Atindra D. Shukla

Controlling wettability and hydrophobicity of organoclays modified with quaternary ammonium surfactants

The montmorillonite clays were modified with quaternary ammonium salts (QASs) having different alkyl chain lengths and a benzyl substitute group. The modified organoclays were characterized by different analytical techniques. The wettability and hydrophilicity/hydrophobicity of the modified clays was evaluated using water or oil penetration (adsorption) and contact angle measurements. The loading of QASs was in the range of 0.60-0.75 mmol/g per clay, irrespective of the type of QAS used for the modification of the clay. From the analytical investigations, it was elucidated that the modification of clay with QAS affected the structural, textural, and surface properties. Moreover, it should be noted that the modification with QAS having benzyl substitute group resulted in water-non-wettable and superhydrophobic surface, whereas clays modified with QAS without benzyl substitute group became more water-wettable and hydrophilic than the pristine clay. The presence of benzyl groups on the clay prevents water from penetration into the inter-clay or interlayer spacing, which yields the hydrophobic surface. These behaviors can arise from molecular arrangement of QAS on clay but not be attributable to the amount of QASs, and the surface area, size, and zeta potential of particles.

Redox responsive binuclear complexes using 5,6-dihydroxy-1,10-phenanthroline as a bridging ligand: synthesis, characterization and physicochemical studies

Abstract A set of new binuclear complexes has been synthesized and characterized using redox active bridging ligands (BL), 5,6-dihydroxy-9,10-phenanthroline (dpcatH2) and M(bipy)2Cl2 (M is Ru(II) or Os(II))/Ru(tpzea)Cl2 (tpzea is tris(N-pyrazolyl ethyl) amine) as the building blocks. For all binuclear complexes a large degree of stabilization of the semiquinone (sq) state for the Ru(bpy)2(dpsq)+ fragment has been observed. UV–Vis, electrochemical and spectroelectrochemical studies clearly show that the first two redox processes involve only the bridging ligand. Further experimental results show that auxiliary ligands coordinated to the metal center do not have any appreciable effect on the redox behavior of the bridging ligand.

Selective capture of CO2 by poly(amido amine) dendrimer-loaded organoclays

Clay loaded poly(amido amine) dendrimers were explored for capture and storage of CO2. The loading of dendrimer was promotive in the order of laponite > hydrotalcite > sericite and depended on the surface area of the clays. The CO2 adsorption on organoclays of laponite and sericite with cationic dendrimer increased with the amount of loaded dendrimer. While CO2 on pristine laponite was completely released in the desorption process, CO2 on organo laponite remained in part after the desorption equilibrium. Since the removal of CO2 from organo laponite was almost comparable to that from pristine clay, it can be mentioned that CO2 adsorbed on the binding site of laponite is almost desorbed but CO2 on the binding site of dendrimer is conserved in organoclay. In contrast, in the case of the CO2 adsorption on the organoclay of hydrotalcite with an anionic dendrimer, the diminution of adsorption sites on hydrotalcite owing to the occupation by dendrimer was observed. It can be mentioned that the cation-exchanged organo laponite loaded amine-terminated dendrimer is a valuable solid adsorbent with a highly selective capture capacity for CO2.

Novel Strategy Involving Surfactant–Polymer Combinations for Enhanced Stability of Aqueous Teflon Dispersions

Among various polymers, the Teflon surface possesses extreme hydrophobicity (low surface energy), which is of great interest to both industry and academia. In this report, we discuss the stability of aqueous Teflon dispersions (particle size range of 100-3000 nm) formulated by a novel strategy that involves distinct combinations of surfactant and polymer mixtures for dispersion stabilization. As a first step, the hydrophobic Teflon particles were wetted using a range of surfactants (ionic, Triton, Brij, Tween, and Pluronic series) bearing different hydrophobic-lipophilic balance (HLB) and further characterized by contact angle and liquid penetration in packed powder measurements. The interaction between hydrophobic chains of surfactants and the Teflon particle surface is the driving force resulting in wetting of the Teflon particle surface. Further, these wetted particles in aqueous solutions were mixed with various polymers, for example, poly(vinyl alcohol) (PVA), polyvinylpyrrolidone (PVP), hydroxyethyl cellulose (HEC), and hydroxypropyl methyl cellulose (HPMC). The rate of sedimentation for the final dispersions was measured using a pan suspended into the dispersion from a transducer recording the increase in weight with time. A significant stability was noticed for Teflon particles suspended in surfactant + polymer mixtures, which was linearly proportional to the concentration of added polymer. The observed phenomenon can be possibly explained by molecular interactions between the hydrophobic chains of surfactant molecules and polar groups in the polymer architecture. Brij-O10 + HEC mixture was found to be the best surfactant-polymer combination for decreasing the sedimentation of the Teflon particles in the final dispersion. As measured by dynamic light scattering (DLS), the hydrodynamic volume of the Teflon particles increases up to ∼55% in the final formulation. These dispersions could be further explored for various technological applications such as paints, inks, protective coatings, and so forth.

Multicomponent Zn-tetraphenylporphyrins: syntheses, characterization and their self assembly in the solid state

Two multicomponent Zn-tetraphenylporphyrin (ZnTPP) based building blocks have been synthesized for designing microporous crystalline solids. Reactions of ZnTPP with 4,4′-bipyridine (4,4′-bpy) in two different molar ratios gave two coordination complexes namely [(ZnTPP)3{μ-4,4′-bpy}2] I and [(ZnTPP)2{μ-4,4′-bpy}] II. Complexes I and II were characterized by spectroscopic and analytical methods. Inclusion complexes of I with toluene, II with toluene, and also with nitrobenzene have been prepared and are characterized by 1H NMR and single crystal X-ray diffraction techniques. Supramolecular structural systematics of TPP-based materials are conserved in these crystals.

Redox switchable NIR dye derived from ruthenium–dioxolene–porphyrin systems

Newly synthesised Ru(bp)2(sq)(+)-derivatives, covalently linked to a porphyrin-core, show very high epsilon values in the NIR region; which exhibit fast on/off switching depending on the redox state of the coordinated dioxolene functionality.

Quality by Design approach for an in situ gelling microemulsion of Lorazepam via intranasal route

The present study illustrates the application of the concept of Quality by Design for development, optimization and evaluation of Lorazepam loaded microemulsion containing ion responsive In situ gelator gellan gum and carbopol 934. A novel approach involving interactions between surfactant and polymer was employed to achieve controlled drug release and reduced mucociliary clearance. Microemulsion formulated using preliminary solubility study and pseudo ternary phase diagrams showed significantly improved solubilization capacity of Lorazepam with 54.31±6.07nm droplets size. The effect of oil to surfactant/cosurfactant ratio and concentration of gelling agent on the drug release and viscosity of microemulsion gel (MEG) was evaluated using a 32 full factorial design. The gel of optimized formulation (MEG1) showed a drug release up to 6h of 97.32±1.35% of total drug loaded. The change in shear-dependent viscosity for different formulations on interaction with Simulated Nasal Fluid depicts the crucial role of surfactant-polymer interactions on the gelation properties along with calcium ions binding on the polymer chains. It is proposed that the surfactant-polymer interactions in the form of a stoichiometric hydrogen bonding between oxyethylene and carboxylic groups of the polymers used, provides exceptional ME stability and adhesion properties. Compared with the marketed formulation, optimized MEG showed improved pharmacodynamic activity. Ex vivo diffusion studies revealed significantly higher release for MEG compared to microemulsion and drug solution. MEG showed higher flux and permeation across goat nasal mucosa. According to the study, it could be concluded that formulation would successfully provide the rapid onset of action, and decrease the mucociliary clearance due to formation of in situ gelling mucoadhesive system.

Mechanism of organic pollutants sorption from aqueous solution by cationic tunable organoclays

A variety of quaternary ammonium salts with different carbon chain lengths were used to modify and convert montmorillonite clay to organoclays. The surface modification attributing to the wettability was investigated using various techniques. The zeta-potential values of all clays showed pH dependency. The prepared organoclays were utilized for phenol (protic) and nitrobenzene (aprotic) adsorption in a batch system, with pH and contact time as variables. The optimized removal for phenol and nitrobenzene within 30 min of equilibrium observed at pH 9.0 and 5.0, respectively. Observed equilibrium data followed the Langmuir monolayer adsorption kinetics with two adsorption sites (outer-layer and interlayer) for purified clay, and third additional sites for organoclays. Kinetic studies revealed that the adsorption of phenol was in the order of mono- > tetra- > di- > benzyl-substituted organoclays, which is similar to the hydrophilicity order. However, the exact opposite trend of adsorption was observed for nitrobenzene. From the performed quantitative study, it is inferred that -OH functional phenol and -NO2 functional nitrobenzene have an affinity towards the hydrophilic clay surface and hydrophobic surfaces of quaternary ammonium salts as its first preference, respectively. This study brings promising observations and implications to the vital structural-property relationship for selective adsorption of pollutants from aqueous solutions.

Role of stabilizing agents in the formation of stable silver nanoparticles in aqueous solution: Characterization and stability study

ABSTRACT The stability of silver nanoparticles is controlled mainly by two major factors, namely, aggregation and oxidation. In the present study, silver nanoparticles were synthesized by using different series of reducing agents like a strong reducing agent (sodium borohydride), a mild reducing agent (tri-sodium citrate), and a weak reducing agent (glucose) with different capping agents, namely, polyvinyl pyrrolidone (PVP K 30), starch, and sodium carboxyl methyl cellulose (NaCMC). The synthesized silver nanoparticles were characterized by UV-Visible absorption spectroscopy, dynamic light scattering (DLS), atomic force microscopy (AFM), and anti-microbial activity. The particle size of silver nanoparticles varies in the following order: sodium borohydride < tri-sodium citrate < glucose. Combination of sodium borohydride–polyvinyl pyrrolidone and tri-sodium citrate-polyvinyl pyrrolidone yields stable silver nanoparticles compared to other combinations of reducing agents and capping agents. The stability results confirmed that a refrigerated condition (8°C) was more suitable for storage of silver nanoparticles. Anti-microbial activity of silver nanoparticles synthesized in a sodium borohydride–polyvinyl pyrrolidone mixture shows a larger zone of inhibition compared to other silver nanoparticles. Anti-microbial results confirmed that the anti-microbial activity is better with smaller particle size. The size and stability of silver nanoparticles in the presence of different combinations of stabilizing and capping agents are reported. GRAPHICAL ABSTRACT

Kinetics of oxidation of ascorbic acid and 1,4-dihydroxybenzene by semiquinone radical bound to ruthenium(II)

In 40% (v/v) MeOH–H2O media, containing [H+] (0.001–0.038 mol dm−3), the semiquinone (sq) radical, bound to Ru(II) in [Ru(bpy)2(sq)]+1, oxidises ascorbic acid (H2A) to dehydroascorbic acid (A), and 1,4-dihydroxybenzene (H2Q) to p-benzoquinone (Q); 1 is itself reduced to [Ru(bpy)2(Hcat)]+2H. The reactions are centred at sq not Ru(II). The sq/cat couple in 1 is reversible and its E1/2 increases with increasing [H+]; rate of chemical reduction of 1 to 2H increases in parallel. Rate increases also with increasing mole percent of D2O in the solvent suggesting a preliminary protonation equilibrium producing 1H, in which a H+ binds to the π-electron cloud of Ru(II)-bound sq. Under the experimental conditions, the kinetically significant species are 1H, H2Q, H2A and HA−. The kinetic activity of HA− ion is only ≈200 times more than that of H2A. This testifies against a purely outer-sphere mechanism and suggests significant electronic interaction between the redox partners. Increased percentage of MeOH in the solvent decreases λmax for the LMCT band; reaction rate for ascorbic acid decreases in parallel.