Green Surfactant APG in Dishwashing Applications

I. How are grease and oil removed from dishes?

Dishwashing is the process of removing grease and other dirt from the surface of tableware and often requires the use of dishwashing detergents, of which surfactants are an essential component. Surfactant molecules usually contain a long hydrophobic chain and a hydrophilic group. The hydrophobic chain is in an unstable state in water, and has a tendency to migrate to the two-phase or multi-phase interface to minimise the contact area with water, resulting in a lower interfacial tension, which produces wetting, solubilisation and other functions. When the concentration of surfactant is large enough, surfactant molecules gather in water, hydrophobic chains together with each other to form a colloid, and hydrophilic groups outward to form the surface of the colloid. The hydrophilic groups form the surface of the colloid. The colloid can encapsulate the oil and make the solubility of the oil in water increase significantly (i.e. solubilisation). By applying mechanical action, the oil can be removed from the surface of the tableware.

There are many surfactants commonly used in dishwashing detergents, such as sodium dodecylbenzene sulfonate (LAS) and aliphatic alcohol polyoxyethylene ether sulfate (AES), but they are not satisfactory in terms of safety, effect on skin, biodegradation performance and so on. Here we introduce a green surfactant APG with more ideal performance.

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II. What is APG?

APG stands for AlkylPolyglucoside, which is usually an alkyl glycoside compound of monosaccharides and oligosaccharides. The hydrophobic chain R in the molecule is usually a C8~C12 alkyl group, and the hydrophilic group is a monosaccharide or oligosaccharide glycoside (the degree of polymerisation is usually 1~3, rarely more than 5). Since the sugar molecule has multiple hydroxyl groups, for example, glucose has 5 hydroxyl groups, sucrose, maltose has 8 hydroxyl groups, so the structure of alkyl glycosides is extremely complex, there are a variety of isomers.

III. Detergent Performance of APG

There is no strong ionisation group in APG molecule, which belongs to non-ionic surfactant, and its surface activity is among the highest, which is stronger than common non-ionic surfactants (e.g. aliphatic alcohol polyoxyethylene ether), and it can still maintain high surface activity in concentrated alkali and electrolyte (e.g. sodium silicate, etc.) solution. The lowest concentration at which a surfactant can form micelles is called the critical micelle concentration (cmc), which is an important parameter for measuring the surface activity of a surfactant. the smaller the cmc, the higher the surface activity. The smaller the cmc, the higher the surface activity. When the alkyl group is the same, the cmc of the commonly used anionic sodium dodecylbenzene sulfonate and sodium dodecyl sulfate are 1.2x10-3mol and 9.3x10-3molL, respectively, and the cmc of the nonionic lauryl polyoxyethylene ether C12H250(OC2H4)9H is 1.0x10-4mol, and the various APGs can be as small as 8.0x10-5mol-2.2x10-4mol/L. The lowest concentration of micelles is called critical micellar concentration (cmc), and it is a very important parameter for measuring the surface activity of the surfactant. 4mol/L.

APG also has other advantages, including rich foam, foaming ability and anionic surfactants are almost the same; and other surfactants have a synergistic effect, can improve the surface activity and reduce the amount of irritating surfactants: feel gentle, less irritation to the skin. Because it is non-toxic, easy to degrade, and 100% produced with regenerative natural raw materials, it has its unique advantages in energy consumption, physiology, environmental compatibility, and so on, and is therefore known as a green surfactant. Its disadvantage is that it is greatly affected by water quality, and as water hardness increases, the decontamination ability decreases.

IV. Synthesis of APG

APG is usually prepared by condensation of natural starch and its hydrolysis products with fatty alcohols. The hydrolysis products of starch can be glucose or oligosaccharides such as disaccharides and trisaccharides. There are various methods of synthesis, but at present, the main method is chemical synthesis method.

Two of the most established industrial synthesis methods are described here. One is often referred to as the one-step method, which employs a direct reaction of C8 to C16 fatty alcohols with starch hydrolysis products to produce alkylglycosides, usually requiring heat and a catalyst. Commonly used catalysts are sulphuric acid, p-toluenesulphonic acid, and acids with emulsifying properties (e.g., dodecylbenzenesulphonic acid) are also used, which cause the feedstock to form stable, fine droplets that aid in the glycosidisation reaction. Reaction with glucose as feedstock.

After the reaction is completed, the fatty alcohol is removed, and then after neutralisation and bleaching, the product APG is obtained.This method is relatively simple, but due to the poor solubility of high-carbon alcohols and sugar, the reaction time is long, and the reaction conditions need to be carefully controlled, or else it is very easy to form agglomerated coke lumps. Foreign countries mainly use this method to synthesise alkyl glycosides.

Another synthesis method is often called two-step method. Take the transacetalisation method as an example, it is in the presence of sulfuric acid, p-toluenesulfonic acid and other acidic catalysts, first let the low carbon fatty alcohols (generally butanol) and starch (or its hydrolysis products) to generate a low carbon chain of alkyl glycosides, and then high carbon chain fatty alcohols instead of glycosides in the low carbon chain, so as to obtain a high carbon chain of alkyl glycosides. The reaction with glucose as raw material is.

At present, the main domestic production with this method, commonly used conditions are: the first step of the reaction for 2 hours, at 100 ~ 120 ℃, under vacuum as soon as possible to remove n-butanol, and then the second step of the reaction. After 40 minutes of reaction, the product is neutralised with Na2CO3 and the dodecanol is removed at 165°C and 0.27kPa. The two-step method is characterised by short reaction time, simple operation, and good solution to the problem of alcohol-sugar compatibility, but the large-scale production needs to increase the separation of low-carbon alcohol processes and equipment, and the product purity is not as good as the one-step method. In addition to the chemical method, APG can also be synthesised by enzymatic method, which has the advantages of good selectivity, mild conditions, simple process, high product purity, etc. However, the conversion rate is low, the reaction speed is slow, and it is complicated to establish a suitable enzyme system, so it is still in the laboratory research stage.

V. Formulation of dishwashing detergents with APG

Usually, dishwashing detergents formulated with APG also contain other surfactants, so that the formulated detergents are rich in foam, good in hard water resistance, easy to rinse and gentle to the touch. It should be noted that there is no direct relationship between foam and detergent performance, but the phenomenon of foam gradually decreasing during the washing process can indicate a decrease in the active ingredients of the washing solution and a decrease in cleaning ability. Due to the relatively high price of APG, the amount of APG used in general formulations is low (usually in the range of 1% to 15%) in order to reduce costs. NaCl is commonly used as a thickener in commercial production to further reduce cost and increase viscosity.

The following is an experimental scheme for the preparation of a simple formula: weigh 0.70g of APG (50%) and 1.20g of AES (70%), add water to 10g respectively, and slightly heat it with a water bath (or heating jacket) to dissolve it. Mix the two solutions, add NaC1 saturated solution 3.5m, gently stir evenly, the product is obtained. The product has strong decontamination ability, and the washing effect in hard water is still very good after it is diluted 5 times.

In the experiment, it is necessary to pay attention to the following matters: (1) AES has weak acidity, which is irritating to the skin, and direct contact with the skin should be avoided as much as possible. (2)When preparing the detergent, APG and AES should be dissolved separately before mixing, otherwise the dissolution will be more difficult. (3) APG is difficult to dissolve in water, but vigorous stirring will produce a lot of foam, and the solubilising effect is not obvious. Therefore, it should not be stirred violently when dissolving, and can be heated slightly to promote dissolution. (4)NaCl should be added in the form of saturated solution, directly added to the solid will cause the dissolution of difficult.The amount of NaCl should not be too large, otherwise it will lose the thickening effect, instead of making the detergent thinner. (5) Most of the experimental supplies are flammable organic materials, so the whole process can not have an open flame.

In order to meet different needs, can be added to the detergent humectant (generally a little glycerin) to avoid drying the skin after washing, can be added to the spice (lemon spice 0.02g) to improve the smell, but also to add an appropriate amount of preservatives (sodium dehydroacetate 0.01g) to extend the shelf life of the product.

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