A comparison of didodecyldimethylammonium bromide adsorbed at mica/water and silica/water interfaces using neutron reflection

Abstract

The layer structure of the dichain alkyl ammonium surfactant, didodecyldimethylammonium bromide (DDAB), adsorbed from water on to silica and mica surfaces has been determined using neutron reflection. Although sometimes considered interchangeable surfaces for study, we present evidence of significant differences in the adsorbed layer structure below the critical micelle concentration. A complete DDAB bilayer was assembled at the water/mica interface at concentrations below the critical micelle concentration (CMC). In contrast it is not until the CMC was reached that the complete bilayer structure formed on the oxidised silicon crystal. Removal of the complete bilayer on both surfaces was attempted by both washing and ion exchange yet the adsorbed structure proved tenacious.

Self-assembly and adsorption of cetyltrimethylammonium bromide and didodecyldimethylammonium bromide surfactants at the mica–water interface

“…'Inner' and 'outer' refer to the simulated surfactant layers in contact with solution and mica, respectively. Experimental values for the adsorbed-layer thickness are included for comparison [38][39][40][41] Surfactant concentration Layer agreement with that from experiment, although the features in the experimental profile are more pronounced in the wave vector range 0.03 Å À1 r Q r 0.2 Å À1 . To put that in context, Fig.…”

“…These concentrations are well above the CMC, because the focus is on the structure of the solid-liquid interface, and it would be completely infeasible to do all-atom simulations near the CMC. (The CMC of DDAB is 8 Â 10 À5 mol L À1 , [39][40][41] equivalent to about 0.004 wt%.) The compositions of these systems are summarised in Table 1.…”

“…Clarke and co-workers have measured the NR profiles of CTAB and DDAB solutions in water, and pure water, in contact with mica surfaces. [38][39][40][41]56,57 In connection with the NR and SLD profiles, 'water' means D 2 O. The surfactant concentration was fixed at or near the CMC.…”

“…37 Howard and Craig proposed that surface-adsorbed aggregates are the cause of slow adsorption in the case of CTAB, and that this may also apply to other surfactants. 12 Clarke and co-workers used NR to study the aggregation of CTAB 38 and DDAB [39][40][41] at the mica-water interface, and observed bilayer formation of the surfactants in both cases.…”

Preliminary assessment of modified Kazakh natural zeolites as possible sorbents for MnO4- removal from aqueous solutions.

“…The main feature giving information about the shape and geometry of it, is its packing parameter of 0.620 [2]. Therefore, DDAB is expected to form on the surface of the absorbent flat structures (either lamellae or vesicle), when present in concentrations above the critical micelle concentration (CMC) [18] (1 CMC stands for 0.1 mM). Type (2) and (3) refer to sorbents that were treated prior to DDAB surface modification, with hydrochloric acid or with sodium chloride, respectively.…”