A study of the synthesis of MCM-41 mesoporous molecular sieves in fluoride media, having no alkaline metal ions, was performed by changing the gel composition and crystallization temperature and time. X-ray diffraction and nitrogen adsorption analyses showed that highly ordered MCM-41 samples were obtained from gels with a NH4OH/SiO2 molar ratio in the 3.25-4.3 range (room temperature synthesis) or in the 4.3-20 range (24 hours at 373 K). During calcination, unit cell shrinkage, caused by high temperature polycondensation of the SiOH groups, was observed for all samples. The samples synthesized at high temperature (373 K) or using low pH gels (7.5) underwent lower unit cell shrinkage than those obtained at room temperature or high pH (9.0), indicating that the former samples had lower SiOH groups content than the latter. These highly-ordered samples showed large surface area (ca. 1100 m²/g) and pore volume (ca. 0.80 cm³/g), also presenting a narrow pore size distribution. Due to higher silicate polycondensation and a thicker pore wall, the samples synthesized at 373 K were more hydrothermally stable than those obtained at room temperature.

Medidas de potencial elétrico espontâneo (PE) foram realizadas no interior de poços e na superfície da terra, ao longo de seções transversais, em aqüíferos fissurais em rochas cristalinas do Estado da Bahia. Estas medidas revelam a presença de fortes anomalias de PE relacionadas à movimentação da água subterrânea. Com base em um modelo geométrico-hidráulico simplificado para um aqüífero freático, numa estrutura essencialmente bi-dimensional, e usando as equações básicas para os acoplamentos de fluxos de fluidos (ou de calor) e de concentração iônica, é mostrado que os efeitos eletrocinéticos desenvolvidos ao longo das paredes das fraturas podem produzir anomalias de PE comparáveis àquelas observadas no campo. Por outro lado, as estimativas obtidas para os acoplamentos associados aos efeitos eletroquímico e termo-elétrico são bem menores do que os estimados para o efeito eletrocinético, o que sugere que este último constitui a fonte dominante das anomalias de PE observadas em zonas fraturadas de rochas cristalinas. Com base neste resultado, foi feita a interpretação integrada de seções de resistividade aparente e perfis de PE de campo. Nesta interpretação, ficou evidenciado que o método de PE pode ser usado como uma técnica geofísica complementar ao método de eletro-resistividade na distinção de áreas com permeabilidade relativamente alta no interior de aqüíferos com porosidade fissural.

Self-potential electric (SP) measurements made in wells and at the surface along traverses crossing aquifers in fractured crystalline rocks in the state of Bahia, Brazil, revealed the presence of strong anomalies related to groundwater movements. By considering a simple geometric-hydraulic model for a phreatic aquifer, in an essentially 2-D structure, and using basic equations for ionic concentration and fluid (or heat) coupled flows, it was found that electrokinetic effects developed along fracture walls can produce anomalies of amplitude comparable to those observed in the field. Estimates obtained for other coupled processes, such as the electrochemical and the thermo-electric effects, suggest that the electrokinetic effect is the most important component for setting up SP currents in fractured zones of crystalline rocks. Based on this result, a joint interpretation of pseudoresistivity sections and SP profiles was performed and the results show that the SP method can be used as a complementary geophysical technique to the resistivity method, allowing to distinguish areas of relative high permeability within fractured rocks.