Publication: Cinética de la cristalización
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2022-03-16
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Universidad Complutense de Madrid
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Abstract
La cristalización de un líquido consta de varias etapas, empezando por que éste se encuentre en condiciones de metaestabilidad. Por ejemplo, un caso típico es el de un líquido que se enfría más allá de su temperatura de coexistencia. Por lo tanto, la temperatura y la presión del mismo correspondan a la región cristalina en el diagrama de fases de la sustancia por lo que el cristal es la fase más estable y por lo tanto la que cabría esperar. Sin embargo, esta no es condicion suficiente para observar la transición de forma espontánea dado que se trata de un proceso activado que puede tomar un tiempo considerable en función de las condiciones termodinámicas 1. Este tipo de procesos se caracterizan por la transición entre estados separados por una barrera de energía. En este caso, dicha barrera se forma por la competición energética entre estabilidad termodinámica y la penalización de crear interfase sólido-líquido. La tensión interfacial es una variable clave en el proceso pues es la que se opone a la transición y da lugar a que el proceso sea activado logrando que un líquido metaestable pueda sobrevivir durante largo tiempo. Cuando la cristalización emerge en una sustancia pura el proceso se denomina nucleación homogénea mientras que en presencia de impurezas o sustratos, el proceso se denomina nucleación heterogénea. En ambos casos, son fluctuaciones de orden local las responsables del inicio de la transición pero mientras que en la nucleación homogénea ocurren en el seno del fluido, en la nucleación heterogénea ocurren también próximas a una intefase lo cual facilita la nucleación 24...
Liquids crystallize in three steps. First, the liquid should be metastable. A good example would be a liquid that has been supercooled beyond its temperature of coexistence. This means that its temperature and pressure should correspond to the crystalline phase in thephase diagram of the substance. Thus, the crystal would be more stable and one should expect the liquid to transform into it. However, this condition is not enough to observe the phase transition spontaneously. This is due to the fact that this is an activated process which may take long depending on the thermodynamic conditions 1. In this kind of process there is a transition between states that are separated by a free energy barrier. In this particular case, the free energy barrier appears due to the competition between thermodynamic stability and the energy cost of creating solid-liquid interface. The interfacial free energy is akey variable in the process since it is the one that goes against the transition allowing a metastable liquid to survive long times. When crystallization originates within a pure substance,the process is known as homogeneous nucleation but, in the presence of impurities or substrates, it is known as heterogeneous nucleation. In both cases, local uctuations are responsible for triggering the phase transition although whereas in homogeneous nucleationthey occur in the bulk of the metastable liquid, in the heterogeneous case they occur also near an interface which facilitates the nucleation 24...
Liquids crystallize in three steps. First, the liquid should be metastable. A good example would be a liquid that has been supercooled beyond its temperature of coexistence. This means that its temperature and pressure should correspond to the crystalline phase in thephase diagram of the substance. Thus, the crystal would be more stable and one should expect the liquid to transform into it. However, this condition is not enough to observe the phase transition spontaneously. This is due to the fact that this is an activated process which may take long depending on the thermodynamic conditions 1. In this kind of process there is a transition between states that are separated by a free energy barrier. In this particular case, the free energy barrier appears due to the competition between thermodynamic stability and the energy cost of creating solid-liquid interface. The interfacial free energy is akey variable in the process since it is the one that goes against the transition allowing a metastable liquid to survive long times. When crystallization originates within a pure substance,the process is known as homogeneous nucleation but, in the presence of impurities or substrates, it is known as heterogeneous nucleation. In both cases, local uctuations are responsible for triggering the phase transition although whereas in homogeneous nucleationthey occur in the bulk of the metastable liquid, in the heterogeneous case they occur also near an interface which facilitates the nucleation 24...
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Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Químicas, leída el 17-12-2021