Soaps

Abstract

Soap is one of the oldest known manufactured chemical substances and was first produced thousands of years ago through the reaction of animal fats with the ashes from plants. Early soaps were used primarily for the cleansing of clothing, not for personal hygiene, because of cultural as well as practical reasons. For instance, these animal fat soaps possessed almost unbearable odors and probably contained excessive amounts of unreacted caustics. Today soaps are produced using a variety of processes, including kettle boiling, continuous saponification, and hydrolysis/neutralization, as well as different fats and oils feedstocks, yielding finished materials that possess specifically desired properties for application as personal cleansing products. Soap is one example of a broader class of materials known as surface‐active agents, or surfactants. Surfactant molecules contain both a hydrophilic or water‐liking portion and a separate hydrophobic or water‐repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the aliphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air–water interfaces. It is this special chemical structure that leads to the ability of surfactants to clean dirt and oil from surfaces and produce lather. Although soaps have many physical properties in common with the broader class of surfactants, they also have several distinguishing factors. First, soaps are most often derived directly from natural sources of fats and oils. Fats and oils are triglycerides, i.e., molecules comprising a glycerol backbone and three ester‐linked fatty oils. Other synthetic surfactants may use fats and oils or petrochemicals as initial building blocks, but generally they require additional chemical manipulations such as sulfonation, esterification, sulfation, and amidation. Second, soaps form insoluble complexes, commonly referred to as curd, in the presence of calcium and magnesium ions in solution. Calcium and magnesium ions are the principal metal ions found in water and their level is commonly referred to as the hardness of the water; hard water has high levels of both of these ions, soft water has very low levels. This curd reduces the effectiveness of soap as a surfactant and gives rise to other undesirable properties during use, e.g., precipitation on surfaces. Many synthetic surfactants are considerably less susceptible to water hardness. This water hardness insensitivity has led to the replacement of soap by synthetic surfactants in a variety of applications, such as dish and laundry detergents and shampoos. Although soap is still the predominant material used in personal cleansing products, e.g., facial, body, and hand cleansing, soap‐based personal cleansing products are being rapidly replaced by products that contain increasing amounts of synthetic surfactants to meet changing consumer needs, such as rinsing and lather in hard water and improved mildness to the skin.

Product and process design for structured products

“…The process of making soap bars can be thought of as in two stages: soap making and soap finishing (Villela and Surányi, 1996; Spitz et al, 1996; Spitz, 1996; Burke, 1996). Consider process synthesis for each of these.…”

“…Soap making takes crude fats and oils, aqueous caustic soda, and water as inputs, and produces neat soap, crude glycerin, and waste as outputs (Villela and Surányi, 1996). Neat soap is a lamellar liquid‐crystalline phase, typically comprised of 65% soap and 35% water (Hill and Moaddel, 2004; Burke, 1996). It is the most concentrated form of soap that is easily pumpable, and, hence, serves as a good dividing point between soap making and soap finishing.…”

The Effects of Cold Saponification on the Unsaponified Fatty Acid Composition and Sensory Perception of Commercial Natural Herbal Soaps

“…Formulation of specialty hand-made natural soap bars by artisan soap makers involves a skillful combination of the ingredients, thoughts, and artistic creativity to produce high-quality soap bars with superior sensory characteristics that resonates with consumers [ 1 ]. These sensory characteristics include fragrance, color, lather ability, moisturizing capabilities, hardness, skin compatibility, and chemical stability during storage and use [ 1 , 2 , 3 , 4 ]. Of these, aroma (fragrance) and moisturizing capabilities are considered the most influential determinants of consumer preference for natural soap products.…”

“…Natural soaps are generally defined as alkali salts of fatty acids derived primarily from vegetable or plant oils used as soap feedstock, and contained natural fragrances and/or organic ingredients included as additives. Commercially, natural soaps are manufactured via either a cold or hot saponification process, where triglycerides in fats, oils, and/or free fatty acids used as feedstock are converted in the presence of a base (typically sodium or potassium hydroxide) to form fatty acid salts (soaps), glycerol, and free fatty acids [ 1 , 3 ]. Many artisanal soap makers prefer the cold saponification process due to the enhanced aesthetics of the finished product, potential superiority in retaining antioxidants, or the fragrance from essential oils, and creative flexibilities to customize each ingredient including the addition of fresh ingredients, such as fruits and vegetable purees, to obtain a desired end product.…”

“…Many artisanal soap makers prefer the cold saponification process due to the enhanced aesthetics of the finished product, potential superiority in retaining antioxidants, or the fragrance from essential oils, and creative flexibilities to customize each ingredient including the addition of fresh ingredients, such as fruits and vegetable purees, to obtain a desired end product. Cold saponification uses the heat generated from the combination of the fatty acids (acid) in the melted oils and fats with sodium hydroxide (base) to facilitate the saponification process, which takes 18–24 h to complete, and a further 3–4 weeks to cure the finished soaps [ 3 ]. The sensory and chemical characteristics of natural soaps are dependent on the manufacturing process, and the chemical composition of the feedstock materials used during formulation.…”