M.R. Pereira

Determination of deacetylation degree of chitosan: a comparison between conductometric titration and CHN elemental analysis

Chitosan is a polysaccharide used in a broad range of applications. Many of its unique properties come from the presence of amino groups in its structure. A proper quantification of these amino groups is very important, in order to specify if a given chitosan sample can be used in a particular application. In this work, a comparison between the determination of chitosan degree of deacetylation by conductometry and CHN elemental analysis was carried out, using a rigorous error analysis. Accurate expressions relating CHN composition, conductometric titration, and degree of deacetylation, in conjunction with their associated errors, were developed and reported in this note. Error analysis showed conductometric analysis as an inexpensive and secure method for the determination of the degree of deacetylation of chitosan.

Surface charging and dimensions of chitosan coacervated nanoparticles.

Chitosan nanoparticles have been used in several systems destined to controlled release of active agents. In this manuscript the process of formation of chitosan nanoparticles, obtained employing the coacervation method with sodium sulfate is analyzed using zeta potential and small angle X-ray scattering (SAXS) measurements. Dispersions were obtained at pH=1 and pH=3 and presented a behavior, in terms of surface charging, that was independent of pH. However, SAXS results indicated a dependence of size-related behavior on pH. The difference in terms of behavior was explained through the influence of enthalpic and entropically driven components.

Rheology and dynamic light scattering of octa-ethyleneglycol-monododecylether/chitosan solutions

In this work we used rheometry and DLS to probe relaxation phenomena in solutions of chitosan and octa-ethyleneglycol-monododecylether. The dispersions had a marked pseudoplastic behavior, which became less evident, as surfactant concentration was increased. Arrhenius plots showed that systems with surfactant presented a characteristic temperature at which apparent enthalpy of activation (varying from 3 to 40 kJ mol(-1)) changed: this change was correlated to a possible transition of colloidal aggregates to a wormlike configuration. DLS intensity correlation functions were described by KWW equation: pure chitosan solutions had relaxation rate distributions centered at a characteristic relaxation rate around 4.6×10(-6) μS(-1); as surfactant was added, a new component, with a faster characteristic relaxation rate with a magnitude order of 10(-3) μs(-1), appeared. It was shown that the dependence between these relaxation rates and surfactant concentration could be used to describe DLS-related relaxation phenomena as an Arrhenius-activated process, agreeing with results obtained using rheometry.

Viscosity–temperature behavior of chitin solutions using lithium chloride/DMA as solvent

Solutions of chitin in Li(+)/N,N-dimethylacetamide systems were studied via viscometry, using LiCl concentrations of 3% and 5% (m/v) and chitin concentrations ranging from 0.075 to 0.375 gL(-1). The reduced viscosity number versus concentration plot showed a minimum that was related to the formation of Li(+)-OC complex moieties along chitin macromolecular chains. Viscosity behavior was affected by temperature according to the Eyring model: concentration dependence of flow enthalpy of activation was correlated to polymer-polymer interactions and flow entropy of activation to the stiffness of the complexed chitosan macromolecular chain.

Equilibrium and kinetic aspects of sodium cromoglycate adsorption on chitosan: Mass uptake and surface charging considerations

Chitosan has more and more been suggested as a material for use as adsorbent in the treatment of effluents as well as in the synthesis of drug-loaded nanoparticles for controlled release. In both cases, a good understanding of the process of adsorption, both kinetically and in terms of equilibrium, has an importance of its own. In this manuscript we study the interaction between sodium cromoglycate, a drug used in asthma treatment, and chitosan. Equilibrium experiments showed that Sips (or Freundlich-Langmuir) isotherm described well the resultant data and adsorption possibly occurred as in multilayers. A model based on ordinary reaction-rate theory, compounded of two processes, each one with a correlated velocity constant, described the kinetics of sorption. Kinetic and equilibrium data suggested the possibility of surface rearrangement, favored by the increase of temperature.

Formation and structure of chitosan–poly(sodium methacrylate) complex nanoparticles

ABSTRACT Interpolyelectrolyte complex (IPEC) dispersions were prepared from chitosan and poly(sodium acrylate), NaPMA, by mixing their solutions, at different carboxyl-to-aminium molar ratios, rCA. Gyration radius was determined by small angle x-ray scattering (SAXS) and showed that, as rCA was increased, IPEC dimensions decreased and reached a minimum at rCA = 0.75, which was considered the ratio at which IPEC cluster dimensions were minimum, following collapse, phase segregation, nucleation, and growth of larger particles. Pair distance distributions, P(r), became narrower up to rCA = 0.75, increasing its width from this point. Relaxation-related parameters from dynamic light scattering (DLS) intensity correlation functions (ICFs) identified three main relaxation processes. The fast process, related to free polyelectrolyte molecules random motion disappeared as rCA, was increased. The other two relaxation processes also were a function of rCA and presented marked changes at rCA = 0.75. At the same value of rCA, the energy of activation for the average relaxation rate showed the occurrence of a clear change in the nature of IPEC-related interactions. As hydrodynamic diameter, determined by DLS, was much larger than the gyration radius determined by SAXS, IPEC particles could be described as being composed by a core, rich in segregated, insoluble material, enveloped by IPEC soluble clusters, possibly in the form of water-rich gels. GRAPHICAL ABSTRACT

Preparation and characterization of dispersions based on chitosan and poly(styrene sulfonate)

Interpolyelectrolyte complexes (IPECs) were obtained by the mixture of solutions of chitosan and poly(sodium 4-styrene-sulfonate). The resultant dispersions were characterized using small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Gyration radius from SAXS increased as sulfonate to aminium molar ratio, rSA, increased, while pair distance distributions, P(r), shifted to higher values of r, describing the process of particle formation as resultant from collapse of solubilized IPEC clusters in the form of solid particles, followed by nucleation of larger ones. Intensity correlation curves from DLS were used to obtain parameters related to Brownian motion of (or within) IPEC clusters. IPEC cluster dimension growth was related to characteristic relaxation rate and relaxation rate distribution width. Association of DLS-derived results to SAXS-resultant data yielded a description of nanoparticles made of solid IPEC clusters stabilized by solubilized IPEC species.

RHEOLOGY OF INVERSE EMULSIONS WITH CHITOSAN, CTAB, AND CYCLOHEXANE

Inverse emulsions consisting of chitosan solution as the dispersed phase and cyclohexane as the continuous phase were obtained, using cetyl trimethylammonium bromide (CTAB) as surfactant. The obtained dispersions clearly presented pseudoplastic character, which was evidenced by an intense drop in apparent viscosity as shear rate was increased. This pseudoplastic behavior was related to the formation of chitosan loops from the micelles to the continuous phase, promoting intermicellar interactions. Apparent shear modulus determination supported the presence of temporary cross‐linking and results from creep experiments were analyzed using Burgers model, in order to separate viscous and elastic intermicellar interactions.

Performance Studies of a Corrosion Inhibitor for Stainless Steel in Acid Conditions and High Temperatures

Abstract A commercial corrosion inhibitor named CI-11 was evaluated for stainless steel alloys (Fe-13Cr and Fe-21.5Cr) in acid conditions (10% acetic acid [CH3COOH]/1.5% hydrofluoric acid [HF]) and temperatures ranging from 168°C to 198°C. The inhibitor performances were investigated by weight-loss measurements and optical microscopy. The main variables analyzed were inhibitor concentration, test temperature, and heat treatment of the studied alloys. The results showed that the Fe-21.5Cr alloy presented superior corrosion resistance under the conditions investigated, considering temperature and inhibitor concentration. According to this study, the temperature to be used for stress-relief annealing is 400°C.

Anionic and cationic drug sorption on interpolyelectrolyte complexes

Interpolyelectrolyte complexes of chitosan and poly(sodium 4-styrenesulfonate) [NaPSS] were synthesized and obtained in the form of solid particles, with two different sulfonate to aminium molar ratios: 0.7, resulting in particles with positive zeta potential (IPEC+), and 1.4, yielding particles with negative zeta potential (IPEC-). Both particles were characterized as potential drug sorbents using differently charged drugs: sodium cromoglycate (negatively charged), and tetracycline hydrochloride (positively charged). The adsorption isotherm for cromoglycate and tetracycline on IPEC+ was adequately described by the Langmuir model, while the IPEC- sorption of tetracycline followed the Redlich-Peterson isotherm without the occurrence of cromoglycate sorption. The sorption kinetics consisted of two processes, one fast and the other slow, which were correlated to purely surface-related interactions and processes that resulted in diffusion and/or destruction/rearrangement on the particle surface and subsurface, respectively. Charge build up equilibrium and kinetics were also monitored via zeta potential measurements, and the differences between mass drug uptake and particle charging were used to propose adsorption mechanisms for the systems studied in this work.