Liquid Detergents (Surfactant Science Series)
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Because air is not hydrophilic, detergents are also foaming agents to varying degrees. Detergents are classified into three broad groupings, depending on the electrical charge of the surfactants. Typical anionic detergents are alkylbenzenesulfonates. The alkylbenzene portion of these anions is lipophilic and the sulfonate is hydrophilic. Two different varieties have been popularized, those with branched alkyl groups and those with linear alkyl groups.
The former were largely phased out in economically advanced societies because they are poorly biodegradable. Bile acids , such as deoxycholic acid DOC , are anionic detergents produced by the liver to aid in digestion and absorption of fats and oils. Cationic detergents that are similar to the anionic ones, with a hydrophilic component, but, instead of the anionic sulfonate group, the cationic surfactants have quaternary ammonium as the polar end.
The ammonium sulfate center is positively charged. Non-ionic detergents are characterized by their uncharged, hydrophilic headgroups. Typical non-ionic detergents are based on polyoxyethylene or a glycoside. Common examples of the former include Tween , Triton , and the Brij series. These materials are also known as ethoxylates or PEGylates and their metabolites, nonylphenol. Glycosides have a sugar as their uncharged hydrophilic headgroup. Examples include octyl thioglucoside and maltosides. See surfactants for more applications.
SAS are seen as special anionic surfactants for consumer products. SAS include high solubility, fast wetting properties, chemical stability and they are very similar to LAS in terms of detergency properties and water hardness sensitivity. They are completely insensitive to hydrolysis, even at extreme pH values, a result of the presence of the stable carbon—sulfur bond.
Journal of Surfactants and Detergents (v, #6) | ybaruxelyl.cf
The share of nonionic surfactants in overall surfactant production and use has been increasing steadily since the s [ 3 ]. The major contributors to this increase have been ethoxylates of fatty alcohols, oxo-alcohols, and secondary alcohols obtained by reaction of the corresponding alcohols with ethylene oxide. The higher use of nonionic surfactants in detergent formulations has partly been concomitant with the trend to wash at lower temperatures and with changes in the production shares of different fibers. The most important nonionic surfactants are:.
AE are the most important nonionics in detergent formulations. By varying the length of carbon chain and the degree of ethoxylation, these nonionic surfactants can be tailor-made with respect to the washing temperature. APE are based on p -octyl-, nonyl-, and dodecylphenol poly ethylene glycol ethers. They achieved an early success due to their exceptional detergency properties, particularly their oil and fat removal characteristics. The usage of APE has largely declined, especially in Europe since , due to a self-obligation of the industries to abandon their use.
Their low biodegradability and the fish toxicity of certain metabolites resulting from partial biodegradation caused considerable environmental problems [ 13 ]. FAA alone have little application in laundry detergents. Their most important feature is foam boosting, i. This property is not desirable for horizontal-axis drum-type washing machines employed, e.
Nevertheless, small amounts of FAA as co-surfactants are capable of enhancing the soil removal properties of the classical detergent components at low washing temperatures. AO are produced by oxidation of tertiary amines with hydrogen peroxide.
2 Surfactants and their applications
They show cationic behavior at acidic conditions and behave as nonionic surfactants at neutral or alkaline conditions. Despite good detergent properties, they are rarely included in laundry detergent formulations. The reasons for this are high costs and low thermal stability. NMG are a new type of nonionic surfactants that has been introduced in detergents in the s. They are increasingly used as co-surfactants in powder and liquid detergent formulations. APG consisting on an alkyl chain hydrophobic and sugar derivates hydrophilic have distinct lathering characteristics, especially in combination with anionic surfactants [ 14 , 15 ].
Due to their good foaming properties APG are predominantly used in dishwashing detergents, liquid detergents, and special detergents for fine fabrics. Since APG are completely based on natural resources, they ultimately biodegrade to carbon dioxide and water under aerobic conditions.
Cationic surfactants in detergent formulations are used as fabric softener in washing processes [ 3 ]. The most important ones are quarternary nitrogen compounds [ 16 ]:. The first surfactant developed in this category was DTDMAC, introduced in as a fabric softener for cotton diapers and presented to the U. Due to their ester bonds which are potential breaking points, esterquats are readily biodegradable in contrast to DTDMAC. Esterquats possess favorable ecotoxicological and toxicological properties.
Alkylated imidazoline derivatives are also used as fabric softeners and, due to dermatological compatability, as body care products. Amphoteric surfactants possess both anionic and cationic groups in the same molecule even in aqueous solution [ 3 , 18 ]. Despite excellent detergent properties, these surfactants are only rarely employed in laundry detergents, primarily for cost reasons. Among amphoterics, the betains are of economic importance.
Betains are insensitive to water hardness, are only slightly toxic and compatible with the skin. They are mainly used in manual dishwashing and body care products. The most important types of amphoterics are:. The surfactants are not only used in the detergent industry but in other fields such as cosmetics, metal working, paper and leather industry.
Half of the total surfactant consumption belongs to household application which is the largest market for surfactants. A brief summary of the surfactant statistics in Europe from is shown in Figure 2. Most produced surfactants belong to the anionic and nonionic group, together they cover nearly half of the production volume. The other surfactant types are produced with much lower volumes. The nonionics, especially the ethoxylates, have passed the anionics in production volume and captive use. Biodegradation means the microbial degradation of organic substances.
Depending on the degradation result, biodegradation with respect to surfactants is defined as follows [ 5 ]:. Primary biodegradation means the structural change transformation of a surfactant by micro-organisms resulting in the loss of its surface-active properties due to the degradation of the parent substance and consequential loss of the surface-active property. Ultimate biodegradation means the level of biodegradation achieved when the surfactant is totally used by micro-organisms resulting in its breakdown to inorganic end-products such as carbon dioxide, water and mineral salts of any other elements present mineralization and new microbial cellular constituents biomass.
Ready aerobic biodegradability is an arbitrary classification of surfactants which have passed certain specified screening tests for ultimate biodegradability; these tests are so stringent that it is assumed that such compounds will rapidly and completely biodegrade in aquatic environment under aerobic conditions. In opposite to former Detergent Guidelines which only required a primary biodegradability of anionic and nonionic surfactants the actual EU legislation prescribes the ultimate aerobic biodegradability of all surfactant types to be used in the detergent industry.
Data on toxicity and biodegradability of surfactants have been collected in the Detergent Ingredient Database DID-list [ 20 ]. Anaerobically biodegradable surfactants are included on the DID-list. Anaerobic biodegradation means the microbial degradation of organic compounds under conditions free of molecular oxygen. In opposite to aerobic biodegradation pathways, where organic compounds often are mineralized by one type of microorganisms, the anaerobic biodegradation of a substance up to inorganic end-products always requires the co-operation of different types of microorganisms.
The mixed culture works like a food chain, where the produced metabolites of one organism are the substrate for the next one In the first step, complex or polymeric organic compounds are utilized by fermentative bacteria. Products of hydrolysis and acidification are metabolites of low molecular weight such as alcohols and short-chain fatty acids C2—C4 organic acids.
Acetogenic bacteria subsequently utilize these fermentation products as substrate and transform them to acetate, carbon dioxide and molecular hydrogen. At the end of the food chain, the methanogenic bacteria use acetic acid, carbon dioxide and hydrogen for the production of biogas—a mixture of methane and carbon dioxide. Carbonate can be used as hydrogen acceptor.
Methanogenic bacteria are often the bottleneck of anaerobic biodegradation processes due to slow growth rates. These bacteria additionally are very sensitive against acidic condition; the optimal range of pH is 7—8. At pH lower than 6. Another bottle neck of anaerobic biodegradation may be the first reaction step hydrolysis and acidification , especially if organic compounds of low bioavailability are used Figure 3.
Anaerobic biodegradation pathway [ 21 ]. In the first step, complex or polymeric organic compounds are utilized by fermentative bacteria. Another bottle neck of anaerobic biodegradation may be the first reaction step hydrolysis and acidification , especially if organic compounds of low bioavailability are used. In the presence of relevant amounts of nitrate and sulfate, alternative biodegradation pathways may occur like denitrification and sulfate-reducing processes, where nitrate and sulfate serve as hydrogen acceptors instead of carbonate anoxic reactions.
In marine sediments, the sulfate-reducing process is the predominant biodegradation pathway compared to methanogenesis. In contrast to the obligate anaerobic methanogenic and sulfate-reducing bacteria, the nitrate-reducing bacteria in general are facultative anaerobic, meaning that they are able to grow under aerobic and anaerobic conditions. The biodegradation is influenced by several factors [ 21 ]:. For degradation of organic compounds at significant rates, an appropriate number of relevant microorganisms are needed.
In biodegradation tests and technical biodegradation processes, the reaction can be started with an initial supply of microorganisms which are adapted to special conditions e. During metabolization of the compound, the microorganisms proliferate and adapt to the special reaction conditions. Microbial biodegradation in continuously operated reactors can be considered as a self optimizing system.
Convenient ambient conditions are a prerequisite for an optimal biodegradation process. Sufficient water content is the major factor for all biological processes. Temperature and pH are also important factors influencing microbial metabolism, and microorganisms differ greatly in their specific optimum in temperature and pH-value.
Nutrition with macro and micro nutrients e. If a complex organic substrate is used as reaction matrix, no further nutrition may be needed. Inhibition of microorganisms, which can be caused by the substrate itself or by degradation metabolites or products, may also occur. Most of the surfactants are known to cause microbial inhibition effects. Reduced bioavailability of the organic compound is often a limiting factor in biodegradation processes. The bioavailability of an organic substrate mainly depends on its chemical fate, its dissolution rate and the mass transfer e.
Surfactants tend to adsorb to solid particles, and some of the surfactants show rapid precipitation with water hardness ions such as calcium and magnesium [ 12 ]. Only water soluble molecules can be metabolized by microorganisms, so that biodegradation of adsorbed surfactants can be a function of the mass transfer rates rather than the degradation rates.
The bioavailability of a compound, especially the adsorption behavior, has to be considered in a biodegradation test design. The biosphere is mainly aerobic. Anaerobic conditions in the environment occur where the oxygen consumption by biological oxidation processes exceeds the oxygen supply. This can either happen in small anaerobic sectors in an otherwise aerobic system, or in large and stable compartments such as marine or freshwater sediments, moorlands, and poorly drained soils. An overview and description on anaerobic compartments is given in [ 6 ].
Soils are typically aerobic systems, even though anaerobic micro sites may arise in poorly drained soils and may have a short depth of aerobic layer. As a general rule an anaerobic environment can be found in soils at a depth below 1 meter, the so called terrestrial subsurface.
Ionic Liquid-Based Surfactant Science: Formulation, Characterization, and Applications
Moorlands are a typical anaerobic environment. The oxygen is used by microorganisms which may cause oxygen deficiency. Flooding of soil may also lead to anaerobic conditions for a short time. Surfactants may reach the soil environment by application of surfactant loaded sewage sludge to agricultural land or landfill [ 22 ]. Since nearly all the surfactants in household detergents go the waste water pathway, the aquatic environment is an important environmental compartment for potential surfactant pollution [ 6 , 23 , 24 , 25 ].
Freshwater environments include rivers with high exchange of water and lakes where water exchange may be limited by seasonal or permanent hydrographic conditions. In sediments with high oxygen consumption an anaerobic water body may arise.
Freshwater lakes with depths greater than 10 m usually generate an anaerobic layer above the sediment. The largest anaerobic water bodies can be found in marine ecosystems such as the Black Sea being anaerobic from a depth of about meters to 2, meters. In marine sediments, sulfate-reducing bacteria dominate the anaerobic biodegradation processes. Generally, lakes are much more sensitive to organic pollutant than rivers. Because of limited oxygen supply due to lower water exchange, discharge of waste water highly loaded with organic compounds into lakes results in fast oxygen consumption with subsequent anaerobic conditions.
Sulfate-reducing processes result in production of hydrogen sulfide which affects higher organisms living in the lake. In freshwater sediments the aerobic biodegradation is the predominant decomposition process. Influenced by season, organic load, water depth and flow, the sediment in rivers and lakes is usually anaerobic some mm below the surface. In rivers, the sediments are subject to dynamic processes involving sediment generation, transport, and erosion, influenced by several factors such as water flow rate, particle size of solid matter, and turbulent flow.
Because of these processes, oxygen is brought to the sediments stimulating aerobic degradation processes. Lake sediments are less exposed to dynamic changes compared to river sediments which results in settlement of solid particles in much higher amount. If organic material is available in relevant concentration, microbial oxygen consumption effects broad anaerobic sediment zones.
Compounds persistent under anaerobic conditions may be fixed in lake sediments and can be used as tracers to reconstruct the history of their release into the environment over some decades [ 26 ]. The aerobic zone of marine sediments can vary from a few mm in coastal areas to more than 1 m in deep sea sediments, depending on oxygen diffusion into sediment pore water, tidal flushing etc. In the anaerobic zones, the biological sulfate-reducing process is the predominant step in biodegradation of organic matter. Groundwaters may become anaerobic if they are contaminated with organic compounds which are biodegraded by aerobic bacteria.
Therefore, the fate of surfactants in WWTP is of great importance. The biodegradation of organic waste water compounds in WWTP is usually performed under aerobic conditions but there are some special treatment steps working under anaerobic conditions such as:. Septic tanks which act as settling tanks for solids in domestic sewage that need to be periodically emptied periodically e.
In the food processing industry, waste water with high organic pollution is often treated anaerobically. In most cases a post-aerobic treatment is used as a second step to reduce the anaerobic non-degraded residues including surfactants which may present. Several systems for testing biodegradability are available [ 28 , 29 ]. Most of them have been developed for determination of aerobic biodegradability of substances and only a few for testing biodegradability under anaerobic conditions. In both cases it has to be distinguished between screening tests for determination of basic biodegradability under stringent conditions and test systems at simulation level for the assessment of biodegradability under more realistic conditions Figure 4.
On the first test level screening tests are performed. These are characterized by a simple test design batch test making them suitable for routine testing. The test conditions may differ considerably from realistic environment situations.
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It is a common feature of screening tests aerobic and anaerobic that they are more stringent e. Positive degradation results of these tests are mainly independent on real environmental conditions and can be considered as highly predictive for good biodegradation of the tested compound in different environments. If positive results of ultimate biodegradation are achieved in screening tests, further testing is not necessary. Feature of testing biodegradation aerobic or anaerobic [ 10 ].
Otherwise, a poor degradation result in screening tests is not necessarily a proof of recalcitrance in real environment. In this case more representative tests under real-world conditions simulation tests should be performed on the second test level. Simulation tests are continuously working systems with more expenditure of test design, chemical analysis and test duration. Positive results of ultimate biodegradation in simulation test are indicators of ultimate biodegradation of the substance in the environment. If primary biodegradation is measured only, further assessment has to be made for study of possible metabolite behavior.
If neither ultimate nor primary biodegradation is determined in the simulation test, this is highly indicative that the test substance is not degraded in the environment. A comparison of main characteristics of screening and simulation test is given in Table 1.
Characteristics of screening and simulation test [ 10 ]. Because of the complex test design, the high expenditure, and long test duration, simulation tests are not suitable for routine testing. Most of surfactants have been tested using screening tests. Depending on analytical methods different biodegradation types can be evaluated. The different parameters describing anaerobic degradation are shown in Table 2. Specific analysis of the test substance is a parameter for determination of primary biodegradability.
A decrease of test substance measured by specific analysis may be caused by several reasons and is not specific for biodegradation. Metabolites of the degradation process are usually not determined. The measurement of organic carbon content in liquid and solid phases covers degradation processes, adsorption, production of metabolites, and transformation of test substance in biomass.
Additionally, carbon analysis in liquids and solids and calculation of carbon content in biogas allow a calculation of a carbon balance. Biogas production is a parameter of anaerobic mineralization, i. In such mineralization tests the ultimate biodegradability of an organic compound is determined. Therefore, testing mineralization is the most reliable method for the evaluation of biodegradation. Parameters of anaerobic biodegradation [ 10 ]. The required threshold limits of biodegradation depend on the kind of measured parameter.
Testing the mineralization by measuring the biogas production, the degradation rate is not referred to initial substance concentration but to calculated maximum theoretical biogas production. It has to be considered that these theoretical values are usually not reached since a relevant part of the test substance is transformed to biomass and not to biogas.
A lot of data are available about anaerobic biodegradability of surfactants [ 30 ]. Since standardized screening test methods for determination of the biodegradability of different organic compounds under anaerobic conditions have been available Table 3. Standard screening tests for anaerobic biodegradation [ 31 , 32 , 33 , 34 , 35 , 36 , 37 ]. All these tests build on the determination of biogas generation manometric or volumetric measurement as final product of anaerobic biodegradation process. The ultimate anaerobic biodegradability is determined. The biogas production is measured manometrically.
Pressure-resistant vessels are fitted with gastight septum. The increasing pressure caused by biogas generation is measured, e. In ISO [ 36 ], a volumetric test system is described as an alternative method to the manometric one. The reaction vessel is connected to a graduated gas collecting tube, which is filled with acidified salt solution barrier solution. The glass tube is connected via flexible gastight tubing to an expansion tank which is filled with barrier solution.
Moving the tank up- and downwards, the solution surface of the tank can be adjusted to the one of the gas collecting tube and enable reading of gas volume at atmospheric pressure. Both methods were found to be suitable and practicable to perform anaerobic biodegradation screening tests. Most of the tests are screening methods for the evaluation of basic biodegradability in an aqueous medium. A definite mineral salt medium with volumes between —1, mL is used.
Typical concentrations of test substance vary from 20— mg of organic carbon per liter. The amount of solids biomass inoculum is adjusted to g of dry matter per liter, corresponding to an organic carbon to solids ratio ranged from 7 to mg per g of dry matter. Blank controls without test substance are necessary to record the endogenous biogas production of the inoculum.
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