Negatively Charged Lipids Exhibit Negligible Effects on the Water Repellency of Montmorillonite Films.

Amphiphilic molecules can alter the wettability of soil minerals. To determine how the headgroup chemistry of amphiphiles determines these effects, we investigate a system of the clay montmorillonite with long-chain phospholipids. We use phosphatidylglycerol (PG) phospholipids to contrast with our previous work using phosphatidylethanolamine (PE) lipids. Zwitterionic PE lipids can sorb to the negatively charged montmorillonite surface, whereas negatively charged PG lipids cannot. Employing a suite of techniques from molecular dynamics, atomic force microscopy, fluorescence microscopy, and contact angle measurements, we define sample characteristics from molecular-scale structure to the macroscopic wettability. We find that PG lipids do not significantly alter montmorillonite wetting characteristics, such as the contact angle, flow viscosity, and the characteristic time scale for droplet imbibition. On comparing PE and PG lipid/clay films, we find that, among the phospholipids compared, they must have three characteristics to change clay/lipid film wettability: they must bind to the mineral surface, be solid at room temperature, and have a relatively continuous distribution throughout the film.

Connecting wettability, topography, and chemistry in a simple lipid-montmorillonite system.

HYPOTHESIS: While soil water repellency causes a variety of undesirable environmental effects, the underlying mechanism is unknown. We investigate the coupled effects of chemical characteristics and surface topology in a simple model system of two lipids, DSPE (1,2-distearoyl-sn-glycero-3-phosphoethanolamine) and DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine), and a clay substrate. These closely-related lipids allowed the study of how a small change in chemical structure influences the surface hydrophobicity. EXPERIMENTS: Techniques ranging from molecular (simulations) to nanoscopic (atomic force microscopy) to microscopic (fluorescence microscopy) to macroscopic (contact angle measurements) were used to explore interactions at all length scales. The wettability was assessed from initial contact angle and time-dependent changes in droplet shape. FINDINGS: The lipid distribution depended on the lipid's melting temperature: solid lipids did not spread evenly through the film, while liquid ones did. However, the initial contact angle did not change appreciably with the addition of DSPE or DOPE. Only DSPE heated above its melting temperature induced significant changes. In addition to the initial contact angle, quantitative variables extracted from the change in droplet shape over time correlated with the film topography or lipid distribution. These results define a new quantitative approach to investigating partially-wettable soils and provide a potential rationale for why clays can remediate water-repellent soils.