Formulation is an ancient concept, although the word has been used only recently. The first formulations made our civilization advance by inventing bronze, steel, and gunpowder; then, it was used in medieval alchemy. When chemistry became a science and with the golden age of organic synthesis, the second formulation period began. This made it possible to create new chemical species and new combinations “à la carte.” However, the research and developments were still carried out by trial and error. Finally, the third period of formulation history began after World War II, when the properties of a system were associated with its ingredients and the way they were assembled or combined. Therefore, the formulation and the systems’ phenomenology were related to the generation of some synergy to obtain a commercial product. Winsor’s formulation studies in the 1950s were enlightening for academy and industries that were studying empirically surfactant-oil-water (SOW) systems. One of its key characteristics was how the interfacial interaction of the adsorbed surfactant with oil and water phases could be equal by varying the physicochemical formulation of the system. Then, Hansen’s solubility parameter in the 1960s helped to reach a further understanding of the affinity of some substances to make them suitable to oil and water phases. In the 1970s, researchers such as Shinoda and Kunieda, and different groups working in Enhanced Oil Recovery (EOR), among them Schechter and Wade’s group at the University of Texas, made formulation become a science by using semiempirical correlations to attain specific characteristics in a system (e.g., low oil-water interfacial tension, formulation of a stable O/W or W/O emulsion, or high-performance solubilization in a bicontinuous microemulsion system at the so-called optimum formulation). Nowadays, over 40 years of studies with the hydrophilic-lipophilic deviation equation (HLD) have made it feasible for formulators to improve products in many different applications using surfactants to attain a target system using HLD in its original or its normalized form, i.e., HLDN. Thus, it can be said that there is still current progress being made towards an interdisciplinary applied science with numerical guidelines. In the present work, the state-of-the-art of formulation in multiphase systems containing two immiscible phases like oil and water, and therefore systems with heterogeneous or micro-heterogeneous interfaces, is discussed. Surfactants, from simple to complex or polymeric, are generally present in such systems to solve a wide variety of problems in many areas. Some significant cases are presented here as examples dealing with petroleum, foods, pharmaceutics, cosmetics, detergency, and other products occurring as dispersions, emulsions, or foams that we find in our everyday lives.
Physicochemical formulation [
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3] has been used by humanity to attain products with tailored properties to meet particular needs [
4,
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6,
7]. The properties of the formulated product can be its stability or instability over time, its reactivity with the environment or with a particular substance, its safety of use, its conditioning and presentation, its thermal or electrical conductivity, its viscosity or rheology, its wettability, its appearance, texture, color, and smell, etc. [
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Formulation combines two aspects: First, the knowledge that relates the product content with the desired effects and properties, which are associated in general with chemistry, physics, and physical chemistry [
23,
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26,
27]. Equilibrated systems are essential in this case, and their behavior has been studied by thermodynamics at a high scientific level. Second, formulation includes the operations used to manufacture the product that involves the association of the ingredients and the final conditioning of the product [
9,
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30]. This often has to do with temporary, out-of-equilibrium, and irreversible phenomena, the outcome of which usually depends on the history of manufacture [
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36]. These aspects are superficially studied in classical university education in science and engineering because their scientific management requires the use of non-continuous or derivable functions. It is the case of non-equilibrium phenomena, including hysteresis [
37], and although they are currently used in practice, they are sometimes difficult to explain and, eventually, contrary to elementary logic. In any case, formulation is intended to obtain a product (usually commercial) with a well-defined objective, and capable of satisfying a list of diverse requirements that imply, in many cases, a multidisciplinary collaboration [
1,
38,
39].
In general, the industrial sectors that deal with formulation are highly specialized and associated with know-how representing a high percentage of the product’s commercial value. This is either because they are complicated (requiring a high scientific level) or unique, confidential, and often protected by patents.
Industries that manufacture commodities (caustic soda, fuels, iron, and non-ferrous metals, etc.), or “fine” specialties, but common chemicals (acetyl salicylic acid, sodium hydroxide, trichloroethylene, vinyl chloride, soaps, etc.), are inserted into a competitive market. Thus, their value depends on production operations (raw material, extraction, chemical synthesis) and not on formulation [
40].
On the contrary, industries that make products whose value does not depend on the availability of ingredients, but on their association and combination to produce synergies, with a lot of confidential know-how and long experience, require the use of formulation fundamentals and applications. When there are several levels of quality for a product, such as in paints, foods, detergents, household and hygiene products, perfumes, etc., basic or ordinary products are economical. On the other hand, products with high performance and a high price belong to the formulation industry.
This entry is adapted from the peer-reviewed paper 10.3390/encyclopedia2020054