by Claudia S. Copeland, Ph.D.
Oil and water don’t mix — unless they have help, that is. That help is an increasingly sophisticated set of options for oil-in-water emulsion driven by the field of nanotechnology. Nowadays, it’s possible to create oil-in-water beverages and other products that provide maximum bioavailability in an emulsion that is optically clear and environmentally stable. Before understanding how this can be done, though, we first must understand the nature of polar and nonpolar substances and why they don’t naturally mix.
Charge, Polarity and Solubility
The words “polar” and “nonpolar” refer to the nature of all substances as either being fully or partially charged (for example, water) or having no charge (for example, oil). When it comes to chemicals, opposites attract: Substances with a positive charge are attracted to substances with a negative charge. The overall charge of pure water (H2O) is zero — all negatively charged electrons are balanced by positively charged protons — but that belies water’s true, dynamic nature. In reality, the oxygen end of the H2O molecule has a partial negative charge as a result of electrons spending more of their time orbiting around it. This leaves the hydrogen atoms relatively electron-free, giving them a partial positive charge. This polarity of H2O molecules causes them to “stick” together, with the negatively charged oxygen of one water molecule being attracted to the positively charged hydrogen end of another water molecule.
Now, if you add another charged or polar substance to water, such as table salt, that substance will easily dissolve. This is because the positively charged sodium ions (Na+) of table salt (NaCl) will be attracted to the partial negative charge of the oxygen end of the water molecule, while the negatively charged chloride ions (Cl–) will be attracted to the partially positive hydrogen ends. Both fully charged and partially charged substances will easily dissolve in water because of the attraction of opposite charges.
Oil and fat-soluble compounds — unlike water, salts and acids — have no charge. These compounds are traditionally called hydrophobic — water-fearing — but it’s not that they fear water molecules or that water molecules are repulsed by them. It’s just that water molecules do not care about them — they are indifferent — and at the same time, water molecules are highly attracted to each other and to other polar molecules. What happens as a result is that the nonpolar compounds become isolated — at first in bubbles and eventually in a separate layer. Like oil-and-vinegar salad dressing, you can shake polar and nonpolar compounds and get them to mix, but they will eventually separate again, forming different layers.
The Quest for Oil-in-Water Stability
Since antiquity, humanity has been trying to get nonpolar substances to dissolve in water-based solutions, a process known as emulsification. Egg yolk, which contains a high amount of phosphatidylcholine, or lecithin, is among the most ancient of emulsifiers. (In those days, users — from cooks to physicians — did not understand the concept of emulsification, but they knew that egg yolks helped fat-soluble substances dissolve in water.) This quest goes far beyond making stable Italian dressing and cake batter. Far more important challenges, such as absorption of drugs from the polar human intestinal environment, call for the development of ways to allow oil to mix with water. Pharmacists have long been concerned with this process, as evidenced by this book section from 1911, on “things all pharmacists should know about the emulsification of oils.”
So, what is an emulsifier? Quite simply, an emulsifier (also known as a surfactant) is a compound that can “stick” to both polar and nonpolar substances, allowing fat-soluble substances to dissolve in water. Emulsifiers are found throughout nature and, increasingly, in sophisticated, cutting-edge food science technologies.
Emulsifying the Way Nature Does It
Although the shorter name lecithin stems from the Greek word for egg yolk, phosphatidylcholine is actually in all living cells, not just chicken eggs. Its structure consists of a negatively charged phosphate backbone from which long-chain fatty acids are suspended — in other words, a polar end bonded to long, nonpolar ends. Phosphatidylcholine and other types of phospholipids form the matrix of the membranes enclosing all cells — the phospholipid bilayer.
Traditional emulsifiers, such as lecithin, can help oil and water mix. However, in order to take an oil-in-water emulsion to the next level — to produce a highly stable oil-in-water emulsion without added or altered taste or adverse appearance and with the highest level of bioavailability — we must build macromolecular structures to efficiently confer the properties we seek. The technological approach of working with molecules of this size is called nanotechnology. Two types of useful emulsions that can be created through nanotechnology are microemulsions and nanoemulsions.
Oil-in-water microemulsions consist of suspensions of tiny droplets with structures quite similar to those enclosing living cells. Among the most important of these are micelles. Micelles, like cell membranes, are made of phospholipids. Instead of being arranged in flat bilayers, though, the phospholipids of micelles are arranged in spherical structures, with their polar “heads” facing outward (in an aqueous solution) and their nonpolar “tails” facing inward. Micelles are like tiny bubbles that spread out in water in an even mixture called a colloid. (A natural example of a colloid is milk. It is opaque, but the fats and lipophilic nutrients in the milk are evenly dispersed in the aqueous base of the milk and do not separate over time.) When fat-soluble substances are placed inside micelles, it’s like they are placed in comfy polar carriages that disperse evenly in water, with their oily passengers stably tucked away inside them.
Nanotechnologists use a variety of carriers that function in a similar way — encapsulating a nonpolar compound of interest (such as a drug or nutraceutical food item) to facilitate solubility, stability of the compound in adverse environmental conditions and absorption of the compound from the GI tract into the body. These carriers include biopolymers, liposomes, solid-lipid nanoparticles and nanofibers, to name a few.
How to Make a Nanoemulsion
Both microemulsions and nanoemulsions consist of particles less than 100 nanometers in size and improve key features such as long-term stability, optical clarity and bioavailability. Nanoemulsions contain particles with different sizes, whereas the particles in microemulsions will be uniform. The difference between nanoemulsions and microemulsions, however, is not so much one of size as of their functional features and how they are made. While both types of emulsion require similar ingredients — oil, water and one or more surfactants — microemulsions, which are thermodynamically stable, will spontaneously self-assemble. In contrast, nanoemulsions are made using technological processes like mechanical shearing or sonication to create droplets with wavelengths shorter than the wavelengths of visible light (and therefore seen by human eyes as clear). The creation of these tiny particles requires the input of energy via technology such as ultrasound, high pressure valve homogenizers or microfluidizers. Then, because they are not thermodynamically stable when they are first created, a stabilizer must be added. Unfortunately, this process is not straightforward and is complicated by other considerations such as controlling the rate of degradation of the active ingredient, seeking temperature and pH stability, and dealing with the overwhelming bitterness that accompanies nanoemulsions loaded with active ingredients such as CBD or THC.
Ready-to-Use Oil-in-Water Emulsion Products
The field of nanotechnology is one of vast possibilities and the methods for achieving oil-in-water stability can be dizzying in number as well as complexity. If you are a small company looking to maximize the stability of lipophilic substances in a water base, research into nanotechnological solutions is most likely prohibitively expensive. Fortunately, Axiomm has done the bulk of the research for you. With products like μGOO, μSHOT and μMIX, you can start off with pre-developed carriers designed to create stable, oil-in-water nanoemulsions infused with your product. Just add your unique product components and follow the directions. You’re free to focus on issues like taste, color and texture, while Axiomm takes over the technical work of the nanoemulsion. The result? An easy way to develop the best version of your own unique product for your customers.