Abstract
Interactions of bioorganic moieties with clay minerals have attracted attention not only from the perspective of novel bioclay materials but also because they play a crucial role in our understanding of physical and chemical processes in soils. The aim of the present article is to explore the interactions responsible for the formation of a phosphatidylcholine-kaolinite bioclay by employing a series of classical molecular dynamic simulations. Detailed analysis of the structure and energies of the resulting bioclays reveals that the phosphatidylcholine molecules bind to the kaolinite surface either via their zwitterionic heads or hydrophobic aliphatic tails, depending on the kaolinite surface characteristics and the density of organic coating. The phosphatidylcholine molecules have a tendency to form irregular layers with a preferred parallel orientation of molecules with respect to the kaolinite surface. The tails exhibit varying degrees of flexibility and disorder depending on their distance from the surface and the density of surface coating. Significant differences in the binding can be spotted with respect to the two types of kaolinite basal surfaces, i.e., the hydrophobic siloxane surface, which possesses a considerable dispersion character, and the hydrophilic alumina surface, polarized by the surface hydroxyl groups.
Highlights
Based on a constant demand for novel, preferably cheap and environmentally-friendly materials, the combination of various organic cations and minerals became crucial in a number of applications ranging from sorbents for environmental damage remediation [1,2] to nanofillers that alter the physico-chemical properties of polymeric matrices [3,4].Recently, significant attention has been paid to the preparation of organoclays, using mainly quarternary ammonium [5] and phosphonium cations [4] with a variety of clay minerals
The simulations were performed in the slab-model where the periodicity in the z-direction was removed and a correction term was applied to the Particle–Particle Particle–Mesh (PPPM) solver [42]
A full surface coverage of a similar system, composed of phosphatidylcholine molecules on a montmorillonite surface has been previously estimated as 0.1 μg cm−2 [29] which in this case corresponds to 59 POPC molecules
Summary
Based on a constant demand for novel, preferably cheap and environmentally-friendly materials, the combination of various organic cations and minerals became crucial in a number of applications ranging from sorbents for environmental damage remediation [1,2] to nanofillers that alter the physico-chemical properties of polymeric matrices [3,4].Recently, significant attention has been paid to the preparation of organoclays, using mainly quarternary ammonium [5] and phosphonium cations [4] with a variety of clay minerals. The exact molecular arrangement at the atomic scale far beyond experimental resolution can be obtained from molecular simulations, which reveal the structure [13,14,15,16,17,18], phase transitions [19,20] and effects associated with various types of organic cations [17,21] or diffusion processes [22,23]. Common organoclays prepared with the aid of synthetic surfactants are considered environmentally harmful due to the byproducts associated with their manufacture and the time required for their natural degradation [25]. An environmentally-friendly alternative is the replacement of synthetic surfactants by molecules of a biological origin. The group of phospholipids has been successfully used in the preparation of stable bioclays either by their intercalation into the interlayer space of a clay mineral [26,27] or by their adsorption onto its outer surface [28]
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