18 research outputs found
Conformation and phase behavior of sodium carboxymethyl cellulose in the presence of mono- and divalent salts
We report a small-angle neutron scattering (SANS) study of semidilute aqueous solutions of sodium carboxymethyl cellulose (NaCMC), in the presence of mono- (Na+) and divalent salts (Mg2+, Ca2+, Zn2+, and Ba2+). A degree of substitution of 1.3 is selected to ensure that, in salt-free solution, the polymer is molecularly dissolved. We find that Na+ and Mg2+ salt addition yields H-type phase behavior, while Ca2+, Zn2+, and Ba2+ instead yield a mixed H/L-type phase behavior dependent on the NaCMC concentration (cp), in the decreasing order of the salt concentration required to induce turbidity (at a fixed cp). Charge screening by addition of NaCl induces the disappearance of the characteristic polyelectrolyte correlation peak and eventually yields scattering profiles with a qā1 dependence over nearly 3 decades in the wavenumber q. By fitting a descriptive model to data with excess Na+, we obtain a correlation length Ī¾ā² = 1030 cpā0.72 Ć
with cp in g Lā1. Addition of Mg2+, which does not interact specifically with NaCMC carboxylate groups, yields an analogous screening behavior to that of Na+, albeit at lower salt concentrations, in line with its higher ionic strength. At low salt concentration, addition of specifically interacting Ca2+, Zn2+, and Ba2+ yields a comparatively greater screening of the polyelectrolyte correlation peak, and at concentrations above the phase boundary, results in excess scattering at low-q, compatible with the formation of 20ā40 nm clusters. This behavior is interpreted as due to the reduction in charge density along the chain, promoting interchain association and multichain domain formation resulting in visible turbidity. Overall, drawing analogies with NaCMC at a lower degree of substitution, where hydrophobic association takes place, our findings provide a framework to describe the solution structure and phase behavior of NaCMC in salt-free and salt solutions
Design and fabrication of polymer microparticles and capsules using microfluidics
Since the advent of microfluidics in the late 1990s, microfluidic approaches to polymer microparticle and capsule formation have become widespread. They benefit from the precise spatio-temporal control attainable over single and multi-phase channel flows, coupled with a range of solidification strategies, which enable the predictive and reproducible design and manufacture of unprecedented polymeric and composite particles. The control over particle shape, microstructure and architecture, monodispersity and regularity, provides unique chemical, biological, bio-medical and physical opportunities for the complex assembly and functionality of these materials. In this chapter, we summarise recent developments of the use of microfluidics for particle and capsule formation, providing an overview of the main approaches available for their manufacture. We describe key mechanistic and design considerations, including system compatibility and demonstrated capability, seeking to establish limitations and identify unexplored opportunities for these methods. We conclude with an outlook on future directions in terms of scalability, functionality, phase space mapping and commercial and societal impact, of this creative and rapidly evolving soft matter field
Growth of Myelin Figures from Parent Multilamellar Vesicles
We examine the formation and growth of isolated myelin figures and microscale multilamellar tubules from isotropic micellar solutions of an anionic surfactant. Upon cooling, surfactant micelles transform into multilamellar vesicles (MLVs) whose contact is found to trigger the unidirectional growth of myelins. While the MLV diameter grows as dMLV ā t1/2, myelins grow linearly in time as LM ā t1, with a fixed diameter. Combining time-resolved small-angle neutron scattering (SANS) and optical microscopy, we demonstrate that the microscopic growth of spherical MLVs and cylindrical myelins stems from the same nanoscale molecular mechanism, namely, the surfactant exchange from micelles into curved lamellar structures at a constant volumetric rate. This mechanism successfully describes the growth rate of (nonequilibrium) myelin figures based on a population balance at thermodynamic equilibrium
Precision polymer particles by flash nanoprecipitation and microfluidic droplet extraction
We comparatively review two versatile approaches employed in the precise formation of polymer particles, with length scales from 10s of nm to to 100s Ī¼m, from ternary polymer(s), solvent and nonsolvent mixtures. Flash nanoprecipitation (FNP) utilizes an opposing jet arrangement to mix a dilute polymer solution and a nonsolvent in confinement, inducing a rapid (ā¼millisecond) chain collapse and eventual precipitation of nanoparticles (NPs) of 10ā1000 nm diameters. FNP of polymer mixtures and block copolymers can yield a range of multiphase morphologies with various functionalities. While droplet solvent extraction (DSE) also involves the exposure of a polymer solution to a nonsolvent, in this case the polymer solution is templated into a droplet prior to solvent extraction, often using microfluidics, resulting in polymer particles of 1ā1000 Ī¼m diameter. Droplet shrinkage and solvent exchange are generally accompanied by a series of processes including demixing, coarsening, phase inversion, skin formation, and kinetic arrest, which lead to a plethora of possible internal and external particle morphologies. In the absence of external flow fields, DSE corresponds effectively to nonsolvent induced phase separation (NIPS) in a spherical geometry. In this review, we discuss the requirements to implement both approaches, detailing consequences of ternary solution phase behavior and the interplay of the various processes underpinning particle formation and highlighting the similarities, differences, and complementarity of FNP and DSE. In addition to reviewing previous work in the field, we report comparative experimental results on the formation of polystyrene particles by both approaches, emphasizing the importance of solution phase behavior in process design
SANS Study of PPPO in mixed solvents and impact on polymer nanoprecipitation
We investigate the conformation of poly(2,6-diphenyl-p-phenylene oxide) (PPPO) in good and mixed solvents by small-angle neutron scattering (SANS) across its ternary phase diagram. Dichloromethane was selected as a āgoodā solvent and heptane as a āpoorā solvent whose addition eventually induces demixing and polymer precipitation. Below the overlap concentration c*, the polymer conformation is found to be well described by the polymer-excluded volume model and above by the OrnsteināZernike expression with a correlation length Ī¾ which depends on the concentration and solvent/nonsolvent ratio. We quantify the decrease in polymer radius of gyration Rg, increase in Ī¾, and effective Ļ parameter approaching the phase boundary. Upon flash nanoprecipitation, the characteristic particle radius (estimated by scanning electron microscopy, SEM) is found to scale with polymer concentration as well as with nonsolvent content. Significantly, the solution volume per precipitated particle remains nearly constant at all polymer concentrations. Overall, our findings correlate ternary solution structure with the fabrication of polymer nanoparticles by nonsolvent-induced phase separation and precipitation
Surface-Induced Crystallization of Sodium Dodecyl Sulfate (SDS) Micellar Solutions in Confinement
We investigate the role of confinement on the onset of crystallization in subcooled micellar solutions of sodium dodecyl sulfate (SDS), examining the impact of sample volume, substrate surface energy, and surface roughness. Using small angle neutron scattering (SANS) and dynamic light scattering (DLS), we measure the crystallization temperature upon cooling and the metastable zone width (MSZW) for bulk 10ā30 wt% SDS solutions. We then introduce a microdroplet approach to quantify the impact of surface free energy (18ā65 mN/m) and substrate roughness (RĪ± ā 0ā60 Ī¼m) on the kinetics of surface-induced crystallization through measurements of induction time (ti) under isothermal conditions. While ti is found to decrease exponentially with decreasing temperature (increasing subcooling) for all tested surfaces, increasing the surface energy could cause a significant further reduction of up to ā¼40 fold. For substrates with the lowest surface energy and longest ti, microscale surface roughness is found to enhance crystal nucleation, in particular for RĪ± ā„ 10 Ī¼m. Finally, we demonstrate that tuning the surface energy and microscopic roughness can be effective routes to promote or delay nucleation in bulk-like volumes, thus greatly impacting the stability of surfactant solutions at lower temperatures
Pure and mixed aqueous micellar solutions of Sodium Dodecyl sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO): Role of temperature and composition
Aqueous mixtures of anionic and nonionic/cationic surfactants can form non-trivial self-assemblies in solution and exhibit macroscopic responses. Here, we investigate the micellar phase of pure and mixed aqueous solutions of Sodium Dodecyl Sulfate (SDS) and Dimethyldodecyl Amine Oxide (DDAO) using a combination of Small Angle Neutron Scattering (SANS), Fourier-Transform Infrared Spectroscopy (FTIR) and rheological measurements. We examine the effect of temperature (0ā60 Ā°C), on the 20 wt% SDS micellar solutions with varying DDAO (5 wt%), and seek to correlate micellar structure with zero-shear solution viscosity. SANS establishes the formation of prolate ellipsoidal micelles in aqueous solutions of pure SDS, DDAO and SDS/DDAO mixtures, whose axial ratio is found to increase upon cooling. Elongation of the ellipsoidal micelles of pure SDS is also induced by the introduction of the non-anionic DDAO, which effectively reduces the repulsive interactions between the anionic SDS head-groups. In FTIR measurements, the formation of elongated mixed ellipsoidal micelles is confirmed by the increase of ordering in the hydrocarbon chain tails and interaction between surfactant head-groups. We find that the zero-shear viscosity of the mixed surfactant solutions increases exponentially with decreasing temperature and increasing DDAO content. Significantly, a master curve for solution viscosity can be obtained in terms of micellar aspect ratio, subsuming the effects of both temperature and DDAO composition in the experimental range investigated. The intrinsic viscosity of mixed micellar solutions is significantly larger than the analytical and numerical predictions for Brownian suspensions of ellipsoidal colloids, highlighting the need to consider interactions of soft micelles under shear, especially at high concentrations
Solution structures of anionic-amphoteric surfactant mixtures near the two-phase region at fixed pH
We examine the solution structures in a mixed surfactant system of sodium dodecyl sulfate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO) in water, on both sides of the two-phase boundary, employing dynamic light scattering, small-angle neutron scattering, and Fourier transform infrared spectroscopy. The precipitate phase boundary was accessed by lowering pH to 8, from its floating pH 9.5 value, and was experimentally approached from the monomeric and micellar regions in three ways: at fixed DDAO or SDS concentrations and at a fixed (70:30) SDS:DDAO molar ratio. We characterize the size, shape, and interactions of micelles, which elongate approaching the boundary, leading to the formation of disk-like aggregates within the biphasic region, coexisting with micelles and monomers. Our data, from both monomeric and micellar solutions, indicate that the two phase structures formed are largely pathway-independent, with dimensions influenced by both pH and mixed surfactant composition. Precipitation occurs at intermediate stoichiometries with a similar SDS:DDAO ratio, whereas asymmetric stoichiometries form a re-entrant transition, returning to the mixed micelle phase. Overall, our findings demonstrate the effect of stoichiometry and solution pH on the synergistic interaction of mixed surfactants and their impact on phase equilibrium and associated micellar and two-phase structure