Microstructure, morphology, and ultrafast dynamics of a novel edible microemulsion

Fourth-Generation Antibiotic Gatifloxacin Encapsulated by Microemulsions: Structural and Probing Dynamics

To overcome the increased disease rate, utilization of the versatile broad spectrum antibiotic drugs in controlled drug-delivery systems has been a challenging and complex consignment. However, with the development of microemulsion (μE)-based formulations, drugs can be effectively encapsulated and transferred to the target source. Herein, two biocompatible oil-in-water (o/w) μE formulations comprising clove oil/Tween 20/ethylene glycol/water (formulation A) and clove oil/Tween 20/1-butanol/water (formulation B) were developed for encapsulating the gatifloxacin (GTF), a fourth-generation antibiotic. The pseudoternary phase diagrams were mapped at a constant surfactant/co-surfactant (1:1) ratio to bound the existence of a monophasic isotropic region for as-formulated μEs. Multiple complementary characterization techniques, namely, conductivity (σ), viscosity (η), and optical microscopy analyses, were used to study the gradual changes that occurred in the microstructure of the as-formulated μEs, indicating the presence of a percolation transformation to a bicontinuous permeate flow. GTF showed good solubility, 3.2 wt % at pH 6.2 and 4.0 wt % at pH 6.8, in optimum μE of formulation A and formulation B, respectively. Each loaded μE formulation showed long-term stability over 8 months of storage. Moreover, no observable aggregation of GTF was found, as revealed by scanning transmission electron microscopy and peak-to-peak correlation of IR analysis, indicating the stability of GTF inside the formulation. The average particle size of each μE, measured by dynamic light scattering, increased upon loading GTF, intending the accretion of drug in the interfacial layers of microdomains. Likewise, fluorescence probing sense an interfacial hydrophobic environment to GTF molecules in any of the examined formulations, which may be of significant interest for understanding the kinetics of drug release.

Micellar and bicontinuous microemulsion structures show different solute–solvent interactions: a case study using ultrafast nonlinear infrared spectroscopy

Using aqueous and organic probes to simultaneously explore the structural dynamics of reverse micellar and bicontinuous microemulsion structures.

Encapsulation of Antibiotic Levofloxacin in Biocompatible Microemulsion Formulation: Insights from Microstructure Analysis

Microemulsions (μEs) are unique systems that offer exciting perspectives in biophysical research for mimicing biomembranes at the molecular level. In the present study, biocompatible μE formulation of a new oil-in-water (o/w) system comprising clove oil/Tween 20/2-propanol/water was accomplished for encapsulating an antibiotic, levofloxacin (LVF). The pseudoternary phase diagram was delineated at a constant cosurfactant/surfactant (2:1) ratio to meet the economic feasibility. The gradual changes occurring in the microstructure of the as-formulated four-component μEs were explored via multiple complementary characterization techniques. The results of electrical conductivity (σ), viscosity (η), and optical microscopic measurements suggested the existence of a percolation transition to a bicontinuous structure in the microregions of the as-formulated μE. LVF displayed a high solubility (5.0 wt %) at the pH of 6.9 in an optimum μE formulation comprising 2-propanol (36.4%), Tween 20 (18.2%), clove oil (20.7%), and water (24.7%). The LVF-loaded μE composition showed long-term stability for over 6 months of storage. Fourier transform IR analysis showed that LVF was stable inside the μE formulation, indicating the absence of any possible aggregation of LVF. Dynamic light scattering revealed that the average particle size of drug-free μE (64.5 ± 3.4 nm) increases to 129.7 ± 5.8 nm upon loading of LVF, suggesting the accumulation of LVF in the interfacial layers of the micelles. Moreover, fluorescence measurements indicated that LVF might be localized in the interfacial film of μE system, which may result in a controlled release of drug.

Physicochemical investigation of mixed surfactant microemulsions: water solubilization, thermodynamic properties, microstructure, and dynamics

In this contribution, we report on a systematic investigation of phase behavior and solubilization of water in water-in-heptane or decane aggregates stabilized by mixtures of polyoxyethylene (20) cetyl ether (Brij-58) and cetyltrimethylammonium bromide (CTAB) surfactants with varying compositions in conjugation with 1-pentanol (Pn) at fixed surfactant(s)/Pn ratio and temperature. Synergism in water solubilization was evidenced by the addition of CTAB to Brij-58 stabilized system in close proximity of equimolar composition in both oils. An attempt has been made to correlate composition dependent water solubilization and volume induced conductivity studies to provide insight into the solubilization mechanism of these mixed systems. Conductivity studies reveal the ascending curve in water solubilization capacity-(Brij-58:CTAB, w/w) profile as the interdroplet interaction branch indicating percolation of conductance and the descending curve is a curvature branch due to the rigidity of the interface in these systems. The microstructure of these systems as a function of surfactant composition has been determined by dynamic light scattering (DLS) and Fourier transform infrared spectroscopy (FTIR) measurements. FTIR study reveals increase and decrease in relative population of bound and bulk-like water, respectively, with increase in Brij-58:CTAB (w/w). DLS measurements showed that the droplet hydrodynamic diameter (Dh) decreases significantly with the increase in Brij-58:CTAB (w/w). Further, the interfacial composition and energetic parameters for the transfer of Pn from bulk oil to the interface were evaluated by the dilution method. Formation of temperature-insensitive microemulsions and temperature invariant droplet sizes are evidenced in the vicinity of the equimolar composition. The results are interpreted in terms of a proposed mechanism.

Fluorescence resonance energy transfer in microemulsions composed of tripled-chain surface active ionic liquids, RTILs, and biological solvent: an excitation wavelength dependence study

In this article we have reported the fluorescence resonance energy transfer (FRET) study in our earlier characterized surface active ionic liquids (SAILs)-containing microemulsion, i.e., N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([P13][Tf2N])/[CTA][AOT]/isopropyl myristate ([IPM]) and N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide ([N3111][Tf2N])/[CTA][AOT]/[IPM] microemulsions (Banerjee, C.; Mandal, S.; Ghosh, S.; Kuchlyan, J.; Kundu, N.; Sarkar, N. J. Phys. Chem. B 2013, 117, 3927-3934). The occurrence of effective FRET from the donor, coumarin-153 (C-153) to the acceptor rhodamine 6G (R6G) is evident from the decrease in the steady state fluorescence intensity of the donor with addition of acceptor and subsequent increase in the fluorescence intensity of the acceptor in the presence of donor. The excitation wavelength dependent FRET from C-153 to R6G has also been performed to assess the dynamic heterogeneity of these confined systems. In time-resolved experiments, the significant rise time of the acceptor in the presence of the donor further confirms the occurrence of FRET. The multiple donor-acceptor (D-A) distances, for various microemulsions, obtained from the rise times of the acceptor emission in the presence of a donor can be rationalized from the varying distribution of the donor, C-153, in the different regions of the microemulsion. Time-resolved measurement reveals that with increasing excitation wavelength from 408 to 440 nm, the contribution of the faster rise component of FRET increases significantly due to the close proximity of the C-153 and R6G in the polar region of the microemulsion where occurrence of FRET is very high. Moreover, we have also studied the FRET with variation of R (R = [room temperature ionic liquids (RTILs)]/[surfactant]) and shown that the effect of excitation wavelength on FRET is similar irrespective of R values.

Unique characteristics of ionic liquids comprised of long-chain cations and anions: a new physical insight

We have designed a unique class of surface active ionic liquids (SAILs) and utilized them to prepare IL-in-oil microemulsions as well as large unilamellar vesicles (LUVs). The IL-in-oil microemulsions were characterized by a phase behavior study, regular swelling behavior, and also by spectral shift of coumarin-480 probe molecules. The LUVs were characterized by dynamic light scattering and transmission electron microscope measurements. Our work opens up the possibility of creating a huge number of IL-in-oil microemulsions as well as LUVs simply by replacing the cation of NaAOT with a long chain cation.

Effect of hydrophobic interaction on structure, dynamics, and reactivity of water

The effect of hydrophobic interaction on water is still controversial and requires more detailed experimental and theoretical investigation. The interaction between organic-water molecular complexes might be indicative of the perturbation of hydrogen-bond network in the tetrahedral structure of bulk waters, due to hydrophobic effect. In this contribution, femto/picosecond-resolved solvation dynamics techniques have been adopted to explore the dynamical modification of water clusters in hydrophobic solvent methyl tert-butyl ether (MTBE). The dynamical evolution of water molecules at the surface of micelle-like MTBE has also been studied. Dynamic light scattering techniques have been employed to determine the size of the molecular clusters being formed in respective solvents. Fourier transform infrared (FTIR) spectroscopy well measures the changes in O-H vibration frequency of water induced by MTBE. We have also monitored temperature dependent picosecond-resolved solvation dynamics in order to explore the energetics associated with water solvation in bulk MTBE. Using detailed ab initio calculations at the MP2 level, our study attempts to predict the possible structures, energies, and thermochemical parameters of corresponding MTBE-water molecular complexes in more detail. The chemical reactivity of water further confirms the effect of the hydrophobic interaction on water molecules. The results impart an understanding on hydrophobic interaction imposed by a biomolecule on the structure and reactivity of water, significant for the in vivo cellular condition.

Picosecond-resolved solvent reorganization and energy transfer in biological and model cavities

Water molecules in hydrophobic biological cleft/cavities are of contemporary interest for the biomolecular structure and molecular recognition of hydrophobic ligands/drugs. Here, we have explored picosecond-resolved solvation dynamics of water molecules and associated polar amino acids in the hydrophobic cleft around Cys-34 position of Endogenous Serum Albumin (ESA). While site selective acrylodan labeling to Cys-34 allows us to probe solvation in the cleft, Förster resonance energy transfer (FRET) from intrinsic fluorescent amino acid Trp 214 to the extrinsic acrylodan probes structural integrity of the protein in our experimental condition. Temperature dependent solvation in the cleft clearly shows that the dynamics follows Arrhenius type behavior up to 60 °C, after which a major structural perturbation of the protein is evident. We have also monitored polarization gated dynamics of the acrylodan probe and FRET from Trp 214 to acrylodan at various temperatures. The dynamical behavior of the immediate environments around the probe acrylodan in the cleft has been compared with a model biomimetic cavity of a reverse micelle (w0 = 5). Using same fluorescent probe of acrylodan, we have checked the structural integrity of the model cavity at various temperatures using picosecond-resolved FRET from Trp to acrylodan in the cavity. We have also estimated possible distribution of donor-acceptor distances in the protein and reverse micelles. Our studies reveal that the energetics of the water molecules in the biological cleft is comparable to that in the model cavity indicating a transition from bound state to quasibound state, closely consistent with a recent MD simulation study.