Arturo Carranza

pH Wave-Front Propagation in the Urea-Urease Reaction

The urease-catalyzed hydrolysis of urea displays feedback that results in a switch from acid (pH ~3) to base (pH ~9) after a controllable period of time (from 10 to >5000 s). Here we show that the spatially distributed reaction can support pH wave fronts propagating with a speed of the order of 0.1-1 mm min(-1). The experimental results were reproduced qualitatively in reaction-diffusion simulations including a Michaelis-Menten expression for the urease reaction with a bell-shaped rate-pH dependence. However, this model fails to predict that at lower enzyme concentrations, the unstirred reaction does not always support fronts when the well-stirred reaction still rapidly switches to high pH.

Processing of lignin in urea–zinc chloride deep-eutectic solvent and its use as a filler in a phenol-formaldehyde resin

The main goal of our research deals with a new greener and more efficient lignin modification method to optimize its structural performance as a phenol-formaldehyde resin filler. Through the appropriate increase in the phenolic hydroxyl content, added value as a raw material was intended. For that, a series of mixtures of zinc chloride–urea with different molar ratios was prepared to obtain deep-eutectic solvents (DESs). Two heating methods (oil bath and microwave heating) were compared and optimized. Microwave helps in reducing the preparation time but to the detriment of temperature control. On the other hand, heating with an oil bath provided better temperature control and homogeneity of the mixture. The preferable molar ratio of ZnCl2–urea was 3 : 10 (Tg = −26.3 °C). The structural changes of the pretreated lignin samples were investigated by Fourier transformed infrared and X-ray photoelectron spectroscopies, scanning electron microscopy, induced coupled plasma and X-ray diffraction. Thermogravimetric analysis demonstrated significant differences in the thermal behavior of the recovered lignin as a result of DES treatment. The weight of lignin recovered was 4 times that of original lignin, indicating that the structure of lignin was transformed through the integration of Zn. The integration of Zn enhanced the thermal stability and enhanced lignins reactivity towards phenol-formaldehyde resin formation. Phenol-formaldehyde resin containing the recovered lignin exhibited lower thermocuring temperatures and better thermostability than those without filler. To the best of our knowledge, this is the first report on the investigation of the structural transformations of lignin through its dissolution in urea–ZnCl2 DES and subsequent use as filler for phenol formaldehyde resin synthesis.

Porous monoliths synthesized via polymerization of styrene and divinyl benzene in nonaqueous deep-eutectic solvent-based HIPEs

Stable nonaqueous high internal phase emulsions (HIPEs) were prepared and thermally polymerized to yield poly(HIPEs). The internal phase accounting for 80 vol% of the HIPE consisted of a deep-eutectic solvent (DES) while the continuous one comprised styrene and divinyl benzene in a 10 : 1 molar ratio. DESs with different viscosities were used as an internal phase: choline chloride combined with urea, glycerol or ethylene glycol in a 1 : 2, salt : hydrogen bond donor molar ratio, respectively. HIPEs were stabilized with different amounts of the surfactant Span 60 (10, 20 and 50 wt% with respect to the total amount of monomers). DESs viscosity and the amount of surfactant employed impact the morphology and mechanical properties of poly(HIPEs). Resulting poly(HIPEs) showed interconnected porosity and high thermal stability above 310 °C. Its worth noting that DES was recovered from 89 to nearly 95 wt% and the monomer conversion was as high as 0.96. In addition, water-in-oil HIPEs were stabilized and then polymerized under the same conditions, but the porous structure of the resulting poly(HIPEs) collapsed. This research demonstrates that DESs are a suitable internal phase for HIPEs thus expanding on the range of monomers forming polymerizable DES-based HIPEs.

Deep-eutectic solvents as a support in the nonaqueous synthesis of macroporous poly(HIPEs)

This study demonstrates the formation and polymerization of high internal phase emulsions (HIPEs) with (meth)acrylic monomers as a continuous phase and urea–choline chloride deep-eutectic solvent as a favorable nonaqueous internal phase. After recovery of DES, the resultant poly(HIPEs) showed interconnected macroporosity which can be tuned by varying the experimental conditions.

Zinc-based deep eutectic solvent-mediated hydroxylation and demethoxylation of lignin for the production of wood adhesive

Choline chloride–ZnCl2 deep-eutectic solvent (ChCl–ZnCl2 DES), mole ratio 1:2, was used to improve the chemical reactivity of wheat straw alkali lignin under different temperatures and times of pretreatment. The chemical structure of the resulting modified lignin was studied by UV, FT-IR, 1H, 13C and 31P-NMR spectroscopies, TGA, NALDI-TOF MS and ICP. Interestingly up to 10 wt% of lignin can be readily dissolved in ChCl–ZnCl2 DES under the optimized pretreatment conditions (80 °C, 1 h) yielding ca. 65% of modified lignin upon precipitation using water as an antisolvent. As a result of the chemical modification of lignin occurring during its dissolution in DES, the total phenolic hydroxyl of the fraction precipitated increased ca. 1.9-fold while methoxyl moieties were reduced between 1.6 and 2.2-fold when compared with untreated lignin. Thus, in the fraction of lignin precipitated, phenolic hydroxyl formation took place at the expense of selective methoxyl cleavage from the aromatic ring, judging by the decrease of S units, whereas β-O-4′ linkages and molecular weight remain unchanged. Finally modified lignin was used as a phenol replacement in the synthesis of phenol-formaldehyde (PF) adhesives. Remarkably, the strength of the modified resin (1.3 MPa) compared with PF resin was practically the same when 40 wt% of the phenol was replaced by the modified lignin. This work shows that lignin can be readily modified in a DES – improving its reactivity – further advancing its prospective use in the wood industry.

Sustainable-solvent-induced polymorphism in chitin films

We report a simple route to produce chitin films with different crystalline structures. The use of a green deep-eutectic-solvent (DES) choline chloride : urea (CCU) or hexafluoroisopropanol (HFIP) allows fine-tuning of the crystallization process to produce films with β-dihydrated- or γ-chitin structures. With the advent of a Grazing-Incidence X-Ray Diffraction (GIXD) technique, we propose a new and fully indexed structure of γ-chitin. This new γ-structure is modeled as a super-cell, in contrast to the idea of a simple physical mixture of α and β phases. The influence of purification, dissolution or dispersion media on chitins crystallinity was investigated by FTIR, solid-state NMR and XRD. Results presented here may trigger applications in which a well-defined crystalline structure is required.

On the stability and chemorheology of a urea choline chloride deep-eutectic solvent as an internal phase in acrylic high internal phase emulsions

High internal phase emulsions are an interesting emulsion subset attainable by surpassing the critical volume of uniform spherical arrangement resulting in a new metastable polyhedral motif. By profiting from their stability time frame and through the introduction of a polymerizable phase, these unique structures can be “locked” in place, affording interconnected hierarchically porous polymers. Rheological exploration has establishing several emulsion stability key parameters including surfactant concentration, internal phase volume fraction (ϕ), interfacial tension (σ), phase polarity, and temperature. Because the majority of HIPEs studied are aqueous, additional parameters such as internal phase viscosity have not been studied in detail. Deep-eutectic solvents (DES) are a new generation of green solvents sharing several ionic liquid properties. DES provide an ideal opportunity to study nonaqueous polar internal phases of increased viscosity while expanding on the conditions for polymerization to potential scale up applications. This study presents the first detailed investigation on the DES-non ionic surfactant HIPE systems. Shear stability of non-aqueous HIPEs was evaluated taking into account continuous phase viscosity as well as monomer and surfactant molecular nature (i.e. chain length and functionality). HIPE polymerization was evaluated through isothermal oscillatory time-sweep experiments. Longer tail methacrylic monomers presented preferential stability over acrylic or short tail monomers. Furthermore, emulsions showed improved stability and elasticity compared to aqueous HIPEs due to their high internal phase viscosity. Internal phase viscosity mediated by hydrogen bonding increased activation energy as estimated by complex viscosity plots.

Europium-doped aluminum oxide phosphors as indicators for frontal polymerization dynamics

In this study, we present an inexpensive and practical method that allows the monitoring and visualization of front polymerization, propagation, and dynamics. Commercially available europium-doped aluminum oxide powders were combined with video imaging to visualize free-radical propagating polymer fronts. In order to demonstrate the applicability of this method, frontal copolymerization reactions of propoxylated glycerin triacrylate (EB53), pentaerythritol triacrylate (PETA), and pentaerythritol tetra-acrylate (PETEA) with 1,1-Bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane (Luperox 231®) as an initiator were studied and compared to the results obtained by IR imaging. Systems exhibiting higher filler loading, higher EB53 content, and less acrylated monomers showed a marked decrease in front velocity, while those with more acrylated monomers and higher crosslinking density showed a marked increase in front velocity. Finally, in order to show the potential of the imaging technique, we studied fronts propagating in planar and spherical geometries.