A. F. C. Campos

Chemical profiling of cocaine seized by Brazilian Federal Police in 2009-2012: major Components

Perfis quimicos de cocaina podem fornecer informacoes relevantes para autoridades da area de seguranca publica. Desde 2006, a Policia Federal tem trabalhado em seu proprio perfil quimico de impurezas da cocaina (projeto PeQui). No esforco de estabelecer rotinas de perfil quimico, este trabalho descreve os resultados obtidos para identificacao de componentes majoritarios (pureza da cocaina, grau de oxidacao e farmacos utilizados como adulterantes), atraves da analise por cromatografia gasosa com detector de ionizacao de chama (GC-FID) de 210 amostras apreendidas em diferentes estados brasileiros entre 2009 e 2012. A pureza media observada para cocaina foi de 71% (expressa como base) e o grau de oxidacao, determinado pela medida relativa entre cis/trans-cinamoilcocaina e cocaina, mostrou-se dependente do local de apreensao. A maioria das amostras nao oxidadas foram apreendidas nos estados que fazem fronteira com os paises produtores. A forma de base livre e a mais comumente encontrada (59%) e mais de 50% das amostras analisadas nao apresentaram nenhum adulterante majoritario. Dentre os farmacos adulterantes identificados, fenacetina foi o mais abundante (30% das amostras). Levamisol, cafeina e lidocaina tambem foram identificados. O projeto PeQui tem sido utilizado regularmente para prover informacoes tecnicas cientificamente embasadas para a analise de inteligencia em seguranca publica e de dados estatisticos que podem contribuir para um melhor entendimento do trafico de cocaina.

Influence of the spatial confinement at nanoscale on the structural surface charging in magnetic nanocolloids.

In this work we focus on the surface charging properties of core shell ferrite nanoparticles dispersed in water, namely magnetic nanocolloids. This structural charge results from the Brönsted acid-base behavior of the particles surface sites and is achieved through hydrolysis reactions. It can be modeled by considering identical charged sites behaving as weak diprotic acids. Then, electrochemical techniques could be implemented to study the acid-base equilibrium between the particle surface and the colloid bulk solution. Simultaneous potentio-conductimetric titrations are therefore performed to determine the thermodynamical constants of the p H-dependent reactions and to obtain the p H variations of the surface charge density. The results reveal that the saturation value of the structural charge strongly depends on the nanoparticle mean size. For large particles, the surface tends to be fully ionized whereas for smaller particles the saturated structural charge decreases drastically. This surface charge reduction is attributed to the existence in smaller particles of metallic surface sites, which cannot be accessible to the proton charge. The existence of such dead sites would be related to hydroxo-bonded sites with very low acidity combined with a quantum size effect, which would affect the charging/discharging process at the surface of the semiconductor ferrite quantum dot.Graphical abstract

Surface charge density determination in electric double layered magnetic fluids

We analyze potentiometric and conductimetric measurements, simultaneously performed in several complex systems (Bronsted acid-base-type). The results show an excellent agreement between reported and obtained values of the dissociation constants of the involved equilibria. A similar analysis is proposed to account, in electric double layered magnetic fluids (EDL-MF), for the charging of the particle surface based on a proton transfer proccess at the interface with the bulk dispersion. This model allows to relate the pH dependence of the phase diagram to the variations of the nanoparticle surface charge density, which leads to a useful method to monitor the colloidal stability of EDL-MF.

Size dependence of the surface charge density in EDL-MF

Abstract We determine the surface charge density of electric double layered magnetic fluids based on manganese ferrite nanoparticles of two different sizes using simultaneous potentiometric–conductimetric titrations. The saturation superficial density of charge is reduced for smaller particles.

Electrodic reduction of core–shell ferrite magnetic nanoparticles

We present square-wave voltammetry (SWV) and controlled potential coulometry (bulk electrolysis) experiments performed on maghemite and ferrite nanoparticles of core–shell-type MFe2O4@γ-Fe2O3 (where M = Mn2+, Co2+, Cu2+ or Zn2+), dispersed in aqueous media. The potential position of the iron(III) reduction peak identified in SWV measurements does not depend on the core chemical composition. No variations with size and polydispersity are detected. The shift of the reduction peak observed in coordinated configurations is related to the iron oxidizing strength. The electrolysis performed at a controlled potential of NPs indicates that only a fraction of iron(III) is reduced. Using the individual net charge deduced from the controlled potential coulometry measurements, we determine the thickness of the surface shell, which is electrolyzed.

X-DLVO Interactions between nanocolloidal magnetic particles: the quantitative interpretation of the pH-dependent phase diagram of EDL-MF

The phase behavior of acidic samples of EDL-MF based on cobalt ferrite nanoparticles with controlled mean sizes was investigated at constant temperature and in absence of magnetic field. By monitoring the nanoparticle charge by pH adjustments, we constructed an experimental pH-dependent phase diagram for all samples that revealed sol, gel thixotropic or coagulated phases in different pH regions. Then, by using an extended DLVO potential we analyzed quantitatively the observed phase diagram in function of pH and nanoparticle mean size.

Core-Shell Bimagnetic Nanoadsorbents for Hexavalent Chromium Removal from Aqueous Solutions

Novel nanoadsorbents based on core-shell bimagnetic nanoparticles (CoFe2O4@ɣ-Fe2O3) with two different mean sizes were elaborated, characterized and applied as potential sorbents for Cr(VI) removal from aqueous solutions through magnetically assisted chemical separation. The nanoadsorbents were characterized by XRD, TEM, FTIR, XPS, potentiometric-conductometric titrations, BET and vibrating sample magnetometry. The influence of contact time, shaking rate, pH, pollutant concentration, temperature and competing ions on Cr(VI) adsorption were evaluated. The results were analyzed in the framework of Langmuir and Freundlich models to evaluate the maximum adsorption capacity and the extent of affinity. The nanoadsorbents showed a good selectivity for Cr(VI) adsorption and were more effective at pH = 2.5, with a shaking rate of 400 RPM. The adsorption process was spontaneous, endothermic and presented an increased randomness. The contact time required to reach the equilibrium was relatively short and the kinetic date followed the pseudo-second-order model. The maximum adsorption capacity was nearly 40% higher for the nanoadsorbent of smaller mean size due to its higher surface area. Regeneration studies revealed that the nanoadsorbents can be recovered for reuse. These results indicate that prepared nanoadsorbents can be used as a powerful tool for Cr(VI) removal from contaminated water.