Theoretical study of the interaction of surfactants and drugs with natural zeolite

Microporous and Mesoporous Materials 91 (2006) 181–186 www.elsevier.com/locate/micromeso

Theoretical study of the interaction of surfactants and drugs with natural zeolite A. Lam *, A. Rivera Zeolites Engineering Laboratory, Institute of Materials and Reagents (IMRE), Faculty of Physics, University of Havana, 10400 Havana, Cuba Received 2 December 2004; received in revised form 21 November 2005; accepted 22 November 2005 Available online 10 January 2006

Abstract The presence of surfactant in a solid surface can modify its properties and improve the adsorption of certain organic molecules. In the present work, different combined systems formed by surfactants, drugs, water and a clinoptilolite channel model have been studied using semiempirical calculations. We modeled the interaction of each organic molecule with the external surface of clinoptilolite model. The results indicate that the cationic surfactant is well adsorbed on the clinoptilolite model unlike the anionic surfactant. The most polar drug, metronidazole, is the best adsorbed on the zeolite model, followed by aspirin and sulfamethoxazole. Taking into account this fact, we also model another system formed by surfactant–drug–water (S–D–W) in order to reproduce the interaction of the different drugs with a cationic surfactant in solution and to evaluate the role of the surfactant in the drug adsorption process. In this system the order of the drug adsorption is opposite to that obtained for the zeolite alone: the adsorption of an hydrophobic molecule like sulfamethoxazole is favored. Moreover, for aspirin and sulfamethoxazole the adsorption enthalpies are higher in the S–D–W system. This fact suggests that the presence of surfactant on the external surface of clinoptilolite could improve the adsorption of some drugs on this zeolite.  2005 Elsevier Inc. All rights reserved. Keywords: Zeolites; Clinoptilolite; Surfactants; Drugs and theoretical methods

1. Introduction Medical and pharmaceutical applications of natural zeolite type clinoptilolite have enjoyed considerable attention over the last decade, due to the good performance of this material in ion exchange, adsorptive and biocatalytic processes, together with its high chemical stability. It is known that purified natural clinoptilolite from Tasajeras deposit in Cuba, NZ, does not cause damage to humans [1,2]. It has been used as raw material in the pharmaceutical industry for the therapy of some pathologies, both in animals and in humans [1,2], using different pharmaceutical forms [3– 5]. The possible interaction between organic molecules of pharmaceutical interest and clinoptilolite has been studied using experimental and theoretical tools [5–10]. Aspirin, *

Corresponding author. Tel.: +53 7 8783647; fax: + 53 7 333758/53 7 8707666. E-mail address: anabel@fisica.uh.cu (A. Lam). 1387-1811/$ - see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.11.035

metronidazole and sulfamethoxazole has been the evaluated drugs based on their wide use and because they are molecules with different chemical-structural characteristics. In addition, these drugs produce side effects associated with gastrointestinal disturbances [11,12], which could be attenuated by the clinoptilolite. Semiempirical calculations, using zeolite window models, suggested a weak physical adsorption of metronidazole and aspirin by the clinoptilolite model. In any case, the groups with biological activity have been affected in the interaction with the models [7,9]. The window models were improved, resulting in the channel model, which represents the cavity of clinoptilolite. In this model the drug adsorption process and the interaction with water molecules occluded in the framework were studied [8,10]. Regarding experimental studies, the results suggest a poor adsorption of these drugs on the natural clinoptilolite and their ion exchanged modified forms in good agreement with the calculations [5,6]. In order to develop drug

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support systems, a new potential application of purified natural clinoptilolite has been explored, which involves the modification of the zeolite surface with surfactants [13]. The obtained composites can coadsorb organic molecules, as sulfamethoxazole and metronidazole, due to the variation of the hydrophilic character of the zeolite. Theoretical methods have been used to study surfactants and their interactions with solvents [14–19], biomembranes [20] and solid surfaces [21–23]. Molecular dynamics is the method preferentially used to simulate such processes—like the aggregation process that lead to micelle formation [14,15,17,18,20,21]—which involved several hundred atoms and thus it is not profitable to model using quantum approaches. Some efforts have been conducted in order to simulate the adsorption of surfactants on the external surfaces of minerals using quantum methods, specially ab initio and DFT [24–26]. However, the number of atoms to be considered are limited due to the computational time required. In addition, DFT has an important limitation reproducing the reliable energies for the van der Waals interactions which dominate adsorption–desorption process [27]. Semiempirical quantum chemical methods were developed in order to resolve the problems of time required in ab initio methods, replacing the most demanding terms in the energy expression with empirical parameters. These approximations permit to calculate systems formed by a bigger number of atoms. Specially, the AM1 method was developed in order to improve the heats of formation of a variety of molecules and to model molecules that forms hydrogen bonds. Specially, semiempirical methods have been employed to simulate surfactants and their interactions with surfaces [28,29]. Huibers used the semiempirical methods MINDO/ 3, AM1, PM3 and MNDO/d to estimate the charge distribution of ionic and amphoteric surfactants [28]. The results allowed to understand the distribution of the charge in the headgroup, a-methylene group and the alkyl tail, and potentially explain the polarity of micelle cores. In the same way, Pradip et al. employed both, MNDO method and force field (UFF), to study the interaction of disphophonic acid based surfactant with different calcium surfaces [29]. In this work we used semiempirical calculations to study the interactions of the clinoptilolite channel model with two surfactants (surfactant–zeolite system) and three drugs (drug–zeolite system). Due to the sizes of these guest organic molecules are bigger than the dimension of the clinoptilolite channels the interaction will be on the external surface of the model. We have improved the channel model, which includes more aluminum atoms in the framework and 16 water molecules inside the channel, resulting a model closer to the real structure of clinoptilolite. The benzalkonium chloride (BC) and the sodium lauryl sulfate (LS) were the studied surfactants in order to evaluate the interaction of clinoptilolite with two ionic surfactants (cationic and anionic). The selected drugs were aspirin, metronidazole and sulfameth-

oxazole. An additional system was modeled, consisting in surfactant–drug–water (S–D–W). This allows to reproduce the interaction of the different drugs with surfactant in solution, and contributes to evaluate the role of the surfactant in the drug adsorption process. To our knowledge, in spite of the relatively high number of studies about surfactants available in the literature, there is no report studying surfactants on zeolite surfaces through atomistic simulation techniques. 2. Methodology As it is known, the channel system of clinoptilolite is formed by two parallel channels, 10 and 8 ring (along the c axis), connected with a third one of 8 ring (parallel to the a axis). Inside these channels we can find water molecules and compensating cations [31,32]. The present channel model is formed by 30 silicon, 4 aluminum, 46 oxygen and 4 sodium atoms (see Fig. 1). This

Fig. 1. Channel model with 16 water molecules, (a) along c axis, (b) along a axis. Sodiums and aluminums are labeled; sodiums and oxygens are black, aluminums gray and silicons, hydrogens and capped bond are white.

A. Lam, A. Rivera / Microporous and Mesoporous Materials 91 (2006) 181–186

model does not include all atoms of the unit cell but it could be considered as a model to represent the clinoptilolite cell. It contains the 10 and 8 ring windows (parallel to c and a axes, respectively), with a silicon/aluminum ratio of 7.5. Although it is not the experimentally reported Si/Al ratio for NZ (5.3) [4], it is more realistic than that used in previous theoretical works (Si/Al = 16) [8,10]. The aluminum is placed at the T2 position, as was suggested by diffraction studies [31,33] and modeling by Ruiz-Salvador et al. [34,35]. Sodium was the cation selected to compensate the charge of the framework. The valences of the tetrahedral atoms were completed using Capped Bond pseudoatoms recommended by MOPAC program [36]. A set of 22 configurations was prepared by random orientation of 16 water molecules inside the cavity of clinoptilolite, and a full geometry optimization was performed. In the lowest energy configuration, each organic molecule—benzalkonium chloride, sodium lauryl sulfate, aspirin, metronidazole and sulfamethoxazole—was placed and optimized. The sizes of these guest organic molecules are bigger than the dimension of the clinoptilolite channels, and therefore, the interaction will be on the external surface. For example the average diameter of the cationic sur˚ , bigger than the dimensions factant is approximately 12 A ˚ · 3.1 A ˚ of the biggest zeolitic channel, which are 7.5 A [30]. Therefore, the surfactant molecules are not expected to penetrate into the zeolite. Benzalkonium chloride was the surfactant selected to study its interaction with the drugs and water. The system was formed by 44 water molecules, one molecule of benzalkonium chloride and one molecule of drug. The initial geometries were built in two forms. In the first one, the surfactant–drug combination was optimized and after that water molecules were placed around it. In the second one, the surfactant, the drug and the water molecules were randomly placed, and after that the system was optimized. It is important to note that the number of water molecules used in our simulations to solvate the guest molecules and to reproduce the water present inside the zeolite is far from the real number in an experimental system. However, it is not computationally possible to include the right number of molecules. It is worth noting that we calculated a large number of initial starting positions and orientations for all guest molecules relative to the different channel model sites, to ensure that we identified the lowest energy adsorbate/substrate structure rather than a local minimum. The modeled systems are complex potential energy surfaces (PES), where exits multiple energy local minima, that is why we should do a careful scan of the hypersurface. Here we present and discuss the most stable geometries obtained. The zeolitic framework atoms were placed in the position determined experimentally [37], and only the sodium cations were optimized. However, all guest molecules were optimized, i.e., water molecules, surfactants and drugs. The calculations were performed using the semiempirical AM1 hamiltonian [38] implemented in the MOPAC

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program, version 6.0 [36]. The adsorption enthalpy (DHa) was the property used to compare the results. For the guest molecules in the channel model it was calculated according to Eq. (1): DH a ¼ DH system  ðDH zeolite þ DH adsorbate Þ

ð1Þ

where DHsystem is the enthalpy of the zeolite model with water and guest molecule, DHzeolite is the enthalpy of the channel model with the 16 water molecules, and DHadsorbate is the enthalpy of the guest molecule. In the surfactant–drug–water system it was calculated as: DH a ¼ DH system  ðDH BC–water þ DH drug Þ

ð2Þ

where DHsystem is the enthalpy of the system formed by BC with drug and water molecules; DHBC–water is the enthalpy of the BC–44 water system and DHdrug is the enthalpy of the drug. It is worth noting that the more negative magnitude of the adsorption enthalpy (DHa) indicates more favorable adsorption process of the guest molecules on clinoptilolite model or the drugs in the BC–water system. 3. Results and discussion The first step in our study was to arrange the sodium cations and 16 water molecules inside the channel model. It is worth noting that the enthalpy values of the water adsorption process depend on the orientation of water molecules inside the cavity. In the most stable geometry, see Fig. 1b, the water molecules are forming two clusters inside the 10-ring window channel, avoiding the center, where the sodium cations are located. In these two clusters, waters are interacting between them and with the framework oxygens by hydrogen bonds. This was the geometry used for randomly oriented surfactants and drugs. The results obtained are discussed below. 3.1. Surfactant–zeolite systems The values of adsorption enthalpies obtained for the two ionic surfactants are very different for each case, see Table 1. BC, is the best adsorbed on the clinoptilolite model. Taking into account the nature of this surfactant (i.e., cationic surfactant) and the negative charge density of the clinoptilolite framework, it is reasonable to expect the adsorption of BC in this zeolite. That is why the value of Table 1 Adsorption enthalpies of drugs and surfactants in the clinoptilolite channel model Molecules

DHa (kcal mol1)

BC LS Metronidazole Aspirin Sulfamethoxazole

31.6 4.9 26.4 19.1 9.0

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Similar orientations have been observed in NMR studies [39] and Molecular Dynamics simulations [22] of surfactants intercalated in clays. It is important to note that our calculations match well the results obtained in the liquid phase adsorption studies of these surfactants on clinoptilolite [13]. The adsorption enthalpies obtained show that BC is strongly adsorbed on the clinoptilolite channel model. This fact confirms that the adsorbed BC could modify the external surface of clinoptilolite improving the adsorption of organic molecules, such as drugs. In order to explore this possibility we model the interaction of BC with drugs in the presence of water. The results are discussed in a further section. 3.2. Drug–zeolite systems

Fig. 2. (a) Benzalkonium chloride in front of the 8-ring window channel. (b) Sodium lauryl sulfate parallel to 8-ring window channel. Nitrogen, chlorine, aluminum and sodium cations are labeled in the figure. Nitrogen, chlorine, sodiums and oxygens are black. Silicons, hydrogens and capped bond are white, and carbons and aluminum are gray.

the adsorption enthalpy of the anionic surfactant is much lower. BC is oriented almost perpendicular to the 8-ring channel (the average tilt angle is 71), with a preferential orientation of the amide group and especially the methyl groups towards the 8-ring window channel, see Fig. 2a. We do not observe any significant change in the position of cations and water molecules by the presence of BC in the system. In our simulations, the presence of one molecule of surfactant on the clinoptilolite model does not cover all the surface, so only minor effects in the intrazeolite chemistry are observed. Unlike the BC, in the case of LS, it was necessary to explore a great number of geometries to find just one where the LS was adsorbed on the clinoptilolite model. LS adsorption on the model is not favored by charge reasons. It is an anionic surfactant, that is why a contribution of hydrophobic interaction between the surfactant and the zeolite should be present. This interaction is weaker than those involved in the BC adsorption process and it would explain the small value of the adsorption enthalpy for LS. In the final geometry (see Fig. 2b), the LS molecule is parallel to the 8-ring channel; i.e., the hydrogens of the hydrocarbon tail are oriented towards the zeolitic framework. Similar orientations have been reported previously for the surfactant adsorption in solids. When surfactants are adsorbed in solid surfaces at low concentration (well below the cmc) an adsorbed layer is formed with the surfactant tails oriented parallel to the plane of the substrate [21].

The most negative adsorption enthalpies obtained when the drugs interact with the zeolite model are presented in Table 1. Metronidazole is the best adsorbed, followed by aspirin and sulfamethoxazole, which is expected if we consider the polarity of these molecules and the hydrophilic character of the zeolite. It is worth noting that the adsorption enthalpies of aspirin and metronidazole are more negative than those obtained in previous works using window models and a simpler channel model [7–9]. This result can be associated to the improvement in the clinoptilolite models, using more compensating cations and water molecules, which contributes to the interaction process. The metronidazole molecule oriented the –CH2–CH2– OH group in front of the 8-ring channel and was not observed any hydrogen bond interaction in this final geometry. The electrostatic interaction between the negative charge density of the clinoptilolite model and the –CH2– CH2–OH group of the drug plays the dominant role. Aspirin is interacting with water molecules in front of the 10-ring window. In particular, the ester group interacts with an oxygen from water forming an hydrogen bond (the ˚ ). It is important to note that aspirin H–O distance is 2.38 A is the only drug that forms hydrogen bonds, however this interactions is not strong enough to permit that this drug is the better adsorbed on the channel model. Sulfamethoxazole is placed in front of the 8-ring channel. Unlike the other drugs, which oriented the groups with more hydrogens, sulfamethoxazole oriented the sulphon group (–SO2–) in front of the 8-ring window, near the sodium cation. The calculated charge of these oxygens is 0.9, and they are more negative than the charges of the oxygens of the –CH2–CH2–OH group of metronidazole and the acid group of aspirin (0.35 and 0.30, respectively). Therefore, a repulsive interaction between the sulphon group and the framework oxygens is present. This repulsion affect the adsorption process of sulfamethoxazole on clinoptilolite, leading to the lowest values of adsorption enthalpy. The order of adsorption of the drugs calculated by us are in good agreement with the experimental behavior observed in references [6,13], which support our theoretical results and vice versa.

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Table 2 Adsorption enthalpies of the drugs in the surfactant–drug–water system Drugs

DHa (kcal mol1)

Sulfamethoxazole Aspirin Metronidazole

33.2 29.4 19.0

3.3. Surfactant–drug–water systems Considering that BC was the surfactant with strongest interaction with clinoptilolite we decided to model the interaction of BC with the selected drugs in the presence of water. The lowest adsorption enthalpies obtained when the drugs interact with the BC–water system are shown in Table 2. The strongest interaction is with sulfamethoxazole, followed by aspirin and finally by metronidazole. In all cases, the groups with oxygen—sulphon group of sulfamethoxazole, acid group of aspirin, and the –CH2– CH2–OH group of metronidazole—were interacting with the amide of BC, especially with the nitrogen. The O–N ˚ , 3.80 A ˚ and 3.91 A ˚ for the sulfamethdistances were 3.82 A oxazole, aspirin and metronidazole, respectively. The charges of the oxygens in the sulphon group are 0.97 and 0.98, those of the oxygens in the acid group of aspirin are 0.35 and 0.31 and that of the oxygen of the –CH2– CH2–OH group of metronidazole is 0.42. There is a direct relation between the value of the negative charge of the oxygens, the O–N distances and the value of the adsorption enthalpy for the drugs. Small O–N distance and more negative charge value of the oxygens are associated with strong interaction between the drug and the surfactant. There exits a distinct difference between the charge of the oxygens of sulfamethoxazole and the other drugs. However, this difference is small between the oxygens of aspirin and that of metronidazole. In the final geometry of the BC–metronidazole–water system a hydrogen bond between the oxygen of the –CH2–CH2–OH group and the hydrogen of a water molecule was established. That is why the oxygen of this group has a more negative charge than the obtained for the acid group of the aspirin. The surrounding waters interact with drugs and BC through hydrogen bonds. Five water molecules are interacting with sulfamethoxazole (see Fig. 3), four waters with aspirin and just one with metronidazole. Other water molecules are interacting with the surfactant tail. Taking into account this fact and the adsorption enthalpy values, it is evident that the hydrogen bond improves the drug interaction process resulting in a higher stability of the system. It is surprising that sulfamethoxazole, that is the most hydrophobic drug, is forming the larger number of hydrogen bonds, which could be determined by the presence of BC. In order to discern the role of BC we decided furthermore to model the interaction of each drug with water (using the same number of molecules, 44). Analyzing the results we did not find any hydrogen bond interaction in

Fig. 3. BC–sulfamethoxazole–water. Nitrogen and sulfur are labeled in the figure, oxygens are black, hydrogens are white and carbons are gray.

the sulfamethoxazole–water system, as it is expected taking into account the low solubility of this drug in water. However, aspirin and metronidazole formed five hydrogen bonds with water in solution. Therefore, it is clear that the presence of BC favors the interaction between the sulfamethoxazole and the water, through hydrogen bond interactions and improves the solubilization process in around 17 kcal mol1. In the case of aspirin and metronidazole, the number of hydrogen bond interactions decrease with the presence of BC, but the interaction process is favored for aspirin in 15 kcal mol1. In the BC–metronidazole–water system, the surfactant just interacts with the metronidazole molecule but does not improve notably the solubilization process (it just favors the solubilization in 1 kcal mol1). The order of adsorption enthalpy values in the surfactant–drug–water (S–D–W) system is opposite to that obtained in the study of interaction of drugs with the zeolitic framework. Moreover, for sulfamethoxazole and aspirin these values are higher in the S–D–W system, which could be associated with the existence of more hydrogen bond interactions, due to the presence of BC in this system. Our calculations confirm the previous experimental results [13], i.e., that the modification of the external surface of clinoptilolite with a cationic surfactant, in particular with BC, improves the adsorption process of some drugs on clinoptilolite, suggesting new applications for these materials. 4. Conclusions We have modeled two different systems: guest organic molecules–clinoptilolite and surfactant–drug–water. Benzalkonium chloride (cationic surfactant) is well adsorbed on clinoptilolite model unlike sodium lauryl sulfate (anionic surfactant). The adsorption enthalpy values of the

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drugs: aspirin, metronidazole and sulfamethoxazole exhibit different trends in each system. On clinoptilolite, the most polar molecules are better adsorbed, whereas a better interaction surfactant–hydrophobic molecule is observed. Moreover, for aspirin and sulfamethoxazole the adsorption enthalpies are higher in the S–D–W system. This fact suggests that the presence of surfactant in the external surface of clinoptilolite could improve the adsorption of some drugs on this zeolite. Acknowledgements The authors thanks to Dr. A.R. Ruiz-Salvador and T. Farı´as for helpful discussions, and Professor E. Altshuler for the revision of the manuscript. This work was partially supported by Alma Mater Research Grant from University of Havana (37–2001). References [1] NRIB, 1152: Quality Requirements, Natural Zeolites for Pharmaceutical Industry, Drug Quality Control of Cuba (1992). [2] G. Rodrı´guez-Fuentes, M.A. Barrios, A. Iraizoz, I. Perdomo, B. Cedre´, Zeolites 19 (1997) 441. [3] B. Concepcio´n-Rosabal, G. Rodrı´guez-Fuentes, R. Simo´n-Carballo, Zeolites 19 (1997) 47. [4] A. Rivera, G. Rodrı´guez-Fuentes, E. Altshuler, Micropor. Mesopor. Mater. 24 (1998) 51. [5] A. Rivera, M.L. Rodrı´guez-Albelo, G. Rodrı´guez-Fuentes, E. Altshuler, in: A. Galarneau, F. Di Renzo, F. Fagula, J. Vedrine (Eds.), Zeolites and Mesoporous Materials at The Dawn of the 21st Century, Stud. Surf. Sci. Catal., Elsevier Science, Amsterdam, 2001, p. 32-P-07. [6] T. Farı´as, A.R. Ruiz-Salvador, A. Rivera, Micropor. Mesopor. Mater. 61 (2003) 117. [7] A. Lam, L.R. Sierra, G. Rojas, A. Rivera, G. Rodriguez-Fuentes, L.A. Montero, Micropor. Mesopor. Mater. 23 (1998) 247. [8] A. Lam, A. Rivera, in: A. Galarneau, F. Di Renzo, F. Fagula, J. Vedrine (Eds.), Zeolites and Mesoporous Materials at The Dawn of the 21st Century, Stud. Surf. Sci. Catal., Elsevier Science, Amsterdam, 2001, p. 32-P-08. [9] A. Lam, A. Rivera, G. Rodrı´guez-Fuentes, Micropor. Mesopor. Mater. 49 (2001) 157. [10] A. Lam, A. Rivera, in: P. Misaelides (Ed.), Zeolite’02: 6th International Conference on the Occurrence, Properties and Utilization of Natural Zeolites (Book of Abstracts), Thessaloniki, Greece, 2002, pp. 193.

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