Conductivities and dielectric constant measurements in water at 25 °C have been made on the amphiphilics sodium n-dodecyl sulfate, n-dodecyltrimethylammonium bromide, and chlorpromazine hydrochloride. By using the conductivity/concentration data, critical micelle concentrations (cmc) have been determined by applying the Williams definition and two forms of the Phillips method. This first Phillips form consists of an approximation to Gaussians of the second derivative of the conductivity/concentration data followed by two consecutive integrations. The second form, which is proposed here, consists of the application of a combination of the Runge-Kutta numerical integrations method and the Levenberg-Marquardt leastsquares fitting algorithm. The proposed method permits the determination of the cmc in systems with low aggregation numbers and with slow variations of physical property/concentration curves allowing the determination of the so-called second cmc. A comparative study with results obtained by dielectric constant measurements has been carried out. With this new technique, the cmc's (first and second) are directly obtained as singular points in the dielectric constant/concentration curves, and thus, this technique is an alternative to the determination of cmc's from conductivities.
Dynamic light scattering and electrophoretic mobility measurements have been used to characterize the size, size distribution and zeta potentials (zeta-potentials) of egg yolk phosphatidylcholine (EYPC) liposomes in the presence of monovalent ions ( Na(+) and K(+)). To study the stability of liposomes the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has been extended by introducing the hydrated radius of the adsorbed ions onto the liposome surfaces. The decrease of liposome size is explained on the basis of the membrane impermeability to some ions which generate osmotic forces, which leads to evacuate water from liposome inside.
The self-association of the penicillins cloxacillin, dicloxacillin, and flucloxacillin in water and in the
presence of added electrolyte (0.025−0.40 mol kg-1 NaCl) at 30 °C has been examined by light-scattering
and NMR techniques. Inflections in the data from both techniques were observed at a single critical
concentration for solutions of cloxacillin and at two critical concentrations for dicloxacillin and flucloxacillin.
Aggregation numbers and effective micellar charges were calculated from the static light-scattering data
for the stable aggregates formed at the first critical concentration. Application of the valance-generalized
light-scattering theory for multicomponent systems to data at concentrations above the second critical
concentration provided an estimate of the aggregate size of the associated species present at high solution
concentration. The interaction between aggregates was interpreted from diffusion data from dynamic
light-scattering using DLVO theory. Micellar properties have been determined by the application of mass
action theory to the concentration dependence of 1H NMR chemical shifts, confirming the results obtained
by the light-scattering technique.
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