title: Salt dependent stability of stearic acid Langmuir-Blodgett films exposed to aqueous electrolytes.
abstract: We use contact angle goniometry, imaging ellipsometry, and atomic force microscopy to study the stability and wettability of Langmuir-Blodgett (LB) monolayers of stearic acid on silica substrates, upon drying and exposure to aqueous solutions of varying salinity. The influences of Ca(2+) and Na(+) ions are compared by varying their concentrations, both in the subphase before the LB transfer, and in the droplets to which the dried LB layers are exposed. Ca(2+) ions in the subphase are found to enhance the stability, leading to contact angles up to 100°, as compared to less than 5° for Na(+). Consistent with the macroscopic wettability, AFM images show almost intact films with few holes exposing bare substrate when prepared in the presence of Ca(2+), while subphases containing Na(+) result in large areas of bare substrate after exposure to aqueous drops. The observations on varying the composition of the droplets corroborate the stabilizing effect of Ca(2+). We attribute these findings to the cation-bridging ability of Ca(2+) ions, which can bind the negatively charged stearate groups to the negatively charged substrates. We discuss the relevance of our findings in the context of enhanced oil recovery.
authors.affiliation: Physics of Complex Fluids Group and MESA+ Institute, Faculty of Science and Technology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands.
authors.forename: Naveen
authors.initials: N
authors.lastname: Kumar
authors.forename: Lei
authors.initials: L
authors.lastname: Wang
authors.forename: Igor
authors.initials: I
authors.lastname: Siretanu
authors.forename: Michel
authors.initials: M
authors.lastname: Duits
authors.forename: Frieder
authors.initials: F
authors.lastname: Mugele
mesh_terms: D002118:Calcium / Q000737:chemistry; D004573:Electrolytes / Q000737:chemistry; D010316:Particle Size; D012492:Salts / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry; D012964:Sodium / Q000737:chemistry; D013229:Stearic Acids / Q000737:chemistry*; D013499:Surface Properties; D014867:Water / Q000737:chemistry; D017155:Wettability
chemical_list: D004573:Electrolytes; D012492:Salts; D013229:Stearic Acids; D014867:Water; C031183:stearic acid; D012822:Silicon Dioxide; D012964:Sodium; D002118:Calcium