3582
J . Phys. Chem. 1988, 92, 3582-3586
Fluorescence and Conductometric Studies of Potassium 2-(p-Toluidinyl)naphthalene-6-sulfonate/Cyclodextrin/Surfactant Systems David J. Jobe, Ronald E. Verrall, Department of Chemistry, Uniuersity of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0 WO Ramamurthy Palepu, Department of Chemistry, University College of Cape Breton, Sydney, Nova Scotia, Canada B1 P 6L2 and Vincent C. Reinsborough* Department of Chemistry, Mount Allison L'niuersity, Sackuille, New Brunswick, Canada EOA 3CO (Received: November 2, 1987)
Time-resolved fluorescence and conductometric measurements of potassium 2-(p-toluidinyl)naphthalene-6-sulfonate(TNS) in aqueous solutions of P-cyclcdextrin (6-CD) were made to investigate the formation and stoichiometry of the inclusion complexes formed between 6-CD and TNS. Both 1:l and 2:1 p-CDITNS complexes were found, and the binding constants for the formation of these complexes were determined from both the conductometric and fluorescence measurements. Multiple fluorescence lifetimes for TNS were found in these solutions, supporting the theory that there is an initial interaction between the inclusate and the host before the TNS is inserted into the p-CD cavity. The surfactants tetradecyltrimethylammonium bromide (TTAB) and sodium dodecyl sulfate (SDS) also were added to these solutions (at concentrations below the critical micelle concentration (cmc) of the surfactant) to investigate the effects that these surfactants would have on the inclusion processes. For low p-CD concentrations where [surfactant] 2 [TNS], the surfactant is found to bind preferentially to p-CD. However, as the p-CD concentration increases and exceeds that of the surfactant, the free surfactant is depleted, and inclusion complexes between p-CD and TNS are then formed
T h e fluorescence lifetime of T N S (2-@-toluidiny1)naphthalene-6-sulfonate)' varies from 60 ps in pure water to 8.7 ns in pure ethanol. The emission wavelength maximum and the quantum yield of T N S also show similar strong dependence on the variation of solvent polarity. These properties thus make TNS a potentially sensitive probe for assessing the relative polarity of its local environment. Kondo et a1.* found a pronounced fluorescence enhancement when P-cyclodextrin (p-CD) was added to TNS solutions and deduced that both 1:1 and 2: 1 P-CD/TNS inclusion complexes are formed. Such stoichiometries are often found for complexes between cyclodextrins (a-CD, @-CD,and y-CD in their order of increasing annulus and, hence, cavity size) and a variety of other compounds, including dyes and surfact a n t ~ . ~ What - ~ is not so well-known is the binding site of the inclusate in the cyclodextrin cavity. This is especially important since kinetic measurements with compounds similar to ThrS indicate that inclusates appear to occupy different hydrophobic environments4 6-9 Kitamura et aI.'O obtained fluorescence lifetimes for T N S included in various carbohydrate complexes and found only one lifetime for T N S in 0-CD, thus inferring only one inclusion site for T N S . However, multiple inclusion sites for TNS in y-CD and the other carbohydrates were found. Unfortunately, in their study, the half-width of the excitation pulse was relatively long (2.5 ns), and shorter lifetimes, therefore, would not have been ( 1 ) Auerbach, R . A,: Synowiec, J . A,: Robinson, G. W. Springer Ser. Chem. Phys. 1980, 14. 215. ( 2 ) Kondo, H.; Nakatani, H.; Hiromi, K. J . Biochem. Jpn. 1976, 79. 393. (3) Sanenger, W. Inclusion Compounds; Atwood, J. L.; Davies, J . E. I).. MacNicol, D. D., Eds.; Academic: London, 1984; p 234. (4) Hersey, A.: Robinson, B. H.; Kelly, H. C. J . Chem. Soc., Faraday Trans. 1 1986, 82, I211. ( 5 ) Palepu, R.; Reinsborough, V . C. Can. J . Chem., in press. ( 6 ) Clarke, R . J.: Coates, J. H.: Lincoln, J. Carbohydrate Res. 1984. 127.
181. (7) Schiller, R. L.: Coates, J. H.; Lincoln, J. J . Chem. Soc., Faraday Trans. I 1984, 80, 1257. (8) Clarke, R. J.; Coates. J. H.: Lincoln. J. J . Chem. Soc., Faraday Trans. I 1984, 80, 3 1 19. (9) Hersey. A.: Robinson, B. H. J . Chem. Soc.. Faraday Trans. 1 1984, 80. 2039. (10) Kitamura, S.; Matsumori, S.; Kuge, T. J . Inclusion Phenom. 1984, 2, 725.
0022-3654/88/2092-3582$01.50/0
observed. Given the fact that Kondo et a1.* proposed two types of complexes between TNS and 0-CD, on the basis of steady-state measurements, and that Kitamura et a1.I0found only one fluorescent lifetime for T N S in these solutions (indicating that potentially only one complex was being formed), it seemed important to reexamine the lifetime measurements with a system capable of exploring the shorter time domain. Thus, an examination of the fluorescent properties of T N S in (3-CD solutions was undertaken with the object of verifying both the stoichiometries assigned to the inclusion complexes by Kondo et al.* and determining whether T N S can occupy a number of different sites in p-CD solutions. Since surfactants are known to form strong 1:l complexes with P-CD,5 they were added to the TNS/P-CD systems to determine what effect they may have on the binding of T N S to cyclodextrin. Conductometric measurements were also carried out because they are an extremely simple, sensitive means of obtaining independent stoichiometry and association constant data for the inclusion complexes with c y c l o d e ~ t r i n . ~ ~ ~ Experimental Section Materials. T N S (Sigma) was recrystallized from ethanol and dried under vacuum before use. Both tetradecyltrimethylammonium bromide (TTAB, Sigma) and sodium dodecyl sulfate (SDS, Sigma) were twice recrystallized from ethanol and dried under vacuum. The concentration of T N S was constant and equal to 2 X lo-' M for all fluorescence measurements. All solutions were prepared by using Millipore Super-Q water. Fluorescence Measurements. Lifetime measurements were made by using the single-photon counting technique. The excitation source was a synchronously pumped, cavity dumped, frequency-doubled rhodamine 6G picosecond dye laser exciting at 295-nm wavelength." The full width at half-maximum (fwhm) of the excitation pulse was 1 2 0 ps. Polarization bias was eliminated by using an emission polarizer placed at 54.7' to the exciting light. Steady-state fluorescence polarization measurements were made by using a microprocessor-controlled Spexfluorolog spectrofluorimeter. The time-resolved and steady-state ( 1 I ) Demmer, D. R.; James, D. R.: Steer, R. P.: Verrall. R. E. Photochem. Phoiobiol. 1987, 45, 39.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Voi. 92, No. 12, 1988 3583
TNS/@-Cyclodextrin/Surfactant Systems
TABLE I: Values of Preexponential Terms A , Lifetimes T i , and xz for TNSIB-CD Solutions with and without Surfactant 1041a-c~1M ,
A2
A,
rI, ns
A3
72,
ns
73,
x2
ns
No Surfactant 0.00 2.82 7.05 28.2 56.4 84.6 113 141
1 0.566 0.261 0.291 0.37 1 0.274 0.254 0.262
0.379 0.687 0.659 0.538 0.624 0.644 0.638
0.00
0.435 0.353 0.558 0.250 0.253 0.261 0.227 0.212
0.565 0.647 0.442 0.7 12 0.701 0.666 0.658 0.672
0.056 0.053 0.05 1 0.091 0.101 0.102 0.100 [TTAB] = 5 X
2.82 7.05 28.2 56.4 84.6 113 141
0.038 0.047 0.074 0.1 I4 0.116 [SDS] = 5 X
0.00 2.82 7.05 28.2 56.4 84.6 113 141
1 0.659 0.391 0.323 0.304 0.321 0.321
0.309 0.577 0.651 0.653 0.628 0.619
0.032 0.032 0.027 0.043 0.052 0.060
O.06Oa 0.108 0.248 0.249 0.121 0.21 1 0.300 0.279
1.332 1.481 1.428 1.340 1.376 1.456 1.452
mol dm-3 3.490 2.291 1.310 0.425 0.472 0.615 0.253 0.432
2.875 3.290 3.515 3.164 3.679 4.067 4.230
1.385 1.429 1.069 1.109 1.137 1.139 1.099
8.532 7.785 6.874 1.449 1.532 1.543 1.381 1.490
3.767 4.297 4.186 3.840 4.133
1.845 1.346 1.857 1.211 1.132 1.211 1.072 1.008
1.334 1.441 1.576 1.554 1.624 1.597
2.820 3.346 4.874 5.138 5.132 5.157
1.468 1.391 1.181 1.186 1.166 1.115
mol dm-3 0.060" 0.108 0.223 0.452 0.356 0.470 0.428
value for no p-CD present taken from ref 1. emissions of T N S were monitored at 520 nm in a 1-cm path length fluorescence cell. The fluorescence decay curves were analyzed by the pulse-shape mimic technique described elsewhere."J* The fluorescence decay profiles collected for T N S were fitted to the expression n
I ( t ) = EAi exp(-t/Ti) i= I
(1)
where Z ( t ) is the intensity of the fluorescence at time t , A iis the preexponential factor for the fraction of fluorescence intensity, T~ is the fluorescence lifetime for the emitting species i and n is the total number of emitting species. The analysis was carried out by using a nonlinear least-squares fitting procedure based on the Marquardt algorithm. The analysis provided estimates of the values of the residuals, autocorrelation, and reduced x square (x2). The steady-state polarization emission intensities were collected by using the L format. The degree of polarization ( P ) was calculated from the polarization ratio (Pr):
TABLE 11: Fluorescence Parameters for TNS in 8-CD Solutions with and without Surfactant No Surfactant 0.00 2.82 7.05 28.2 56.4 84.6 113 141
505.1 479.0 475.2 474.2 469.3 468.4 467.6 465.3
(2)
0.00 2.82 7.05 28.2 56.4 84.6 113 141
[TTAB] = 5 X 454.3 455.5 454.6 470.8 470.8 469.6 467.8 466.3
where Pr = I w l h h / l v ) , l h v ) and, for example, Zvh is the intensity of the emission when the exciting light is vertically polarized and the emitting light is horizontally polarized, both with respect to a common frame of reference. All intensity values were averaged over a period of 60 s. In analyzing the steady-state polarization data, it was assumed that the total polarization was the weighted sum of the polarization from the individual emitting species:
0.00 2.82 7.05 28.2 56.4 84.6 113 141
[SDS] = 5 X 504.2 502.4 478.2 478.5 476.5 475.2 470.5 466.5
P = (Pr - 1)/(Pr
+ 1)
0.10 1.54 2.35 3.52 4.66 4.75 4.86 5.16
0.214 0.124 0.119 0.114 0.109 0.106 0.105 0.101
mol dm-3 42.2 46.4 27.6 4.74 4.99 4.99 6.47 6.99
0.025 0.086 0.112 0.109 0.103 0.090 0.106 0.104
mol dm-3 0.14 0.35 1.22 2.8 1 3.95 4.10 5.09 5.29
0.221 0.207 0.1 17 0.101 0.098 0.092 0.09 1 0.094
n
p = CcViPi0 i= 1
(3)
where ai is the mole fraction and Pi"is the steady-state polarization value for species i in its microenvironment. The value for the limiting polarization, Po,for T N S was determined at -20 "C in propylene glycol and was found to be 0.331. A symmetrical prolate ellipsoid model with an axial ratio of 3.2:l was used to estimate the molecular volume of T N S = 430 A3. (12) Demmer, D. R.; James, D. R.; Verrall, R. E.; Steer, R. P.Reu. Sci. Insfrum. 1983, 54, 1121.
These two quantities are required to calculate microviscosity values by using the Perrin equation. Conductometric Measurements. The conductometric measurements were made at 25 f 0.1 OC by using a Radiometer CDM83 bridge.
Results and Discussion Inspection of the fluorescence decay traces obtained for T N S indicated the presence of more than one emitting species for all solutions studied. These traces were analyzed according to eq 1, and the best statistical fits were obtained for n = 3. The values for Ai,T ~and , x2 are given in Table I. Table I1 shows values of
3584
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
the emission wavelength maximum A,,
Jobe et al.
the relative intensity,
If,and the steady-state fluorescence polarization, P. It should be noted from Table I1 that as the p-CD concentration increases, the fluorescence intensity is enhanced relative to T N S in H 2 0 , in both the presence and absence of SDS. However, in the presence of TTAB the intensity first increases to a maximum then rapidly decreases to a minimum for [p-CD] > 1 X lo-, M. A similar large initial fluorescence enhancement has been reportedI3 for the system cetyltrimethylammonium bromide (CTAB)/TNS, with the maximum correlating to the stoichiometry of a 1:l complex between CTAB and TNS. The enhanced fluorescence intensity from T N S in the presence of p-CD alone indicates that the fluorophore is in a more hydrophobic environment compared to T N S in the absence of p-CD. In general, it appears that T N S forms only a weak complex with p-CD since at the highest concentrations of fi-CD, where the molar , is still a detectable change ratio of p-CD to TNS ~ 7 0 0 there in the fluorescence intensity. If the complex were strong, Le., similar to those formed between p-CD and long-chain surfactant^,^ then the intensity changes would be expected to be negligible at these concentrations. In the presence of 5 X M SDS, the fluorescence intensity from T N S also increases with increasing p-CD concentration. However, the increase is comparable to that observed in the case of SDS only when [p-CD] > [TNS]. This shift in the onset of increased fluorescence is due to (3-CD complexing more strongly with SDS than with TNS, giving rise to fewer fluorescing pCD/TNS species. Once all of the SDS is complexed with 6-CD, then the fluorescence increases as in the absence of SDS. It is also significant to note that T N S and SDS are both anionic and will have little tendency to associate except, possibly, hydrophobi~ally.'~.~~ On the other hand, the unusual fluorescence behavior of T N S in the TNS/TTAB/P-CD system can be qualitatively explained by considering the additional competitive equilibria introduced as T N S is Coulombically attracted to TTAB.I5 As long as [TTAB] 5 (p-CD], the association between 6-CD and TTAB (C,) dominates over that between T N S and p-CD (C2) or between TNS and TTAB (CJ. Increasing the concentration of 6-CD effectively depletes [TTAB] through the formation of C I , which in turn reduces the concentration of C,. The maximum in the fluorescence intensity indicates an optimum p-CD concentration for the formation of C,.', Finally, the decrease of the fluorescence intensity with further additions of p-CD, to values similar to those observed in the absence of TTAB, reflects the complete depletion of unassociated TTAB in solution and the complexation of unassociated TNS with the excess of p-CD to form C2. To a first approximation, the magnitude of the preexponential factor, A,, for each of the species in eq 1 can be assumed to be proportional to the population of T N S bound to different sites of 8-CD. With the exception of 8-CD/TNS/TTAB systems at the lowest concentrations of p-CD, the fluorescence lifetimes of T N S in site 1 range between 100 and 500 ps. By comparison with T N S in water ( T = 60 ps'), it is apparent that T N S is in a less polar environment in site 1. Furthermore, the lifetimes for TNS in sites 2 and 3 are 1.5 and 3-5 ns, respectively, indicating that the relative order of increasing hydrophobicity of the three sites i s 3 > 2 > I. The nature of the binding sites was examined further by calculating the microviscosity of the environment for each of the fluorescing species. Assuming that the fraction of species occupying site i is CY( and equal to A,, eq 3 can be rearranged in a linear form to yield p/a3
= (aI/a3)PIo
+ ( ~ 2 / ~ 3 ) p 2+' p3'
(4)
(13) Hautala, R . R.; Schore, N. E.; Turro, N . J. J . A m . Chem. SOC.1973, 95, 5508. (14) Satake, I.; Ikenoue, T.; Takeshita, T.; Hayakawa. K.; Maeda, T. Bull. Chem. SOC.Jpn. 1985, 58, 2746. (15) (a) Chaing, H.-C.; Lukton, A. J . Phys. Chem. 1975. 79, 1935; (b) 1977, 81, 936. (16) Hamai, S. Bull. Chem. SOC.Jpn. 1986, 59, 2721.
8
0
4
1
1
2 3 4 6 1 IO-CDI / I O J m o l d~? Figure 1. Molar conductivity of TNS in 6-CD solutions at 25 "C: ( 0 ) 1.00 X mol dm-J TNS; (0)2.00 X low3mol dm-3 TNS. 94 0
-
0
88-
.-.#
> 86-
2 0
84v
82 !
I
0
2
t
I
4 6 [ O-CDI/
Figure 2. Molar conductivity of 4.00 solutions at 25 "C.
J
I
I
6 1 0 1 2 m o l dE3 X
4
mol d m 3 TNS in p-CD
TABLE III: Formation Constants for 1:l and 2 1 D-CD/TNS Complexes Obtained Conductometricallyat 25 "C 1 O3 [TNS], mol dm-3 K , , dm3 molP K,, dm3 mol-' 1 .oo 2290 91 2.00 1650 51 4.00 970 28
Linear regression of eq 4 using the values of P , A i , A2, and A, given in Table I yielded values of P I o = 0.13 f 0.01, P20 = 0.086 f 0.01, and P,' = 0.14 f 0.06 with a correlation coefficient of 0.98 and standard deviations of the coefficients as shown. With these values of PIoand the lifetime values, T , , from Table I, the microviscosity, vl,around each of the sites of TNS can be calculated from the Perrin equation: (l/p,")-
'/3
= ( ( l / P O ) - Y,H1 + k7-7/(7?,1/3)
(5)
where k is the Boltzmann constant, T i s the absolute temperature, V is the molecular volume of T N S (assumed to be the same magnitude for each site and equal to 450 A,), and Po is the limiting polarization for T N S (0.331). The results obtained for sites 1, 2, and 3 are v1 = 1.9 i 0.2, q2 = 4.3 f 0.5, and q3 = 30 f 9 CP, respectively. Here we note that there is an increase in the ''local'' viscosity that correlates with increasing hydrophobicity of the sites in agreement with the interpretation based on lifetime data. Before the binding sites of T N S with p-CD can be more positively identified, it is important to know the stoichiometry of the inclusion complexes formed and the relative strength of the complexes, i.e., the magnitude of the association constants. Conductivity measurements of the p-CD/TNS systems indicate there are two complexes formed, with P-CDITNS stoichiometries of 1:l and 2:l (Figures 1 and 2). Breaks occur in the conductivity curves for T N S concentrations of 1 .OO, 2.00, and 4.00 mmol dm-, at 1.4, 2.5, and 3.9 mol dm-3 p-CD, respectively, with second breaks at 2.6,4.4, and 8.5 mmol dm-3 p-CD, respectively, assuming that molality is approximately equivalent to molarity for dilute solutions of p-CD. The binding constants for the 1 : l and 2:l complexes can be estimated by using the method of Satake et al.'4,'7 Table I11 lists the values of the constants K , and K 2
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3585
TNS/P-Cyclodextrin/Surfactant Systems
-
TABLE IV: Binding Constants for Cvclodextrin Inclusion Compounds inclusate K , , mol dm-) system K I , , mol dm-3 400 TNS 1270 f 40 8-CD 6-CD b-CD 6-C D / SDS 6-CDITTAB CY-CD CY-CD a
TNS TNS TNS TNS PADA MY7
1500 1600 1550 f 120 1350 f 800 3180 2890
530 350 670 430
Klb
K2, mol dm-3
methodo
2.1 f 0.4
3.3 f 1.7 14 60 2.6 f 0.6 9.4 f 0.2
F F
1.9 & 0.2 2.8 f 0.3 3.1 + 0.3 5.7 f 0.3
C
F F K K
ref this 2 this this this 4 4
work work work work
F = fluorescence, C = conductivity, K = kinetic.
Figure 3. Model for inclusion of T N S in @-CD.
calculated at 25 OC for the formation of both the 1:l and 2:l (3-CD/TNS inclusion complexes for the processes T N S p-CD F? TNSIP-CD Kl
+ TNS/P-CD + p-CD e P-CD/TNS/P-CD
K2
The binding constant K I is much stronger than K2, but both decrease in magnitude with increasing inclusate (TNS) concentrati~n.~J~ The % ' value ~ of the binding constant for the first complex is in good agreement with that obtained from the fluorescence data reported here and elsewhere2 (Table IV). The binding constant for the 2:l complex, K2, is also in agreement with the literature2 value, notwithstanding the rather gross assumptions that are made in its calculation. The molar conductivities of 2.00 mmol dm-3 T N S in p-CD solutions were measured at four temperatures to obtain estimates of AHo and AS" for the two associative processes. From the expansion of AG as a polynomial and by a plot of In K versus 1/ T, an average estimate of AHi" and AS,"could be obtained. It was found that AH10 = -3 f 1 kJ mol-' and AS,' = 85 f 5 J mol-I K-l for the formation of the 1:l complex, while the formation of the 2:l complex gives values of AH20 = -30 f 10 kJ mol-I and ASZo= -40 f 20 J mo1-I Kt'. The large favorable AS,"for the first process is consistent with the transfer of T N S into the hydrophobic cavity of P-CD.'* The unfavorable AS2' for the formation of the 2:l complex may simply result from the loss of rotational motion for the T N S molecule as the second p-CD encapsulates T N S and induces rigidity in the molecule through hydrogen bonding with the hydroxyl groups at the mouth of the cavity. A number of other factors may be responsible for the negative a S Z o , l 8 but it is clear from the thermodynamic data that TNS experiences a significant entropy-driven change on formation of the first complex. A general mechanism4 for the initial inclusion of dye molecules into cavities has postulated, as a first step, an encounter complex in which the forward rate constant is presumed close to the diffusion-controlled limit, followed by a second step in which the dye penetrates deeper into the p-CD torus with concomitant expulsion of water. It would appear that the time-resolved fluorescence, microviscosity, and conductance data reported here are not inconsistent with this mechanism. The loose encounter to TNS with P-CD (site l ) , placing it in a slightly more hydrophobic environment than in water, and entry of T N S into the 8-CD cavity (site 2), leading to a much more hydrophobic milieu, are consistent with our results. However, the observation here of a third inclusion site for T N S supports previous suggestions of higher order complexes.2 While the low population of site 3, which is most hydrophobic, and the apparent negative entropy change for the second process, K2, are consistent with a possible deeper penetration of T N S into the t o r ~ s ,formation ~,~ of a 2:l P-CD/TNS complex where both ends of the T N S molecule are encapsulated by P-CD tori cannot be (17) Satake, I.; Yoshida, S.; Hayakawa, K.; Maeda, T.; Kusumoto, Y. Bull. Chem. SOC.Jpn. 1986, 59, 3991. (18) Komiyama, M.; Bender, M. L. J . Am. Chem. SOC.1978, ZOO, 2259.
I O - C D I / $ m o l dmJ Figure 4. Plots of l / ( N - A 3 N 2 - )versus 1/[6-CD]: (0)6-CDITNS; (0)(3-CDITNSISDS; ( 0 )6-CD/TNS/TTAB.
65 1.0 1.5 IO-C 01/ Ib'mo I d m-3 Figure 5. Plots of A3/(1 - A,) versus [p-CD]: (0) p-CDITNS; (0) -0
P-CDITNSISDS; ( 0 )P-CD/TNS/TTAB.
ruled out. In fact the latter process appears to be more consistent with the fluorescence and conductivity data reported here. Figure 3 depicts these various equilibria. In terms of the equilibria postulated for the conductivity data (see above), the first process encompasses steps 1 and 2 of Figure 3 or K , = Kla(1 + Klb),where K,, is the association complex for the loose encounter and Klbis the association constant for the formation of the 1:1 complex from the loose association complex.' Klbcan be estimated from the fluorescence data of Table I, where Klb = A2/Al. K1 can be calculated from the intensity data by means of eq 6 , 2 where A I is the difference in the values for the 1 /AZ = l/AZjm
+ 1/(AZj"Kj[P-CD])
(6)
fluorescenceintensity for T N S in solution with p-CD and the value found for T N S in water, AI," is the maximum change in fluorescence intensity when all the T N S is combined in its ith complex,2and Ki is the association constant for the complex. Since two complexes are formed, the fraction of intensity contributed by the 2:l complex (A3AZZm)must be subtracted from the total intensity to obtain the change in intensity due to the formation of the 1: I complex. Therfore, the expression for the evaluation of the overall 1:l association complex ( K , ) becomes l/(AZ-.43AZ2-)
= l/AZ,"
+ l/(AZ,"K,[P-CD])
-
(7)
where AZ2= can be obtained by extrapolating to the y axis for plots m). of l/AZ versus l/[P-CD] (Le., when [p-CD] In the presence of surfactant, the [p-CD] in eq 7 differs from the initial concentration of (3-CD by the amount that is complexed 4 depicts the to the surfactant to form a 1:l c o m p l e ~ .Figure ~ linear plots that are obtained from eq 7 for the results reported here. K1, can be evaluated by using the value of K, obtained f r o m these plots.
J . Phys. Chem. 1988, 92, 3586-3588
3586
Since it is possible that the 2:l complex may be formed from the addition of a p-CD to the loose complex (la) as well as through the more associated complex ( l b ) , the concentration of the 1:l complex could be considered as including both species. Therefore, assuming most of the T N S is included in some form of complex with 6-CD, the overall formation constant for the 2:l complex can be given as K2 = [(P-CD),TNS]/([P-CD][(P-CD)TNS]) = A3/((AI + A2)[P-CD]) and can be derived from the slope of a plot of A3/(l - A 3 ) versus [p-CD] (Figure 5). Table IV lists the values of K 1 ,Kl,, Klb,and K2 determined from the fluorescence and conductometric data for the three systems studied in this work. Values obtained from previous work on similar systems also are presented for comparison. Generally, there is good agreement between the values of the binding constants for these systems
obtained from different sources, which suggests that the proposed model is reasonable. The larger value of K 2 found in the pCD/TNS/TTAB system compared to the value found in the absence of TTAB may reflect interactions between the TTAB/ p-CD and TNS/P-CD complexes; however, any additional speculation as to the formation of such a complex would be unfounded without further study.
Acknowledgment. Acknowledgment is made to the Natural Sciences and Engineering Research Council of Canada for financial support. We thank Dr. B. Skalski and Prof. R. P. Steer for helpful discussions and technical assistance during this study. Registry No. KTNS, 32752-10-6; @-CD,7585-39-9; TTAB, 11 1997-7; SDS, 151-21-3.
Aluminum-Gallium Substitution in Zeolite Li-A(BW) J. Yang; D. Xie, W. B. Yelon, University of Missouri Research Reactor, Columbia, Missouri 6521 1
and J. M. Newsam* Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801 (Received: November 2, 1987)
Powder neutron diffraction (PND) and 29Sisolid-state NMR have been used to characterize an aluminogallosilicatezeolite, LiAlo,SGao,sSi04~H20, with the ABW framework. Rietveld refinement indicates complete Si-Al/Ga segregation, consistent with Loewenstein’s rule, but random AI/Ga substitution on the AI(Ga) site. The width of the Si-4(Al/Ga) peak observed in the 29SiNMR spectrum is also consistent with random AI-Ga substitution. The PND measurements were made on a fully hydrated material, and despite the large incoherent scattering background, complete structure refinements were possible.
Introduction pattern (recorded on a Siemens D500 diffractometer, Cu K a radiation, X(a,) = 1.540 56 A), compared with those of the pure The scope and structural consequences of T-atom substitution alumino- or gallosilicate analogues, showed a distinct intensity ( T = tetrahedral species, Si, Al, Ga, B, P, Fe, etc.) in zeolite distribution but comparable peak widths. ( d spacings and norframeworks are of considerable practical interest. In aluminosilicate zeolites, the effects of silicon substitution by aluminum have been widely in~estigated.l-~Other substitutions have been less well explored structurally, but data on a l u m i n o p h ~ s p h a t e s ~ ~ ~ (1) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley: London, 1973; reprinted by Krieger: Malabar, FL, 1984. and gal lo silicate^^^^ are accumiilating and limited information for (2) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular other elements such as B, Fe, etc. is available. Such T’ for Si Sieves; Academic: London, 1978). substitutions (T’ # Si) are governed by the constraints of firsts (3) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic: and, to a more subtle extent, second T’-neighbor9 avoidance rules. London, 1982. (4) Meier, W. M.; Olson, D. H. Atlas ofzeolite Structure Types; 2nd ed.; In the case of simultaneous substitution by more than one cation, Structure Commission of the International Zeolite Association; Butterworths: T’ and T“ (T” # Si, T’), T’-T” ordering effects are also possible, Cambridge, UK, 1987. although investigationshave to date been limited to a small number (5) Mortier, W. J. Compilation ofExtra Framework Sites in Zeolites; of materials in the Si-A1-P We describe here Structure Commission of the International Zeolite Association; Butterworths: Guildford, Surrey, UK, 1982. structural characterization of an aluminogallosilicate zeolite with (6) Bennett, J. M.; Dytrych, W. J.; Pluth, J. J.; Richardson, J. W.; Smith, the ABW framework. Accurate structural details are already J. V. Zeolites 1986, 6, 349-360. available for the analogous gallo-’*J3 and a l ~ m i n o s i l i c a t e l ~ - ~ ~ (7) Newsam, J. M.; Vaughan, D. E. W. In New Developments in Zeolite ABW-framework zeolites. Science and Technology; Murakami, Y., Iijima, A,, Ward, J. W,, Eds.; KoExperimental Section The crystallization of an aluminogallosilicate ABW-framework zeolite from a gel of composition 2.OLi20.Al20,.Ga2O,. 4.0Si02.100H20 was described previously (ref 13; sample 107C). Elemental analysis by inductively coupled plasma emission spectroscopy (ICPES) and thermogravimetric analysis (TGA) gave a formula, ~ ~ ~ 2 ~ ~ l . 8 l ~ ~ ~ 2 ~ 3 ~ 1 . 0 4 ~ ~ ~ 0)3,60,13indicating that the AI:Ga ratio in the gel is reproduced in the crystallized zeolite. The powder X-ray diffraction (PXD) Permanent address: Institute of Atomic Energy, Beijing, People’s Republic of China.
0022-3654/88/2092-3586$01.50/0
dansha: Tokyo, 1986, pp 457-464. (8) Loewenstein, W. Am. Mineral. 1954, 39, 92-96. (9) Dempsey, E. In Molecular Steues; Barrer, R. M., Ed.; Society of Chemical Industry: London, 1968; p 293. (10) Artioli, G.; Pluth, J. J.; Smith, J. V. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, C40, 214-217. (11) Ito, M.; Shimoyama, Y . ; Saito, Y . ; Tsurita, Y . ; Otake, M. Acta Crystallogr., Sect. C: Crysf.Struct. Commun. 1985, C41, 1698-1700. (12) Newsam, J. M. J . Chem. SOC.,Chem. Commun. 1986, 1295-1296. 2 ~ 3 ~(13) 0 .Newsam, 9 s ~ ~ J. ~ M. ~ 2J .~Phys. 4 . 0Chem. 0 ~ ~1988, 2 - 92, 445-452. (14) Kerr, I. S. Z . Kristallogr. 1974, 139, 186-195. (15) Krogh Andersen, E.; Ploug-Sorensen, G.Z . Kristallogr. 1986, 176, 67-73. (16) Norby, P.; N ~ r l u n dChristensen, A.; Krogh Andersen, I. G. Acta Chem. Scand, Ser. A 1986, A40, 500-506.
0 1988 American Chemical Society
