Roger Rymden

Substrate binding to cyclodextrins in aqueous solution: A multicomponent self-diffusion study

It is demonstrated that substrate binding to α- and β-cyclodextrins (CD) in solution can conveniently and directly be monitored from multicomponent self-diffusion data on these solutions, using the Fourier Transform NMR pulsed-gradient spin-echo technique. Included are aromatics and a series of alcohols ranging from methanol to octanol. Experimentally it was found thatn-alcohols associate more strongly with α-CD than with β-CD. As the bulkiness of the alcohol increased, binding to β-CD was enhanced while the reverse effect was observed in the case of α-CD. For both cyclodextrins it was found thatn-alcohol complexation in the homologous series was attributable to an increment in standard free energy of complexation of ∼ −3.0 kJ/mol for each −CH2− group, suggesting that the binding mechanism is of a hydrophobic nature.

Counterion binding in aqueous solutions of mixed micellar aggregates from self-diffusion measurements

Abstract Measurements of counterion self-diffusion in mixed anionic/nonionic surfactant aqueous solution have been performed to study the dependence of micellar ion binding on micellar composition. The systems studied were mixtures of lithium dodecyl sulfate (LiDS) and ethylene glycol dodecyl ethers having different ethylene oxide chain lengths. A mixed micellar system comprising LiDS and a zwitterionic surfactant was also included in the study. Surfactant and micellar self-diffusion coefficients were obtained to estimate changes in micellar size accompanying the variation in the surfactant mixing ratio. The self-diffusion measurements were performed using the Fourier transform NMR pulsed gradient spin-echo technique. Counterion binding was found to be strongly dependent on mixed micellar composition in terms of micellar surface charge density, aggregate size/shape, and chemical nature of the nonionic surfactant headgroup.

A study of ion binding in aqueous latex dispersions from osmotic pressure measurements

Abstract Osmotic pressure measurements on aqueous dispersions of carboxylated latex particles have been performed for different surface charge densities (up to ∼0.48 C m −2 ) and latex volume fractions. The thermodynamic nonideality exhibited by the solutions was interpreted in terms of counterion binding and it was found that a very substantial fraction of the counterions could be considered bound (in a thermodynamic sense). A behavior reminiscent of counterion condensation was also observed for surface charge densities exceeding ∼0.10 C m −2 , i.e., the osmotic pressure became independent of latex surface charge density. The general behavior of the osmotic factor, φ osm , with respect to increasing latex volume fraction, was an initially rapid decrease in φ osm whereupon it leveled off and became almost independent of bulk concentraiton at elevated latex volume fractions. Calculated osmotic pressures based on the promitive model of polyelectrolytes reveal a qualitiative agreement with the present experimental data whereas quantitative agreement is poor.

Self diffusion of small molecules in aqueous solution of poly(vinylpyrrolidone)

Abstract The self-diffusion coefficients of small penetrants were measured in aqueous solutions at varying concentrations of poly(vinylpyrrolidone). Measurements have been performed using the n.m.r. pulsed-gradient spin-echo (PGSE) technique and the classical gradient diffusion (CGD) method, modified for ternary systems. A good agreement was found between the two, confirming the validity of the latter. The results have been quantitatively analysed by a free-volume approach adapted for diffusion of a solute in moderately concentrated polymer solutions. From this model a linear relationship is predicted between ln D D o and ∅−1, the reciprocal volume fraction of solvent, which was also found experimentally for all diffusants studied. An enhanced concentration dependence of penetrant diffusion with increasing size of the diffusing molecules was observed. These findings are in agreement with predictions from the free-volume theory.

Diffusion of glucose derivatives in aqueous solutions of hydroxypropyl cellulose

Abstract The diffusion behaviour of glucose and three of its derivatives in aqueous solutions of hydroxypropyl cellulose was studied as a function of polymer concentration. A shearing type diffusion cell was used together with a schlieren optical system. The presence of polymer did not affect the diffusion of glucose. The derivatives, however, exhibited a linear decrease in diffusion coefficient with increasing polymer concentration. The results were discussed in terms of solvation and obstruction.

An experimental study of ionic interactions in solutions of mixed lecithin/dicetylphosphate vesicles

Abstract Ion binding to mixed ionic/nonionic vesicles was determined from 133Cs NMR chemical-shift measurements and compared to a model of ion binding based on the Poisson-Boltzmann (PB) equation. A good quantitative agreement was observed between experimental and PB model predicted degrees of ion binding at high vesicular surface charge densities, whereas in the low surface charge density region the experimental fraction of bound ions is significantly smaller than predicted by the PB calculations. The competitive ion binding of Cs+ and a number of divalent ions to vesicles of different charge densities was investigated. Small but yet significant differences in membrane interaction were observed for Mg2+, Ca2+, and Ba2+. Addition of moderate amounts of the paramagnetic ions Mn2+ and Co2+ induced a downfield shift of the Cs+ resonance which was attributed to ion-pair formation at the vesicular surface. Penetration of the vesicular membrane by divalent ions was evident from the effect of paramagnetic ions on the NMR chemical shift of both 133Cs and 31P. The ability of divalent ions to penetrate the vesicle membrane was also evident from the influence of added Ca2+ on the time-averaged self-diffusion coefficient of internal and external Li+. Vesicle aggregation, induced by divalent ions, was monitored by measurements of vesicle diffusion using dynamic light scattering.