Martins, P.; Caparros, C.; Gonçalves, R.; Manuel, P.; Benelmekki, M.; Botelho, G.; Lanceros-Mendez, S. Journal of physical chemistry Vol. 116, num. 29, p. 15790-15794 DOI: 10.1021/jp3038768 Data de publicació: 2012-06-26 Article en revista
The electroactive ß-phase of poly(vinylidene fluoride) (PVDF) can be nucleated by introducing CoFe2O4 nanoparticles within the polymer matrix, leading to electroactive materials with large potential for sensor and actuator applications. The effects of the CoFe2O4 nanoparticle electrostatic charge on the phase crystallization of PVDF polymer is reported. For this purpose, CoFe2O4 nanoparticles were coated with anionic (SDS), nonanionic (Triton X-100), and cationic (CTAB) surfactants, and the obtained coated nanoparticles were used as fillers. It is found that the piezoelectric ß-form of the polymer increases when CoFe2O4 nanoparticles with higher negative electrostatic charge are added. This behavior is attributed to the interaction between the negatively charged magnetic particles and the polymer CH2 groups, having a positive charge density. Further the relationship between the ß-phase content and the piezoelectric response has been demonstrated. The magnetostriction of the ferrite nanoparticles and the proven piezoelectricity of the polymer allows the use of the material in piezoelectric and magnetoelectric sensors or/and actuators.
Preat, J.; Teixeira-Dias, Bruno; Michaux, C.; Perpete, E.; Aleman, C. Journal of physical chemistry Vol. 115, num. 46, p. 13642-13648 DOI: 10.1021/jp2076676 Data de publicació: 2011-10-08 Article en revista
Kinetic investigation of 1-octene bromination in AOT-isooctane-water microemulsions (13 = w = [H2O]/[AOT] = 24 and 6 = z = [IO]/[AOT] = 57) shows that the reaction is first-order in alkene and first-order in bromine, as usually found in protic media. Although both reagents are mainly located in the isooctane phase (Ktr, transfer coefficients from isooctane to water, are 1.5 × 10-5 and 8.8 × 10-3 for alkene and bromine, respectively), bromination occurs in an aqueous microenvironment, as illustrated by the high sensitivity of the bromination rate to the water content of the microemulsion. A kinetic pseudophase model describes the rate constant dependence on microemulsion composition satisfactorily by assuming competition between reactions at the interface and in the aqueous phase. Reasonable values for the coefficients of reagent partition between the interface and the two microphases and for the local bromination rate constants are obtained from the kinetic equations derived from the model. In particular, spectroscopically observed AOT-bromine complexation is in agreement with the high bromine concentration at the interface (K2, bromine partition coefficient from isooctane to interface, = 6.8). The water-phase bromination rate constant, kw = 1 × 108 M-1 s-1, is in the same range as that measured in bulk water. The lower limit for the interfacial rate constant, ki, is 103 M-1 s-1, a value close to that observed in poorly aqueous methanol (MeOH/H2O = 95/5 v/v). These data are compared with those recently obtained in the same microemulsions for solvolysis, a reaction which, like bromination, is water-promoted but supposed to take place at the interface only. The results are discussed in terms of the chemical properties of the water molecules encased in the microemulsion droplets.
Molecular dynamics is applied to analyze the association-dissociation process that takes place between contact and solvent-separated ion pairs for aqueous sodium chloride. A flexible single point charge model for water has been assumed. The reactive flux method has been used to compute the forward and backward rate constants. Activated trajectories were sampled according to the constrained reaction coordinate dynamics technique. A detailed study of the influence of the polarization state of the solvent on the reaction rate and of kinetic properties of transition-state recrossings is carried out. The time-dependent friction coefficient on the reaction coordinate has been calculated in order to test the GroteHynes theory. Theoretical predictions are in good agreement with computer simulation results.
We performed molecular dynamics simulations of single Na’ and F ions in aqueous solutions. Two single point charge water models with and without internal degrees of freedom were considered. Structural (radial distribution functions, orientation angles), dynamical (translational, vibrational, and reorientational motions), and other microscopic properties (hydration numbers, residence times) of ions and water molecules of their hydration shell were calculated. Our results are compared both with experimental data and with other simulation results using different interaction models. The influence of the flexibility of water molecules on the different properties is carefully discussed.