Tandem effects of lipid head and tail chemistry on the wettability of clay

Abstract

Organic matter can drastically alter soil behavior, including by changing how soil interacts with water. In particular, organic matter can reduce a soil’s ability to absorb and retain water, a phenomenon called soil water repellency. Soil water repellency leads to a variety of detrimental environmental effects ranging from increased erosion to decreased crop yield. However, the molecular mechanisms that drive soil water repellency have not been determined, which hampers the development of remediation protocols. Researchers hypothesize that amphiphilic molecules (such as phospholipids) could drive seasonal changes in water repellency. To determine the chemical factors that influence amphiphile-induced water repellency, I developed a model system consisting of one mineral, the clay montmorillonite, mixed with several phospholipids selected to test specific chemical characteristics and their relationship to water repellency. Phospholipids have a polar headgroup and two tails of carbon chains. I tested two distinct head groups, phosphoethanolamine (zwitterionic) and phosphoglycerol (negative). The head groups were selected to test the relationship of lipid-mineral binding and wettability, as they will interact differently with the negatively-charged montmorillonite clay surface. Within each category of headgroup, I investigated two types of tails: a saturated tail that is solid (gel) at room temperature, and an unsaturated tail that is liquid at room temperature, in order to determine if the lipids’ physical state – solid or liquid – influences wettability. I employed a suite of techniques designed for measurements at a range of length scales. For the smallest scale I utilized molecular dynamics, and at the mesoscale I used atomic force microscopy and fluorescence microscopy. To probe the macroscopic wettability, I utilized contact angle measurements and adapted a kinetic model to extract information on how the model system absorbed water. I measured three metrics of wettability: the initial contact angle of a water droplet on the sample surface, the kinetics of the droplet spreading across and imbibing into the sample, and the viscosity of the imbibing flow. All three metrics were altered by lipids that exhibited binding to the mineral surface, were solid at room temperature, and were distributed throughout the mineral film. In addition to determining which lipid characteristics changed wettability, this study provides unprecedented detail on the assembly of phospholipids on mineral surfaces and on the distribution of phospholipids in a model substrate. These results can help illuminate why clay alleviates water repellency and inform the design of remediation strategies.