Physico-chemical phenomena in soft monolayers: chiral recognition, microfluidics and protein interactions

[cat] En aquesta tesi s’han estudiat diferents fenòmens físico-químics en l’àmbit de les monocapes de Langmuir. En una primera secció, s’han caracteritzat monocapes de Langmuir d’un derivat d’azobenzè que presenta una separació de fases entre els seus dos possibles isòmers. S’ha explorat la inducció de quiralitat en dominis condensats aïllats, rics en isòmer trans, mitjançant l’acció d’una força quiral química (un dopant quiral), i una de física (un vòrtex). Els resultats obtinguts s’han comparat amb els d’un altre estudi realitzat prèviament en el grup, fet amb un derivat d’azobenzè homòleg. A part, s’ha estudiat un fenomen de reconeixement quiral en monocapes, degut a la interacció de dos enantiòmers quirals d’un altre derivat azobenzènic. Una segona secció de la tesi es centra en el camp de la microfluídica bidimensional, i mostra l’estudi de diferents fenòmens de transport que ocorren en la interfase entre dues monocapes que flueixen al llarg d’un canal. En concret, s’han fet experiments de difusió entre monocapes que co-flueixen per un canal, per tal de trobar les condicions en les que s’obté un increment de la difusió, traduït en un augment del valor del coeficient de difusió mesurat. D’aquesta manera, els experiments realitzats en circuits amb forma de serpentí, o en els que s’hi ha creat vòrtexs, presenten un augment en la difusió. En canvi, l’efecte de l’augment del gradient de concentració existent entre les dues monocapes no ha resultat en canvis apreciables en els coeficients de difusió mesurats. D’altra banda, s’ha visualitzat un procés de dissolució en dues dimensions que segueix el model de dissolució clàssic, i s’han fet experiments per tal d’aconseguir visualitzar una reacció en la línia de contacte entre dues monocapes. A més, s’ha estudiat la formació d’un flux de retorn en la subfase com a resultat del flux d’una monocapa en la interfase, i s’ha pogut relacionar amb perfils de velocitat interficials anòmals obtinguts. En l’última secció, s’han adaptat els protocols experimentals utilitzats en la microfluídica bidimensional, per tal d’estudiar la inserció i difusió d’una proteïna en una monocapa de fosfolípids.[eng] In this thesis, several physico-chemical phenomena occurring at the two-dimensional environment of a Langmuir monolayer are studied. In the first section, the characterization of Langmuir monolayers of an azobenzene derivative is presented, together with the induction of chirality by the action of a chemical and a physical force, inside labile condensed domains formed by the trans isomer. This has been compared with results of another study done in the group for a homologue azobenzene molecule. In addition, we have studied a heterochiral recognition phenomenon in monolayers by means of the interaction of two enantiomeric chiral azobenzene derivative surfactants. The following section focuses on the field of two-dimensional microfluidics, and encompasses the analysis of different transport processes occurring at the interface between two monolayers that are coflowing through a channel. In this context, studies of the diffusion between co-flowing monolayers have resulted in a slight increase in the diffusion coefficient value for experiments with curvilinear channels, and when vortices are introduced in the system. In contrast, increasing the concentration gradient between co-flowing monolayers has not been successful at producing a noticeable impact in the measured diffusion coefficient values. On the other hand, a two-dimensional dissolution process has been visualized, with a rate that follows a classic dissolution model. Different unsuccessful attempts to realize a chemical reaction between coflowing monolayers are presented. Moreover, we have addressed the flow of monolayers in open microchannels driven by surface tension gradients, determining that drag at the interface and lateral confinement result in the formation of backflow patterns that carry the subphase liquid upstream through self-organized micro-ducts. This backflow patterns depend on channel geometry and are bound by velocity stagnation surfaces, whose shape and location has been related to the interfacial velocity profiles by means of the combination of a simple model and particle image velocimetry measurements. In the last section, we adapt the protocols used in two-dimensional microfluidics to study the insertion and diffusion of a protein in a phospholipid monolayer

Thermodynamics and mesoscopic organisation in Langmuir monolayers of an azobenzene derivative

We have carried out the analysis of liquid crystalline Langmuir monolayers at the air-water interface composed of the amphiphilic azobenzene derivative 8Az5COOH. By varying the temperature and the isomeric (trans-cis) composition, the monolayer behaviour has been studied in comparison with a shorter homologue, 8Az3COOH, by measuring the surface pressure-area isotherms along with Brewster angle microscopy (BAM). Our data with the pure trans isomer enable a posterior thermodynamic analysis, which was not feasible with the shorter homologue. For the mixed trans-cis monolayers, BAM observations reveal a phase segregation with trans enriched domains surrounded by a cis enriched matrix. Line tension between the two phases is lower than in the shorter homologue. The organisation of the rodlike molecules inside the trans domains results in highly symmetric textures that make the quantitative analysis of the BAM images possible, and a better understanding of the microscopic structure of the monolayer can be achieved

Measurement of a Structured Backflow in an Open Small Channel Induced by Surface-Tension Gradients

We present experiments in which the laterally confined flow of a surfactant film driven by controlled surface tension gradients causes the subtended liquid layer to self-organize into an inner upstream microduct surrounded by the downstream flow. The anomalous interfacial flow profiles and the concomitant backflow are a result of the feedback between two-dimensional and three-dimensional microfluidics realized during flow in open microchannels. Bulk and surface particle image velocimetry data combined with an interfacial hydrodynamics model explain the dependence of the observed phenomena on channel geometry

Supramolecular organization and heterochiral recognition in Langmuir monolayers of chiral azobenzene surfactants

We study the self-assembly of novel azobenzene-based chiral surfactants at the air/water interface, and find that while the pure enantiomers lack the ability to organize in ordered mesophases, the racemic mixture spontaneously forms a hexatic phase at low lateral pressures, which we detect by means of Brewster angle microscopy. This work provides a unique example of heterochiral recognition in which the racemic monolayer is not only condensed with respect to the pure enantiomers, but causes an ordered mesophase to form. Although hexatic order vanishes at high surface pressures, long-range orientational order is regained for all compositions upon monolayer collapse, which proceeds through the formation of birefringent trilayers with a well-defined lateral microstructure, as revealed by atomic force microscopy

Thermodynamics and mesoscopic organisation in Langmuir monolayers of an azobenzene derivative

We have carried out the analysis of liquid crystalline Langmuir monolayers at the air-water interface composed of the amphiphilic azobenzene derivative 8Az5COOH. By varying the temperature and the isomeric (trans-cis) composition, the monolayer behaviour has been studied in comparison with a shorter homologue, 8Az3COOH, by measuring the surface pressure-area isotherms along with Brewster angle microscopy (BAM). Our data with the pure trans isomer enable a posterior thermodynamic analysis, which was not feasible with the shorter homologue. For the mixed trans-cis monolayers, BAM observations reveal a phase segregation with trans enriched domains surrounded by a cis enriched matrix. Line tension between the two phases is lower than in the shorter homologue. The organisation of the rodlike molecules inside the trans domains results in highly symmetric textures that make the quantitative analysis of the BAM images possible, and a better understanding of the microscopic structure of the monolayer can be achieved

Measurement of a Structured Backflow in an Open Small Channel Induced by Surface-Tension Gradients

We present experiments in which the laterally confined flow of a surfactant film driven by controlled surface tension gradients causes the subtended liquid layer to self-organize into an inner upstream microduct surrounded by the downstream flow. The anomalous interfacial flow profiles and the concomitant backflow are a result of the feedback between two-dimensional and three-dimensional microfluidics realized during flow in open microchannels. Bulk and surface particle image velocimetry data combined with an interfacial hydrodynamics model explain the dependence of the observed phenomena on channel geometry

Supramolecular organization and heterochiral recognition in Langmuir monolayers of chiral azobenzene surfactants

We study the self-assembly of novel azobenzene-based chiral surfactants at the air/water interface, and find that while the pure enantiomers lack the ability to organize in ordered mesophases, the racemic mixture spontaneously forms a hexatic phase at low lateral pressures, which we detect by means of Brewster angle microscopy. This work provides a unique example of heterochiral recognition in which the racemic monolayer is not only condensed with respect to the pure enantiomers, but causes an ordered mesophase to form. Although hexatic order vanishes at high surface pressures, long-range orientational order is regained for all compositions upon monolayer collapse, which proceeds through the formation of birefringent trilayers with a well-defined lateral microstructure, as revealed by atomic force microscopy

Supramolecular organization and heterochiral recognition in Langmuir monolayers of chiral azobenzene surfactants

We study the self-assembly of novel azobenzene-based chiral surfactants at the air/water interface, and find that while the pure enantiomers lack the ability to organize in ordered mesophases, the racemic mixture spontaneously forms a hexatic phase at low lateral pressures, which we detect by means of Brewster angle microscopy. This work provides a unique example of heterochiral recognition in which the racemic monolayer is not only condensed with respect to the pure enantiomers, but causes an ordered mesophase to form. Although hexatic order vanishes at high surface pressures, long-range orientational order is regained for all compositions upon monolayer collapse, which proceeds through the formation of birefringent trilayers with a well-defined lateral microstructure, as revealed by atomic force microscopy

Supramolecular Organization and Heterochiral Recognition in Langmuir Monolayers of Chiral Azobenzene Surfactants

We study the self-assembly of novel azobenzene-based chiral surfactants at the air/water interface, and find that while the pure enantiomers lack the ability to organize in ordered mesophases, the racemic mixture spontaneously forms a hexatic phase at low lateral pressures, which we detect by means of Brewster angle microscopy. This work provides a unique example of heterochiral recognition in which the racemic monolayer is not only condensed with respect to the pure enantiomers, but causes an ordered mesophase to form. Although hexatic order vanishes at high surface pressures, long-range orientational order is regained for all compositions upon monolayer collapse, which proceeds through the formation of birefringent trilayers with a well-defined lateral microstructure, as revealed by atomic force microscopy