10.4.3 Detergents

Role and Composition of Detergents

Detergents are formulated cleaning agents designed to remove dirt, grease, and stains from surfaces, textiles, and hard materials under everyday conditions (often in water of varying hardness, temperatures, and contamination levels). In contrast to individual surfactants, detergents are complex mixtures in which surfactants are combined with a range of auxiliaries to optimize cleaning performance, fabric and material protection, user safety, and environmental compatibility.

Detergents are typically optimized for a specific application field:

• Laundry detergents (powder, liquid, gel, pods)
• Dishwashing detergents (manual and automatic)
• Household cleaners for hard surfaces (floors, bathrooms, kitchens)
• Special-purpose detergents (delicate fabrics, carpets, industrial cleaning)

While surfactants provide core cleaning action, the characteristic performance of a detergent arises from the interplay of multiple components.

Typical Ingredients of Detergent Formulations

A modern detergent formulation may contain:

• One or more classes of surfactants
• Builders and water softeners
• Bleaching agents and bleach activators
• Enzymes
• Polymers and soil-release agents
• Optical brighteners
• Corrosion inhibitors and fabric-care additives
• Solvents and hydrotropes
• pH regulators and buffers
• Fragrances, dyes, and preservatives

Only some of these are present in any given product; composition depends on purpose (e.g. heavy-duty laundry vs sensitive-skin detergent).

Surfactant Systems in Detergents

Detergents rarely rely on a single surfactant. Instead, blends are used to balance cleaning performance, foaming behavior, mildness, and cost.

Common surfactant types in detergents:

• Anionic surfactants
• Strong grease and particulate soil removal
• Typical examples:
• Linear alkylbenzene sulfonates (LAS)
• Alkyl sulfates (AS) and alkyl ether sulfates (AES)
• Often primary cleaning agents in laundry and dishwashing detergents
• Nonionic surfactants
• Good performance on oily soils, often less foam, better at low temperatures
• Typical examples:
• Alcohol ethoxylates
• Alkyl polyglucosides (more “green” / bio-based)
• Amphoteric / zwitterionic surfactants
• Mild to skin, good foam stability
• Often used in hand dishwashing liquids and personal-care oriented cleaners
• Cationic surfactants
• Less common as primary cleaning surfactants in detergents
• Used more for fabric softening and antimicrobial purposes

Blends are chosen to:

• Extend cleaning power over a wide temperature range
• Work in both soft and hard water
• Control or reduce foam according to application (e.g. low-foaming for automatic dishwashers)

Builders and Water Softeners

Builders improve washing performance, especially in hard water, by binding calcium and magnesium ions and modifying the washing liquor.

Main functions:

• Complexation of hardness ions to prevent surfactant precipitation:
  $$\text{Ca}^{2+} + \text{builder}^{n-} \rightarrow [\text{Ca–builder}]^{(2-n)-}$$
• Stabilization of dirt in suspension to avoid redeposition
• pH control (often alkaline in heavy-duty detergents to promote fat hydrolysis and soil swelling)

Typical builder substances:

• Phosphates (e.g. sodium tripolyphosphate, historically widely used)
• Zeolites (e.g. zeolite A as phosphate replacement)
• Citrates, carbonates, and silicates
• Polycarboxylates (synthetic polymers with multiple carboxylate groups)

Environmental concerns over eutrophication have led to strong reduction or banning of phosphates in many household detergents, especially laundry and dishwashing formulations.

Bleaching Agents and Bleach Activators

For stain removal and hygiene, especially in laundry and automatic dishwashing detergents, chemical bleaches are incorporated.

Common oxygen bleach sources:

• Sodium perborate (historically) and sodium percarbonate
• In water, release hydrogen peroxide:
  $$\text{Na}_2\text{CO}_3 \cdot 1.5 \,\text{H}_2\text{O}_2 \rightarrow \text{Na}_2\text{CO}_3 + 1.5 \,\text{H}_2\text{O}_2$$
• Hydrogen peroxide reacts with stains by oxidation.

Bleach activators:

• Designed to make bleaching effective at lower temperatures (e.g. 30–60 °C)
• React with hydrogen peroxide to form peracids, which are more reactive oxidants:
  $$\text{H}_2\text{O}_2 + \text{activator} \rightarrow \text{peracid} + \text{byproducts}$$
• Typical activators: TAED (tetraacetylethylenediamine), NOBS (nonanoyloxybenzene sulfonate)

Chlorine-based bleaches are less common in household laundry but may appear in some cleaning products and disinfecting agents.

Enzymes in Detergents

Biocatalysts are widely used to break down specific stain components under mild conditions.

Common enzyme types:

• Proteases – degrade protein-based stains (blood, egg, milk)
• Amylases – break down starches (sauces, pasta)
• Lipases – attack fats and oils
• Cellulases – modify cotton fiber surfaces, helping to remove microfibrils and brighten colors
• Pectinases and mannanases – help remove plant-based and thickener-based stains

Characteristics:

• Active in specific pH and temperature ranges chosen for the detergent
• Denature at too high temperatures or extreme pH
• Often stabilized by additives (e.g. calcium salts, polyols, borates)

Enzymes allow effective cleaning at lower wash temperatures, reducing energy consumption.

Polymers and Soil-Release Agents

Polymers in detergents serve multiple roles:

• Anti-redeposition polymers
• Keep detached dirt dispersed in the washing liquor
• Prevent particles from re-attaching to fabric or surfaces
• Soil-release polymers
• Form thin layers on fibers (especially synthetic textiles), making future soil easier to wash off
• Scale inhibitors
• Reduce formation and deposition of calcium carbonate and other mineral scales on fabrics and inside washing machines

Typical examples include polycarboxylates, acrylic/maleic copolymers, and polyester-based soil-release polymers.

Optical Brighteners and Color Care Additives

Optical brighteners (fluorescent whitening agents) are added to many laundry detergents.

• Absorb ultraviolet light and re-emit it as blue-visible light
• Compensate yellowish tints in fabrics, making whites appear “brighter”
• Chemically, they are often stilbene or coumarin derivatives with conjugated systems, binding to textile fibers.

Color-care additives:

• Stabilize dyes on fabrics
• Reduce dye transfer between garments (common in “color detergents”)
• Often based on specific polymers or complexing agents that bind loose dyes.

Solvents, Hydrotropes, and Other Auxiliaries

To obtain stable liquid formulations and fully dissolve surfactants and additives, detergents may contain:

• Solvents (e.g. short-chain alcohols, glycols)
• Hydrotropes (e.g. sodium xylene sulfonate) to improve solubility of hydrophobic components
• pH regulators and buffers (e.g. sodium carbonate, citric acid)
• Corrosion inhibitors (e.g. silicates, phosphonates) to protect metals
• Anti-foaming agents (e.g. silicones) in low-foam products such as automatic dishwashing detergents
• Fragrances and dyes to provide a pleasant product experience and help distinguish products
• Preservatives to prevent microbial growth in water-rich formulations

Types of Detergents by Application

The composition of a detergent is strongly tailored to its intended use. Some major categories illustrate different priorities.

Laundry Detergents

Laundry detergents are formulated to clean textiles of natural and synthetic fibers.

Subtypes:

• Heavy-duty detergents (often powders or concentrates)
• High surfactant content
• Builders for hard water
• Bleaches and activators for stain removal and hygiene
• Enzymes for protein, starch, and fat stains
• Color detergents
• Often without bleaches to protect colors
• Color-stabilizing polymers, anti-redeposition agents
• Delicate and wool detergents
• Milder surfactants (frequently more nonionic and amphoteric)
• Neutral or mildly acidic pH
• No (or minimal) bleaching
• Formulations adapted to protein-based fibers (wool, silk)
• Liquid detergents and capsules
• Higher proportion of nonionic surfactants
• Limited use of solid builders (replaced by soluble sequestrants or higher surfactant levels)
• Often well suited for low-temperature washing

Dishwashing Detergents

Two fundamentally different product types have distinct chemical requirements.

Hand Dishwashing Liquids

Requirements:

• Strong grease removal
• High, stable foam (perceived as cleaning power by users)
• Mildness to skin (hands are in direct contact with solution)
• Pleasant fragrance

Typical formulation aspects:

• Mixtures of anionic and amphoteric/nonionic surfactants
• No bleaching agents or strong alkalinity
• Skin-conditioning additives and pH near neutral or slightly acidic

Automatic Dishwashing Detergents

Requirements:

• Effective soil removal at elevated temperatures (typically 50–70 °C)
• Low foaming to avoid machine problems
• Prevention of spotting and filming on glass and dishes
• Protection of machine parts and decor (glass, metals, glazes)

Typical formulation aspects:

• Low-foam nonionic surfactants
• Strong builder systems (e.g. phosphates historically, now often phosphonates, citrates, carbonates, silicates)
• Oxygen bleaches and activators
• Enzymes (proteases, amylases)
• Anti-corrosion agents and glass protectants
• Rinse aids (sometimes separate products) with surfactants that promote rapid water runoff

Household and Hard Surface Cleaners

These are formulated for cleaning floors, tiles, bathrooms, kitchens, and other surfaces.

Composition and emphasis:

• Surfactants (often anionic + nonionic) for general soil removal
• Solvents for degreasing and cleaning shiny surfaces (e.g. alcohol-based “glass cleaners”)
• Acids (citric, formic) or bases (ammonia, sodium hydroxide) depending on application:
• Descalers and bathroom cleaners: acidic to dissolve carbonates and soap scum
• Oven and grill cleaners: strongly alkaline to saponify fats
• Disinfectants in some formulations (e.g. quaternary ammonium compounds)
• Fragrances and dyes characteristic of the product category

Performance Factors and Mechanistic Aspects

Detergent performance is influenced by both formulation and external use conditions.

Influence of Temperature

Temperature affects:

• Solubility of surfactants, builders, and soils
• Reaction rates of bleaching and enzymatic processes
• Viscosity of fats and oils
• Micelle structure and dynamics

At higher temperatures:

• Greases soften, become more easily emulsified
• Oxygen bleaches act more rapidly
• Some enzymes may become inactivated at too high temperatures

Modern detergents are optimized for effective cleaning at low to moderate temperatures (e.g. 20–40 °C) to save energy, often through tailored surfactants and robust enzyme systems.

Influence of Water Hardness

Water hardness (mainly Ca²⁺ and Mg²⁺ ions) affects:

• Precipitation of anionic surfactants as insoluble salts
• Formation of deposits on textiles and in washing machines
• Efficiency of cleaning and clarity of rinse water

Builders and sequestrants are therefore key to:

• Bind hardness ions
• Maintain surfactant molecules in solution
• Prevent grey, stiff fabrics and machine scaling

Mechanical Action and Time

Besides chemical action, detergents work in combination with:

• Mechanical forces (drum rotation, water jets, manual scrubbing)
• Contact time (duration of the wash cycle, soaking time)

Cleaning effectiveness is often discussed in terms of the “Sinner’s circle” (not derived here): a balance between mechanical action, chemistry (detergent formulation), temperature, and time. Optimizing detergent chemistry can compensate, to some extent, for lower temperature or shorter wash cycles.

Environmental and Health Aspects of Detergents

Detergent chemistry is strongly influenced by environmental regulations and consumer health considerations.

Biodegradability and Aquatic Impact

Key environmental issues:

• Biodegradability of surfactants and organic components
• Linear alkylbenzene sulfonates and alcohol ethoxylates are designed to be readily biodegradable.
• Eutrophication by phosphates
• Leads to algal blooms and oxygen depletion in aquatic systems
• Many regions have restricted or banned phosphate-containing household detergents.
• Toxicity to aquatic organisms
• Some surfactants and additives can be harmful at certain concentrations
• Regulatory frameworks limit allowed substances and concentrations.

Environmental assessments consider:

• Primary biodegradation (breakdown of the original molecule)
• Ultimate biodegradation (mineralization to CO₂, H₂O, and inorganic ions)
• Persistence and bioaccumulation of components

Human Exposure and Safety

Main routes of exposure:

• Skin contact during hand dishwashing or handling detergents
• Inhalation of aerosols or dust (powdered detergents)
• Accidental ingestion, especially by children (risk with highly concentrated capsules)

Formulation measures:

• Use of milder surfactants for hand-contact products
• pH adjustment and buffer systems to reduce irritation potential
• Inclusion of bittering agents in pods to deter accidental ingestion
• Clear labeling and child-resistant packaging

Allergic reactions and sensitivities often relate to:

• Fragrances
• Preservatives
• Certain dyes or enzymes

Therefore, “fragrance-free” or “hypoallergenic” variants are offered for sensitive users.

Trends and Innovations in Detergent Chemistry

Detergent development is an active research field driven by performance, cost, regulation, and sustainability goals.

Some current directions:

• Lower washing temperatures and shorter cycles
• More efficient enzymes and surfactant systems adapted to cold water
• Concentrated and compact products
• Less packaging, smaller transport volumes
• Higher active ingredient content per unit volume or weight
• Bio-based and renewable raw materials
• Surfactants from plant oils or sugars (e.g. alkyl polyglucosides)
• Builders and solvents from renewable feedstocks
• Improved biodegradability and reduced toxicity
• New surfactant structures that break down more quickly and benignly
• Smart dosing and sensor-based systems
• Washing machines and dishwashers that adjust chemical dosage to load, soil level, and water hardness
• Microplastic and solid residue reduction
• Replacement of insoluble polymer additives by biodegradable alternatives

These developments illustrate how detergent chemistry connects fundamental concepts from surfactants, physical chemistry, environmental chemistry, and industrial process design into practical products used daily.