Analysis of the phase diagram and microstructural transitions in phospholipid microemulsion systems using high-resolution ultrasonic spectroscopy

High-Resolution Ultrasound Spectroscopy for the Determination of Phospholipid Transitions in Liposomal Dispersions

High-resolution ultrasound spectroscopy (HR-US) is a spectroscopic technique using ultrasound waves at high frequencies to investigate the structural properties of dispersed materials. This technique is able to monitor the variation of ultrasound parameters (sound speed and attenuation) due to the interaction of ultrasound waves with samples as a function of temperature and concentration. Despite being employed for the characterization of several colloidal systems, there is a lack in the literature regarding the comparison between the potential of HR-US for the determination of phospholipid thermal transitions and that of other common techniques both for loaded or unloaded liposomes. Thermal transitions of liposomes composed of pure phospholipids (dimyristoylphosphatidylcholine, DMPC; dipalmitoylphosphatidylcholine, DPPC and distearoylphosphatidylcholine, DSPC), cholesterol and their mixtures were investigated by HR-US in comparison to the most commonly employed microcalorimetry (mDSC) and dynamic light scattering (DLS). Moreover, tramadol hydrochloride, caffeine or miconazole nitrate as model drugs were loaded in DPPC liposomes to study the effect of their incorporation on thermal properties of a phospholipid bilayer. HR-US provided the determination of phospholipid sol-gel transition temperatures from both attenuation and sound speed that are comparable to those calculated by mDSC and DLS techniques for all analysed liposomal dispersions, both loaded and unloaded. Therefore, HR-US is proposed here as an alternative technique to determine the transition temperature of phospholipid membrane in liposomes.

Narrow Band Solid-Liquid Composite Arrangements: Alternative Solutions for Phononic Crystal-Based Liquid Sensors

Periodic elastic composite structures attract great attention. They offer the ability to design artificial properties to advance the control over the propagation of elastic/acoustic waves. In previous work, we drew attention to composite periodic structures comprising liquids. It was shown that the transmission spectrum of the structure, specifically a well-isolated peak, follows the material properties of liquid constituent in a distinct manner. This idea was realized in several liquid sensor concepts that launched the field of phononic crystal liquid sensors. In this work we introduce a novel concept—narrow band solid-liquid composite arrangements. We demonstrate two different concepts to design narrow band structures, and show the results of theoretical studies and results of experimental investigations that confirm the theoretical predictions. This work extends prior studies in the field of phononic crystal liquid sensors with novel concepts and results that have a high potential in a field of volumetric liquid properties evaluation.

Ultrasonic Monitoring of Biocatalysis in Solutions and Complex Dispersions

The rapidly growing field of chemical catalysis is dependent on analytical methods for non-destructive real-time monitoring of chemical reactions in complex systems such as emulsions, suspensions and gels, where most analytical techniques are limited in their applicability, especially if the media is opaque, or if the reactants/products do not possess optical activity. High-resolution ultrasonic spectroscopy is one of the novel technologies based on measurements of parameters of ultrasonic waves propagating through analyzed samples, which can be utilized for real-time non-invasive monitoring of chemical reactions. It does not require optical transparency, optical markers and is applicable for monitoring of reactions in continuous media and in micro/nano bioreactors (e.g., nanodroplets of microemulsions). The technology enables measurements of concentrations of substrates and products over the whole course of reaction, analysis of time profiles of the degree of polymerization and molar mass of polymers and oligomers, evolutions of reaction rates, evaluation of kinetic mechanisms, measurements of kinetic and equilibrium constants and reaction Gibbs energy. It also provides tools for assessments of various aspects of performance of catalysts/enzymes including inhibition effects, reversible and irreversible thermal deactivation. In addition, ultrasonic scattering effects in dispersions allow real-time monitoring of structural changes in the medium accompanying chemical reactions.

Improved Method for Solid Lipid Nanoparticle Preparation Based on Hot Microemulsions: Preparation, Characterization, Cytotoxicity, and Hemocompatibility Evaluation

The ease of application and no requirement of extra energy input make the microemulsion method favorable for solid lipid nanoparticles (SLNs) production. Very limited data are available to date on preparation of SLNs from pre-screened microemulsion phase diagrams. The purpose of this study was to investigate the microemulsion formation area with solid lipids using hot ternary phase diagrams at elevated temperatures and to use selected microemulsions for SLN production. Also, we aimed to characterize obtained SLNs in terms of physicochemical properties, in vitro cell toxicity, and hemolysis. Phase diagrams of solid lipids were screened at elevated temperatures and oil-in-water microemulsion regions were determined. Microemulsions were selected, and SLNs were produced by modification of the microemulsion dilution method and characterized in terms of visual appearance, turbidity, particle size, and zeta potential. Cytotoxicity of nanoparticles was tested on L929 mouse skin fibroblast cells. Hemolytic potential was assessed in vitro using freshly isolated erythrocytes. The phase diagram screening and the modified hot microemulsion dilution method enabled production of SLNs with particle size below 100 nm. We found evidence that the solid lipids in the SLNs produced by the new method remain in supercooled liquid state. Nanoparticles prepared by the new method exhibit lower toxicity on L929 cells and have lower hemolytic potential than the formulations prepared by direct mixing of the components. The method can be used to prepare SLNs with controllable composition and small particle size below 100 nm. These SLNs are low toxic and can be used for drug delivery purposes.

Acoustic spectroscopy: A powerful analytical method for the pharmaceutical field?

Acoustics is one of the emerging technologies developed to minimize processing, maximize quality and ensure the safety of pharmaceutical, food and chemical products. The operating principle of acoustic spectroscopy is the measurement of the ultrasound pulse intensity and phase after its propagation through a sample. The main goal of this technique is to characterise concentrated colloidal dispersions without dilution, in such a way as to be able to analyse non-transparent and even highly structured systems. This review presents the state of the art of ultrasound-based techniques in pharmaceutical pre-formulation and formulation steps, showing their potential, applicability and limits. It reports in a simplified version the theory behind acoustic spectroscopy, describes the most common equipment on the market, and finally overviews different studies performed on systems and materials used in the pharmaceutical or related fields.

High-resolution ultrasonic spectroscopy study of interactions between hyaluronan and cationic surfactants

Interactions in a cationic surfactant-hyaluronan system in water and in sodium chloride solution were investigated by high-resolution ultrasonic spectroscopy at 25 °C. Two alkyltrimethylammonium bromide surfactants of different chain lengths (tetradecyl and hexadecyl) were used; hyaluronan molecular weight ranged from 10 to 1750 kDa. Two main parameters-ultrasonic velocity and attenuation-were measured in the titration regime. Up to six different regions could be identified in the velocity titration profiles in water in a narrow interval of surfactant concentration. These regions differed primarily in the compressibility of structures formed in the system. The number of detected transitions was higher for the tetradecyl surfactant; therefore, the increased length of the hydrophobic chain simplified the details of the structure-forming behavior. The measurement of attenuation was much less sensitive and detected only the formation of microheterogeneous structures or visible phase separates. The richness of the titration profiles was depressed in salt solution, where essentially only two principal regions were observed. On the other hand, the effect of hyaluronan molecular weight on the positions of boundaries between regions was more significant in the presence of salt. Besides electrostatic interactions, hydrophobic interactions are also relevant for determining the behavior of hyaluronan-surfactant systems and the properties of formed complexes (aggregates).

Volumetric and Diffusion Properties of Water/Surfactant/n-Propanol/4-Allylanisole Micellar Systems

Sol-gel encaged [(C8H17)3NCH3][RhCl4] catalyses the double bond isomerization in the flavoring agent 4-allylanisole in aqueous microemulsions. In order to provide optimal composition of the reaction medium water/n-propanol/surfactant/4-allylanisole micellar systems were formulated. The surfactants were sodium dodecyl sulfate, cetyltrimethylammonium bromide, sucrose monolaurate, and polyethylene glycol (7) glyceryl cocoate. The ratio (w/w) of n-propanol/surfactant equals 2/1. The extent of the microemulsions region as function of temperature was determined. The micellar systems were characterized by the volumetric parameters, density, excess volume, ultrasonic velocity and isentropic compressibility. The micellar densities increase with the increase in the water volume fraction. Ultrasonic velocities increase with the increase in water volume fraction up to 0.8 then decrease. Ultrasonic velocities increase with temperature for water volume fractions below 0.8 and decrease for water volume fractions above 0.8. Quantitative analysis of the volumetric parameters enabled the characterization of structural transition along the micellar phase. The particle hydrodynamic diameter of the oil-in-water systems was determined as function of temperature. The particle hydrodynamic diameter decreases in the case of the ionic surfactants while in the case of nonionic surfactants it increases.

Characterization of cephalexin loaded nonionic microemulsions

Water/propylene glycol/sucrose laurate/ethoxylated mono-di-glyceride/isopropyl myristate/peppermint oil U-type microemulsions were used to solubilize cephalexin. Microemulsion dilution and interfacial factors contributing to the cephalexin solubilization were evaluated. Cephalexin solubilization capacity increases with the increase in the aqueous phase volume fraction (φ) up to 0.4 then decreases. Electrical conductivity of drug loaded and drug free microemulsions increases with φ. The hydrodynamic radius measured by dynamic light scattering of the oil-in-water loaded microemulsions decreases with temperature. The microemulsions were characterized by the volumetric parameters, density, excess volume, ultrasonic velocity and isentropic compressibility. The microemulsion densities increase with φ up to 0.8 then decrease. The excess volume decreases with φ up to 0.8 then stabilizes. Ultrasonic velocities increase with the increase in φ while isentropic compressibility decreases. Analysis of the volumetric parameters enabled the characterization of structural transition along the microemulsion phase region. The presence of water-in-oil, bicontinuous and oil-in-water microemulsions, at aqueous phase volume fractions below 0.2, between 0.3 and 0.7 and above 0.8, respectively were found. Interfacial properties and dynamic structure of the monolayer for drug loaded and drug free microemulsions, were studied by electron paramagnetic resonance spectroscopy employing the nitroxide spin probe 5-doxylstearic acid. The rigidity of the interface was affected by the water content and also the presence of cephalexin.

Ultrasonic analysis of kinetic mechanism of hydrolysis of cellobiose by β-glucosidase

High-resolution ultrasonic spectroscopy (HR-US) was applied for real-time analysis of enzymatic hydrolysis of cellobiose by a β-glucosidase from Aspergillus niger (Novozyme 188) at 50°C and pH 4.9. This technique is noninvasive, it does not require optical transparency and is suitable to continuously monitor the time dependence of the reaction progress in a broad range of experimental conditions. The time profiles of the amount of glucose released and the reaction rate were obtained from the time profile of ultrasonic velocity. The results are in good agreement with a discontinuous glucose assay (hexokinase method). The kinetic parameters of the reaction were estimated by fitting the ultrasonic time profiles of the reaction rates to several inhibition models. In addition, the equilibrium constant for the reaction of hydrolysis of cellobiose and the molar Gibbs free energy of hydrolysis were determined from the ultrasonic time profiles of concentration of glucose in the reverse reaction (glucose condensation). The results suggest the existence of more complex mechanisms regulating the activity of cellobiase than the combination of simple inhibitions. An extended kinetic model based on two sites for the competitive inhibitor (glucose) is proposed.

Ultrafast dynamics in reverse micelles

Recent advances in ultrafast laser technology have spurred investigations of microheterogeneous solutions. In particular, researchers have explored details of reverse micelles (RMs), which present isolated droplets of polar solvent sequestered from a continuous nonpolar phase by a surfactant layer. This review explores recent studies utilizing a variety of ultrafast laser techniques to uncover details about structure and dynamics in various RMs. Using ultrafast vibrational spectroscopy, researchers have probed hydrogen-bond dynamics and vibrational energy relaxation in RMs. These studies have developed our understanding of reverse micellar structure, identifying varying water environments in the RMs. In a plethora of experiments employing probe molecules, researchers have explored the confined environment presented by RMs and their impact on a range of chemical reactions. These studies have shown that confinement, rather than the specific interactions with surfactants, is an important factor determining the impact of the reverse micellar environment on the chemistry.

Quality by design: Characterization of self-nano-emulsified drug delivery systems (SNEDDs) using ultrasonic resonator technology

In the present work, a novel application of ultrasonic measurements is detailed to characterize nano-emulsion formulations as a part of the overall Quality by Design (QbD) goal. Ultrasonic resonator technology (URT) was utilized to measure sound velocity and absorption of self-nanoemulsified drug delivery systems (SNEDDs) consisting of various ratios of oil:surfactant:co-surfactant. A QbD concept was used to create different SNEDDs formulations utilizing sweet orange oil (oil), Emulphor-620 (surfactant), and Capmul (co-surfactant) by dissolving Cyclosporine A in oil. The mixture was emulsified in water and ultrasonic measurements were carried out in an ultrasonic resonator system isothermally for a period of about 15-20min. Compressibility of the individual components in the droplets, hydration of the droplets and the influence of the composition on droplet stability were studied by systematic ultrasonic measurements at a single resonator frequency. The adiabetic compressibilities for the oil, aqueous and interfacial components were 68, 44.6, and 53 [10(-11)Pa(-1)], respectively as calculated using Urick's equation. Also the ultrasonic absorption correlated droplet size of nano-emulsions linearly with R(2) of 0.84 indicating this can be used as an additional technique to measure the droplet size of nano-emulsions. Correlation of ultrasonic data with formulation components indicated that the ultrasonic velocity correlated negatively with increasing oil amount in the formulation as well as surfactant-to-cosurfactant ratios where as droplet diameter correlated positively with these formulation factors. It can be envisioned from the results that the compressibility of the media increases with the addition of the oily component and thus reducing the sound velocity. Thus URT enabled direct and convenient analysis of the physical properties as well as influence of formulation factors of nano-emulsions which is an important indication of stability of these nano-emulsions.