The art of incorporating lipids in to personal care products spans a multitude of product forms including creams, gels, lotions, and serums as examples of aqueous based formulations, while lipsticks, lip balms, and mascaras constitute the anhydrous forms. It should be noted that lipids are incredibly versatile and useful and this chapter will focus on the utilization of fatty acids, fatty alcohols, esters, and glycerin for their application in personal care ingredient formulation. This text will use the international nomenclature of cosmetic ingredients (INCI) to refer to specific lipid moiety examples throughout the text.

Fatty acids and fatty alcohols are the most widely utilized and most consumed lipids. Sourcing is generally natural and mainly from plant origins such as coconut and palm. Fatty acids contain a carboxylic acid and a branched or unbranched aliphatic chain of various lengths. Due to natural sourcing, carbon chain lengths are even in accordance with understood rules of biosynthesis. Industrial production processing usually relies on some form of hydrolysis of triglycerides. Fatty acids in general have two classifications systems. Saturated fatty acids are without double bonds and are considered linear and unsaturated fatty acids are nonlinear and contain one (monounsaturated) or multiple (polyunsaturated) double bond(s). Applications include most product categories ranging from soaps and deodorants to hair and skin care including color cosmetics. Saturated fatty acids and fatty alcohols function mainly as emulsifiers, stabilizers, and tactile sensory modifiers in these product categories. It is not surprising that with the exception of soaps and deodorants the largest application of fatty acids and alcohols are in emulsions systems. Chain lengths from 12 to 20 are typical for cosmetic application in emulsions, soaps, and deodorants as a general rule of thumb (Richter and Knaut 1984, 1985; Reusch 1977; Lehninger 1982; Condea 2000; Voeste and Buchold 1984; Anneken et al. 2006).

The linear nature of saturated fatty acids is known to support spherical interface structure in emulsification applications. Fatty acids are referred to as emulsifiers while the fatty alcohols are referred to as emulsion stabilizers or modifiers when used in this application. Both contribute significantly to tactile sensory properties. The common tactile sensory term to describe fatty acids and fatty alcohols is “waxy.” Fatty alcohol chain lengths are almost always limited to C16–C18 or some blend of the two for cosmetic applications (Anneken et al. 2006; Harry and Rieger 2000; Schlossman 2009). Branched chain fatty alcohols are not common for cosmetic applications. However, cetyl alcohol (1-hexadecanol) is one example of a commonly employed fatty alcohol for cosmetic application with a low-melting point (Windholz 1983). Usage levels for emulsion application are typically 1.0–5.0 % when in the salt form. Fatty acids have low-water solubility. The salt formation with a base, usually sodium hydroxide, will result in a water dispensable structure for emulsion applications. The ideal carbon chain length for fatty acid emulsion application is C16 (palmitic) and C18 (stearic). Usage level is similar at 1.0–5.0 %. Occasionally C14 (myristic) can be found in emulsions for shaving application. The laws of thermodynamic equilibrium indicate that the lowest free energy states are achieved when considering carbon chain lengths of C16 and C18 in spherical structures between 0.1 and 1 µm diameters. It is also no surprise that the typical or common chain length of the fatty acids found in human cell membranes is 16–18. This similar molecular weight range is ideal for spherical structures when designing emulsions (Adkins 1968; Gray et al. 2002; Alberts et al. 2002; Budin and Devaraj 2011). Cetyl and stearyl alcohol are frequently paired with salts of stearic acid and a secondary emulsifier in modern oil-in-water cosmetic emulsions (Harry and Rieger 2000; Schlossman 2009). Cis unsaturated fatty acids are sometimes employed to reduce the spherical nature and increase the fluidity of the interface. Oleic acid, the major fatty acid found in olive oil, is an example of a common unsaturated fatty acid for this purpose. Oleic acid is also found in human cell membranes. The reason is speculated to be similar (Gray et al. 2002; Alberts et al. 2002; Kiritsakis 1998). The essential fatty acids, which are also cis unsaturated, have suggested application as biologically functional ingredients in skin care products. This suggestion is due to their role in mitigating inflammation. They are called essential as they must be ingested as they are not synthesized by the body. They include the omega-6 and omega-3 family of fatty acids and are naturally found in small amounts in the triglycerides of vegetable oils. Linoleic acid and longer chain gamma-linolenic acid and arachidonic acid are part of the omega-6 family. Alpha-linolenic acid and longer chain eicosapentaenoic acid and docosahexaenoic acid are part of the omega-3 family and are considered antiinflammatory (Brenner 2004; Kiritsakis 1998; Williams et al. 2013). It is noteworthy to apply a greater ratio of omega 3 fatty acids over omega 6 essential fatty acids as part of a therapeutic regime for sensitive skin formulations, as omega 6 essential fatty acids are used as a substrate for the creation of arachidonic acid, a pro inflammatory lipid mediator. When formulating products for sensitive skin, this becomes important factor to consider as some traditionally used shorter chain fatty acids such as those found in coconut oil can be irritating and comedogenic to consumers with sensitive skin (National Toxicology Program 2001). In addition, there is no specificity for either omega 3 or omega 6 fatty acids for the desaturase enzyme pool found in skin to process the essential fatty acids so favoring the omega 3’s reduces the potential for arachidonic acid loading in the cell membranes. Shorter chain fatty acids applied at very small usage levels can have therapeutic effect in terms of skin penetration to enhance the delivery of other actives in the formulation. Lauric acid is an example of a shorter chain fatty acid. Higher usage levels above 1.0 % can be irritating to the skin and caution should be considered when formulating (Pornpattananangkul et al. 2010).

The fatty alcohols are speculated to increase emulsion stability through transition temperature (T _g) modulation of the microsphere film. This is a result of increasing the packing efficiency of the fatty acids at the oil and water interface. Product form is a critical design goal and part of the consumer experience during cosmetic product usage and purchase. Product stability ensures proper intended product form specified in the design goals. Fatty acids and fatty alcohols contribute significantly during emulsion design to ensure product stability. Tactile sensory goals are also a critical part of product design. In many cases, it is the single most influential product attribute that consumers will use when making a purchase choice or evaluating the performance of a product. The influence fatty acids and fatty alcohols play in tactile sensory perception is an important role during and after product usage. Fatty acids impart sensory aspects such as slip, tact, glide, and shine.

The film left on the skin or hair after product application of products containing both fatty acids and alcohols are described as “waxy” during sensory evaluation. The waxy film is mostly perceived after a short period of time after product application. During product application fatty acids and alcohols are thought to contribute to tactile sensory attributes such as “dry time” and “play time” (Condea 2000; Harry and Rieger 2000; Schlossman 2009).

Fatty acids and alcohols when combined by the chemical reaction referred to esterification are called esters. The reaction equilibrium is controlled by the removal of water during processing and is therefore considered a condensation process. The reverse of the esterification reaction is referred to as hydrolysis and is catalyzed by heat and extreme pH conditions. Process temperatures and pH requirements of formulations are a critical consideration during the design of cosmetics containing esters (Reusch 1977).

Triglycerides sourced from naturally occurring oils are considered natural esters. Triglycerides are a combination of glycerol and three fatty acids. Cosmetic application of natural esters almost always includes esters from plant sources (Lehninger 1982; Noweck and Grafahrend 2006; Anneken et al. 2006). The lower molecular weight and cyclic esters have aromatic application in the cosmetic industry (Gottschalck and Bailey 2008; Harry and Rieger 2000). Many possible combinations of fatty alcohols and acids result in a plentitude of synthetic esters available to the cosmetic product designer. Currently there are hundreds of ester choices ranging from simple linear to complex branched, and low to high molecular weight and polarity (Gottschalck and Bailey 2008). The dynamic range of polarity includes esters that are water dispersible to esters that are insoluble in some hydrocarbons. Melting points range to yield both liquid and solid physical forms at room temperatures. Cosmetic application includes all categories and product forms. Typical usage level is 1.0–20 %. Esters are one of the most popular and major cosmetic ingredients and in some product categories, such as skin lotions, the major ingredient (Harry and Rieger 2000; Schlossman 2009). Monoglycerol esters function as emulsifiers for both oil-in-water and water-in-oil designs. Typical usage levels for this application range from 1.0 to 5.0 %. The moderate polar esters have application as pigment dispersants and organic sunscreen boosting properties (Gottschalck and Bailey 2008; Harry and Rieger 2000; Schlossman 2009). Very-high molecular weight and extensively branched esters which are synthesized from guerbet alcohol are known as guerbet esters (Reusch 1977). INCI: Trioctyldodecyl citrate is one example of a guerbet ester which despite its high molecular weight (66 carbons) has a low-melting point due to the extensive branching. Guerbet esters also have suggested applications as pigment dispersants and organic sunscreen boosters (Gottschalck and Bailey 2008). The linear and branched esters function as tactile sensory modifiers giving short- and long-term sensory benefits due to their emollient nature. Long-term benefits are consumer perceived as skin softening or emollient. This benefit is believed to be achieved through the plasticization effect esters have on the proteins in the epidermis and hair upon penetration of the surface. The natural esters have an even higher tendency to be absorbed into the skin and hair. This phenomenon is believed to be due to the similarity in polarity, molecular weight, and geometry of the natural esters and sebum (Mackenna et al. 1950; Camera et al. 2010). Natural oils such as coconut and olive have a long history of cosmetic application supported by this belief (Kiritsakis 1998; Nutritional composition of Mediterranean crops; Gode et al. 2012; Keis et al. 2005). Linear and branched esters contribute to the immediate short-term sensory attributes referred to as “cushion” and “slip” during product application. Some high-molecular weight branched esters are perceived to contribute an abundant amount of cushion and slip are suggested as mineral oil replacements. It should be noted that there is a trend in skin care formulations to avoid mineral oil use due to impurities (polyaromatic and polycyclic hydrocarbons), but overall, mineral oil use has been plentiful and safe. It can also be very occlusive preventing other skin care ingredients from penetrating the skin and many consumers are mindful of materials that originate from the petroleum industry as mineral oil does. Molecular weight and/or chain length correlate with “cushion,” while increase branching results in more “slip” (Gorcea and Laura 2010). Reduction of skin friction is considered at least one cause of improved “slip” effect (Sivamani et al. 2005). INCI: Neopentylglycol dicaprate/dicaprylate is an example of a complex high-molecular weight branched ester with application as a mineral oil replacement. Short chain lower-molecular weight simple linear esters deliver a “silky” and “dry” tactile sensory profile. Some have suggested application as a silicone replacement. This is due to the very “silky” and silicone like tactile sensory profile they deliver. INCI: Ethyl palmate and ethyl oleate are two examples of low-molecular weight simple linear esters with suggested application as silicone replacements as well. The popular application of esters in cosmetic formulation across all product categories and forms is partially due to the abundance of these desirable and deliverable sensory benefits. The global cosmetic industry consumption of esters is estimated in the thousands of metric tons (Gorcea and Laura 2010; Gottschalck and Bailey 2008).