%0 Book Section %1 statphys23_0358 %A Robazzi, W.S. %A Mokross, B.J. %B Abstract Book of the XXIII IUPAP International Conference on Statistical Physics %C Genova, Italy %D 2007 %E Pietronero, Luciano %E Loreto, Vittorio %E Zapperi, Stefano %K langmuir lattice mean-field model monolayers phase statphys23 thermodynamic topic-7 transitions %T Thermodynamic description of Langmuir monolayers via a lattice model %U http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=358 %X Langmuir monolayers have been the subject of research for many years. They are formed by amphiphilic molecules which have hydrophobic and hydrophilic ends such that part of the molecule repels water while the other one is attracted to it. Those systems are important as models to understand transport processes in biological membranes and also as the basis for bio-electronic sensors. The basic experimental technique applied to study those systems is to measure the surface tension in function of the compression of the surface. The resulting isotherms show that during the compression the monolayer undergoes several phase transitions that depend on the nature of the molecule and on the temperature. Many studies showed that during the compression the following events are observed: at the beginning the molecules are disperse and do not interact behaving as isolated molecules like in a gaseous phase. The next stage when the molecules begin to feel the presence of their neighbors is designated expanded-liquid, since there is no definitve structural arrangement. With further compression the hydrophobic tails of the molecules lift above the interface with inclinations that depend on the nature of the molecule. In the last stage the tails are ordered in a solid like phase. The theoretical study of the phase transitions allow a better understanding of the nature of the interaction forces that act between the molecules. There are many approaches which explain the transitions. Due to the complexity of the system numerical simulations that consider all intra- and inter-molecular interactions are unfeasible. Therefore more general treatments based on thermodynamic concepts are advisable. In this context we developed a lattice model able to describe the behavior of the monolayers during isothermal compression. The interface where the amphiphilic molecules rest on is taken as a bi-dimensional lattice with a constant number of molecules and where the number of sites changes under compression. These molecules are described by a model analogous to the Flory-Huggins theory for polymer solutions where only interactions between nearest neighbors are considered. The description of the hydrophobic tails lifted above the interface due to compression is done by three different mean-field methods: the first one considers the lifted tails as rigid rods without any flexibility, the second one considers that the segments are distributed in a random way without connectivity and the third one which considers connection among the segments and having flexibility. The results show that, for given values of the interaction energy among nearest-neighbor segments, there are two phase transitions for all three models. Another interesting result that depends on which of the three models is used is the rate at which the water molecules enter the layer below the interface during compression and also, during the second phase transition, the rate at which the segments lift above the interface.

@incollection{statphys23_0358, abstract = {Langmuir monolayers have been the subject of research for many years. They are formed by amphiphilic molecules which have hydrophobic and hydrophilic ends such that part of the molecule repels water while the other one is attracted to it. Those systems are important as models to understand transport processes in biological membranes and also as the basis for bio-electronic sensors. The basic experimental technique applied to study those systems is to measure the surface tension in function of the compression of the surface. The resulting isotherms show that during the compression the monolayer undergoes several phase transitions that depend on the nature of the molecule and on the temperature. Many studies showed that during the compression the following events are observed: at the beginning the molecules are disperse and do not interact behaving as isolated molecules like in a gaseous phase. The next stage when the molecules begin to feel the presence of their neighbors is designated expanded-liquid, since there is no definitve structural arrangement. With further compression the hydrophobic tails of the molecules lift above the interface with inclinations that depend on the nature of the molecule. In the last stage the tails are ordered in a solid like phase. The theoretical study of the phase transitions allow a better understanding of the nature of the interaction forces that act between the molecules. There are many approaches which explain the transitions. Due to the complexity of the system numerical simulations that consider all intra- and inter-molecular interactions are unfeasible. Therefore more general treatments based on thermodynamic concepts are advisable. In this context we developed a lattice model able to describe the behavior of the monolayers during isothermal compression. The interface where the amphiphilic molecules rest on is taken as a bi-dimensional lattice with a constant number of molecules and where the number of sites changes under compression. These molecules are described by a model analogous to the Flory-Huggins theory for polymer solutions where only interactions between nearest neighbors are considered. The description of the hydrophobic tails lifted above the interface due to compression is done by three different mean-field methods: the first one considers the lifted tails as rigid rods without any flexibility, the second one considers that the segments are distributed in a random way without connectivity and the third one which considers connection among the segments and having flexibility. The results show that, for given values of the interaction energy among nearest-neighbor segments, there are two phase transitions for all three models. Another interesting result that depends on which of the three models is used is the rate at which the water molecules enter the layer below the interface during compression and also, during the second phase transition, the rate at which the segments lift above the interface.}, added-at = {2007-06-20T10:16:09.000+0200}, address = {Genova, Italy}, author = {Robazzi, W.S. and Mokross, B.J.}, biburl = {https://www.bibsonomy.org/bibtex/279d06db5ecf7e21e64606b71a635eec8/statphys23}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Pietronero, Luciano and Loreto, Vittorio and Zapperi, Stefano}, interhash = {78851d16cb0faef25333c7820687ab28}, intrahash = {79d06db5ecf7e21e64606b71a635eec8}, keywords = {langmuir lattice mean-field model monolayers phase statphys23 thermodynamic topic-7 transitions}, month = {9-13 July}, timestamp = {2007-06-20T10:16:18.000+0200}, title = {Thermodynamic description of Langmuir monolayers via a lattice model}, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=358}, year = 2007 }