Surfactants at the Design Limit

  • Adam Czajka

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The adsorption and aggregation properties of twenty seven anionic hydrocarbon surfactants have been studied in aqueous systems. They are single-chain sulfonates with sodium or TAA counterions (where TAA = tetraalkylammonium, i.e. tetrapropylammonium), and di-chain sulfosuccinates with sodium counterions. The novel surfactants introduced possess branched tail structures which differ by the extent and position of branching. All
surfactants were synthesised and purified to investigate the relationships between surfactant structure and performance. The aim of this project is to fundamentally improve our understanding of controlling surface tension, and consequently use this to improve the performance of hydrocarbon surfactants to achieve low surface energies.

First, by evaluating the surface coverage of various effective surfactants, a new general property to account for low aqueous surface tension regardless of surfactant type is introduced. Where it is shown that all super-effective surfactants pack effciently at air-water interfaces to generate dense surface coverages. With this general property in mind, the adsorption and aggregation properties of novel surfactants are studied through tensiometry and small-angle neutron scattering (SANS). By making small systematic variations in the surfactant structure, general structure-property relationships of effective hydrocarbon surfactants have been distinguished. The branching position, extent, and chain length are all shown to be both highly sensitive, and critical to generate low surface energies. The structural characteristics of effective hydrocarbon surfactants are encapsulated in a new index to assess potential surfactant performance, based on the molecular structure of the tail alone, H-Gamma.

Novel approaches to improve packing effciency at the surface have been explored, leading to the lowest surface tensions for single-chain, di-chain and mixed hydrocarbon surfactant systems. For example, replacing carbon in the surfactant tail chain-tip with silicon leads to an increased molecular volume, greater packing effciency and thus, lower surface tension. The ability to further improve packing effciency by choice of surfactant counterion is demonstrated. By systematically changing the identity of the head group for various tail structures, the TPA (tetrapropylammonium) counterion is shown to be an effective alternative for all hydrocarbon surfactants. Furthermore, mixing linear and branched surfactants help to generate dense surface coverages, as spaces within the monolayer are effectively filled, generating lower surface energies than either constituent surfactant. The research presented here highlights structural characteristics of effective hydrocarbon surfactants, outlines design principles, developing our understanding and ability to control, surface tension.
Date of Award6 Nov 2018
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJulian Eastoe (Supervisor)

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