Detergents are mixtures of surface-active agents of synthetic origin and other additives which are formulated for the purpose of cleansing. In a broader sense, a detergent is a substance or a mixture of substances intended to assist cleaning.
Detergents are chemical compounds that can effectively remove oil stains and dirt from the surface of solids and cloth materials by breaking them into smaller par- ticles and thereby making them soluble. Defined in this manner, the term covers all substances including soap that are used for the purposes of cleansing dirt and grease from surfaces. In everyday usage and in commercial circles, however, the term is used for a class of substances of synthetic origin used for cleaning purposes.
Therefore, for clarity and for avoidance of doubt, the subsequent discussions will refer to synthetic detergents. In particular, as soaps were discussed in the previous section of this chapter.
Detergents are more resistant to calcium salt formation in hard water and gen- erally provide better wetting, cleansing and surfactancy than soaps. Although the main use of detergents is for cleaning, they are also used in several other products including pharmaceuticals, shampoos, cosmetics, antiseptics, petroleum recovery, foods, cleaning products, toothpaste, vitamin and tablets. The form and composition of a detergent formulation is determined by the application to which it is to be put.
Detergents come in several forms such as powdered, liquid, gel, sticks, sprays, sheets, pastes and bars. The composition of a detergent is determined mainly by the
1.2 Soaps 15
material to be cleaned, the nature of dirt to be cleaned and by the apparatus to be used (manual or machinery). The various formulations are laundry detergents, dish washing detergents and household detergents.
1.3.1 Ingredients Used in the Formulation of Synthetic Detergents
Irrespective of its form or use, detergent products are complex mixtures of several ingredients. The backbone of any detergent formulation, however, is surfactants and builders. Other additives designed to maximize the performance of the main ingredients are also included. These include bleaches, bleach activators, softeners, enzymes, scouring agents, fabric brighteners, anticaking agents, thickening agents, pearlescent agents and opacifying agents, among others. The role of these ingre- dients and specific examples of the ingredients are discussed below.
1.3.1.1 Surfactants
The word surfactant is a shortened form of the expression‘surface active agents’. These are organic chemicals that are capable of changing the properties of water.
They lower the surface tension of water and enable a cleaning solution to wet the surface of clothes and other solid surfaces, so dirt can be readily loosened and removed. Surfactants also emulsify oily soils and keep them dispersed and sus- pended, so that they do not settle back on the surface from which they were removed. Surfactants derive their unique activity from their molecular structure (Fig.1.4). The characteristic features of a molecule of a surfactant are a hydrophilic (water attracting) end and a hydrophobic (oil attracting) tail. Due to this unique structure, a surfactant acts as an emulsifier by bridging the water and oil phases, thereby breaking oil into tiny droplets suspended in water. The disruption of the oil film allows the dirt particles to be solubilized. Surfactants can be tailored for
Fig. 1.4 Molecular structure of surfactant
16 1 Soaps and Detergents
effective use in either aqueous or nonaqueous systems, depending upon their sol- ubility characteristics. An optimum hydrophile–hydrophobe balance generally exists for a specific detergent application and for the compound class used. This optimum composition is arrived at through evaluation of the required hydrophile– hydrophobe characteristics of the application. An example is alkylbenzene, which is slightly surface active in nonaqueous media but insoluble and ineffective in water.
However, upon sulfonation of alkylbenzene into alkylbenzesulfone, it acquires characteristics that make it highly water soluble with excellent surfactant charac- teristics but insoluble in petroleum solvents. If alkylbenzene is di- or tri-sulfonated, the compound becomes more water soluble but loses much of its surface activity.
The two major surfactants used in detergents are linear alkylbenzene sulfonates (LAS) and the alkyl phenol ethoxylates (APE). As shown in Fig.1.5, surfactants are classified into four main groupings, namely, as; anionic, cationic, nonionic or zwitterionic (also known as amphoteric surfactants).
i.Anionic surfactants.
Anionic surfactants are negatively charged molecules that determine the effec- tiveness of the detergent. These anions react with basic components, thereby resulting in better cleansing action. In detergents, anionic surfactants are used for cleaning automobiles, washing metal articles, washing of clothes and as hair cleansers. The main anionic surfactants are:
Sulfonates: The sulfonate group—SO3M, attached to an alkyl, aryl or akylaryl hydrophobe, is a highly effective solubilizing group. Examples are sodium alkyl- benzene sulfonates (ABS), which are the most widely used non-soap surfactants, short-chain alkylarene sulfonates, lignosulfonates, naphthalene sulfonates, o-olefin sulfonates, petroleum sulfonates, sulfonates with ester, ether and amide linkages and fatty acid ester sulfonates. Sulfonates interact moderately with the hardness of Ca2+ and Mg2+ ions. They are used in the manufacture of light and heavy-duty liquid detergents and as spray-dried detergents for institutional and industrial cleaning applications and as car washing compounds. They can be tailored for specific applications by the introduction of double bonds, ester or amide groups either into the hydrocarbon chain or as substituents. Sulfonates are among the high-volume surfactants used in the preparation of detergents.
Hydrophilic portion Hydrophobic portion Fig. 1.5 The different types
of surfactants
1.3 Detergents 17
Sulfates: The sulfates represent the sulfuric acid half ester of an alcohol. The sulfate group is more hydrophilic than the sulfonate group because of the presence of an additional oxygen group. Examples are alcohol sulfates, ethoxylated and sulfated alcohols, ethoxylated and sulfated alkylphenols and sulfated natural oils and fats.
N-Acylsarcosinates: An example in this group is sodium N-lauroylsarcosinate. The amido group in the hydrophobic chain lessens interactions with water hardness ions. They are prepared from the reaction of fatty acids with sarcosine.
Acylated protein hydrolysates: These are mild surfactants used in personal care products. They are prepared by acylation of protein hydrosylates with fatty acids or acid chlorides.
Phosphate esters: Examples are the mono- and di-esters of orthophosphoric acid and their salts. Unlike the sulfonates and the sulfates, the resistance of the alkyl phosphate esters to acids and hard water is poor. That is, their calcium and mag- nesium salts are insoluble. In the acid form, the esters show limited water solubility, although their alkali metal salts are more soluble. The surface activity of phosphate esters is good but somewhat lower than their phosphate-free precursors. At higher temperatures, however, the phosphate surfactant is significantly more effective.
Phosphate surfactantsfind applications in specialty situations due to their high costs and limitations. Their applications include dry-cleaning compositions where solu- bility in hydrocarbon solvents is a particular advantage, textile mill processing where the stability and emulsifying power for oil and wax under highly alkaline conditions are necessary, and in industrial cleaning, compositions where tolerant for high concentrations of electrolyte and alkalinity are required. They are also used as corrosion inhibitors.
The anionic surfactants mostly used in detergents are the alkyl sulfates, alkyl ethoxylate sulfates and soaps.
ii.Cationic Surfactants
Cationic surfactants have positively charged heads when in solution. Three types of cationic surfactants can be distinguished based on their applications. Those used in fabric softeners where they provide soft feel to fabric. An example of cationic used for this purpose is esterquat. In laundry detergents, they are used in conjunction with anionics to boost the performance of anionic surfactants and leading to improved dirt removal systems, and especially very efficient stain removal. An example of a cationic surfactant used in this category is mono-alkyl quaternary system. In household and bathroom cleaners, cationic surfactants play a role as disinfecting and sanitizing properties of detergents. Examples of cationic surfac- tants are primary, secondary or tertiary amines, benzalkonium chloride (BAC), cetylpyridinium chloride (CPC), polyethoxylated tallow amine (POEA) and alkyl trimethylammonium salts, namely, cetyl trimethylammonium bromide (CTAB), hexadecyl trimethylammonium bromide, cetyl trimethylammonium chloride (CTAC).
18 1 Soaps and Detergents
iii.Nonionic surfactants
The surfactants in this category do not carry any electrical charge. This makes them resistant to water hardness deactivation. Among the most commonly used nonionic surfactants are ethers of fatty alcohols. Nonionic surfactants are excellent grease removers. Their major area of application is in laundry detergents, hand dish- washing liquids and in household cleaners. Nonionic surfactants are often used alongside anionic surfactants as they complement each other's cleaning action.
When present, they help in making surfactant systems less hardness sensitive.
Examples include cetyl alcohol, stearyl alcohol, cetostearyl alcohol, oleyl alcohol, octaethylene glycol monododecyl ether, pentaethylene glycol monododecyl ether, glucoside alkyl ethers and glycerol alkyl esters among others.
iv.Amphoteric (Zwitterion) surfactants
These are special class of surfactants. The molecules contain both positive and negative charges. This bestows them with properties of both cationic and anionic surfactants. Amphoteric surfactants anionic at pH > 7, cationic at pH < 7 and have no charge at pH = 7. They are compatible with both cationic and anionic surfac- tants. Amphoteric surfactants offer high performance only at neutral pH. They are, thus, preferred for use in personal care formulations such as hair and body sham- poos, bubble baths and other toiletries and cosmetics. Amphoteric surfactants are relatively expensive because of the raw materials involved and the production costs.
Examples of surfactants in this group are imidazolinium derivatives and amino alcohols and their derivatives, e.g. diethylcarboxymethyl coconut fatty acids, propylamide ammonium betaine, alkylamide betaine and cocomidopropylamine oxide.
Evaluation of Surfactants
Surfactants are evaluated based on their performance in the following areas;
foamability, wetting, emulsifying, detergency, skin and eye irritation and compat- ibility with /other surfactants. For the four surfactants discussed, their performance infive of these indicators is listed in Table1.4.
1.3.1.2 Builders
In addition to surfactants, other backbone substances of detergents are builders.
They are added to detergents to extend their cleaning performance across a wide range of use conditions. Combining surfactants and builders lead to a synergistic effect to boost total detergency and cleaning efficacy as compared with an equal amount of either compound alone. The most important function of detergent builders is as sequestering agents. Other characteristics required of good builders are:
i. It must be able to control water hardness and metal ions in water.
ii. It must contribute to thefinal product alkalinity.
iii. It should be able to provide buffer capacity in the proper pH range.
1.3 Detergents 19
iv. It must have deflocculating capability.
v. It must be compatible with other formulation ingredients and additives.
vi. It must be safe to use by consumers.
vii. It must not hurt the environment.
viii. It should not pose any problem to the processing of the detergent and must itself be easy to process.
ix. Its performance and cost must be adequate.
Builders are used singly or in combination with other builders to contribute unique properties forfinal product performance enhancement. Some of the most commonly used builders are listed below.
i.Phosphates
These include sodium tripolyphosphate-STP (Na5P3O10), which is the major builder ingredient used in heavy duty laundry detergents, automatic dishwashing formu- lations and industrial and institutional cleaners. It has superior detergent processing, solubility and hardness ion sequestering characteristics. Another phosphate builder is tetrapotassium pyrophosphate, which is used in industrial and institutional liquid detergents. It has good solubility and sequestering characteristics. Others in this category are tetrasodium pyrophosphate-TSPP (Na4P2O7), sodium trimetaphos- phate, trisodium phosphate and tripotassium phosphate.
ii.Silicates
Principal among these are sodium silicate and potassium silicate. They are used either as solids or in solution. They find use as emulsification, buffering, defloc- culating and anti-redeposition agents. They also provide corrosion protection to metal paints in washing appliances and enhance the sequestering ability of other builder systems. Their performance depends on the ratio, silicate/alkali (SiO2/ Na2O), selected for the formulation. For example, for dry blending applications, the desired ratio is 1:1. For laundry and automatic dishwashing applications, the desired ratio is 2:1.
Table 1.4 Performance indicators of the different types of surfactants Indicator Ionic surfactant Cationic
surfactant
Nonionic surfactant
Amphoteric surfactant
Foaming Very good Good Low to
average
Very good
Emulsifying Good Poor Very good Very good
Eye and skin irritation
Mild Very mild – –
Detergency Very good Poor Very good Very good
Compatibility Incompatible with cationics
Incompatible with anionics
Very good Very good
20 1 Soaps and Detergents
iii.Sodium citrate (Na3C6H25O7).
It is the preferred builder in liquid consumer laundry detergents. It sequesters water hardness ions and deflocculates soils. It has limited use in powdered products.
ivSodium carbonates
They are used mostly in granular laundry detergents, automatic dishwashing for- mulations, hard surface cleaners and in presoak formulations. It precipitates hard- ness minerals, provides alkalinity for detergents and facilitates soil dispersion. It is sometimes used in combination with zeolite as a substitution for sodium tripolyphosphate.
v.Zeolites (e.g. Na2.Al2O3.2SiO3.4.5H2O)
Zeolites are the most widely used builders for laundry detergents. They are not soluble in water but reduce water hardness by ion exchange. The calcium (Ca) ions pass through the zeolite pore opening and exchange with the sodium (Na) ions.
Magnesium (Mg) ions, because of their larger size are not removed. They are used in conjunction with other builders such as sodium carbonate to control magnesium and other ions present in water.
vi.Sodium nitrilotriacetate (N(CH2COONa)3
It has excellent sequestrant and chelating ability. It is, however, considered to be carcinogenic. Therefore, in some countries, it is not allowed in personal care, hand dish-washing detergents and foods. It is nonetheless widely used in industrial and institutional cleaning products. It is normally used in detergents and as a supple- ment to phosphates in spray-dried detergents.
1.3.1.3 Detergent Additives
Other additives used in detergents and the functions they play are listed in Table1.5.
1.3.2 Laundry Detergents
A laundry detergent is any of the various detergents designed for use in washing clothes. In general, these help in the breakup and removal of dirt and stains in the laundry and are available as liquids, powders, gels, sticks, sprays, pumps, sheets or bars. All laundry detergents are soluble in water. They function in a wide range of temperature and water conditions. They are formulated with the objective that they can remove soil and stain from fabrics to bleach fabrics and as fabric softening agents. They are also used for conditioning fabrics and as disinfectants. Liquid laundry detergents are particularly good at removing greasy and starchy food stains.
Powdered laundry detergent is especially effective on heavily soiled clothing.
Enzymes are often added to laundry detergents to improve their effectiveness in breaking down complex food stains.
1.3 Detergents 21
Like all detergents, the key ingredient in laundry detergents is the surfactant.
Most laundry detergents use anionic or nonionic surfactants or a mixture of the two, although cationic surfactants are sometimes used in laundry detergents. Cationic and anionic surfactants are not used together in detergents as they are incompatible.
The usual content of surfactants in a typical detergent is about 8–18%. Other additives are added to the surfactants to improve its efficiency. For instance, laundry Table 1.5 Additives employed in commercial detergents
Substance Functions in detergents
Sodium sulfate (Na2SO4) Improvesfinished productflow characteristics in dry mix and agglomerated products. In spray-dried products, it acts as an inert‘filler’and aids in density control andflow
characteristics
Sodium chloride (NaCl) Improvesfinished productflow characteristics in dry mix and agglomerated products. In spray-dried products, it acts as an inert‘filler’and aids in density control andflow
characteristics Sodium carboximethyl
cellulose (C8H15NaO8)
It reduces soil redeposition during detergent application (at formulation levels of 0.1 to 0.5%)
Optical brightners They improve the whiteness or brightness of fabrics. They are used in both liquid and powdered products. All detergents contain one or more optical brighteners
Hydrophobes Used in liquid detergents to improve the solubility of less soluble components and also improve storage stability. They are also used occasionally as viscosity modifiers during spray-dried processing. Examples are ammonium, potassium or sodium salts of toluene, xylene or cumene sulfates Pearlescent agents Gives aesthetic feel to fabrics. Examples are bismuth and
titanium dioxide-coated mica
Opacifying agents They reduce translucence, modify the viscosity characteristics of liquids and provide a creamlike texture or pearlescent effect on products. Examples are water-soluble salts of styrene and maleic anhydride
Thickening agents They provide rheological modification to automatic dishwashing liquids. An example is betonite clay. Use level in automatic dishwashing liquids is 0.5 to 1.0%
Anticaking agents Prevent caking in detergent systems, especially in humid climates. Examples include re sodium benzoate, tricalcium oxide, calcium stearate, and microcrystalline cellulose Enzymes They are used to remove stains. They break down
carbohydrates, proteins and other soil materials to forms readily removed by detergent. They are used in presoak detergent systems and some liquid and powdered laundry detergents
Fillers Used to adjust the active matter in the detergent to the doses used. Examples include sodium sulfate in powders, water and other solvents in liquids
22 1 Soaps and Detergents
detergents may have ingredients to help control the pH of the wash water. Sodium carbonate or sodium bicarbonate is often added to solid detergents to neutralize acidic materials that may enter the wash water and, thus, help maintain its pH.
Builders such as trisodium orthophosphate, monosodium orthophosphate or a form of tripolyphosphate (TPP) may be used to enhance (‘build’) the surfactant effect.
They lower the water hardness by scavenging the calcium and magnesium ions and adsorbing them or chelating them. In place of the phosphates, other chelating agents may be used. Sodium carbonate precipitates insoluble calcium carbonate and magnesium carbonate. Organic chemicals similar to ethylenediaminetetraacetic acid (EDTA) can be employed, for example, nitriloacetic acid (NTA). Borates and ion exchange materials may also be used as builders in detergents. The usual content of builders in a typical detergent is about 20–45%.
Many detergents contain bleaches. Sodium hypochlorite-based bleach additives are more commonly used in certain conditions. Peroxide-based bleaches are also used. These contain compounds that release hydrogen peroxide when in use.
Examples of such compounds are sodium percarbonate and sodium perborate.
Peroxide bleaches either need higher temperature (60 °C or more) to become effective or a suitable catalyst or activator (e.g. manganese or iron complexes or Tetraacetylethylenediamine—TAED), which lowers the required temperature down to 40 °C or even to room temperature. The usual content of bleaches in a typical detergent is about 15–30%. Optical brighteners, usually at a concentration of about 0.1%, may also be added to laundry detergents to improve the whiteness of the cloth. These compounds act by converting some ultraviolet radiation to blue light and, thus, optically offsetting the yellowing of the material. Fillers are added to detergents to modify the physical properties of the material. In solid detergents, sodium sulfate or borax can be used to make the powder-free flowing. Other additives used in liquid detergents include alcohols, corrosion inhibitors and anti-foaming agents. Alcohols increase the solubility of the compounds and decrease the mixture’s freezing point. Corrosion inhibitors when present in deter- gents protect the washing machines and prolong their lifetime while anti-foaming agents reduce foam production. The percentage composition offillers in a typical detergent is about 5–45% (wt).
Enzymes are used in some laundry detergents to help in the removal of bio- logical stains. Often, the enzymes produced by the bacteria Bacillus subtilis and Bacillus licheniformisare used. The content of enzymes can be as high as 0.75%.
Some laundry detergents have fabric softeners and a perfume or color ingredients to give better smell or to give a detergent some color.
1.3.3 Dishwashing Products
These include detergents for hand and machine washing as well as some specialty products. They are available in liquids, gels, powders and solids. Hand dishwashing detergents remove food soils, hold soil in suspension and provide long-lasting suds that indicate how much cleaning power is left in the wastewater. Automatic
1.3 Detergents 23