Why ionic liquids are more surface active than common surfactant?
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Ionic liquids (ILs) are considered as nontoxic and greener solvents.(1) Owing to their versatile properties, they find applications in diverse fields of chemistry and biochemistry.(2, 3) In recent times, the conventional surfactants have been turned to IL surfactants commonly called surface-active ILs (SAILs) to achieve desired surface activity and dissolution in water. So far, most of these surfactants have imidazolium and ammonium as a cation.(4-6) They have exhibited superior surface activity, are also capable of forming supramolecular nano- to giant aggregates viz., micelles, vesicles, wormlike micelles, and multilayered vesicles, and have found their uses in industrial, chemical, or pharmaceutical applications,(7-16) but there are issues about their toxicities and biodegradabilities. Therefore, the choice of a bioderived countercation (choline- and amino acid-based) or counteranion (carboxylate and alkyl sulfate) would create more opportunities to introduce biodegradability in IL surfactants for wider practical utility.(17, 18) Moreover, the rise in concerns toward the toxicity of imidazolium-based ILs has further furnished the demand for more biodegradable IL surfactants for their efficient and biofriendly utilization.(19-21) Seeking progress in this direction, herein, we have turned the commercial anionic surfactant sodium dodecylbenzenesulfonate NaDBS to a more eco-friendly and biodegradable surfactant “cholinium dodecylbenzenesulfonate Cho[DBS]”. In brief, we have synthesized Cho[DBS] as a potential nontoxic IL surfactant whose surface-active properties were analyzed by using tensiometry, conductivity, and isothermal titration calorimetry (ITC) and morphological/size analysis has been done using dynamic light scattering (DLS) and transmission electron microscopy (TEM).
Surface-active compounds find a wide range of applications in micellar catalysis, in DNA stability, in encapsulation of drugs, as a protein stabilizer, and in enzyme/protein stabilization in detergent formulations. Among these, the surfactant–protein interaction studies do hold a great relevance if needed to be utilized for practical applications.(22, 23) Therefore, we have explored the stabilization of cellulase (an important laundry enzyme) in micellar solution of Cho[DBS] for its suitability in enzymatic catalysis, protein stabilizer, and biodetergents (enzymes bearing detergents). So far, the enzymatic activities in direct micellar systems have occasionally been investigated, and only a few reports are there till date. It is assumed that organic substrates which have limited solubility in water are localized in the palisade layer or the hydrophobic core of the micelles, whereas the enzymes are dissolved and stabilized in the continuous aqueous phase. These essentially aqueous systems are more appropriate for potential industrial use in enzymatic conversion than colloidal systems of the inverse type.(24) The term micellar enzymology has been introduced to investigate the kinetic properties of cytosolic enzymes in micellar systems resembling to mimic biological environments.(25) Surfactants in their monomeric or aggregated form can act as potential crowding agents,(26) and therefore the presence of surfactants may lead to either the activation(27, 28) or deactivation(29, 30) of enzyme activity. Recently in 2017, Mitra et al. demonstrated the five-fold α-chymotrypsin enzyme activity enhancement in the micellar medium of dodecyltrimethylammonium bromide (DTAB). They also investigated that DTAB does not produce noticeable changes in the protein structure before the postmicellar region.(31) Verma et al. investigated the α-chymotrypsin activity presence of novel cationic amine-based gemini surfactants and observed the effect of the different spacers, chain lengths, and head group sizes with the comparison of cationic surfactants in aqueous buffer media