Capre Masó, Lucas D.

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    Influence of surface composition on functionalized gold nanoparticle adsorption to the oil/water interface
    (2021-07) Capre Masó, Lucas D.; Acevedo Vélez, Claribel; College of Engineering; Acevedo Rullán, Aldo; Pagán Torres, Yomaira J.; Department of Chemical Engineering; Rodríguez Carias, Abner
    The studies presented in this thesis focused on investigating the influence of surface composition on the adsorption and self-assembly of nanoparticles at the toluene/water interface, as a step towards generating knowledge that can be useful in the design of nanoparticle-based materials to manage oil/water interfaces. To this end, gold nanoparticles were functionalized with binary mixtures of polar and nonpolar alkanethiols displaying hydrophobic (methyl) and hydrophilic (hydroxyl, amine, guanidine) groups as a model system. The adsorption behavior of the gold nanoparticles was studied by dynamic interfacial tension measurements and optical microscopy experiments that showed that changing parameters such as the ratio of polar-to-nonpolar terminated thiols, nonpolar thiol chain length, and polar thiol functional group identity and charge strongly influenced nanoparticle adsorption to the interface. Results from dynamic interfacial tensions measurements suggest that: (1) a molar surface composition of 40-50% polar functional groups (e.g., 45% hydroxyl) leads to enhanced affinity of the functionalized gold nanoparticles to the toluene/water interface, (2) short chain nonpolar alkanethiols (i.e., C8) enhance nanoparticle adsorption to the interface, and (3) nanoparticles decorated with binary mixtures of protonated amine/methyl alkanethiols decrease the interfacial tension to a greater extent than hydroxyl/methyl or guanidine/methyl nanoparticles under the same conditions, suggesting an enhanced affinity of these nanoparticles to the hydrophobic interface. Optical microscopy experiments of toluene droplets emulsified in water in the presence of the nanoparticles revealed that these adsorb to the toluene interface as aggregates, with size and arrangements that depend upon the identity of the polar functional group. Hydroxyl nanoparticles adsorb to the droplet interface as small, dispersed clusters, while amine and guanidine nanoparticles form interconnected aggregates on droplet surfaces. Moreover, the aggregates formed by amine nanoparticles are larger than those formed by hydroxyl and guanidine nanoparticles. Overall, the results presented in this thesis show that gold nanoparticles decorated with hydroxyl, amine or guanidine polar functional groups adsorb to the toluene/water interface, and that the self-assembly and dynamics of adsorption at the interface depend upon the identity and charge of the polar group within the mixed monolayer. These results provide insights into the adsorption behavior of chemically functionalized gold nanoparticles at the toluene/water interface and contribute knowledge that can be used towards engineering new and efficient nanoparticle-based materials to manage oil/water interfaces.