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Microheterogeneous mediated electron transfer reaction (ETR) of surfactant cobalt(III) complexes by Fe2+: Effect of pyridine substituent as co ligand
⁎Corresponding author. Tel.: +91 9677836849. naturalnagaraj@gmail.com (K. Nagaraj)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The outer sphere electron transfer reaction of surfactant cobalt(III) complexes, Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 1, Cis-[Co(trien)(4CNP)(C12H25NH2)](ClO4)3 2 and Cis-[Co(trien)(4AMP)(C12H25NH2)](ClO4)3 3 (en: ethylenediamine, trien: triethylenetetramine, 4CNP: 4-cyanopyridine, 4AMP: 4-aminopyridine, C12H25NH2: dodecylamine) have been investigated by Fe2+ ion in liposome vesicles (DPPC) and ionic liquids medium at different temperatures under pseudo first order conditions using an excess of the reductant. In the presence of ionic liquid medium the second order rate constant for this electron transfer reaction was found to increase with increasing concentration of ionic liquids. Below the phase transition temperature of DPPC, the rate decreased with increasing concentration of DPPC, while above the phase transition temperature the rate increased with increasing concentration of DPPC for the same complexes has also been studied. Experimentally the reactions were found to be second order and the electron transfer postulated as outer sphere. The results have been discussed in terms of increased hydrophobic effect, self aggregation and the presence of pyridine ligand containing 4-amino and 4-cyano substituent.
Keywords
Surfactant cobalt(III) complex
Vesicles
Electron transfer
Hydrophobicity
Ionic liquids
Aggregation
1 Introduction
Recently there has been a growing interest in the study of electron transfer reactions in microheterogeneous environment (Rodriguez et al., 1996; Rossi and Liveri, 2009; Weidemaier and Fayer, 1996; Imonigie and Macartney, 1993; Olson et al., 1997), where one or more of the reactants are forced to remain at the surface of micelles (Panigrahi and Sahu, 2004; Das et al., 2001), or in the cavity of cyclodextrins and related compounds, or at the surface of DNA (Sanchez et al., 1998; Jimenez et al., 1997), etc. These studies are of interest because (i) the local concentrations of the reactants can be changed to allow the tuning of the reaction rates, and (ii) the properties of local media can be adjusted to modify the reactivity. The use of micelles as rate promoters or inhibitors in electron transfer reactions has been recently reviewed by Prado-Gotor et al. (2001). The purpose in using micelles in electron transfer reactions is to ascertain how these reactions might behave in biological systems for which micelles are fitting mimicks (Alkaitis et al., 1975; Patterson and Gratzel, 1975; Ponganis et al., 1980). Micelles can also have a major influence on photochemical reactions (Grand and Hautecloque, 1990). An interesting twist in using micelles in electron transfer reactions is to slow rates so that they might be measured conveniently.
A number of studies have been devoted to the understanding of the principles governing the interaction of surfactants with simplified membrane models as phospholipid bilayers. This interaction leads to the breakdown of lamellar structures and the formation of lipid-surfactant mixed micelles. The action of surfactants on the phospholipid bilayer leads to the incorporation of surfactant molecules into these structures. Due to the partition equilibrium between the bilayers and the aqueous phase, this incorporation involves complex perturbations in the physical properties of vesicle membranes, which depend upon the type and amount of surfactant. Electron transfer in these restricted geometry systems attracts great deal of interest because of their potential to prolong lifetime of charge transfer states, a goal of electron transfer studies aiming to utilize solar energy (London Singh, 2012). In recent years, ionic liquids have attracted considerable interest (Heintz, 2005; Isabel et al., 2008; Zhang, 2006), since these compounds have found several applications in catalysis, organic synthesis, electro chemistry, sensing material, lubrication and solar cell, etc (Dupont et al., 2002; Sun et al., 2009). As a solvent, the self-assembly of some surfactants or block copolymers in ionic liquids has been investigated (Anderson et al., 2003; Tran and Yu, 2005). Microemulsions containing hydrophilic and hydrophobic ionic liquids are also prepared (Li et al., 2004; Gao et al., 2005). Some researchers have found that some ionic liquids behave as surfactants owing to the long hydrophobic substituent alkyl groups on the cation. The surface active characteristic of ionic liquids has been the cause of extensive concern recently (Vanyur et al., 2007; Dong et al., 2007). Electron transfer reactions involving cobalt(III) complexes are very well known because the kinetics of reduction of octahedral cobalt(III) complexes is mostly free from complications arising due to reversible electron transfer, aquation, substitution, and isomerization reactions. Diebler and Taube, Watts et al. and other researchers (Diebler and Taube, 1965; Nejo et al., 2002; Espenson, 1982; Haim, 1970) have studied the kinetics and mechanism of reduction of cobalt(III) complexes by Fe2+aq in aqueous and non-aqueous media.
Micelle forming properties and electron transfer reactions of many of these types of surfactant metal complexes have been studied in our laboratories for a long-time (Nagaraj and Arunachalam, 2013a, 2013b, 2013c). In previous papers (Nagaraj and Arunachalam, 2013a, 2013b, 2013c), we have reported the influence of temperature on the behavior of some surfactant cobalt(III) complexes in micelles and β-cyclodextrin solution. In continuation of our works on the same surfactant cobalt(III) complexes in microheterogeneous (liposome vesicles (DPPC) and ionic liquids ((BMIM)Br) media. In these studies we focus on the effect of presence of a more hydrophobic and rigid ligand, triethylenetetramine in the complex on the self aggregation and electron transfer properties compared to those of less rigid and hydrophobic ligand, ethylenediamine, in the complex. Also, the effect of surfactant cobalt(III) complexes containing the pyridine co-ligand as 4-amino and 4-cyano substituent.
2 Experimental
2.1 Materials and methods
All reagents were of analytical grade (Sigma-Aldrich and Merck). Ultra pure water, obtained by deionizing distilled water using a Milli-Q reagent grade water system, was used for preparative work and to make up solutions for all physical measurements. The liposome (dipalmitoylphosphotidylcholine), ionic liquids (1-butyl-3-methylimidazoliumbromide) and dodecylamine were purchased from Sigma-Aldrich Chemical Co. (Bangalore, India) and were used as such. To prepare buffer solutions sodium phosphate dibasic anhydride and sodium dihydrogen orthophosphate were used.
2.2 Preparation of oxidant/reductant
The surfactant cobalt(III) complexes, 1, 2 and 3 used as oxidants were prepared as reported earlier (Nagaraj and Arunachalam, 2013a, 2013b, 2013c). A stock solution of Fe2+ was prepared by dissolving pure iron powder in slight excess of perchloric acid. The concentration of Fe2+ was determined by a method similar to that reported in the literature (Cannon and Gardiner, 1972), and the ionic strength of the solution was adjusted by the addition of sodium per chlorate solution. The structure of surfactant cobalt(III) complexes is shown in Chart 1.
Structure of cis-surfactant cobalt(III) complexes, 1–3.
2.3 Preparation of liposome vesicles (DPPC)
Unilamellar vesicles (ULV) were prepared by ethanol injection method (Batzri and Korn, 1973). This solvent injection method is suitable for preparing SUV or LUV. For our studies only unilamellar vesicles (ULV) were used due to minimum scattering interference obtained. The volume of ethanol injected is always less than 1% v/v in order to avoid any damage to the liposome by ethanol. The lipid was dissolved in organic solvents (ethanol) and injected rapidly with the help of a fine needle to the 10 ml of buffer maintained at 55–65 °C with continuous stirring using magnetic stirrer.
2.4 Kinetic measurements
The rate of the reaction was measured spectrophotometrically using a Shimadzu-1800 UV–Visible spectrophotometer equipped with the water Peltier system. The temperature was controlled within ±0.01 °C. For kinetic measurements the reactant solution containing the desired concentration of ionic liquid/liposome and surfactant cobalt(III) complex, NaClO4, and HClO4, omitting Fe2+, in oxygen free water was placed in a 1 cm cell which was then covered with a serum cap fitted with a syringe needle. This cell was placed in a thermostated compartment in the spectrophotometer and then the solution containing Fe2+ was added anaerobically using the syringe. The decrease in the absorbance at the characteristic absorption maxima of the surfactant cobalt(III) complexes (λmax = 490 nm), as a function of time was taken into account to follow kinetics of electron transfer between the oxidant and reductant. The rate law for the Fe(II) reduction of the cobalt(III) complex may be defined by −d[Co(III)]/dt = k[Co(III)][Fe(II)] and the second order rate constant, k, was calculated from the concentration of Fe(II) and the slope of the linear plot of log (At–Aα) versus time, which is equal to −k[Fe(II)/2.303, where At is the absorbance at time, Aα is the absorbance after all the cobalt(III) complex has been reduced to cobalt(II), and k is the second order rate constant. Usually the value of Aα was measured at times corresponding to ten half-lives. All the first order plots were substantially linear for at least five half-lives. Each rate constant reported was the average result of triplicate runs. Rate constants obtained from successive half-life values within a single run agreed to within ±5%.
2.5 Stoichiometry
The stoichiometry of the reaction was determined by estimating the Fe(III) and Co(II) present in the product mixture. Fe(III) was determined spectrophotometrically by (Kitson, 1950) as the thiocyanate complex having a maximum absorption at 490 nm by reference to the calibration curve, and Co(II) was determined (Link, 1970) as [CoCl4]2− at 690 nm in an excess of hydrochloric acid. The ratio of Fe(III) to Co(II) was found to be 1:1 in the reactions studied, specify 1:1 stoichiometry.
3 Results and discussion
3.1 Electron-transfer kinetics
The reduction of each surfactant cobalt(III) complex, cis-[Co(LL)2(A)(DA)]3+ by Fe2+, proceeds according to the overall reaction as indicated below:
This reaction is postulated as outer-sphere in comparison with such type of reactions in the literature (Miralles et al., 1982) involving ordinary lower primary amine coordinated cobalt(III) complexes similar to the surfactant cobalt(III) complexes of the present study. The complexes of the present study are inert to substitution due to the non-availability of a co-ordination site for inner-sphere precursor complex. Only a non-bridging intermediate is expected from such reactions that are inert to substitution. Already literature reports (Hak et al., 2003; Ghosh et al., 2006) on similar type of complexes supported only outer-sphere redox pathway. The most favorable mechanism for this outer-sphere electron transfer process consists of three elementary steps (Scheme 1): ion pair formation (kip), electron transfer (ket), and product successor dissociation. Accordingly the mechanism is delineated in Scheme 1.
Mechanism of the outer sphere electron transfer reaction.
3.2 Influence of electron transfer in microheterogenous media
3.2.1 Effect of liposome vesicles (DPPC)
The unique ability of liposomes to entrap drugs both in an aqueous and a lipid phase make such delivery systems attractive for hydrophilic and hydrophobic drugs. Because of its amphiphilic nature, DPPC (dipalmitoylphosphotidylcholine) undergoes spontaneous aggregation in aqueous solutions. This leads to the formation of a three-dimensional closed bilayer structure called vesicles (New, 1990). The hydrocarbon chains of DPPC undergo an abrupt transition from a highly ordered, tightly packed arrangement, the so-called gel state, to one which is less ordered and less tightly packed, the liquid-crystalline state, when the temperature is raised (Subuddhi and Mishra, 2006).
The effect of DPPC vesicles on the kinetics of outer-sphere electron transfer between the surfactant cobalt(III) complexes and Fe2+ have been investigated at various temperatures. As ethanol injection method was used, the reaction medium of these electron transfer reactions should contain only unilamellar vesicles. In the presence of these unilamellar vesicles also the outer-sphere electron transfer proceeds with second-order kinetics. The rate constants are given in table (Table 1 and SI Tables 1 and 2) and plots of k against [DPPC] are shown in Fig. 1 and SI Figs. 1 and 2. Two trends have been observed in the behavior of the rate constants with concentration of DPPC. Below the phase transition temperature, the rate constants decrease with increasing [DPPC], whereas above this temperature the rate constants increase with [DPPC]. These trends were observed for the surfactant cobalt(III) complexes used in the present study. It is well known that when a surfactant is added to an aqueous medium containing lipid membranes, the interaction between surfactants and lipids takes place in three ways: part of the added surfactant inserts into the outer membrane leaflet; the surfactant molecules equilibrate between the outer and inner leaflets of the vesicle; and the inner leaflet equilibrates with the interior of the vesicle. Below the phase transition temperature, the lipid is very rigid, so the surfactant cobalt(III) complex molecules are tightly bound to the membrane DPPC, mostly at the inner membrane leaflet. As the concentration of DPPC is increased, more of the surfactant cobalt(III) complex molecules will be accumulated into the DPPC interior, whereas Fe2+ ions will be at the outer surface, so the rate constant decreases (Scheme 2).
[DPPC] × 105 (mol dm−3)
k × 103, dm3 mol−1 s−1
298 K
303 K
308 K
323 K
328 K
333 K
2.0
5.8
6.9
7.8
5.1
5.5
7.0
2.5
5.4
5.7
6.9
5.8
6.2
7.4
3.0
3.8
4.3
4.8
6.1
6.9
7.8
3.5
3.6
4.2
4.6
6.9
7.2
8.0
4.0
2.7
3.0
3.3
7.5
8.0
9.3
4.5
2.5
2.9
3.1
7.9
8.2
9.7
5.0
1.5
1.9
2.7
8.1
8.4
10.5
5.5
0.8
1.2
2.0
8.5
9.0
10.9
![Plot of k against [DPPC] for Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 under various temperatures; Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 = 4 × 10−4 mol dm−3, μ = 1.0 mol dm−3, [Fe2+] = 0.01 mol dm−3.](/content/184/2019/12/8/img/10.1016_j.arabjc.2014.11.062-fig3.png)
Plot of k against [DPPC] for Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 under various temperatures; Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 = 4 × 10−4 mol dm−3, μ = 1.0 mol dm−3, [Fe2+] = 0.01 mol dm−3.
Disposition of phospholipids diacyl chain.
But beyond the phase transition temperature, the rigidity of the DPPC membrane is low; so when the concentration of DPPC is increased, more of the surfactant cobalt(III) complex molecules will move from the membrane interior to the outer surface where the concentration of Fe2+ ion is also high, causing the rate constant to increase. So the main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between vesicles surface and hydrophobic part of the surfactant complexes. In these trends, on comparing the second order rate constant of surfactant cobalt(III) complexes 1 and 2, the complex 2 containing triethylenetetramine ligand the second order rate constant is higher compared to a similar type of complex 1 containing ethylenediamine ligand in all the initial concentrations studied. This is due to lower CMC value for triethylenetetramine containing complex. Increasing hydrophobicity and rigidity of the amine ligand decreases the CMC value of complex 2 which facilitates the formation of micelles, hence an increase in the rate constant. The second order rate constant value for the outer-sphere electron transfer reaction between the complex 3 containing 4-aminopyridine ligand and Fe2+, is found to be remarkably greater compared to that of complex 2 containing 4-cyanopyridine ligand in all the initial concentrations studied. This small difference in the value of second order rate constant may be ascribed due to the fact that the electron releasing amino substituent makes Co3+ ion in complex 3, electron rich center thereby increases the size of 3d orbital of oxidant, thus increases the effective orbital interaction between the oxidant and the reductant leading to the enhancement in the rate of electron transfer; on the other hand, the electron withdrawing cyano substituent makes Co3+ ion in complex 2, electron deficient center thereby reduces the size of 3d orbital of oxidant, thus reduces the orbital interaction between the oxidant and the reductant leading to the lower rate. In our previous work (London Singh, 2012; Heintz, 2005; Dong et al., 2007) we have carried out the kinetics of the electron transfer reaction between the surfactant cobalt(III) complexes and Fe2+ ions in micelles. Comparing with the rate constants of these reactions in micelles rate constants of the same reactions in DPPC media are higher for each complex. This is because the double fatty acid chain DPPC gives the molecule an overall tubular shape due to the intervesicular hydrophobic interaction between vesicles surface and hydrophobic part of the surfactant complexes more suitable for vesicle aggregation which facilitates higher rate and lower activation energy. So the extent of aggregations will be higher in liposome vesicles which enhance the overall rate of the reactions.
3.3 Effect of ionic liquids
Due to their unusual and interesting properties, the role of ionic liquids (ILs) (Werner et al., 2003), as additives in modifying properties of aqueous surfactant systems (Fletcher and Pandey, 2004; Merrigan et al., 2000) may turn out to be crucial. In our recent investigations we have clearly demonstrated the effectiveness of ILs in altering the key physicochemical properties of aqueous surfactant systems composed of cationic surfactants. The effectiveness of IL in changing the properties of an aqueous surfactant system depends in major part to the kind and extent of interaction(s) between the cation/anion of the IL and surfactant head-group. The unique advantage of ionic liquids is that their physical and chemical properties can be readily adjusted by suitable selection of cation, anion, and the substituents of cation. Aggregates such as micelles, liquid crystals and microemulsions formed in ionic liquids have been widely studied recently (Benson and Haim, 1965). The effect of presence of ionic liquids [BMIM]Br in the medium on the kinetics of outer-sphere electron transfer between the surfactant cobalt(III) complexes of the present study with Fe2+ ion have been investigated at various temperatures. The observed second order rate constants are given in tables (Table 2 and SI Tables 3 and 4) and the plots of k against various concentrations of ionic liquid, [BMIM]Br in Fig. 2 and SI Figs. 3 and 4 at 303, 308, 313, 318, 323 and 328 K. As seen from these tables the rate constant of each of the reaction goes on increasing with increase in the concentration of ionic liquid in the medium from 1.4 × 10−3 mol dm−3 to 2.6 × 10−3 mol dm−3. As the cation of the ionic liquid used has an inherent amphiphilicity it can interact with the long aliphatic chain of the surfactant cobalt(III) complexes of the present study thereby the ionic liquid facilitates some more aggregation of the surfactant cobalt(III) complexes. This aggregation leads to higher local concentration of reactants leading to increase in the rate of the reaction. Hence the rate of the outer sphere electron transfer reaction of the present study increases with increase in the concentration of the ionic liquid (see Scheme 3).
[(BMIM)Br] × 104, mol dm−3
k × 102, dm3 mol−1 s−1
303 K
308 K
313 K
318 K
323 K
328 K
1.4
2.0
3.0
4.0
5.0
8.0
13.3
1.6
3.0
3.5
4.5
6.8
11.9
17.7
1.8
3.5
4.0
5.0
8.1
16.6
22.5
2.0
4.0
4.5
7.0
14.8
20.1
24.0
2.2
4.5
6.0
12.6
22.0
25.0
30.3
2.4
5.2
9.0
15.4
26.0
32.0
46.0
2.6
8.5
12.0
17.6
35.0
48.0
71.0
![Plot of k against [BMIM]Br for Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 at various temperatures; Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 = 4 × 10−4 mol dm−3, μ = 1.0 mol dm−3, [Fe2+] = 0.01 mol dm−3.](/content/184/2019/12/8/img/10.1016_j.arabjc.2014.11.062-fig5.png)
Plot of k against [BMIM]Br for Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 at various temperatures; Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 = 4 × 10−4 mol dm−3, μ = 1.0 mol dm−3, [Fe2+] = 0.01 mol dm−3.
Aggregation behavior in ionic liquids.
3.4 Activation parameters and isokinetic plots (ΔS# and ΔH#)
The effect of temperature on rate was studied at six different temperatures for each concentration of ionic liquids and liposome vesicles in order to obtain the activation parameters for the reaction between surfactant cobalt(III) complexes with Fe2+. From the transition state theory (Arulsamy et al., 2001), making use of Eyring equation,
The values of ΔS# and ΔH# were determined by plotting ln(k/T) versus 1/T and the plots are shown in Figs. 3 and 4 and SI Figs. 5–7. The ΔS# and ΔH# values obtained are shown in Table 3 and SI Tables 5–9. As seen from these tables the values of ΔH# are positive for all the reactions (in ionic liquid and liposome vesicles), indicating that the formation of activated complex is endothermic. All these concentrations of DPPC and ionic liquids, the ΔS≠ values are negative, which indicates a more ordered structure in the transition state. In such a case, the transition state complex attracts the surrounding water molecules, the positive and negative charges of the ion pair leading to loss of freedom of the solvent molecules in the transition state (electrostriction). This may be due to the binding of cobalt(III) complexes and Fe2+ ion to the DPPC in the transition state. The binding of cobalt complexes with hydrophobic ligand phenanthroline and bipyridine discourages solvation of the transition state around the ion pair thereby facilitating more freedom of the solvent molecules in the transition state. When we plot of activation enthalpy versus activation entropy values for the reactions in ionic liquids and liposome vesicles we get straight lines (isokinetic plot Figs. 5 and 6 and SI Figs. 8–11) which indicates that a common mechanism exists in all these media (see Table 4). KB = 1.380 6488 × 10−23 J K−1.![Eyring plot for complexes 1, 2 and 3 in [DPPC] medium at below phase transition temperature. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3; [DPPC] = 4.0 × 10−5 mol dm−3.](/content/184/2019/12/8/img/10.1016_j.arabjc.2014.11.062-fig7.png)
Eyring plot for complexes 1, 2 and 3 in [DPPC] medium at below phase transition temperature. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3; [DPPC] = 4.0 × 10−5 mol dm−3.
3 in [BMIM]Br medium. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.](/content/184/2019/12/8/img/10.1016_j.arabjc.2014.11.062-fig8.png)
Eyring plot for Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 in [BMIM]Br medium. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.
[DPPC] × 105 (mol dm−3)
ΔH‡ (kJ mol−1)
ΔS‡ (J K−1)
2.0
0.35
−197.2
2.5
0.57
−188.5
3.0
1.11
−180.2
3.5
1.91
−152.6
4.0
2.97
−134.9
4.5
3.64
−92.3
5.0
5.32
−64.4
5.5
6.41
−17.4
3 by ion(II) in [DPPC] solutions. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.](/content/184/2019/12/8/img/10.1016_j.arabjc.2014.11.062-fig9.png)
Isokinetic plot of the activation parameters for the reduction of Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 by ion(II) in [DPPC] solutions. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.
3 by ion(II) in [BMIM]Br medium. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.](/content/184/2019/12/8/img/10.1016_j.arabjc.2014.11.062-fig10.png)
Isokinetic plot of the activation parameters for the reduction of Cis-[Co(en)2(4CNP)(C12H25NH2)](ClO4)3 by ion(II) in [BMIM]Br medium. [Complex] = 4 × 10−4 mol dm−3; [Fe2+] = 0.01 mol dm−3; [μ] = 1.0 mol dm−3.
[(BMIM)Br] × 103 (mol dm−3)
ΔH‡ (kJ mol−1)
ΔS‡ (J K−1)
1.4
6.81
−50.1
1.6
6.93
−46.9
1.8
7.42
−34.1
2.0
7.56
−26.8
2.2
7.86
−16.8
2.4
8.28
−4.2
2.6
8.54
8.8
4 Conclusion
The effect of ionic liquids and liposome vesicles may have enormous potential as far as modifying the rate of electron transfer reactions of surfactant cobalt(III) complexes in aqueous solutions is concerned. In liposome media we have noticed two types of trends in the behavior of the reaction. It reveals that below the phase transition temperature the rate decreases with increasing concentration of DPPC, which is explained by the accumulation of these surfactant-cobalt(III) complexes inside the vesicles through hydrophobic effects. Above the phase transition temperature, the rate increased with increasing concentration of DPPC, which may be due to release of the surfactant cobalt(III) cobalt(III) complexes from the interior to the exterior surface of the DPPC membrane. The results demonstrate the presence of strong hydrophobic and electrostatic attractive interactions between the ions of ILs and surfactant complex is to be the major reason for these observations. It is noted that the changes in the properties of surfactant complex upon IL addition depend strongly on the nature of the surfactant head group. In ionic liquids media the rate constant increases with increase in concentration of ionic liquid which show that amphiphilicity of surfactant complex and ionic liquids interaction enhance the rate with increasing concentration of ionic liquids.
Acknowledgments
We are grateful to the UGC-COSIST and DST-FIST programmes of the Department of Chemistry, Bharathidasan University, and UGC – RFSMS fellowship to one of the authors, K. Nagaraj, by Bharathidasan University. Financial assistance from the CSIR (Grant No. 01(2461)/11/EMR-II), DST (Grant No. SR/S1/IC-13/2009) and UGC (Grant No. 41-223/2012(SR) sanctioned to S. Arunachalam is also gratefully acknowledged.
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Appendix A
Supplementary material
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2014.11.062.
Appendix A
Supplementary material
Supplementary data 1
Supplementary data 1