Kunal Mali - Academia.edu (original) (raw)
Papers by Kunal Mali
The Journal of Physical Chemistry B, 2007
Rotational diffusion of a cationic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dio... more Rotational diffusion of a cationic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole, DMDPP has been examined in the surfactant-block copolymer system of sodium dodecyl sulfate (SDS) and poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123). In this study, the mole ratio of SDS to P123 was varied from 0 to 5 in steps of one unit, to investigate the role of electrostatic interactions and micellar hydration on solute rotation. It has been noticed that there is a significant enhancement in the average reorientation time of rhodamine 110, when [SDS]/[P123] increased from 0 to 1. This has been rationalized on the basis of migration of rhodamine 110 from the interfacial region of P123 micelles to the palisade layer (corona region) due to the electrostatic interaction with negatively charged head groups of SDS, whose tails are embedded in the polypropylene oxide core. Further increase in the mole ratio of SDS to P123 has resulted in only a marginal decrease in the average reorientation time of rhodamine 110, which is probably due to the solute molecule experiencing a microenvironment similar to the interfacial region of SDS micelles. In contrast, a gradual decrease has been observed in the average reorientation time of DMDPP with [SDS]/[P123], which is due to the increase in hydration levels in the palisade layer (corona region) of the micelle. These explanations are consistent with the structure of the SDS-P123 micellar system that has been deduced from neutron scattering and viscosity measurements recently.
Langmuir, 2007
In an attempt to understand the role of interfacial friction on solute rotation, fluorescence ani... more In an attempt to understand the role of interfacial friction on solute rotation, fluorescence anisotropy decays of a cationic solute, rhodamine 110, have been measured in polymorphic environments of a triblock copolymer, (PEO) 20-(PPO) 70-(PEO) 20 (P123) (PEO) poly(ethylene oxide), PPO) poly(propylene oxide)). It has been noticed that even though rhodamine 110 is located in the interfacial region of the micelles, sol-gel transition does not significantly influence its rotation. Micelle-micelle entanglement, which is responsible for gelation, persists even in the micellar solution phase, perhaps to a lesser degree, and this entanglement is responsible for the observed behavior. This hypothesis has been substantiated by undertaking concentration-dependent studies in which it is shown that the reorientation time of the solute increases with an increase in the micellar concentration. In the case of reverse micelles, it has been observed that an enhancement in the water content facilitates solute rotation, which has been rationalized on the basis of solute migration from the hydrated poly(ethylene oxide) region to the poly(ethylene oxide)-water interface within the core.
Langmuir, 2006
In an attempt to understand the nature of water present in the reverse phases of aggregates forme... more In an attempt to understand the nature of water present in the reverse phases of aggregates formed with the triblock copolymer poly(ethylene oxide) 20-poly(propylene oxide) 70-poly(ethylene oxide) 20 (P123) and also investigate how these confined environments influence the rates of photoisomerization, fluorescence lifetimes and quantum yields of a carbocyanine derivatives3,3′-diethyloxadicarbocyanine iodide (DODCI)swere measured in these systems over the temperature range of 293-318 K. Three different copolymer-oil-water compositions were chosen such that the mole ratio of water to copolymer (W) spans the range of 50-150. In these systems, butyl acetate was used as the oil or the nonpolar component. It has been noticed that in all three systems the fluorescence decays of DODCI comprise a long component whose contribution is 85-90%, and this has been ascribed to the fraction of solute solubilized in the core region where hydrated poly(ethylene oxide) units are present. A short-decay component is associated with the remaining fraction, and its values match with those measured in water, indicating that the water present in these reverse phases is in the form of droplets. The photoisomerization rate constants of DODCI located in the core regions of the reverse phases are identical in the three systems at a given temperature and similar to the ones obtained in normal phases of P123. The reasons for the observed behavior have been discussed.
Journal of Chemical Sciences, 2007
This article describes our ongoing efforts to understand dynamical processes such as rotational d... more This article describes our ongoing efforts to understand dynamical processes such as rotational diffusion and photoisomerization in polymorphic environments of a block copolymer. The objective is to explore how the typical properties of a block copolymer solution such as critical micelle temperature (CMT) and temperature-induced sol-gel transition influence the rotational diffusion of hydrophobic solute molecules. Rotational diffusion of solute molecules differs significantly below and above the CMT of a block copolymer solution, while there is no influence of sol-gel transition on solute rotation. This is rationalized on the basis of the site of solubilization of the solute molecules which is the palisade layer of the micelles in both phases and unaffected by gelation. A similar result has been obtained in case of photoisomerization studies carried out with a carbocyanine derivative in the sol and gel phases of the block copolymer. The isomerization studies have been extended to the reverse phases (sol and gel phases) of the block copolymer to explore the nature of the water present in the cores of the reverse micelles. Our results provide evidence for the existence of water droplets with properties resembling bulk water. In essence, we show that despite having vastly differing bulk properties, both the solution and gel phases (normal as well as reverse) offer identical microscopic environment.
The Journal of Chemical Physics, 2005
In an attempt to understand the nature of interactions between organic solutes and room temperatu... more In an attempt to understand the nature of interactions between organic solutes and room temperature ionic liquids, temperature-dependent rotational relaxation of two structurally similar nondipolar solutes--2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP)--has been examined in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6(-)]). Even with the ionic liquid, where the cation and the anion are strongly associated, the solute DPP experiences specific interactions, which is evident from its reorientation times that are 50%-60% longer in relation to DMDPP. It has been noticed that the reorientation times of both the solutes are faster in [bmim+][PF6(-)] than in glycerol, which is also a strongly associated solvent and whose viscosity is similar to the ionic liquid. This observation has been explained by taking into consideration the relative sizes of the solvents. By comparing the ratios of the reorientation times of DPP to DMDPP, in [bmim+][PF6(-)] and glycerol, it has been deduced that the strengths of the interaction between DPP-[bmim+][PF6(-)] and DPP-glycerol are the same.
The Journal of Chemical Physics, 2005
The Journal of Chemical Physics, 2007
Rotational diffusion of an ionic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo... more Rotational diffusion of an ionic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo͓3,4-c͔pyrrole ͑DMDPP͒ has been investigated in aqueous mixtures of cetyltrimethylammonium chloride ͑CTAC͒ and poly͑ethylene oxide͒ 20-poly͑propylene oxide͒ 70-poly͑ethylene oxide͒ 20 ͑P123͒. The purpose of this work is to understand how an increase in the mole ratio of surfactant to block copolymer from low to high influences the dynamics of ionic and neutral solute molecules. The variation in the mole ratio of CTAC to P123 from low to high has resulted in a drastic increase in the average reorientation time of rhodamine 110. In contrast, an exactly opposite trend has been noticed in the case of DMDPP. In the low mole ratio regime, rhodamine 110 and DMDPP are located at the interface and palisade layer, respectively, of P123 micelle-CTAC complexes. On the other hand, in the high mole ratio regime, both the probes are located in the Stern layer of CTAC-P123 complexes. The enhancement in the average reorientation time of rhodamine 110 with an increase in the mole ratio of surfactant to block copolymer has been rationalized on the basis of formation of rhodamine 110-Cl ion pair, which in turn associates with the cationic head groups of CTAC-P123 complexes. The observed decrease in the average reorientation time of DMDPP with an increase in the mole ratio of CTAC to P123 is a consequence of lower microviscosity of the Stern layer of CTAC-P123 complexes compared to the palisade layer of P123 micelle-CTAC complexes.
![Research paper thumbnail of Erratum: “Polyene photoisomerization rates: Are they distinct in aqueous block copolymer micellar solutions and gels?” [J. Chem. Phys. 124, 054904 (2006)]](https://attachments.academia-assets.com/96447096/thumbnails/1.jpg)
](The Journal of Chemical Physics, 2006
The Journal of Chemical Physics, 2006
Photoisomerization of 3,3Ј-diethyloxadicarbocyanine iodide ͑DODCI͒ has been investigated in water... more Photoisomerization of 3,3Ј-diethyloxadicarbocyanine iodide ͑DODCI͒ has been investigated in water, 5% and 30% aqueous triblock copolymer, poly͑ethylene oxide͒ 20 − poly͑propylene oxide͒ 70 − poly͑ethylene oxide͒ 20 ͑P123͒ by measuring the fluorescence quantum yields and lifetimes in the temperature range 293-318 K. Reports available in literature indicate that 5% aqueous P123 exists as micellar solution, whereas 30% aqueous P123 forms gel due to micelle-micelle entanglement. This study has been undertaken to find out how the polyene photoisomerization rates are influenced in the sol and gel phases. It has been observed that 60%-70% of DODCI is located in the palisade layer of the micelles in the sol as well as gel phases and the photoisomerization rate of this component is identical in both the phases at a particular temperature. The remainder of the probe is located in the interfacial region and isomerization rates of this fraction are slower by a factor of 1.4-1.1 in the gel phase compared with the micellar solution. The retardation of the isomerization rate in the gel phase has been explained on the basis of enhancement in the friction experienced by the probe due to micelle-micelle entanglement at the interface. Compared to the isomerization rates in water, the rates of photoisomerization of DODCI located in the palisade layer, interfacial region of micellar solution, and interfacial region of the micelles in the gel phase are slower by factors of 3.5, 1.5-1.9, and 2, respectively. The outcome of this study validates the point that in organized media photoisomerization rates are sensitive to the localized friction, which is not uniform unlike in a homogeneous solution.
The Journal of Chemical Physics, 2009
The role of cosurfactant and water on the interfacial fluidities of reverse micelles formed with ... more The role of cosurfactant and water on the interfacial fluidities of reverse micelles formed with the cationic surfactant, cetyltrimethylammonium bromide ͑CTAB͒ has been examined by measuring the fluorescence anisotropies of two structurally similar ionic solutes, rhodamine 110 and fluorescein. For this purpose, reverse micellar systems with ͑CTAB/1-pentanol/cyclohexane/water͒ and without a cosurfactant ͑CTAB/chloroform-isooctane/water͒ have been chosen. In this study, the mole ratio of water to surfactant W has been varied in the region of 4-25. Experimental results indicate that the average reorientation time of the probe, which is a measure of the fluidity near the interfacial region, decreases by a factor of 1.5 and 1.4 for rhodamine 110 and fluorescein, respectively, as W goes up from 5 to 25 in CTAB/1-pentanol/cyclohexane/water reverse micellar system. In contrast, the average reorientation time, remains invariant for both the probe molecules in CTAB/chloroformisooctane/water reverse micellar system despite an increase in W from 4 to 24. In case of CTAB/ 1-pentanol/cyclohexane/water reverse micellar system, the added water binds to bromide counter ions and also the hydroxyl groups of the cosurfactant, 1-pentanol, which results in an increase in the effective head group area. Such an increase in the effective head group area leads to a decrease in the packing parameter, and hence an increase in the interfacial fluidity. On the other hand, in CTAB/chloroform-isooctane/water system, the added water merely hydrates the bromide ions, thereby leaving the effective head group area unchanged. Thus, the interfacial fluidities remain invariant upon the addition of water in the absence of a cosurfactant.
The Journal of Chemical Physics, 2008
Photoisomerization of two cyanine derivatives, 3 , 3Ј-diethyloxadicarbocyanine iodide ͑DODCI͒ and... more Photoisomerization of two cyanine derivatives, 3 , 3Ј-diethyloxadicarbocyanine iodide ͑DODCI͒ and merocyanine 540 ͑MC 540͒, has been investigated in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate and aqueous glycerol ͑93 wt % glycerol +7 wt % water͒ by measuring fluorescence lifetimes and quantum yields. The aim of this work is to understand how the rates of photoisomerization of DODCI and MC 540 are influenced by specific solute-solvent interactions besides the viscosity of the medium. For DODCI, it has been observed that the nonradiative rate constants, which represent the rates of photoisomerization, are almost identical in the ionic liquid and aqueous glycerol at given temperature, indicating that viscosity is the sole parameter that governs the rate of photoisomerization. In contrast, the photoisomerization rate constants of MC 540 have been found to be a factor of 2 higher in aqueous glycerol compared to the ionic liquid. The observed behavior is due to the zwitterionic character of MC 540, a consequence of which, the twisted state gets stabilized by the solute-solvent hydrogen bonding interactions in aqueous glycerol, thus lowering the barrier for isomerization.
The Journal of Chemical Physics, 2008
Temperature dependent rotational diffusion of a nonpolar solute, 9-phenylanthracene (9-PA), and a... more Temperature dependent rotational diffusion of a nonpolar solute, 9-phenylanthracene (9-PA), and a dipolar solute, rhodamine 110 (R110), has been examined in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim(+)][PF(6) (-)]) and in a conventional solvent, glycerol. This study has been undertaken to explore how parameters such as solvent size and free volume influence solute rotation in the case of a nonpolar solute, 9-PA. To understand the role of specific solute-solvent interactions, similar measurements have been performed with a dipolar analogue, R110. It has been observed that the viscosity normalized reorientation times of 9-PA are longer by a factor of 1.4-1.6 in glycerol compared to those in [bmim(+)][PF(6) (-)]. While the most commonly used Stokes-Einstein-Debye hydrodynamic theory is not successful in explaining this experimental observation, Gierer-Wirtz and Dote-Kivelson-Schwartz quasihydrodynamic theories could rationalize this trend, albeit in a qualitative manner. Rotational diffusion of R110, on the other hand, follows an exactly opposite trend compared to 9-PA. The normalized reorientation times of R110 are longer by a factor of 1.3-1.4 in [bmim(+)][PF(6) (-)] compared to glycerol, which is due to the formation of stronger solute-solvent hydrogen bonds between the positively charged R110 and the ionic liquid.
The Journal of Physical Chemistry B, 2007
Rotational diffusion of a cationic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dio... more Rotational diffusion of a cationic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole, DMDPP has been examined in the surfactant-block copolymer system of sodium dodecyl sulfate (SDS) and poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123). In this study, the mole ratio of SDS to P123 was varied from 0 to 5 in steps of one unit, to investigate the role of electrostatic interactions and micellar hydration on solute rotation. It has been noticed that there is a significant enhancement in the average reorientation time of rhodamine 110, when [SDS]/[P123] increased from 0 to 1. This has been rationalized on the basis of migration of rhodamine 110 from the interfacial region of P123 micelles to the palisade layer (corona region) due to the electrostatic interaction with negatively charged head groups of SDS, whose tails are embedded in the polypropylene oxide core. Further increase in the mole ratio of SDS to P123 has resulted in only a marginal decrease in the average reorientation time of rhodamine 110, which is probably due to the solute molecule experiencing a microenvironment similar to the interfacial region of SDS micelles. In contrast, a gradual decrease has been observed in the average reorientation time of DMDPP with [SDS]/[P123], which is due to the increase in hydration levels in the palisade layer (corona region) of the micelle. These explanations are consistent with the structure of the SDS-P123 micellar system that has been deduced from neutron scattering and viscosity measurements recently.
Langmuir, 2007
In an attempt to understand the role of interfacial friction on solute rotation, fluorescence ani... more In an attempt to understand the role of interfacial friction on solute rotation, fluorescence anisotropy decays of a cationic solute, rhodamine 110, have been measured in polymorphic environments of a triblock copolymer, (PEO) 20-(PPO) 70-(PEO) 20 (P123) (PEO) poly(ethylene oxide), PPO) poly(propylene oxide)). It has been noticed that even though rhodamine 110 is located in the interfacial region of the micelles, sol-gel transition does not significantly influence its rotation. Micelle-micelle entanglement, which is responsible for gelation, persists even in the micellar solution phase, perhaps to a lesser degree, and this entanglement is responsible for the observed behavior. This hypothesis has been substantiated by undertaking concentration-dependent studies in which it is shown that the reorientation time of the solute increases with an increase in the micellar concentration. In the case of reverse micelles, it has been observed that an enhancement in the water content facilitates solute rotation, which has been rationalized on the basis of solute migration from the hydrated poly(ethylene oxide) region to the poly(ethylene oxide)-water interface within the core.
Langmuir, 2006
In an attempt to understand the nature of water present in the reverse phases of aggregates forme... more In an attempt to understand the nature of water present in the reverse phases of aggregates formed with the triblock copolymer poly(ethylene oxide) 20-poly(propylene oxide) 70-poly(ethylene oxide) 20 (P123) and also investigate how these confined environments influence the rates of photoisomerization, fluorescence lifetimes and quantum yields of a carbocyanine derivatives3,3′-diethyloxadicarbocyanine iodide (DODCI)swere measured in these systems over the temperature range of 293-318 K. Three different copolymer-oil-water compositions were chosen such that the mole ratio of water to copolymer (W) spans the range of 50-150. In these systems, butyl acetate was used as the oil or the nonpolar component. It has been noticed that in all three systems the fluorescence decays of DODCI comprise a long component whose contribution is 85-90%, and this has been ascribed to the fraction of solute solubilized in the core region where hydrated poly(ethylene oxide) units are present. A short-decay component is associated with the remaining fraction, and its values match with those measured in water, indicating that the water present in these reverse phases is in the form of droplets. The photoisomerization rate constants of DODCI located in the core regions of the reverse phases are identical in the three systems at a given temperature and similar to the ones obtained in normal phases of P123. The reasons for the observed behavior have been discussed.
Journal of Chemical Sciences, 2007
This article describes our ongoing efforts to understand dynamical processes such as rotational d... more This article describes our ongoing efforts to understand dynamical processes such as rotational diffusion and photoisomerization in polymorphic environments of a block copolymer. The objective is to explore how the typical properties of a block copolymer solution such as critical micelle temperature (CMT) and temperature-induced sol-gel transition influence the rotational diffusion of hydrophobic solute molecules. Rotational diffusion of solute molecules differs significantly below and above the CMT of a block copolymer solution, while there is no influence of sol-gel transition on solute rotation. This is rationalized on the basis of the site of solubilization of the solute molecules which is the palisade layer of the micelles in both phases and unaffected by gelation. A similar result has been obtained in case of photoisomerization studies carried out with a carbocyanine derivative in the sol and gel phases of the block copolymer. The isomerization studies have been extended to the reverse phases (sol and gel phases) of the block copolymer to explore the nature of the water present in the cores of the reverse micelles. Our results provide evidence for the existence of water droplets with properties resembling bulk water. In essence, we show that despite having vastly differing bulk properties, both the solution and gel phases (normal as well as reverse) offer identical microscopic environment.
The Journal of Chemical Physics, 2005
In an attempt to understand the nature of interactions between organic solutes and room temperatu... more In an attempt to understand the nature of interactions between organic solutes and room temperature ionic liquids, temperature-dependent rotational relaxation of two structurally similar nondipolar solutes--2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP)--has been examined in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6(-)]). Even with the ionic liquid, where the cation and the anion are strongly associated, the solute DPP experiences specific interactions, which is evident from its reorientation times that are 50%-60% longer in relation to DMDPP. It has been noticed that the reorientation times of both the solutes are faster in [bmim+][PF6(-)] than in glycerol, which is also a strongly associated solvent and whose viscosity is similar to the ionic liquid. This observation has been explained by taking into consideration the relative sizes of the solvents. By comparing the ratios of the reorientation times of DPP to DMDPP, in [bmim+][PF6(-)] and glycerol, it has been deduced that the strengths of the interaction between DPP-[bmim+][PF6(-)] and DPP-glycerol are the same.
The Journal of Chemical Physics, 2005
The Journal of Chemical Physics, 2007
Rotational diffusion of an ionic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo... more Rotational diffusion of an ionic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo͓3,4-c͔pyrrole ͑DMDPP͒ has been investigated in aqueous mixtures of cetyltrimethylammonium chloride ͑CTAC͒ and poly͑ethylene oxide͒ 20-poly͑propylene oxide͒ 70-poly͑ethylene oxide͒ 20 ͑P123͒. The purpose of this work is to understand how an increase in the mole ratio of surfactant to block copolymer from low to high influences the dynamics of ionic and neutral solute molecules. The variation in the mole ratio of CTAC to P123 from low to high has resulted in a drastic increase in the average reorientation time of rhodamine 110. In contrast, an exactly opposite trend has been noticed in the case of DMDPP. In the low mole ratio regime, rhodamine 110 and DMDPP are located at the interface and palisade layer, respectively, of P123 micelle-CTAC complexes. On the other hand, in the high mole ratio regime, both the probes are located in the Stern layer of CTAC-P123 complexes. The enhancement in the average reorientation time of rhodamine 110 with an increase in the mole ratio of surfactant to block copolymer has been rationalized on the basis of formation of rhodamine 110-Cl ion pair, which in turn associates with the cationic head groups of CTAC-P123 complexes. The observed decrease in the average reorientation time of DMDPP with an increase in the mole ratio of CTAC to P123 is a consequence of lower microviscosity of the Stern layer of CTAC-P123 complexes compared to the palisade layer of P123 micelle-CTAC complexes.
The Journal of Chemical Physics, 2006
The Journal of Chemical Physics, 2006
Photoisomerization of 3,3Ј-diethyloxadicarbocyanine iodide ͑DODCI͒ has been investigated in water... more Photoisomerization of 3,3Ј-diethyloxadicarbocyanine iodide ͑DODCI͒ has been investigated in water, 5% and 30% aqueous triblock copolymer, poly͑ethylene oxide͒ 20 − poly͑propylene oxide͒ 70 − poly͑ethylene oxide͒ 20 ͑P123͒ by measuring the fluorescence quantum yields and lifetimes in the temperature range 293-318 K. Reports available in literature indicate that 5% aqueous P123 exists as micellar solution, whereas 30% aqueous P123 forms gel due to micelle-micelle entanglement. This study has been undertaken to find out how the polyene photoisomerization rates are influenced in the sol and gel phases. It has been observed that 60%-70% of DODCI is located in the palisade layer of the micelles in the sol as well as gel phases and the photoisomerization rate of this component is identical in both the phases at a particular temperature. The remainder of the probe is located in the interfacial region and isomerization rates of this fraction are slower by a factor of 1.4-1.1 in the gel phase compared with the micellar solution. The retardation of the isomerization rate in the gel phase has been explained on the basis of enhancement in the friction experienced by the probe due to micelle-micelle entanglement at the interface. Compared to the isomerization rates in water, the rates of photoisomerization of DODCI located in the palisade layer, interfacial region of micellar solution, and interfacial region of the micelles in the gel phase are slower by factors of 3.5, 1.5-1.9, and 2, respectively. The outcome of this study validates the point that in organized media photoisomerization rates are sensitive to the localized friction, which is not uniform unlike in a homogeneous solution.
The Journal of Chemical Physics, 2009
The role of cosurfactant and water on the interfacial fluidities of reverse micelles formed with ... more The role of cosurfactant and water on the interfacial fluidities of reverse micelles formed with the cationic surfactant, cetyltrimethylammonium bromide ͑CTAB͒ has been examined by measuring the fluorescence anisotropies of two structurally similar ionic solutes, rhodamine 110 and fluorescein. For this purpose, reverse micellar systems with ͑CTAB/1-pentanol/cyclohexane/water͒ and without a cosurfactant ͑CTAB/chloroform-isooctane/water͒ have been chosen. In this study, the mole ratio of water to surfactant W has been varied in the region of 4-25. Experimental results indicate that the average reorientation time of the probe, which is a measure of the fluidity near the interfacial region, decreases by a factor of 1.5 and 1.4 for rhodamine 110 and fluorescein, respectively, as W goes up from 5 to 25 in CTAB/1-pentanol/cyclohexane/water reverse micellar system. In contrast, the average reorientation time, remains invariant for both the probe molecules in CTAB/chloroformisooctane/water reverse micellar system despite an increase in W from 4 to 24. In case of CTAB/ 1-pentanol/cyclohexane/water reverse micellar system, the added water binds to bromide counter ions and also the hydroxyl groups of the cosurfactant, 1-pentanol, which results in an increase in the effective head group area. Such an increase in the effective head group area leads to a decrease in the packing parameter, and hence an increase in the interfacial fluidity. On the other hand, in CTAB/chloroform-isooctane/water system, the added water merely hydrates the bromide ions, thereby leaving the effective head group area unchanged. Thus, the interfacial fluidities remain invariant upon the addition of water in the absence of a cosurfactant.
The Journal of Chemical Physics, 2008
Photoisomerization of two cyanine derivatives, 3 , 3Ј-diethyloxadicarbocyanine iodide ͑DODCI͒ and... more Photoisomerization of two cyanine derivatives, 3 , 3Ј-diethyloxadicarbocyanine iodide ͑DODCI͒ and merocyanine 540 ͑MC 540͒, has been investigated in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate and aqueous glycerol ͑93 wt % glycerol +7 wt % water͒ by measuring fluorescence lifetimes and quantum yields. The aim of this work is to understand how the rates of photoisomerization of DODCI and MC 540 are influenced by specific solute-solvent interactions besides the viscosity of the medium. For DODCI, it has been observed that the nonradiative rate constants, which represent the rates of photoisomerization, are almost identical in the ionic liquid and aqueous glycerol at given temperature, indicating that viscosity is the sole parameter that governs the rate of photoisomerization. In contrast, the photoisomerization rate constants of MC 540 have been found to be a factor of 2 higher in aqueous glycerol compared to the ionic liquid. The observed behavior is due to the zwitterionic character of MC 540, a consequence of which, the twisted state gets stabilized by the solute-solvent hydrogen bonding interactions in aqueous glycerol, thus lowering the barrier for isomerization.
The Journal of Chemical Physics, 2008
Temperature dependent rotational diffusion of a nonpolar solute, 9-phenylanthracene (9-PA), and a... more Temperature dependent rotational diffusion of a nonpolar solute, 9-phenylanthracene (9-PA), and a dipolar solute, rhodamine 110 (R110), has been examined in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim(+)][PF(6) (-)]) and in a conventional solvent, glycerol. This study has been undertaken to explore how parameters such as solvent size and free volume influence solute rotation in the case of a nonpolar solute, 9-PA. To understand the role of specific solute-solvent interactions, similar measurements have been performed with a dipolar analogue, R110. It has been observed that the viscosity normalized reorientation times of 9-PA are longer by a factor of 1.4-1.6 in glycerol compared to those in [bmim(+)][PF(6) (-)]. While the most commonly used Stokes-Einstein-Debye hydrodynamic theory is not successful in explaining this experimental observation, Gierer-Wirtz and Dote-Kivelson-Schwartz quasihydrodynamic theories could rationalize this trend, albeit in a qualitative manner. Rotational diffusion of R110, on the other hand, follows an exactly opposite trend compared to 9-PA. The normalized reorientation times of R110 are longer by a factor of 1.3-1.4 in [bmim(+)][PF(6) (-)] compared to glycerol, which is due to the formation of stronger solute-solvent hydrogen bonds between the positively charged R110 and the ionic liquid.