- Email: [email protected]
Partitioning of thiophene derivatives between solvent and micellar media of cationic surfactant, cetyl trimethyl ammonium bromide
Journal of Molecular Liquids 240 (2017) 389–394
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Partitioning of thiophene derivatives between solvent and micellar media of cationic surfactant, cetyl trimethyl ammonium bromide Rizwan Saeed a, Muhammad Usman a,⁎, Nasir Rasool a, Matloob Ahmad a, Zulfiqar Ali Khan a, Zahoor Hussain Farooqi b, Mohammad Siddiq c, Ameer Fawad Zahoor a a b c
Department of Chemistry, Government College University, Faisalabad 38000, Pakistan Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistan Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
a r t i c l e
i n f o
Article history: Received 12 March 2017 Received in revised form 7 May 2017 Accepted 14 May 2017 Available online xxxx Keywords: Thiophene derivatives Surfactant Partitioning Spectroscopy Micelle Partition coefficient
a b s t r a c t This manuscript reports the interaction of thiophene derivatives i.e. 5-(2-(benzyloxy) phenyl) thiophene-2-sulfonamide (BPTS) and 5-bromothiophene-2-sulfonamide (BTS) with micellar media of cationic surfactant, cetyl trimethylammonium bromide (CTAB) using UV/visible spectroscopy. Quantitative measurement of the degree of solubilization and binding has been calculated in term of partition coefficient (Kx) and binding constant (Kb) respectively while the values of free energy of partition (ΔGp), and free energy of binding (ΔGb) give information about spontaneity of both processes. © 2017 Elsevier B.V. All rights reserved.
1. Introduction One of the most unique properties of surfactants is formation of micelles, due to non-covalent forces, above critical micelle concentration (cmc) and increase in solubility of insoluble materials in micellar solution. Hydrophilic surface and hydrophobic core impart heterogeneous structure to micelle and, thus, enabling them to act as biomembranes because they have ability to interact with both hydrophilic and lipophilic additives. The said property makes micellar media of surfactants valuable for a number of applications in laboratory and industry i.e. micellar catalysis, detergency, emulsion polymerization, enhanced oil recovery, dry cleaning and the most important of all drug delivery and removal of pollutants from aqueous media [1,2]. Thus the selection of the most suitable surfactant for the said purpose is of utmost importance. The calculation of parameters like binding constant, Kb, partition coefficient, Kx, free energies of binding and partition help to take decision in this regard. The values of aforementioned parameters not only enable us to understand interactions of additives with bio membranes but also to establish structure activity relationship [1]. Non-covalent forces may also induce self-aggregation in some nonamphiphilic organic compounds but this aggregation does not deserve to be called micellization rather called open association as there is no ⁎ Corresponding author. E-mail address: [email protected] (M. Usman).
http://dx.doi.org/10.1016/j.molliq.2017.05.062 0167-7322/© 2017 Elsevier B.V. All rights reserved.
sharp change in their physical properties at cmc [3]. This open association may cause shift in λmax of these compounds thus exhibiting red or blue shift. Red shift is produced by J-aggregates formed due to end to end stacking while H-aggregates cause blue shift being formed by plane to plane stacking. The value of tilt angle is less than 32° for J-aggregates and greater for H-aggregates [4]. Control release rate, less degradation of drug, increase in solubility and decrease in toxicity level of drugs make micelles excellent drug carriers [5]. Hydrophobic effects play major role to decide the locus of solubilizate within micelle, although, role of hydrophilic and electrostatic forces is not ignorable [6]. Thiophene is a five membered heterocyclic aromatic compound. Its chemical stability, easy synthesis and easy processing make its derivatives among one of the most studied organic compounds. Their applications in drug design, electronic devices, bio diagnostics and sensory devices have invited attention of many researchers. Many research groups have turned their attention toward oligomers and polymers of thiophenes due to their semiconductor, luminescence and sensory properties [7]. In our previous work we have reported solubilization of Chloroquine Diphosphate [8], Quinacrine 2HCl [9], Pefloxacin mesylate [10], benzothiazole [11], Benzalkonium Chloride [12] reactive blue 19 [13] and reactive red 223 and reactive orange 122 [14] in micellar solution of selected surfactants. In present work we want to explore the effect of structure on partitioning of two thiophene derivatives i.e. BPTS and
390
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
Fig. 1. Structures of chemicals used.
BTS between solvent and micellar media of CTAB, a cationic surfactant. Fig. 1 shows structures of chemicals being used in study while resonance structures of both thiophene derivatives are given in Schemes 1 and 2.
employed differential absorbance method to calculate partition coefficient using Kawamura equation [15]. 1 1 1 þ ¼ ΔA K c ΔA∞ C a þ C mo ΔA ∞ s
ð1Þ
2. Parameters calculated 2.1. Partition and binding parameters The molecules of Thiophene derivatives (i.e. BTS and BPTS) partition themselves between solvent and micellar media. Partition coefficient is the quantitative measure of degree of solubilization and we have
In Eq.uation (1), Ca is concentration of additive (BPTS and BTS) in denotes analytical concentration of surfactant mol·dm−3 and Cmo s (CTAB) after micellization calculated by subtracting cmc of CTAB from its experimental concentration [16]. ΔA and ΔA∞ represent value of differential absorbance at experimental and infinite concentration of CTAB respectively. Kc is partition constant in unit of dm3 per mol. Kc is
Scheme 1. Resonance structures of BPTS.
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
391
Scheme 2. Resonance structures of BTS.
converted to a dimensionless quantity partition coefficient, Kx by multiplying with nw (number of moles of water per dm3 i.e. 55.555 mol·dm−3). The free energy of partition was calculated using the following relation; ΔGp ¼ −RTlnK x
ð2Þ
Surfactant-additive binding constant, Kb was calculated using following equation; CsCa Cs 1 ¼ þ ΔA Δεl K b Δεl
ð3Þ
3.3.1. UV/visible absorption spectroscopy UV/visible absorption spectra were recoded using Hitachi U-2800 double beam spectrophotometer. For simple absorption spectra solvent (50% ethanol) was taken in reference cell and for differential absorption spectra thiophene solution was used for same purpose. However, for both cases solution of CTAB prepared in thiophene solution was placed in sample cell.
4. Results and discussion 4.1. Interaction of CTAB with BPTS
Δε is difference of absorption coefficient; l is path length, while Kb stands for the binding constant [16–20]. The free energy of binding was calculated from Eq.uation (4); ΔGb ¼ −RTlnK b
3.3. Experimental methods
ð4Þ
3. Materials and methods 3.1. Material used Two bioactive derivatives of thiophene i.e. 5-bromothiophene-2sulfonamide (BTS) and 5-(2-(benzyloxy)phenyl)thiophene-2-sulfonamide (BPTS) were synthesized and purified. Cationic surfactant, cetyl trimethyl ammonium bromide (CTAB) and ethanol were purchased from sigma Aldrich and used as received. 3.2. Preparation of solutions Solution of thiophene derivatives was prepared in equimolar mixture of distilled water and ethanol. CTAB was dissolved in aforementioned solution of thiophene in submicellar and micellar concentration range.
Simple UV/visible spectrum of BPTS, in presence and absence of CTAB, is given in Fig. 2a showing maximum absorbance at 300 nm and red shift is observed in the presence of CTAB due to transfer of BPTS from polar solvent to nonpolar core of micelle. This bathochromic shift can be explained on the basis of relative difference of energy between molecular levels of solubilizate in polar and non-polar environment. In polar environment of solvent polar anti bonding π* molecular orbitals and nonbonding molecular n-orbitals are more stabilized. In nonpolar micellar environment, however, bonding π molecular orbitals are stabilized and consequently energy gap between π and π* increases while that between n and π* decreases causing an overall bathochromic effect [21]. Electrostatic and hydrophobic interactions are cumulatively responsible for incorporation of BPTS molecules within micelles. The absorbance increases with CTAB concentration, as shown in Fig. 2b, because large amounts of BPTS molecules are being encapsulated within the micelle [8–18]. The lateral pressure, a downward force faced by solubilized molecules, tends to push solubilized molecule deep in palisade layer. This pressure directly depends on bulkiness and area of cross section of molecules. The molecules having branches and aromatic rings face greater lateral pressure than do straight chain molecules with same hydrophilic
Fig. 2. (a) Simple UV/visible spectra of BPTS in absence and presence of CTAB. (b) Plot of simple absorbance of BPTS versus concentration of CTAB.
392
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
Fig. 3. (a) Change in differential absorbance of BPTS in presence of CTAB. (b) Plot for calculation of partition constant for BPTS/CTAB system. (c) Plot for calculation of binding constant for BPTS/CTAB system.
part. Aromatic character of BPTS and its lateral pressure cause it to be penetrated relatively deeper in palisade layer of micelle [2,8–18] than BTS. Absorbance increases rapidly till cmc and then a saturation plateau is observed due to maximum solubilization of BPTS. Very slow increase in absorbance after cmc is due to encapsulation of BPTS by newly born micelles. Environment of BPTS molecules undergoes continuous change as they transfer themselves from polar solvent to nonpolar core of micelle. After cmc, however, no change in environment and, thus, no change in absorbance is observed [18–20]. Dynamic nature of solubilization causes BPTS molecules to not occupy fix position in micelle. The high value of partition coefficient indicates large scale inclusion of additive molecules due to dominant hydrophobic forces owing to presence of aromatic rings [6,10,18–22]. Differential spectroscopy is the best technique for qualitative and quantitative measurement of partitioning of solubilizate between solvent and micelle. It gives us an opportunity to have quantitative measure of degree of solubilization in term of partition constant. The increase in differential absorbance of BPTS versus concentration of CTAB, as evident in fig. 3a, is due to its continuous inclusion in CTAB micelles [4–10]. The presence of three aromatic rings makes hydrophobic forces more dominant and causes additive molecules to move inside micelles. BPTS molecules, therefore, orient themselves in such a way that delocalized negative charge keeps one part of molecule near surface while aromatic rings are in nonpolar environment near core [4–10]. Fig. 3b and c display plots for calculation of partition and binding constants respectively. Table 1 displays the values of binding and partition parameter calculated for BPTS/CTAB system. The large value of Kx (3.61 × 106) indicates large scale partitioning of BPTS molecules between solvent and micellar media, while the value of Kb (3333) indicates strong binding between additive and surfactant. The negative value of ΔGp (− 37.41 kJ/mol) and ΔGb (−20.1 kJ/mol) indicates that partitioning and binding is spontaneous process giving rise to stable system. Scheme 3a shows binding between BPTS and CTAB while 4a unveils the locus of BPTS within CTAB micelle, as guessed from value of Kx [8–11,18–22].
in differential absorbance with CTAB concentration, as visible in Fig. 5a [12–22]. Delocalized negative charge on BTS molecule causes binding between BTS and CTAB molecules as evident in Scheme 3b [12–17]. The plots to calculate binding constant and partition coefficient for BTS/CTAB system has been displayed in Fig. 5b and c respectively and values of parameters calculated from these plots are given in Table 1. The value of binding constant for BTS/CTAB is larger (1 × 104 dm 3 mol− 1 ) than for BPTS/CTAB system (3.33 × 103 dm3 mol− 1) because smaller size of BTS causes stronger binding. Similarly higher value of Kx for BTS/CTAB system (11.5 × 106) indicates that BTS is partitioned in CTAB micelle to much greater extent because of its small size which causes its molecules to be accommodated very close to surface where large space is available and large numbers of molecules are accommodated (Scheme 4b). The negative value of ΔGp (− 40.27 kJ/mol) and ΔGb (− 22.82 kJ/mol) discloses that both process are more spontaneous and system is more stable than BPTS/CTAB system [9,12,20–22]. 4.1.2. Comparison of interaction of BPTS and BTS The values of binding and partition parameters of a particular system depend largely on structure of additive and surfactant. Smaller size of BTS and less delocalization of negative charge causes it to bind more strongly with CTAB than BPTS. Larger molecular size, higher delocalization of negative charge and higher aromaticity of BPTS makes difficult for its molecules to be accommodated in micelle and force it to be deep penetrated toward core of micelle where less space is available and lower value of partition constant is resulted. In case of BTS, however, smaller size, less aromatic character and less delocalization of negative charge causes it to be short penetrated near surface where large space is available and higher degree of partitioning is observed.
4.1.1. Interaction of CTAB with BTS CTAB causes λmax of BTS to undergo red shift, as evident in Fig. 4a, due to change in polarity of environment as BTS moves from polar solvent to less polar micellar media. The increase in absorbance of BTS is rapid till cmc and becomes very slow in micellar region (Fig. 4b). Continuous inclusion of BTS molecules within micelle also causes increase
Table 1 Binding constant, Kb, free energy of binding, ΔGb, Partition coefficient, Kx and free energy of partition, ΔGp for BPTS/CTAB system and BTS/CTAB system. Systems
Kb (dm3 mol−1)
ΔGb (kJ/mol−1)
Kx
ΔGp (kJ/mol−1)
BPTS/CTAB BTS/CTAB
3333 10,000
−20.1 −22.82
3.61 × 106 11.5 × 106
−37.41 −40.27
Scheme 3. Binding of CTAB with (a) BPTS (b) BTS.
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
393
Fig. 4. (a) Simple UV/visible spectra of BTS in absence and presence of CTAB. (b) Plot of simple absorbance of BTS versus concentration of CTAB.
Fig. 5. (a) Change in differential absorbance of BTS in presence of CTAB. (b) Plot for calculation of partition constant for BTS/CTAB system. (c) Plot for calculation of binding constant for BTS/ CTAB system.
The red shift of λmax in both cases confirms the migration of additive molecules, in both cases, from solvent (more polar) to micellar (less polar surface or nonpolar core) phase. More polar orbitals are stabilized to greater extent in polar environment therefore n → π* transitions will cause bathochromic shift [4–9,12,20–22]. 5. Conclusion Partitioning of thiophene derivatives 5-bromothiophene-2-sulfonamide (BTS) and 5-(2-(benzyloxy) phenyl)thiophene-2-sulfonamide (BPTS) was studied by UV/visible spectroscopy to find the extent of solubilization in term of partition coefficient (Kx), free energy of partition (ΔGp), binding constant (Kb) and free energy of binding (ΔGb). Both
compounds showed red shift with CTAB due to incorporation of additives from solvent (polar) to micellar phase (non-polar). Kx and Kb have higher values for BTS due to its smaller size and less delocalization of negative charge. BPTS has less partitioning and binding capability due to larger size, more aromaticity, extensive delocalization and less charge density. It is, thus, concluded that bulky substituents and higher degree of delocalization on thiophene nucleus may decrease its ability to be solubilized. Acknowledgments This manuscript is a part of M.Phil thesis of Mr. Rizwan Saeed. All authors contributed at various stages of planning, execution and write up.
Scheme 4. Solubilization of (a) BPTS and (b) BTS by CTAB micelle.
394
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
References [1] O. Cudina, K.K. Rajic, R.I. Bugarcic, I. Jankovic, Colloids Surf. A Physicochem. Eng. Asp. 256 (2005) 225–232. [2] M.J. Rosen, J.T. Kunjappu, Surfactants and Interfacial Phenomenon, fourth ed. John Wiley & Sons, USA, 2012. [3] D. Attwood, Adv. Colloid Interf. Sci. 55 (1995) 271–303. [4] K. Behera, P. Mohapara, S. Patel, K. Mehera, J. Photochem. Photobio. A Chem. 169 (2005) 253–260. [5] S.K. Mehta, K.K. Bhasin, S. Dham, S. Kumar, Colloids Surf. A Physicochem. Eng. Asp. 278 (2006) 17–25. [6] B. Naseem, A. Sabri, A. Hasan, S.S. Shah, Colloids Surf. A Physicochem. Eng. Asp. 35 (2004) 7–13. [7] G. Barbarella, M. Melucci, G. Sotgiu, Adv. Mater. 17 (2005) 1581–1593. [8] M. Usman, M. Siddiq, J. Chem. Thermodyn. 58 (2013) 359–366. [9] M. Usman, M. Siddiq, Spectrochim. Acta A 113 (2013) 182–190. [10] M. Usman, M.A. Rashid, A. Mansha, M. Siddiq, Thermochim. Acta 573 (2013) 18–24. [11] S. Hanif, M. Usman, A. Hussain, N. Rasool, M. Zubair, U.A. Rana, J. Mol. Liq. 211 (2013) 7–14.
[12] K. Mahmood, M. Shakeel, M. Siddiq, M. Usman, Tenside Surfactant Deterg. 53 (2016) 195–204. [13] N. Haq, M. Usman, A. Mansha, U.A. Rana, M.A. Rashid, M. Majid, J. Mol. Liq. 196 (2014) 264–269. [14] M. Irfan, M. Usman, A. Mansha, N. Rasool, M. Ibrahim, U.A. Rana, M. Siddiq, M.Z. Haq, H.Z.E. Jaffar, Sci. World J. 2014 (2014) 1–8. [15] H. Kawamura, M. Manabe, Y. Miyamoto, Y. Fujita, S. Tokunaga, J. Phys. Chem. 93 (1989) 5536–5540. [16] S.W.H. Shah, K. Naeem, B. Naseem, S.S. Shah, Colloids Surf. A Physicochem. Eng. Asp. 331 (2008) 227–231. [17] M.F. Nazar, M. Abid, M. Danish, M. Ashfaq, A.M. Khan, M.N. Zafar, S. Mehmood, A. Asif, J. Mol. Liq. 212 (2015) 142–150. [18] M.F. Nazar, F. Mukhtar, S. Chaudry, M. Ashfaq, S. Mehmood, A. Asif, U.A. Rana, J. Mol. Liq. 200 (2014) 361–368. [19] O. Duman, S. Tunç, B. Kancı, Fluid Phase Equilib. 301 (2011) 56–61. [20] M.F. Nazar, F. Mukhtar, M. Ashfaq, H.M.A. Rahman, M.N. Zafar, S.H. Sumrra, Fluid Phase Equilib. 406 (2015) 47–54. [21] M. Younis, Organic Spectroscopy, Ilmi Kitab Khana, Lahore, Pakistan. [22] M.F. Nazar, S.S. Shah, M.A. Khosa, J. Surfactant Deterg. 13 (2010) 529–537.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Partitioning of thiophene derivatives between solvent and micellar media of cationic surfactant, cetyl trimethyl ammonium bromide Rizwan Saeed a, Muhammad Usman a,⁎, Nasir Rasool a, Matloob Ahmad a, Zulfiqar Ali Khan a, Zahoor Hussain Farooqi b, Mohammad Siddiq c, Ameer Fawad Zahoor a a b c
Department of Chemistry, Government College University, Faisalabad 38000, Pakistan Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistan Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
a r t i c l e
i n f o
Article history: Received 12 March 2017 Received in revised form 7 May 2017 Accepted 14 May 2017 Available online xxxx Keywords: Thiophene derivatives Surfactant Partitioning Spectroscopy Micelle Partition coefficient
a b s t r a c t This manuscript reports the interaction of thiophene derivatives i.e. 5-(2-(benzyloxy) phenyl) thiophene-2-sulfonamide (BPTS) and 5-bromothiophene-2-sulfonamide (BTS) with micellar media of cationic surfactant, cetyl trimethylammonium bromide (CTAB) using UV/visible spectroscopy. Quantitative measurement of the degree of solubilization and binding has been calculated in term of partition coefficient (Kx) and binding constant (Kb) respectively while the values of free energy of partition (ΔGp), and free energy of binding (ΔGb) give information about spontaneity of both processes. © 2017 Elsevier B.V. All rights reserved.
1. Introduction One of the most unique properties of surfactants is formation of micelles, due to non-covalent forces, above critical micelle concentration (cmc) and increase in solubility of insoluble materials in micellar solution. Hydrophilic surface and hydrophobic core impart heterogeneous structure to micelle and, thus, enabling them to act as biomembranes because they have ability to interact with both hydrophilic and lipophilic additives. The said property makes micellar media of surfactants valuable for a number of applications in laboratory and industry i.e. micellar catalysis, detergency, emulsion polymerization, enhanced oil recovery, dry cleaning and the most important of all drug delivery and removal of pollutants from aqueous media [1,2]. Thus the selection of the most suitable surfactant for the said purpose is of utmost importance. The calculation of parameters like binding constant, Kb, partition coefficient, Kx, free energies of binding and partition help to take decision in this regard. The values of aforementioned parameters not only enable us to understand interactions of additives with bio membranes but also to establish structure activity relationship [1]. Non-covalent forces may also induce self-aggregation in some nonamphiphilic organic compounds but this aggregation does not deserve to be called micellization rather called open association as there is no ⁎ Corresponding author. E-mail address: [email protected] (M. Usman).
http://dx.doi.org/10.1016/j.molliq.2017.05.062 0167-7322/© 2017 Elsevier B.V. All rights reserved.
sharp change in their physical properties at cmc [3]. This open association may cause shift in λmax of these compounds thus exhibiting red or blue shift. Red shift is produced by J-aggregates formed due to end to end stacking while H-aggregates cause blue shift being formed by plane to plane stacking. The value of tilt angle is less than 32° for J-aggregates and greater for H-aggregates [4]. Control release rate, less degradation of drug, increase in solubility and decrease in toxicity level of drugs make micelles excellent drug carriers [5]. Hydrophobic effects play major role to decide the locus of solubilizate within micelle, although, role of hydrophilic and electrostatic forces is not ignorable [6]. Thiophene is a five membered heterocyclic aromatic compound. Its chemical stability, easy synthesis and easy processing make its derivatives among one of the most studied organic compounds. Their applications in drug design, electronic devices, bio diagnostics and sensory devices have invited attention of many researchers. Many research groups have turned their attention toward oligomers and polymers of thiophenes due to their semiconductor, luminescence and sensory properties [7]. In our previous work we have reported solubilization of Chloroquine Diphosphate [8], Quinacrine 2HCl [9], Pefloxacin mesylate [10], benzothiazole [11], Benzalkonium Chloride [12] reactive blue 19 [13] and reactive red 223 and reactive orange 122 [14] in micellar solution of selected surfactants. In present work we want to explore the effect of structure on partitioning of two thiophene derivatives i.e. BPTS and
390
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
Fig. 1. Structures of chemicals used.
BTS between solvent and micellar media of CTAB, a cationic surfactant. Fig. 1 shows structures of chemicals being used in study while resonance structures of both thiophene derivatives are given in Schemes 1 and 2.
employed differential absorbance method to calculate partition coefficient using Kawamura equation [15]. 1 1 1 þ ¼ ΔA K c ΔA∞ C a þ C mo ΔA ∞ s
ð1Þ
2. Parameters calculated 2.1. Partition and binding parameters The molecules of Thiophene derivatives (i.e. BTS and BPTS) partition themselves between solvent and micellar media. Partition coefficient is the quantitative measure of degree of solubilization and we have
In Eq.uation (1), Ca is concentration of additive (BPTS and BTS) in denotes analytical concentration of surfactant mol·dm−3 and Cmo s (CTAB) after micellization calculated by subtracting cmc of CTAB from its experimental concentration [16]. ΔA and ΔA∞ represent value of differential absorbance at experimental and infinite concentration of CTAB respectively. Kc is partition constant in unit of dm3 per mol. Kc is
Scheme 1. Resonance structures of BPTS.
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
391
Scheme 2. Resonance structures of BTS.
converted to a dimensionless quantity partition coefficient, Kx by multiplying with nw (number of moles of water per dm3 i.e. 55.555 mol·dm−3). The free energy of partition was calculated using the following relation; ΔGp ¼ −RTlnK x
ð2Þ
Surfactant-additive binding constant, Kb was calculated using following equation; CsCa Cs 1 ¼ þ ΔA Δεl K b Δεl
ð3Þ
3.3.1. UV/visible absorption spectroscopy UV/visible absorption spectra were recoded using Hitachi U-2800 double beam spectrophotometer. For simple absorption spectra solvent (50% ethanol) was taken in reference cell and for differential absorption spectra thiophene solution was used for same purpose. However, for both cases solution of CTAB prepared in thiophene solution was placed in sample cell.
4. Results and discussion 4.1. Interaction of CTAB with BPTS
Δε is difference of absorption coefficient; l is path length, while Kb stands for the binding constant [16–20]. The free energy of binding was calculated from Eq.uation (4); ΔGb ¼ −RTlnK b
3.3. Experimental methods
ð4Þ
3. Materials and methods 3.1. Material used Two bioactive derivatives of thiophene i.e. 5-bromothiophene-2sulfonamide (BTS) and 5-(2-(benzyloxy)phenyl)thiophene-2-sulfonamide (BPTS) were synthesized and purified. Cationic surfactant, cetyl trimethyl ammonium bromide (CTAB) and ethanol were purchased from sigma Aldrich and used as received. 3.2. Preparation of solutions Solution of thiophene derivatives was prepared in equimolar mixture of distilled water and ethanol. CTAB was dissolved in aforementioned solution of thiophene in submicellar and micellar concentration range.
Simple UV/visible spectrum of BPTS, in presence and absence of CTAB, is given in Fig. 2a showing maximum absorbance at 300 nm and red shift is observed in the presence of CTAB due to transfer of BPTS from polar solvent to nonpolar core of micelle. This bathochromic shift can be explained on the basis of relative difference of energy between molecular levels of solubilizate in polar and non-polar environment. In polar environment of solvent polar anti bonding π* molecular orbitals and nonbonding molecular n-orbitals are more stabilized. In nonpolar micellar environment, however, bonding π molecular orbitals are stabilized and consequently energy gap between π and π* increases while that between n and π* decreases causing an overall bathochromic effect [21]. Electrostatic and hydrophobic interactions are cumulatively responsible for incorporation of BPTS molecules within micelles. The absorbance increases with CTAB concentration, as shown in Fig. 2b, because large amounts of BPTS molecules are being encapsulated within the micelle [8–18]. The lateral pressure, a downward force faced by solubilized molecules, tends to push solubilized molecule deep in palisade layer. This pressure directly depends on bulkiness and area of cross section of molecules. The molecules having branches and aromatic rings face greater lateral pressure than do straight chain molecules with same hydrophilic
Fig. 2. (a) Simple UV/visible spectra of BPTS in absence and presence of CTAB. (b) Plot of simple absorbance of BPTS versus concentration of CTAB.
392
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
Fig. 3. (a) Change in differential absorbance of BPTS in presence of CTAB. (b) Plot for calculation of partition constant for BPTS/CTAB system. (c) Plot for calculation of binding constant for BPTS/CTAB system.
part. Aromatic character of BPTS and its lateral pressure cause it to be penetrated relatively deeper in palisade layer of micelle [2,8–18] than BTS. Absorbance increases rapidly till cmc and then a saturation plateau is observed due to maximum solubilization of BPTS. Very slow increase in absorbance after cmc is due to encapsulation of BPTS by newly born micelles. Environment of BPTS molecules undergoes continuous change as they transfer themselves from polar solvent to nonpolar core of micelle. After cmc, however, no change in environment and, thus, no change in absorbance is observed [18–20]. Dynamic nature of solubilization causes BPTS molecules to not occupy fix position in micelle. The high value of partition coefficient indicates large scale inclusion of additive molecules due to dominant hydrophobic forces owing to presence of aromatic rings [6,10,18–22]. Differential spectroscopy is the best technique for qualitative and quantitative measurement of partitioning of solubilizate between solvent and micelle. It gives us an opportunity to have quantitative measure of degree of solubilization in term of partition constant. The increase in differential absorbance of BPTS versus concentration of CTAB, as evident in fig. 3a, is due to its continuous inclusion in CTAB micelles [4–10]. The presence of three aromatic rings makes hydrophobic forces more dominant and causes additive molecules to move inside micelles. BPTS molecules, therefore, orient themselves in such a way that delocalized negative charge keeps one part of molecule near surface while aromatic rings are in nonpolar environment near core [4–10]. Fig. 3b and c display plots for calculation of partition and binding constants respectively. Table 1 displays the values of binding and partition parameter calculated for BPTS/CTAB system. The large value of Kx (3.61 × 106) indicates large scale partitioning of BPTS molecules between solvent and micellar media, while the value of Kb (3333) indicates strong binding between additive and surfactant. The negative value of ΔGp (− 37.41 kJ/mol) and ΔGb (−20.1 kJ/mol) indicates that partitioning and binding is spontaneous process giving rise to stable system. Scheme 3a shows binding between BPTS and CTAB while 4a unveils the locus of BPTS within CTAB micelle, as guessed from value of Kx [8–11,18–22].
in differential absorbance with CTAB concentration, as visible in Fig. 5a [12–22]. Delocalized negative charge on BTS molecule causes binding between BTS and CTAB molecules as evident in Scheme 3b [12–17]. The plots to calculate binding constant and partition coefficient for BTS/CTAB system has been displayed in Fig. 5b and c respectively and values of parameters calculated from these plots are given in Table 1. The value of binding constant for BTS/CTAB is larger (1 × 104 dm 3 mol− 1 ) than for BPTS/CTAB system (3.33 × 103 dm3 mol− 1) because smaller size of BTS causes stronger binding. Similarly higher value of Kx for BTS/CTAB system (11.5 × 106) indicates that BTS is partitioned in CTAB micelle to much greater extent because of its small size which causes its molecules to be accommodated very close to surface where large space is available and large numbers of molecules are accommodated (Scheme 4b). The negative value of ΔGp (− 40.27 kJ/mol) and ΔGb (− 22.82 kJ/mol) discloses that both process are more spontaneous and system is more stable than BPTS/CTAB system [9,12,20–22]. 4.1.2. Comparison of interaction of BPTS and BTS The values of binding and partition parameters of a particular system depend largely on structure of additive and surfactant. Smaller size of BTS and less delocalization of negative charge causes it to bind more strongly with CTAB than BPTS. Larger molecular size, higher delocalization of negative charge and higher aromaticity of BPTS makes difficult for its molecules to be accommodated in micelle and force it to be deep penetrated toward core of micelle where less space is available and lower value of partition constant is resulted. In case of BTS, however, smaller size, less aromatic character and less delocalization of negative charge causes it to be short penetrated near surface where large space is available and higher degree of partitioning is observed.
4.1.1. Interaction of CTAB with BTS CTAB causes λmax of BTS to undergo red shift, as evident in Fig. 4a, due to change in polarity of environment as BTS moves from polar solvent to less polar micellar media. The increase in absorbance of BTS is rapid till cmc and becomes very slow in micellar region (Fig. 4b). Continuous inclusion of BTS molecules within micelle also causes increase
Table 1 Binding constant, Kb, free energy of binding, ΔGb, Partition coefficient, Kx and free energy of partition, ΔGp for BPTS/CTAB system and BTS/CTAB system. Systems
Kb (dm3 mol−1)
ΔGb (kJ/mol−1)
Kx
ΔGp (kJ/mol−1)
BPTS/CTAB BTS/CTAB
3333 10,000
−20.1 −22.82
3.61 × 106 11.5 × 106
−37.41 −40.27
Scheme 3. Binding of CTAB with (a) BPTS (b) BTS.
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
393
Fig. 4. (a) Simple UV/visible spectra of BTS in absence and presence of CTAB. (b) Plot of simple absorbance of BTS versus concentration of CTAB.
Fig. 5. (a) Change in differential absorbance of BTS in presence of CTAB. (b) Plot for calculation of partition constant for BTS/CTAB system. (c) Plot for calculation of binding constant for BTS/ CTAB system.
The red shift of λmax in both cases confirms the migration of additive molecules, in both cases, from solvent (more polar) to micellar (less polar surface or nonpolar core) phase. More polar orbitals are stabilized to greater extent in polar environment therefore n → π* transitions will cause bathochromic shift [4–9,12,20–22]. 5. Conclusion Partitioning of thiophene derivatives 5-bromothiophene-2-sulfonamide (BTS) and 5-(2-(benzyloxy) phenyl)thiophene-2-sulfonamide (BPTS) was studied by UV/visible spectroscopy to find the extent of solubilization in term of partition coefficient (Kx), free energy of partition (ΔGp), binding constant (Kb) and free energy of binding (ΔGb). Both
compounds showed red shift with CTAB due to incorporation of additives from solvent (polar) to micellar phase (non-polar). Kx and Kb have higher values for BTS due to its smaller size and less delocalization of negative charge. BPTS has less partitioning and binding capability due to larger size, more aromaticity, extensive delocalization and less charge density. It is, thus, concluded that bulky substituents and higher degree of delocalization on thiophene nucleus may decrease its ability to be solubilized. Acknowledgments This manuscript is a part of M.Phil thesis of Mr. Rizwan Saeed. All authors contributed at various stages of planning, execution and write up.
Scheme 4. Solubilization of (a) BPTS and (b) BTS by CTAB micelle.
394
R. Saeed et al. / Journal of Molecular Liquids 240 (2017) 389–394
References [1] O. Cudina, K.K. Rajic, R.I. Bugarcic, I. Jankovic, Colloids Surf. A Physicochem. Eng. Asp. 256 (2005) 225–232. [2] M.J. Rosen, J.T. Kunjappu, Surfactants and Interfacial Phenomenon, fourth ed. John Wiley & Sons, USA, 2012. [3] D. Attwood, Adv. Colloid Interf. Sci. 55 (1995) 271–303. [4] K. Behera, P. Mohapara, S. Patel, K. Mehera, J. Photochem. Photobio. A Chem. 169 (2005) 253–260. [5] S.K. Mehta, K.K. Bhasin, S. Dham, S. Kumar, Colloids Surf. A Physicochem. Eng. Asp. 278 (2006) 17–25. [6] B. Naseem, A. Sabri, A. Hasan, S.S. Shah, Colloids Surf. A Physicochem. Eng. Asp. 35 (2004) 7–13. [7] G. Barbarella, M. Melucci, G. Sotgiu, Adv. Mater. 17 (2005) 1581–1593. [8] M. Usman, M. Siddiq, J. Chem. Thermodyn. 58 (2013) 359–366. [9] M. Usman, M. Siddiq, Spectrochim. Acta A 113 (2013) 182–190. [10] M. Usman, M.A. Rashid, A. Mansha, M. Siddiq, Thermochim. Acta 573 (2013) 18–24. [11] S. Hanif, M. Usman, A. Hussain, N. Rasool, M. Zubair, U.A. Rana, J. Mol. Liq. 211 (2013) 7–14.
[12] K. Mahmood, M. Shakeel, M. Siddiq, M. Usman, Tenside Surfactant Deterg. 53 (2016) 195–204. [13] N. Haq, M. Usman, A. Mansha, U.A. Rana, M.A. Rashid, M. Majid, J. Mol. Liq. 196 (2014) 264–269. [14] M. Irfan, M. Usman, A. Mansha, N. Rasool, M. Ibrahim, U.A. Rana, M. Siddiq, M.Z. Haq, H.Z.E. Jaffar, Sci. World J. 2014 (2014) 1–8. [15] H. Kawamura, M. Manabe, Y. Miyamoto, Y. Fujita, S. Tokunaga, J. Phys. Chem. 93 (1989) 5536–5540. [16] S.W.H. Shah, K. Naeem, B. Naseem, S.S. Shah, Colloids Surf. A Physicochem. Eng. Asp. 331 (2008) 227–231. [17] M.F. Nazar, M. Abid, M. Danish, M. Ashfaq, A.M. Khan, M.N. Zafar, S. Mehmood, A. Asif, J. Mol. Liq. 212 (2015) 142–150. [18] M.F. Nazar, F. Mukhtar, S. Chaudry, M. Ashfaq, S. Mehmood, A. Asif, U.A. Rana, J. Mol. Liq. 200 (2014) 361–368. [19] O. Duman, S. Tunç, B. Kancı, Fluid Phase Equilib. 301 (2011) 56–61. [20] M.F. Nazar, F. Mukhtar, M. Ashfaq, H.M.A. Rahman, M.N. Zafar, S.H. Sumrra, Fluid Phase Equilib. 406 (2015) 47–54. [21] M. Younis, Organic Spectroscopy, Ilmi Kitab Khana, Lahore, Pakistan. [22] M.F. Nazar, S.S. Shah, M.A. Khosa, J. Surfactant Deterg. 13 (2010) 529–537.