Pablo Martı́nez-Landeira

Aggregation energies of some amphiphilic antidepressant drugs

Abstract The critical micelle concentrations (cmc) of the amphiphilic antidepressant drugs doxepin and butriptyline hydrochlorides in aqueous solution were determined over the temperature range 288.15–313.15 K by conductivity data and using a method based on deconvolution into Gaussians of the second derivative of the measured specific conductivity data. For fully characterization of the thermodynamics of aggregation process, the variation of the standard Gibbs energy, Δ G m 0 , the standard enthalpy, Δ H m 0 , and the standard entropy, Δ S m 0 , of micellization were obtained from the application of the mass action model and Phillips definition of the cmc.

Interactions Between Liposomes and Cations in Aqueous Solution

Abstract An investigation on the dependence of electrophoretic mobilities of unilamellar vesicles of phosphatidylcholine–cholesterol–phosphatidylinositol (PC–Chol–PI) on the concentration of several cations with variations in the relation charge/radius in the range Na+, K+, Cs+, Mg2+, Ca2+, Ba2+, Al3+, and La3+ has been realized. Plots of zeta potential against ion concentration exhibit a maximum for all the cations under study, the position of the maximum is greatly affected by the charge of the ion. From the feature of these plots two phenomenon were observed: an initial binding of cations into the slipping plane for ion concentration below the maximum and a phenomenon of vesicle association for concentration above the maximum. To confirm these observations measurements on dynamic light scattering were performed to obtain the corresponding size distribution of the liposomes at different ion concentrations. Finally the ability of the Stern isotherm to describe the adsorption of the cations to vesicles was tested by two methods. The two main parameters of the theory: the total number of adsorption sites per unit area, N1, and the equilibrium constant, K; (and consequently the free energy of adsorption, ) were calculated for the different ions, showing good agreement. The equilibrium constants of adsorption have been found to obey a linear relationship with ion radius the slope of which decreases with the ion charge.

Surface behaviour of C5, C6, C7 and C8 lecithins at the aqueous solution/air interface

Abstract Surface tensions of aqueous solutions of diacylphosphatidylcholines (diCnPC) with n=5, 6, 7 and 8 have been measured at 25xa0°C and pH 3.2, 7.4 and 10.0, respectively. From these data critical micelle concentrations (cmc), Gibbs energies of micellization (ΔG°m), of adsorption (ΔG°ads), of transfer from micelles to surface (ΔG°trans) and surface area minima per molecule have been obtained. The increase of the cmc with the pH was more significant for diC5PC, decreasing conform the carbon atoms number in the acyl chain raise. Surface areas decrease with increasing carbon number. The micellization is most favorable at low pH. The contributions to ΔG°m and ΔG°ads were of 1.8RT and 2RT per mol of CH2, respectively. The variation of ΔG°trans indicates that the processes of micellization and adsorption are most favorables conform the acyl chain increases.

A study of the behaviour of ampicillin in aqueous solution and thermodynamic characterization of its aggregation

The aggregation of ampicillin in water has been examined by conductivity measurements over the temperature range 288.15–313.15 K and light scattering. These measurements indicate the formation of two critical concentrations over the range 0–0.35 mol kg−1. Aggregation number and effective aggregate charge were calculated from the static light scattering data according to the Anacker and Westwell treatment. Thermodynamic parameters of aggregate formation were obtained from a form of the mass action model applicable to systems of low aggregation number. This method was applied at both critical concentrations. A valence of one was used for the monomers present in the first equilibrium. The second equilibrium was between aggregates of two different sizes, in this case, the valence of the aggregates being the effective charge calculated from the Anacker and Westwell treatment. Experimental results show that free energies of micellization for ampicillin are higher for the first critical concentration and in the same range, but lower than for other penicillins. The enthalpies of micellization become negative when the temperature is increased, but the variation is three times greater for the first critical concentration than for the second.

The micellization of dioctanoylphosphatidylcholine at low pH: a laser light scattering study

Abstract Laser light scattering measurements have been performed on aqueous solutions of dioctanoylphosphatidylcholine at pH 1.2 in the presence of added electrolyte (0.05–2.0 M KCl) at 25xa0°C. Static laser light scattering was used to obtain molecular mass and the critical micelle concentration (CMC) of the micelles. Aggregation numbers and number of charges per micelle were calculated from static light scattering according to the Anacker and Westwell treatment. The predicted aggregation numbers were in good agreement with those obtained experimentally from the molecular mass. The interaction between micelles was interpreted from diffusion data from dynamic laser light scattering using the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory.

The surfactant characteristics of short-chain lecithins analyzed through lecithin-lecithin and lecithin-biopolymer interactions

The micellar behaviour of the binary mixed systems dihexanoylphosphatidylcholine (diC6PC)/diheptanoylphosphatidylcholine (diC7PC), diC7PC/dioctanoylphosphatidylcholine (diC8PC) and diC6PC/ diC8PC has been studied. Critical micelle concentrations of the mixtures were quantitatively estimated from surface tension measurements versus solution composition plots. The micellar composition in the micelles was determined by the Motomura thermodynamic model. The nature and the strength of the interaction between two surfactants in the mixture were estimated by calculating the values of their β 12 M parameter from Holland and Rubingh’s treatment. DiC6PC and diC7PC mix ideally with an interaction parameter near zero. DiC7PC/ diC8PC and diC6PC/diC8PC show nonideality for about 75 and 40% diC8PC in the mixture, respectively. The nonideality has been attributed to differences in the acyl chain length, the size of the micelles formed by pure components and the composition of mixed micelles. Zeta potential and microcalorimetric measurements were used to investigate the effect of diC8PC on human serum albumin. The results revealed that the interaction is hydrophobically driven and depends on lecithin concentration.

Thermodynamics of micellization of C7 and C8 lecithins

The surface tensions of the lecithins diheptanoyl- and dioctanoylphosphatidylcholine in phosphate medium, pH 7.4, have been measured over the temperature range 288.15–313.15K. Critical micelle concentrations, thermodynamic micellization parameters, adsorption parameters, transfer data of micelles to the aqueous surface and surface area minima per molecule were obtained from these measurements. Critical micelle concentrations slightly decrease with temperature. The micellization processes became increasingly exothermic with increase in temperature. The variation of Gibbs energies of transfer indicates that the processes are most favourable at low temperatures. From a study of entropy-enthalpy compensation phenomenon a compensation temperature of 300−1 K−1 was found, and from the intercept an analysis of the hydrophobic-hydrophilic balance was carried out.