Enhanced Protonation of Cresol Red in Acidic Aqueous Solutions

J. Phys. Chem. B 2006, 110, 1277-1287

1277

Enhanced Protonation of Cresol Red in Acidic Aqueous Solutions Caused by Freezing Dominik Heger,† Jana Kla´ nova´ ,‡ and Petr Kla´ n*,† Department of Organic Chemistry, Faculty of Science, Masaryk UniVersity, Kotlarska 2, CZ - 611 37 Brno, Czech Republic, and Recetox, Faculty of Science, Masaryk UniVersity, Kamenice 126/3, CZ - 625 00 Brno, Czech Republic ReceiVed: September 21, 2005; In Final Form: NoVember 25, 2005

The protonation degree of cresol red (CR) in frozen aqueous solutions at 253 or 77 K, containing various acids (HF, HCl, HNO3, H2SO4, and p-toluenesulfonic acid), sodium hydroxide, NaCl, or NH4Cl, was examined using UV/Vis absorption spectroscopy. CR, a weak organic diacid, has been selected as a model system to study the acid-base interactions at the grain boundaries of ice. The multivariate curve resolution alternating least-squares method was used to determine the number and abundances of chemical species responsible for the overlaying absorption visible spectra measured. The results showed that the extent of CR protonation, enhanced in the solid state by 2-4 orders of magnitude in contrast to the liquid solution, is principally connected to an increase in the local concentration of acids. It was found that this enhancement was not very sensitive to either the freezing rate or the type of acid used and that CR apparently established an acid-base equilibrium prior to solidification. In addition, the presence of inorganic salts, such as NaCl or NH4Cl, is reported to cause a more efficient deprotonation of CR in the former case and an enhanced protonation in the latter case, being well explained by the theory of Bronshteyn and Chernov. CR thus served as an acid-base indicator at the grain boundaries of ice samples. Structural changes in the CR molecule induced by lowering the temperature and a presence of the constraining ice environment were studied by the absorption and 1H NMR spectroscopies. Cryospheric and atmospheric implications concerning the influence of acids and bases on composition and reactivity of ice or snow contaminants were examined.

1. Introduction Hydrophobic and hydrophilic compounds are known to become spontaneously segregated at grain boundaries of ice at the phase transition when their aqueous solutions are frozen.1,2 Such a solute-concentration-enhancing effect is now well established, for example, in connection with the acceleration of some heterogeneous reactions3-5 or with specific intermolecular photoreactions of halogenated aromatic compounds.6-8 Several investigations indicated that organic impurities in natural ice or snow undergo heterogeneous reactions with various trace compounds,9-15 the reaction efficiency of which unquestionably depends on the local concentrations of reactants. Recently, a UV-Vis spectroscopic technique was shown to provide insight into the composition of frozen aqueous solutions of a model organic compound, methylene blue.16 The extent of the dye aggregation at the grain boundaries of ice matrix was found to be dependent on the freezing rate and temperature. While the local concentration increased by approximately 3 orders of magnitude upon fast freezing at 77 K compared to the liquid phase, it raised at least by 6 orders of magnitude upon slow freezing at 243 K. In addition, temperature-dependent restrictions of the molecular motion have to be considered to evaluate any chemical transformation in a constraining medium. Diffusion and surface transport of small particles, such as protons or inorganic ions, is relatively effective in/on ice;17-20 moreover, * To whom correspondence should be addressed. Phone: +420549494856. Fax: +420-549492688. E-mail: [email protected]. † Department of Organic Chemistry, Masaryk University. ‡ Recetox, Masaryk University.

diffusion of larger species, such as benzyl radicals, was also found to be remarkably efficient at the grain boundaries above 223 K.21 Acid-base processes are of obvious importance in chemistry. Mineral acids and halogen molecules play an important role in the heterogeneous chemistry on the surface of ice crystals in the atmosphere22,23 or the snowpack.9 There has been extensive interest in characterizing HCl adsorption and ionization on the ice surface, representing a key step in the heterogeneous chemical reactions in the ozone destruction cycle because HCl degradation may lead to the production of the chlorine radicals.24-26 Some organic monocarboxylic acids (such as formic and acetic acid)27 and various dicarboxylic acids (C2C8)28,29 were hypothesized to be photochemically produced in the snowpack. They contribute a significant fraction of free acidity in precipitation in remote regions,30 and their concentrations in the Artic snowpack can reach 0.1-10 µg L-1.29 The presence of acids (protons) may play a significant role in chemical processes in/on ice or snowpack; however, only limited information is available to date. Fourier transform infrared (FTIR) spectroscopy study has, for example, revealed that acidbase reaction between HCl and NH3 on crystalline ice produces the ammonium ion (NH4+) to a limited extent between 80 and 140 K, while its formation is very efficient above 140 K.31 The major nitrate photolysis process in ice or snow, producing NO2 and •OH,15,32 was found to be affected by pH.13 In this work, we present a study of the protonation degree of cresol red (CR), a common acid-base indicator, in frozen aqueous solutions in the presence of various acids (HF, HCl, HNO3, H2SO4, and p-toluenesulfonic acid), NaOH, NaCl, or NH4Cl. Three forms of aqueous CR (Scheme 1) are known to

10.1021/jp0553683 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/04/2006

1278 J. Phys. Chem. B, Vol. 110, No. 3, 2006

Heger et al.

SCHEME 1

have absorption bands in the visible region with λmax at 518 (A, orange-red), 434 (B, yellow), and 573 (C, red) nm,33 respectively, which can be easily distinguished by absorption spectroscopy. The structure of CR has been well established by X-ray analysis,34 and unlike phenolphthalein possessing a γ-lactone (colorless) ring system,35 all CR forms in aqueous solutions have zwitterion structures with the prolonged conjugation system responsible for their color.36 This weak organic diacid has been selected as a model system to study the acidbase interactions at the grain boundaries of ice because its proton dissociation equilibria in water are generally well accepted. Our study focused on examining the extent of CR protonation which can be affected by (a) sample temperature, (b) freezing rate, (c) acid equilibrium and nonequilibrium solubility in ice, and (d) CR or acid accumulation (aggregation) in the layer of liquid, quasiliquid, or solid solution surrounding the ice crystal walls of the polycrystalline state. The multivariate curve resolution alternating least-squares (MCR-ALS) method37 was used to determine the relative abundances of CR forms in ice samples from their overlaying visible absorption spectra. 2. Experimental Section CR (for analysis, J. Knittl Co.) was used without further purification, and each experiment was performed with a freshly prepared stock solution. Water was purified by the reverse osmosis process on an Aqua Osmotic 03 and its quality complied with U.S. Pharmacopeial Standards (USP). Hydrofluoric acid (38-40%, puriss; Spolek pro chemickou a hutni vyrobu, Co.), hydrochloric acid (35%, purum; ONEX), H2SO4 (96%, p.a.; ML Chemica), nitric acid (65%, p.a.; ML Chemica), p-toluenesulfonic acid monohydrate (p.a.; MERCK), sodium hydroxide (p.a.; Lachema), NaCl (p.a.; Lachema), and NH4Cl (p.a.; Lachema) were used as received. The CR aqueous solutions of different pH were prepared by dissolving concentrated (aq) or pure chemicals, and subsequently by adding a corresponding amount of the stock CR solution. The resulting concentration of CR used in this work was either ∼1 × 10-5 or ∼3 × 10-6 mol L-1. The pH of the solutions was measured after the addition of the dye using a SenTix 97 T pH-combined electrode with an integrated temperature sensor of a pH 320 Microprocessor pH-meter (WTW).

The solidified samples containing CR solutions in Plastibrand cuvettes (transparent at >280 nm) were prepared by freezing either quickly at 77 K (a liquid nitrogen bath) or slowly at 253 K (ethanol cooling bath). The spectra of liquid aqueous solutions were measured on a Unicam UV4 spectrometer (Cambridge, UK) against a pure water sample in quartz cells with the optical path length of 1 cm. The absorption spectra in methanol at low temperatures were recorded using Hellma optical fibers (Hellma, 041.002-UV) and an immersion quartz Suprasil probe (Hellma, 661.500-QX). The spectra of frozen samples and the reference spectra of pure ice were measured on a Lambda 19 UV/VIS/ NIR spectrophotometer (Perkin-Elmer) using a 60-mm integrating sphere (the slit width was set to 1 nm and the scan speed to 480 nm min-1 or lower) immediately after removing the cuvettes from the cold environment. Although the sample temperature was not controlled during the absorption measurements, no changes of the spectra were observed within the time period necessary for duplicate consecutive experiments. The averaged spectral background of pure ice was subtracted from each spectrum, and the spectra shown are averaged from at least three independent measurements. The final spectra were smoothed using an adjacent averaging method when necessary. 1H NMR spectra were obtained on a Bruker Avance spectrometer 500 (500 MHz for 1H). Chemical shifts in the NMR spectra are reported in ppm (δ) relative to an internal standard (tetramethylsilane) at 0.00 ppm. CR (303 K): 1H NMR (CD3OD) δ ) 8.18-8.16 (m, 1 H), 7.79-7.76 (m, 1 H), 7.707.66 (m, 1 H), 7.42-7.39 (m, 4 H), 7.25-7.23 (m, 1 H), 6.976.96 (m, 2 H), 2.25 (s, 6 H). CR (303 K; HCl (aq) addition): 1H NMR (CD OD) δ ) 8.18-8.16 (m, 1 H), 7.85-7.81 (m, 1 3 H), 7.74-7.71 (m, 1 H), 7.484-7.479 (m, 1 H), 7.47-7.46 (m, 1 H), 7.45-7.44 (m, 2 H), 7.27-7.25 (m, 1 H), 7.23-7.11 (m, 2 H), 2.27 (s, 6 H). MATLAB 6.5.1 was utilized as the graphical and statistical software. The MCR-ALS has been used as a model-free mathematical method for recovery of the concentration profiles and pure spectra of the spectroscopically active species, based on the Lambert-Beer law and a least-squares minimization, in the same way as it was described in our preceding article.16 The first step in the spectra recovery was finding the number of species in the sample by a singular value decomposition of

Enhanced Protonation of Cresol Red

J. Phys. Chem. B, Vol. 110, No. 3, 2006 1279

Figure 1. Representative absorption spectra of three CR forms (A, B, and C from Scheme 1) in liquid aqueous solutions of different pH adjusted by HCl or NaOH and measured at 293 K. The insert assigns the corresponding pH values to the spectra; the values with an asterisk are the molar concentrations of HCl in the most acidic solutions.

the data in MATLAB. The evolving factor analysis (EFA) routine was applied for estimation of the species concentrations, and subsequently used for MCR-ALS with specific constrains applicable in this calculation (e.g., no negativity, unimodality, or closure), developed by Tauler and his collaborators.37 The step by step filter program38,39 was applied to determine the number of peaks in the spectra. 3. Results CR: The Study System. The pKa1 and pKa2 of cresol red (Scheme 1), corresponding to the first and second proton exchange equilibria (transition points) in aqueous solutions, have been reported to be 1.10 (in glycine-NaCl-HCl buffer systems40) or 1.11 ( 0.0141 and 8.15 (in phosphate buffer systems)42 or 7.87 ( 0.01,43 respectively. Sulfonephthaleins, including CR, are inherently weak acids or bases and can therefore somewhat change the pH of the sample in diluted solutions. The pKa2 value of CR is known to exhibit a weak temperature dependence (eq 1; temperature T is in K)44

pKa2 )

913.4 + 2.049 + 1.266log T T

(1)

For example, increasing temperature from 253 to 323 K decreases the pKa2 from 8.70 to 8.05.44 It was also found that pKa2 decreased with increasing ionic strength in some sulfonephthaleins. The pKa2 for thymol blue, for example, dropped from 9.20 at ionic strength of µ ) 0.00 M to ∼8.9 at µ ) 0.10 M.45 The shifts of the absorption maxima of all three forms have been observed under various conditions, for example, in the presence of cetyltrimethylammonium bromide (CTAB) in aqueous solutions.46 Such observations were explained, in addition to electrostatic interactions between the positively charged nitrogen of CTAB and a negative charge at CR, by

formation of very closely packed ion pairs due to strong hydrophobicity of the oppositely charged dye and surfactant ions enabling a more efficient deprotonation.46 It was found by using the infrared absorption spectroscopy that sulfonephthaleins in the solid KBr matrix display the quinone-like structure,36 in contrast to a prevailing charge separated moiety in solutions. Also, inclusion of sulfonephthaleins to cyclodextrins, studied by UV-Vis and NMR spectroscopy, revealed that the formation of a lactone ring is induced in the cavity.47 CR Liquid Solutions at 293 K. As a first step to establish the validity of the proposed method, we determined the transition points of CR forms in aqueous solutions with a different pH using the corresponding concentrations of HCl or NaOH. Absorption spectra of CR (c ) 1.3 × 10-5 mol L-1) in the range of 350-650 nm (Figure 1) distinguished well three equilibrated forms (A, B, C) shown in Scheme 1. The molar concentrations of HCl are depicted in the figure for most acidic solutions, when the solution pH was outside the range of the pH meter performance. The absorption maxima (λmax) of the doubly protonated (A), singly protonated (B), and deprotonated (C) forms were found at approximately 520, 434, and 573 nm, respectively. The existence of two isosbestic points in the overlaid spectra (λi ) 474 and 486 nm), corresponding well to the literature values,33 authenticated that the CR forms were in equilibrium. The latest release of MCR-ALS user-friendly graphical interface was used to resolve spectra and concentration profiles by a mathematical least-squares minimalization.37 The closure was set to 1, and the non-negative least-squares and unimodality (with an average implementation and the constraint tolerance equal to 1.05) were applied to all spectra. Singular value decomposition revealed the presence of 3 species as expected. The calculated spectra of the CR forms from the MCR-ALS analysis are shown in Figure 2 and their abundances in Figure 3. The form A was prevailing below pH ) 1.1, B was observed


Figure 2. Calculated spectra of three CR forms (A, blue; B, green; C, red) in liquid aqueous solutions at 293 K obtained from MCR-ALS method using the input data calculated by the EFA.

Figure 3. The calculated relative concentration-dependent abundance profiles of three CR forms (A, blue; B, green; C, red) in liquid aqueous solutions at 293 K as a function of the pH, obtained from the MCRALS method based on data shown in Figure 1. The lines are visualized trends of the corresponding calculated values (circles).

in the pH range of 1.1 to 7.9, and C was the most abundant form in more basic solutions. The pKa can be roughly estimated from the transition point (pHT), which is the corresponding pH at which two forms have the same molar concentrations. The obtained values, pHT1 ≈ 1.1 (pKa1 equivalent) and pHT2 ≈ 8.0 (pKa2 equivalent), corresponded accurately to the literature values.40-42 Frozen CR Liquid Solutions (HCl, NaOH) at 253 K. The data obtained from the spectral analysis of the frozen aqueous CR solutions containing HCl or NaOH were used to identify all three forms present in the constrained medium of ice to determine the protonation degree of the dye (Figure 4). A relatively slow freezing rate of the CR samples, the pH values of which were measured in the liquid state, was achieved by immersing them to an ethanol bath at 253 K. The spectra were measured immediately after removing from the cooling medium. The absorption maxima and isosbestic points did not change their position significantly compared with those in the liquid (293 K) samples. A new minor absorption maximum (560 nm) at the shoulder of a band A, however, appeared in the spectra of samples with the corresponding pH in the interval of 2.8

Heger et al. and 5. Heterogeneity, lower transparency, light scattering, and reflection of the polycrystalline ice samples decreased the signalto-noise ratio. The singular value decomposition indicated the presence of three most abundant species again (Figure 5). The resolved spectra of the CR forms in frozen solutions at 253 K are comparable to those measured in the liquid phase (Figure 2). While no shifts of the absorption maxima comparing to liquid samples were observed, the transition point (pHT1), in this case the corresponding pH (liquid) value at which the molar concentration ratio of A and B equals to 1, was found to be ∼4.2 (Figure 6). In contrast to data shown in Figure 3, protonation of the form B in the slowly frozen samples occurs at an HCl concentration lower by approximately 3.5 orders of magnitude. The partitioning between the forms B and C, on the other hand, occurred at the same pHT2 ∼8.0 in both liquid (293 K) and solid (253 K) phases. Frozen CR Liquid Solutions (HCl, NaOH) at 77 K. The character of the absorption spectra measured changed when the samples were prepared by fast freezing at 77 K (Figure 7). The absorption maximum of the form A at the corresponding highest HCl concentration was red shifted by ∼3 nm (523 nm). Furthermore, the absorption band split in two for the solution pH range of 1.2-1.7: one maximum remained at λmax ≈ 523 nm; the second appeared at 498 nm. This change was found to be reversible with temperature. Increasing the temperature of the sample from 77 to 253 K caused that the bands coalesced into one with λmax ≈ 523 nm, observed also in slowly frozen solutions (253 K), but two bands reappeared when the temperature dropped to 77 K. A new minor absorption maximum (560 nm) at the shoulder of the band A, also observed at 253 K, was apparent at 77 K at the corresponding solution pH of 2-4.85. In addition, an absorption enhancement at 375 nm was also observed. The form B prevailed at pH ) 4.9-9.0, having λmax shifted to 447 nm. For pH > 9.3, the band of the form C appeared (λmax ≈ 582 nm), and at pH > 10.4, two apparent absorption maxima were observed (546 and 574 nm). A reversible coalescence of these two bands after slow heating the samples to 253 K was discerned as well. Two isosbestic points were observed at approximately same wavelengths as those at 293 K (Figure 7). The singular value decomposition of all spectra revealed 3 important species present in the whole pH range and the MCRALS analysis subsequently generated practically identical spectra of the main CR forms (λmax ≈ 524 nm (A), 444 nm (B), 574, and 549 nm (C)), where two bands (A and C) are, however, composed evidently of two closely overlying spectra (Figure 8). Because of the fact that appearance of the double bands is temperature dependent and their coalescence is reversible (vide infra), only three species A, B, and C were introduced to the minimalization procedure. The corresponding pHT1 value at 77 K was found to be ∼4.4, while pHT2 ≈ 9.3 (Figure 9); thus, the latter was somewhat higher than that measured at 253 or 293 K. Frozen CR Liquid Solutions in the Presence of Various Acids. Protonation degree of CR has also been studied in the presence of some other acids, such as HF, HNO3, H2SO4, or p-toluenesulfonic acid. Table 1 lists the pHT1 of various samples measured at 293, 253, and 77 K. The character of an acid used had only minor effects on pHT1 and no effects on the band maxima, however, an overall increase of the pHT1 values when temperature dropped from 253 to 77 K was apparent. Temperature Effects on CR Spectroscopic Behavior. Unexpected appearance of an absorption band splitting in the



Figure 4. Representative absorption spectra of CR (A, B, C from Scheme 1) frozen at 253 K. HCl or NaOH were used to adjust the pH. The insert assigns the corresponding pH values (liquid) to the spectra; the value with an asterisk is the molar concentration of HCl in the most acidic solution.

Figure 5. Calculated normalized spectra of three CR forms (A, blue; B, green; C, red) in frozen aqueous solutions at 253 K obtained from MCR-ALS method using input data calculated by the EFA.

spectra of frozen aqueous solutions in several cases and their reversible temperature coalescence urged us to study temperature-dependent structural changes of CR in detail. The absorption spectra of CR methanol solutions containing HCl (c ) 0.1 mol L-1) were measured in the temperature interval of 176 and 303 K. CR in this methanol solution exists as a doubly protonated form (A), and the absorption spectrum did not reveal any band splitting. Only a red shift of λmax from 521 to 526 nm was observed when temperature dropped from 303 to 200 K. The same shift was observed in frozen aqueous solutions. When the absorption spectrum was measured immediately after a sample frozen at 175 K was thawed, a new minor absorption maximum at ∼560 nm appeared. In the case of CR methanol solutions in the absence of HCl, the absorption maximum of a monoprotonated form (B) exhibited a larger red shift from 419

Figure 6. The calculated relative concentration-dependent abundance profiles of three CR forms (A, blue; B, green; C, red) in samples at 253 K as a function of the pH (liquid), obtained from the MCR-ALS method based on data shown in Figure 4. The lines are visualized trends of the corresponding calculated values (circles).

to 440 nm, when temperature dropped from 303 to 200 K. The latter λmax value corresponds to that found in ice at 77 K. In addition to the absorption measurements in methanol solutions, 1H NMR (CD3OD) spectra were obtained for neutral and acidic samples (HCl (aq) addition) in the temperature interval of 175-303 K. Figure 10 shows NMR spectra of the form A. The aromatic hydrogen signals corresponding to the rings Y, assigned by Yoshida et al.,47 exhibited a strong temperature dependence, in contrast to those of the ring X. The line broadening was visible already at 233 K and developed extensively at 175 K. The same behavior was observed for the form B in pure methanol (not shown). The CHD2OD shifts,


Heger et al.

Figure 7. Representative absorption spectra of CR frozen at 77 K. HCl or NaOH were used to adjust the pH. The insert assigns the corresponding pH values (liquid) to the spectra; the values with an asterisk are the molar concentrations of HCl in the most acidic solutions.

Figure 8. Calculated normalized spectra of three CR forms (A, blue; B, green; C, red) in frozen aqueous solutions at 77 K obtained from MCR-ALS method using input data calculated by the EFA.

calibrated on temperature,48 were used as a reference for measuring the 1H NMR spectra at low temperatures. Effects of Salt Addition on CR Spectroscopic Behavior in Frozen Aqueous Solutions. Figure 11 shows the absorption spectra of CR in aqueous frozen solutions containing NaCl or NH4Cl at 77 K, compared to that of a liquid CR solution. Addition of a salt caused new absorption maxima emerged: λmax ) 578 and 538 nm in the presence of NaCl and NH4Cl, respectively. The former is consistent with a band of the deprotonated CR (form C), while the latter with a doubly protonated form A. We were unable to measure the spectra of samples frozen at 253 K because the liquid phase containing salt was completely separated from the ice phase.

Figure 9. The calculated relative concentration-dependent abundance profiles of three CR forms (A, blue; B, green; C, red) in samples at 77 K as a function of the pH (liquid), obtained from the MCR-ALS method based on data shown in Figure 7. The lines are visualized trends of the corresponding calculated values (circles).

4. Discussion In the first part of this study, UV/Vis absorption changes in aqueous cresol red solutions at various pH (Figure 1) in the absence of buffers (buffers cannot be used in the subsequent low-temperature measurements) were followed to obtain the acid-base equilibrium constants. The evolving factor analysis routine and MCR-ALS computational method16,37 proved to be an excellent tool for determining three CR forms (Scheme 1) and their concentrations from the overlaying absorption spectra measured (Figures 2). The pKas calculated from the abundance profiles (Figure 3) were found to be identical with those known from the literature: pKa1 ≈ 1.1 and pKa2 ≈ 8.0, respectively.40-42 As a result, the same method was used in the analysis of the



TABLE 1: Transition Points of the First Protonation Step of CR in the Presence of Various Acids in Liquid and Frozen Aqueous Solutionsa acid

pHT1 (293 K)

pHT1 (253 K)

pHT1 (77 K)

HF HCl H2SO4 HNO3 p-toluenesulfonic acid

1.1

3.6 4.2 4.2 4.6 4.7

4.0 4.4 4.4 4.9 5.0

a The transition point pHT1 refers to the pH of the corresponding liquid CR solution, in which the concentrations of A and B forms (Scheme 1) are equal.

absorption spectra of frozen aqueous CR solutions. The results are described in terms of the transition points (pHT), the pH values of the corresponding liquid solutions, at which the molar concentrations of two CR forms, being in equilibrium, are equal. Using the pKa values has no meaning in the frozen solution. Spectral Characterization of CR Forms in Ice. The absorption bands corresponding to three CR forms were still clearly observable upon freezing CR samples at 253 K (Figure 4) and their maxima had practically the same wavelengths as those measured in the liquid phase. The acid form (A) had an additional distinct shoulder absorption band at 560 nm in samples with the corresponding pH of 2.8-5. The subsequent evolving factor analysis then provided the absorption spectra of individual forms (Figure 5) and the abundance profiles (Figure 6). This calculation was not able to separate the bands corresponding to B and C forms completely, therefore that of B possesses a second (minor) maximum at ∼580 nm. The changes of absorption spectra were, however, significant in samples frozen at 77 K (Figure 7). The band maxima (λmax) shifted comparing to 253 K samples to some extent, and some new significant absorption bands appeared. As in the previous case, a shoulder with λmax ) 560 nm appeared in most acidic solutions. The peak corresponding to the form B was largely red shifted with a maximum at 447 nm. Moreover, a splitting of the C band was clearly evident in most alkaline samples. Interestingly, the evolving factor analysis again provided relatively pure spectra of individual forms (Figure 8) and the abundance profile (Figure 9), although A and C were evidently composed of at least two overlaying bands. To explain the appearance of new absorption bands or shifts of their maxima, a further investigation was necessary. To evaluate the temperature effect, the spectra of CR in methanol were measured at low temperatures (293-175 K). All three forms exhibited comparable spectra to those obtained in aqueous solutions. Only a shoulder absorption in the A band (λmax ) 560 nm) was observed at 175 K (methanol is very viscous at this temperature). This absorption should not be related to the form C at such acid concentrations. Despite the fact that no major band splitting was observed in methanol, we assumed that spectral changes in ice at 77 K can be connected to a molecular confinement in the solid matrix and to temperature. It is now well established that heterogeneous environment often causes remarkable (cage) effects on chemistry due to restricted translational or conformational motion of molecules.49 For example, we have recently shown that restrictions of an alkyl chain dynamics in alkyl phenyl ketones can cause a decrease in the intramolecular photoreaction efficiency.50 Scheme 1 suggests that the conjugation in the CR molecule is connected via a (partial) double bond between the central carbon, bearing the positive charge, and the benzene rings. It is obvious that slowing down the rotation along this bond, because of lower temperature or escalating dipolar interactions with other mol-

ecules, would support the contribution of the quinone-like structure. This was, for example, demonstrated by the infrared absorption spectroscopy measurement in the solid KBr matrix at 293 K;36 however, the absorption bands and their maxima of solid CR silicate films were shown to be same as those in aqueous solutions at 293 K.51 Absorption properties of such an electronic isomer should be different and unquestionably affected by temperature. Indeed, the absorption band splitting at 77 K reversibly disappeared when temperature reached 253 K, which suggests on a rather simple, least-motion transformation. On the basis of the facts described above, we cannot fully exclude the possibility of very fast intermolecular reactions (such as a proton exchange); however, we tentatively suggest that the appearance of new absorption bands at 77 K is connected to a charge/electron redistribution in the CR molecule because of a restricted conformational motion. In this context, a specific signal broadening in a series of temperature 1H NMR measurements of CR in methanol (Figure 10) clearly showed that only one part of the molecule, corresponding to phenol/phenolate rings, exhibited significant structural changes. Conformational restriction, because of a partially developing double bond in the quinone-like form, is the most probable explanation again. Other insignificant changes in the spectra and minor shifts of the absorption band maxima in samples at 77 K, including an enhancement of the band at λmax ≈ 375 nm, were considered to be an inherent property of the system induced by specific intra- or intermolecular interactions. The evolving factor analysis calculations of the concentration profiles in 77 K ice samples (Figure 9) were thus based on a presumption that an absorption band splitting is connected to different conformations or electronic isomers of only one protonated CR form. Transition Point (pHT) in Ice. Table 1 shows the influence of temperature and the acid type (HF, HCl, HNO3, H2SO4, and p-toluenesulfonic acid) on the transition points (pHT1) of the first CR protonation step (A T B; Scheme 1). While the known pKa1 (∼pHT1) value of ∼1.1 was measured in all liquid acidic solutions at 293 K, the pHT1 for the frozen solutions raised to 3.6-4.7 at 253 and to 4.0-5.0 at 77 K, depending on the acid type used. A significant increase of the pHT1 values in frozen samples (by 2-4 orders of magnitude) is unquestionably related to enhanced protonation of the form B, in contrast to liquid solutions. Freezing the aqueous solutions is known to be accompanied by exclusion of most of the solutes from the growing ice phase resulting in increased concentrations at the grain boundaries of the polycrystalline state.4,5 Such a concentration enhancement by 3-6 orders of magnitude was determined, for example, using methylene blue,16 which is a similar cationic organic dye to CR. Only few impurities, such as HF or HCl, could be a part of the hydrogen-bonded structure of ice to generate protonic point defects.52 HF can be incorporated substantially, which affects the electrical properties of the ice matrix,53 and besides, it readily diffuses.54 Thibert and Domine´ have studied thermodynamic and kinetic behavior the HClwater-ice system.20 The maximum solubility of HCl in ice was found to be ∼10-5 mole fraction at 253 K, while the acid was predominantly a part of the solid solution below 200 K. In our experiments, HCl in the frozen solutions with pH corresponding to 4.2 (cHCl ≈ 5 × 10-5 mol L-1; cCR ) ∼3 × 10-6 mol L-1) should be mostly incorporated in the ice solid solution. At this HCl concentration, addition of CR to the solution still had no detectable effects on the pH measured in the liquid phase. We assume that CR forms at the grain boundaries at 253 K are in equilibrium, because protons55 or simple organic molecules21


Heger et al.

Figure 10. Temperature dependence of 1H NMR spectra of an acidic CR solution in d-methanol. The corresponding hydrogen atoms are assigned in the figure.


Figure 11. Normalized representative spectra of CR in a liquid aqueous solution (black) and those of frozen aqueous solutions containing NaCl (red; c ) 0.0024 mol L-1) and NH4Cl (green; c ) 0.0013 mol L-1) at 77 K.

are known to migrate efficiently in ice at even lower temperatures. The value of pHT1 ) 4.2 for HCl solutions at 253 K means that CR was protonated approximately 1000 times more efficiently comparing to liquid solutions. To explain this, an increase in the local concentration of acid (and CR) molecules during the freezing process must be considered. Thus, while sample temperature decreased, a local concentration of the acid molecules dynamically increased and protons preferentially interacted with CR. When HCl was replaced by other acids, pHT1 changed only insignificantly (Table 1). A somewhat lower value of pHT1 ) 3.6 in the case of HF could be related to its better solubility in ice; the presence of other acidssHNO3, H2SO4, and p-toluenesulfonic acidscaused only a negligible protonation enhancement, in contrast to our expectations. HNO3 is, for example, much less soluble56 in ice than HCl, and H2SO4 should be found essentially only at the grain boundaries of ice.57 Additionally, p-toluenesulfonic acid is quite different system than the other acids. As an organic molecule it should be excluded from the ice phase completely. Such a lower sensitivity of the pHT1 values to the acid structure shows that their concentrations at grain boundaries increased significantly rather than being incorporated in the ice phase in all cases. The affinity of CR toward protons as well as still efficient diffusion of the solutes was apparently the main reason of the effect observed. The CR concentration at the grain boundaries certainly became very high. The aggregation (self-organization) of such a molecule is promoted by electrostatic and dispersion forces in addition to hydrophobic effects. These interactions might enhance the CR deprotonation to some extent because of the formation of closely packed intermolecular ion pairs as it was observed in the case of CTAB addition.46 Interestingly, very similar pHT1 values were obtained when the samples were frozen in a liquid nitrogen bath (77 K). The extent of the protonation enhancement was higher only by a factor of ∼1.1, compared to that found at 253 K (Table 1), possibly reaching the maximum value. Immersing the samples in liquid nitrogen could be considered as fast freezing, but an equilibrium prior to solidification was obviously achieved quickly enough to allow this magnitude of protonation. The fact that all pHT1 values obtained at 77 K were higher while the enhancement was modest compared to 253 K experiments means that the system in the liquid nitrogen was pre-equilibrated

J. Phys. Chem. B, Vol. 110, No. 3, 2006 1285 at a relatively higher temperature, which still allows diffusion. This is, moreover, supported by the observation of two isosbestic points in the spectra at nearly the same wavelengths as those at 293 K. Several groups have showed that quasibrine layer, the unfrozen NaCl solution phase on ice crystals, exists at temperatures below the eutectic point.58-60 Some relaxation processes were also observed below liquid glass transition temperatures in supercooled aqueous propylene glycol, glycerol, or polyethylene glycol solutions.61 We do not know the eutectic point for CR solutions (the values for common organic molecules such as urea or citric acid62 are close to 260 K), but CR molecules should have some additional time to establish the protonation equilibrium even at temperatures below 253 K. When temperature drops to 77 K, any molecular transport or motion is critically constrained and intermolecular reactions can be almost excluded. Furthermore, the extent of the acid dissociation is temperature dependent. While hydrogen chloride, a strong acid, dissociates completely in liquid solutions, ionization was found to be limited on the surface of ice at very low temperatures.63 This will also have a restrictive effect on the acid-base interactions at very low temperatures. The pHT2 ≈ 8.0, corresponding to the second equilibrium step (BTC; Scheme 1), was found to have nearly the same value in the liquid phase and at 253 K, but a somewhat higher value (∼9.3) at 77 K. The solubility of alkali hydroxides in ice is known to be low.52 Ice prepared, for example, by freezing a KOH solution, contains inclusions of a concentrated KOH solution which freezes at 210 K to an eutectic mixture of almost pure ice and KOH‚4H2O.64,65 An insignificant increase in CR deprotonation at 77 K might be explained by enhanced OHconcentration during the freezing process, but it seems that the hydroxyl ions exhibit a more specific access to the OH group of CR than protons or we simply observe a temperature effect only as was demonstrated elsewhere.44 Acid-Base Interactions in the Presence of Inorganic Salts. When an aqueous solution of inorganic salt is frozen, the anions and cations are not necessarily incorporated in the proportion originally present in the solution.66,67 For NaCl, Cl- incorporates more into the ice lattice as HCl, whereas Na+ and OH- remain in the liquid phase.68 A decrease in the proton concentration on the surface of newly formed ice crystals of the polycrystalline state corresponds well to the theory of Bronshteyn and Chernov.69,70 When CR absorption spectra of the frozen (77 K) aqueous solutions containing NaCl were measured, two deprotonated CR forms (B and C) were exclusively found (Figure 11). As expected, the hydroxyl ions excluded from the ice phase deprotonated CR molecules before the layer at the grain boundaries froze. In contrast, when NH4Cl aqueous solutions is frozen, NH4+ can occupy H2O sites in the lattice52 in a greater extent than Cl-, thus the layer excluded from the ice phase is more acidic. The absorption spectra of frozen aqueous CR solutions containing this salt at 77 K revealed only more protonated CR forms (A and B). CR thus served as an excellent acid-base indicator at the grain boundary. Since, in addition to the B form, either A (in the case of NH4Cl) or C (in the case of NaCl) coexisted in frozen samples, we tried to estimate the relative concentrations of the CR forms in each case to evaluate the corresponding pH of the frozen layer. The spectrum with two absorption bands of the frozen NH4Cl sample at 77 K resembled that containing HCl of the corresponding pH ∼4.8 (Figure 7). In contrast, the spectrum of the frozen NaCl solution was similar to that of NaOH of the corresponding pH ≈ 9.1. Such a finding is well in accord with the value measured in

1286 J. Phys. Chem. B, Vol. 110, No. 3, 2006 frozen NaCl solutions, where the corresponding maximum pH of the brine layer was found to be close to 9.70 Cryospheric and Atmospheric Implications. To consider any physical process or chemical reaction that occurs within the natural snowpack or ice, we need information about the initial physical conditions, physical and chemical properties of the contaminants, such as an extent and dynamics of intermolecular interactions to the surrounding molecules, as well as about a chemical exchange between the atmosphere and ice surfaces. The evaluation of acid-base reactions is thus extremely important in order to know what are the forms of the species present in the snow or ice matrix. In general, protonation of organic compounds may have large consequences on their physical and chemical behavior, but it is not clear if an efficient protonation of organic impurities takes place in the environment. Mineral acids in a surface coverage of cirrus cloud ice particles or the snowpack in the cold environments were suggested to have a significant impact on atmospheric chemistry31,71,72 or ground snowpack chemistry.9 Especially nitric acid has a very important role in photochemistry in the polar areas,73,74 but probably only when NO3- is located on/near the surface of ice crystals.75 It was, however, suggested that this anion can be associated with one of the cations available in the snowpack, (e.g., Na+, Ca2+, or Mg2+), being therefore considerably inert with regard to physical exchanges.76 In such a case, protonation of freshly scavenged organic bases would be very limited. The acid-base equilibrium can be, however, established prior solidification. It was shown that the distribution of the cations and the anions during the freezing process is not uniform, producing large freezing potentials.69,70 While NaCl, CaCO3, and NaF produce the positive freezing potential (the ice surface remains basic), NH4Cl and NH4OH generate a large negative freezing potential, causing that the ice surface becomes acidic. As a result, when the phase transition occurs it is important if organic compounds are already present in the snowpack or if they are scavenged from the atmosphere or rain. The partitioning of all compounds between the atmosphere and snow as well as chemical dynamics in these heterogeneous systems must then be considered. 5. Conclusion The aim of this work was to study the acid-base interactions of a model weak diacid, CR, at the grain boundaries of ice containing various acids, bases, and salts. The results showed that the extent of CR protonation, enhanced in the solid state in contrast to the corresponding liquid solutions, is principally connected to an increase in the local concentration of acids. It was found that CR can establish an acid-base equilibrium prior to solidification even if samples are quickly frozen at 77 K and that the extent of protonation enhancement has a limiting value, which is not very sensitive to the freezing rate or the acid type used. The application of CR was successful in the study of preferential ion incorporation in ice samples containing inorganic salts. In light of this work, we hypothesize that trace acids, bases, or salts may also affect (photo)reactions of organic impurities in natural snow or ice. Further study is necessary to confirm or exclude this possibility. Acknowledgment. The project was supported by the Czech Ministry of Education, Youth and Sport (MSM 0021622412) and by the Grant Agency of the Czech Republic (205/05/0819). The authors express their thanks to Jaromir Jirkovsky, Adriena Rokosova, and Jana Topinkova for their help with spectroscopy

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Enhanced Protonation of Cresol Red in Acidic Aqueous Solutions

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