Maria G. Miguel

Rationalizing cellulose (in)solubility: reviewing basic physicochemical aspects and role of hydrophobic interactions

Despite being the world’s most abundant natural polymer and one of the most studied, cellulose is still challenging researchers. Cellulose is known to be insoluble in water and in many organic solvents, but can be dissolved in a number of solvents of intermediate properties, like N-methylmorpholine N-oxide and ionic liquids which, apparently, are not related. It can also be dissolved in water at extreme pHs, in particular if a cosolute of intermediate polarity is added. The insolubility in water is often referred to strong intermolecular hydrogen bonding between cellulose molecules. Revisiting some fundamental polymer physicochemical aspects (i.e. intermolecular interactions) a different picture is now revealed: cellulose is significantly amphiphilic and hydrophobic interactions are important to understand its solubility pattern. In this paper we try to provide a basis for developing novel solvents for cellulose based on a critical analysis of the intermolecular interactions involved and mechanisms of dissolution.

Polymer-vesicle association.

Mixed polymer-surfactant systems have been intensively investigated in the last two decades, with the main focus on surfactant micelles as the surfactant aggregate in interaction. The main types of phase behavior, driving forces and structural/rheological effects at stake are now fairly well understood. Polymer-vesicle systems, on the other hand, have received comparatively less attention from a physico-chemical perspective. In this review, our main goal has been to bridge this gap, taking a broad approach to cover a field that is in clear expansion, in view of its multiple implications for colloid and biological sciences and in applied areas. We start by a general background on amphiphile self-assembly and phase separation phenomena in mixed polymer-surfactant solutions. We then address vesicle formation, properties and stability not only in classic lipids, but also in various other surfactant systems, among which catanionic vesicles are highlighted. Traditionally, lipid and surfactant vesicles have been studied separately, with little cross-information and comparison, giving duplication of physico-chemical interpretations. This situation has changed in more recent times. We then proceed to cover more in-depth the work done on different aspects of the associative behavior between vesicles (of different composition and type of stability) and different types of polymers, including polysaccharides, proteins and DNA. Thus, phase behavior features, effects of vesicle structure and stability, and the forces/mechanisms of vesicle-macromolecule interaction are addressed. Such association may generate gels with interesting rheological properties and high potential for applications. Finally, special focus is also given to DNA, a high charge polymer, and its interactions with surfactants, and vesicles, in particular, in the context of gene transfection studies.

Aggregation and gelation in hydroxypropylmethyl cellulose aqueous solutions

In this work we present an analysis of the thermal behavior of hydroxypropylmethyl cellulose aqueous solutions, from room temperature to higher temperatures, above gelation. We focus on significant aspects, essentially overlooked in previous work, such as the correlation between polymer hydrophobicity and rheological behavior, and the shear effect on thermal gelation. Micropolarity and aggregation of the polymer chains were monitored by both UV/vis and fluorescence spectroscopic techniques, along with polarized light microscopy. Gel formation upon heating was investigated using rheological experiments, with both large strain (rotational) tests at different shear rates and small strain (oscillatory) tests. The present observations allow us to compose a picture of the evolution of the system upon heating: firstly, polymer reptation increases due to thermal motion, which leads to a weaker network. Secondly, above 55 degrees C, the polymer chains become more hydrophobic and polymer clusters start to form. Finally, the number of physical crosslinks between polymer clusters and the respective lifetimes increase and a three-dimensional network is formed. This network is drastically affected if higher shear rates, at non-Newtonian regimes, are applied to the system.

DNA conformational dynamics in the presence of catanionic mixtures

DNA conformational behavior in the presence of non‐stoichiometric mixtures of two oppositely charged surfactants, cetyltrimethylammonium bromide and sodium octyl sulfate, was directly visualized in an aqueous solution with the use of a fluorescence microscopy technique. It was found that in the presence of cationic‐rich catanionic mixtures, DNA molecules exhibit a conformational transition from elongated coil to compact globule states. Moreover, if the catanionic mixtures form positively charged vesicles, DNA is adsorbed onto the surface of the vesicles in a collapsed globular form. When anionic‐rich catanionic mixtures are present in the solution, no change in the DNA conformational behavior was detected. Cryogenic transmission electron microscopy, as well as measurements of translational diffusion coefficients of individual DNA chains, supported our optical microscopy observations.

Modeling of DNA compaction by polycations

In this work we study polycations as efficient compacting agents of a medium size polyanion by means of Monte Carlo simulations. The systems are characterized in terms of a conformational analysis in which shape, overall dimensions, structure factors, radial distribution functions, and the degree of accumulation of the compaction agent near the polyanion are taken into consideration. Results show that the degree of compaction depends on the size of the positive chains and their number. The role of electrostatic interactions is paramount in the compaction process, and an increase in the number of molecules of the compacting agent or in the number of charges of each molecule leads to collapse, which may be followed by some unfolding in situations of overcharging. Compaction is associated with polycations promoting bridging between different sites in the polyanion. When the total charge of the polycations is significantly lower than that of the polyanion, interaction produces only a small degree of intrachain segregation in the latter, allowing for significant translational motion of the compacting agent along the longer chain. However, complete charge neutralization is not mandatory to achieve compact forms.

DNA–cationic amphiphile interactions

DNA shows strong interactions with cationic cosolutes and these have both biological and technological significance. We outline our research on various mixed systems of DNA and cationic amphiphiles including the interaction of DNA with simple cationic surfactants as well as the interaction with catanionic mixtures and positively charged catanionic vesicles. An overview from phase behavior to microstructure will be presented. We will also address DNA compaction and decompaction phenomena in different systems. Finally, simulations on DNA confinement and interaction with cationic polyions are considered.

The structure and thermal behaviour of some long chain cerium(III) carboxylates

The even chain length cerium(III) carboxylates from the octanoate to the octadecanoate have been synthesised by metathesis. Thermogravimetry shows the presence of coordinated water for the short chain homologues, whereas the longer chain ones only contain adsorbed water. X-Ray diffraction and IR spectral measurements show that the solid phase has a lamellar, bilayer structure with planes of the cerium(III) ions coordinated to the carboxylate groups. The phase behaviour of the carboxylates has been studied by DSC and polarized-light microscopy. One or more mesophases are observed over the temperature range 70–120°C and melting occurs between 130 and 150°C. The textures observed on the polarizing microscope clearly show the anisotropic nature of the mesophases. Although the overall enthalpy and entropy of melting of these compounds increase with increasing chain length, the values are considerably lower than expected for complete fusion of the alkyl chains. Competition between melting of the chains and changes in the metal–carboxylate coordination region is the major factor responsible for the differences observed in the phase behaviour between the short and long chain derivatives.

Shear-induced transitions between a planar lamellar phase and multilamellar vesicles: continuous versus discontinuous transformation.

The shear-induced transitions between an oriented lamellar phase and shear-induced multilamellar vesicles (MLVs) in a nonionic surfactant system were studied by deuterium rheo-NMR spectroscopy as a function of time in start-up experiments at several temperatures and shear rates. By starting from an initial state of oriented lamellae and observing the transformation to the final steady state of MLVs and vice-versa, two different mechanisms were found, depending on the direction of the transition and the initial state. The transition is continuous when MLVs are formed, starting from the oriented lamellar phase. On the other hand, a discontinuous nucleation-and-growth process with a coexistence region is observed when transforming MLVs into an oriented lamellar phase.

Vesicle-Templated Layer-by-Layer Assembly for the Production of Nanocapsules

Hollow structures on the submicrometer scale (nm) are obtained with the assembly of polyelectrolytes according to the layer-by-layer (LbL) technique. Following the LbL procedure, polymers alginate and chitosan were alternatively adsorbed on a vesicular template made of didodecyldimethylammonium bromide (DDAB). Evidence for the removal of the vesicular template entrapped in the alginate/chitosan film is presented. The removal of the vesicular template was achieved through interactions between a nonionic surfactant (Triton X100) and the double-chained surfactant forming the vesicles. The application of this approach allowed the production of hollow nanospheres with a mild procedure, avoiding the use of strong acids or other extreme working conditions that can modify the shell integrity. The obtained nanostructures were characterized by means of dynamic light scattering (DLS), the zeta potential, and scanning electron microscopy (SEM). The SEM analysis demonstrated the presence, after the core removal process, of nanocapsules indistinguishable in size and shape from the parent core-shell system. The analysis of the surface charge of the hollow nanocapsules, after the core dissolution, by zeta potential measurements, indicates good aggregate stability. DLS measurements showed that the size of the nanocapsules is on the order of hundreds of nanometers. Moreover, the size of both the core-shell and the hollow particles did not appear to be perturbed by variations in temperature or ionic strength.

Effect of the head-group geometry of amino acid-based cationic surfactants on interaction with plasmid DNA.

The interaction between DNA and different types of amino acid-based cationic surfactants was investigated. Particular attention was directed to determine the extent of influence of surfactant head-group geometry toward tuning the interaction behavior of these surfactants with DNA. An overview is obtained by gel retardation assay, isothermal titration calorimetry, fluorescence spectroscopy, and circular dichroism at different mole ratios of surfactant/DNA; also, cell viability was assessed. The studies show that the surfactants with more complex/bulkier hydrophobic head group interact more strongly with DNA but exclude ethidium bromide less efficiently; thus, the accessibility of DNA to small molecules is preserved to a certain extent. The presence of more hydrophobic groups surrounding the positive amino charge also gave rise to a significantly lower cytotoxicity. The surfactant self-assembly pattern is quite different without and with DNA, illustrating the roles of electrostatic and steric effects in determining the effective shape of a surfactant molecule.