RESUMEN
Complex fluids composed of water, an organic solvent, and a surfactant have been recently employed as cleaning systems to remove hydrophobic materials, such as polymeric coatings, from solid surfaces. The simultaneous presence of surfactants and an organic solvent with good affinity for the polymer was proven necessary for the polymer's removal, but the comprehension of the cleaning mechanism is poorly understood. In this Article, we investigated the mechanism of removal, highlighting the specific role of each component in the interaction with the polymer film. In particular, the results from quartz crystal microbalance with dissipation monitoring (QCM-D) were compared with those obtained by using confocal microscopy to follow in situ the effect of a nanostructured fluid, i.e., a ternary formulation containing water, 2-butanone (MEK) as a good solvent for the polymer, and a nonionic surfactant (C9-11 ethoxylated alcohol, BR) on acrylic copolymer films (Paraloid B72). The results indicate a two-step process: (i) the penetration of the good solvent across the film causes the swelling of the polymer, the weakening of polymer-polymer interactions, and an increase of molecular mobility, followed by (ii) the slow adsorption of amphiphilic aggregates promoting the film detachment from the solid substrate. A different behavior is observed in the presence of similar formulations containing an anionic surfactant (sodium dodecyl sulfate, SDS), where the adsorption of SDS micelles on the surface of the polymeric film hinders solvent access into the polymer layer, rather than promoting its detachment from the solid substrate.
RESUMEN
HYPOTHESIS: The structure, rheology and other physicochemical properties of dilute aqueous dispersions of sodium oleate (NaOL) are well known. This paper is the first report in which a moderately concentrated (13% w/w) dispersion of NaOL in water is investigated. In fact, at this concentration the phase and rheology behavior of the surfactant remarkably deviates from those of its dilute solutions in water and a significant effect is imparted by the addition of potassium chloride. EXPERIMENTAL: The structural, thermal and rheological properties of a 13% w/w dispersion of NaOL in water were investigated by cryo-TEM, rheology, and DSC experiments with and without the addition of potassium chloride. The system is comprised of elongated wormlike micelles that turn into a gel-like more disordered viscous material upon addition of small amounts of KCl (4% w/w). FINDINGS: This paper illustrates the multifaceted behavior of sodium oleate dispersions at intermediate concentrations that depends on the presence of other cosolutes (such as KCl). The results show that viscoelastic aqueous dispersions of NaOL are excellent candidates for the preparation of stimuli-responsive green materials to be used in a number of different applications. We also discuss the genesis of wormlike micelles (WLMs) in terms of the general theory of self-assembly.
RESUMEN
Complex fluids based on amphiphilic formulations are emerging, particularly in the field of conservation of works of art, as effective and safe liquid media for the removal of hydrophobic polymeric coatings. The comprehension of the cleaning mechanism is key to designing tailored fluids for this purpose. However, the interaction between nanostructured fluids and hydrophobic polymer films is still poorly understood. In this study, we show how the combination of confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM) provides interesting and complementary insight into this process. We focused on the interaction between an ethyl methacrylate/methyl acrylate 70:30 copolymer film deposited onto a glass surface and a water/nonionic surfactant/2-butanone (MEK) ternary system, with MEK being a good solvent and water being a nonsolvent for the polymer. Our results indicate a synergy between the organic solvent and the surfactant assemblies: MEK rapidly swells the outer layers of the polymer film allowing for the subsequent diffusion of solvent molecules, while the amphiphile decreases the interfacial energy between the polymeric coating and the liquid phase, favoring dewetting and dispersion of swollen polymer droplets in the aqueous phase. The chemical nature of the surfactant and the microstructure of the assemblies determine both the kinetics and the overall efficiency of polymer removal, as assessed by comparing the behavior of similar formulations containing an anionic surfactant (sodium dodecyl sulfate, SDS).