The aggregation of amphiphilic molecules and their organization on various surfaces are common phenomena in many applications such as surface treatments, cleaners and nano-composite materials. For interaction mechanisms leading to the adsorption equivalent to those ensuring the cohesion of amphiphilic (non-ionic surfactant) molecules in several membrane lyotropic phases, the structure of the aggregates formed may be similar to those in bulk surfactant/water system. Although the structure of such aggregates has been observed on well-defined surfaces such as graphite or silica, it is only recently that Regis Guégan, in collaboration with Javier Perez (SWING beamline), was able to demonstrate the importance of the state of the non-ionic surfactant during its adsorption and organization onto natural layered materials (clay minerals). Beyond its interest in soft matter physics, the results published in ‘Soft Matter’ journal open new ways for the preparation of promising composites.
Adsorption of surfactants on clay minerals
Clay minerals such as montmorillonite are layered materials that exhibit high surface areas, cation exchange capacity and excellent adsorption properties for many chemical compounds. The adsorption of surfactants by clay minerals is based on various interaction mechanisms. Although these mechanisms are relatively well understood for cationic surfactants, where the adsorption takes place through ion exchange with inorganic cations located in the interlayer space, leading to the synthesis of the well-known 'organoclays' widely used notably in the reinforcement of polymer matrices, the problem is more complex for nonionic surfactants.
Previous research has been limited to studies of the adsorption of nonionic surfactants onto clay minerals in a concentration range close to or below the critical micelle concentration (cmc), for which the maximum of adsorption is reached. Although involving other interacting forces, nonionic surfactants aggregate onto clay surface as bulk micellization occurs leading to the adsorption of lateral mono / bilayer(s) covering the whole accessible surface. The resulting composite materials show, like their 'organoclay' analogues prepared using cationic surfactants, a hydrophobic surface but a relatively restricted interlayer spacing (4 or 8 Å resulting to the adsorption of only surfactant lateral layers) thus limiting their potential applications.
Role of surfactant self-organization
The adsorption of n-CnH2n+1(OCH2CH2)mOH, abbreviated to (CnEm) nonionic surfactants has received particular attention because of their ability to self-assemble into several liquid crystalline lyotropic (hexagonal, cubic, lamellar…) phases above the cmc. More specifically, the C10E3-H20 system self-organizes into a lamellar phase characterized by an aggregation of C10E3 bilayers at room temperature, allowing the study in easy experimental conditions, of its adsorption onto surfaces and/or the preparation of composite materials. The latest experiments on the SWING beamline with results obtained by complementary techniques (FTIR and solid-state NMR) have shown that the self-organization of C10E3 into a lamellar phase modifies the adsorption mechanism of C10E3 with the clay mineral, favoring its aggregation into a normal bilayer which increases the interlayer space at large values (of the order of 40 Å), while maintaining the inorganic exchangeable cations within the interlayer space, bestowing a dual hydrophobic/hydrophilic character to the composite materials. Moreover, depending on the state of C10E3 in solution, as monomers or self-assembled in a lamellar phase, the adsorption leads to four separate C10E3 arrangements within the interlayer space of the montmorillonite: (i) a lateral monolayer, (ii) a lateral bilayer (iii) a normal bilayer and (iv) at high concentration, a ‘tilted’ normal bilayer (Figure 1).
These results therefore highlight the importance of the surfactant state in solution during its adsorption on a surface where competition between the forces behind the adsorption and those favoring the surfactant arrangement remains balanced. We intend now to study the adsorption of similar self-organized lyotropic phases (hexagonal and cubic phases) onto both synthetic and natural layered materials for the preparation of promising nanocomposite materials.
Figure 1: Small angle X-ray scattering spectra of: dehydrated montmorillonite exchanged with Na+ ions (red); the "organoclays" or composite materials prepared with the same clay mineral and a nonionic surfactant (C10E3) as monomers leading to intercalation into the interlayer space of lateral mono- and bilayers (yellow and green, respectively) and in a self-assembled lamellar phase at a concentration above the cmc, leading to the aggregation of a normal bilayer tilted within the montmorillonite surface (blue and purple, respectively).