Stabilization of aqueous two-phase systems using charged nanoparticles

Abstract

Aqueous two-phase system (ATPS) is a liquid-liquid separation technique. This technique represents a mixture of two polymers in an aqueous medium that separates into two clear phases. Interestingly, the interfacial tension between these two separated phases is very low, typically 3 to 4 orders lower than any alkane-water systems. This template-based route finds several applications in various fields such as extraction, separation, drug delivery, cosmetics and bio-medical as purification and enrichment of proteins and membranes. Water-in-Water (w/w) emulsion is a well-known example system of ATPS. In this thesis, an attempt has been made to understand the physics of two-phase systems and the equilibrium structures that can be derived from the stabilized products. The commercial grade positively charged (CL-30) and negatively charged (HS-40) nanoparticles have been utilized as stabilizers. Chapter 3 and Chapter 5 focus on stabilizing thermodynamically incompatible systems such as PEO and dextran mixtures using oppositely charged nanoparticles (OCNPs), while Chapter 4 employed pure nanoparticles of type HS-40 to stabilize w/w emulsions. When the stabilization is achieved using OCNPs, the self-assembly phenomenon is exploited to generate clusters or aggregates of varying sizes and compositions. Therefore, it is envisaged that one can easily tune not just the interfacial area but also the contact angle, as these parameters are linked to the size and composition of the aggregates being adsorbed at the respective water-water interface. This additional flexibility provides more advantages in tuning the Gibbs detachment energy. Apart from stabilizing w/w emulsion droplets, this thesis reports a unique pathway to generate a few novel structures, such as bijels (), emulsion-filled gels, and double emulsions formed due to self-assembly and several other underlying factors. Chapter 3 illustrates a simple yet straightforward methodology of stabilizing aqueous two-phase systems (ATPS) using OCNPs. When these OCNPs are mixed using an emulsifier, they are known to self-assemble to form aggregates of varying sizes with net positive, net negative, and neutral charges. The interplay of this size and charge nature promotes stronger adsorption by increasing Gibbs detachment energy and yields exciting 3d bi-continuous network channels, i.e., bijels, depending on the nature of charged aggregates at a particular experimental regime. It will be shown later in Chapter 3 that the resulting clusters with net zero charge will give rise to the formation of such bijels, while the net positive and negative clusters favour the formation of droplets. A detailed phase diagram has been constructed to demonstrate the influence of nanoparticles’ and polymers’ composition on the structural transformation from droplet-bijel-droplet. Therefore, creating such a phase diagram based on the empirical study using turbidity and zeta potential measurements to identify a suitable experimental regime for generating bijels without performing any complicated surface modifications is a noteworthy contribution. Several experimental studies reveal that the formation of bijels will be most likely when the parameter M (ratio of weight fraction of positively charged nanoparticles to negatively charged nanoparticles) is chosen between 0.7-4. Nevertheless, the w/w droplets stabilized by OCNPs showed good resilience under high centrifugal action irrespective of M, while bijels produced in this route remained stable for a long time. The proposed route offers a simple pathway to fabricate the bijels with three-dimensional hierarchical-bicontinuous network channels. Chapter 4 demonstrates the simple yet straightforward procedure to generate stable w/w emulsions and emulsion-filled gels stabilized by pure HS-40 silica nanoparticles. The rheological studies well support the hypothesis presented in this section. The focus of this chapter slightly shifts to altering the molecular weight of one of the polymers in the emulsion mixtures, i.e., PEO. It will be shown later in this chapter that the increase in the molecular weight of PEO increases the stability of the resulting emulsions owing to a synergistic effect. Like the droplet-bijel-droplet transition shown in Chapter 3 at different M, Chapter 4 also captures a distinct sol-gel transition in a particular experimental regime through repeated experimental studies at different variables such as molecular weights, storage times, and polymer compositions. The phase diagram created using the combination of these variables helps identify the distinct regimes for generating the emulsion-filled gels and w/w emulsion droplets. The time evolution of shear-induced structures reveals that the viscoelastic properties of these emulsion-filled gels correlate directly with storage time, molecular weight, and polymer compositions. Chapter 5 describes a single-step approach for synthesizing Water-in-Water-in-Water (W/W/W) Pickering double emulsions. The proposed method involves varying both ’M’ and the molecular weight of the polymers in both phases. The results indicate that the molecular weight of the PEO and dextran used in the aqueous phase is a crucial parameter in deciding the structural formation of the emulsion and the generation of stable double emulsions. Additionally, this chapter discusses the effect of scalability by batch size on the generation of double emulsions. The findings reiterate that the formation of double emulsions is independent of size (volume of the batch) and dependent on the concentration of nanoparticles used. The proposed method offers a unique methodology of generating double emulsions in single-step with excellent stability for at least 30 days. This thesis thoroughly investigates the stability and diverse morphological characteristics of aqueous two-phase systems (ATPS) stabilized by charged nanoparticles, unveiling their extensive applications in drug delivery, cosmetics, and other industries. The primary objective was to synthesize and meticulously characterize intricate structures, including bijels, emulsion-filled gels, and double emulsions. These unique structures possess exceptional properties that position them as highly promising candidates for various applications.