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Revealing the surprising complexity of skin creams

8 January 2020

Scientists from King's College London and Manchester University, in collaboration with scientists at ISIS Neutron and Muon Source (ISIS), have carried out the first-ever investigations of the microstructure of aqueous creams. The results are not what they expected and could change the way such creams are formulated and even lead to new ways to deliver drugs.

If you have ever suffered from irritated, dry or raw skin you probably know the relief that can come from applying copious amounts of a soothing lotion. Many of these – particularly those used to treat skin conditions like eczema and psoriasis – are non-greasy, water-based lotions known as aqueous creams.

While it’s easy to appreciate the skin-calming effects of aqueous creams, it’s a fair bet that you’ve probably not given much thought to their molecular structure. Such creams are essentially an emulsion of oil and water (rather like mayonnaise) in which droplets of oil are held in suspension within a watery matrix. While that might sound simple, the outward appearance of creamy mushiness belies a fairly complex molecular structure – a structure that the team has discovered through studies at ISIS to be even more complex than previously imagined.

Although the creams you buy in the shops may contain as many as 20 ingredients, only a few of them are essential to create an emulsion and keep it stable: oil, water, a surfactant and one or two co-surfactants. Surfactants are molecules with a hydrophilic (water-loving) head and a hydrophobic (water-hating) tail. When surfactants come into contact with oil, their hydrophobic tails are drawn to the oil and their hydrophilic heads are drawn to the water. As such, they gather around oil droplets to form spheres (micelles) in which the outward-facing, water-soluble, hydrophilic heads allow the oil drops to be dispersed (co-surfactants are molecules that aid in the formation of micelles and in keeping them stable).

According to established wisdom, the surfactant and co-surfactant molecules in aqueous creams surround the oil droplets to keep them dispersed, whilst excess molecules form bilayer membranes that trap water between their layers. But the new study has revealed that the actual picture is even more complex than that.

'Our work has revealed that the textbook view of skin cream structure is, in some respects, rather naïve, and in other respects totally incorrect,' said Delaram Ahmadi of King's College London. Together with David Barlow, also from King's College, and Jayne Lawrence of the University of Manchester, Delaram Ahmadi’s team is the first to carry out detailed investigations of the molecular structure of skin creams using neutron scattering.

The team focused on a very simple formulation made up of five ingredients (paraffin, water, a surfactant and two co-surfactants). The ingredients were synthesised in ISIS’ Deuteration Lab where the hydrogen atoms within the chemical’s molecules were replaced with deuterium, or heavy hydrogen, atoms (a process called deuteration). In the experiments, the neutrons were only able to ‘see’ the deuterated molecules, everything else appearing invisible.

The SANS2D instrument was used to perform neutron scattering experiments that were able to determine each component’s molecular arrangement. Separate experiments were run as each component was deuterated in turn, making it possible to build up a picture of the cream’s molecular structure in unprecedented detail. Complementary x-ray scattering studies were also carried out using facilities in the Materials Characterisation lab at ISIS.

‘We wanted to improve the science around cream formulation so that companies could more rationally formulate them to get exactly what they want,’ said David Barlow.

During the experiments the team made several discoveries about the structure of aqueous creams that could have implications for their formulation and perhaps even their applications in the future.

Their first surprise was to find that the bilayer membranes in the cream contained primarily the co-surfactants (with little evidence of surfactant) and the second surprise was that these membranes also contained oil. The team also discovered, however, that the surfactant (and co-surfactants) gives rise to an abundance of elliptical structures called bicelles – the first time that these have been seen in aqueous creams.

Bicelles are similar to micelles in that they are made up of an envelope of surfactant molecules but, whereas micelles are spherical, bicelles are elliptical, or disk-like, structures. It is possible that these bicellar structures could be used to encapsulate lipophilic drugs (drugs that are easily absorbed and metabolised by the human body) – potentially creating a new way of delivering drugs.

The second surprise came when the team added an antimicrobial preservative called pentanediol. Such preservatives are routinely added to skin creams because it is believed that they dissolve within the layers of water between the membranes of surfactants – the place where bacteria are thought to thrive. In fact, the team discovered that, instead of dissolving and dispersing through the watery layers, the preservative doesn’t dissolve and instead sticks to the membranes. This means that the preservative is likely to have no effect and skin creams may not need to contain a preservative at all because they are, in effect, self-preserving.

The team hopes that their work might help manufacturers better predict how each component changes the cream’s properties – allowing them to design more effective creams – and help them do away with unnecessary ingredients.

‘At the moment, the recipes for creams are based largely on trial and error,’ explains Ahmadi. This means it’s hard for manufacturers to predict how a lotion’s properties change when they add ingredients like sunscreens, preservatives or antioxidants.

The team plan on expanding their work to study more complex formulations and a variety of antimicrobial preservatives.

Last updated: 08 January 2020

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