The most often purchased personal care items are shampoos and body washes. All of these items are designed to clean the skin and hair by removing excess oil and debris. However, a shampoo or body wash should do more than just clean. The modern customer also anticipates that these items will clean and condition the skin, make washing the body more accessible, and leave a pleasant, lingering scent. 2 In Chapter 20, Dr. Herman discusses the sense of smell. The fundamental scientific principles of sanitation will be discussed here. The current crop of hygiene products is mainly concerned with eliminating smells and the microorganisms that produce them from the human body's outermost layers. However, the favorable impacts of skin colonization by a broad milieu of microorganisms are starting to be recognized, and the relevance of the symbiotic connections connected with the human microbiome is becoming more well recognized. To be clear, one must not emit a perspiration stench. Sweat's distinctive odor is produced when bacteria combine with apocrine gland secretions. 4–6 Cleansing products may help by neutralizing the smell and the microorganisms that cause it. A lipid-rich material called sebum is secreted by sebaceous glands next to hair follicles. Sebum, a semifluid secretion of mammalian sebaceous glands composed chiefly of fat, keratin, and cellular material, is often removed for aesthetic purposes. 7 To keep the skin and hair from drying out, sebum is produced. Facially anaerobic bacteria like Propionibacterium acnes thrive in sebaceous secretions. Free fatty acids are released onto the skin when P. acnes hydrolyzes the triglycerides in sebum, which may be found in abundance in the pores of the skin. 10,11 The skin's surface pH is acidic because of the released fatty acids. This prevents the spread of several bacteria and viruses, including Staphylococcus aureus and Streptococcus pyogenes (see references 12, 13). 
14 The symbiotic microorganisms that sebum provides a home for may be good for the skin. However, modern consumers view sebum buildup on the skin and hair as "unclean" and undesirable. In addition, the sebum layer can attract dirt and dust particles, contributing to an overall uncleanliness impression. One of the goals of this chapter is to cover the physical and chemical sciences that provide the basis for modern washing products and processes, with the end goal being the elimination of oils, dirt particles, and germs from the surface of the skin and hair. Sebum is an oil; washing it off with water alone won't work. This is why most products designed for this purpose also include surfactants or surface-active agents. In addition to dispersing particles, surfactants may also reduce the interfacial tension between the soil and substrate, emulsify oily soils, and/or solubilize soils. It's essential to grasp why oil and water don't mix before diving into the surfactants' mechanism of action. For instance, sugars and salts dissolve because water's interaction with their component ions or molecules is more favorable than the interaction between the salt ions or sugar molecules themselves. The propensity for solute molecules to diffuse through a solid and into a solution diminishes as the solute concentration rises. When the solute's tendency to escape the solution (in thermodynamics, this is referred to as the chemical potential) is equal to its propensity to separate from the solution (precipitate), saturation has been attained. Insoluble in water may refer to a number of various properties. Sand, clay, and glass are all insoluble in water for the same reason: the attraction between sand molecules is more potent than that between water molecules. This is because the free energy required for water to interact with the sand's individual silicate groups is more significant than that needed for the silicate groups to interact with each other. 
Surfactants' role is to improve wetting and facilitate dispersion for such particles. When exposed to water, oils and waxes remain insoluble because of their hydrophobic nature. 15,16 When oils are introduced to water, they are quickly pushed out in order to reduce the water-oil interface area since the intermolecular interactions between oil molecules are much lower than those between water molecules. This structure of water at the oil-water interface reduces entropy in the system. To combat this loss of entropy, the system forces the oil to split from the water, which decreases the surface area on which the two substances may interact. Surfactants serve this goal by dissolving oils and waxes and decreasing surface and interfacial tensions. When intermolecular forces between molecules are unbalanced at the gas-liquid interface, the result is surface tension. The majority of molecules in a liquid are attracted to each other from all directions. However, surface molecules experience an unbalanced set of forces since they are drawn to the liquid molecules below but have little to no contact with the gas molecules beyond the liquid-vapor border. Surface tension is the two-dimensional force at the surface caused by this mismatch. Linear units (such as millinewtons per meter) are often used to describe surface tension. In terms of joules per square meter, surface energy is defined as the amount of labor required to cover a particular area. Surface tension and surface energy have the same dimensions and have the same absolute value. In comparison, water's surface energy and surface tensions are 0.072 J/m2 and 0.072 N/m, respectively. One may infer the intensity of intermolecular forces from the amount of surface tension. Water's surface tension is relatively high (0.072 N/m at ambient temperature) due to the presence of hydrogen bonds, dipole-dipole interaction, and dispersion forces between its molecules. 
Surface tension in hydrocarbons is minimal (0.020-0.030 N/m) because only dispersion forces are present between the molecules. The surfactant molecules have two moieties: a water-repellent hydrophobic segment and a water-interacting hydrophilic segment. For this reason, molecules of surfactants are referred to be amphipathic (Amphi meaning "dual" and pathic from the same root as pathos, which may be translated as "suffering") since they "suffer" both oil and water. The hydrophilic moiety is more at home in the water phase, whereas the hydrophobic moiety may mix with oil. There might be either nonionic or ionic, or cationic hydrophilic groups. The hydrophilic moiety is often a hydrocarbon; however, silicones and fluorocarbons are also viable alternatives. Surfactant molecules in oil-free aqueous phases are pushed to the surface by adsorption at deficient concentrations due to their amphipathicity. Hydrophobic contact, which prevents the hydrocarbon from dissolving in water, is the main factor behind surface adsorption. As a result of relatively strong interactions between the hydrophilic moieties and water at the surface, the adsorbed surfactant molecules stay in close contact with water. Polar, ionic, Lewis acid/Lewis base, and London dispersion forces are all examples of strong interactions. 
Because of this adsorption, the concentration of surfactant at the surface is substantially higher than that towards the center of the solution. Traube's rule states that at very dilute concentrations, the adsorbed surfactant is much higher than the concentration in the solution. According to Traube's government, the surface concentration rises by a factor of three relative to the bulk concentration for every CH2 group in an alkyl chain. 17 Gibbs created the term "surface excess concentration" to describe the difference between a surfactant's adsorbed concentration and its bulk concentration. 18 Traube's rule states that the excess surface concentration of dodecyl-chain soap must be more than 500,000,000 times that of the bulk solution. Surfactant molecules on the surface behave like a two-dimensional gas at deficient concentrations. When surfactant concentration is high enough, molecules begin to interact, but they still move freely in the plane. This is because they are still acting as two-dimensional liquids. At even greater concentrations, the surfactant saturates the surface, and the hydrophobic groups orient off of the surface plane, causing the surfactant monolayer to behave like a two-dimensional solid due to interactions between surrounding hydrophobic groups. 19 When a monolayer is formed by the adsorption of a sufficient number of surfactant molecules, the hydrophobic groups of the surfactant become the dominant surface property, and the surface energy becomes dominated by the surface energy associated with hydrophobic group interaction. 
Pure liquids have very high surface energy and are resistant to surface expansion. It is well known that surfactant solutions readily form foams because their surface adsorption promotes interface expansion. The adsorbed surfactant provides surface structure to the foam, which increases its stability. 20
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