Editor’s note: Emulsion science is widespread throughout the personal care industry, providing the means to formulate creams or lotions that contain both oil-miscible and water-soluble components. However, conventional methods for processing such emulsions require significant quantities of energy and time; it has been estimated that heating and cooling alone accounts for over 90% of the total energy cost for the production of an emulsion.1 With the current market focus on eco-friendly materials and processes, cold process emulsion technologies have gained popularity, reducing both the energy demand and manufacturing time required. In this article, traditional emulsions and emulsion processing are reviewed and compared with cold-processed emulsions.
What is an emulsion? As most readers know, an emulsion can be considered a dispersion of one material inside of another, non-miscible phase. Generally, in cosmetics and personal care, the two phases in an emulsion are oil and water/aqueous phases. Emulsion science provides the personal care industry with the means to formulate a cream or lotion that contains both oil-miscible and water-soluble components. For example, emollient components tend to be lipophilic in nature, whereas moisturizers demonstrate mostly hydrophilic characteristics. Other components such as natural extracts, active ingredients, essential oils, fragrances, preservatives, colors and tints all exhibit a preference to either the oil or water phase. Theoretically, oil and water phases are completely immiscible, but there is always a statistical possibility that some oil may dissolve in the water phase and vice versa. The same probability can be applied to other ingredients, so there may be some dissolution, to a minor extent, into the less-favored phase.
Emulsions are considered thermodynamically metastable systems, i.e., they can exist in a long lived-state that is not its most stable form. In fact, Gibbs stated, in reference to emulsion stability, “the only point in time where an emulsion is stable is when it is completely separated.”2 In the real world, this implies that emulsions will always have a tendency to separate into their oil and water phases, although viscosity and stabilizing components slow this separation process. Figure 1 shows an emulsion separating into its oil and water phases.
The cloudy region in the emulsion is the manifestation of creaming in the system, where the oil droplets are attracted to each other. The upper yellow layer shows the oil phase, appearing after the oil droplets have coalesced. Creaming of the oil droplets can be reversed by re-mixing the emulsion; however, coalescence can only be reversed by re-formulating the emulsion.
Emulsifiers are molecules that contain both hydrophilic and lipophilic chemical groups. From a thermodynamic standpoint, the emulsifier reduces the surface energy of the dispersed phase, thus increasing the thermodynamic drive to form a stable emulsion. While droplet size, temperature and entropy also contribute to the thermodynamic stability of the emulsion, the use of emulsifiers provides a simple route to form a stable product.
Numerous types of emulsifiers are available and the selection criteria can be based on: chemical functionality—i.e., esters, silicones, ethoxylates, etc.; hydrophilic or lipophilic chemical behavior; efficiency; and/or cost. Further, Griffin defined the hydrophilic-lipophilic balance (HLB) theory to provide formulators with a tool to assist in the selection of the correct balance of emulsion system, although this theory is not without flaws or exceptions.3
When formulating an emulsion, one can tailor the texture, body and sensory characteristics of the finished product by choosing specific ingredients. Common ingredients used in emulsion systems are: water; oils/waxes; emulsifiers; emollients, often lipophilic; moisturizers, often hydrophilic; natural extracts; active ingredients; essential oils; fragrances, which are volatile and temperature-sensitive; preservatives; and colors or tints.
Conventional emulsion processing involves separately mixing and heating water and oil phases, bringing the two phases together and mixing with high shear. In this case, temperature-sensitive ingredients must be added to the formulation once the emulsion has cooled. Why might heat be required to make an emulsion? The aqueous phase may include a gum that must be hydrated at an elevated temperature; or the oil phase may contain waxes that require melting. Waxes and gums are used in emulsions to increase the viscosity of the oil phase, improve stability and enhance the aesthetics of an emulsion.
The drawbacks inherent with this process are the time and energy it takes to heat the two phases and the emulsion; although energy prices are on the rise, the largest cost component for the conventional manufacture of creams and lotions is the time involved for the heating and cooling stages of the process. In fact, it has been calculated that these stages account for nearly 40% of the total processing time.1 Another critique of the conventional process arises when processing a high viscosity cream, as the post-emulsification addition of temperature- and shear-sensitive ingredients, if using inefficient mixing systems, can lead to non-homogeneity within the finished formulation.
Cold Process Emulsification
Cold process emulsions are prepared from base raw materials preferably without any external heat—i.e., cold. In an ideal world, this process would simply involve premixing the oil and water phases separately, then bringing them together with an efficient mixing system. To successfully eliminate the requirement for heat in the process, all raw materials must be liquids or soluble within their respective phases at ambient temperature. In fact, any increase in temperature generated would only be the direct result of the mixing process. To some, the separate blending of emulsifiers and low melting of waxes and oils using heat could be considered part of this process; however, this author believes they cannot be considered part of a truly cold process.
Advantages of cold processing include reduced energy cost, manufacturing times and equipment costs; greater flexibility for formulation development; and the absence of deleterious effects on temperature and shear sensitive ingredients. Temperature and shear sensitive ingredients such as fragrances, active compounds, preservatives, etc., can be added to the relevant phase prior to forming the emulsion. Cold processing potentially allows the formulator to construct entire phases separately, and to simply bring them together with a suitable mixing system.
From a practical standpoint, taking a simple formulation of water, oil, emulsifiers, preservative, essential oil, plant extract, viscosity modifier, pH adjusters and a fragrance, the conventional process would require separately heating the oil and emulsifier; and the water, emulsifier and viscosity modifier; then bringing the two phases together with efficient mixing, and allowing the batch to cool. Once cold, the preservative, essential oil and plant extract could be added and finally, the pH adjusted to meet the product specifications. In contrast, cold processing allows all the oil-compatible components to be premixed in the oil phase, and likewise for the water-compatible components and the water phase. The phases can then be mixed together and the pH adjusted to create the finished product in a fraction of the time.
Calculating the potential time and energy saved using cold emulsification is somewhat tricky and will be specific to an individual company’s equipment. Dependent factors include: heater/chiller unit efficiency, cooling/heating demand of the system, flow rate through the heater chiller, cooling/heating transfer efficiency in the processing vessel, the ambient temperature’s contribution to heating and cooling, the power rating of the stirrer motor and high shear mixer (if required), current energy pricing, batch size, viscosity of the product, local labor costs and overall time of processing. However, the potential cost savings could be in the order of thousands of dollars. As a simple example, the energy requirement to heat and cool 1 tonne of water from 20°C to 70°C and back is approximately 120 kWh, accounting for over 90% of the total energy consumption for processing an emulsion.1
In addition, cold process emulsions require less equipment since external heating or cooling is not necessary. In the case of natural viscosity modifiers or powdered acrylates, however, efficient or high shear mixing systems may be required to ensure complete wetting of the thickener. An efficient mixing configuration is also needed when manufacturing higher viscosity emulsions, to ensure homogeneity of the finished product. Also for viscous creams, the ability to apply a partially positive pressure within the reaction vessel will facilitate emptying the product. In the case where wax is an essential component of the oil phase and some heat is required to melt it, blending the wax into a warm oil phase in a melt vessel prior to mixing in the cold bulk aqueous phase is an option and only necessitates heating one phase; however, the authors do not consider this to be a true cold process. For low viscosity emulsions, a fine, uniform droplet size is easily achievable using low shear stirring.4
Cold Process Considerations
Readers should note there are some limitations to cold process formulating,5 including the fact that all raw materials must be liquid or readily soluble at room temperature. Waxes or wax-like materials cannot be incorporated into the oil phase, and viscosity modifiers are required to achieve the desired body and feel in the end formulation. Also, pasteurization of natural viscosity modifiers cannot be employed as a means to remove any pre-existing microbes and bacteria. Further, the range of available emulsifiers is limited; cold process emulsifiers tend to have a specific purpose or application. However, there are ways to circumvent these issues.
Viscosity modifiers: With respect to the development of texture and viscosity in the absence of waxes, numerous synthetic or natural viscosity modifiers are available that either immediately increase viscosity or require activation via pH adjustment or the addition of electrolytes. The range of thickeners available is large, including polyacrylate based thickeners that provide excellent texture, feel and viscosity; although one major drawback is their intolerance towards electrolytes. Another range of thickeners worthy of mention are cold process waxes.
Natural viscosity modifiers, such as xanthan or guar gums, often receive criticism for their finished texture and appearance and they also play host to a number of microbes and bacteria. Pasteurizing the water phase at 72°C is therefore the general method of choice for killing the microbes and bacteria. Unfortunately, this is not an option for true cold processing, but there are a number of preservatives and blends that provide a means to sterilize the system. One advantage of the cold process is that a robust formulation can tolerate the addition of preservatives early in the process. Hence, the judicious choice of preservative system and its early inclusion in the formulation will overcome these issues caused by natural viscosity modifiers.
Emulsifiers: The number of cold process emulsifiers available is relatively small and has been exhaustively reviewed.5 Generally, they fall into distinct groups: those for o/w emulsions, often polyglycerol esters, or those for w/o emulsions, often alkoxylated alcohols and phosphorous compounds, and a small group of silicone based emulsifiers. Emulsifiers provide stability by reducing surface energy, although it has been noted5 that acrylate polymer thickeners can hold the internal phase droplets in suspension. While the majority of cold process emulsifiers tend to have a specific application, one range of emulsifiers based on polyglycerol ester derivatives of vegetable oils has been developed that is compatible with both oil and water phases, allowing for the use of just one emulsifier.
As previously mentioned, emulsions are unstable systems, and all possess a thermodynamic drive to return to a state of lowest energy6—i.e., to completely separate into the individual oil and water phases. Figure 1 demonstrates flocculation and coalescence; however, other effects of emulsion instability include: sedimentation, which is similar to creaming except the denser phase settles to the bottom; flocculation, when oil droplets floc together to form a cloudy suspension; phase inversion; and Ostwald ripening.
Phase inversion: Phase inversion is said to occur when the emulsion inverts from one type to another, for example, an o/w inverts to a w/o emulsion. Phase inversion arises when the emulsifier becomes more soluble in the dispersed phase than in the continuous phase. Such changes can occur upon on change in temperature and/or increase in electrolyte content. While phase inversion can be considered emulsion system instability, it also can be used as an energy-efficient method to prepare a stable emulsion. In phase-inverted emulsions, the interfacial tension is extremely low at the inversion temperature, which relates to the existence of a micro emulsion phase at the temperature of inversion. This physical behavior results in the formation of very small droplets with little mechanical energy input.7
Within the cold process paradigm, the ability to pre-mix individually homogenous phases is a prerequisite to applying the phase inversion technique. If the inversion from w/o to o/w occurs by reducing the temperature, the aqueous affinity of the emulsifier will increase as the temperature drops. Phase inversion can also be concentration-dependent; for example, a prepared oil phase containing an emollient, emulsifier and microemulsion booster4 will proceed through a phase inversion state as the level of water increases. It has been reported that for this oil phase: below 10% water content, the product is a clear solution; between 10–32% water, a w/o emulsion forms; between 32–72%, a microemulsion forms; and above 72%, the emulsion reverts to an o/w type. While this is a specific example, it demonstrates that within the sphere of cold process emulsion science, the ratio of oil to water phase will have a significant impact on the type of emulsion formed. Since the physiochemical performance of emulsions varies with respect to components and their levels, it is difficult to develop a predictive rule to indicate that X oil phase plus Y water phase would give a specific emulsion type.
Ostwald ripening: Ostwald ripening tends to occur in solid solutions or liquid sols, where the small crystals or sol particles dissolve and redeposit themselves as larger particles. This is because, from a thermodynamic standpoint, larger particulates or droplets are more stable.8 In the case of emulsions, molecules diffuse from small droplets to form large ones within the continuous phase. Generally, Ostwald ripening occurs in w/o emulsions.
Stokes’ law is a mathematical expression relating the frictional force exerted on a sphere in a viscous medium.9 This expression supports the theory that the more viscous the formulation, the less likely the oil droplets will separate from the water phase. The increased viscosity reduces the mobility of the suspended droplets and reduces the capacity for flocculation and eventual coalescence. This theory also implies that the choice of viscosity modifier is critical for the successful formulation of cold process emulsions.
From the previous paragraph, the logical assumption would be that a stable, water-thin emulsion should be a difficult product to formulate. However, using emulsifiers, water, oil, preservative and a small amount of viscosity modifier, a water-thin emulsion was developed that demonstrates long-term stability (see Formula 1). Figures 2 and 3 are microscopy images comparing samples on the day of preparation with a sample stored for four weeks in an oven at 40°C, respectively. Generally, the micelle size distribution in emulsions can range from nanometer to micron scale and is influenced by numerous factors including ratio of the oil and water phases, levels of emulsifiers, efficiency of mixing, etc.10 Figure 4 shows the microscopy image of the same water-thin emulsion prepared by a heated process.
These images show there is little or no alteration in micelle size or distribution after storing the sample at 40°C for four weeks. Additionally, little or no difference in physical appearance between the two samples was observed, as shown in Figure 5.
The associated variations in preparing an emulsion, including formulation, stirring, ambient temperature, processing temperature, etc., all lead to emulsified products that may possess differing properties or characteristics. Therefore, careful control and processing conditions must be adhered to during all stages of any emulsion preparation.1 This means that for any meaningful comparison of emulsion systems, one must consider a single formulation prepared at different temperatures. An in-house panel test comprising of 37 members, when asked to consider rub-in, pick-up, texture, appearance, tack and skin feel, could not detect any difference between a hot-processed or cold-processed lotion, based upon Formula 1 (data not shown).
Given the continual rising global energy costs, and the adoption of green policies by leading manufacturers, cold processing can assist the manufacturer in offsetting production costs and provide a route to directly reducing the carbon footprint of operations. Although the number of currently available cold process raw materials is limited, increasing interest in this technology will ultimately lead to greater offerings of such materials. Cold processing also provides a direct route to improve the speed of manufacture, therefore providing a measurable improvement in operational efficiencies. As more and more manufacturing companies determine a need for green initiatives, investment in research and development will increase the understanding of these advantages and their limitations, also raising the profile of the technology.
Further, the role of the formulator in developing cold-processed products will be critical. Namely, reproducibility must be ensured across the spectrum of research scale, pilot batch and full plant production. The focus must include key product performance factors including sensory and stability attributes. Also, one key ingredient to any cold-process formulation will be the viscosity modifier; be it a melted wax, a synthetic polymer or a natural gum. This choice will significantly impact the product’s appearance and performance. One particular advantage of this process for the formulator is the ability to include preservatives early in the preparation, which ensures the entire batch will be sterilized for nearly the entirety of its processing.
The potential for cold process technology is great, and in coming years, a greater understanding of the science will increase, along with the availability of compatible raw materials, ultimately leading to an increased number of cold-processed finished products.
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