Josep Claret

University of Barcelona, Barcino, Catalonia, Spain

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Publications (89)380.42 Total impact

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    ABSTRACT: A chiral selection process in a self-assembled soft monolayer of an achiral amphiphile as a consequence of its interaction with chiral species dissolved in the aqueous subphase, is reported. The extent of the chiral selection is statistically measured in terms of the enantiomorphic excess of self-assembled submillimeter domains endowed with well-defined orientational chirality that is unambiguously resolved using optical microscopy. Our results show that the emergence of chirality is mediated by electrostatic interactions and significantly enhanced by hydrophobic effects. This chiral chemical effect can be suppressed and even reversed by opposing a macroscopic physical influence, such as vortical stirring. This result gives evidence for the crucial role of hydrodynamic effects in supramolecular aggregation.
    Chemistry - A European Journal 06/2014; 20(24). DOI:10.1002/chem.201400248 · 5.73 Impact Factor
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    ABSTRACT: 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.
    Physical Review Letters 05/2013; 110(21):214506. DOI:10.1103/PhysRevLett.110.214506 · 7.51 Impact Factor
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    ABSTRACT: Chirality, the absence of mirror symmetry, can be equally invoked in relation to physical forces and chemical induction processes, yet a competition between these two types of influence is rarely reported. Here we present a self-assembled soft matter system in which chiral selection is controlled by the combined independent action of a chiral dopant and vortical stirring, which are arbitrarily coupled, either constructively or destructively. In the latter case, perfect compensation, that is, absence of a net chiral effect, is realized. The induced enantiomorphic excess is measured in terms of the statistical imbalance of an ensemble of submillimetre domains, where achiral molecules self-assemble with a well-defined orientational chirality that is unambiguously resolved using optical microscopy. The possibility of combining top-down and bottom-up strategies to induce a chiral predominance in a supramolecular system of achiral components should be recognized as a new twist in the process of chiral recognition, selection and control.
    Nature Communications 08/2012; 3:1001. DOI:10.1038/ncomms1987 · 11.47 Impact Factor
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    ABSTRACT: Herein, we report and interpret a new chiral-selection phenomenon in the orientational ordering of soft-assembled domains that arise spontaneously in Langmuir monolayers of an azobenzene derivative at the air/water interface. The orientational chirality of isolated sub-millimeter domains was unambiguously assessed by optical microscopy. The selection process, quantified by using an enantiomeric excess parameter, was controlled by stirring the aqueous subphase. We have studied the dependence of this process on stirring rate and handedness, stirring time, temperature, and on the initial state of the monolayer. The influence of the concomitant cis/trans isomerization on the reported chiral-selection process is also discussed.
    Chemistry - A European Journal 03/2012; 18(13):3975-80. DOI:10.1002/chem.201102358 · 5.73 Impact Factor
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    ABSTRACT: We use Langmuir-Blodgett molecular monolayers and nematic liquid crystals as model two- and three-dimensional orientationally ordered systems to study the stability and healing of topological defects at their contact interfaces. Integer-strength defects at the monolayer induce disclinations of similar strength in the nematic that, however, do not propagate deep into the bulk, but rather form single- or double-split arch-shaped loops pinned to the interface. This behavior is qualitatively independent of the far-field director orientation and involves either half-integer singular or twist-escaped unity-strength nonsingular nematic disclinations. These two defect configurations can be selected by varying sample preparation given their comparable free energy, consistently with direct probing by use of laser tweezers.
    Physical Review Letters 10/2011; 107(17):177801. DOI:10.1103/PhysRevLett.107.177801 · 7.51 Impact Factor
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    ABSTRACT: We originally introduce a device to achieve monitored flows of Langmuir monolayers on predesigned wet circuits imprinted on a solid support. The hydrophilic track is first engraved in contact with a hydrophobically coated metallic plate that is fitted into a three-compartment Langmuir trough. Two different designs are tested to confirm flow stability and control. In particular and as a first utility, we measure the diffusion coefficient of a fluorescent probe using Y-junction geometry.
    Langmuir 03/2010; 26(7):4613-5. DOI:10.1021/la100319x · 4.46 Impact Factor
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    ABSTRACT: We report a novel phenomenon of chiral selection in self-assembled condensates of achiral amphiphiles. The handedness of chiral textures, reproducing the collective rotational component of the molecular orientation inside submillimeter circular domains, is correlated with the sign of a vortical stirring in the aqueous subphase. We propose an explanation based on the distinctive kinetics of topological defect annihilation during domain coalescence at the initial coarsening stage of a phase-separating monolayer.
    Physical Review Letters 12/2009; 103(23):237802. DOI:10.1103/PhysRevLett.103.237802 · 7.51 Impact Factor
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    ABSTRACT: We report on the dynamical behavior of paramagnetic ellipsoidal particles dispersed in water and floating above a flat plane when subjected to an external precessing magnetic field. When the magnetic field and the long axis of the particles are on the same plane, two clear regimes are distinguished in which the particles follow the magnetic modulation synchronously or asynchronously. Both regimes are also observed when the field precesses at an angle theta<90 degrees with respect to the normal to the confining plane, while the transition frequency increases with decreasing precession angle. We combine experimental observations with a theoretical model to characterize the particle dynamics. The possibility to control and/or reorient microscopic elongated particles by changing the frequency or strength of the applied field makes them suitable in microfluidic devices such as microgates for microchannels or active fluid mixers when placed close to channel junctions.
    Physical Review E 03/2009; 79(2 Pt 1):021501. DOI:10.1103/PhysRevE.79.021501 · 2.29 Impact Factor
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    ABSTRACT: An anomalously long transient is needed to achieve a steady pressurization of a fluid when forced to flow through micronarrowed channels under constant mechanical driving. This phenomenon, known as the "bottleneck effect" is here revisited from a different perspective, by using confined displacements of interfacial fluids. Compared to standard microfluidics, such effect admits in this case a neat quantitative characterization, which reveals intrinsic material characteristics of flowing monolayers and permits to envisage strategies for their controlled micromanipulation.
    Physical Review Letters 04/2008; 100(13):134503. DOI:10.1103/PhysRevLett.100.134503 · 7.51 Impact Factor
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    ABSTRACT: Condensed matter physics is facing new challenges and opportunities raising from the vast and increasingly attractive field of soft materials and complex fluids. Here, we concentrate on one of the simplest experimental examples by reviewing different structural and dynamical properties of spread (Langmuir) monolayers. Our interest goes beyond an equilibrium description to stress experiments and models oriented to unveil dynamic responses either following relaxation or externally controlled forcing processes. We choose to focus on mesoscopic phenomena that reflect the pattern of self-assembly of the constituent molecules. We start by reviewing basic structural and textural aspects of Langmuir monolayers. Probably the most classical nonequilibrium phenomena one can envisage when referring to complex fluids concern flow properties. Thus, rheological experiments conducted with Langmuir monolayers are briefly commented next. Finally, new scenarios of collective self-organization are discussed from experiments and models, revealing unexplored dynamic self-assembling capabilities of Langmuir monolayers.
    Physics Reports 08/2007; 448(5-448):163-179. DOI:10.1016/j.physrep.2007.02.016 · 20.03 Impact Factor
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    ABSTRACT: We look at bulk-like viscoplastic properties of a metastable Langmuir mono layer of an alkanethiol derivative close to collapse conditions. By performing controlled creep experiments (decrease of molecular area at constant surface pressure) we were able to evidence two regimes of plastic flow, below and above a pseudo two dimensional melting point, with distinctive temperature dependencies.
    The European Physical Journal Special Topics 03/2007; 143(1):165-170. DOI:10.1140/epjst/e2007-00084-0 · 1.40 Impact Factor
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    ABSTRACT: Continued irradiation of smectic-C-like domains of photosensitive Langmuir monolayers from azobenzene derivatives induces the nucleation and propagation of orientational travelling waves as observed with Brewster angle microscopy (BAM). BAM image analysis has allowed to identify different dynamical behaviors involving the generation and propagation of such waves. A model based on the coupling between an orientational and a composition field proposes a scenario for dynamic self-assembly that accounts for most of the observed phenomena, and allows to pinpoint the relevance of boundary defects in wave-emitting structures.-1.
    The European Physical Journal E 11/2006; 21(2):111-6. DOI:10.1140/epje/i2006-10049-5 · 1.76 Impact Factor
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    ABSTRACT: Propagation of localized orientational waves, as imaged by Brewster angle microscopy, is induced by low intensity linearly polarized light inside axisymmetric smectic-C confined domains in a photosensitive molecular thin film at the air/water interface (Langmuir monolayer). Results from numerical simulations of a model that couples photoreorientational effects and long-range elastic forces are presented. Differences are stressed between our scenario and the paradigmatic wave phenomena in excitable chemical media.
    Physical Review E 03/2006; 73(2 Pt 2):026225. DOI:10.1103/PhysRevE.73.026225 · 2.29 Impact Factor
  • Josep M. Ribó · Carles Acero · Josep Claret
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    ABSTRACT: The electrochemical and chemical oxidative α,α'-coupling of 3-methoxy-4-methyl-2-pyrrolecarboxylic acid (2) and its ethyl ester 4 is compared with that of the corresponding 4-methoxy-2-pyrrolecarboxylates (1 and 3 respectively). The β-alkoxy substituent exerts a strong directing influence on its vicinal α-position, which in the case of compounds 2 and 4 determines the 2,5'-coupling and allows the isolation of the corresponding 1H,2'H-2,2'-bipyrrole 6 in the case of 4. For 2 the polypyrrole can be obtained by exhaustive electrochemical or chemical coupling, whereas in the case of 1 only the corresponding tetramers are formed. The structures of these tetrapyrroles, already described in the literature, here obtained by oxidation of 4-alkoxy-2-pyrrolecarboxylic acid (1), have been reexamined: they are mixtures of tetramers with zwitterionic structure, have at least two substitution patterns and are partially oxidized to the corresponding bipyrrolylidenes.
    Berichte der deutschen chemischen Gesellschaft 01/2006; 126(10):2269 - 2273. DOI:10.1002/cber.19931261017 · 2.94 Impact Factor
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    ABSTRACT: In a photoresponsive Langmuir monolayer comprised of smectic-C-like domains of mesogenic trans-azobenzene derivative embedded within an isotropic matrix of its cis isomer counterpart, several structurally differing circular droplets were irradiated with linearly polarized light. This report describes the structural rearrangements that occurred in these droplets upon illumination followed by Brewster angle microscopy analysis. Starting from initial well-characterized and symmetric states, final photoaligned situations were reached in which the azimuth angles of the rod-shaped elongated molecules were found to be perpendicular to the electric component of the excitation light. The dynamical aspects of the photoalignments, including their transient patterns, are captured by a theoretical model that couples a relaxational principle incorporating long-range elastic forces with a kinetic formalism presenting an anisotropic rate law.
    The Journal of Chemical Physics 07/2005; 122(24):244722. DOI:10.1063/1.1927518 · 2.95 Impact Factor
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    ABSTRACT: The Mn oxidation state of two water soluble Mn porphyrins, MnTMPyP and MnTPPS4, was studied as a function of the aqua or hydroxo ligands of the Mn atom. In NaOH solutions, long-lived O=Mn(IV) species were detected in the presence of O2. Conversely, the dihydroxo Mn(III) porphyrin reduces spontaneously to the Mn(II) species in the absence of O2. In alkaline solutions, these Mn porphyrins were able to electrocatalyze the 4-electron reduction of O2 to H2O on a vitreous carbon electrode.
    Journal of Porphyrins and Phthalocyanines 06/2005; 09(06). DOI:10.1142/S108842460500054X · 1.40 Impact Factor
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    ABSTRACT: We report here a photoswitch process that involves collective molecular reorientation in a monolayer of an azobenzene derivative. Using polarized light we force the transition between two clearly distinguishable orientational mesoscopic configurations that can be monitored by reflection optical microscopy. A model that combines thermodynamic and kinetic arguments is proposed, and it is able to reproduce both the two states and the mechanism involved in the transition. We conclude that the phenomenon reported here is essentially different from the usual electric-field-induced molecular alignment often found in liquid crystalline materials and devices. Instead, it involves a photoexcitation concomitant with an H-aggregation process.
    Journal of the American Chemical Society 05/2005; 127(15):5296-7. DOI:10.1021/ja0502066 · 12.11 Impact Factor
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    ABSTRACT: Preparation of Langmuir monolayers of a mixture of trans- and cis-isomers of an azobenzene derivative, 4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid, results in the segregation of birefringent trans-isomer domains embedded in an isotropic medium of cis-isomers. Brewster angle microscopy observations allow us to identify different textures inside the domains depending on surface pressure, temperature, and domain size. The evolution of the monolayer in the dark, from initial droplets formed after spreading to a stable stripe texture, is described. The dynamics of domain coalescence and some morphological transitions induced by temperature and surface pressure changes are also discussed. A simple theoretical model is included to supplement some of these experimental observations.
    Langmuir 04/2005; 21(7):2948-55. DOI:10.1021/la0473557 · 4.46 Impact Factor
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    ABSTRACT: Droplet smectic-C domains in films of surfactant molecules exhibit different orientational textures. For these systems we formulate a kinetic model based on a free energy functional containing bulk (elastic) and surface interactions. Numerical simulations for the corresponding relaxational equation show the existence of two different equilibrium configurations with a centered defect. In particular, when the elastic terms dominate, bend-shaped textures appear, whereas for strong boundary effects mixed bend/splay conformations are displayed. A variational analysis for the free energy functional confirms the validity of the above numerical results. The stability of textures with centered defects with respect to the formation of periferic defects (boojums) is also discussed qualitatively. The above theoretical predictions are compared with experimental results from Brewster angle microscopy imaging of azobenzene Langmuir monolayers.
    The Journal of Chemical Physics 12/2004; 121(18):9066-76. DOI:10.1063/1.1805515 · 2.95 Impact Factor
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    ABSTRACT: We report a reorientational phenomenon that takes place inside confined smectic-C (SmC) domains in a phase-segregated Langmuir film. Observations of texture changes by means of Brewster angle miscroscopy (BAM) refer to an azobenzene derivative whose monolayer behavior depends strongly on its molecular cis/ trans configuration. Well-developed droplets of a trans-rich mesophase adopt mixed bend/splay structures whose splay sign is reversed through complete droplet reordering when varying the surface pressure. Numerical simulations of a simple relaxational model based on a free-energy functional fully reproduce the experimental observations.
    The Journal of Physical Chemistry B 11/2004; 108:17274. DOI:10.1021/jp046182r · 3.30 Impact Factor

Publication Stats

794 Citations
380.42 Total Impact Points


  • 1985–2014
    • University of Barcelona
      • • Department of Physical Chemistry
      • • Departament de Química Orgànica
      Barcino, Catalonia, Spain
  • 2011
    • Indian Broiler (IB) Group India
      Bhānpuri, Chhattisgarh, India
  • 1984
    • University of Alicante
      • Departamento de Química Física
      Alicante, Valencia, Spain