Surfactants at the Design Limit

Key words: Intermolecular bonding, surfactants, amphiphilic molecules, surface tension

Molecules may be considered as either polar or non-polar. Polar molecules will readily mix with other polar molecules, but not so well with other non-polar molecules (and vice versa). An easily visualised example of this is water, a polar liquid, mixing poorly with oil, a non-polar liquid. If a molecule is polar it is hydrophilic ("water-loving"), whereas if a molecule posse no polarity then it may be hydrophobic (“water-hating” or also "oil-loving", lypophilic). Surfactants, a contraction of the term surface-active agents, are amphiphilic (dual-natured) molecules because they possess both hydrophilic and hydrophobic groups, see Figure 1. Surfactant molecules posse both a polar “water-loving” headgroup attached to a non-polar “water-hating” (or “oil-loving”) tail. Due to their dual nature, they are associated with many useful interfacial phenomena, and as such are key components for many diverse industrial products and processes, including but not limited to: detergents, cosmetics, paints and inks, herbicides, insecticides, firefighting foams, medicine and oil recovery.

‌‌As surfactants combine both polar and non-polar groups into a single molecule, they can adsorb (or locate) at interfaces, thereby altering significantly the physical properties of those surfaces. The term “interface” is commonly employed here to describe the boundary in liquid/liquid, solid/liquid and gas/liquid systems. This can be visualised by considering the specific example of surfactant molecules at the air-water surface, shown in the figure right. The polar hydrophilic heads remain solvated in polar water whilst the non-polar tails can partition into the air, which is by nature non-polar. This means we can use surfactants to control surface tension!‌‌‌ ‌

Controlling surface tension is imperative to many processes, both natural and industrial, such as breathing, or removing crude oil. Water possesses strong polar interactions; it has a high surface tension ≈ 72 mN m-1 at 25 oC. Surfactants made from fluorine and carbon, termed fluorosurfactants (see Fig. 3) are the best performing surfactants ever made. They can reduce the surface tension of water from 72 -> 15-25 mN m-1. However, we now realise fluorosurfactants (FS for short) are environmentally hazardous and there is a need to develop a replacement. That’s where my research comes in.

Hydrocarbon surfactants have always been outperformed by FS. Recently it has been shown that through branching the tail structure, hydrocarbon surfactants can achieve surface tensions which rival FS. Current research focuses on developing hydrocarbon surfactants to rival and replace FS. This requires the design and synthesis of novel surfactant structures, see Figure 3. A variety of experiments, such as measuring the surface tension it generates in an aqueous solution (Figure 4), can be performed. The use of particle accelerators allows the study of novel surfactants directly in solution. By combining the data from pulsed neutron sources and laboratory data, accurate conclusions about the structure-property relationships of hydrocarbon surfactants and surface tension can be drawn. [500 words]

Adam Czajka is a final year PhD in the Eastoe Group at the School of Chemistry, University of Bristol. During his PhD, he intends to improve our fundamental understanding of controlling surface tension, attempt to produce the lowest surface tensions ever achieved by hydrocarbon surfactants and point to new ways to design commercially available, environmentally friendly, 21st century surfactants.