Hand Sanitizer, Moisturizer and Emulsion

In my previous blog, we talked about the long-lasting higher demand for moisturizers and hand sanitizers. We then discussed what they are made of most of them are emulsions with hydrophobic droplets dispersed in a continuous hydrophilic phase or vice versa.

Now, what do we care about when developing moisturizers and hand sanitizers? We care about what consumers care about. That’s simple then, we are all consumers! So just imagine applying some moisturizers on your hands.

Efficacy of Active Ingredients and Droplet Characterizations

The number one thing we care about is: Does it work as it claims? Many skin care products contain some active ingredients to achieve some functional effects, e.g. anti-aging, acne-preventing, etc. A simple example could be vitamin E, a fat-soluble molecule whose benefits to skin are widely accepted, it is often an active ingredient in hand sanitizers. Such ingredients are often contained inside emulsion droplets. Droplet size as well as surface properties, like zeta potential, can significantly affect the efficacy of such ingredients on the skin [3] as well as the product shelf life (see below). It is suggested that emulsion systems with reduced droplet size may have elevated penetration and can provide more overall available interfacial area for possible chemical reactions [4].

Expiration Date and Droplet Characterizations

Once we have a product that spreads well on the skin with a proper layer thickness and actually meets its claim, a long expiration date would make it into our must-have list. Droplet colloidal stability is one of the most important determinants of an emulsion product’s longevity. The hydrophilic and hydrophobic phases of an emulsion can separate over time. When two droplets bump into each other, it’s likely that they flocculate and then collapse into one droplet. A collection of such events eventually leads to a phase separation. To elongate the expiration date, we need to prevent droplets from bumping into each other.

Zeta Potential is probably the most important parameter when it comes to emulsion stability. It is defined as the potential difference between the particle surface and the bulk of the liquid. Closely related to surface charge, it is a measure of the magnitude of the electrostatic repulsion/attraction between particles. For example, emulsions with zeta potential values of −11 mV tend to agglomerate/coalesce, while emulsions with zeta potential values of  −50 mV indicate good stability [5]. Generally speaking, an emulsion formulation with a zeta potential greater than 30 mV in absolute value would be considered stable. [Optimizing Silicone Emulsion Stability Using Zeta Potential]

Zeta potential can be engineered from many aspects. First, emulsifiers can be anionic, nonionic, cationic, and amphoteric. The type of emulsifier used directly affects the surface charge of a droplet. Next, the environment the droplets are in ionic strength, pH, etc. For further details, please check out our technical note.

Droplet Size is another key parameter that determines emulsion stability. Comparing emulsions with the same oil to water ratio, the one with a smaller droplet size is considered to be more stable [4][6]. One reason is that smaller particles tend to undergo faster Brownian Motion and, therefore, are less affected by gravitational effects. In addition, the overall interfacial surface tension is lowered in an emulsion with smaller droplets [7]. This is because smaller particles have larger interfacial area ratio, therefore, more absorption of the stabilizer.

“COVID-19 is impacting the way consumers approach beauty and personal care product, especially when they consider ingredient safety, cleanliness and shelf life” said Mintel, the marketing intelligence agency.

Rheological Properties and Droplet Characterizations

One of the first experiences we have using an emulsion product, like a hand cream, include ease of getting out of package, spreadability, layer thickness, etc. These are usually described as rheological properties [1]. Emulsion rheology can be influenced by many factors. Droplet size and size distribution are one of the most key ones. For example, a study compared emulsions of the same oil and water volume fractions but with different droplet sizes – one has a small droplet size of 4 µm verses the other of 28 µm. The former has a much higher viscosity at low shear stress and a much stronger shear-thinning effect [2].

So, what does it mean to product development and consumer expectation?

As discussed in the previous blog, to make an emulsion (e.g. cream or lotion), we are mixing a hydrophobic phase with a hydrophilic phase with the help of emulsifiers. Here we have the volume fraction of the two phases the same in hand cream A and B, but A has a smaller droplet size than B. When they are sitting inside the packages, A would appear to be more solid-like, or, more like a cream rather than a lotion, but when we squeeze some out of a tube (or scoop some out of a jar) and spread it on our hands, A would feel as fluidic as B, thanks to the smaller droplets inside.

Droplet Size and How to Control

For your reference, here are some examples of emulsion droplet sizes of different products. A hand sanitizer is reported to have droplets of 5 microns or less [8]. A silicone emulsion used in producing skincare gels is reported to have droplets of around 100 nm in size [9]. An anti-aging application where antioxidants were loaded into nanoparticles has a narrow size distribution around 140 nm [10].

How can we control droplet size in emulsions? There are a lot of things we could explore: concentration of ingredients, e.g. oil phase, water phase, emulsifiers [11], homogenize pressure [12], adding rate, mixing rate [6]; pH and temperature are also shown to affect droplet size, etc. The cited references provide some good examples.

How to Characterize Droplet Size and Zeta Potential

Speaking of emulsion droplet characterization, The Zetasizer products from Malvern Panalytical are the world’s most widely used systems for the measurement of particle size (in the nanometer to small micron range) and charge characterizations. Zetasizer measures particle size and zeta potential using dynamic light scattering. Briefly, the fluctuation of scattered light reflects the motion of particles: Brownian motion reveals particle size; electrophoretic mobility reflects zeta potential. Here at Malvern Panalytical, we offer a range of options, please visit our website to find out more information.

However, there are some potential issues with Zetasizer and the measurement of large droplets. The size range achievable on Zetasizer is usually 0.3 nm – 10 μm. Gravitational effects start to dominate particle motions with particles of 5 μm and beyond. Therefore, the larger the particle, the less measurable on Zetasizer. In that case, it would be a better idea to measure particle size with a Mastersizer using laser diffraction. Briefly, the smaller the particle is the wider the diffraction angle will be. The measurable size range on Mastersizer is usually from 10nm up to 3.5mm.

It is always a good idea to check the particle size with an orthogonal technique such as microscopy against dynamic light scattering and diffraction to make sure the results are correctly interpreted. Seeing is Believing. But, is it really? A problem with microscopy is whether the data is statistical. Morphologi 4-ID is here to help. It is a combination of automatic microscopy with a powerful software that allows measurements of tens of thousands of particles per second. The “ID” part is a Raman spectroscopy that enables probing the chemical composition of each droplet measured.

Finally, dilution is another complicated factor when characterising emulsion droplets. Ideally, we would want to characterize droplets as how they appear inside the final product. However, it is hardly achievable with light scattering which assumes a single scattering event (for more information click here). When it comes to making dilutions, it is important to make equilibrium dilutions in a liquid with the same pH, Ionic strength, surfactant concentration, etc. Microscopy would stand the highest chance in avoiding dilution. A sample microscopy image of a cetrimide cream can be found with this link, where we can see that the droplets can be well distinguished against each other. In this case, statistical size measurement can be achieved with Morphologi 4 without any dilution needed.

Great news! Both our Zetasizer and Mastersizer can “work from home” now. Check it out. At this time, we are trying to be as creative as we can to provide the support you need. Just contact us and let us figure out the rest.

References