Influence of hydrogen bonds and temperature on dielectric properties

Dielectric properties are evaluated by means of molecular dynamics simulations on two model systems made up of dipolar molecules. One of them mimics methanol, whereas the other differs from the former only in the ability to form hydrogen bonds. Static dielectric properties such as the permittivity and the Kirkwood factor are evaluated, and results are analyzed by considering the distribution of relative orientations between molecular dipoles. Dipole moment–time correlation functions are also evaluated. The relevance of contributions associated with autocorrelations of molecular dipoles and with cross-correlations between dipoles belonging to different molecules has been investigated. For methanol, the Debye approximation for the overall dipole moment correlation function is not valid at room temperature. The model applies when hydrogen bonds are suppressed, but it fails upon cooling the nonassociated liquid. Important differences between relaxation times associated with dipole auto- versus cross-correlations as well as their relative relevance are at the root of the Debye model breakdown.Postprint (author's final draft

Dinàmica orientacional i estructura local en liquids moleculars sobrerefredats

The aim of this thesis is to study, by means of molecular dynamics simulations, the behaviour of two molecular models at different temperatures, approaching the glass transition. One of them mimics methanol molecules, whereas the other differs from the former only in the ability of forming hydrogen bonds. The analysis carried out is focused on the reorientational dynamics, the dielectric response and various orientational properties that characterize the local structure. All time correlation functions show a two-stage relaxation, separated by a plateau at the lowest temperatures. Translational relaxation times are always larger than reorientational ones when removing hydrogen bonds. A coupling between reorientation and translation has been found: higher mobility in translation is associated with larger reorientation angles. Rotational dynamics around a principal inertial axis has also been investigated in the non-associated system. Rotational diffusion coefficients can be fit to a mode coupling law with a critical temperature which is considerably lower than that of translation. That means that reorientation is still active at temperatures characterized by a hindered translational dynamics. Results show that the Stokes-Einstein relation breaks at a higher temperature than the Stokes-Einstein-Debye relation does. Hydrogen bonds strongly restrict the orientation of molecular dipole moments of neighbouring molecules, so that both Kirkwood factor and dielectric permittivity decrease as the ability to establish them is suppressed. At room temperature, and in the system without hydrogen bonds, the most important contribution to the total dipole correlation function is the autocorrelation of molecular dipoles. In addition, relaxation times associated with the autocorrelation and the cross-correlations are similar, and the Debye’s model of dielectric relaxation reproduces reasonably well the behaviour of the system. In methanol, correlation between orientations of different molecules is much more relevant, and the Davidson-Cole model is more appropriate. At lower temperatures, the behaviour of both systems is best represented with the Davidson-Cole model. The longitudinal and transverse components of the dipole density for the smallest wave vectors compatible with the size of the system, have allowed to recover the value of the dielectric constant. The anisotropy of the rotational dynamics in methanol has also been analyzed and rotation around the three main molecular axes of inertia has been studied. It has been found that rotational spectra contain higher frequency contributions than those detected for the translational spectrum. Its dependence has been analyzed for different hydrogen bonded states. As the number of hydrogen bonds established by a molecule increases, the relevant peaks shift towards larger frequencies. Upon cooling, no significant changes in the frequencies involved are encountered, but rather in the relevance of the peaks. Several rotational relaxation models have been tested, which predict analytical relationships between angular momentum relaxation times and reorientational times. The Langevin model satisfactorily reproduces the behaviour of the liquid without hydrogen bonds, even in the supercooled state. Finally, the local structure of both systems has been investigated. The Voronoi polyhedra have been calculated and their topological and metric properties have been studied. This analysis has allowed to quantify the increase in local order in both systems upon cooling: a decrease in the number of different types of polyhedra and an increase in the frequency of the most frequent polyhedra have been encountered.L'objectiu fonamental d’aquesta tesi ha estat estudiar, mitjançant la dinàmica molecular, el comportament de dos models moleculars a diferents temperatures, acostant-se les més baixes a la de transició vítria. S’ha estudiat un sistema que modelitza el metanol i un altre en tot similar al primer excepte en la capacitat d’establir ponts d’hidrogen. L’anàlisi efectuada s’ha centrat en la dinàmica de reorientació, la resposta dielèctrica i diverses propietats orientacionals i que caracteritzen l’estructura local. Totes les funcions de correlació temporal analitzades mostren una relaxació en dues etapes, separades per un plateau a les temperatures més baixes. Els temps de relaxació translacionals són superiors que els reorientacionals en el sistema sense ponts d’hidrogen. S’ha detectat que les molècules amb més mobilitat translacional també han patit reorientacions més grans. En el sistema sense ponts d’hidrogen s’ha investigat la dinàmica rotacional al voltant d’un eix principal d’inèrcia. S’ha comprovat que els coeficients de difusió rotacional segueixen una llei tipus mode coupling amb una temperatura crítica menor que la corresponent a la difusió en translació. Per tant, a baixes temperatures, quan la dinàmica translacional es veu fortament obstaculitzada. la rotació encara és activa. Els resultats mostren que la relació Stokes-Einstein deixa de satisfer-se a una temperatura superior que la de Stokes-Einstein-Debye. S’ha analitzat també l’anisotropia de la dinàmica de rotació en metanol, considerant la rotació al voltant dels tres eixos principals d’inèrcia moleculars. S’ha obtingut que els espectres rotacionals contenen contribucions de freqüències més grans que les detectades en l’espectre translacional. S’ha analitzat la seva dependència amb el nombre de ponts d’hidrogen en què participen les molècules. A mesura que aquest nombre creix, els pics principals es desplacen cap a freqüències més altes. En disminuir la temperatura no s’observa un canvi important en les freqüències involucrades, però sí en la importància dels pics de les distribucions. S’han analitzat diferents models de relaxació rotacional que donen lloc a relacions analítiques entre els temps de relaxació del moment angular i els de reorientació. El model del Langevin reprodueix satisfactòriament el comportament del líquid sense ponts d’hidrogen, fins i tot a l’estat sobrerefredat. Els enllaços per pont d’hidrogen restringeixen fortament l’orientació dels moments dipolars moleculars de molècules veïnes, de manera que tant el factor de Kirkwood com la permitivitat dielèctrica són menors en suprimir-se la capacitat d’establir-los. A temperatura ambient, i en el sistema sense ponts d’hidrogen, la contribució més important a la correlació del moment dipolar total es deu a l’autocorrelació dels dipols moleculars, els temps associats a les autocorrelacions i a les correlacions creuades són similars i el comportament es reprodueix raonablement bé amb el model de Debye de la relaxació dielèctrica. En el sistema amb ponts d’hidrogen, la correlació entre orientacions de diferents molècules és molt més rellevant, i el model de Davidson-Cole resulta més adient. A temperatures més baixes, el comportament de tots dos sistemes queda millor representat amb el model de Davidson-Cole. Les components longitudinal i transversal de la densitat de moment dipolar en funció dels vectors d’ona més petits compatibles amb la mida del sistema, han permès recuperar el valor de la constant dielèctrica. Finalment s’ha investigat l’estructura local dels sistemes. S’ha realitzat el càlcul dels poliedres de Voronoi, dels quals s’han analitzat les propietats topològiques i mètriques. Aquesta anàlisi ha permès quantificar l’augment de l’ordre local en tots dos sistemes en disminuir la temperatura: s’ha observat una disminució en el tipus de poliedres detectats i un augment en la freqüència dels poliedres més freqüents.Postprint (published version

Orientational dynamics in methanol: influence of temperature and hydrogen bonding

Rotational diffusion has been investigated in methanol by means of molecular dynamics simulations at several temperatures, approaching the supercooled state. Angular velocity autocorrelation functions in the molecule-fixed coordinate frame have been evaluated, and the principal components of the rotational diffusion tensor have been obtained. Rotational dynamics of molecules belonging to different hydrogen-bonded states has also been analyzed. It has been found that out-of-plane rotation is favoured at room temperature. Upon cooling, out-of-plane rotational diffusion coefficients decrease faster than in-plane ones, which become largest in the supercooled state. Nevertheless, differences on the microscopic leading mechanisms of rotational diffusion at low temperatures have been identified. For molecules that establish only one hydrogen bond, out-of-plane dynamics is still the most relevant in the supercooled state. Rotational relaxation has also been studied and the available theoretical models have been tested.Postprint (author's final draft

Influence of hydrogen bonds and temperature on dielectric properties

Dielectric properties are evaluated by means of molecular dynamics simulations on two model systems made up of dipolar molecules. One of them mimics methanol, whereas the other differs from the former only in the ability to form hydrogen bonds. Static dielectric properties such as the permittivity and the Kirkwood factor are evaluated, and results are analyzed by considering the distribution of relative orientations between molecular dipoles. Dipole moment–time correlation functions are also evaluated. The relevance of contributions associated with autocorrelations of molecular dipoles and with cross-correlations between dipoles belonging to different molecules has been investigated. For methanol, the Debye approximation for the overall dipole moment correlation function is not valid at room temperature. The model applies when hydrogen bonds are suppressed, but it fails upon cooling the nonassociated liquid. Important differences between relaxation times associated with dipole auto- versus cross-correlations as well as their relative relevance are at the root of the Debye model breakdown

Rotational dynamics of a dipolar supercooled liquid

We study the rotational dynamics of a supercooled molecular liquid by means of molecular dynamics simulations. The system under investigation is composed of rigid diatomic molecules with an associate dipole moment. At room temperature, orientational correlations decrease rapidly with increasing distances. Upon cooling, angles between dipole moments of molecules within the first coordination shell decrease. As for the dynamical properties, rotational diffusion coefficients decrease with temperature at a smaller rate than translational diffusion coefficients do, and the critical temperature associated with the former is lower than the one corresponding to their translational counterparts. Translation and rotation about an inertial axis are uncorrelated, whereas some coupling between translation and dipole reorientation is obtained

Prediction of outcomes in subjects with type 2 diabetes and diabetic foot ulcers in Catalonian primary care centers : a multicenter observational study

Altres ajuts: Primary Care Diabetes Europe (PCDE2019/4).Diabetic foot and lower limb complications are an important cause of morbidity and mortality among persons with diabetes mellitus. Very few studies have been carried out in the primary care settings. The main objective was to assess the prognosis of diabetic foot ulcer (DFU) in patients from primary care centers in Catalonia, Spain, during a 12-month follow-up period. We included participants with type 2 diabetes and a new DFU between February 2018 and July 2019. We estimated the incidence of mortality, amputations, recurrence and healing of DFU during the follow-up period. A multivariable analysis was performed to assess the association of these outcomes and risk factors. During the follow-up period, 9.7% of participants died, 12.1% required amputation, 29.2% had a DFU recurrence, and 73.8% healed. Having a caregiver, ischemia or infection were associated with higher mortality risk (hazard ratio [HR]:3.63, 95% confidence interval [CI]:1.05; 12.61, HR: 6.41, 95%CI: 2.25; 18.30, HR: 3.06, 95%CI: 1.05; 8.94, respectively). Diabetic retinopathy was an independent risk factor for amputation events (HR: 3.39, 95%CI: 1.37; 8.39). Increasing age decreased the risk for a DFU recurrence, while having a caregiver increased the risk for this event (HR: 0.97, 95%CI: 0.94; 0.99). The need for a caregiver and infection decreased the probability of DFU healing (HR: 0.57, 95%CI: 0.39; 0.83, HR: 0.64, 95%CI: 0.42; 0.98, respectively). High scores for PEDIS (≥7) or SINBAD (≥3) were associated with an increased risk for DFU recurrence and a lower probability of DFU healing, respectively. We observed high morbidity among subjects with a new DFU in our primary healthcare facilities. Peripheral arterial disease, infection, and microvascular complications increased the risk of poor clinical outcomes among subjects with DFU. The online version contains supplementary material available at 10.1186/s13047-023-00602-6