Dish Soap and Surfactants

1.Introduction

With the development of the chemical industry, people’s living standards have been continuously improved. While life has been greatly enhanced, it has also caused severe environmental problems, even endangering human health and safety. As people’s demands for health keep rising, the safety of chemical products ubiquitous in daily life has attracted widespread public attention. Detergents, as chemical substances widely used in daily life and industrial production, have drawn particularly high public concern over their safety.

The safety of chemical products has once fallen into a credibility crisis. This situation arises on the one hand from the heavy reliance of detergent production on traditional raw materials, and on the other hand from the public’s lack of professional knowledge about chemical production processes.

Against this backdrop, guided by the core concept of green chemistry — “reducing and eliminating environmental pollution at the source” — this study designs and develops new detergent formulations. Environmentally friendly surfactants and chemical reagents capable of inhibiting microorganisms in water are adopted in this detergent formulation.

2.Current Development Status of Detergents

Since humanity entered civilized society, washing activities have always been an indispensable part of human life. Around 5,000 years ago, humans began to collect natural washing-friendly substances such as Chinese honeylocust fruits and alkaline components in plant ash for washing purposes. Three hundred years later, surfactants were artificially produced by humans. More than a century ago, soap was invented. Since then, soap made from grease, alkali, salt, spices and pigments has become a traditional detergent. The first artificially synthetic detergent, alkyl naphthalene sulfonate, emerged during World War I. It was developed by Germany’s BASF in 1917 and officially put into production in 1925. The popularization of synthetic detergents took place after sodium alkyl benzene sulfonate and tetrapropylene alkyl benzene were discovered and officially launched into production between 1935 and 1939.

3.Effective Ingredients and Action Mechanism of Detergents

3.1 Washing Principle

Washing in the general sense refers to the process of removing dirt from the surface of a carrier. During washing, the action of detergent weakens or eliminates the interaction between dirt and the carrier, converting the bonding state of dirt and carrier into the bonding state of dirt and detergent. Eventually, the dirt is separated from the carrier through rinsing and other methods. The basic process of washing action can be expressed by the following simple relationship:

Carrier·Dirt + Detergent → Carrier + Dirt·Detergent

The adhesion of dirt to objects is divided into physical adhesion and chemical adhesion. Physical adhesion further includes mechanical adhesion and electrostatic adhesion.

Chemical adhesion mainly refers to the adhesion achieved through chemical bonds. For instance, protein stains and rust adhered to fiber articles belong to chemical adhesion. Since the chemical interaction force of this type of adhesion is generally strong, the dirt is firmly combined with the substrate and extremely difficult to remove, requiring special treatment methods.

The interaction force between dirt attached by physical adhesion and the substrate is relatively weak, making it easier to remove compared with chemical adhesion. Dirt with mechanical adhesion is easy to remove; it is only difficult to eliminate when the dirt particles are small (<0.1 μm). Electrostatic adhesion manifests as the interaction between charged dirt particles and opposite charges. This force is stronger than mechanical force, resulting in relatively difficult dirt removal.

The washing process of dirt removal is generally considered to include the following stages:

A. Adsorption: Surfactants in detergents undergo directional adsorption at the interface between dirt and the carrier.

B. Wetting and Penetration: Due to the interfacial directional adsorption of surfactants, the detergent can penetrate between dirt and the carrier, wet the carrier, and reduce the adhesion force between dirt and the carrier.

C. Dispersion and Stabilization of Dirt: Dirt detached from the carrier surface is dispersed, emulsified or solubilized in the detergent solution, ensuring that the detached dirt will not reattach to the cleaned surface.

3.1.1 Types of Soil

Soil refers to greasy substances adhering to carriers as well as adhesives of such greasy substances, featuring an extremely complex composition. Based on different forms, it can be roughly classified into solid soil, liquid soil and special soil.

Common solid soils include rust, dust, carbon black particles and the like. The surfaces of these substances usually carry negative charges, making them prone to adhering to substrates. Most particulate solid soils are water-insoluble, yet they can be easily dispersed in aqueous solutions containing detergents; larger solid particles are easier to remove. Most common liquid soils are oil-soluble and can undergo saponification with alkaline solutions, which explains why most detergents are alkaline. Special soils mainly refer to stubborn stains such as bloodstains, plant sap and human secretions. This type of soil is primarily removed by bleaches, as the strong oxidizing property of bleaches can destroy their chromophoric groups.

3.2 Active Ingredients in Detergents

Surfactants, also known as surface-active substances, are the primary functional components in detergents. They dissolve rapidly in water and exhibit excellent properties including decontamination, foaming, solubilization, emulsification, wetting and dispersion.

3.2.1 Surfactants: Origin and Development

Experiments have shown that adding certain substances to water can alter its surface tension, and different substances exert varying effects on water surface tension.

In terms of the property of reducing surface tension, the ability to lower the surface tension of a solvent is defined as surface activity, and substances with surface activity are called surface-active substances. Substances that can significantly change the interfacial state of a solution system when added in small amounts are referred to as surfactants.

A surfactant is a substance that, when added to a solvent in a tiny dosage, can markedly reduce the solvent’s surface tension and change the interfacial state of the system. This gives rise to a series of functions such as wetting or dewetting, emulsification or demulsification, dispersion or flocculation, foaming or defoaming, solubilization, moisturizing, sterilization, softening, water repellency, antistatic property and corrosion resistance, to meet practical application demands.

Soap-based surfactants first appeared in ancient Egypt around 2500 BC, where ancient Egyptians made cleaning products from a mixture of mutton fat and plant ash. Around 70 AD, Pliny of the Roman Empire created the first bar of mutton fat soap. Soap did not gain widespread popularity until 1791, when French chemist Nicolas Leblanc discovered the method of producing caustic soda via electrolysis of sodium chloride. A product of the second stage of surfactant development is Turkey Red Oil, also known as Sulfonated Castor Oil. It is synthesized by reacting castor oil with concentrated sulfuric acid at a low temperature, followed by neutralization with sodium hydroxide. Turkey Red Oil boasts outstanding emulsifying power, permeability, wettability and diffusibility, and outperforms soap in resistance to hard water, acid and metal salts.

3.2.2 Structure of Surface Activity

The unique properties of surfactants stem from their special molecular structure. Surfactants are generally linear molecules that contain both hydrophilic polar groups and lipophilic non-polar hydrophobic groups.

Hydrophobic groups have diverse structures such as straight chains, branched chains and cyclic structures. The most common are hydrocarbon chains including alkanes, alkenes, cycloalkanes and aromatic hydrocarbons, with most carbon atom numbers ranging from 8 to 20. Other hydrophobic groups include fatty alcohols, alkylphenols, and atomic groups containing fluorine, silicon and other elements. Hydrophilic groups are categorized into anionic, cationic, amphoteric ionic and non-ionic types. Ionic surfactants can ionize in water to carry electric charges, while non-ionic surfactants cannot ionize in water but possess polarity and water solubility.

3.2.3 Common Harmful Surfactants

Surfactants are widely used in human daily life, yet they are undeniably chemical substances. Many raw materials for surfactants possess certain toxicity and pollution properties. Inevitably, they cause harm to the environment; upon human contact, they can irritate the skin, and some even feature strong toxicity and corrosiveness, inflicting severe damage to the human body. The following introduces several common harmful surfactants:

A. APEO

APEO is a common type of non-ionic surfactant, composed of an alkyl moiety and an ethoxy moiety. Varying carbon chain lengths of the alkyl part and different addition quantities of the ethoxy part result in numerous existing forms of APEO with significant performance differences among different forms. In the synthesis process of APEO, the main product is non-carcinogenic, but its by-products are corrosive to the skin and eyes, and some can even cause cancer in severe cases. Though it does not directly harm organisms, APEO poses an environmental hormone risk. Such chemical substances enter the human body through various routes, exert estrogen-like effects, disrupt normal human hormone secretion, and further reduce male sperm count. It is not only harmful to humans; reports indicate that its synthetic raw material NPEO also causes substantial damage to fish.

B. PFOS

PFOS, fully named Perfluorooctane Sulfonate, is a general term for a class of perfluorinated surfactants. It has an environmental amplification effect. Due to its special physical and chemical properties, PFOS is extremely difficult to degrade and is regarded as one of the most recalcitrant substances. After entering animals and the human body via the food chain, it accumulates in large quantities and severely threatens biological health.

C. LAS

LAS is a major organic pollutant that causes great harm to the environment. It can alter the physical and chemical properties of soil, such as changing soil pH value and water content, thereby inhibiting plant growth. In addition, when entering water bodies, LAS can combine with other pollutants to form dispersed colloidal particles and exhibits toxicity to juvenile higher organisms and lower organisms.

D. Fluorocarbon Surfactants

PFOA and PFOS are the two main traditional fluorocarbon surfactants. Relevant studies have shown that such compounds have high toxicity, cause persistent environmental pollution, and accumulate massively in organisms. Consequently, they were listed as Persistent Organic Pollutants (POPs) by the United Nations in 2009.

4 Green and New-type Surfactants

A. Amino Acid-based Surfactants

Amino acid-based surfactants are mainly made of biomass raw materials with abundant sources. They feature low toxic and side effects, mild properties, low irritation to organisms, and excellent biodegradability. According to the charge properties of hydrophilic groups after ionization in water, they can also be classified into four categories: cationic, anionic, non-ionic and amphoteric. Common types include N-alkyl amino acid type, amino acid ester type and N-acyl amino acid type.

B. Pineapple Enzyme Surfactants

Pineapple enzyme surfactants are produced by fermenting camellia seed meal and oil cake left after oil extraction, pineapple peel, together with yeast powder, pectinase and other microorganisms. Although the molecular structure of their active ingredients remains unclear, experimental data proves they have favorable washing performance.

C. SAA

SAA is a palm oil derivative. As a product made from renewable plant raw materials, it has attracted widespread attention. Its production process is eco-friendly. Moreover, in hard water with high calcium and magnesium ion content, it precipitates calcium salts much more slowly than commonly used surfactants such as LAS and AS, meaning it delivers outstanding detergency in practical applications.

5 Prospect of Detergent Development

Across the global detergent market, countries differ in development priorities and trends, yet the general research direction for detergent products remains consistent. The concentration and liquefaction of detergants have become mainstream trends, while water conservation, safety, energy saving, professionalism, environmental friendliness and multi-functionality have emerged as popular development directions. Surfactants, the core raw materials of detergents, are evolving toward mildness, compound formulation and environmental compatibility. Enzyme preparations, boasting high efficiency, specificity and eco-friendliness, have become a research hotspot in detergent development. Overall, the development trends of the detergent industry are summarized as follows:

Diversification, specialization and segmentation of detergent products. Detergents can be divided into solid, powder, liquid and gel types by form; concentrated type and regular type by active ingredient content; and various categories by packaging, color and fragrance.

Liquid detergents will become the most promising product category. Compared with solid detergents, liquid detergents perform better in low-temperature washing, feature more flexible formula design and simpler production processes. They also require less equipment investment and consume less energy during production.

Progressive concentration of detergent products. Since 2009, concentrated detergents have evolved into three major categories: concentrated washing powder, concentrated laundry pods and concentrated liquid detergent. Concentrated detergents have remarkable advantages over traditional products, including high active substance content, strong detergency and energy conservation. Additionally, they save packaging materials, reduce transportation costs and occupy less warehousing space due to their concentrated formula.

Human safety orientation. With the improvement of living standards, people no longer merely evaluate detergents by stain removal performance. Human safety, non-toxicity and mild non-irritation have become crucial criteria for detergent selection.

Eco-friendly product development. Eutrophication caused by phosphorus-containing detergents and adverse environmental impacts of bleaching agents have drawn widespread public concern. In response to the requirements of green chemistry, raw material selection for detergents is gradually shifting toward environmentally friendly and mild options.

Multi-functionalization. Multi-functionality is a prevailing development trend for various social products, and multi-purpose daily necessities have become common in life. In the future, detergents will integrate stain removal with functions such as sterilization, disinfection and bleaching.