Multilayering of Calcium Aerosol-OT at the Mica/Water Interface Studied with Neutron Reflection: Formation of a Condensed Lamellar Phase at the CMC

Abstract

Using specular neutron reflection, the adsorption of sodium and calcium salts of the surfactant bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT or AOT) has been studied at the mica/water interface at concentrations between 0.1 and 2 CMC. The pH dependence of the adsorption was also probed. No evidence of the adsorption of Na(AOT) was found even at the critical micelle concentration (CMC) while the calcium salt was found to adsorb significantly at concentrations of 0.5 CMC and above. This interesting and somewhat unexpected finding demonstrates that counterion identity may be used to tune the adsorption of anionic surfactants on anionic surfaces. At the CMC, three condensed bilayers of Ca(AOT) were adsorbed at pH 7 and 9 and four bilayers adsorbed at pH 4. Multilayering at the CMC of Ca(AOT) on the mica surface is an unusual feature of this surfactant/surface combination. Only single bilayer adsorption has been observed at other surfaces at the CMC. We suggest this arises from the high charge density of mica which must provide an excellent template for the surfactant.

Potassium, Calcium, and Magnesium Bridging of AOT to Mica at Constant Ionic Strength

“…The added ionic strength will lead to a reduced Debye length from greater screening between charged species, and could therefore inuence closer packing than has been seen in some pure salt solutions where bilayer thicknesses have been observed in the range 31 -38 Å for the calcium system. 2,3 Dense packing is also consistent with the unhydrated nature of the layer from the ts to the data.…”

“…It has been shown previously that Ca(AOT) 2 can form multilayered structures on mica at the CMC (0.5 mM) with up to four bilayers and intervening water, and signicantly more layers at twice the CMC. 3 In the system studied here the Ca 2+ concentration is the same as in that work, but the [AOT ] is higher in our case, at 2.5…”

“…Numerous examples of divalent ions bridging have been reported, including on silica, 2 mica 3 and montmorillonite. 4 This form of binding has been suggested as a mechanism of crude oil adsorption onto mineral surfaces.…”

Neutron Reflectometry for Studying Corrosion and Corrosion Inhibition

“…In many cases, the reflection data can be collected in absolute units; however, the intensity below the critical angle should be unity, providing a convenient intensity scaling. There are cases where this is not appropriate, for example where there is significant beam attenuation [31,56,57]. Representative simulation of neutron reflectometry data, illustrating the total reflection at low Q and the fringes that arise from an adsorbed layer at higher Q.…”

“…The period of the fringes is therefore related to the film thickness and their amplitude to the composition of the thin film. In some cases where the films are very thick, the calculation of the reflected signal is more complex [31,56,57]. However, for the corrosion-related work discussed here, the thin film approximation is almost universally used.…”