A quartz crystal microbalance characterization of metal-oil interfaces and interactions with wax molecules

The solid-liquid interface between stainless steel and model petroleum fluids is investigated at isothermal conditions using a quartz crystal microbalance. AISI 316 (Fe/Cr18/Ni10/Mo3) stainless steel is chosen to represent the metal surface. Paraffin components dissolved in dodecane constitute the petroleum fluid phase. Commercial macro-crystalline and micro-crystalline waxes provide primarily linear and branched paraffin components, respectively. Paraffin solubility conditions are established through a van't Hoff relationship. Model fluids prepared with the single-component alkanes n-C-36 or n-C-30 paraffin provide well-defined solubility conditions. Monitored changes in resonance frequency and energy dissipation of the quartz crystal resonator immersed in the model fluids confirm that no continual deposition of paraffin components occurs at isothermal conditions. Solid paraffin crystals dispersed in solution show no adherence to the stainless steel surface. The absence of attractive interactions between the stainless steel surface and the dispersed paraffin crystals suggests that a surface adsorption and/or surface nucleation mechanism is responsible for the formation of incipient paraffin wax deposits under nonquiescent conditions.