Effects of fatty acids on polyelectrolyte–surfactant interactions

Phase Transitions of Isotropic to Anisotropic Biocompatible Lipid-Based Drug Delivery Systems Overcoming Insoluble Benznidazole Loading

Previous studies reported low benznidazole (BNZ) loading in conventional emulsions due to the weak interaction of the drug with the most common oils used to produce foods or pharmaceuticals. In this study, we focused on how the type of surfactant, surfactant-to-oil ratio w/w (SOR) and oil-to-water ratio w/w (OWR) change the phase behavior of different lipid-based drug delivery systems (LBDDS) produced by emulsion phase inversion. The surfactant mixture composed of soy phosphatidylcholine and sodium oleate (1:7, w/w, hydrophilic lipophilic balance = 16) stabilized medium chain triglyceride in water. Ten formulations with the clear aspect or less turbid dispersions (five with the SOR ranging from 0.5 to 2.5 and five with the OWR from 0.06 to 0.4) were selected from the phase behavior diagram to assess structural features and drug-loading capacity. The rise in the SOR induced the formation of distinct lipid-based drug delivery systems (nanoemulsions and liquid crystal lamellar type) that were identified using rheological measurements and cross-polarized light microscopy images. Clear dispersions of small and narrow droplet-sized liquid-like nanoemulsions, Newtonian flow-type, were produced at SOR from 0.5 to 1.5 and OWR from 0.12 to 0.4, while clear liquid or gel-like liquid crystals were produced at SOR from 1.5 to 2.5. The BNZ loading was improved according to the composition and type of LBDDS produced, suggesting possible drug location among surfactant layers. The cell viability assays proved the biocompatibility for all of the prepared nanoemulsions at SOR less than 1.5 and liquid crystals at SOR less than 2.5, demonstrating their promising features for the oral or parenteral colloidal delivery systems containing benznidazole for Chagas disease treatment.

Preparation and properties of EDC/NHS mediated crosslinking poly (gamma-glutamic acid)/epsilon-polylysine hydrogels

In this paper, a novel pH-sensitive poly (amino acid) hydrogel based on poly γ-glutamic acid (γ-PGA) and ε-polylysine (ε-PL) was prepared by carbodiimide (EDC) and N-hydroxysuccinimide (NHS) mediated polymerization. The influence of PGA/PL molar ratio and EDC/NHS concentration on the structure and properties was studied. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) proved that hydrogels were crosslinked through amide bond linkage, and the conversion rate of a carboxyl group could reach 96%. Scanning electron microscopy (SEM) results showed a regularly porous structure with 20 μm pore size in average. The gelation time in the crosslink process of PGA/PL hydrogels was within less than 5 min. PGA/PL hydrogels had excellent optical performance that was evaluated by a novel optotype method. Furthermore, PGA/PL hydrogels were found to be pH-sensitive, which could be adjusted to the pH of swelling media intelligently. The terminal pH of swelling medium could be controlled at 5 ± 1 after equilibrium when the initial pH was within 3-11. The swelling kinetics was found to follow a Voigt model in deionized water but a pseudo-second-order model in normal saline and phosphate buffer solution, respectively. The differential swelling degrees were attributed to the swelling theory based on the different ratio of -COOH/-NH2 and pore size in hydrogels. The results of mechanical property indicated that PGA/PL hydrogels were soft and elastic. Moreover, PGA/PL hydrogels exhibited excellent biocompatibility by cell proliferation experiment. PGA/PL hydrogels could be degraded in PBS solution and the degradation rate was decreased with the increase of the molar ratio of PL. Considering the simple preparation process and pH-sensitive property, these PGA/PL hydrogels might have high potential for use in medical and clinical fields.

Composition and structural transitions of polyelectrolyte-surfactant complexes in the presence of fatty acid studied by NMR and cryo-SEM

Insoluble complexes formed when a cationic polyelectrolyte is neutralized by the oppositely charged surfactant sodium dodecylethersulfate (SDES) in the presence and absence of lauric acid (LA) have been examined directly using NMR spectroscopy and cryo-SEM. Below the SDES critical micelle concentration (CMC') the insoluble complex contains about 10 times more water than just above CMC'. This is related to a structural transition of the complex, where water is contained mainly in larger compartments below CMC' and then mainly in narrower compartments above CMC'. The structure of the complex's solid matrix was monitored by recording two-dimensional T2-diffusion correlation spectra of the water proton resonance, which reveal the presence of several different water environments which correspond to different complex structures. Structural features in the micrometer range were confirmed using cryo-SEM. When LA is present, the larger water compartments seen below CMC' are to some extent present in the entire SDES concentration range, which is not the case in the absence of LA. Furthermore, the inclusion of LA into the SDES aggregates above CMC' leads to a lamellar sheetlike organization of the polyelectrolyte-stabilized surfactant phase. In the absence of LA, a stringy network of fibers is seen in cryo-SEM images, indicating a spherical or rodlike SDES phase. Consequently, the complex without LA holds about 1.7-1.9 times more water than the complex with LA above the SDES CMC'. T1 relaxation, (13)C chemical shifts, and (1)H resonance line widths of SDES in the system support the above observations. The combination of MAS NMR, T2-diffusion correlation, and cryo-SEM proved to be an effective method for studying structural transitions in the surfactant-polyelectrolyte(-LA) insoluble complexes.

The nature of fatty acid interaction with a polyelectrolyte-surfactant pair revealed by NMR spectroscopy

The interaction mechanisms of an oppositely charged polyelectrolyte-surfactant pair and dodecanoic (lauric) acid (LA) were experimentally investigated using a combination of nuclear magnetic resonance (NMR) techniques. It is observed that LA significantly affects the interaction between the anionic surfactant sodium dodecylethersulfate (SDES) and the cationic polymer guar modified with grafted hydroxypropyl trimethylammonium chloride (Jaguar C13 BF). Typically, oppositely charged polymers and surfactants interact electrostatically at a certain surfactant concentration known as the critical aggregation concentration (CAC). Once the polymer is neutralized by the surfactant, an insoluble complex (precipitate) is observed (phase separation), and, at concentrations beyond the surfactant critical micellar concentration (CMC'), the system returns to a one phase entity. In a system in which a mixture of SDES-LA is added to the polymer, NMR data show that below the neutralization onset, some of the polymer interacts with SDES, while some of the polymer is adsorbed at the surface of LA solid aggregates present in the system. Furthermore, SDES is found to aggregate in a lamellar-like structure at the polymer side chain prior to the SDES CMC'. Above the SDES (CMC'), LA is solubilized and incorporated at the palisade region of SDES micelles. Analysis of (1)H resonances provided estimated concentrations of all species in the system phases at all stages of interaction.