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Fuerzas impulsoras para la asociación de poliones de carga opuesta en soluciones acuosas: entálpica, entrópica, pero
Jingcheng Fu1, Joseph B Schlenoff1
1Department of Chemistry and Biochemistry, The Florida State University , Tallahassee, Florida 32306, United States.
Journal of the American Chemical Society
|January 16, 2016
Resumen
Las fuerzas impulsoras para la asociación de polímeros no son únicamente electrostáticas. La perturbación del agua, influenciada por el emparejamiento específico de iones, impacta significativamente los cambios de entalpía, con la energía libre impulsada principalmente por la entropía.
Área de la Ciencia:
- Ciencia de los Polímeros
- Química Física
- Química de la solución
Sus antecedentes:
- La asociación de polímeros con carga opuesta en solución se atribuye tradicionalmente a las interacciones electrostáticas.
- Estas fuerzas electrostáticas comprenden componentes entrópicos (liberación de counterion) y entálpicos (atracción de Coulombic).
- Las teorías de la electrostática continua a menudo interpretan las pequeñas exotermas / endotermas como evidencia de las interacciones coulombinas.
Objetivo del estudio:
- Comparar los mecanismos de emparejamiento de iones electrostáticos y específicos para la asociación de polielectrolitos.
- Investigar el origen de los cambios de entalpía durante la complejación policatión-polianión.
- Para aclarar el papel de los contraiones y la estructura del agua en la asociación de polímeros.
Principales métodos:
- Comparación frontal de modelos de emparejamiento de iones electrostáticos y específicos.
- Se utilizó una serie de contadores Hofmeister con poli (diallidimetilamonio) (PDADMA).
- Utilizó la dispersión de Raman para estudiar las vibraciones OH del agua y evaluar la perturbación del agua.
Principales resultados:
- Se descubrió que los cambios de entalpía se originan en alteraciones en la perturbación del agua, no solo en la atracción de Coulomb.
- La espectroscopia de Raman reveló cambios en la estructura del agua correlacionados con la identidad de los counteriones.
- La energía libre de complejación fue predominantemente entrópica a través de varias concentraciones de sal.
Conclusiones:
- Los pares de iones específicos y los mecanismos de intercambio iónico, que implican cambios en la estructura del agua, son cruciales para comprender la asociación de polielectrolitos.
- El modelo electrostático tradicional puede no capturar completamente las fuerzas motrices termodinámicas.
- Los efectos específicos de los counteriones en la estructura del agua juegan un papel importante en la energética de la complejación del polímero.
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