Investigación

 

Aggregate structures of the sorbitan monooleate (SPAN80) surfactant at TiO₂(rutile)/water interfaces by computer simulations

 

E. Nuñez-Rojas and H. Domínguez

 

Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, México, D.F. 04510, México, e-mail: eeddgar@yahoo.com.mx; hectordc@servidor.unam.mx.

 

Received 21 March 2013
Accepted 26 July 2013

 

Abstract

Molecular dynamics simulations were carried out to study the behavior of a nonionic surfactant close to TiO₂ surfaces at three different crystallographic orientations of rutile, (001), (100) and (110). Sorbitan Monooleate (SPAN80) molecule was used as nonionic surfactant and it was observed that these molecules seemed to aggregate in similar ways. Namely, the hydrocarbon chains of the surfactant molecules were attached at the solid surfaces. Structure of the molecules and surfactant adsorption on the surfaces were studied in terms of tails and head groups density profiles as well as surface coverage. From density profiles and angular distributions it was possible to determine the influence of the solid surface. For instance, on the three surfaces the surfactant molecules formed molecular layers parallel to the surface. Besides, it was found that in the solids (100) and (110), where there were oxygen atoms exposed on the surface, surfactants were attached to the surfaces along the sites between the lines of these oxygen atoms. Finally, diffusion coefficients for the aggregates were calculated in order to determine molecular mobility on the surfaces. These results were compared with those of the Sodium Dodecylsulfate (SDS) molecules on the same rutile surfaces and it was found that the SPAN80 molecules were more attached to surfaces than the SDS molecules. On the other hand the diffusion coefficients calculated in the present work were also compared with those obtained in a SDS/graphite system and we observed that SDS molecules on graphite showed a significant mobility compared with the same molecules on rutile.

Keywords: Computer simulations; SPAN80 surfactant; adsorption; rutile.

 

Resumen

Se realizaron simulaciones de dinámica molecular para estudiar el comportamiento de un surfactante no-iónico sobre una superficie de TiO₂ en las tres diferentes caras en la estructura de rutilo, (001), (100) y (110). Como surfactante se usó la molécula Sorbitan Monooleate (SPAN80) y se observó que las moléculas se adsorben sobre las diferentes superficies de manera similar. Para cada una de las superficies se vio que los surfactantes se agregaron con las colas hidrocarbonadas adsorbidas sobre las placas de rutilo. Se realizaron estudios de los perfiles de densidad y distribuciones angulares para estudiar la adsorción y se observó que las moléculas de surfactante se depositaron en capas paralelas sobre la superficie. En particular, se encontró que para las caras (100) y (110) los surfactantes se adsorbieron en el sólido entre los espacios de los átomos de oxígeno expuestos sobre las superficies. También se realizaron estudios del coeficiente de difusión para calcular la mobilidad del agregado formado sobre las superficies. Estos resultados se compararon con estudios previos del surfactante sulfato dodecil de sodio (SDS) sobre las mismas superficies y se encontró que las moléculas de SPAN80 se anclaban mas sobre las superficies que el SDS. Sin embargo, cuando se compararon estos resultados, del coeficiente de difusión, de moléculas de SDS sobre una superficie de grafito se encontró que en este caso el SDS tenía una mobilidad mucho mayor en esta superficie que en las superficies de rutilo.

Descriptores: Simulaciones por computadora; surfactante SPAN80; adsorción; rutilo.

 

PACS: 68.08De; 68.43.Hn; 68.43.Jk

 

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Acknowledgments

We acknowledge support from grants DGAPA-UNAM-Mexico, IN102812 and Conacyt-Mexico, 154899. We also thank DGTIC-UNAM for KamBalam supercomputer facilities. ENR acknowledges the scholarship and support from CONACyT Mexico.

 

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