The systematic description of the complex photophysical behavior of pyrene in surfactant solution... more The systematic description of the complex photophysical behavior of pyrene in surfactant solutions in combination with a quantitative model for the surfactant concentrations reproduces with high accuracy the steady-state and the time resolved fluorescence intensity of pyrene in surfactant solutions near the cmc, both in the monomer and in the excimer emission bands. We present concise model equations that can be used for the analysis of the pyrene fluorescence intensity in order to estimate fundamental parameters of the pyrene-surfactant system, such as the binding equilibrium constant K of pyrene to a given surfactant micelle, the rate constant of excimer formation in micelles, and the equilibrium constant of pyrene-surfactant quenching. The values of the binding equilibrium constant K TX100 = 3300 10 3 M -1 and K SDS = 190 10 3 M -1 for Triton X-100 (TX100) and SDS micelles, respectively, show that the partition of pyrene between bulk water and micelles cannot be ignored, even at relatively high surfactant concentrations above the cmc. We apply the model to the determination of the cmc from the pyrene fluorescence intensity, especially from the intensity ratio at two vibronic bands in the monomer emission or from the ratio of excimer to monomer emission intensity. We relate the finite width of the transition region below and above the cmc with the observed changes in the pyrene fluorescence in this region.
A quantitative empirical model is presented, which relates the total surfactant concentration wit... more A quantitative empirical model is presented, which relates the total surfactant concentration with the fluorescence intensity and the mean translational diffusion coefficient of dyes in micellar solutions, as determined by Fluorescence Correlation Spectroscopy (FCS). Based on this model a systematic data analysis method is defined for the precise determination of the dye binding equilibrium constant, the critical micelle concentration, the translational diffusion coefficients, and the hydrodynamic radii of dyes 10 and micelles and related properties. The method can be used for the routine and automatic determination of the cmc of surfactant solutions. The model is applied to dyes with very different hydrophobicity in aqueous solutions of the non-ionic surfactant Triton X-100. As a reference the cmc of Triton X-100 is also determined directly from changes in the absorbance of its phenyl-ring applying a ratiometric method. 65 characterization of a surfactant solution, for example for the determination of the cmc, it is imperative to take the dye exchange equilibrium into account.
An empirical model for the concentrations of monomeric and micellized surfactants in solution is ... more An empirical model for the concentrations of monomeric and micellized surfactants in solution is presented as a consistent approach for the quantitative analysis of data obtained with different experimental techniques from surfactant solutions. The concentration model provides an objective definition of the critical micelle concentration (cmc) and yields precise and well defined values of derived physical parameters. The use of a general concentration model eliminates subjective graphical procedures, reduces methodological differences, and thus allows one to compare directly the results of different techniques or to perform global fits. The application and validity of the model is demonstrated with electrical conductivity, surface tension, NMR chemical shift and self-diffusion coefficient data for the surfactants SDS, CTAB, DTAB, and LAS. In all cases the derived models yield excellent fits of the data. It is also shown that there is no need to assume the existence of different premicellar species in order to explain the chemical shifts and self-diffusion coefficients of SDS as claimed recently by some authors.
We have studied the complexation between an adamantane derivative labelled with the fluorescent p... more We have studied the complexation between an adamantane derivative labelled with the fluorescent probe Alexa 488 and the three natural cyclodextrins (-, and -CD) by Fluorescence Correlation Spectroscopy (FCS), demonstrating the ability of this technique to detect association and to determine the corresponding equilibrium constants with no need for changes in the fluorescence properties of the guest. At low CD concentrations the observed increase of the diffusion time of the probe is mainly due to the complexation of the adamantyl moiety, but further changes are observed when increasing CD concentration that are attributed to the formation of CD aggregates. These aggregates appear at quite low CD concentrations and seem to be formed by a small number of CD molecules. These results show the potential of FCS for the study of CD self-assembly, a recently-recognized phenomenon that could be used to improve certain applications of CDs.
The systematic description of the complex photophysical behavior of pyrene in surfactant solution... more The systematic description of the complex photophysical behavior of pyrene in surfactant solutions in combination with a quantitative model for the surfactant concentrations reproduces with high accuracy the steady-state and the time resolved fluorescence intensity of pyrene in surfactant solutions near the cmc, both in the monomer and in the excimer emission bands. We present concise model equations that can be used for the analysis of the pyrene fluorescence intensity in order to estimate fundamental parameters of the pyrene-surfactant system, such as the binding equilibrium constant K of pyrene to a given surfactant micelle, the rate constant of excimer formation in micelles, and the equilibrium constant of pyrene-surfactant quenching. The values of the binding equilibrium constant K TX100 = 3300 10 3 M -1 and K SDS = 190 10 3 M -1 for Triton X-100 (TX100) and SDS micelles, respectively, show that the partition of pyrene between bulk water and micelles cannot be ignored, even at relatively high surfactant concentrations above the cmc. We apply the model to the determination of the cmc from the pyrene fluorescence intensity, especially from the intensity ratio at two vibronic bands in the monomer emission or from the ratio of excimer to monomer emission intensity. We relate the finite width of the transition region below and above the cmc with the observed changes in the pyrene fluorescence in this region.
A quantitative empirical model is presented, which relates the total surfactant concentration wit... more A quantitative empirical model is presented, which relates the total surfactant concentration with the fluorescence intensity and the mean translational diffusion coefficient of dyes in micellar solutions, as determined by Fluorescence Correlation Spectroscopy (FCS). Based on this model a systematic data analysis method is defined for the precise determination of the dye binding equilibrium constant, the critical micelle concentration, the translational diffusion coefficients, and the hydrodynamic radii of dyes 10 and micelles and related properties. The method can be used for the routine and automatic determination of the cmc of surfactant solutions. The model is applied to dyes with very different hydrophobicity in aqueous solutions of the non-ionic surfactant Triton X-100. As a reference the cmc of Triton X-100 is also determined directly from changes in the absorbance of its phenyl-ring applying a ratiometric method. 65 characterization of a surfactant solution, for example for the determination of the cmc, it is imperative to take the dye exchange equilibrium into account.
An empirical model for the concentrations of monomeric and micellized surfactants in solution is ... more An empirical model for the concentrations of monomeric and micellized surfactants in solution is presented as a consistent approach for the quantitative analysis of data obtained with different experimental techniques from surfactant solutions. The concentration model provides an objective definition of the critical micelle concentration (cmc) and yields precise and well defined values of derived physical parameters. The use of a general concentration model eliminates subjective graphical procedures, reduces methodological differences, and thus allows one to compare directly the results of different techniques or to perform global fits. The application and validity of the model is demonstrated with electrical conductivity, surface tension, NMR chemical shift and self-diffusion coefficient data for the surfactants SDS, CTAB, DTAB, and LAS. In all cases the derived models yield excellent fits of the data. It is also shown that there is no need to assume the existence of different premicellar species in order to explain the chemical shifts and self-diffusion coefficients of SDS as claimed recently by some authors.
We have studied the complexation between an adamantane derivative labelled with the fluorescent p... more We have studied the complexation between an adamantane derivative labelled with the fluorescent probe Alexa 488 and the three natural cyclodextrins (-, and -CD) by Fluorescence Correlation Spectroscopy (FCS), demonstrating the ability of this technique to detect association and to determine the corresponding equilibrium constants with no need for changes in the fluorescence properties of the guest. At low CD concentrations the observed increase of the diffusion time of the probe is mainly due to the complexation of the adamantyl moiety, but further changes are observed when increasing CD concentration that are attributed to the formation of CD aggregates. These aggregates appear at quite low CD concentrations and seem to be formed by a small number of CD molecules. These results show the potential of FCS for the study of CD self-assembly, a recently-recognized phenomenon that could be used to improve certain applications of CDs.
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Papers by Lucas Maseda