The nematic cylindrical (Nc)−nematic discotic (Nd) phase transitions in three amphiphile/decanol/water systems, occurring as a function of the decanol/amphiphile molecular ratio (M d), are correlated with a change of micellar form from spherocylinder (SC) to square tablet (ST), which occurs geometrically in a continuous way, with an intermediate biaxial object. The relative stability of these forms is measured by the micellar elastic bending energy, calculated in the surfactant parameter model, as a function of the molecular quantity p = v/al (where v, l, and a are respectively the volume and length of the hydrophobic chain and the area per polar head at the apolar−polar interface) and of the decanol/amphiphile mixing. A SC−ST transition is predicted for increasing M d and p 0 values, because the amphiphile (spontaneous p 01) and decanol (p 02) molecules prefer zones with different curvatures. A good agreement is obtained by comparison of the predicted SC−ST transformation with experimental results obtained at the Nc−Nd transitions for KL (potassium laurate), SDS (sodium decyl sulfate), and SLS (sodium dodecyl sulfate).

Benzoporphyrins, chlorin‐type compounds, are efficient new drugs in photodynamic therapy. In this work, some aspects of the benzoporphyrin‐1,3‐diene dimethyl ester (DiesterB) aggregation were investigated in water–organic solvent mixtures, using visible spectrophotometry. The effects of solvent mixtures, water with acetonitrile, dioxane, methanol and ethanol, were compared taking the calculated aggregation equilibrium constant (K) and the aggregation number. The K dependence on temperature, DiesterB concentration and water content suggest that, in water with acetonitrile, the predominant species is a dimer, whereas in dioxane, methanol and ethanol systems, there are multiple equilibria, with a pre‐equilibrium between the monomer and dimer (small amount), followed by trimer formation. Not only the solvent polarity but also the nature of the organic solvent are important for the aggregation process, driving the aggregate size and the magnitude of the equilibrium constant. Copyright © 2004 John Wiley & Sons, Ltd.

The kinetics of the aggregation process of DiesterB, a homologous coproduct generated in Verteporfin synthesis (the drug used in the medication Visudyne applied in photodynamic therapy), was investigated by visible spectrophotometry in several aqueous organic solvents (dimethyl sulfoxide (DMSO), acetonitrile, dioxane, methanol, and ethanol). The monomeric form of DiesterB is stable in pure organic solvents, showing a characteristic peak at 690 nm. In water-rich medium, an aggregation process is induced, giving rise to a new band in the 720-740 nm region. In aqueous DMSO and acetonitrile solvents this process is very fast and leads to the formation of dimers, while in dioxane-, methanol-, and ethanol-water mixtures the absorption intensities show a sigmoidal time profile, suggesting a slow initial reaction (lag phase) followed by a rapid aggregation (log phase), characteristic of autocatalyzed reactions. The proposed final species in these solvents is a trimer as the main aggregate (supported by resonance light scattering and small-angle X-ray scattering experiments). The experimental absorbance values, taken at monomer or aggregate peaks during the reaction, were fitted using a nonconventional treatment proposed by Pasternack. This model allows evaluating two rate constants, due to a first (k 0 , noncatalytic) and a second (k c , catalytic) step, as well as a parameter (m), related to the size and amount of the catalyst nucleus. Although the model was originally applied to large porphyrin arrays growing on templates, an excellent accordance was obtained between its formalism and our kinetic experimental data. The global mechanism seems to start with a dimeric nucleus formation (lag phase), which acts, despite its small amount, as a catalytic center driving to trimers (log phase). The effects caused by water content and DiesterB concentration on the kinetic results support the proposed multistep equilibrium. The absence of isosbestic points during the process reinforces the presence of more than one step. The most unusual feature is the effect of the temperature on the rate constants. As the temperature is raised, the constants increase up to a maximum, and decrease for higher temperatures. The effect is more pronounced for k c than for the k 0 rate constant. The model proposed states that at high temperatures the equilibrium is shifted toward monomers, reducing the catalyst nucleus formation and resulting in an overall reaction velocity decrease.

Relações entre ciência, tecnologia e sociedade em livros didáticos Relações entre ciência, tecnologia e sociedade em livros didáticos Relações entre ciência, tecnologia e sociedade em livros didáticos Relações entre ciência, tecnologia e sociedade em livros didáticos de RESUMO. Neste trabalho analisou-se o uso das relações Ciência, Tecnologia e Sociedade (CTS) no tema Radioatividade em livros didáticos de Química. Foram consultados professores da rede pública do Ensino Médio, que indicaram as obras mais utilizadas em sua prática docente. Os títulos sugeridos foram analisados dentro de quesitos elaborados com base no Programa Nacional do Livro Didático e nos Parâmetros Curriculares Nacionais. Estes quesitos foram divididos em dois grupos, um referente à estrutura geral da obra e outro referente às relações CTS. Os resultados mostram que as obras não têm preocupação em adotar abordagens que contemplem as relações CTS e que o assunto Radioatividade é tratado de modo descontextualizado e não atende às recomendações dos Parâmetros Curriculares Nacionais. Por fim, são feitas sugestões de reestruturação de forma e conteúdo, com a intenção de deixar os livros mais adequados para o uso do aluno, mesmo quando o professor não se faz presente.Palavras-chave: radioatividade, relações ciência, tecnologia e sociedade, livros didáticos. ABSTRACT. Relations between science, technology and society in chemistry textbooks.This work deals with the subject 'Radioactivity' as it appears in high school Chemistry textbooks. We verified whether they followed a Science, Technology and Society (STS) approach in dealing with nuclear energy. Textbooks were chosen after a survey with public high school teachers. The criteria of analysis were based on the Plano Nacional do Livro Didático (PNLD), on the Plano Nacional do Livro de Ensino Médio (PNLEM) and on the PCN (Parâmetros Curriculares Nacionais). The general structure, proposed activities, as well as the treatment given to STS relations was analyzed. The results show that the STS relations are poorly discussed. The lack of these debates constrains the possibility of students taking a critical attitude in view of the necessity of nuclear power plants in our country. Some suggestions are given, in order to improve the quality of the textbooks, re-orienting towards a STS discussion.

O presente trabalho teve como objetivo investigar as potencialidades didáticas de uma atividade experimental investigativa sobre ácidos e bases. A atividade foi desenvolvida em uma turma do segundo ano do Ensino Médio. A intervenção pedagógica envolveu reflexão sobre como reconhecer produtos ácidos e alcalinos presentes no dia a dia, a leitura e discussão de um texto sobre origem e características físicas e químicas de substâncias ácidas e alcalinas, realização do experimento utilizando várias substâncias cotidianas de caráter ácido e básico, bem como uma escala de pH preparada com extrato de repolho roxo e soluções aquosas de ácido clorídrico e hidróxido de sódio. Após o experimento, mediante discussão, os alunos, em grupos, responderam questões com vistas à análise dos dados observados. Na sequência responderam individualmente a uma questão que tinha o intuito de verificar a aplicação do conhecimento. Os resultados evidenciam que os alunos tiveram boa compreensão acerca dos conhecimentos desenvolvidos, o que nos leva a concluir que experimentos investigativos, como o discutido aqui, favorecem a aprendizagem de conhecimentos científicos.