Colloids and Surfaces B: Biointerfaces

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1 Colloids and Surfaces B: Biointerfaces 2 (203) Contents lists available at ScienceDirect Colloids and Surfaces B: Biointerfaces journal homepage: Role of -methyl-3-octylimidazolium chloride in the micellization behavior of amphiphilic drug amitriptyline hydrochloride Abbul Bashar Khan a, Maroof Ali b, Nisar Ahmad Malik b, Anwar Ali b, Rajan Patel a, a Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia (A Central University), New Delhi, India b Department of Chemistry, Jamia Millia Islamia (A Central University), New Delhi, India article info abstract Article history: Received 7 June 203 Received in revised form August 203 Accepted 3 August 203 Available online 30 August 203 Keywords: Mixed micellization Amphiphilic drug Imidazolium ionic liquid Synergistic interaction The mixed micellization behaviour of amitriptyline hydrochloride (AMT) with ionic liquid (IL) -methyl- 3-octylimidazolium hydrochloride, mim][cl], have been investigated using electrical conductivity, at different temperatures. The non-ideal behaviour (i.e., synergistic interaction) of AMT mim][cl] binary mixtures, explained by the deviations in critical micelle concentration (cmc) from ideal critical micelle concentration (cmc*) and micellar mole fraction (X m ) from ideal micellar mole fraction (X ideal ) values. The values of interaction parameter (ˇ) and activity coefficients (f and f 2 ), also confirm the synergistic interaction. The excess free energy ( G ex ) for the AMT mim][cl] binary mixtures explains, stability of mixed micelles in comparison to micelles of pure, AMT and mim][cl]. The calculated thermodynamic parameters (viz., the standard Gibbs energy change, Gm, the standard enthalpy change, H m, the standard entropy change, Sm ), suggest the dehydration of hydrophobic part of the drug at higher temperatures (>33 K), not only in case of AMT but also in the presence of mim][cl]. 203 Elsevier B.V. All rights reserved.. Introduction In recent years amphiphilic ionic liquids (ILs), i.e., a class of salts composed of bulky organic cation and appropriate anion exists in a molten state around room temperature, are of immense importance. They possess significant promise in miscellaneous industrial applications, where high surface areas, modification of the interfacial activity or stability of colloidal systems are required. The low volatility, non-flammability, wide electrochemical window, high thermal stability, and wide liquid range [ 4] are unique properties of these salts that are applied for catalysis [5], electrochemistry [6], chemical separation [7 9] and as a novel solvent in organic synthesis [0,]. Additionally, the self-assemblies of amphiphilic molecules in a solvent have many potential applications such as nanomaterial synthesis [2 4], drug delivery [5,6], separation process, pharmaceutical formulation, and other dispersant technologies [7]. A typical imidazolium IL analyzed by the structure activity relationship (SAR), guide the assumption that ILs could acquire surface active properties similar to surfactants and would allow the ILs to form micelles in aqueous solution [8,9]. As shown in Scheme (a), the anion or cation of IL consist of a charged hydrophilic head group Corresponding author. Tel.: ; fax: addresses: rpatel@jmi.ac.in, rajanpatelpcy@gmail.com (R. Patel). and a hydrophobic tail domain, suggesting that IL have properties analogous to amphiphiles. Extensive work has been done related to the surfactant self assemblies in imidazolium based room temperature ILs (RTILs). Anderson et al. [5] has been reported the formation of micelles by anionic and nonionic surfactants in -butyl-3- methylimidazolium chloride [Bmim][Cl] and hexafluorophosphate [Bmim][PF 6 ]. Fletcher and Pandey have also studied the aggregation behaviour of anionic SDS, cationic CTAB, nonionic Brij, Triton X-00 and Tween 20 in -ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Emim][Tf 2 N] by using solvatochromic probe technique [6]. Tang et al. [8] has also been reported the temperature dependent self-assembly of Brij surfactant in [Bmim][BF 4 ]. Currently, the ILs have been combined with active pharmaceutical ingredients (APIs), and supposed to be a third generation of ILs [20]. Such IL API compounds, offer new and improved properties like stability, solubility, permeability and drug delivery, as compared to the corresponding solid pharmaceutical forms. Many drugs, particularly those with the local anesthetic, tranquillizing, antidepressant and antibiotic actions, are amphiphilic in nature, and exert their activity by interaction with biological membranes [2]. The tricyclic antidepressant drugs are a family of structurally similar compounds possessing an almost planar tricyclic ring system with a short hydrocarbon chain carrying a terminal, charged nitrogen atom (Scheme (b)). It has been shown that these drugs form aggregates (or micelle) of approximately /$ see front matter 203 Elsevier B.V. All rights reserved.

2 A.B. Khan et al. / Colloids and Surfaces B: Biointerfaces 2 (203) Although the mixed micellization study of the drug-surfactant system in various concentrations and temperature range [4 44] have been reported and sufficient literature also available for the surfactants micellization [45 52] and its thermodynamic study [53,54] with various ILs. Up to our knowledge, no work has been done related to the role of ILs in the micellization of amphiphilic drugs, therefore, herein we show the micellization behaviour and thermodynamics of AMT, in the pre- and post-micellar range of mim][cl] at different temperatures. Due to the amphiphilic character of AMT and mim][cl], the formation of the mixed micelles are expected and hence the conductivity measurements are done for AMT, at pre- and post-micellar region of mim][cl], at different temperatures. 2. Materials and methods The amphiphilic drug, amitriptyline hydrochloride (AMT) ( 98%) and -methyl 3-octylimidazolium chloride mim][cl] ( 97%), Sigma, USA, were used without further purification to prepare aqueous solutions in doubly distilled water. Conductances were measured with a Labindia Pico + conductivity bridge having a cell constant.02. The specific conductance of doubly distilled water is.82 S/cm. The temperature in all the experiments was maintained by circulating water from an electronically controlled water bath (Julabo, Germany) with a temperature stability of ±0.0 K. The conductivity was measured after every addition of AMT solution in stock solution of mim][cl] and then specific conductance plotted against the molar concentration of the AMT. The reproducibility of conductance measurements was estimated to be ±0.5%. 3. Results and discussion Scheme. Ball and stick models of (a) -methyl 3-octylimidazolium chloride and (b) amitriptyline hydrochloride. 6 2 monomers [2 24]. Tricyclic antidepressant amitriptyline hydrochloride (AMT), a first generation antidepressant drug, which suffers from several drawbacks like anticholinergic, cardiovascular, and antiarrhythmic side effects [25]. To reduce these side effects, the antidepressants are used with a drug carrier. Recently, researchers found the phase separation (clouding) phenomena of some tricyclic amphiphilic antidepressant drugs [26 29]. It was found that like ionic surfactants, these amphiphilic drugs undergo ph, concentration, and temperature dependent phase separation. The thermodynamic parameters also evaluated at cloud point [30 34]. Generally, surfactants are considered as good carriers as they form micelles and can solubilize drugs in their core. Although these drugs are amphiphilic in nature, and they are not much hydrophilic that can be used without a carrier. Among various compounds used as carrier, surfactants possess a number of unbeaten advantages [35,36]. Micelle size permits the extravasation and accumulation in a variety of pathological sites. Also, they are easy to be prepared on a large scale. Since, AMT molecules themselves posses capacity of self-aggregation in aqueous medium, and it is worthwhile to study mixed micellization involving this drug with surfactants. Mixed micelles are studied extensively, in the past decades as a well-known complex organized system [37 40], due to the better surface properties of mixtures as compared to their individual components. Synergism in the properties of the binary mixtures, that may be exploited in their application in industrial preparations and pharmaceutical formulations, may be due to nonideality of mixing in binary mixtures of amphiphilic compounds [37]. Representative plots of specific conductivity,, as a function of molar concentration of AMT (i.e., [AMT]) in the presence of IL at different temperatures are shown in Fig.. At each temperature, the electrical conductivity increases with a gradual decrease in the slope and the break point in the plot originates from the inception of micellization. The change in slope at critical micelle concentration (cmc) is due to an effective loss of ionic charges because a fraction of the counterions are confirmed to the micellar surface. With the assistance of conductivity data here in, we discussed the effect of temperature on cmc, mixed micellization and thermodynamics of micellization of an amphiphilic drug with the alkyl imidazolium chloride. 3.. Effect of temperature on cmc The cmc value is governed by two opposing forces, i.e., () van der Waals forces between the hydrophobic part of an amphiphile that stabilizes the micelles and (2) hydration of hydrophilic part that destabilizes the micelles, at a particular temperature while rise in temperature affects micellization in two different ways, i.e., increase in dehydration and increase in thermal solubility of the amphiphiles. As shown in Fig. 2, the unusual behaviour of cmc values of AMT, mim][cl], and AMT mim][cl] with temperature. In most cases, for ionic amphiphiles, the cmc first decreases at low temperatures, while at high temperatures cmc again increases [55], while in case of non-ionic surfactants, the cmc decreases with increasing the temperature [56]. In addition for ionic systems, continuous increase in cmc with temperature is also reported in some cases [57,58]. However, in our systems, the cmc values first increase with temperature and then decrease (i.e., an inverted U-shaped behaviour)

3 462 A.B. Khan et al. / Colloids and Surfaces B: Biointerfaces 2 (203) / (ms/cm) (a) [54,55]. It is explained because of the fact that below T max (temperature at which cmc value is maximum), thermal solubility predominates over dehydration and cmc of AMT, mim][cl] and AMT mim][cl] systems increases while above T max, the high temperature dehydrates micelles more and this factor outweighs the solubility factor. Hence, cmc again decreases. Generally, the energetics of adsorption and micellization are discussed in terms of different forces like hydrophobic interaction, dispersion, attraction between hydrocarbon chains, and the electrostatic and van der Waals interactions between the head groups. κ K 303 K 308 K 35 K 38 K 3.2. Mixed micellization of AMT mim][cl] For binary amphiphilic mixtures, under the equilibrium condition of micelle formation, the ideal cmc is related to individual cmc s by Eq. () [59]. / (ms/cm) κ (b) [AMT] / (mm) 298 K 303 K 308 K 35 K 38 K [AMT] / (mm) Fig.. Plot of specific conductivity, vs. [AMT] in presence of 50 mm (a) and 50 mm (b) mim][cl] at different temperatures, respectively. cmc / (mm) Pure mim][cl] Temprature / (K) Fig. 2. Variation of cmc with temperature for pure and mixed systems. cmc = cmc + ( ) cmc 2 () where, cmc*, cmc, and cmc2 are the mole fraction of mim][cl] in the bulk, ideal cmc of mixture, cmc of mim][cl], and cmc of AMT, respectively. The experimentally obtained cmc values for pure and binary mixtures of mim][cl] and AMT as a function of temperature are shown in Fig. 2. As shown in Table and Fig. 2 that the experimental cmc values, for AMT-IL in pre-micellar region as well as in post-micellar region, are deviating from the predicted ideal behaviour, which indicates synergism in the mixed micelle formation. However, this deviation is more in post-micellar range as compared to pre-micellar region that means stronger synergism is observed in post-micellar region. Rubingh s procedure based on the Regular Solution Theory (RST) [60 62] for the mixed micelles not only characterizes the interaction parameter (ˇ) but also explains the deviation from ideality. The activity coefficients in the mixed micelles, according to the RST, are explained by the following equations: f = exp{ˇ( X ) 2 } (2) f 2 = exp{ˇ(x ) 2 } (3) where f, f 2, ˇ, and X are activity coefficients of the two components, interaction parameter in mixed micelles, and mole fraction of IL in the micelles, respectively. With the help of the RST a quantitative interpretation of micellar mole fraction can be calculated iteratively from the following equation: (X ) 2 ln(cmc /cmc X ) ( X ) 2 ln{cmc( )/cmc 2 ( X )} = (4) In ideal state, the micellar mole fraction [63], was calculated by the following equation: X ideal cmc 2 = (5) cmc 2 + ( )cmc As shown in Table, both X and X ideal decreases with the increase in temperature up to a maximum temperature then increases with the increase in temperature, that indicate micelle formation favoured up to a particular temperature then start to hinder because of the reason as discussed in earlier section in case of cmc variation with temperature. Non-ideality in the mixed micelles can also be explained by deviation of X from the corresponding X ideal values. The variation of X and X ideal, as reported in Table that in the pre-micellar region, higher X values than the corresponding X ideal indicate that the mixed micelles are rich in [C 8 mim][cl], while in the post-micellar region, lower X than corresponding X ideal indicate that mixed micelles are rich in AMT.

4 A.B. Khan et al. / Colloids and Surfaces B: Biointerfaces 2 (203) Table The physicochemical parameters for AMT [C8mim][Cl] mixed systems at various temperatures. Temp. (K) cmc exp cmc ideal X X ideal ˇ f f 2 G ex (kj/mol) AMT AMT The X values obtained from the above equation are used for the calculation of the interaction parameter, ˇ by the following equation: ˇ = ln(cmc /cmc X ) ( X ) 2 (6) The mixed micelle formation, due to the attractive and repulsive interactions are indicated by negative and positive ˇ values, respectively while a value close to zero refers to an ideal behaviour [62]. The negative values of ˇ means, the attractive interaction between AMT and IL is stronger than the individual components. For synergism in the mixed micelle formation, two conditions must be followed: () ˇ must be negative and (2) ˇ > ln(cmc /cmc 2 ) [64]. Also, the higher the magnitude of ˇ the greater would be the interaction whether attractive or repulsive. As shown in Table, in the pre-micellar region of mim][cl] the ˇ value is almost constant, i.e., very slight change in magnitude of ˇ value with increase in temperature but at higher temperature there is a significant increase in magnitude of ˇ value, while at post-micellar concentration there is a decrease in magnitude of ˇ value, to be observed with the rise in temperature from 298 K to 308 K and at 33 K the ˇ value again increases as in pre-micellar region. The values of f and f 2, calculated using Eqs. (3) and (4), are, in all cases less than unity, indicating non-ideality. The excess free energy of mixing, G ex [65], can be calculated using X i and f i by the following equation: G ex = RT[X ln f + X 2 ln f 2 ] (7) All the negative G ex values show that AMT mim][cl] mixed micelles are more stable than the micelle of mim][cl] and AMT. Their absolute values are more negative in post-micellar region as compared to in pre-micellar range of imidazolium IL. In addition, as shown in table the G ex values, are almost constant in pre-micellar region and slight decrease in post-micellar region of imidazolium IL with increase in temperature from 208 K to 308 K, while at 33 K there is an increase in magnitude that is also supported by the same trend of X and ˇ values. It means that in the pre- as well as in postmicellar region the micellization is more favourable at 33 K as compared to lower temperatures and, in post-micellar region with the increase in temperature from 208 K to 308 K the micellization become more difficult as shown in Table, while in pre-micellar region almost no effect of temperature on micellization process Thermodynamics of micellization In turn to estimate the thermodynamic parameters that are associated to micellization process, the standard Gibbs energy change, Gm, the standard enthalpy change, H m, the standard entropy change, Sm, were calculated by using the following equations: Gm = (2 g)rt ln X cmc (8) [ ] ln Hm = (2 g)rt 2 Xcmc (9) T Sm = H m G m (0) T where X cmc is the mole fraction of the drug at cmc and g is the degree of counter ion dissociation, which was calculated from the relation, g = S 2 /S, where S and S 2 are the slopes in the pre- and post-micellar regions determined from conductivity plots. This is quite simple satisfactory method to provide quantitative estimation of g, as reported by Buckingham and co-workers [66]. Later on integrity of this method was verified by Kale et al. [67] and also by Bandyopathyay and Moulik [68] who have estimated g by using ion-selective membrane electrode and found that the values of g thus obtained are in good agreement with those obtained conductometrically. Hence the degree of counterion dissociation (g) is experimental technique dependent [69], like cmc [70,7]. Counterions are bound primarily by the electric field created by head group and also by specific interactions that depend upon the head group and counterion type. It has been accepted that a fraction, g, of counterions of an ionic amphiphile are dissociated from the micelles, leaving the micelles charged. In ionic micelles, most of the counterions are bound strongly to the Stern layer. As some of the counterions remain bound to the amphiphile molecules even above the cmc, the ratio of values of slopes gives the degree of counterion dissociation. The value of g increases with increase in temperature while decreases with increase in electrolyte concentration [72] as shown in Fig. 3. Also, it decreases with increase in micellar growth [73]. Generally the g values at all temperature for drug are higher as compared to the values in the presence of IL. Increase in g (or decrease in counterion association) is observed in case of ionic surfactants also [74,75]. There are following opposing forces, i.e., van der Waals forces between the hydrophobic part of an amphiphile that stabilizes the micelles, and hydration of hydrophilic part that destabilizes the micelles, at a particular temperature are responsible to govern the change in cmc values, while rise in temperature affects micellization in two different ways, i.e., increase in dehydration and increase in thermal solubility of the amphiphiles. The negative Gm values are always indicating that the process of micelle/mixed micelle formation to be spontaneous as shown in Fig. 4 and Table 2. Gm values for pure drugs agree well with the literature [76,77]. The spontaneity for micelle formation process is more for mixed micelles as compare to pure ones at 298 K, while with the increase in temperature the spontaneity of micelle formation slightly decrease in case of AMT as well as in mixed micelle of AMT mim][cl], it may be due to the trend of g value, i.e.,

5 464 A.B. Khan et al. / Colloids and Surfaces B: Biointerfaces 2 (203) g Temprature (K) Fig. 3. Variation of g with temperature. increases with the increase in temperature. However, the overall process is spontaneous. During the process of micellization, the destruction of the water structure (or the icebergs ) around the amphiphilic molecules gives a positive entropy change while the destruction of hydrogen bonding in water structure gives a positive enthalpy change [78]. Therefore, the micellization process is governed primarily by the entropy gain and the driving force for the process is the tendency of the hydrophobic group of the AMT to transfer from the solvent environment to the interior of the micelle. The negative enthalpy values suggest the importance of London dispersion interactions as an attractive force for micellization [79]. The Hm values show a change from negative to positive with increasing temperature as shown in Fig. 5 and Table 2. In all cases for pure components as well as in mixed system of drug and ionic liquid, at 308 K, Hm values are negative and on further increase in temperature at 33 K it become positive. This may be due to the difference in the hydration between aromatic and saturated parts of the AMT and mim][cl]. Release of water molecules from hydrophobic part may change the process from exothermic to endothermic at high temperatures. ΔG 0 m / (kj/mol) Pure mim][cl] Temprature / (K) Fig. 4. Variation of Gm for pure and mixed systems with temperature. Table 2 Thermodynamic parameters for pure AMT, mim][cl] and AMT [C8mim][Cl] mixed systems. Temp. (K) G m (kj/mol) H m (kj/mol) S m (kj/mol) T S m (J/kmol) Pure mim][cl] AMT AMT ΔH o m / (kj/mol) Pure mim][cl] Temprature / (K) Fig. 5. Variation of Hm for pure and mixed systems with temperature. The Sm values are positive at all temperatures (Table 2), indicating that the micellization process is entropy dominated in these systems, particularly when entropy change is high. Obviously, this is caused by the particular structure of AMT, which is the principal component of the mixed micelles. Ostensibly, the key lies in the difference in the hydration between the saturated and aromatic hydrocarbon parts of the drug molecule. The high increase in entropy suggests a strong liberation of water, which probably is the water associated with the aromatic ring of the drug. This, in turn, must increase the hydrophobicity of AMT molecules causing the reduction of cmc. As obvious from (Table 2), the magnitude of Sm is higher for AMT as compared to mim][cl] and again higher in pre-micellar region as compared to post-micellar region. 4. Conclusion The role of alkyl imidazolium ionic liquid mim][cl], in the mixed micellization with amphiphilic drug, AMT have been studied

6 A.B. Khan et al. / Colloids and Surfaces B: Biointerfaces 2 (203) at different temperatures by using electrical conductivity. In view of above discussion there are following conclusions: (i) With the rise in temperature the cmc values increases upto a certain temperature, on further increase in temperature cmc values start to decrease (i.e., an inverted U-shaped behaviour) for all the systems. (ii) As obvious from (Table ), the values X >X ideal in pre-micellar region of mim][cl] indicates that the mixed micelles are rich in ionic liquid, while in the post-micellar region, X >X ideal indicate that mixed micelles are rich in AMT. (iii) The negative G ex values support that AMT mim][cl] mixed micelles are more stable than the micelle of mim][cl] and AMT, trend also supported by the X and ˇ values. (iv) The Gm values come out to be negative for the all systems. However, Hm for AMT and mim][cl], as well as AMT mim][cl] systems is negative at low temperature and positive at high temperature. The Sm values are positive, and their magnitude being more at T = 33 K. Acknowledgements Dr. Abbul Bashar Khan is thankful to Council of Scientific and Industrial Research, New Delhi, India, for providing a research grant (No. 9/466(050) 2K2-EMR-I) and Dr. Maroof Ali is also thankful to UGC for Dr. D.S. Kothari fellowship. References [] T. Welton, Chem. Rev. 99 (999) 207. [2] C.M. Gordon, Catal. Appl. A. Gen. 222 (200) 0. [3] C. Lagrost, D. Carrié, M. Vaultier, P. Hapiot, J. Phys. Chem. A 07 (2003) 745. [4] A.M. Scurto, S.N.V.K. Aki, J.F. Brennecke, Chem. Commun. (2003) 572. [5] J.L. Anderson, V. Pino, E.C. Hagberg, V.V. Sheares, D.W. Armstrong, Chem. Commun. (2003) [6] K.A. Fletcher, S. Pandey, Langmuir 20 (2004) 33. [7] C. Patrascu, F. Gauffre, F. Nallet, R. Bordes, J. Oberdisse, N. de Lauth-Viguerie, C. Mingotaud, Chem. Phys. Chem. 7 (2006) 99. [8] J. 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