Binary Phase Diagrams at the Air-Water Interface: An Experiment for Undergraduate Physical Chemistry Students

• Autores: Molly C. Larsen
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 91, Nº 4, 2014, págs. 597-601
• Idioma: inglés
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• Resumen

  • An experiment is described where students determine the binary phase diagram of mixed monolayer films of pentadecanoic acid (C15H30O2) and isopalmitic acid (C16H32O2). They do this by using Langmuir–Blodgett troughs to measure isotherms, surface pressure (π) measurements as a function of surface area, of the mixed films above an aqueous surface. The experiment is appropriate for third-year physical chemistry students. Students determine the surface pressure of the start of the phase transition between the liquid expanded (LE) phase and the tilted condensed (TC) phase from their isotherms and use it to obtain a boundary line between the LE and coexistence region. They also calculate the mole fraction of the solution in the tilted condensed phase at the pressure where the beginning of the LE to TC transition occurs to obtain the boundary line between the coexistence region and the TC region, allowing the students to create a binary phase diagram for the LE to TC transition. Students also integrate their isotherms to determine the free energy of mixing of their solutions at two surface pressures. This experiment allows for several variations with different types of phase diagrams, and students seemed to enjoy the more modern take on the classic binary phase diagram experiment.