8 41.5 26.0
decane mol % 0
note a
T41 7.2 0 6.6 30.0 1.6 18.9 25.4 10.5
+ DEC
a
T42
+ DEC
b C
d e
“Ternary systems. bCompositions of the three components BE, H20, and C16TABare nearly the same as those in T42. cT14 + CI6TABwhere the composition of T14 has been presented previously.” dCompositions of the three components BE, H 2 0 , and C16TABare intermediate to those of T33 and T49. eRatio of BE/H20 is equal to that of T41. TABLE V Ultrasonic Relaxation Parameters for Quaternary Systems Consisting of CgnTAB,BE, H20.and DEC at 25 OC A , x 1017, A2 X lOI7, B x 1017, system At, MHz cm-I s2 A*>MHz cm-I s2 cm-I s2 u, m s-I pmax2 x io3 pmx, x io3 T4 1 205 7.7 246 87 134 1435 1.13 15.4 Q1 257 7.7 246 66 127 1385 1.37 11.2 42 436 8.0 228 65 115 1367 2.38 10.1 43 540 7.6 228 67 92 (1 349)‘ 2.77 10.3 T42 173 8.9 150 108 127 1406 1.os 11.4 Q4 180 8.9 148 94 122 1367 1.09 9.51 Q5 24 1 8.5 150 84 114 1343 1.38 8.46 Q6 260 9.1 50 69 92 47 495 8.0 220 70 128 150 12 41 83 92 Q8 12 42 83 92 160 Q9 QlO 483 7.1 149 52 123 Mole ratio of B E / H 2 0 is equal to 0.23.
Mole ratio of B E / H 2 0 is equal to 0.45.
The relaxation frequency f,,shifts to lower frequencies with increasing addition of oil whilef,, does not change. Table V summarizes the ultrasonic relaxation parameters calculated for these quaternary systems at 25 OC. An attempt was made to observe the effect of inorganic salts on the sound absorption spectra in quaternary systems 4 6 and QlO using sodium chloride and calcium chloride. The temperature dependence of the ultrasonic absorption also was measured for quaternary systems, with and without added salts. Figure 4 shows the effect of both temperature and added salt on the ultrasonic absorption spectra for the quaternary system QlO. The salt is seen t o have only a slight effect on the ultrasonic absorption a t low frequencies, and it falls within the experimental error of measurement. Table VI lists the relaxation parameters obtained from the analysis of the absorption data for these systems. PCS Data. Table VI1 shows the values obtained for the apparent Stokes radius, r,, of diffusion, assuming spherical aggregates, in several of the ternary and quaternary systems. The values
note a
b
Extrapolated.
700
500
300
100
3
5
7
10
2 0 30 f MHz
50 70 100
2 0
Figure 4. Ultrasonic absorption as a function of frequency and temperature for quaternary system QlO (0)and quaternary systems having salts, 4 1 2 ( 0 )and 4 1 3 (a).
Ultrasonic Relaxation of Ternary and Quaternary Systems
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4131
TABLE VI: Effects of Temperature and Added Salts on the Ultrasonic Relaxation Parameters for Quaternary Systems temp, A~ x 1017, fr,, A~ x 1017, fr2' B x 1017, system OC cm-' s2 MHz cm-I sz MHz cm-I s2 25 260 9.1 50 69 92 Q6 35 186 12.3 55 78 80 QlO 25 483 7.1 50 123 149 35 320 7.5 154 65 99 45 181 12 (160)' (80)' (44Y Q1 Id 25 260 9.1 50 69 90 35 186 12 55 65 79 Q12d 25 483 7.1 149 52 123 35 320 7.5 154 65 99 Q13d 25 48 3 7.1 149 52 123 35 320 7.5 154 65 99 " 4 6 + 1.35 m NaCI. bQIO + 0.472 m NaC1. cQIO + 0.338 m CaCI2. relaxation parameters from the two lowest temperatures. TABLE VII: Values of the Apparent Stokes Radius (r,) for Systems of BE-H20-DEC and BE-H20-DEC-CI6TAB at Various Values of Rap system RBE mol 7% decane r,, nm group T12 T13 T14 T6 T7 T8 T26 T21 T28 T41 Q1 Q2 T42 44 Q5 46
0.23 0.5 1.6 0.43 8.6 11.2 0.75 0.23 0.43
+ 0.5 m C16TAB + 0.5 m
9.8 17.6 2.5 5.0
C16TAB 3.3 6.6 30.0
10 4.7 110 40 31 16 21 20 5.8 5.8 8.7 28 4.2 4.7 4.8 25
A
B
note
a
b C
the presence of added salt. 'Estimated from data analysis and
TABLE VIII: Activation Enthalpies for Ternary (BE-DEC-H20) and Quaternary (BE-DEC-H20-CI6TAB) Systems system AH,',kJ mol-' AH2', kJ mol-' T15 24.8 f 1.0 14.3 f 0.4 13.3 f 6.4 T26 21.7 f 0.9 T27 23.2 f 7.0 16.5 f 4.6 T28 15.3 f 4.0 13.3 f 1.2 QlO 19 f 10 14 f 0.4"
Weighted from data for the two lowest temperatures. C D
E
given for groups A and B are for ternary systems where the mole ratio of BE/H20, RBE, is kept constant with increasing mole fraction of decane. The data in groups E and F are for solutions having the values of RBE= 0.23 and 0.43, respectively, and which are 0.5 m in C16TABwith respect to the mixed solvent BE HzO.
t
j T \* ,
+
Discussion Assignment of Observed Ultrasonic Relaxation Processes in the Ternary Systems. From the results obtained in this study and one previously reported," it appears that the ultrasonic relaxation processes observed in the BE-water-DEC systems, containing relatively high concentrations of alcohol, can be assigned to the merging of "clathrate-like" structures of alcohol-water and self-association of the alcohol. There are several observations arising from the results of this work that appear to support this tentative conclusion: (1) The magnitude of the relaxation frequencies and relaxation amplitudes observed (Table 11) are very similar to those reported" previously for C16TAB-BE-water systems. (2) The temperature variation of sound absorption in the systems reported here show a decrease in absorption with increasing temperature (cf. Figure 2 and Table 111), a result usually associated with equilibrium processes involving hydrogen-bonding structures and not with processes due to critical solution phenomena." (3) Values of the activation enthalpies (Table VIII) derived from temperature studies of both ternary systems are very similar and are of the same magnitude obtained in the binary BE-water systems." Figure 5 shows the dependence of relaxation frequenciesf,, and f,, vs decane concentration (mol %) for ternary systems having constant mole ratios of BE to water. Relaxation frequencies for ternary systems composed of C16TAB-BE-water1 are included for comparison. It is seen that both relaxation frequencies are approximately independent of oil concentration up to a specific (13) Fanning, R. J.; Kruss, P. K. Can.J . Chem. 1970, 48, 2052.
0
I
I
1
I
2
4
6
8
10
C16TAB or Decane m o l %
Figure 5. Relaxation frequenciesf,, and& as a function of CI6TABor decane compositions for ternary systems of CI6TAB-BE-water and decane-BE-water. Filled and open symbols represent surfactant (C16TAB) systems and oil (decane) systems, respectively.
mole percent and then they decrease with increasing addition of oil, toward the onset of phase separation. Furthermore, it is to be noted that the independence of the relaxation frequencies on oil concentration appears to be maintained to higher concentrations when the mole ratio of BE/water is greater. On the other hand, it is observed" that the ultrasonic relaxation frequencies decrease almost linearly with increasing C16TABconcentration in systems in which DEC has been replaced by surfactant at the same high mole ratios of BE/water. A possible, though somewhat speculative, explanation for the C16TAB-BE-water results was presented previously." It was proposed that units of CI6TAB-BEwater begin to form, thus depleting the amount of associated alcohols and merged clathrates of BE-water. The estimated stoichiometry of the CI6TAB-BE-water units is consistent with low aggregation numbers found for surfactants in alcohol-water mixtures of relatively higher alcohol concentration^.'^
4132
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 a
E
"
I
Kat0 et al.
b
6001
looo~/
5
800
400
'0 Decane mol %
u 2 4 6 8 1 0 Decane mol %
Figure 6. (a) Relaxation amplitudes A , ( 0 )and A2 (0) as a function
of decane composition for ternary systems composed of decane-BEwater. (b) Relaxation amplitudes A I ( 0 )and A2 (0)as a function of decane compositions for quaternary systems composed of C,,TAB-BEdecane-water. The narrow monophasic region of the BE-H204ecane system limited ultrasonic studies to low oil compositions. In this region there is an apparent invariance off,, andf,, with increasing oil composition. It appears that decane can be solubilized to form monophasic solutions and yet not have any obvious effect on the aggregates postulated to be present in the BE-water mixed solvent. There have been reports's-'8 of 2-propanol exhibiting surfactant-like properties in some ternary systems composed of oilalcohol water. It has been reported also that thermodynamic properties of the ternary systems 2-propanol-ben~ene-water'~,'~ or 2-buto~yethanol-benzene-water~~ show all the features of microemulsion systems. By analogy with the 2-propanol behavior and considering the fact that BE has a similar chemical structure to that of poly(oxyethy1ene)-alkyl alcohols, known to be typical nonionic surfactants, it could be expected that BE may act as a surfactant in these ternary systems with the BE molecules residing almost entirely at the oil-water interface. A similar model has been presented by Lianos et aLzoand other investigators,2'*22and it has been widely accepted to represent the solubilization of oil in alcohol-water systems. A comparison of the ultrasonic relaxation data obtained here for the BE-H20-DEC system with those of our previous study of BE-H@-Cl6TAB is in agreement with this model. It has been shownz3that the addition of nonpolar additives, such as decane, to miceller solutions results in no change in the relaxation frequency of the surfactant. However, the addition of polar compounds, which are adsorbed at the surface of the micelle, causes the relaxation frequency of the surfactant exchange process to change. If BE is considered as a "pseudosurfactant", then the low-frequency relaxation process could be similar to that of the fast exchange of surfactant monomers between the bulk phase and the micelle. The addition of decane to solutions with RBE = 0.43 is seen to have no effect on the BE exchange process; however, the addition of C16TABto these same solutions causes a decrease in the relaxation frequency. These results are consistent with BE behaving as a "pseudosurfactant". As noted in the preceding section, the compositions of the systems T9, T10, T11, and T28 are in the vicinity of the phase (14) Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 7 2 , 171. (15) Smith, G. D.; Donelan, C. E.; Barden, R. E. J. Colloid Interface Sci. 1977, 60, 488. (16) Keiser, B. A,; Varie, D.; Barden, R. E.; Holt, S.L. J . Phys. Chem. 1979, 83, 1276. (17) Lara, J.; Perron, G.; Desnoyers, J. E. J . Phys. Chem. 1981, 85, 1600. (18) Lara, J.; Avedikian, L.; Perron, G.; Desnoyers, J. E. J. Solution Chem. 1981, 10, 301. (19) Lara, J. Ph.D. Thesis, Universitt de Sherbooke, 1981. (20) Lianos, P.; Lang, J.; Zana, R. J . Phys. Chem. 1982, 86, 4809. (21) Shinoda, K. Solution andSolubility (in Japanese Yoeki to Yokaido); Maruzen: Tokyo, 1980; p 212. (22) Biais, J.; Bothorel, P.; Clin, B.; Lalanne, P. J . Colloid Interface Sci. i981,80, 138. (23) Jobe, D. J.; Reinsborough, V. C.; White, P. J. Can. J . Chem. 1982, 60, 279.
Decane mol % Figure 7. Relaxation frequencies f,, and f,, as a function of decane
compositions for quaternary systems composed of C,,TAB-BEAecanewater. Oil concentration was changed by adding decane to ternary systems having constant compositions of surfactant (C,,TAB), cosurfactant (BE), and water. Variation of relaxation frequencies with composition of decane (cf. Table VII) in the systems T41-Q3 (series D, 0 ) and in the systems T42-Q6 (series E, 0),respectively. 4 8 and Q9 ( 0 ) are for values of RBE= 0.9 and the concentration of surfactant is 0.16 M with respect to the mixed solvent. The dashed line is an extrapolation to ternary system having corresponding composition. separation line where critical phenomena occur. Ultrasonic relaxation due to critical phenomena has been reported by a number of investigators.*28 Generally, the ultrasonic absorption becomes extremely large near the critical point. Although ultrasonic relaxation processes arising from critical phenomena cannot be well-expressed by a single relaxation equation, it is interesting to note that the ultrasonic absorption values measured at higher decane concentrations are seen to be greatest at low frequencies, Le., 20 MHz, one may conclude that there is no significant change within the experimental error. It has been shown3s that the addition of salts increases the relaxation frequency of the (29) Lang, J.; Djavanbakht, A.; Zana, R. In Microemulsions; Robb, I. D., Ed.; Plenum: New York, 1981; p 233. (30) Aniansson, E. A. G.;Wall, S . N . J . Phys. Chem. 1974, 78, 1024. (31) Ito, N.; Fujiyama, T.; Udagawa, Y. Bull. Chem. SOC.Jpn. 1983, 56, 379. (32) Lang, J.; Tondre, C.; Zana, R.; Bauer, R.; Hoffmann, H.; Ulbricht, W. J . Phys. Chem. 1975, 79, 276. (33) Ikeda, S.; Ozeki, S.; Tsunoda, T. J . Colloid Interface Sci. 1980, 73, 21. (34) Porte, G.; Appell, J.; Poggi, Y. J . Phys. Chem. 1980, 84, 3105. (35) Ozeki, S.; Ikeda, S . Bull. Chem. SOC.Jpn. 1981, 54, 552. (36) Ozeki, S.; Ikeda, S . J . Colloid Interface Sci. 1982, 87, 424. (37) Lang, J.; Zana, R. J . Phys. Chem. 1986, 90, 5258. (38) Zana, R.; Yiv, S . Can. J . Chem. 1980, 58, 1780.
The Journal of Physical Chemistry, Vol. 92, No, 14, 1988 4133 surfactant exchange between micelles and the bulk phase. The apparent independence of the relaxation frequency on salt composition supports the conclusion drawn here and elsewhere" that these processes are associated with exchange of cosurfactant and do not involve relaxation processes with the surfactant. These experimental results also rule out the possibility that the relaxation process, AI, in the quaternary systems may be ascribed to an ion-binding process between the counterion and the micelle. In such a case the expected ultrasonic relaxation spectra would be strongly affected by the replacement of sodium chloride with calcium chloride39 and would depend on the salt concentration. Such processes probably occur at higher frequencies, beyond the frequency range investigated. PCS Data Analysis. A comparison of the results from this and a previous study" enables several conclusions to be drawn. The addition of C16TABto ternary systems of decane-BE-H20 increases the solubility of decane, possibly through the formation of bicontinuous phases. The data in Table VI1 indicate that, in the presence of CI6TAB,an almost constant particle size is obtained, regardless of the oil concentration. This is not the case for systems which do not contain CI6TAB. The stabilizing effect of C16TABon particle size has been observed in previous studies with the BE-H20 systems." The addition of decane to the binary BE-HzO system results in an apparent decrease in the average size of BE-HzO aggregates followed by (in the case of RBE = 0.23) an increase as the critical demixing concentration is approached. This is also indicated by a sharp increase in the value of the ultrasonic parameter Al. For those solutions with a higher BE composition (RBE = 0.43 and RBE = 0.75) a decrease in aggregate size is observed with increasing decane concentration, but an increase in aggregate size does not appear at the highest compositions studied. This may be due to a shift in the critical demixing concentration due to the increased RE composition. Because of the strong similarities of aggregate size dependence on composition for the ternary systems studied here and previously," one may conclude that the structures present in these systems are similar. This conclusion is also strongly supported by the similarities in the ultrasonic data for the two studies. Although the interpretation of PCS results can often be difficult, the relative trends can still be used to indicate, qualitatively, possible structural changes occurring in these systems. The above conclusions cannot be made solely on the basis of the PCS data. However, together with the ultrasonic data from this and a previous study." it may be concluded that aggregate structures found in the binary systems may also exist in ternary and quaternary systems containing surfactant and/or oil, especially when the concentration of cosurfactant is high. These systems obviously are very complex and difficult to interpret. However, it is felt that this study provides a qualitative picture that will help to better resolve the nature of these aggregates in future work.
Conclusions The ultrasonic and PCS results for these systems show strong similarities with those reported previously" for other related systems. The addition of surfactant to ternary systems of BEDEC-H20 and binary BE-HzO appears to lead to stabilization of aggregates of a maximum average size, suggesting that similar structures may be present in all systems. Also, in both ternary (BE-HzO-C16TAB) and quaternary (BE-C 16TAB-H20-DEC) systems there is a strong concentration dependence of the ultrasonic relaxation frequencies with surfactant composition, especially at higher alcohol concentrations. On the other hand, in the ternary system (BE-HzO-DEC), i.e., in the absence of C16TAB, there is very little change in the magnitude of the ultrasonic relaxation frequencies with increasing oil concentration. This may imply that these aggregates may be similar to those believed to occur in the binary BE-H20 system. (39) Atkinson, G.; Baumgartner, E.; Fernandez-Prini, R. J . Am. Chem. SOC.1971, 93, 6436.
J . Phys. Chem. 1988, 92, 4134-4141
4134
It is important to note that CI6TABis relatively insoluble in BE and decane as compared to H20. Therefore, solubilization of CI6TABrequires a “water-rich” environment. Consequently, as suggested in our previous study,” the addition of CI6TABto BE-H20 systems at high alcohol compositions promotes the breakdown of merged clathrate structures and hence may give rise to the observed decrease of the ultrasonic relaxation frequencies with increasing CI6TABcomposition. The breakdown of merged clathrates to form bicontinuous phases may also explain the origin of the dependence of the high-frequency ultrasonic relaxation process on DEC composition in the quaternary systems BE-H20-DEC-C16TAB. Recently, Evans et have suggested a bicontinuous-phase model where the oil and water can coexist as separate phases with the surfactant
and/or cosurfactant acting as an “interfacial buffer” between these two phases. Although it would be interesting to interpret the data in terms of this model, additional conductivity and diffusion studies of the systems could be required to enable such an analysis.
Acknowledgment. The financial assistance of the Alberta Oil Sands Technology and Research Authority and the Natural Sciences and Engineering Research Council of Canada is acknowledged. S.K. and N.P.R. thank Nagoya University and Sri Venkateswara University, respectively, for allowing a leave of absence to perform this work. S.K. also thanks the Yoshida Foundation of Science and Technology for financial support in the form of a travel grant. Registry No. C16TAB,57-09-0;DEC, 124-18-5;BE, 111-76-2:NaCI, 7647-14-5;CaC12, 10043-52-4. Supplementary Material Available: Tables of ultrasonic absorption data for various systems (6 pages). Ordering information is given on any current masthead page.
(40) Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1986, 90, 226. (41) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S . J . Phys. Chem. 1986, 90, 842.
Dynamic Properties of p-Xylene Adsorbed on Na-ZSM-5 Zeollte by Deuterium and Proton Magic-Angle Sample Spinning NMR I. Kustanovich, D. Fraenkel, Z. Luz, S. Vega,* The Weizmann Institute of Science, Rehovot 76100, Israel
and H. Zimmermann Max- Planck-Institut fur Medizinische Forschung, 0-6900 Heidelberg. West Germany (Received: August 26, 1987; In Final Form: December 12, 1987)
Solid-state deuterium and proton magic-angle sample spinning (MASS) NMR measurements as function of temperature, on two deuteriated p-xylene species, CH3C6D,CH3and CD3C6H4CD3,sorbed in Na-ZSM-5 zeolite are presented. The results are interpreted in terms of possible dynamic state and sorption sites of the sorbed molecules. Altogether five species are identified, whose relative abundances vary with the loading level and the temperature of the sample. The various species differ in the dynamic state of the phenyl ring and the molecular axis, but in all of them, at least down to --I50 O C , there is fast reorientation of the methyl groups. The five dynamic states are (1) static molecules, (2 and 3) molecules whose phenyl rings undergo, respectively, dicrete 180’ flips and free continuous rotation about their para axis, and (4 and 5 ) molecules whose molecular axes exhibit twofold jumps through 90” and 112O, respectively. The various species can be attributed to (1) rigidly sorbed molecules in the zeolite channels, (2 and 3) molecules that undergo local reorientations about their para axis, and (4 and 5) molecules sorbed near intersections between channels and undergoing two-site jumps between neighboring segments of the straight and zigzag channels or between neighboring zigzag segments.
I. Introduction Among the several dozens or so of zeolites known to date,’ the various forms of ZSM-5 occupy a unique place because of their industrial importance, including in particular the manufacture of gasoline from methanol and the catalytic production of p-xylene in the polymer industry.2 The high catalytic selectivity of ZSM-5 (as indeed of other zeolites) is attributed to the unique topology and dimensions of the inner channels and cavities in the zeolite framework. This framework serves as a molecular sieve that allows only molecules with certain shapes and sizes to penetrate into and diffuse through the zeolite’s pores. Although the crystallographic structure of the ZSM-5 zeolites is well understood through X-ray and neutron diffraction ~ t u d i e s much ,~ less is known about the nature of the intercalation sites and the modes of motions of sorbed molecules within the zeolite channels. The present work is aimed to fill this gap. It is concerned with a study of the structure and dynamics of p-xylene sorbed in the zeolite Na-ZSM-5, with use
of solid-state N M R technique^.^^^ ZSM-5 is a highly siliceous, thermally stable, synthetic zeolite with the unit cell formula NanA1,Sig6-,01g2 ( n == 3). It has orthorhombic (P-) symmetr with unit cell dimensions a = 20.07, b = 19.92, and c = 13.42 Its structure is based on pentasil building blocks1that are linked together into a three-dimensional network to form two interconnected sets of ]O-ring opening channel^.^ One set consists of straight nearly circular channels (5.4-5.6 8,in diameter) that run along the crystallographic [OlO] axis, while a second set of elliptical channels (5.1-5.5 A) runs along the perpendicular [ 1001 direction. It consists of zigzag channels with 112’ between segments. These channels are connected at right angles to the straight channels at the zigzag corners. There are three intersections per unit cell. It is generally accepted that these characteristics of ZSM-5 are responsible for its unique sorption and catalytic activities, in particular with respect to the p-xylene isomers.2s6 The molecular
(1) Breck, D. W. Zeolite Molecular Sieues; R. E . Krieger: FL, 1984. (2) Meisel, S . L.; McCullough, J. P.; Lechthaler, C. H.; Weisz, P. B. Chem. Technol. 1976, 6, 86. (3) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M . J . Phys. Chem. 1981, 85, 2238.
(4) Mehring, M. Principles of High Resolution N M R in Solids; Springer-Verlag: New York, 1983. Fyfe, C. A. Solid-Stare N M R f o r Chemists; CFC Press: Guelph, Ontario, 1983. Gerstein, B. C.; Dybowski, C. R. Transient Techniques in N M R of Solids; Academic: New York, 1985. (5) Klinowski, J. Prog. N M R Specrrosc. 1984, 16, 237.
0022-3654/88/2092-4134$01.50/0
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0 1988 American Chemical Society
