Molecular dynamics of liquid-liquid equilibrium and interfacial properties of aqueous solutions of methyl esters.

In this work, the liquid-liquid phase equilibria and interfacial properties of methyl ester + water binary mixtures are determined at atmospheric pressure and from 278 to 358 K combining the direct coexistence technique and molecular dynamics simulations. Methyl esters are modelled using new parametrizations based on the united atom TraPPE model force field proposed recently by us [E. Feria, J. Algaba, J. M. Míguez, A. Mejía, P. Gómez-Álvarez and F. J. Blas, Phys. Chem. Chem. Phys., 2019, 22, 4974-4983] that are able to predict the vapour-liquid interfacial properties of pure methyl esters with high accuracy. In the case of water, we consider the well-known TIP4P/2005 model, the most popular rigid and non-polarizable model to describe the interfacial properties of pure water. The simulations are performed using the direct coexistence technique in the isothermal-isobaric or NPzT ensemble in combination with molecular dynamics. We obtain density profiles, temperature-densities and temperature-composition projections of the phase diagrams, and interfacial tensions. The liquid-liquid interfacial tension is calculated from the normal and tangential components of the pressure tensor according to the mechanical virial route. We pay attention particularly to the ability of the molecular models in predicting the experimental behavior of the systems. Simulation results are able to account for the liquid-liquid phase equilibria of these binary mixtures, in good agreement with the experimental data taken from the literature. Unfortunately, experimental values for interfacial tensions are substantially overestimated by predictions from computer simulations in all cases. To our knowledge, this is the first time that the liquid-liquid phase equilibrium and interfacial properties of methyl ester + water mixtures have been predicted from computer simulations.

Influence of preventive sex education programmes in compulsory secondary education students: a descriptive observational study.

BACKGROUND: Sex education programmes conducted by health professionals and educators are essential for young people to adopt healthy habits and attitudes towards their sexuality. The Forma Joven Program, promoted by the Andalusian Regional Government's Ministry of Health and Families and Education, is a good example of this. The aim of the study is to determine if different "degrees of intervention" in the informative consultancies of the Forma Joven Program imply differences in knowledge and attitudes towards sexuality. METHODS: This descriptive observational study analysed 27 Compulsory Secondary Education high schools in Huelva. These were included in the Program and belonged to a Primary Care Health District. Out of the 17 institutes initially selected because they met the inclusion criteria (4.256 students), finally 14 (3.596 students) participated. During the 2018/2019 school year, students from 3rd, 4th year of Secondary Compulsory Education (ESO), 1st, and 2nd Baccalaureate of the selected centres were asked to fill in a questionnaire of knowledge and attitudes towards sexuality. It collected variables such as age, sex, school year, institute, educational level and employment status of the father or mother and profile of the professional who delivers the counselling. RESULTS: A total of 1.237 students completed the questionnaire, which represents a participation rate of 34.4%. The average age was 15.59 years (SD 1.26) and 39.9% were girls. In some evaluated questions, we found statistically significant differences between the groups with different levels of exposure to counselling and the acquisition of knowledge and attitudes towards students' sexuality, although in most of them no such differences were found. The results of this study suggest the importance of the quality of counselling over quantity. Some classic myths persist in relation to sexuality and in some situations, they can be decisive when adopting preventive measures to avoid risks related to pregnancy and contagion of STIs. CONCLUSIONS: A greater number of counselling sessions does not imply acquiring a higher level of knowledge or better attitudes towards sexuality. Perhaps the quality of the education is more important than the quantity of counselling sessions.

Vapour-liquid phase equilibria and interfacial properties of fatty acid methyl esters from molecular dynamics simulations.

We have determined the phase equilibria and interfacial properties of a methyl ester homologous series (from methyl acetate to methyl heptanoate) using direct simulations of the vapour-liquid interfaces. The methyl esters are modelled using the united atom approach in combination with transferable parameters for phase equilibria (TraPPE) force fields for alkanes, alkenes, carbon dioxide, ethers, and carboxylic acids in a transferable way. This allows us to take into account explicitly both dispersive and coulombic interactions, as well as the repulsive Pauli-exclusion interactions. Simulations are performed in the NVT or canonical ensemble using molecular dynamics. Vapour-liquid surface tension is determined using the virial route, i.e., evaluating the normal and tangential components of the pressure tensor along the simulation box. We have also calculated density profiles, coexistence densities, vapour pressures, surface entropies and enthalpies, and interfacial thickness as functions of temperature, as well as the normal boiling temperatures and the critical temperatures, densities, and pressures for each member of the series. Special attention is paid to the comparison between experimental data taken from the literature and our results obtained using molecular dynamics simulations. We also analyze the effect of increasing the molecular weight of the methyl esters (at fixed temperature) on all the properties considered, with special emphasis on phase equilibria envelopes and surface tension. The TraPPE force fields transferred from other molecules and chemical families are able to predict very accurately the experimental vapour-liquid phase envelopes of methyl esters. We also compare the results obtained from simulations of the surface tension, with experimental data taken from the literature. To our knowledge, this is the first time that vapour-liquid phase equilibria and interfacial properties, and particularly surface tension, of this methyl ester homologous series are obtained using computer simulation.