Flexible liposomes have proved to be efficient for transcutaneous administration of drugs. In this context, the bending elastic modulus $\kappa$ of the membrane turned to be a measurable physical parameter that unambiguously reflects the elastic property of the membrane. The spin-lattice relaxation rate dispersion of protons in unilamellar liposomes has been recently studied using the fast field-cycling (FFC) NMR technique [1-5]. Results were interpreted in terms of a model that considers the lipid dynamics within the membrane. An important feature of the approach is that it allows inferring about its elastic properties, as the elastic constant $\kappa$ is fully involved in the model. The methodology and the model were validated through measurements of $\kappa$ in liposomes containing both cholesterol and different surfactants within the membrane, that is, at different elastic conditions of the specimen. In this work we focus in the temperature dependence and the behavior of flexible liposomes formulated with different surfactants. Specifically, we analyzed four different temperatures within the range 291-328K for liposomes of radius of 50nm, composed of SPC (soy phosphatidylcholine) with different additives that are commonly used to enhance the membrane flexibility, at different concentrations. As previously observed in the literature, for the lowest temperatures, the elastic modulus of the membrane decrease with the addition of surfactants. However, the observed behavior is not linear for none the temperature and concentration, nor with the type of surfactant. A correlation analysis reveals that specific properties of the surfactants have a direct influence on the elastic properties as well as in the lipid molecular dynamics.
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