title: Phase Diagram of Fatty Acid Langmuir Monolayers from Rheological Measurements.
abstract: Langmuir monolayers of fatty acids and alcohols are two-dimensional systems with a rich equilibrium phase diagram. We have explored the temperature and surface-pressure-dependent shear response of monolayers formed by fatty acids of different chain lengths and a fatty alcohol. This has been accomplished with an interfacial shear rheometer utilizing magnetic tweezers and equipped with a refined temperature control and acquisition system. Our rheological results have allowed us to draw a phase diagram from the viscoelastic properties of these 2-D systems and show new phenomena that strongly depend on temperature: the existence of a maximum in viscosity at the L2' phase, the behavior of the elastic modulus to the storage modulus ratio at the L2 phase, and the increase or decrease in viscosity at the L2-LS phase transition. In addition, we unambiguously show that the LS phase displays a counterintuitive behavior in which the loss modulus increases with temperature. We demonstrate, through isothermal surface pressure sweeps and isobaric temperature sweeps, that the exponential dependence of the loss modulus on temperature at the LS phase appears for all hydrophobic tail lengths studied and for both acid and alcohol head groups.
authors.affiliation: Departamento de Física Fundamental, Universidad Nacional de Educación a Distancia , 28040 Madrid, Spain.
authors.forename: J
authors.identifier: 0000-0002-4085-7610
authors.initials: J
authors.lastname: Tajuelo
authors.affiliation: Departamento de Química Física I, Universidad Complutense de Madrid , 28040 Madrid, Spain.
authors.forename: E
authors.initials: E
authors.lastname: Guzmán
authors.affiliation: Departamento de Química Física I, Universidad Complutense de Madrid , 28040 Madrid, Spain.|Instituto Pluridisciplinar, Universidad Complutense , 28040 Madrid, Spain.
authors.forename: F
authors.initials: F
authors.lastname: Ortega
authors.affiliation: Departamento de Química Física I, Universidad Complutense de Madrid , 28040 Madrid, Spain.|Instituto Pluridisciplinar, Universidad Complutense , 28040 Madrid, Spain.
authors.forename: R G
authors.initials: RG
authors.lastname: Rubio
authors.affiliation: Departamento de Física Fundamental, Universidad Nacional de Educación a Distancia , 28040 Madrid, Spain.
authors.forename: M A
authors.initials: MA
authors.lastname: Rubio