Size and pH effect on electrical and conformational behavior of poly(acrylic acid): Simulation and experiment
ABSTRACT Monte Carlo simulations, experimental titrations and fluorescence correlation spectroscopy experiments were used to investigate the conformational and electrical properties of polyacrylic acids (PAA). On the one hand, titration curves were calculated to get an insight into the role of pH on the degree of ionization and conformation of PAA chains. On the other hand, experimental potentiometric titrations of PAA were also achieved for different PAA molecular weights and compared to the calculated titration curves obtained by Monte Carlo coarse grained simulations. It was found that for a large range at intermediate PAA ionizations, a good correlation is obtained between experimental and simulations data thanks to the prominence of electrostatic interactions in this domain. The effect of ionic concentration and PAA molecular weight on the titration curves was also investigated. In order to get a better understanding of PAA conformational behavior, we also investigated PAA diffusion properties in aqueous solutions as a function of pH and ionic strength by fluorescence correlation spectroscopy (FCS), thanks to its high sensitivity to measure diffusion coefficients of tracer solutes. Good qualitative agreements were observed between experimental diffusivities and polymer properties calculated from MC simulations. It was shown that the high molecular weight PAA chains display more significant changes in diffusivity in agreement with the ionization degrees and conformational changes observed in the simulations.
Article: Polymer HandbookNature 01/1972; 238(5358):56-56. · 38.60 Impact Factor
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ABSTRACT: Monte Carlo simulations of linear, weak polyacids and explicit simple ions have been performed in a spherical cell model to study the shift in the apparent dissociation constant. The simulations are performed in a canonical ensemble with the apparent dissociation constant calculated in a procedure based on Widom's particle-insertion method. The effects of model parameters are discussed, for example, the influence of various distances of closest approach, and the results are compared to experiments as well as simulations with effective pair-potentials between monomers, both the screened Coulomb potential and an extended version that takes into account a distance of closest approach between monomers and simple ions. Results of the latter potential are also compared to the experimental data, which are from poly(acrylic acid), poly-d,l-glutamic acid and carboxymethyl cellulose.Macromolecules. 08/2000; 33(19).
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ABSTRACT: This paper describes the first experimental application of fluorescence correlation spectroscopy, a new method for determining chemical kinetic constants and diffusion coefficients. These quantities are measured by observing the time behaviour of the tiny concentration fluctuations which occur spontaneously in the reaction system even when it is in equilibrium. The equilibrium of the system is not disturbed during the experiment. The diffusion coefficients and chemical rate constants which determine the average time behaviour of these spontaneous fluctuations are the same as those sought by more conventional methods including temperature-jump or other perturbation techniques. The experiment consists essentially in measuring the variation with time of the number of molecules of specified reactants in a defined open volume of solution. The concentration of a reactant is measured by its fluorescence; the sample volume is defined by a focused laser beam which excites the fluorescence. The fluorescent emission fluctuates in proportion with the changes in the number of fluorescent molecules as they diffuse into and out of the sample volume and as they are created or eliminated by the chemical reactions. The number of these reactant molecules must be small to permit detection of the concentration fluctuations. Hence the sample volume is small (10−8 ml) and the concentration of the solutes is low (∼ 10−9M). We have applied this technique to the study of two prototype systems: the simple example of pure diffusion of a single fluorescent species, rhodamine 6G, and the more interesting but more challenging example of the reaction of macromolecular DNA with the drug ethidium bromide to form a fluorescent complex. The increase of the fluorescence of the ethidium bromide upon formation of the complex permits the observation of the decay of concentration fluctuations via the chemical reaction and consequently the determination of chemical rate constants.Biopolymers 01/2004; 13(1):29 - 61. · 2.88 Impact Factor