Foaming and anti-foaming of nonionic surfactant solutions
Foam is important for many processes such as enhanced oil recovery and firefighting. Although a large volume of foam is desirable for these particular applications, for other applications it needs to be suppressed, for example, in washing machines. In order to suppress foam formation, an anti-foam agent is added to the foaming solution. An anti-foam agent may take the form of oil, solid particles (generally of the micron (Jlm) size range), or a combination of both. This thesis is concerned with understanding the foaming and anti-foaming of nonionic surfactants of the polyoxyethylene glycol ether type of general structure H-(CH₂)n(O-CH₂-CH₂)mOH (abbreviated to CnEm). In the first results chapter, the foaming of the homologous series of CnEm surfactants is investigated in the absence of any additives (Le. anti-foam agents). The foamability of the surfactant is determined by measuring the volume of foam generated by bubbling nitrogen gas, through the surfactant solution as a function of surfactant concentration. It is found that the surfactant concentration corresponding to the transition from non-foaming to foaming behaviour, C(1I2), is less than the critical micelle concentration cmc for short tailed surfactants (low n) and greater than the cmc for long tailed surfactants (higher n). This is explained in terms of two requirements which must both be fulfilled before a surfactant can stabilise foam. Firstly, the rate of adsorption of the surfactant must be sufficiently fast to stabilise a foam bubble as it is being formed and secondly, the level of surfactant adsorption must be sufficiently high relative to the adsorption isotherm such that there is sufficient surfactant surrounding the bubble for it to be stable. Chapter 4 examines a new mechanism for the way in which oil may affect the stability of aqueous foam. It is known that alkane vapours can co-adsorb with surfactant at the air-water surface to form mixed alkane/surfactant monolayers. The effects of alkane vapours on the foamability and foam stability for foams stabilised by CnEm surfactants of different head and tail chain lengths has been systematically investigated. The addition of alkane vapours within the gas stream during foam formation increases C(1I2) to concentrations in excess of the cmc, i.e. oil vapours inhibit foamability. In addition to the effects on foamability, oil vapours also accelerate the decay rates of foams (i.e. reduce the foam stability). Chapter 5 looks at the effects that nanometre (run)-silica particles of different hydrophobicities have on aqueous foams stabilised by C₁₂E₅. In the past, most studies have focussed on solid particles in the Jlm size range. It is found that the nonionic surfactant can adsorb onto the surface of the silica particles, lowering the equilibrium surfactant concentration. This adsorption is the highest for the most hydrophobic silica particles. Reducing the equilibrium surfactant concentration lowers the foamability of the system. Whilst the foamability is decreased, the foam stability is increased however. This is explained in terms of the silica particles blocking the drainage channels in the foam by networking together in solution and thus slowing liquid drainage from the foam. Finally in chapter 6, the effects that oil and run-sized silica particles have when used in combination on foam stabilised by C₁₂E₅ are investigated. Here it is found that a synergistic anti-foam action is observed for some systems i.e. the oil and particles are more effective when used together than individually. When silica particles of an intermediate hydrophobicity are used, there is a sharp increase in the foamability and foam stability for many of the systems. These differences in foaming behaviour are explained in terms of the hydrophobicity of the overall entity which is formed when surfactant, oil and run-sized silica particles of different initial hydrophobicities are shaken together to produce foam.