Optimization of surfactant-solvent systems

Abstract

The surfactant-solubility and self-assembly in nonpolar solvents are selective – i.e., the stability of surfactant-based colloidal systems in nonpolar solvents may be guided by both the surfactant and solvent chemical nature. Thus, understanding the compatibility of surfactant-solvent chemical combinations for colloidal stability is vital. Little is understood regarding this aspect, however. This thesis aims to improve this understanding. A class of dichain anionic surfactant analogues of the much-studied aerosol-OT (AOT) has been synthesized. The hydrophilic sulfosuccinate headgroup and sodium counterion were constant, whereas the hydrophobic tail architecture was varied. Single-phase (L2) dilute water-in-oil microemulsions (W/O-μEs) were utilized as model colloidal systems; these systems facilitate focus exclusively on the surfactant-chemical compatibility with hydrocarbon solvents (linear, cyclic, and aromatic). There are two key advantages of W/O-μEs over two component “dry” (no water) surfactant-solvent mixtures – i) the self-assembly of surfactants that are not necessarily soluble in hydrocarbon solvents can be studied; this is because the surfactant molecules reside at the oil-water (O-W) interfaces – forming thermodynamically stable surfactant monolayers at O-W interfaces in the process. For this, W/O-μEs are efficient model systems for probing the surfactant film-properties at liquid-liquid interfaces, ii) their phase behaviour can be monitored over a wide temperature–composition range; this facilitates understanding chemical effects on the colloidal stability boundaries of surfactant-solvent combinations.
The synthesized surfactants cover variation of hydrophobic chain-length, secondary branching, and inclusion of other chemical groups at the chain-tips. Not all surfactants form in W/O-μEs alone – they often require mixing with a second AOT-analogue. Some surfactants form W/O-μEs only in a specific class of solvents and not in the others. Based on the current observations, and relevant literature – a set of chemical criteria for surfactants to become efficient microemulsifiers were proposed. The solvents could be generalized based on solvent physicochemical parameters such as density and Hansen solubility parameters (HSPs). In terms of solvents, it is possible to account for L2-μE domains on an HSP–composition plot – suggesting that W/O-μEs having adequate water-content (characterized by W ≥ 20, where W = [H2O]/[surfactant]) can be “optimized” between an HSP window ~ 14.5 – 18.0 MPa1/2. This is irrespective of surfactant- and solvent-chemical identities (i.e., independent of the chemical structures).
Small-angle neutron scattering (SANS) measurements showed that, under the dilute approximation, the water core droplets can be approximated as hard spheres – at least under ambient conditions (298 K, 1 atm). The solvent chemical nature often affects interdroplet attractive interactions. However, this is probably linked to the relative L2-domain locations with respect to the microemulsification failure-boundaries. In addition, the solvent chemical nature has only subtle effects on the surfactant headgroup area (i.e., molecular area). This argument is equally applicable for surfactants having hydrocarbon (carbon and hydrogen atoms only) and silicon (Si)-containing silylcarbon hydrophobic chains – alone or mixed. The trimethylsilyl (TMS) hedgehogs were of specific interest as they have not been tested at O-W interfaces before. Three TMS-derivatives were studied; it was found that this class of surfactants (depending on the hydrophobic chain-length) can exhibit both solvent-specific and solvent-independent microemulsifying ability. Temperature-SANS measurements showed that interdroplet attraction may, or may not, increase (at least in terms of the droplet correlation length, ξ) on increasing temperature – depending on the L2-domain relative-location on the T-C landscape. In addition, for “overlapping” L2-domains having chemically different surfactant (or mixed surfactants)-solvent (or solvent blend) combinations – the interdroplet attraction is very similar in strength and extent. These results suggest that for dilute W/O-μEs – the chemical effects can, both macroscopically and microscopically, be optimized.

Date of Award	3 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Julian Eastoe (Supervisor) & Paul Bartlett (Supervisor)