ISSN 1061933X, Colloid Journal, 2010, Vol. 72, No. 6, pp. 764–770. © Pleiades Publishing, Ltd., 2010. Original Russian Text © N.K. Gaisin, O.I. Gnezdilov, T.N. Pashirova, E.P. Zhil’tsova, S.S. Lukashenko, L.Ya. Zakharova, V.V. Osipova, V.I. Dzhabarov, Yu.G. Galyametdinov, 2010, published in Kolloidnyi Zhurnal, 2010, Vol. 72, No. 6, pp. 755–761.
Micellar and LiquidCrystalline Properties of Bicyclic FragmentContaining Cationic Surfactant N. K. Gaisina, O. I. Gnezdilovb, T. N. Pashirovac, E. P. Zhil’tsovac, S. S. Lukashenkoc, L. Ya. Zakharovac, V. V. Osipovaa, V. I. Dzhabarova, and Yu. G. Galyametdinova a
Kazan State Technological University, ul. Karla Marksa 68, Kazan, 420015 Tatarstan, Russia Zavoiskii Physicotechnical Institute, Kazan Scientific Center, Russian Academy of Sciences, Sibirskii trakt 10/7, Kazan, 420029 Tatarstan, Russia c Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, ul.Akademika Arbuzova 8, Kazan, 420088 Tatarstan, Russia b
Received November 11, 2009
Abstract—NMR spectroscopy is employed to study the aggregation of a cationic surfactant, 4aza1cetyl 1azoniabicyclo[2.2.2]octane bromide, in aqueous solutions. Selfdiffusion coefficients are determined for micelles and monomers and the hydrodynamic radius and aggregation number are calculated for micelles. Polarization microscopy data demonstrate that the examined compound is an amphotropic substance. It is found that the lyotropic liquidcrystalline system is characterized by a wider temperature range of mesophase existence as compared to the thermotropic system. DOI: 10.1134/S1061933X10060062
INTRODUCTION
phism in compounds of this class in the presence of solvents remains to be studied.
Amphiphilic derivatives of 1,4diazabicy clo[2.2.2]octane (DABCO) quaternary salts belong to the new class of cationic surfactants whose colloidal properties have been poorly studied. At the same time, numerous examples of the practical application of unsubstituted DABCO and its derivatives have been described in the literature [1–5]. The possible scope of application of their hydrophobized analogs can be widened via the formation of supramolecular struc tures. Organized systems based on amphiphilic com pounds are widely used in genetic therapy, pharmacol ogy, nanoparticle synthesis, production of liquidcrys talline devices, oilrefining industry, catalysis, etc., [6–10]. Cationic surfactants possessing liquidcrystal line properties and their coordination compounds are promising materials for optoelectronics and informa tion devices. The feasibility of realizing lyotropic mesomorphism, which manifests itself in the presence of solvents, broadens the practical applications of these substances in the template synthesis of nano products, biotechnology, catalysis, medicine, cosme tology, etc. [11–13]. Cationic surfactants that exhibit thermotropic mesomorphism include quaternary ammonium salts, such as 1,4dialkyl1,4diazoniabi cyclo[2.2.2]octane dibromide, which was first obtained by Nogami et al. [14] and later studied by Ohta [15]. A new type of thermotropic smectic mesophase ST [11] and double melting phenomenon were revealed for such compounds. Simon et al. called this type of crystals as tegma crystals [16]. However, the possibility of realizing the lyotropic mesomor
N
+
N
C16H C H3333BrBr– −
Scheme 1. Chemical formula of DABCO16.
In this work, we describe the synthesis of a hexade cyl derivative of DABCO, 4aza1cetyl1azoniabi cyclo[2.2.2]octane bromide (DABCO16) (Scheme 1), and study its micellization properties and the abil ity to form thermotropic and lyotropic liquid crystals. DABCO16 contains two polar groups, one of which represents a quaternized nitrogen atom. The presence of the positively charged nitrogen atom allows us to attribute this compound to cationic surfactants, while, due to the presence of two polar fragments, the micel lization of DABCO16 and the properties of aggre gates can differ from those of ordinary cationic surfac tants. According to the tensiometry and conductome try data, the critical micelle concentration (CMC) of DABCO16 is 1 mM. Aggregation was studied by NMR spectroscopy, which is rather informative for studying the physical properties, structure, and dynamics of particles in micellar systems. Fourier transform (FT) pulsedfield gradient (PFG) NMR spectroscopy makes it possible to measure the selfdiffusion coefficients of compo nents in micellar solutions, from which, the hydrody namic radii of micelles and variations in the concen trations of free and aggregated components may be determined as functions of surfactant concentration in
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solutions. The theoretical foundations of this approach were formulated in [17, 18]. EXPERIMENTAL DABCO16 was obtained through the alkylation of 1,4diazabicyclo[2.2.2]octane with hexadecyl bro mide in acetone; yield, 77%; melting temperature, 176–178°C. The structure of DABCO16 was deter mined by IR and 1H NMR spectroscopy and elemen tal analysis. IR (KBr): ν 2919, 2849, 1467, 1376, 1098, 1058, 850, 797, 720 cm–1. 1 H NMR (CDCl3): δ 0.87 (t, 3 H, СН3, J = 6.9), 1.24–1.34 (m, 26 H, СH2CH2(СН2)13СН3]; 1.73–1.75 (m, 2 H, СH2 CH2 (СН2)13СН3]; 3.26 (t, 6 H, N(СH2CH2)3, J = 7.4), 3.48–3.51 (m, 2 H, N+CH2]; 3.67 (t, 6 H, N+(СH2CH2)3, J = 7.8) ppm. Elemental analysis data: Found (%): C, 63.23, H, 10.86; N, 6.71; Br, 19.15. For C22H45N2Br anal. calcd. (%): C, 63.29; H, 10.87; N, 6.42; Br, 19.07. The concentration dependence of the observed selfdiffusion coefficient (SDC) Dobs was measured for surfactant molecules in aqueous solutions with differ ent concentrations using an AVANCE 400 FT NMR spectrometer (Bruker) operating in the PFG mode. The spectrometer was equipped with a pulsedfield gradient attachment generating gradients as high as 0.53 T m–1. SDCs were estimated from the integral intensity decay of stimulated spin echo signals attrib uted to protons of different groups of alkyl and aro matic fragments induced by changes in the field gradi ents in a sequence of three 90° pulses: 2 I ( G ) = I 0 exp – ( γδG ) D ⎛ Δ – δ ⎞ , ⎝ 3⎠
where γ is the gyromagnetic ratio of a nucleus (pro ton), δ is the gradient pulse duration, and Δ is the time interval between the gradient pulses. Depending on the values of the measured SDCs, the constant values of times δ and Δ are varied in the ranges of 5–10 and 50–70 ms, respectively. These time intervals are noticeably longer than the duration of the molecule exchange between the free and micellar components of a solution. The SDC of a molecule was determined by averaging the values obtained from the resonance signals due to protons of its different fragments. The errors in the SDC measurement were nearly 2 and 5% at high and low surfactant concentrations, respec tively. The temperature of the samples (25°C) was maintained using the thermostating system of the spectrometer. The experimental data were analyzed in terms of the pseudophase model [17–19]. According to this model, CMC is considered as the formation point of a new micellar pseudophase. The molecules of all micellar solution components (water, monomeric sur COLLOID JOURNAL
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factant molecules, and micelles) are involved in a cha otic diffusion motion and occur in a dynamic equilib rium. A fast exchange takes place between the free (f) and micellar (m) components of the surfactant. According to [20], the lifetime of a surfactant mole cule in a micelle is 10–5–10–7 s, thus being markedly shorter than the time Δ of SDC measurement. There fore, during the SDC measurement, a molecule repeatedly passes from the free to the bound (micellar) state. Under the conditions of the fast exchange between the free and micellar components in a solution, the observed SDC of surfactant molecules is expressed as a weightaverage value of SDCs of free Df and micellar Dm surfactant components [17]: Dobs = pfDf + pmDm,
(1)
where pf and pm are the molar fractions of the mono meric surfactant and micellized surfactant (surfactant contained in micelles), respectively, and cf, cm, and сt are the molar concentrations of the monomeric and micellized surfactant and the total surfactant concen tration, respectively. pf = 1 – pm ;
c p f = f ; ct
c p m = m. ct
(2)
From Eqs. (1) and (2), we obtain D obs – D m c f = c t – c m = c t . Df – Dm
(3)
According to [21], at concentrations above CMC, the SDC of free molecules Df is estimated by introducing a small correction for the intermicellar interaction into the SDC value found in the vicinity of CMC (Df, CMC): ϕ D f = D f, CMC ⎛ 1 + ⎞ , ⎝ 2⎠ –1
where ϕ is the volume fraction of the micellized sur M ( C – CMC, ) M is the molar mass of the factant, ϕ = 1000ρ surfactant (g/mol); and ρ is its density (g/ml). The selfdiffusion coefficient of micelles Dm was determined from the decay of the NMR line attributed to solubilisate protons using hexamethyldisiloxane (HMDS, (CH3)3SiOSi(CH3)3) as a hydrophobic probe, whose molecules are solubilized by the micelles and diffuse with the latter. Liquidcrystalline behavior was studied by polarization optical microscopy using a NAGEMAK8 polarizing microscope equipped with a Boetius warm stage and an electronic temperature sensor. Temperature was controlled with an accuracy of 0.2°C.
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Dobs × 1010, m2/s 4
4.0 3
1
3.5 3.0
2
2.5
1
2.0 0
1.5
2 4 6 8 10 12 14 16 18 20 22 1/ct, mM
1.0 0.5
2
0 0.1
1
10
ct, mM
Fig. 1. Concentration dependences for observed selfdiffusion coefficients (Dobs) of (1) DABCO16 and (2) HMDS at 25°C. The inset demonstrates the selfdiffusion coefficient of DABCO16 as depending on the inverse surfactant concentration.
RESULTS AND DISCUSSION Micellization in Aqueous DABCO16 Solution Figure 1 illustrates the concentration dependences of SDCs for molecules of DABCO16 and HMDS in dilute heavy water solutions. In the case of the surfac tant, the dependence consists of two regions. In the initial region of surfactant concentrations, the curve is parallel to the abscissa axis, and the SDC of the mole cules remains unchanged within the experiment error, thus corresponding to the monomeric state of the sur factant. The concentration at the inflection point may be considered to be CMC. Above CMC, SDC decreases, and, at high concentrations (≥50 mM) the coefficients of become equal for DABCO16 and HMDS. According to the twostate model, the precise CMC value is determined as the intersection point of the linear regions of the dependence plotted in the Dobs–1/сt coordinates [17]. The CMC value thus determined (Fig. 1, inset) is 0.85 mM, which is in good agreement with the previous tensiometry and conductometry data. The concentration dependence of SDC of HMDS molecules comprises three regions (Fig. 1). At con centrations below CMC, the SDC of HMDS molecules has a minimum constant value of 6.2 × 10–12 m2/s. In the region of CMC, SDC rises; however, as the surfactant concentration is further increased, the SDC reaches a maximum, then somewhat decreases. The region below CMC is most likely relevant to a heterogeneous system. The abnormally low SDC of HMDS mole cules in this concentration range cannot be attributed to the monomeric state of the probe for two reasons. First, hydrophobic HMDS molecules are waterinsol uble and, second, because of their smaller sizes and masses, HMDS molecules that are not bonded to
micelles would have a higher SDC than surfactant molecules. Most likely, HMDS forms emulsion drop lets with masses and sizes that are substantially larger than those of individual molecules. In the vicinity of CMC, the SDC of HMDS molecules should be con sidered to be the weightaverage value of the selfdiffu sion coefficients of the emulsion droplets and micelles. Upon reaching CMC, the rise in SDC explained by the solubilization of HMDS molecules in surfactant micelles is smooth, which is most likely due to the low micelle concentration, which is insufficient for the solubilization of the total amount of HMDS. As the surfactant concentration is further elevated, the solubilization of the hydrophobic probe increases, which is reflected in the enhancement of the effective SDC value of HMDS. The onset of the third region corresponds to the state in which solubilisate mole cules completely pass into micelles and diffuse together with them. In this concentration range, the SDC of HMDS coincides with that of micelles. The pattern of the concentration dependence of surfactant SDC (Fig. 1) confirms the suppositions of the pseudophase model of micellization and is ade quately explained using Eq. (1). As the concentration is elevated, SDC varies insignificantly for both mono meric molecules and micelles. The variation ofDobs is explained by a rise in micelle concentration. This con clusion is supported by the data in Fig 2, in which the molar fractions of the monomeric (curve 1) and micel lized (curve 2) surfactant are presented as functions of the total surfactant concentration ct. In the initial region of curve 1 cf = ct; i.e., all surfactant molecules dissolved in water occur in the monomeric state. The end of this region corresponds to CMC and coincides with the intersection of curve 2 with the abscissa axis. COLLOID JOURNAL
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The concentration dependence of micelle SDC can be used to calculate the sizes of aggregates and the aggregation numbers of micelles through the Stokes– Einstein relation
767
p, % 100
2
80
kT , R h = 0 6πηD m
(4) 60
where Rh is the hydrodynamic radius of a micelle, η is the viscosity of deuterated water (1.1 cP [22]), and 0 D m = 7.4 × 10–11 m2/s is the micelle selfdiffusion coefficient that is obtained by the extrapolation of the concentration dependence of Dm to the zero micelle concentration. This extrapolation is carried out to eliminate the influence of intermicellar interaction on the selfdiffusion coefficient. The hydrodynamic radius of micelles calculated via formula (4) is 26.8 Å. Note that hydrodynamic radius Rh is different from the proper micelle radius. A charged micelle diffuses in a solution together with a solvation shell of head groups; i.e., SDC of water molecules bonded to micelles is equal to SDC of micelles per se. If the hydrate shell is represented as a water monolayer sur rounding a micelle, the proper micelle radius may be estimated from the difference between the hydrody namic radius and water molecule diameter. The diam eter of D2O molecules, as determined from the molar volume, is 4 Å. Hence, the radius of DABCO16 micelles is equal to 22.8 Å. Dividing micelle volume 4 Vm = πR3 by the volume of a surfactant molecule 3 M Vmol = , we obtain the aggregation number ρN A N = 72. In the latter formula, M is the molar mass (417 kg/kmol), ρ is the density of the surfactant (1 × 103 kg/m3), and NA is Avogadro’s number. Parameters of HMDS emulsion droplets deter mined in the same way (M = 162 kg/kmol and ρ = 764 kg/m3) are Rh = 230 Å and N = 3.9 × 105. It is obvi ous that HMDS droplets are much larger than micelles and contain a huge number of molecules; therefore, their SDC is lower than that of surfactant micelles. Let us compare the results of calculations per formed according to the pseudophase model of micel lization with the data obtained based on the law of mass action. Let surfactant monomers be in equilib rium with micelles having aggregation number N [23]. c 'm The equilibrium constant is K = , where c 'm is N ( cf ) micelle concentration and ct = cf + N c 'm , c 'm = c m /N, COLLOID JOURNAL
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40 20
1
0 0
2
4
6
8
10
12 ct, mM
Fig. 2. Molar fractions of (1) free and (2) micellized DABCO16 molecules as functions of surfactant concen tration at 25°C.
With allowance for Eqs. (1), (2), and (5), we derive –1 D obs = ( c f D f + Nc 'm D m ) ( c t ) .
(6)
To make the calculations more convenient, let us transform the coordinate system in Fig. 1. We place the abscissa axis to the line corresponding to region 1 of curve 1 and direct the y axis vertically downward. Given this, Df = yf = 0. Let us introduce the following denotations: ym = Df – Dm and yn = Df – Dobs. Then, yn = (cfyf + N c 'm ym)(ct)–1. A number of transformations yields the following expression: N
N
c t y n = NK ( c t ) ( y m – y n ) ( y m )
(1 – N)
,
after taking the logarithm of which, we arrive at ln ( c t y n ) = ln N + ln K + N ln [ c t ( y m – y n ) ] – ( N – 1 ) ln y m .
(7)
According to Eq. (7), aggregation number N is determined from the slope of the ln(ctyn) – [ln[ct(ym – yn)]] dependence, which is nonlinear for the examined system (Fig. 3), and its slope decreases with a rise in the total surfactant concentration. This decrease seems to be associated with an increase in the mutual effect of micelles on the selfdiffusion coefficients, which is disregarded in the model based on the law of mass action. Therefore, it seems to be reasonable to determine the aggregation number in the vicinity of CMC, where the intermicellar interaction is insub stantial and the curve is steepest. The aggregation number thus found is equal to 17. This value is notice ably lower than that calculated in terms of the pseudophase model. With regard to the literature data
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ln(ct yn) 4
3
2
1
0
–1 0
0.2
0.4
0.6
0.8 1.0 ln[ct(ym – yn)]
Fig. 3. Data on selfdiffusion coefficients of DABCO16 presented in ln(ctyn)–ln[ct(ym – yn)] coordinates (see text for explanations).
on the aggregation numbers of cationic surfactants containing quaternary ammonium groups [24], the data obtained according to the pseudophase model should be considered to be more correct. However, the aggregation of DABCO16 can be somewhat different from the aggregation of classical cationic surfactants, because the polar moiety of its molecule consists of two head groups, i.e., the positively charged and uncharged nitrogen atoms separated by a rigid bicyclic fragment. Therefore, we cannot exclude that DABCO16 has a specific morphology predetermined by a looser packing of surfactant monomers with lower aggregation numbers. Organized systems based on amphiphilic mole cules are widely used in modern technologies as tem plates for creation of nanomaterials (nanoparticles, mesostructured metal oxides, etc.), the properties of which are to a high extent governed by the morphology of aggregates [13]. Therefore, the ability of amphiphilic compounds to form different supramo lecular structures is of great interest. In addition to micellar solutions, liquidcrystalline systems based on DABCO16 were studied in this work in condensed states (a solid phase and an aqueous solution). Study of Thermotropic and Lyotropic Mesomorphisms of Monosubstituted 4Aza1cetyl1 azoniabicyclo[2.2.2]octane Bromide As was shown in [14–16], in addition to thermo tropic mesomorphism, substituted quaternary azobi cycloammonium salts possess a rather interesting property. When heated, they melt at a certain temper
Fig. 4. Confocal texture of thermotropic mesophase at 100°С; 96fold magnification.
ature; then, as the temperature is elevated, they crys tallize and, upon further heating, the final melting takes place. This behavior may be explained by a high conformational flexibility of molecules and their capability of association. The latter circumstance enables us to assume that micellar aggregates that were studied in the first part of the work can be precursors of liquidcrystalline mesophases. Let us consider the properties of the synthesized substance (in both the condensed solidphase state and in mixtures with a solvent) concerning the existence of lyotropic mesophases. Polarization microscopy is the most adequate method for the primary identification of the type of mesomorphism and the establishment of the tempera ture interval mesophase existence. In this method, textures observed in the polarized light are compared with tabulated textures [25]. In this work the possible formation of thermotropic and lyotropic mesophases involving DABCO16 was studied. During the first heating, the crystals, which were formed immediately after the recrystallization, melt at 103°C to yield a liquidcrystalline phase, which is transformed into an isotropic liquid at 179°С. If the substance is instantaneously heated to 110°С, it partly melts to yield domains of isotropic liquid and crystals. Upon subsequent cooling, the substance forms a liq uidcrystalline mesophase with a confocal texture (Fig. 4) that is typical of smectic A mesophase, which exists in a temperature interval of 99–108°С. This behavior, which is characterized by double melting, which depends on the rate of variations in tempera ture, was observed previously for 1,4diazoniabicy clo[2.2.2]octane derivatives [26]. The observed phase transition temperatures are presented in Scheme 2. COLLOID JOURNAL
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Cr
Cr
103
Tg
179
769
I
99
SA
108
I
Scheme 2. Phase transition temperatures observed for 4aza1cetyl1azoniabicy clo[2.2.2]octane bromide: Cr is a crystal, Tg is a tegmamesophase, SA is a smectic A mesophase, and I is an isotropic liquid. Solid and dashed arrows refer to enantiotropic and monotropic phase transitions, respectively.
The lyotropic mesophase is formed as a result of the interaction between 4aza1cetyl1azoniabicy clo[2.2.2]octane bromide and water at a surfactant concentration of 0.72 M. The examination of the sys tem in the polarized light indicated the occurrence of lyotropic mesomorphism yielding a wedge texture (Fig. 5), which is inherent in the hexagonal packing of micelles in a mesophase [27, 28]. The crystalline state is observed for the system at temperatures below 9°С. (The minimum value of the lower limit of crystalline phase existence, which is confined by the potential of the temperature attachment is 5°С.) The transition from the liquidcrystalline state to the isotropic liquid takes place at 116°С (i.e., the mesophase exists throughout a temperature range of ΔТ = 107°С). The reproducible data obtained after several heating–cool ing cycles attest to the stability of the system. Thus, 4aza1cetyl1azoniabicyclo[2.2.2]octane bromide belongs to amphotropic substances [29] that are capable of forming thermotropic and lyotropic mesophases. Lyotropic liquidcrystalline systems are characterized by a wider temperature range of mesophase existence, as compared with thermotropic systems. The existence of a lyomesophase in a wide temper ature range including room temperature widens the scope of application of the obtained systems as tem plates for the synthesis of nanomaterials [30–32] and as model membranes in biological studies [33]. CONCLUSIONS NMR spectroscopy has been used to study the con centration dependence of selfdiffusion coefficients for 4aza1cetyl1azoniabicyclo[2.2.2]octane bro mide molecules in aqueous solutions. At concentra tions above CMC, a solution of the surfactant contains a mixture of monomeric molecules and micelles, between which a fast exchange takes place, as a result of which the observed selfdiffusion coefficient becomes equal to the weightaverage value of the self diffusion coefficients of monomeric molecules and micelles. The selfdiffusion coefficients of micelles and monomers have been determined using a hydro phobic probe (hexamethyldisiloxane), and the con tents of free and micellized surfactant molecules, the hydrodynamic radius of micelles, and their aggrega tion number have been calculated. An examination of COLLOID JOURNAL
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Fig. 5. Wedge texture of lyotropic mesophase; 96fold magnification.
samples in the polarized light has demonstrated that 4aza1cetyl1azoniabicyclo[2.2.2]octane bromide belongs to amphotropic substances. A wider tempera ture range of the mesophase existence has been found for the lyotropic liquidcrystalline system, as com pared with the thermotropic system. The wide tem perature interval of lyomesophase existence makes it possible to use it as a template for the synthesis of nanostructures. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research (project nos. 080300984a and 090312260ofim) and the programs MK 2332.2009.3 and NSh4531.2008.2. REFERENCES 1. Price, K.E., Broadwater, S.J., Jung, H.M., and McQuade, D.T., Org. Lett., 2005, vol. 7, p. 147. 2. Li, J.H., Li, J.L., Wang, D.P., et al., J. Org. Chem., 2007, vol. 72, p. 2053. 4. Ray, R.S., Misra, R.B., Farooq, M., and Hans, R.K., Toxicol. in Vitro, 2002, vol. 16, p. 123. 5. Nakagawa, K. and Tajima, K., Langmuir, 1998, vol. 14, p. 6409. 6. Yadava, P., Buethe, D., and Hughes, J.A., Polymeric Drug Delivery, ACS Symp. Ser., 2006, vol. 923. 7. Papell, S.S., US Patent 3215572, 1965. 8. Mustafina, A.R., Fedorenko, S.V., Konovalova, O.D., et al., Langmuir, 2009, vol. 25, p. 3146. 9. Vriezema, D.M., Aragone`s, M.C., Elemans, J.A.A.W., et al., Chem. Rev., 2005, vol. 105, p. 1445. 10. Taubert, A., Napoli, A., and Meier, W., Curr. Opin. Chem. Biol., 2004, vol. 8, p. 598. 11. Hegmann, T., Qi, H., and Marx, V.M., J. Inorg. Orga nomet. Polym. Mater., 2007, vol. 17, p. 483.
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