Synthesis and Characterization of Bio-inspired Glycolipid Surfactants

Abstract

Surfactants are a commodity in the human civilization. Their unique ability of powerfully affecting the interfacial tension and their tendency of forming supramolecular aggregates in solution have been utterly exploited in several technological products now indispensable in the modern society. Evidence of their technological application date back to 5000 years ago. Currently, surfactants are used in practically all the industrial sectors. Due to their importance and demand, the science and technology to manufacture and characterize surfactant systems is highly developed. Nevertheless, there is a large room for improvement given the environmental challenges we are facing. For example, the surfactants that currently dominate the market are not readily biodegradable and their building blocks come from non-renewable sources like oil. For this reason, there is a growing interest in learning how to design and manufacture greener surfactant molecules. Bio-surfactants figured for several decades as potential greener replacements of the synthetic commercial counterparts. These materials have outstanding surfactant performances, and since most bio-surfactants are glycolipids, their biodegradability is favorable and the building blocks can be obtained from renewable and abundant feedstocks. However, their manufacture is problematic: biotechnological methods produce unpredictable mixtures with irreproducible yields, while their chemical synthesis is often impractical to be performed at scales relevant to the demand. As part of this quest for determining the best way to make bio-surfactants a feasible option, our laboratories developed a cost-effective methodology to chemically synthesize rhamnolipids, a class of bio-surfactant, allowing to explore its structure-performance relationship. However, despite its undisputable and unbeatable advantages, due to the structural complexity of rhamnolipids, this synthetic method has similar limitations to the mentioned above that precludes the manufacturing of naturally occurring rhamnolipids at industrial scales and at competitive costs with the oil-based counterparts. The present dissertation examines and demonstrates the possibility of employing what is known from the structure-performance relationship of rhamnolipid bio-surfactants to design molecules that are easier to manufacture while keeping similar or better performance. In sequence, a thorough exploration of the structure-performance relationship of these novel bio-inspired materials is presented to define the limits of their tailorability. This exploration includes the contributions to understand the relationship of surfactant performance of bio-inspired rhamnolipids with: i) the absolute configuration of the carbinols at the lipid tails and ii) the symmetry of the lipid tails of diastereomeric mixtures. These studies involved chemical synthesis and the characterization of the physicochemical properties of colloidal aqueous solutions. Evidence of the synthesis and isolation of the final products and building blocks is provided in form of nuclear magnetic resonance. The thermodynamics of their surface activity are described by surface tensiometry at the air-water interface at pH 8. Characteristics of their aggregation behavior in aqueous solutions including hydrodynamic radius, aggregation number, and aggregate morphology are determined using dynamic light scattering and time-resolved fluorescence quenching. The overall impact of this work is to push the frontier to make glycolipid surfactants a feasible alternative for their eventual introduction to wider markets.