Modes of Interaction of Simazine with the Surface of Model

Article pubs.acs.org/JPCC

Modes of Interaction of Simazine with the Surface of Model Amorphous Silicas in Water Serena Esposito,† Filomena Sannino,‡ Michele Pansini,† Barbara Bonelli,§ and Edoardo Garrone*,§ †

Department of Civil and Mechanical Engineering, Università degli Studi di Cassino e del Lazio Meridionale, Via G. Di Biasio 43, 03043 Cassino (FR), Italy ‡ Department of Agriculture, Università di Napoli “Federico II”, Via Università 100, 80055 Portici, Italy § Department of Applied Science and Technology and INSTM Unit of Torino-Politecnico, C.so Duca degli Abruzzi 24, Politecnico di Torino, I-10129 Torino, Italy ABSTRACT: Simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) was adsorbed from aqueous solutions on three types of amorphous silica, one commercial nonporous and two sol−gel with different porosity. Adsorption takes place on both types of solids in a narrow pH interval around 5.5. This fact, pH changes along adsorption, and IR spectroscopic data concerning dried samples provide evidence that adsorption involves proton transfer from acidic SiOH species to N atoms at ethylamino chains, those at the ring being practically not basic. Two adsorbate species are involved with markedly different interaction enthalpy, causing the presence of two regions in the pseudoisotherms, one irreversibly held, the other partially reversible. Possibilities for the occurrence of two species are discussed. The most likely explanation seems to be a two steps process, in which first a protonated molecule is formed, onto which a second molecule may anchor so forming a dimer, held together by either sharing of the positive charge or hydrophobic interactions. The available surface appears to have basically the same properties in all three cases. Microporosity, however, introduces diffusional constraints and limits the extent of surface available to adsorption.

1. INTRODUCTION Simazine (2-chloro-4,6-bis(ethylamino)-s-triazine (Scheme 1) is the second pesticide most commonly detected in waters in

Gaining information on the surface properties of silica in water is of interest for several reasons. Silica is by far the most used of all adsorbents,17 but notwithstanding the immense literature on the subject, fundamental questions concerning the strength and density of its acid centers are still obscure. Recent literature seems to favor a bimodal distribution of acidic silanols into two families, with pKa ca. 4.5 and 8.5, respectively (the former being as acidic as acetic acid),18 and recent computational results seem to support such a view.19 Agreement on such view, however, is not universal.19 Besides, the interaction of hydrated silica with organic molecules can help in the understanding of biological processes at such a surface.20−22 The system simazine/water/silica has been studied in the present work through adsorption/desorption experiments from aqueous solutions at room temperature and DRIFT spectra of the samples dried at room temperature. To investigate the role of porosity in the adsorption process, three types of silica have been adopted, namely, a commercial nonporous one and two homemade porous specimens. From a general point of view, the interplay among simazine, water, and silica can be represented by a Born−Haber cycle (Scheme 2), where the reference level is that of the three unmixed components. Two paths are possible: the former goes through the vapors of simazine and water, each of them

Scheme 1. Structure of Simazine [2-Chloro-4,6bis(ethylamino)-1,3,5-triazine]

the Western world. For remediation, several methods have been proposed, among which outstands adsorption on widely available, low cost adsorbents, such as activated carbon and clay minerals.1−6 Adsorption of simazine on other solids such as mesoporous metal oxides and zeolites has recently been studied, as well as the effect7,8 of acid treatments of clay minerals, causing an increase in hydroxyl species.9−14 The present article deals with the adsorption of simazine on model amorphous silicas, already partially reported as it concerns the technological aspects of water remediation:15 here, we show that it is possible to use simazine as a molecule for probing the surface properties and to understand the modalities of interaction of a small organic molecule with a hydroxyl-rich surface. Note that this is a subject not so much developed because the literature favors the study of larger adsorbate molecules, like polymers and biomolecules.16 © XXXX American Chemical Society

Received: February 26, 2013 Revised: May 1, 2013

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dx.doi.org/10.1021/jp401997s | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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L−1. All sorption experiments were carried out by adding 0.3 mg sorbent to 3.0 mL simazine solution in glass vials with Teflon caps at 25 °C. After 24 h, the samples were centrifuged at 7000 rpm for 20 min, and the concentration in the supernatant solution (C) was determined as detailed below. The amount of adsorbed simazine was calculated as the difference between the initial amount and that at the steady state. During adsorption, a slight increase was observed in pH, which was adjusted by the addition of 0.10 or 0.01 mM HCl solution. A first set of experiments was carried out by keeping constant both simazine and solid amount, while pH values ranged between 3.0 and 7.0. In a second set, after determining the optimal pH conditions, different volumes of herbicide stock solution (20 μmol L−1) were mixed with pure water to a final volume of 3 mL, in order to obtain an initial simazine concentration ranging from 0.50 to 20 μmol L−1. This solution (3 mL) was added to each silica sample (0.3 mg). Finally, in a number of cases, samples loaded with simazine obtained immediately after adsorption from 20 (for SG800 and Aerosil) and 10 μmol L −1 (only for SG800) initial concentration, respectively, were put in contact with different volumes of pure water at standard pH value, so to determine the amount of released simazine. In particular, 3 mL of supernatant removed for adsorption analysis was replaced with 3 mL of pure water. After shaking at 25 °C for 24 h, the suspensions were centrifuged, and in the supernatant, the released herbicide concentration was determined. The same procedure as described above was employed by replacing 1.0, 1.5, and 2 mL of supernatant with equal volume of pure water. 2.2.2. Simazine Evaluation. Simazine content was measured by means of an Agilent 1200 Series HPLC apparatus (Wilmington U.S.A.), equipped with a DAD array and ChemStation Agilent Software, using a Macharey−Nagel Nucleosil 100-5 C18 column (stainless steel 250 × 4 mm). The mobile phase, comprising a binary system of 65:35 acetonitrile/water, was pumped at 1 mL·min−1 flow in an isocratic mode. The detector was set at 220 nm, and the injection volume was 20 μL. Quantitative determination was done on the basis of a calibration curve in the concentration range between 0.15 and 20 μmol/L. 2.2.3. Diffuse Reflectance IR Fourier Transform Spectroscopy (DRIFTS). For DRIFTS measurements, an amount of simazine corresponding to the maximum sorption value as obtained from the pseudoisotherm reported below was added to each adsorbent at the optimal sorption pH. After incubation, the samples were centrifuged and the precipitates washed twice with H2O, then dehydrated by lyophilization. Finally, 2.0 mg of sample were mixed with 200 mg of KBr (FTIR grade, Aldrich, Chemical, Co., Milwaukee, WI, USA). The mixture was finely ground in an agate mortar and transferred to a sample holder. Its surface was smoothed with a microscope glass slide, and DRIFT spectra were recorded (on a Perkin-Elmer Spectrum One FT-IR Spectrometer at a resolution of 1 cm−1).

Scheme 2. Born−Haber-Like Cycle Concerning the ThreeComponents System (Simazine, Water and Silica) and Relationships among the Different Possible States of the System

successively adsorbed on the solid; the latter envisages the simazine/water mixture to form a solution, which is then contacted with the solid. Unfortunately, one cannot go along the cycle completely from a practical point of view because of the poor volatility of simazine (vapor pressure 8.1 × 10−7 Pa at 20 °C).23 Measurements here reported were therefore carried out by following the second path, i.e., by contacting the solids with the desired simazine solution, then drying the system to the desired point.

2. EXPERIMENTAL SECTION 2.1. Materials. The commercial nonporous silica studied was pyrogenic Aerosil 250 (Degussa). Two porous silica samples were prepared by using either a conventional sol−gel route or a modified method, where tetraethoxysilane, Si(OC2H5)4 (TEOS), was hydrolyzed at 50 °C without any alcoholic solvent.24,25 In the conventional route, instead, TEOS was hydrolyzed in anhydrous ethanol (EtOH) at room temperature using water and concentrated hydrochloric acid with the molar ratios: TEOS/EtOH/H2O/ HCl = 1:4:2:0.01. High water/alkoxide ratio (20:1) is employed in the modified sol−gel method to obtain highly porous silica sample.26 All gelled systems were kept 3 days at room temperature, then fully dried in air at 110 °C in an electric oven for 12 h. Both dried samples were heated (10 °C/min heating rate) up to 400 °C and kept at this temperature for 1 h. The temperature of 400 °C was chosen on the basis of thermal analysis results (data not reported) so to allow the combustion of all organic residues. Samples will be referred hereafter as SG400 and SG800, where the numbers indicate the specific surface area. Silica powders were characterized by N2 adsorption at −196 °C on ca. 100 mg sample previously outgassed at 250 °C for 3 h to remove atmospheric contaminants (Quantachrome Autosorb 1 instrument). Specific surface area (SSA) was calculated according to the BET (Brunauer−Emmett−Teller) method for Aerosil and SG800; that of SG400 was obtained through the Langmuir equation. The pore size distribution was calculated by applying the nonlocal-density functional theory (NL-DFT) method to isotherm adsorption branches. 2.2. Methods. 2.2.1. Adsorption Experiments. A stock solution was prepared by dissolving 2 mg of simazine in 500 mL of doubly distilled water. The obtained concentration 20 μmol L−1 is close to saturation, which takes place at ca. 25 μmol

3. RESULTS 3.1. Porosity of the Studied Adsorbents. N2 adsorption−desorption isotherms of SG400 and SG800 samples are reported in Figure 1a: SG400 has a type I isotherm, reaching saturation at low P/P0 values, and without hysteresis loop, as B



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system N2−silica with cylindrical pores to adsorption branches of the corresponding isotherms, are reported in Figure 1b: in agreement with isotherm shapes, SG400 does not show pores larger than 1.0 nm, whereas SG800 shows both larger micropores and a certain amount of mesopores. In conclusion, SG400 is a microporous silica; SG800 has both microporosity and disordered mesoporosity, whereas Aerosil is nonporous. 3.2. Effect of pH on Adsorbed Amounts. The amount adsorbed in fixed conditions, when contacting 0.3 mg of silica with 3.0 mL of 13 μmol L−1 solution, markedly depends on the solution pH, as illustrated in Figure 2 for both SG800 and

Figure 2. Dependence of simazine uptake per unit mass of solid as a function of the pH for Aerosil (triangles) and SG800 ( squares).

Aerosil. The two samples behave closely: with SG800, adsorption does not take place for pH values larger than 6.5 or smaller than 4.5, and actually shows a rather sharp maximum at pH = 5.5. In the case of Aerosil, a wider distribution of pH values is observed, as well as some asymmetry. As a consequence, all subsequent measurements have been carried out at pH = 5.5. 3.3. Adsorption/Desorption Measurements (Pseudoisotherms). Figures 3 and 4 illustrate for SG800 and Aerosil the dependence of the uptake per unit mass A (adsorbed amount) on the concentration C of simazine in the supernatant, measured as described above. Full symbols represent data obtained at increasing C, and empty symbols, those obtained with decreasing C. Note that, for SG800, a first desorption experiment was run starting from a concentration corresponding to the inflection point in the pseudoisotherm. For both SG800 and Aerosil, desorption was studied at the end of the adsorption run. Quite evidently, the adsorption/desorption data do not coincide, so that the use of the term isotherm will be avoided since the systems are not at equilibrium. A number of points can, however, be established. SG800 shows first a concave region, followed by a sharp increase at about the same value of C. A plateau region follows. Aerosil, instead, shows a convex first region, followed by a point of inflection at much larger C values than with the other two cases, and finally a plateau region as in the other two cases. Desorption of simazine from SG800 at the inflection point does not take place, i.e., the whole amount adsorbed is strongly

Figure 1. (a) N2 adsorption−desorption isotherms at −196 °C on SG400 (circles) and SG800 (triangles); full symbols and empty symbols correspond to adsorption and desorption run, respectively. (b) Corresponding pore size distribution, as calculated by means of the NL-DFT method.

typical of a microporous material. SG800 also shows a type I isotherm, in comparison to SG400; however, a much higher N2 volume is adsorbed, and the smooth isotherm knee indicates the presence of a wide range of pore diameters, so showing that also some mesopores contribute to N2 adsorption, despite the absence of any hysteresis loop. The corresponding values of specific surface areas (SSA), total pore volume (Vp), and micropore volume (VMP) are reported in Table 1 along with data concerning Aerosil. NL-DFT pore size distributions (PSD) of SG400 and SG800, as obtained by applying a kernel for a Table 1. Sample Properties As Derived from N2 Sorption Isotherms at −196°C sample

SSA (m2 g−1)

total pore volume (cm3 g−1)

micropore volume (cm3 g−1)c

pore diameter (Å)

SG400 SG800 Aerosil

400a 800b 250b

0.156 0.465

0.135 0.150

2−10 10−50

a

As calculated by applying Langmuir equation. bAs calculated according to the BET algorithm. cAs obtained by NL-DFT data. C



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Figure 3. Dependence of the simazine uptake per unit mass of SG800 after 24 hours contact with solutions at different solute concentration. Full squares: data obtained by increasing the starting solute concentration; empty squares: data obtained after desorption by contacting the solid, loaded at the maximum simazine concentration with pure water; full triangles: data obtained after desorption by contacting the solid, loaded at the simazine concentration corresponding to inflection point, with pure water.

Figure 4. Dependence of the simazine uptake per unit mass of Aerosil after 24 hours contact with a solution at different solute concentration. Full symbols: data obtained by increasing the starting solute concentration; empty symbols: data obtained after desorption by contacting the solid, loaded at the maximum simazine concentration, with pure water.

Figure 5. (a) DRIFT spectra in the 4000−700 cm−1 range in air of simazine-loaded SG800 after drying (curve 1) and pure SG800 (curve 2). (b) Comparison of curves (1) and that of solid simazine (3) in the 1800−1200 cm−1 range.

held. From both Aerosil and SG800 at the end of the adsorption run, instead, desorption has been observed, and a relevant fraction of the adsorbate may leave the surface. Data concerning SG400 are not reported for brevity but will be included in Figure 6 below. 3.4. DRIFT Spectra of the Samples. Figure 5a compares DRIFT spectra of SG800 sample at the highest simazine load, corresponding to the end of the adsorption run (curve 1) with that of simazine free SG800 treated in the same way (soaked with pure water and then dehydrated by lyophilization (curve 2). Another comparison is made in Figure 5b, at high magnification, involving curves (1) and that of solid simazine (3). At low magnification, the bands of simazine are hardly visible, whereas the changes in the intense bands due to O−H vibrations are evident. A decrease in the intensity of the stretching mode of O−H is observed, together with the increase of absorption at ca. 1050 cm−1, which can be ascribed to the vibration of Si−O− species in interaction with positive groups. An increase of absorption below 3200 cm−1 is also seen. At high magnification, comparison is possible between the modes of simazine both adsorbed (1) and in the solid state (3):

in both cases, bands are seen at 1638, 1407, 1373, 1346, and 1304 cm−1. The bands at 1444, 1543, and 1558 cm−1 in the solid simazine undergo a shift to 1439 and 1568 when adsorbed, respectively.

4. DISCUSSION Three different pieces of evidence strongly suggest that the main process governing adsorption is proton transfer, according to the equation Si−OH + C7H12ClN5 ⇄ Si−O−···C7H13ClN5+

(1)

where dots represent electrostatic interaction. These are as follows. 4.1. Evidence A. The strict pH dependence of the adsorbed amounts in Figure 2 is per se evidence that adsorption involves proton transfer. Actually, simazine shows one single pKa value at ca. 1.65, above which value the molecule is expected to be present in the solution as a neutral species. For pH values below 1.65, single-charged species are present. As to silanols, they are expected to undergo dissociation in water being slightly acidic according to D


The Journal of Physical Chemistry C Si−OH + H 2O ⇄ Si−O− + H3O+

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ring is in full agreement with the very poor solubility of simazine in water. One is, therefore, left with the two ethyl-amino groups, which display the basicity of secondary amines, according to the reaction

(2)

The classical view of the surface of hydrated silica assumes that it can be thought as a giant polyprotic acid, for which a distribution of pKa is expected,17 the most acidic species being characterized by a pKa value between 6 and 7. According to the recent view, instead, two families are present, with ca. 4 unit difference in their pKa, one of which really acidic and comparable to classical weak acids on the lab shelf. If the measurements were run in equilibrium conditions, one could use the classical Henderson−Hasselbach relationship, according to which the pKa value marks the pH at which an acidic species has equal populations of dissociated and undissociated forms, to evaluate the pKa of silanols involved in reaction 1. The result would be ca. 6 for SG800 and 6.5 for Aerosil. Apparently, reaction 1 could take place from the top of the volcano plot down to pH 1.65. Its occurrence is, however, probably limited by the protonation of silanols in acidic media: Si−OH + H3O+ ⇄ Si−OH 2+ + H 2O

R−NH−R′ + H3O+ ⇄ R−NH 2−R′+ + H 2O

(4)

For simazine, a single pKa value has been reported close to 1.65,29 which is indeed in the range expected for a secondary amine. Secondary amines have an out of plane deformation mode at ca. 730 cm−1, where the solid is opaque, so that possible changes concerning this mode cannot be detected. Fortunately, the in-plane bending mode of the secondary N−H group is fully visible at 1543 cm−1. This band is nearly absent in the spectrum of silica loaded with simazine and substituted by a band at 1568 cm−1 (Figure 5b) due to the bending mode of a NH2+ group, so providing clear evidence of protonation at the N atoms of the ethyl-amino substituents.30 Minor changes occur in the 1500−1600 cm−1 range, where the ring vibrations fall: protonation taking place so close to the ring itself somewhat affects the aromatic system. Figure 5a shows that the presence of simazine causes some shift of the O−H stretching band to lower frequency and therefore is ascribable to H-bonding. This apparently amounts to only ca. 100 cm−1. If this value was meaningful, it would provide evidence that interaction occurs either with the electronic cloud of the aromatic ring or the poorly basic N atoms in the ring itself: a similar value ca. 100 cm−1 has been indeed observed for the interaction of isolated silanols with benzene.31 A more appropriate interpretation is, however, as follows. H-bonding can only take place with available OH species, e.g., those either isolated or terminal species in a chain. Isolated silanols would absorb at 3750 cm−1, which is not the case, as spectra in Figure 5a show no absorption at such a frequency. The stretching mode of terminal silanols in a chain lies at lower frequency, not exceeding, however, 3650 cm−1.32 Being the increase in absorption in the range 2900−3000 cm−1, it is concluded that the shift undergone by the O−H stretching mode is on the order of ca. 600 cm−1, a value compatible with the interaction of the simazine molecule through the aminic N atoms. Indeed, the shift observed with molecular ammonia is ca. 650 cm−1.33 In summary, spectra in Figure 5 indicate that, at least on the samples dehydrated at room temperature, the aminic N atoms in simazine interact with silanols via both H-bond and Htransfer. Figure 6 compares the adsorption runs on the three solids. Normalization with respect to the surface area as calculated from the N2 isotherms (Table 1) brings data for SG400 and SG800 nearly to coincide. In particular, both the plateau value and the point of inflection are rather close. The curve for Aerosil is still rather different. In particular, three aspects of the curve concerning Aerosil seem worth of note: (i) the initial branch of the related curve is convex; (ii) the point of inflection is at higher values of concentration; (iii) the plateau value is nearly the double. The discrepancy between the plateau values in the case of Aerosil, on the one hand, and SG400 and SG800, on the other hand, is likely to be due to the simple fact that, because of the different porous properties, not all the surface of the latter solids is actually available to adsorption of a bulky molecule like simazine. This does not hamper adsorption on Aerosil, which can display all the surface to adsorption.

(3)

Indeed, because of reaction 3, the zero-point charge of silicas is in the range 2.5−4.5.17,27 As a result, the pH range in which reaction 1 takes place is limited. It has been noted, however, that, as detailed below the following measurements are not at equilibrium. Also, the pH measured in solution is necessarily not the same at the surface because of the presence of the double layer giving rise to the ζ potential. Moreover, the measurement of the amount adsorbed is hampered by slow diffusional processes, as shown below, so that the above considerations have only a semiquantitative meaning. Indeed, with Aerosil, where diffusional limitations seem to be less severe, the adsorption extends over a wide range on the low-pH side. The similarity of the results for the two types of silica lends support to the idea that the chemical nature of the two surfaces is similar. 4.2. Evidence B. The increase in pH observed along the adsorption process also indicates the occurrence of reaction 1. The increase in the population of dissociated silanols Si−O− because of reaction 1 is bound to shift the equilibrium in eq 2 to the left, with an increase in the pH of the system. 4.3. Evidence C. IR results (Figure 5) provide ample direct evidence about protonation. IR results refer to the dried sample because IR measurement are not possible in the massive presence of water, but protonation is most likely to be even more favored in the presence of solvating water molecules in the liquid phase. Protonation may be followed in two ways. On the one hand, the loss of protons from silanols, turning these into Si−O− species, is shown by the decrease in intensity of the related stretching band and the increase of absorbance at ca. 1050 cm−1. On the other hand, changes in the IR spectrum of adsorbed simazine are observed (Figure 5b). The question arises what are the atoms that undergo protonation in simazine. Actually, five N atoms are present in the simazine molecule. Contrary to what is reported sometimes in the literature,14 there is evidence that N atoms in the heterocyclic ring do not have any basic properties: indeed, considering 1,3,5-triazine, the conjugated C3N3H4+ acid is a strong one, the corresponding pKa being less than zero.28 This property is related to the number of N atoms in the aromatic ring because pyrazine (two N atoms) and pyridine (one N atom only) are progressively more basic, as shown by the pKas of conjugate acids. The nonbasic nature of the N atoms in the E



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The two species could correspond to the formation of the same monoprotonated species involving two silanol species with different acidity. If the recent views on the acidity at the surface of hydrated silica are correct, the two types of adsorption should have a difference in their standard free enthalpy of ca. 4 × 2.3 × RT, which amounts to ca. 23 kJ, an amount large enough to separate the two processes in the adsorption run. The maximum load is, however, ca. 4 molecules per 100 nm2, partitioned nearly evenly in the two forms. It is hard to understand, thus, why simazine molecules would prefer a weak interaction with the surface with so many strong sites still available. One is left with the possibility that the two species differ because one species is protonated (structure a in Scheme 3), while the other is not and implies instead only H-bonding, e.g., two simultaneous such interactions at the two aminic N atoms

Figure 6. Comparison of the adsorption runs on the three silicas after normalization to unit surface area. Triangles: SG400; Full squares: SG800; Empty squares: Aerosil.

Scheme 3. Possible Configurations for the Adsorbed Molecule: (a) Singly Protonated; (b) Doubly H-Bonded to the Surface through the Aminic N Atoms; (c) a Second Molecule Interacting via Hydrophobic Interactions with a Protonated Molecule Anchored to the Surface

Similarly, the discrepancy in the first stage of adsorption and the difference in the inflection points of the curves are probably amenable again to the presence of microporosity in SG400 and SG800. With microporous silicas, the same adsorbed amount corresponds to a higher concentration in the liquid, i.e., to a larger difference in the chemical potential of the solute. In other terms, to overcome diffusional constraints, a higher chemical potential of the solute is required. Coming to the shape of the initial part of the curves, one may note that the lack of diffusional constraints makes the curve for Aerosil closer to what is actually expected for an adsorption isotherm, i.e., with a convexity not far from the reference Langmuir isotherm. The fact that at later stages of adsorption a fraction of adsorbed simazine can be brought back in solution, whereas the molecules adsorbed in the first stages can not, seems to indicate that two modes of adsorption are possible, one relatively strong, yielding species irreversibly held in the adopted conditions, and the other relatively weak, so to allow desorption. The presence of the second jump in the curves would just so correspond to the filling of weak sites, so giving rise to a sort of stepped isotherm. The presence of two adsorption modes with markedly different enthalpy of interaction may be accounted for in different ways. First, it could be that the strong species is doubly protonated and that the (relatively) weak species is so only once. If, by mere sake of reference, the protonation enthalpies of piperazine are assumed, the first adsorption would correspond to ca. 75 kJ/mol, and the latter to ca. 50 kJ/mol. The difference in energy would be large enough to discriminate between the two processes and cause a stepped isotherm, and also their relative magnitude would account for an irreversible first step and to near reversibility in the second. As already quoted, however, only one protonation is commonly reported in the literature for simazine. The reason could be that, with simazine, the electronic effect of the first protonation on the second one is expected to be substantial (e.g., much larger than with piperazine) because the two external N atoms are connected through an aromatic system. In conclusion, the successive protonation of the two N atoms occurs, if any, under probably harsh conditions of pH and not at pH = 5.5. If double protonation is to be ruled out, the IR data in Figure 5 are to be interpreted that, of the two basic N atoms in the lateral chains, one is protonated and the other is engaged in H-bonding. F



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by the surface (structure b in Scheme 3). Occurrence of Hbonding interactions at the aminic N atoms has been provided above. An objection to this interpretation, however, rises from the observation that, whereas a particularly acidic silanol is needed for protonation, the availability of silanols for Hbonding does not seem to be a particular stringent condition. The amount of molecules adsorbed in the second mode is thus expected to be not markedly limited. Data in Figure 6 seem, instead, to suggest that the two modes of adsorption involve about the same number of molecules. The most satisfactory interpretation, accounting for this last observation, is that species (a) acts as an adsorption center for another molecule. The overall process can be represented by the following reactions: SiOH + Sim(aq) → SiO−···SH+ads

(5)

SH+ads + Sim(aq) ⇄ Sim2H+ads

(6)

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AUTHOR INFORMATION

Corresponding Author

*(E.G.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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In the first instance, one can assume that stabilization in the Sim2H+ads species is provided by hydrophobic interactions between the two aromatic rings, in agreement with the behavior of poorly soluble molecules. The corresponding structure is tentatively described in species (c) in Scheme 3. However, it is worth recalling that ammonium species are known to form adducts with ammonia with formula N2H7+, where the transferred proton is shared between the two ammonia moieties. Formation of such an adduct is readily studied in zeolites.34 Therefore, such an interaction can easily takes place between two secondary amino groups, one of which protonated, leading to the formation of a protonated dimer of the simazine.

5. CONCLUSIONS A perfunctory look at the structure of simazine molecule could suggest a markedly basic behavior, also involving the ring N atoms. Instead, the polar character of simazine is strictly limited to the secondary aminic functionalities, and all the rest of the molecules behave hydrophobic: this accounts for the very poor solubility in water. Not surprisingly, hydrophobicity is also displayed toward the hydrated silica surface. Adsorption takes place in the first instance by reaction with a few, most acidic silanols (pKa ≈ 5.5) with formation of protonated species, for which good evidence exists. Such protonated species, anchored via electrostatic interaction to the surface, act successively as centers for molecular adsorption via sharing of the transferred proton or hydrophobic interactions. Whereas the former type of adsorption is strong and irreversible, the latter is relatively weaker and partially reversible. The adsorption process is largely dominated by slow diffusional processes at room temperature, which hamper to some extent the study, even with nonporous silicas. When microporosity is present, not only diffusional processes are more important but also a fraction of the surface is even excluded to simazine, probably present in water as clustered species. As a whole, the amount of simazine adsorbed is rather limited, amounting to ca. 4 molecules per 100 nm2. This seems to be an indication that a relatively acidic silanol is not enough to constitute the site: a full explanation of such feature probably requires a knowledge of the properties of silica not yet available. G



Article

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H


Modes of Interaction of Simazine with the Surface of Model

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