Calorimetric Evidence of the Formation of Half-Cylindrical Aggregates of a Cationic Surfactant at the Graphite/Water Interface

Abstract

The adsorption of a cationic surfactant, dodecyltrimethylammonium bromide (C 12 TAB), from aqueous solutions on graphitized carbon black has been investigated from 288.15 to 318.15 K using a versatile, automated measuring system, which has been designed for the simultaneous measurement of the adsorption isotherm and the calorimetric enthalpies of displacement at the solid/liquid interface. At low concentrations, the surfactant molecules form a flat monolayer on the graphite surface. The enthalpy of monolayer formation is apparently not (or only slightly) dependent on the temperature and is independent of the surface coverage (-61 kJ‚mol -1 ). The adsorption proceeds further as the concentration in the bulk solution is increased to the cmc. In this region, the enthalpy of displacement is again nearly independent of the surface coverage but depends strongly on the temperature: -10.7, -17.5, and -29 kJ‚mol -1 at 288.15, 298.15, and 318.15 K, respectively. The mechanism of the adsorption and the morphology of the high-density adsorbate structure have been analyzed in terms of the classical reorientation model (reorientation of the adsorbed surfactant molecules from a horizontal to a vertical orientation, accompanied by further adsorption from the bulk solution) and in terms of a recent concept of interfacial aggregation (formation of half-cylindrical surface micelles templated by an epitaxially bound surfactant monolayer). The results of the thermodynamic analysis strongly support the formation of C 12 TAB half-cylinders at the graphite/water interface.

Structure and Dynamics of Surfactant and Hydrocarbon Aggregates on Graphite: A Molecular Dynamics Simulation Study

“…The SDS or C 12 molecules were initially placed in a square lattice with their molecular axis perpendicular to the plane of graphite in bulk water, see Figure 1. For the single-molecule simulations, the graphite slab has dimensions 4.57 × 5.00 nm 2 (912 carbon atoms in each 3 layers) and for the multiple molecule simulations the box is increased to 6.74 × 7.09 nm 2 (1904 carbon atoms in each 3 layers). The simulation box dimension perpendicular to the graphite plane, that is, the combined thickness of the graphite and the water slab, was set as z ) 4.5 nm for the simulations involving a single SDS or a C 12 molecule and z ) 8.5 nm for the multiple-molecule simulations.…”

Surfactant Aggregates at Rough Solid−Liquid Interfaces

“…16,19 More recently, experimental efforts applying ellipsometry, [20][21][22] optical reflectometry, 23,24 electrochemical methods, 9 quartz crystal microbalances, 25 and Fourier transform infrared spectroscopy 26 have added information about the dynamic properties of surfactant adsorption. Fluorescence decay, 27 neutron reflection, 28 and grazing-incidence small-angle neutron scattering techniques 29 have provided general information about the length scales of the surface aggregates, and microcalorimetry [30][31][32][33][34] has been used to determine the corresponding heats of adsorption. None of these techniques, however, provide high enough spatial resolution to visualize surfactant adsorption at the molecular level.…”

Supramolecular Structure of Surfactants Confined to Interfaces

“…1 The temperature variation in the adsorption enthalpy at high coverages also closely follows the temperature variation of micellization enthalpy, providing further evidence for this mechanism. 19 While these studies show strong evidence for aggregative adsorption as the mechanism, they do not by themselves definitively exclude the possibility of uniform monolayers and bilayers as the final equilibrium structure once surface saturation has been reached. Findenegg et al have taken a step in this direction by comparing the theoretical enthalpy of half-cylindrical aggregation on graphite (the AFM result) to the enthalpy associated with horizontal-to-vertical reorientation of surfactants (the "standard model"); 19 only the former is found to be consistent with their calorimetry data.…”

“…Experiments on both hydrophilic (silica) 16,17,18 and hydrophobic (graphite) 15,19 surfaces show that the adsorption is strongly exothermic at very low surface coverages but becomes weakly exothermic or even endothermic at coverages exceeding -10% of saturation. The former step is therefore energy-driven and should be dominated by surfactant-surface interactions (since the coverage is very low), whereas the latter step is more entropydriven and should be dominated by intermolecular interactions.…”