Competitive adsorption of surfactant foaming agents to nanoclays added to cement foams for enhanced strength

Abstract

The adsorption affinity of a surfactant foaming agent (SFA), α-olefin sulphonate (AOS)—used for generation of foam for low density concrete—to organically-modified montmorillonite (OMMT) has been investigated. OMMT has been proposed as an additive to cement and concrete for improved strength and durability. Similar thermodynamic processes are involved in the generation and stabilisation of foam and in the compatibilisation and stabilisation of organic particles in aqueous environments, so interaction between SFA and OMMT particles is likely. Association of foaming agent molecules with organoclay may lead to poor foaming performance and potential instability of the nanoparticles due to displacement of dispersants from the particle surface by foaming agent. Adsorption isotherms determined using a combination of ion-pair reverse phase high performance liquid chromatography (RP-HPLC) and gravimetric methods revealed that there is a relatively high affinity of AOS for the organoclay particles. This is a dynamic process, with smaller molecules adsorbing quickly but being displaced by larger molecules at higher surfactant loading. From the adsorption isotherm it was possible to calculate the minimum AOS addition that will ensure the full foaming performance in the cement formulation. Relative adsorption affinity and competitive adsorption at the particle surface of AOS with non-ionic and anionic surfactants commonly used as wetting and dispersing agents, was studied. The dispersants displayed considerably higher relative adsorption onto the organoclay than AOS, particularly in the case of the anionic species. There is evidence that some AOS adsorption takes place in particle systems stabilised by non-ionic dispersants; displacement of high adsorption affinity dispersants by the lower affinity AOS from the OMMT particle surface was not observed.