Influence of divalent ion concentration on the adhesion behaviour of sulfonate self-assembled monolayers (SAM)

Surfactant-assisted oil recovery is usually employed for the production of residual oil after primary and secondary recovery techniques have been exhausted. The loss of injected surfactants via adsorption on to the porous media surfaces impede the efficiency of the procedure and greatly impact process economics. To address this issue, the choice of surfactant and aqueous fluid composition are varied. In particular, the divalent ion concentration is a property that potentially influences surfactant adsorption significantly. It is this aspect which has been addressed in this study. In this study, atomic force microscope (AFM). tips were functionalised with a selfassembled monolayer (SAM) of the alkanethiol, sodium 11-mercaptoundecanesulfonate (otherwise known as MUS) with the -SO3-headgroups oriented outwards towards the aqueous solutions, this is a model for the wellstudied surfactant, sodium dodecyl sulfate, SDS. The adhesion forces between these "surfactant-functionalised" tips and Bandera Brown sandstone surfaces were measured in brine solutions of varying CaCl2 concentrations. Observations deviate from previous studies where adsorption of surfactant headgroups in the bulk solution is enhanced via Ca2+ bridging with the surface. Rather, measured adhesion forces decreased with increase in CaCl2 concentration. The results are reversible and also occurs in a reducing environment when the iron in the Bandera brown is reduced for iron (III) to iron (II). The observed behaviour is interpreted in terms of Ca2+ preferentially bridging between neighbouring headgroups on the tip due to the dense packing of molecules non-representative of respective bulk behaviours and acting as a barrier to adhesive contact. Thus, this supposed micellization at the tip aided by the formation of Ca2+ salt bridges between alternate headgroups more closely mimics a surfactant micelle, rather than dispersed individual monomers. The findings of this study provide insight into nanoscale anionic surfactant interactions in general and within sandstone reservoirs in particular where surfactant adsorption is driven largely by monomers rather than micelles. Investigating surfactant adsorption using microscopic techniques such as AFM allow for the determination of interactions at mineral-fluid interfaces and constitute an effective screening methodology for ascertaining enhanced oil recovery process feasibility.