In this work, a smectite clay from the State of Paraiba, Brazil, was treated with six different types of ammonium salts, which is an usual method to enhance the affinity between the clay and polymer for the preparation of nanocomposites. The clays, before and after modification, were characterized by X ray diffraction. The conformation of the salts within the platelets of the clay depended on the number of long alkyl chains of the salt. The thermal stability of the clays was also studied. The ammonium salts thermal decomposition was explained in light of their position within the organoclays.

No abstract

. Professsor Lineu Prestes, 748, 05508-900 São Paulo -SP Recebido em 26/5/01; aceito em 20/12/01Cyclic voltammetry was used to study 3,4-dihydroxybenzaldehyde (3,4-DHB) electropolymerization processes on carbon paste electrodes. The characteristics of the electropolymerized films were highly dependent on pH, anodic switching potential, scan rate, 3,4-DHB concentrations and number of cycles. Film stability was determined in citrate/phosphate buffer solutions at the same pH used during the electropolymerization process. The best conditions to prepare carbon paste modified electrodes were pH 7.8; 0.0 £ E apl £ 0.25 V; 10 mV s -1 ; 0.25 mmol L -1 3,4-DHB and 10 scans. These carbon paste modified electrodes were used for NADH catalytic detection at 0.23 V in the range 0.015 £ [NADH] £ 0.21 mmol L -1 . Experimental data were used to propose a mechanism for the 3,4-DHB electropolymerization processes, which involves initial phenoxyl radical formation.

IIOs eletrodos modificados foram utilizados, durante 3 dias, para determinação amperométrica de NADH a 0,23 V (vs Ag/AgCI, KCI sat), obtendose resposta linear no intervalo de 0,015 s [NADH] s 0,21 mmol L-1 . NADH foi igualmente determinado usando-se sistema de análise por injeção em fluxo (FIA). A_ s condições otimizadas foram: alça de amostragem com volume 100 µL e vazão de O, 7 ml min-1 . As amostras de NADH foram preparadas em solução tampão PIPES 0,01 mal L-1 (pH 6,8), enquanto o fluído transportador utilizado foi solução 0,01 mal L-1 de tampão fosfato, pH 6,8. Resposta linear foi observada no intervalo de 0,005 s [NADH] s O, 15 mmol L-1 , com limite de detecção estimado em 0,4 µmal L-1 (3S/R) e freqüência analítica de 55 determinações/hora. III Abstract Cyclic voltammetry and deposition at controlled potential were used to study the 3,4-dihydroxybenzaldehyde (3,4-DHB) electropolymerization process at carbon paste electrode surfaces. The electropolymerized film characteristics were highly dependent on pH; anodic switching potential; scan rate; 3,4-DHB concentrations and scan numbers. The film stability was determined in citrate/phosphate buffer solutions at the sarne pH used during electropolymerization process. The best films were obtained by using cyclic voltammetry at the conditions: pH 7.8; O.O ~ Eapt ~ 0.25 V; 1 O mvs-1; 0.25mmoll-1 3,4-DHB and 1 O scans.The electropolymerization process was totally inhibited in presence of NADPH and partially inhibited in presence of NADH containing solutions. Since NADPH is a known phenoxyl radical scavenger in biological systems, the fenoxyl free radical was the responsible specie for the initial step in the electropolymerization process. From these results were concluded that it is not possible to immobilize enzymes or co-factors (NADH and NADPH) during the electropolymerization process.The modified surface was quickly lost after use in NADPH solutions, but showed upper stability in NADH solutions. These results were indicativa that NADPH and NADH catalytic oxidations could be involve the phenolic groups · present in the electropolymerized film as described by the mechanism proposed in this work. 3. 9.4 Influência do volume da alça de amostragem nos sistemas em 88 fluxo. 3.9.5 Determinação de NADH e NADPH. 89 3.1 O Estabilidade da solução de NADH 96 4 CONCLUSÕES REFERÊNCIAS BIBLIOGRÁFICAS 97 99 VIII