Dan F. Anghel

Effect of the surfactant head group length on the interactions between polyethylene glycol monononylphenyl ethers and poly(acrylic acid)

Abstract The behavior of homogeneous and polydisperse ethoxylated nonylphenols in the presence of poly(acrylic acid) (PAA) was investigated by fluorescence, surface tension, viscometric and pH measurements. Critical micelle concentration (CMC), critical aggregation concentration (CAC or T 1 ) and various other critical points of interaction such as the appearance of free surfactant molecules at the air/water interface ( T 2 ′) and of micelles into solution ( T 2 ) were determined. For polydisperse surfactants, the surface excess, areas per molecule and the composition of polymer–surfactant complexes were also established. T 1 was lower than CMC and was not influenced significantly by the length and dispersity of the poly(ethylene oxide) (PEO) chain. For the polydisperse surfactants, T 2 ′ and T 2 decrease with increasing PEO chain length. The PAA practically does not affect the area per molecule, irrespective of the ethoxylation degree. The results suggest that complexes originate from hydrogen bonding and are further stabilized by hydrophobic interactions of the surfactant tails.

Analysis of alkylphenol-based non-ionic surfactants by high-performance liquid chromatography

Abstract Ethoxylated non-ionic surfactants derived from octyl- and nonylphenols were analyzed by normal-phase HPLC. A silica column (Si-100, 5 μm) with gradient elution and UV detection at 280 nm were used for oligomer determination. Baseline separation of the ethylene oxide adducts of alkylphenols with average ethoxymer numbers as high as 40 was achieved. The elution system and the gradient profile used depend on the average ethoxymer number of the surfactant. The method can provide full information for the quantitative calculation of the oligomer distribution from ethoxylated surfactants. The hydrophobic moiety of the surfactants was characterized by reversed-phase chromatography. An octadecylsilica column (RP-18, 5 μm) and methanol—water (8:2, v/v) as eluent were used. The procedure is useful for the rapid identification of surfactants according to the alkylphenyl chain.

EFFECTS OF CETYLTRIMETHYLAMMONIUM BROMIDE (CTAB) SURFACTANT UPON THE DIELECTRIC PROPERTIES OF YEAST CELLS

The dielectric properties of yeast cells in the absence and presence of cetyltrimethylammonium bromide (CTAB) were investigated. The surfactant concentration range was between 0.0 and 1.0 mM. The experimental permittivity and conductivity spectra of frequency were analyzed by means of the two-shell electrical cell model (Irimajiri et al., Bull. Inst. Chem. Res., Kyoto Univ. 69 (1991) 421-438), and the electrical phase parameters of cells were subsequently evaluated. The cytoplasm conductivity and the conductivity of the vacuole interior decreased drastically by treating the cells with surfactant. The apparent capacitance of the plasma membrane increased systematically from 0.65 microF/cm2, for untreated cells, up to about 0.75 microF/cm2, at 0.3 mM CTAB. This growth was ascribed to the increase in the folding of the membrane surface associated with the surfactant-induced cell shrinkage. A further addition of the surfactant entailed a gradual decrease of the capacitance that was assigned to the membrane solubilization by the surfactant molecules. Within the accuracy of the data, the specific capacitance of the vacuole membrane was nearly constant (0.544+/-0.021 microF/cm2) over the whole surfactant concentration range. Also, the cytoplasm permittivity remained constant at 64.3+/-4.5.

Some critical points in the interaction between homogeneous non-ionic surfactants and poly(acrylic acid)

Abstract Single hexaethylene glycol mono(n-dodecyl) ether or octaethylene glycol mono(n-dodecyl) ether in aqueous solution and their mixtures with poly(acrylic acid) (PAA) were studied by surface tension, fluorescence, dye solubilization, viscosity and pH methods. The CMC and several critical surfactant concentrations for the interaction were determined. The starting point of micellar aggregation on the PAA chain (T1) and the point at which free micelles appeared in the solution were found and identified by the various methods. The concentration at which free surfactant molecules reappear in the system, was a transitional range rather than a critical point and was identified with certainty only by surface tension measurements. The data were discussed taking into account the interaction of both ethoxylated and hydrophobic moieties of the surfactant with PAA (i.e. hydrogen bonding and hydrophobic bonding respectively). Finally, the relationship between T1 and the ethoxylated and hydrophobic moieties of the surfactants was considered.

Development and Evaluation of New Microemulsion-Based Hydrogel Formulations for Topical Delivery of Fluconazole

The aim of the present investigation was to develop and evaluate microemulsion-loaded hydrogels (MEHs) for the topical delivery of fluconazole (FZ). The solubility of FZ in oils, surfactants and cosurfactants was evaluated to identify the components of the microemulsion. The pseudo-ternary phase diagrams were constructed using the novel phase diagram by micro-plate dilution method. Carbopol EDT 2020 was used to convert FZ-loaded microemulsions into gel form without affecting their structure. The selected microemulsions were assessed for globule size, zeta potential and polidispersity index. Besides this, the microemulsion-loaded hydrogel (MEH) formulations were evaluated for drug content, pH, rheological properties and in vitro drug release through synthetic membrane and excised pig ear skin in comparison with a conventional hydrogel. The optimised MEH FZ formulations consisting of FZ 2%, Transcutol P 11.5% and 11%, respectively, as oil phase, Lansurf SML 20-propyleneglycol 52% and 50%, respectively, as surfactant–cosurfactant (2:1), Carbopol EDT 2020 1.5% as gelling agent and water 34.5% and 37%, respectively, showed highest flux values and high release rate values, and furthermore, they had low surfactant content. The in vitro FZ permeation through synthetic membrane and excised pig ear skin from the studied MEHs was best described by the zero-order and first-order models. Finally, the optimised MEH FZ formulations showed similar or slightly higher antifungal activity as compared to that of conventional hydrogel and Nizoral® cream, respectively. The results suggest the potential use of developed MEHs as vehicles for topical delivery of FZ, encouraging further in vitro and in vivo evaluation.

A combined binding mechanism of nonionic ethoxylated surfactants to bovine serum albumin revealed by fluorescence and circular dichroism.

The study systematically investigates aqueous mixtures of fixed bovine serum albumin (BSA) and various ethoxylated nonionic surfactants belonging to a homologous series or not. Mono-disperse tetra-(C12E4), hexa-(C12E6) and octa-ethyleneglycol mono-n-dodecyl ether (C12E8), and poly-disperse eicosa-ethyleneglycol mono-n-tetradecyl ether (C14EO20) are respectively employed. Fluorescence and circular dichroism measurements are performed at surfactant/protein molar ratios (rm)s lower and higher than one. We aim to get new insights into the binding mechanism of these species and to differentiate among the interaction abilities of these surfactants. The relative magnitude of the binding thermodynamic parameters by fluorescence, and the increase of α-helix prove that hydrogen bonding drives the interaction next to the hydrophobic attraction. C12En (n=4,6,8) develop more H bonds with the albumin than C14EO20 owing to a zigzag conformation of their short ethyleneoxide chains. Among the homologous surfactants, C12E6 has a slightly stronger interaction with BSA due to a maximal number of H bonds at a minimal hindering. Static fluorescence and dynamic fluorescence indicate an inter-conversion between the tryptophan (Trp) rotamers which happens around the surfactants critical micellar concentration. For C14EO20, the meander conformation of the polar group determines a less evident conversion of the Trp rotamers and smaller α-helix rise. Binding isotherms of the homologous surfactants and the fluorescence quenching mechanism by C12E6 are also provided.

Effect of substrate hydrophobicity and electrolytes upon the micellar hydrolysis of p-nitrophenyl esters

Basic hydrolysis of p-nitrophenyl acetate, propionate and butyrate was studied in the presence of micellar hexadecyltrimethylammonium bromide. The influence of potassium bromide upon the reaction was also considered. The kinetics show that the overall reaction rate increases in the micellar systems. To explain the effect of micelles upon the reactions, the pseudo-phase ion-exchange model was used. From the fitting of data, the solubilization and the ion-exchange constants were obtained. The micellar second-order rate constants were not greater than those in aqueous systems. Hence, the micelles affect the rate by bringing the reactants together into a small volume, and not by providing a more favorable microenvironment for the reaction.

Improving the Properties of CdS Nanoparticles by Adding Polymers

The properties of cadmium sulfide (CdS) nanoparticles generated in the aqueous phase by adding a specially obtained maleic anhydride/styrene copolymer and polymers with different electrical charges—like alginate, chitosan, carrageenan, and polyvinyl pyrrolidone—were improved in order to reduce their toxicity. The cytotoxic effect of CdS semiconductor nanoparticles coated with polymers on cultures of Vero cells was investigated. CdS semiconductor nanoparticles were synthesized and stabilized in aqueous-polymeric system by mixing cadmium nitrate with sodium sulfide into aqueous polymeric solution, with nitrogen gas bubbled throughout the system. The fluorescence properties of nanoparticles depend on the nature of capping polymer. Transmission electronic microscopy gives the information about the size and size distribution of the nanoparticles. The computed size diagrams of CdS nanoparticles have different frequency depending of the used polymer. The nanoparticles coated with polymers presented low toxicity against cultures of Vero cells for a determined period of time. The toxicity diminished up to 10 times by covering the semiconductor nanoparticles with polymers.

Antioxidant activity of rosemary extracts in solution and embedded in polymeric systems

The rosemary extract was encapsulated in polyethylene or in covalently-based network gels. The covalent gels were obtained by the reaction of isocyanate end-capped polyethylene glycol (PEG) with β-cyclodextrin or glycerol. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay was used to evaluate the antioxidant activity (AA) of rosemary extract entrapped in polymeric structures and in ethanol or water solutions. The AA of the rosemary extract was determined using a DPPH radical for samples prepared in ethanol, and a water-soluble derivative, the sulphonated DPPH radical (DPPH-SO3Na), for the rosemary extract in water. Formulation of the rosemary extract in polymeric gels ensures a rapid release which determines the AA values similar to those in solution.

Catechin oxidation products: mechanistic aspects and kinetics

The oxidation of catechin in the presence of atmospheric oxygen in a methanol/buffer model solution was studied by investigating generated products using LC/MS and spectrophotometry. It was proved that catechin was slowly converted to dimeric species with similar structures and properties. The catechin autoxidation was investigated spectrophotometrically at different pH ranging between 5 and 10. Using the initial linear dependence of the kinetic curves [Dimers] = f(t) the initial reaction rates were estimated for the autoxidation of catechin in methanol/buffered solutions. It was found that the dimer formation is favored at pH 8.