Rewa Rai

Ionic Liquid-Induced Unprecedented Size Enhancement of Aggregates within Aqueous Sodium Dodecylbenzene Sulfonate

Physicochemical properties of aqueous micellar solutions may change in the presence of ionic liquids (ILs). Micelles help to increase the aqueous solubility of ILs. The average size of the micellar aggregates within aqueous sodium dodecylbenzene sulfonate (SDBS) is observed by dynamic light scattering (DLS) and transmission electron microscopy (TEM) to increase in a sudden and drastic fashion as the IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) is added. Similar addition of [bmim][PF(6)] to aqueous sodium dodecyl sulfate (SDS) results in only a slow gradual increase in average aggregate size. While addition of the IL [bmim][BF(4)] also gives rise to sudden aggregate size enhancement within aqueous SDBS, the IL 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF(4)]), and inorganic salts NaPF(6) and NaBF(4), only gradually increase the assembly size upon their addition. Bulk dynamic viscosity, microviscosity, dipolarity (indicated by the fluorescent reporter pyrene), zeta potential, and electrical conductance measurements were taken to gain insight into this unusual size enhancement. It is proposed that bmim(+) cations of the IL undergo Coulombic attractive interactions with anionic headgroups at the micellar surface at all [bmim][PF(6)] concentrations in aqueous SDS; in aqueous SDBS, beyond a critical IL concentration, bmim(+) becomes involved in cation-π interaction with the phenyl moiety of SDBS within micellar aggregates with the butyl group aligned along the alkyl chain of the surfactant. This relocation of bmim(+) results in an unprecedented size increase in micellar aggregates. Aromaticity of the IL cation alongside the presence of sufficiently aliphatic (butyl or longer) alkyl chains on the IL appear to be essential for this dramatic critical expansion in self-assembly dimensions within aqueous SDBS.

Effect of Ionic Liquid on J-Aggregation of meso-Tetrakis(4-sulfonatophenyl)porphyrin within Aqueous Mixtures of Poly(ethylene glycol)

Aqueous mixtures of poly(ethylene glycol) (PEG) of different compositions offer widely varying physicochemical properties that may support porphyrin aggregation. Aggregation behavior of a common water-soluble porphyrin, meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS), is investigated within aqueous PEG mixtures constituted of PEGs of average molecular weights 200 (PEG200), 400 (PEG400), 600 (PEG600), and 1000 (PEG1000) using UV-vis molecular absorbance, steady-state fluorescence, and resonance light scattering techniques. No aggregation of TPPS is observed in neat PEGs; addition of 10 wt % water to PEG at pH 1.0 is found to trigger TPPS into significant J-aggregation. The J-aggregation is observed to be most efficient within an aqueous mixture of 90 wt % PEG1000 at pH 1.0. The effect of ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]), as additives on the J-aggregation efficiency of TPPS within aqueous mixtures of PEG400 at pH 1.0 is investigated and compared with the effect of salts NaCl, NaPF(6), and NaBF(4) as additives on the J-aggregation of TPPS under the same conditions. In an aqueous mixture of 10 wt % PEG400 at pH 1.0, ionic liquids are observed to increase the J-aggregation efficiency more than the salts at lower concentrations. The efficiency of J-aggregation decreases upon further addition of [bmim][BF(4)] due to reduced dissociation of this ionic liquid in the mixture. While the three salts show limited solubility, the two ionic liquids are completely miscible in a 90 wt % PEG400 mixture in water at pH 1.0. The J-aggregation efficiency of TPPS increases rapidly and reaches a maximum before decreasing gradually as more and more ionic liquid is added to the mixture. The results draw attention to the unique dual role of ionic liquids as additives in affecting the J-aggregation of TPPS within aqueous mixtures of PEG as well as to their proficiency over common salts in J-aggregation.

Self-Aggregation of Sodium Dodecyl Sulfate within (Choline Chloride + Urea) Deep Eutectic Solvent

Deep eutectic solvents (DESs) have shown tremendous promise as green solvents with low toxicity and cost. Understanding molecular aggregation processes within DESs will not only enhance the application potential of these solvents but also help alleviate some of the limitations associated with them. Among DESs, those comprising choline chloride and appropriate hydrogen-bond donors are inexpensive and easy to prepare. On the basis of fluorescence probe, electrical conductivity, and surface tension experiments, we present the first clear lines of evidence for self-aggregation of an anionic surfactant within a DES containing a small fraction of water. Namely, well-defined assemblies of sodium dodecyl sulfate (SDS) apparently form in the archetype DES Reline comprising a 1:2 molar mixture of choline chloride and urea. Significant enhancement in the solubility of organic solvents that are otherwise not miscible in choline chloride-based DESs is achieved within Reline in the presence of SDS. The remarkably improved solubility of cyclohexane within SDS-added Reline is attributed to the presence of spontaneously formed cyclohexane-in-Reline microemulsions by SDS under ambient conditions. Surface tension, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), density, and dynamic viscosity measurements along with responses from the fluorescence dipolarity and microfluidity probes of pyrene and 1,3-bis(1-pyrenyl)propane are employed to characterize these aggregates. Such water-free oil-in-DES microemulsions are appropriately sized to be considered as a new type of nanoreactor.

Quorum Sensing Biosensors

Quorum sensing is a cell density-dependent phenomenon, which at high cell density induces expression of target genes in a bacterial population. In bacteria, quorum sensing is facilitated by cell-to-cell signaling molecules referred as autoinducers (AIs). Among Gram-negative bacteria, quorum sensing is mediated primarily by two classes of AIs: AI-1 and AI-2. Further, AI-1 has tens of subtypes and each bacterium responds to a very limited subtypes of AI-1. These signaling molecules, at high cell density, regulate several physiological processes among bacteria, including bioluminescence, biofilm, and virulence.

Controlling excited-state prototropism via the acidity of ionic liquids

Proton transfer involving acridine dissolved in neat ionic liquids (ILs) as a function of temperature is studied with the help of electronic absorbance and steady-state fluorescence, as well as time-resolved fluorescence, and is compared with the proton transfer observed within water and common organic solvents. The acidity of the IL cation is found to control the excited-state proton transfer involving acridine. Fluorescence corresponding to the protonated form of acridine is observed in the temperature range of 10–90 °C when the solubilizing IL contains an imidazolium cation possessing C2–H functionality. This is attributed to the acidity associated with the C2 proton of the imidazolium cation. When the C2 proton of the imidazolium cation is substituted with a methyl group in the IL (C2–CH3 substituted imidazolium ILs), the fluorescence from the dissolved acridine is found to depend on temperature. The fluorescence corresponds to neutral acridine at room and lower temperatures, whereas it is again from the cationic form of acridine at slightly higher temperatures. This is attributed to the weaker acidity associated with the C2–CH3 protons of the imidazolium cation as compared to the C2–H proton; the acidity of the C2–CH3 protons is found to increase with increasing temperature. In contrast, the observed fluorescence corresponds exclusively to the neutral acridine in the temperature range of 10–90 °C when the solubilizing IL contains a pyrrolidinium cation possessing no such acidic protons. Control of the excited-state proton transfer using the acidity of the cation of the solubilizing IL is amply demonstrated.

Quorum Sensing in Competence and Sporulation

In several Gram-positive bacteria, competence and sporulation are few of several physiological processes controlled by quorum sensing (QS). Competence is a phenomenon wherein a bacterium acquires extracellular DNA for its maintenance. Only a fraction of cells (10–20 %), in a population, develop competence, at a particular window of growth phase, and in response upregulate expression of genes involved in the uptake and processing of extracellular DNA. Sporulation, second QS-controlled phenotype, occurs under extreme stress and nutritional scarcity. Prolonged nutrient deprivation compels the cell to enter the process of sporulation, the outcome of which is the production of a metabolically dormant endospore that resumes growth once the conditions become favorable again. Spore formation is a complex and tightly regulated phenomenon, where several hundred genes are directly and indirectly involved. Regulation of competence and sporulation is a complex and temporally regulated process. In present chapter, we will discuss QS driven regulation of competence and sporulation in different Gram-positive bacteria.

Surfactant Self-Assembly Within Ionic-Liquid-Based Aqueous Systems

Ionic liquids (ILs) have received increased attention from both academic and industrial research communities all over the world due to their unusual properties and immense application potential in various fields of science and technology. During the past decade, ionic-liquid-based systems have become the subject of considerable interest as a promising media for extraction and purification of several macro-/biomolecules. ILs are attractive designer solvents with tunable physicochemical properties. Using IL-based systems as alternative solvents for forming surfactant self-assemblies has several advantages. For example, the properties of surfactant self-assemblies in these media can be easily modulated by tuning the structure of ILs; ILs can dissolve a large variety of organic and inorganic substances and their properties are designable to satisfy the requirements of various applications. This may enhance the application potential of both ILs and surfactants in many important fields. Consequently, the study on surfactant self-assemblies within IL-based aqueous systems has attracted considerable attention in recent years. This chapter overviews the investigation carried out on the formation of surfactant self-assemblies within IL-based aqueous systems and their applications in various fields.