pubs.acs.org/Langmuir © 2010 American Chemical Society
Adsorption of Aromatic Alcohols into the Walls of Hollow Polyelectrolyte Capsules Diptangshu Chakraborty,†,‡ Rudra Prosad Choudhury,†,‡,§ and Monika Sch€onhoff*,† †
Institute of Physical Chemistry, Westf€ alische Wilhelms- Universit€ at M€ unster, Corrensstr. 28-30, D-48149 M€ unster, Germany, and ‡International NRW Graduate School of Chemistry (GSC-MS), D-48149 M€ unster, Germany. § Current address: School of Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0295 Received May 7, 2010. Revised Manuscript Received June 23, 2010 Hollow polyelectrolyte microcapsules prepared by layer-by-layer assembly of polyelectrolytes onto colloidal particles and subsequent core removal are investigated concerning their uptake capacity and the exchange dynamics of aromatic alcohols, that is, hydroquinone and phenol. Diffusion coefficients of the alcohols in the dispersion are determined by pulsed field gradient (PFG) NMR spectroscopy. In addition, spin relaxation rates are determined, which characterize the molecular dynamics. Alcohol molecules in capsule dispersions occur as a bound fraction that is adsorbed to the wall and as a free fraction in the aqueous phase. According to a previously established procedure, from diffusion and relaxation data, population fractions and exchange times are calculated using a two-site model. The adsorbed amounts are well described by Langmuir isotherms, where for hydroquinone as compared to phenol the equilibrium constant is about a factor of 3 larger, and the maximum adsorbed amount about a factor of 3 lower. This indicates the relevance of H bonds for adsorption as well as size effects controlling the uptake capacity of the wall for small molecules.
Introduction Following the development of layer-by-layer self-assembly of oppositely charged polyelectrolytes as a method to prepare polyelectrolyte multilayers (PEMs),1 the same approach was applied to coat even colloidal objects.2 After a subsequent core dissolution, hollow polymeric capsules remain, which find application as drug carriers or microreactors.3,4 Key properties are their uptake capacity and their wall permeability for different types of guest molecules. The permeability on the one hand is relevant for the release kinetics and is expected to depend on the porosity and the polymer network density of the capsule wall. The multilayer wall was shown to exhibit nanopores,5 and the permeation for molecules of different size depends on both their charge and their size.6-10 Nevertheless, the wall is partially permeable even for macromolecules or larger conjugated dye molecules, and tuning of the permeability by several external parameters has been achieved.11,12 To investigate the wall permeability of hollow PEM capsules, confocal laser scanning microscopy and fluorescence recovery after photobleaching (FRAP) are the most commonly applied techniques, where the permeation of fluorescent guest molecules *To whom correspondence should be addressed. E-mail: schoenho@ uni-muenster.de. Telephone: þ49-2518323419. Fax þ49-2518329138.
(1) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831. (2) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; M€ohwald, H. Colloids Surf., A 1998, 137, 253. (3) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; M€ohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (4) Sukhorukov, G.; Fery, A.; M€ohwald, H. Prog. Polym. Sci. 2005, 30, 885. (5) Vaca Chavez, F.; Sch€onhoff, M. J. Chem. Phys. 2007, 126. (6) Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287. (7) Hoshi, T.; Saiki, H.; Kuwazawa, S.; Tsuchiya, C.; Chen, Q.; Anzai, J. Anal. Chem. 2001, 73, 5310. (8) Toutianoush, A.; Tieke, B. Mater. Sci. Eng., C 2002, 22, 135. (9) Liu, X. Y.; Bruening, M. L. Chem. Mater. 2004, 16, 351. (10) Jin, W. Q.; Toutianoush, A.; Tieke, B. Appl. Surf. Sci. 2005, 246, 444. (11) Ibarz, G.; D€ahne, L.; Donath, E.; M€ohwald, H. Chem. Mater. 2002, 14, 4059. (12) Antipov, A. A.; Sukhorukov, G. B.; Leporatti, S.; Radtchenko, I. L.; Donath, E.; M€ohwald, H. Colloids Surf., A 2002, 198, 535.
12940 DOI: 10.1021/la101836a
into capsules is monitored.13 However, these methods are limited to either dye molecules themselves or macromolecules, which are sufficiently large not to be influenced by dye labeling. As an alternative, pulsed field gradient (PFG)-NMR diffusion experiments can be employed to study the equilibrium exchange dynamics of guest molecules between the interior and exterior of the capsules. With this method, so-called diffusion-exchange experiments, different polymers such as dextrane or poly(ethylene oxide)s have been studied in capsule dispersions.14,15 For example, scaling laws of the permeation of chains through the nanoporous wall could be established.16 Since in diffusion-exchange experiments it is simply a nuclear spin, for example, 1H, acting as the label to be detected, the studies can even be extended to small nondye molecules.17 In such studies, the equilibrium distribution of guest molecules between the capsules and the aqueous phase can be extracted, as well as exchange time scales of the guest molecules between different locations. In our previous study on phenol in capsule dispersion, we showed that a large fraction of phenol adsorbs into the capsule wall.17 This leads to a loss of the liquid state NMR signal of the adsorbed fraction of the guest molecule due to its loss of mobility, making the bound fraction undetectable. Though this would in principle render a diffusion-exchange analysis impossible, we could show that the indirect effect of the exchange between bound and free guest molecules, in combination with additional spin relaxation experiments, can serve as an alternative method for analyzing exchange in this particular case, where the adsorbed site acts as a relaxation sink.17 (13) Georgieva, R.; Moya, S.; Hin, M.; Mitlohner, R.; Donath, E.; Kiesewetter, H.; M€ohwald, H.; B€aumler, H. Biomacromolecules 2002, 3, 517. (14) Adalsteinsson, T.; Dong, W. F.; Sch€onhoff, M. J. Phys. Chem. B 2004, 108, 20056. (15) Qiao, Y.; Galvosas, P.; Adalsteinsson, T.; Sch€onhoff, M.; Callaghan, P. T. J. Chem. Phys. 2005, 122, 214912. (16) Choudhury, R. P.; Galvosas, P.; Sch€onhoff, M. J. Phys. Chem. B 2008, 112, 13245. (17) Choudhury, R. P.; Sch€onhoff, M. J. Chem. Phys. 2007, 127, 234702.
Published on Web 07/07/2010
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Having demonstrated the validity of this approach, it is now possible to investigate different guest molecules. In the present paper, we apply this methodology to hydroquinone (HQ), in order to investigate and compare the adsorption of two different aromatic alcohols, phenol and hydroquinone, into polyelectrolyte multilayer walls. Aromatic alcohols can act as model systems for typical drug molecule structures, and they exhibit the possibility to study the effect of substituents. We show that the adsorbed amount in both cases follows a Langmuir isotherm. In particular, the second -OH group exhibited by hydroquinone as compared to phenol has an effect on the local interactions and thus on the adsorbed amount: We find that the maximum adsorbed amount is much larger for phenol, which can be attributed to its lower size. On the other hand, the affinity of the alcohol to an adsorption site is larger for HQ, as documented by a larger equilibrium constant of the adsorption reaction, pointing at the relevance of the -OH group.
Materials and Methods Materials. Poly(allylamine hydrochloride), PAH (Mw =
70 000 g mol-1), and poly(diallyldimethyl ammonium chloride), PDADMAC (Mw = 100 000-150 000 g mol-1), were purchased from Aldrich and used without further purification. Poly(sodium4-sulfonate), PSS (Mw = 70 000 g mol-1, ACROS) was purified by filtration (pore size 20 μm), followed by dialysis (membrane cutoff Mw=10 000-20 000 Da) against pure water to remove low molecular weight fractions. Monodisperse silica particles (diameter 400 nm) were purchased from Microparticles GmbH (Berlin). Deuterium oxide (99.9% isotopic purity) and hydroquinone (purity g99%) were purchased from Aldrich, and 2,4,6-d3phenol from Cambridge Isotope Laboratories. Hydrofluoric acid (HF, 40% solution) was bought from Gr€ ussing (Filsum, Germany), and NaCl (analytical grade) from Merck. Ultrapure water purified by a three stage water purification system (Milli Q, Millipore) with a resistivity of around 18 MΩ cm was employed in all procedures. Polycations and polyanions were dissolved in 0.5 M aqueous NaCl solution, respectively, with polyelectrolyte concentrations of 0.2 wt %. Capsule Preparation. Hollow polyelectrolyte multilayer (PEM) capsules are prepared in two steps. At first, the multilayers are assembled by the layer-by-layer technique. Each polyelectrolyte layer is adsorbed by adding 30 mL of the respective polyelectrolyte solution to 5 mL of the silica dispersion. The polyelectrolyte is allowed to adsorb for 20 min under stirring. After this, the excess polyelctrolyte is removed and the dispersion is washed in water by centrifugation-decant-redispersion steps and repeated three times before the adsorption of a new layer. The process is repeated until a layer system of PDADMAC-(PSSPAH)4-PSS is prepared. As the first layer, PDADMAC is used, because it produces a stable layer on the surface of the silica particles. Each adsorption step is confirmed by determining the particle size and zeta potential (ζ) in a Zetasizer 3000 HAS or Zetasizer 4 (Malvern) instrument. In the second step, the silica cores are dissolved by hydrofluoric acid, yielding hollow capsules. These capsules are washed with water until the pH of the dispersion is neutral. The quality of the capsules is checked by transmission electron microscopy (TEM); a typical image was published previously.14 Then the aqueous phase of the dispersion is replaced by D2O for NMR experiments. This is done by washing the capsule dispersion in D2O again in a centrifugation-decant-redispersion process, applied five to six times. The volume of the capsule dispersion is finally reduced to 2 mL, and the final capsule concentration is calculated assuming that no particles are lost in the centrifugation-decant-redispersion process. Dispersions with hydroquinone concentrations varying from 0.15% to 1.25 wt % are prepared by addition of HQ solutions to a fixed volume of capsule dispersion (25 μL). Langmuir 2010, 26(15), 12940–12947
NMR Measurements. 1H diffusion and relaxation experiments are performed on a 400 MHz Avance NMR spectrometer (Bruker) in a probe head equipped with field gradient coils (DIFF 30, Bruker), which provides a maximum gradient strength of 12 T/m. The 1H NMR spectrum of hydroquinone in D2O shows two peaks; one peak at 4.8 ppm is due to residual water protons, exchanging with the proton of the -OH group. The other peak at 6.8 ppm is due to the four equivalent aromatic protons of hydroquinone. In all HQ diffusion and relaxation experiments, the signal intensity of the four equivalent nuclei (6.8 ppm) is evaluated. In phenol experiments, the deuterated form is employed to avoid spin coupling and the proton signals of the 3,5 position on the ring at 7.3 ppm are evaluated. All measurements are performed at room temperature (295 K). The gradient coils are cooled via a water circulation unit (Haake) which is also used to control the sample temperature. Under these conditions, no influence of convection on the diffusion experiment is observed as ensured by control experiments. For diffusion experiments, the pulsed field gradient stimulated echo sequence (PFG-STE) [π/2-τ-π/2-T-π/2-τ-echo] is employed in combination with two gradient pulses of duration δ and gradient strength g, which are applied during each delay τ. The time between the two gradient pulses is the observation time (Δ), which is varied from 20 to 1000 ms. τ is kept constant, τ = 5 ms. The spin-lattice relaxation decay constant (T1) is determined by inversion recovery experiments [π-τ-π/2-acquisition]. The spin-spin relaxation decay constant (T2) is determined by the Carr-Purcell-Meiboom-Gill sequence [π/2-(τ/2-π-τ/2)n-echo].
Results Hydroquinone in D2O. At first, the properties of hydroquinone (HQ) in D2O are determined. Concentrations of HQ are varied from 0.15 wt % to 1.25 wt %. In this range, the diffusion coefficient is determined as D = (6.2 ( 0.1) 10-10 m2s -1, independent of concentration and observation time (Δ), as expected. The spin-lattice relaxation decay constant (T1) and the spin-spin relaxation decay constant (T2) of hydroquinone are (6.1 ( 0.2) and (5.4 ( 0.2) s, respectively, independent of concentration in the above range. Corresponding values for phenol are D = (7.5 ( 0.1) 10-10 m2 s-1, T1 = (15.8 ( 0.2) s, and T2 = (11.8 ( 0.1) s, as determined earlier.17 This indicates slightly slower dynamics of HQ in comparison to phenol, which can be attributed to the second -OH group, leading to a slightly larger molecular size, as well as to stronger hydrogen bonds with the surrounding water; both are effects which slow down the translational as well as the rotational dynamics of the alcohol molecule. Diffusion of HQ in Capsule Dispersion. Diffusion experiments are performed for different observation times (Δ), ranging from 20 to 1000 ms. All diffusion experiments show monoexponential echo decays (see Figure 1a). The relative intensity of the data point with the lowest k value is normalized to 1. The echo decays can be evaluated using the monoexponential echo decay equation
where
IðkÞ ¼ A expð - kDapp Þ
ð1aÞ
δ k ¼ ðγδgÞ Δ 3
ð1bÞ
2
Here, γ is the gyromagnetic ratio of the nucleus, δ is the duration of the gradient pulses, g is the gradient strength, and A is a constant. Equation 1a holds for Gaussian diffusion in a homogeneous medium. Since here we deal with diffusion in a heterogeneous medium, the diffusion coefficient extracted in this DOI: 10.1021/la101836a
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Figure 2. Apparent diffusion coefficients of hydroquinone in capsule dispersion. Numbers in the legend are showing the concentration of HQ in wt %. Symbols, Dapp extracted from monoexponential fits of single echo decays; lines, Dapp obtained from global fitting as described in the Analysis and Discussion section. Star shaped data points and the horizontal straight line represent the diffusion coefficient of hydroquinone in D2O.
Figure 1. (a) Echo decays of hydroquinone (0.25 wt %) in capsule dispersion for different observation times Δ. Symbols, experimental echo decays; lines, results obtained by global fitting as described in the Analysis and Discussion section. (b) Apparent diffusion coefficients (Dapp) of hydroquinone (0.25 wt %) in capsule dispersion in dependence of diffusion time (Δ). Symbols, Dapp extracted from monoexponential fitting of single echo decays, eq 1a; lines, Dapp calculated according to eq 2 with the parameters extracted from global fitting as described in the Analysis and Discussion section.
manner is denoted as an apparent diffusion coefficient, Dapp. Apparent diffusion coefficients of hydroquinone in capsule dispersion (0.25 wt %) are shown in Figure 1b. They are clearly dependent on the observation time (Δ). Dapp is decreasing with increasing observation time. For the other concentrations of hydroquinone in capsule dispersion, the results are very similar, and Dapp values in dependence on observation time (Δ) are given in Figure 2. Relaxation Experiments of HQ in Capsule Dispersion. Spin-lattice and spin-spin relaxation experiments are performed in dependence on hydroquinone concentration. Spin-lattice relaxation data and the normalized spin-spin echo decays are shown in Figure 3. All decay curves are well described by single exponentials, from which T1 and T2 values are extracted and given in Table 1. The relaxation decay constants of hydroquinone in capsule dispersion are shorter than those of hydroquinone in D2O. In addition, they become dependent on hydroquinone concentration. With increasing concentration, the relaxation decay constants increase. Furthermore, the capsules have a larger influence on T2 than on T1. While T1 values are on the second scale, T2 values are reduced even to the millisecond scale. These results show that the motion of hydroquinone is strongly influenced by the presence of capsules. 12942 DOI: 10.1021/la101836a
Figure 3. (a) Spin-lattice relaxation decay constant (T1) of different concentrations of hydroquinone in capsule dispersion. Symbols, experimental data; line, exponential fit function employed for extrapolation to determine T1B. (b) Normalized spin-spin relaxation echo decay curves of HQ at different concentrations in capsule dispersion. Numbers in the legend represent the concentration of HQ in wt %. Symbols, experimental data; lines, results obtained by global fitting of all samples as described in the text in the section on relaxation analysis. Langmuir 2010, 26(15), 12940–12947
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Table 1. T1 and T2 Relaxation Decay Constants of Different Concentrations of Hydroquinone in Capsule Dispersion (Capsule Concentration 0.6 vol %) cHQ (wt%)
T1 (s)
T2 (ms)
0.15 0.25 0.40 0.60 0.75 1.25
2.7 ( 0.1 3.3 ( 0.1 3.7 ( 0.1 4.2 ( 0.1 4.6 ( 0.1 5.0 ( 0.1
62 ( 2 72 ( 2 80 ( 2 92 ( 2 102 ( 2 112 ( 2
Since all echo decays are monoexponential, it can be concluded that the exchange time of hydroquinone is fast compared to the relaxation decay constant, and relaxation rates can be interpreted as averages of relaxation rates of free and bound hydroquinone.
Analysis and Discussion From the observed results, it is clearly understood that HQ in capsule dispersion behaves similar to phenol in our previous study,17 and qualitatively we can conclude on similar regimes of the relevant time scales, as explained in the following: A fraction of hydroquinone is present in free solution ( pA), and another fraction of HQ is bound to the capsules (pB). The exponential echo decays in diffusion and relaxation experiments indicate that the exchange time (τex) is fast compared to diffusion and relaxation decay constants. On the other hand, Dapp is decreasing with Δ, which is a signature of a system not being in fast exchange with respect to the time scale of Δ, a seeming contradiction to the finding of single exponential echo decays. However, the relaxation results can explain the diffusion behavior: The short value of the average T2 of hydroquinone in capsule dispersions implies that the relaxation decay constant of bound HQ, T2B, is very short, indicating that the bound fraction is significantly immobilized. This further implies that the bound fraction represents HQ adsorbed by the capsule wall, instead of encapsulated in the aqueous interior. For the diffusion echo decays this means that, due to the very short T2B values, only the signal of free HQ is detected. As a result, for very small diffusion times it is Dapp ∼ DA. As the diffusion time increases, the probability of desorption from the wall during Δ increases, such that previously bound molecules, which contribute with a small value of D to the average diffusion coefficient, can contribute to the signal. As a result, Dapp decreases as Δ increases. This is a signature of intermediate exchange, which occurs when the exchange time scale τex is on the order of Δ. Finally, when Δ is very large (Δ > 900 ms), the regime of fast exchange (τex , Δ) is entered and Dapp seems to approach a constant value. Thus, from the experimental results, it can be qualitatively concluded that it is T2B < τex e Δ. Analysis of Diffusion Experiments. A quantitative analysis of the diffusion echo decays is performed in a two-site-model of intermediate exchange. The model, the corresponding equations, and the fitting procedure have been described in detail earlier and were shown to yield a quantitative analysis in the regime of T2B < τex < Δ.17 In the model, the free site (pA) and the bound site (pB), respectively, are characterized by their corresponding diffusion coefficients and relaxation decay constants (T1A,B; T2A,B), and the average residence times τA and τB in the respective site. In a system where a spin can exist in two separate environments with the same Larmor frequency and can exchange from one site to another, the most general description of diffusion echo decays Langmuir 2010, 26(15), 12940–12947
is a superposition of two exponential functions as derived by K€arger.18,19 IðkÞ ¼ PA expð - kaA Þ þ PB expð - kaB Þ
ð2aÞ
where
( 1 Δ 1 1 1 1 aA, B ¼ þ þ þ ðDA þ DB Þ þ 2 k TB TA τA τB vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi9 u" #2 2 = u Δ 1 1 1 1 4Δ - þ þ ð2bÞ -t ðDB - DA Þ þ τ A τ B k2 ; k TB TA τ B τ A
are apparent decay constants. TA and TB are the effective relaxation decay constants for the probe molecule in sites A and B, respectively, given by20 1 TA, B
¼
T=Δ 2τ=Δ þ T1A, B T2A, B
ð2cÞ
The apparent populations are given by PB ¼ 1 - PA ¼
1 Δ Δ þ pB aB þ - aA ð2dÞ pA aA þ aB - aA TA k TB k
where pA, B ¼
τ A, B τA þ τB
ð2eÞ
are the true population fractions in site A and site B, respectively. The exchange time (τex) is given by 1 1 1 ¼ þ τex τA τB
ð2fÞ
For fitting the diffusion echo decays by eq 2a-2d, several parameters of the model are set to fixed values: The diffusion coefficient and the relaxation decay constants of hydroquinone in the free site are given by the values determined for HQ in D2O, that is DA = (6.2 ( 0.1) 10-10 m2 s-1, T1A = (6.1 ( 0.2) s, and T2A = (5.4 ( 0.2) s. The diffusion coefficient of adsorbed HQ, DB, is that of the capsules as calculated from the Stokes-Einstein equation under the assumption that the capsule radius is similar to the original particle radius, and thus, DB = (9.2 ( 0.7) 10-13 m2 s-1. The spin-lattice relaxation decay constant of hydroquinone in the bound site (T1B) is also taken as a fixed parameter. It is determined by extrapolating the T1 values of different concentrations of hydroquinone toward zero concentration employing an exponential fit function (see Figure 3a). The assumption behind this procedure is that when the concentration of the probe molecule tends to zero, almost all probe molecules are present in the bound site. The result is T1B = 1.4 s. Since T1B as well as T1A are in the s range and thus much larger than all other time scales, any errors in the T1 values are negligible in the analysis. Therefore, in the analysis of echo decays at different Δ, the set of fixed parameters is Pin = (DA, DB, T1A, T2A, and T1B) and the set of floating parameters is Pfree = (T2B, τA, and τΒ). The latter is (18) K€arger, J. Ann. Phys. (Weinheim, Ger.) 1969, 24, 1. (19) K€arger, J. Ann. Phys. (Weinheim, Ger.) 1971, 27, 107. (20) Sch€onhoff, M.; S€oderman, O. J. Phys. Chem. B 1997, 101, 8237.
DOI: 10.1021/la101836a
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Table 2. Parameters Calculated from Global Fitting of Diffusion Data Sets of Different Concentrations of Hydroquinone in Capsule Dispersion (Capsule Concentration 0.6 vol %)a cHQ (% wt)
T2B (ms)
τex (ms)
pB
0.15 0.6 ( 0.2 19 ( 2.3 0.58 ( 0.01 0.25 0.6 ( 0.1 15 ( 0.9 0.47 ( 0.01 0.40 1.1 ( 0.1 26 ( 0.6 0.36 ( 0.01 0.60 1.0 ( 0.1 15 ( 0.1 0.25 ( 0.01 0.75 1.2 ( 0.3 20 ( 5.7 0.22 ( 0.01 1.25 1.3 ( 0.2 17 ( 2.2 0.15 ( 0.01 a Errors shown here give only the contribution arising from the fit procedure itself, and the variation of the input parameters within their error range.
optimized to fit the whole set of echo decays with the set of fixed input parameters. The influence of errors of the input parameters is checked by varying the input parameters within their error range. For example, DA = (6.2 ( 0.1) 10-10 m2 s-1, and therefore, the input parameter DA is varied from 6.1 10-10 to 6.3 10-10 m2 s-1. Similarly the other input parameters are also varied in their error range. It is observed that only DA has a significant influence in the fit analysis. The solid lines in Figure 1a show the results of global fitting applied to echo decays of hydroquinone (0.25 wt %) in capsule dispersion. The resulting fit parameters allow one to calculate a function Dapp(Δ), which is shown as the solid line in Figure 1b. This line agrees well with the results obtained from monoexponential fitting of single echo decays (data points in Figure 1b). Diffusion data obtained from other hydroquinone concentrations in capsule dispersion are treated similarly, and the comparison of the results of global fitting to those obtained from exponential fitting are shown in Figure 2. From the resulting parameters τA and τB, the exchange time (τex) and the bound fraction (pB) are calculated for each concentration using eq 2f and 2e, respectively. The results are given in Table 2. Here, errors of the parameters T2B and τex are rather large, as can be directly seen from the scatter of the values. Fit errors calculated as the effect of variation of all input parameters in their respective error range do not account for the large variation of the results. However, in the case of the bound fraction, pB, fit errors are rather small ((0.01), since the correlation with the fixed parameters is small. Thus, the diffusion analysis yields a precise determination of pB, but not of T2B and τex. The results of global fitting show that the diffusion data are well described by the two-site model. A significant fraction of hydroquinone is associated to the capsule. With an increase in concentration, the bound fraction decreases monotonously. The very small value of T2B shows that the dynamics of the bound hydroquinone is strongly changed as compared to the free hydroquinone, where it is T2A = 5.4 s. This motional restriction is an indication of adsorption into the capsule wall, as opposed to encapsulation into the aqueous interior. Though errors are large, a trend of an increase of T2B with concentration is evident; that is, the mobility of HQ is slightly increasing with concentration. This might be an effect of the swelling of the wall material by the alcohol, which also becomes evident in the exchange rates, as dicussed further below. The values of τex are in the range of ∼15-26 ms, but a systematic dependence on concentration cannot be concluded due to large errors. The time range, however, is generally on the order of ∼10 ms, and thus of the same order of magnitude as the shortest diffusion times (Δ ∼ 20 - 50 ms). Thus, hydroquinone shows an intermediate exchange with respect to the diffusion time (Δ) and the spin-spin relaxation decay constant; but since T2B , τex, no signal of the bound fraction is detected in diffusion and 12944 DOI: 10.1021/la101836a
spin-spin relaxation experiments. This leads to monoexponential echo decays in diffusion and relaxation experiments. At low diffusion time (∼20-50 ms) due to very short T2B, relaxation of bound hydroquinone is rapid and its magnetization does not contribute to the diffusion echo decays. As a result, it is Dapp ∼ DA. However, as Δ increases, exchange events during Δ become more likely and the bound fraction can indirectly contribute to the diffusion echo decays; consequently, Dapp decreases. Finally, at very large Δ (Δ ∼ 900 ms), it is τex , Δ and the exchange shifts to the limit of fast exchange; as a result, Dapp proceeds to converge toward a constant value. These variations of Dapp become less prominent with increasing concentration because with increasing concentration pB decreases. This indirect influence of the bound fraction enables us to extract the time scale of the exchange process. Thus, we can conclude that T2B < τex < Δ, with the latter inequality being valid at least for the larger diffusion times, showing that the exchange is intermediate. In order to extract more precise results for T2B and τex, an analysis of the relaxation data is necessary and is described in the next section. Analysis of T2 Relaxation Experiments. To analyze T2 relaxation data of Figure 3 quantitatively, the model intermediate exchange in relaxation experiments has been employed. The model and the procedure have been described in detail earlier.17 For a relaxation experiment in a two-site system, the mathematical expressions of apparent relaxation rates and the apparent population fractions are21
C1 ¼
1 C2 ¼ 2
a A ¼ C1 þ C2
ð3aÞ
a B ¼ C1 - C2
ð3bÞ
1 1 1 1 1 þ þ þ 2 T2A T2B τA τB
ð3cÞ
1=2 1 1 1 1 4 þ þ T2A T2B τA τB τA τ B
ð3dÞ
1 1 1 1 1 1 ðpA - pB Þ þ þ PA ¼ 1 - PB ¼ 2 4C2 T2A T2B τA τB ð3eÞ Equation 3a-3e is fitted by Levenberg-Marquard algorithm to data sets consisting of six spin-spin relaxation echo decays at different hydroquinone concentrations. T2A is a fixed input parameter, set to T2A = (5.4 ( 0.2) s, the value determined for HQ in D2O. The fractions pAi, pBi for each hydroquinone concentration cHQ,i are taken from the results of the diffusion analysis and treated as fixed parameters as well. With the approximation that the relaxation decay constant of adsorbed HQ, T2B, can be considered independent of the adsorbed amount, a global set of parameters Pfree = (T2B, τAi) (i = 1-6) is optimized to fit the whole set of echo decays. The optimized parameters are shown in Table 3. The errors result from the variation of the input parameters within their error range. The resulting exchange times confirm the order of magnitude of τex extracted from diffusion analysis. However, here τA, τΒ, and thus τex are extracted quite precisely, while the error in T2B is very large. There is a systematic dependence of the residence times on concentration. Generally, τex lies between the T2 relaxation decay constants of bound and free hydroquinone. (21) Woessner, D. E. Concepts Magn. Reson. 1996, 8, 397.
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Table 3. Parameters Calculated from Global Fitting of Spin-Spin Relaxation Data Sets of Different Concentrations of Hydroquinone in Capsule Dispersion (Capsule Concentration 0.6 vol %)a cHQ (% wt)
τA (ms)
T2B (ms)
τB (ms)
τex (ms)
0.15 66 ( 4 92 ( 4 38 ( 2 0.25 74 ( 4 66 ( 4 35 ( 2 0.40 1.0 ( 0.4 82 ( 4 46 ( 4 29 ( 2 0.60 96 ( 4 33 ( 4 24 ( 2 0.75 103 ( 4 29 ( 4 22 ( 2 1.25 115 ( 4 20 ( 4 16 ( 2 a Errors shown here give only the contribution arising from the fit procedure itself, and the variation of the input parameters within their error range.
Since T2B is very small, the bound species is not directly detected and thus only exponential echo decays resulting from the free fraction are observed (Figure 3b). Furthermore, τA increases and τB decreases with increasing concentration. As a result, the exchange time of hydroquinone decreases with increasing concentration and its value is in the range of 15-40 ms (Table 3). The mean residence times of hydroquinone in the free site (τAi) (Table 3) agree very well with the apparent relaxation decay constants (T2i) which were extracted from single exponential fits of the decays in Figure 3b (see Table 1). This has been predicted for apparent relaxation decay constants in the regime where T2B < τex , T2A, and Woessner found that in this regime τA can be extracted directly from the single echo decays.21 Discussion of Aromatic Alcohols Interaction with Capsules. The analysis of diffusion and relaxation data sets clearly show that, in capsule dispersion, hydroquinone is found in two sites, that is, adsorbed and free, with fractions of similar magnitude. There is, probably, HQ also present in a third site, C, the capsule interior, where it shows slow spin-spin relaxation, T2C = T2A, and slow diffusion, DC = DB. Since the system is in thermodynamic equilibrium, the chemical potential and consequently the concentration in the capsule interior should be the same as those in the continuous aqueous phase. For polymers in capsule dispersion, this was found true for poly(ethylene oxide),16 while an enrichment in the interior was found for dextrane and attributed to chain entanglement.14 Assuming the concentrations cC and cA to be identical in the case of HQ, at a capsule volume fraction of 0.6%, the encapsulated fraction amounts to 0.3% to 0.5% and is thus negligible. Therefore, employing a two-site model is justified, and in the following we will discuss the equilibrium between the adsorbed and the free component only, neglecting HQ in the capsule interior. From the bound fraction, it is possible to determine the adsorbed amount of hydroquinone. The simplest model for adsorption is described by the Langmuir isotherm, where the fraction of occupied adsorption sites, θ, is given in dependence on concentration as θ ¼
Kc 1 þ Kc
ð4aÞ
with K as the equilibrium constant of the adsorption-desorption reaction. The inverse isotherm given by 1 1 ¼ 1þ θ Kc
ð4bÞ
can be easily employed to determine the equilibrium constant as the inverse slope. Figure 4a shows a fit of the adsorbed amount of hydroquinone by the inverse Langmuir isotherm (eq 4b), yielding Langmuir 2010, 26(15), 12940–12947
Figure 4. (a) Inverse adsorption isotherm and (b) adsorption isotherm of hydroquinone in capsule dispersion (capsule conc. 0.6% vol). The solid lines are fits with eq 4b and 4a, respectively. Concentrations of free hydroquinone in solution (cA) are represented in weight fractions and are therefore dimensionless.
a very good description of the data. The ordinate in this plot is rescaled, such that an extrapolation of 1/θ to infinite concentration (1/cA f 0) yields a value of 1; from the rescaling factor, the number of sites, that is, the number of alcohol molecules, adsorbed by one capsule can be determined. The equilibrium constant results in KHQ =1374 ( 16. Since the concentration in eq 4a is defined in weight fraction, K is dimensionless. Figure 4b shows the adsorbed fraction after rescaling and fitting with the Langmuir isotherm, eq 4a. Figure 4a suggests that when 1.25 wt % of HQ is added to the capsule dispersion, the adsorbed amount has practically reached the saturation value. Since a direct measurement of the capsule concentration is hard to perform, the capsule concentration was derived from the original silica particle concentration under the assumption of no particle losses during the centrifugation-decantation processes of the multilayer preparation. This yields a capsule concentration of 0.6% vol in the final dispersion, possibly lower due to losses of capsules in the preparation process. Thus, a direct comparison of the present results for HQ to previously published results for phenol17 cannot be done quantitatively. In order to compare the adsorption behavior of HQ and phenol quantitatively without dependence on the capsule concentration, phenol experiments are repeated on the same capsule dispersion employed for HQ adsorption. The resulting isotherms are shown in comparison in DOI: 10.1021/la101836a
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Figure 5. (a) Inverse adsorption isotherms and (b) adsorption isotherms of phenol and hydroquinone in capsule dispersion (capsule conc. 0.6%). Circles, hydroquinone; squares, phenol; lines, respective fits with a Langmuir isotherm.
Figure 5, represented by the total number of adsorbed molecules, Nads. In both cases, the Langmuir isotherm yields a fairly good description. The equilibrium constant for phenol adsorption, KPh = 496 ( 7 is about a factor of 3 lower as compared to that of HQ, KHQ = 1374 (see above). This suggests a lower affinity of phenol to the capsule wall at low concentrations. On the other hand, the maximum adsorbed amount is larger for phenol as for HQ by again about a factor of 3, as it is (Nads)max,Ph = 2.3 10-5 mol as opposed to (Nads)max,HQ = 7.5 10-6 mol. These factors of three partially compensate, leading to a similar initial slope of both isotherms at very low concentration, where Nads ≈ Nads,maxKcA (see Figure 5b). In order to compare interactions of the probe molecules with the capsule wall, it is interesting to study the interactions with single chains of PAH or PSS, respectively. The interaction with PDADMAC is neglected, since PDADMAC is only present in the first layer. To investigate the interactions, diffusion coefficients of the probe molecules are determined in solutions of 0.2 wt % probe and 2.0 wt % polyelectrolyte. As a reference, the diffusion coefficients in pure water are (7.5 ( 0.1) 10-10 m2 s-1 for phenol and (6.2 ( 0.1) 10-10 m2 s-1 for HQ (see above). The diffusion coefficient of phenol is (7.0 ( 0.1) 10-10 m2 s-1 in PAH solution and (5.8 ( 0.4) 10-10 m2 s-1 in PSS solution. Hydroquinone diffuses with the diffusion coefficient (5.8 ( 0.1) 10-10 m2 s-1 in PAH solution and with the diffusion coefficient (4.8 ( 0.4) 10-10 m2 s-1 in PSS solution. Decreased values of 12946 DOI: 10.1021/la101836a
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diffusion coefficients indicate that for both probe molecules there is an attractive interaction with the polyelectrolyte, and this interaction is larger with PSS than with PAH. The plausible explanation is that both aromatic probe molecules have strong attractive π-π interaction with the aromatic polymer PSS. A similar interaction by stacking of the π-systems was demonstrated between PSS and Rhodamine B22 and also between PSS and a tetrazolium ion.23 Thus, the interaction with PSS can be expected to dominate the guest molecule binding due to the aromatic ring of PSS. This is further support that the interaction with the single, internal layer of PDADMAC can indeed be neglected. For either polyelectrolyte, the relative decrease of the diffusion coefficient is however independent of the type of probe molecule. Thus, the strength of the interaction with a single chain in aqueous solution is the same for phenol and HQ. Comparing the two probe molecules, hydroquinone has a slightly larger mass and volume. This could be a reason for a much lower number of adsorption sites being accessible for HQ as compared to phenol. Very steep size cutoff values were shown to be relevant for guest molecule permeation through free-standing PEM,9,10 showing that possibly even small differences in guest molecule size can have a large impact on incorporation into PEM. On the other hand, hydroquinone is more hydrophilic than phenol. The octanol-water partition coefficient POW gives a value of log(POW) = 1.46 for phenol, whereas it is 0.59 for hydroquinone. If the wall is considered a hydrophobic material, this implies that the distribution of HQ between the wall and aqueous phase is shifted toward the aqueous phase in comparison to that of phenol. Thus, the adsorbed amount of HQ will be lower. Both of these effects, hydrophobicity as well as size, can contribute to the preferential adsorption of phenol into capsule walls. However, on the other hand, the much larger equilibrium constant of HQ adsorption suggests a larger affinity of HQ to the wall at least at very low concentrations, leading to the similar initial slopes of both isotherms in Figure 5b. This large affinity is interesting, since it indicates that the second OH group of HQ might play a major role in interaction with the multilayer. Though an enhanced interaction of HQ with either of the constituent chains is not detected in solutions, the difference here is clearly pronounced. In comparison to the previous data for phenol, the adsorption dynamics of hydroquinone show quite similar characteristics: The exchange between bound and free HQ is much slower as expected for diffusion controlled adsorption; thus, the time limiting step is probably the release of HQ molecules from the wall and their transport through the wall. With increasing total HQ concentration, the exchange rate increases. This can be understood as an enhancement of the transport within the wall with increasing concentration. This has two possible explanations: It might be that at higher HQ loading a larger number of molecules is situated close to the interface and contributes to the exchange by a fast process, since its diffusion path through the wall is short. In addition, the large amount of HQ adsorbed into the wall might change the wall properties such that, at larger HQ concentration, diffusion within the wall is faster, enhancing the HQ exchange rate. Jin et al. had compared permeation rates of HQ and phenol through polyelectrolyte multilayers of PAH and PSS and found a somewhat larger rate for phenol (16.6 10-6 cm 3 s-1) as compared to HQ (11.6 10-6 cm 3 s-1).10 They attributed this (22) Moreno-Villoslada, I.; Gonzalez, R.; Hess, S.; Rivas, B. L.; Shibue, T.; Nishide, H. J. Phys. Chem. B 2006, 110, 21576. (23) Moreno-Villoslada, I.; Torres, C.; Gonzalez, F.; Soto, M.; Nishide, H. J. Phys. Chem. B 2008, 112, 11244.
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to the difference in size, that is, the long axis of phenol being smaller than that of HQ. Comparing exchange rates, however, we find slightly larger exchange rates for HQ (for example 34 s-1 at 0.4 wt %) as compared to those for phenol (26 s-1 at 0.4 wt %17). While the experiment of Jin et al. is performed in very dilute solutions, in our case the change of the wall properties due to the alcohol has to be taken into account. Since the incorporated fractions are large, the multilayer has to be considered strongly swollen. Possibly in the case of phenol the large alcohol amount in the layer reduces the average exchange time of an alcohol molecule.
Conclusion Combining self-diffusion and spin relaxation experiments according to a method established previously,17 the distribution and exchange of hydroquinone and phenol is investigated in
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capsule dispersions. HQ and phenol partially adsorb into the capsule wall, and the distribution equilibrium between adsorbed and free alcohol is in both cases well described by a Langmuir isotherm with equilibrium constants of KPh = 496 ( 7 and KHQ = 1374. The equilibrium constant of hydroquinone shows a large affinity of HQ to the capsule wall; however, the number of accessible adsorption sites is larger for phenol. Both properties are clearly specific for interaction of the alcohols with the wall, since the interactions with single polyelectrolyte chains in solution are similar for both alcohols. Acknowledgment. We thank Dr. D. Baither, Institute of Material Physics, WWU M€unster, for help with TEM measurements to ensure the capsule quality. D.C. and R.P.C. are grateful to the “International NRW Graduate School of Chemistry (GSC-MS)” for a doctoral scholarship.
DOI: 10.1021/la101836a
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