Physical Properties Of Ionic Liquids Research Articles

Nanostructure of Locally Concentrated Ionic Liquids in the Bulk and at Graphite and Gold Electrodes.

The physical properties of ionic liquids (ILs) have led to intense research interest, but for many applications, high viscosity is problematic. Mixing the IL with a diluent that lowers viscosity offers a solution if the favorable IL physical properties are not compromised. Here we show that mixing an IL or IL electrolyte (ILE, an IL with dissolved metal ions) with a nonsolvating fluorous diluent produces a low viscosity mixture in which the local ion arrangements, and therefore key physical properties, are retained or enhanced. The locally concentrated ionic liquids (LCILs) examined are 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM TFSI), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIM FAP), or 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (BMIM FAP) mixed with 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE) at 2:1, 1:1, and 1:2 (w/w) IL:TFTFE, as well as the locally concentrated ILEs (LCILEs) formed from 2:1 (w/w) HMIM TFSI-TFTFE with 0.25, 0.5, and 0.75 m lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). Rheology and conductivity measurements reveal that the added TFTFE significantly reduces viscosity and increases ionic conductivity, and cyclic voltammetry (CV) reveals minimal reductions in electrochemical windows on gold and carbon electrodes. This is explained by the small- and wide-angle X-ray scattering (S/WAXS) and atomic force microscopy (AFM) data, which show that the local ion nanostructures are largely retained in LCILs and LCILEs in bulk and at gold and graphite electrodes for all potentials investigated.

Predictive understanding of the surface tension and velocity of sound in ionic liquids using machine learning.

Knowledge of the physical properties of ionic liquids (ILs), such as the surface tension and speed of sound, is important for both industrial and research applications. Unfortunately, technical challenges and costs limit exhaustive experimental screening efforts of ILs for these critical properties. Previous work has demonstrated that the use of quantum-mechanics-based thermochemical property prediction tools, such as the conductor-like screening model for real solvents, when combined with machine learning (ML) approaches, may provide an alternative pathway to guide the rapid screening and design of ILs for desired physiochemical properties. However, the question of which machine-learning approaches are most appropriate remains. In the present study, we examine how different ML architectures, ranging from tree-based approaches to feed-forward artificial neural networks, perform in generating nonlinear multivariate quantitative structure-property relationship models for the prediction of the temperature- and pressure-dependent surface tension of and speed of sound in ILs over a wide range of surface tensions (16.9-76.2 mN/m) and speeds of sound (1009.7-1992m/s). The ML models are further interrogated using the powerful interpretation method, shapley additive explanations. We find that several different ML models provide high accuracy, according to traditional statistical metrics. The decision tree-based approaches appear to be the most accurate and precise, with extreme gradient-boosting trees and gradient-boosting trees being the best performers. However, our results also indicate that the promise of using machine-learning to gain deep insights into the underlying physics driving structure-property relationships in ILs may still be somewhat premature.

(Physical and Analytical Electrochemistry Division Max Bredig Award In Molten Salt and Ionic Liquid Chemistry) Of Ionic Liquids and Hydrogen Bonds

Whether it is the solubility of biopolymers, the rates of reactions, the physical properties of ionic liquids, or the formation of Deep Eutectic Solvents, the concept of the hydrogen bond often finds itself at the centre of explanations of ionic liquids’ behaviours. How strange it would seem to someone entering the field today that the idea that hydrogen bonds could form in ionic liquids was even recently controversial. So, how did this transformation come about?In this presentation, you will meet some of the most well-known names in ionic liquids, both past and present. Key insights have only come from a variety of different techniques, both experimental and theoretical, being brought to bear on the question of whether hydrogen bonds exist or are of any use in ionic liquids and their solutions. You will also see that opposing views can turn out to both be correct and controversies are not always solved by one side ‘winning’ over the other.

Simulation of CO2 Capture Process in Flue Gas from Oxy-Fuel Combustion Plant and Effects of Properties of Absorbent

Oxy-fuel combustion technology is an effective way to reduce CO2 emissions. An ionic liquid [emim][Tf2N] was used to capture the CO2 in flue gas from oxy-fuel combustion plant. The process of the CO2 capture was simulated using Aspen Plus. The results show that when the liquid–gas ratio is 1.55, the volume fraction of CO2 in the exhaust gas is controlled to about 2%. When the desorption pressure is 0.01 MPa, desorption efficiency is 98.2%. Additionally, based on the designability of ionic liquids, a hypothesis on the physical properties of ionic liquids is proposed to evaluate their influence on the absorption process and heat exchanger design. The process evaluation results show that an ionic liquid having a large density, a large thermal conductivity, and a high heat capacity at constant pressure is advantageous. This paper shows that from capture energy consumption and lean circulation, oxy-fuel combustion is a more economical method. Furthermore, it provides a feasible path for the treatment of CO2 in the waste gas of oxy-fuel combustion. Meanwhile, Aspen simulation helps speed up the application of ionic liquids and oxy-fuel combustion. Process evaluation helps in equipment design and selection.

Benchmarking machine learning methods for modeling physical properties of ionic liquids

The great importance of the ability to quantitatively predict the properties of ionic liquids (ILs) using quantitative structure–property relationships (QSPR) models necessitates the understanding of which modern machine learning (ML) methods in combination with which types of molecular representations are preferable to use for this purpose. To address this problem, a large-scale benchmarking study of QSPR models built by combining three traditional ML methods and neural networks with seven different architectures with five types of molecular representations (in the form of either numerical molecular descriptors or SMILES text strings) to predict six important physical properties of ILs (density, electrical conductance, melting point, refractive index, surface tension, and viscosity) was carried out. The datasets include from 407 to 1204 diverse ILs composed of various organic and inorganic ions. QSPR models for predicting the properties of ILs at eight different temperatures were built using multi-task learning. The best combinations of ML methods and molecular representations were identified for each of the properties. A unified ranking system was introduced to rank and prioritize different ML methods and molecular representations. It was shown in this study that on average: (i) nonlinear ML methods perform much better than linear ones, (ii) neural networks perform better than traditional ML methods, (iii) Transformers, which are actively used in natural language processing (NLP), perform better than other types of neural networks due to the advanced ability to analyze chemical structures of ILs encoded into SMILES text strings. A special “component-wise” cross-validation scheme was applied to assess how much the predictive performance deteriorates for the ILs composed of cations and anions that are not present in the dataset.

Study on the electrowetting and beam current characteristics of externally wetted ionic liquid electrospray thruster

It is the theoretical basis to analyze the hydrodynamics mechanism of ionic liquid in an electrostatic field to ensure that the ionic liquid electrospray thruster possesses good wettability. Stable and continuous electrowetting emitters are closely related to thrust noise, resolution, and stability. Therefore, it is urgent to explore the law of ionic liquid electrowetting emitters in an electrostatic field to improve the performance of thrusters. The article proposes a new hybrid emitter structure, which consists of a V-blade emitter and a square capillary. It also carries out numerical simulation of emitter electrowetting under normal gravity and atmospheric conditions and emission experiments to explore the process of ionic liquid layer spreading on the external surface of the emitter. The dimensions of the emitter d1, d2, and θ and the physical properties of ionic liquids jointly determine the electrowetting velocity, the threshold of wetting voltage, and the liquid layer thickness. By analyzing the influence of various variables on electrowetting, a complete emitter electrowetting law of ionic liquid electrospray thrusters is summarized, which provides an important basis for the optimization of the emitter structure. Considering the scale of the emitter and bond ≈1, the meniscus in the electrostatic field is mainly affected by gravity, surface tension, viscous force, and electrophoretic force. Accordingly, it is worth noting that electrophoretic force is calculated by the Maxwell stress tensor method that treats ionic liquids as a dielectric to ensure the continuity of the meniscus. Although it can satisfy the electrowetting study at the macroscale, the interaction between anions and cations at the micro-scale is ignored. The numerical simulation results and the experimental results are correlated. It is proved that the method used in this article is accurate enough to simulate electrowetting before the liquid layer breaks up to form ion emission.

Effect of environmental temperature on CO2 selective absorption characteristics by ionic liquid electrospray in flow system

In this study, the effect of ambient temperature on ionic liquid electrospray is experimentally clarified for the selective absorption of CO2 in a flow system with 1-ethyl-3-methylimidazolium acetate. The physical properties of ionic liquids, such as viscosity and surface tension are dependent significantly on the liquid temperature. The spray mode transition voltage increases with temperature primarily because of the decrease in viscosity. Moreover, the ionic liquid feeding rate increases the threshold voltage for the spray mode transition when the liquid temperature is higher. The ionic liquid electrospray enhances selective CO2 absorption in the flow system, and the initial decay of CO2 concentration increases with applied voltage, indicating that widely spread ultrafine droplets of ionic liquid are continuously generated. Although the CO2 reaction rate increases with temperature, the CO2 absorption in the spraying chamber at the steady state is lower because of the lower solubility at higher temperatures.

Specific Heat Capacity of Non-Functional and Functional Ionic Liquids during the Absorption of SO2

Ionic liquids (ILs), a new type of absorbents, are widely used to absorb sulfur dioxide (SO2) in flue gas. However, the lack of data on the physical properties of ILs has severely hampered their in...

Effect of Bulky Anion around the Dication on the Electronic Structure and Normal Frequencies in 1,3-Bis(3-methylimidazolium-1-yl)propane Bis(trifluoromethanesulfonyl)imide Ionic Liquid.

A manifestation of hydrogen bonding between the dication and anions attributed to their relative position of the anions around the cation can influence both the conformational equilibrium and the physical properties of ionic liquids. With this view, we studied the electronic structure and normal frequencies using density functional theory calculations to analyze the hydrogen-bonding interactions in dicationic ionic liquids. The conformers are distinguished based on the hydrogen-bonding sites of the cation and anion. The weak hydrogen bonding between the dication and anions in dication ionic liquids can lead to greater conformational equilibrium compared to the monocation system. Consequences of these interactions for the vibrational spectrum are analyzed to provide an insight into the conformational equilibrium in dicationic ionic liquids at the molecular level.

Potential Application of Ionic Liquids in Pharmaceutical Dosage Forms for Small Molecule Drug and Vaccine Delivery System.

Ionic liquids are salts in which the ions are poorly coordinated, which causes them to exist in liquid form below 100°C, or at room temperature. Therefore, these are also defined as room temperature ionic liquids (RTILs). In ionic liquids, at least one ion has a delocalized charge and one component is organic, which prevents the formation of a stable solid form of crystal lattice. Physical properties of ionic liquids, such as melting point, viscosity, and solubility of starting materials and other solvents, are impacted by the substituents on the organic component and by the counterions. Many ionic liquids have even been developed to address specific synthetic problems and that is the reason these are also termed as "designer solvents". Ionic liquids are considered as "green solvents" that exhibit several unique characteristics such as high ionic conductivity, high solvation power, thermal stability, low volatility, and recyclability. Although very useful with several advantages, ionic liquids have some limitations that include high cost and ease of recycling. Moreover, the toxicity and biodegradability of ionic liquids are not yet well understood. Nonetheless, ionic liquids can potentially be used in the field of pharmacy in drug design and formulation development. In drug or vaccine dosage formulation development, ionic liquids can be used as a solubility enhancer, permeability enhancer, stabilizer, targeted delivery inducer, stealth property provider or bioavailability enhancer. In this article we reviewed the physical properties of ionic liquids and potential application of ionic liquids in developing formulations for vaccines and small molecule drugs (A table has been added).

Refractive index of ionic liquids under electric field: Methyl propyl imidazole iodide and several derivatives**Project supported by the National Natural Science Foundation of China (Grant Nos. 51576054 and 51705210) and the Jiangsu Provincial Planned Projects for Postdoctoral Research Funds, China (Grant No. 2019K195).

Ionic liquids have received wide attention due to their novel optoelectronic structures and devices as an optical means of regulating electricity. However, the quantitative testing and analysis of refractive index of ionic liquids under electric field are rarely carried out. In the present study, an experimental apparatus including a hollow prism is designed to measure the refractive indices of ionic liquids under different electric fields. Five groups of imidazole ionic liquids are experimentally investigated and an inversion is performed to determine the refractive indices under electric fields. The error propagation analysis of the apex angle and the minimum deflection angle are conducted, and the machining accuracy requirements of the hollow prism are determined. The results show that the refractive indices of imidazole ionic liquids change with the light wavelength, following a downward convex parabola. Furthermore, the refractive index decreases with the carbon chain length of ionic liquid at a given wavelength, presenting an order of C3MImI > C4MImI > C5MImI > C3MImBr > C3MImBF4. Notably, the refractive index of imidazole ionic liquid exhibits a nonlinear change with the applied voltage at 546 nm and a monotonical decrease at 1529 nm. Besides, the variation of refractive index at 1529 nm with the applied voltage is larger than that at 546 nm and 1013 nm. Importantly, the variation of refractive index is contrary to that of absorption coefficient under electric field. This study illustrates that the theory of electrode and carrier transport can be used to explain the law of variation of n–k value of ionic liquid under the electric field, and provides the support for the evaluation of physical properties of ionic liquids, the measurement of optical functional parameters and the regulation of electric–optic performances of optical devices.

Structural and electronic properties of CuII, CoII, and NiII-containing chelate-based ionic liquids.

Potential applications of chelate-based ionic liquids depend on the structural and electronic properties of this class of liquid materials. Due to the large size of chelated metal anions, the high number of potential interaction sites could lead to complex intermolecular interactions, but metal-based anions have many degrees of freedom, and they have great potential in the structural modification of the physical properties of ionic liquids. To explore the influence of varying the metal center of anions and the length of carbon chains of cationic species, four single crystal structures of chelate-based ionic liquids, ([C10mim][M(F6-acac)3], M = Cu, Co, and Ni) and [C6mim][Cu(F6-acac)3] were obtained. Taking these as the initial configurations, theoretical efforts were made to understand the structural and electronic properties. The hydrogen bonding energies of the primary hydrogen bonding interaction, in the range of 17.7-20.9 kJ mol-1, follow the order of [C10mim][Ni(F6-acac)3] < [C10mim][Co(F6-acac)3] <[C10mim][Cu(F6-acac)3], and [Cnmim][Cu(F6-acac)3] (n≠ 10) < [C10mim][Cu(F6-acac)3], while the experimental viscosities exhibit an opposite trend. Furthermore, by NBO analysis, the more negative charge on the oxygen atoms of anions shows the stronger hydrogen bonding of imidazolium with C2H. The reliability of the theoretical method was supported by the comparison between the simulated and experimental infrared and UV/vis spectra. This work is useful in increasing the understanding of the structure-property relationship of chelate-based ionic liquids and furthering the rational design of novel ionic liquids.

Synergistic effect of a binary ionic liquid/base catalytic system for efficient conversion of epoxide and carbon dioxide into cyclic carbonates

Carbon dioxide (CO2) mitigation via utilization can be a potential solution to global warming issues such as increasing CO2 concentration. In this study, we present the synthesis and characterisation of 3-(2-hydroxyethyl)-1-vinyl-1H-imidazol-3-ium chloride [EvimOH][Cl] and 3-(2-hydroxyethyl)-1-vinyl-1H-imidazol-3-ium sulphate [EvimOH][HSO4] ionic liquids for cycloaddition reactions of CO2 and epoxides into cyclic carbonates. The synthesized ionic liquids were characterized by FT-IR, 1H- and 13C-NMR spectroscopic methods. Also, we have analysed the physical properties of ionic liquids by using TGA and DTA. Further, the CO2 consumption was done by developing a binary catalytic system with bases such as DBU, TEA, DIPEA, piperidine, pyrrolidine, aniline. The developed binary ([EvimOH][Cl]/base) catalytic system exhibited the excellent catalytic activity of 99.8% conversion and ≥99% selectivity under mild reaction conditions without the aid of solvent and metal co-catalysts. Furthermore, the results show that the improved activity from the binary system is due to synergistic effect among −OH functional group from the cation, reactive acidic C(2)-H in the imidazolium ring and amine containing bases which promote the CO2 absorption. Also, a plausible mechanism of cyclic carbonate formation and kinetic study in the presence of binary catalytic system was demonstrated.

Investigating the effects of mixing ionic liquids on their density, decomposition temperature, and gas absorption

In order to counter the detrimental effect of chemicals on the environment, the academic and industry research interest have shifted towards “green” solvents. Ionic liquids (ILs) are salt-like compounds that, with few exceptions, are known to have negligible vapor pressure and are widely proclaimed as green solvents. Their physical properties can be tailored for the desired purpose by the careful selection of cation and anion or by mixing with other solvents. In this work, we tested the gas absorption of ILs and measured the thermo-physical properties of individual ILs and their binary mixtures. Density and thermogravimetric measurements were performed on ILs choline octanoate ([Ch][Oct]), 1-methyl-3-octylimidazolium trifluoromethanesulfonate ([Omim][Otf]), and 1-methyl-3-octylimidazolium chloride ([Omim][Cl]), and their equal molar (1:1) mixtures. The effect of different anions [Otf−, Cl−] and different cation groups [Omim, Ch] on the gas absorption potential of ILs have been observed. The density and thermal stability tests have been conducted to understand the changes in the physical properties of ILs when they are mixed together in equal ratios. All samples of ILs ([Ch][Oct], [Omim][Otf], [Omim][Cl], [Ch][Oct]+[Omim][Otf], [Ch][Oct]+ [Omim][Cl], and [Omim][Cl]+ [Omim][Otf]) were tested as CO2 absorption media at temperatures 298.15, 305.15, and 308.15K and pressure values ranging from 0 to 50bar. In addition to that, the CO2 sorption results at 298.15K were compared with experimentally measured methane (CH4) and nitrogen (N2) data. It was observed that the IL [Ch][Oct] is significantly better for CO2 absorption than [Omim][Cl] ([Ch][Oct]≥[Omim][Cl]>[Omim][Otf]). On comparing the CO2 absorption profiles with those of CH4 and N2, the similar trends for IL and IL mixtures were obtained: CO2>CH4>N2 and CO2>CH4>N2 respectively.

Surfactant Ionic Liquids for Supercapacitors

Supercapacitors are electrochemical energy storage devices that find use in many electronic devices as well as increased application in large high-power requirement devices. The primary advantages of using a supercapacitor are the fast charge/ discharge rates and the long cycling lifetime. Traditionally supercapacitors have utilized organic or aqueous electrolytes both of which introduce significant drawbacks. For example, organic electrolytes are flammable which creates safety considerations that need to be addressed while aqueous electrolytes are limited to the electrochemical stability of water (1.23 V). Ionic Liquids (ILs) are a growing class of electrolyte that offer increased thermal and electrochemical stability over standard electrolytes. Additionally, ILs are non-flammable and are non-volatile, eliminating the possibility of flammable devices. Despite the advantages of IL electrolytes, they are highly viscous and do not easily access the pores of the electrode material in supercapacitors. Most commonly, high surface area carbon is used as supercapacitor electrodes but is only effective if the pores are filled with electrolyte. It is expected that successful wetting of the electrode surface will make IL electrolytes competitive with the current state of the art. Here, new surfactant ILs are synthesized and assessed as the electrolyte in supercapacitors containing porous carbon electrodes. The physical properties of ILs can be tailored by modification of the anion and cation structures. Here, a mixture of surfactant ILs was used as the electrolyte which is shown to offer enhanced capacitance over the parent ILs. Each surfactant IL was synthesized in house and characterized through NMR, TGA, as well as viscosity and conductivity measurements. Capacitance, calculated from the integration of cyclic voltammograms, is shown to depend on the concentration of each IL and temperature, which is attributed to the changes in viscosity and conductivity of the mixture. Improved capacitance in supercapacitors with porous carbon electrodes can be related to lower surface tension and increased wettability of the porous electrodes the surfactant IL additive. In addition we demonstrate a significant thermal operating window of the proposed electrolyte.

Computer-Aided Design of Ionic Liquids as Absorbent for Gas Separation Exemplified by CO2 Capture Cases

In order to design ionic liquids as absorbents for gas separation, a systematic computer-aided ionic liquid design (CAILD) methodology is applied and demonstrated by three cases of CO2 capture. Mixed-integer nonlinear programming problems are formulated, where a mass-based Absorption-Selectivity-Desorption index (ASDI) integrating the most important thermodynamic properties of ILs (i.e., gas solubility, selectivity, and desorption capacity) is proposed as the objective function and calculated by the COSMO-GC-IL inputted COSMO-SAC model. The physical properties of ionic liquids are implemented as optimization constraints, which are estimated by semiempirical models. The reliability of the thermodynamic method for IL-gas systems is validated first by comparing a large number of experimental and calculated data of Henry’s law constant of different gases in ILs. Then, comparative CAILD studies are performed for CO2 separation from flue gas (CO2/N2) to demonstrate the importance of ASDI for identifying practic...

“Rheological and Physical properties of ionic liquids with ammonium cations as synthetic lubricants’’

Ionic liquids possess different properties which make them a high potential as lubricants. In our work series of five ammonium salts ionic liquids were synthesized by using different C-chain carboxylic acids with ethanol amines and evaluated. The physicochemical properties and rheological behavior investigated for their potential use as synthetic lubricants. The prepared Ils show high thermal stability, good fluidity characters and the viscosity increases with increasing C-number (become thicker) which converted from liquid phase to semisolid phase (grease). Ia-Ic show Newtonian behavior behavior for and pseudoplastic behavior for Id and Ie.

The development of the UNIFAC-CONDUCT model as a novel approach for the estimation of the conductivity of pure ionic liquids

The correlation and prediction of physical properties of ionic liquids (ILs) is important in the development and design of novel structures for a variety of applications. In particular, for the application as electrolytic media for electrochemical applications, the electrical conductivity of ILs is an important property. Herein, we define a novel model called the UNIFAC-CONDUCT for the estimation of IL conductivity which mimics the principles of the UNIFAC-VISCO model. 784 data points for 38 ILs were used to establish this group contribution model, covering a wide range of temperature (248.15–468.15 K) and a broad range of electrical conductivity (0.002–14.54 S·m−1). Three sets of UNIFAC-CONDUCT parameters were determined to calculate the conductivity of pure ILs by three methodologies wherein the parameters of ion VFT equations, A, B and To, and binary interaction parameters, αmn, are selectively either fixed (based on previously reported values) and/or optimized by Marquardt technique. Correlations by the three methods are compared and analyzed, and further correlated with the previously predicted viscosities using the Nernst-Stokes relationship to support the quality of both the UNIFAC-VISCO model, previously proposed by our group, and the UNIFAC-CONDUCT model developed in this paper. Finally, the quality of the proposed method was further assessed through the prediction of the conductivity of one binary system containing two different ILs as the function of the temperature and composition with an accuracy close to 4.1%.

Research on the strengths of electrostatic and van der Waals interactions in ionic liquids

The special properties of ionic liquids are due to the simultaneous presence of Coulombic and van der Waals forces. Evaluation of their interaction strengths is a basic work for explaining the macroscopic physical properties of ionic liquids, understanding the molecular dynamics and microstructure of ionic liquids, and assessing the binding force induced by ions in biological systems. In this paper, a modified Coulomb's law was constructed to calculate the average electrostatic force in ionic liquids through treating an ionic liquid molecule as two charges separated by some organic groups and taking these organic groups as an organic molecule. The modified Coulomb's law was verified with the surface tension data of 117 non-ionic liquids and 153 ionic liquids. According to the modified Coulomb's law, the average strengths of the electrostatic and van der Waals interactions between a pair of cation-anion in the 153 ionic liquids were almost equal, which were about 20pN.

Influence of C2-Methylation of Imidazolium Based Ionic Liquids on Photoinduced Spin Dynamics of the Dissolved ZnTPP Studied by Time-Resolved EPR

Abstract Unusual physical properties of ionic liquids (ILs) can be implemented in many different applications and are very sensitive to the structure of IL. In this work we investigate the spin dynamics of probe molecule Zn tetraphenylporphyrin (ZnTPP) dissolved in a series of ILs using time-resolved electron paramagnetic resonance (TR EPR). We compare the TR EPR characteristics in C2-methylated imidazolium-based ILs [bmmim]BF4 and [bmmim]PF6 and in their C2-protonated analogs [bmim]BF4 and [bmim]PF6 to assess the influence of C2-methylation. The corresponding TR EPR signatures are drastically different in the two types of ILs. The analysis of experimental data allows assumptions that the ZnTPP molecule is distorted in C2-methylated ILs, contrary to other organic media and C2-protonated analogs. The mobility of ZnTPP in C2-methylated ILs is smaller compared to that in C2-protonated analogs, implying different microenvironment formed around dissolved ZnTPP.