A method for synthesizing anionic and zwitterionic composite surfactant

Abstract

The invention belongs to the technical field of daily chemical materials, and discloses a method for synthesizing an anion and zwitterion composite surfactant. The synthesis method comprises: subjecting a fatty acid and an amino acid salt to an amidation reaction at 100-250°C to obtain an anion surfactant of residual fatty acid, then adding N,N-dimethyl-1,3-propylenediamine to continue the reaction, obtaining an anion surfactant containing a fatty amide propyl dimethyl tertiary amine intermediate, then adding a quaternizing agent to carry out a quaternization reaction or adding an oxidant to carry out an oxidation reaction, and obtaining the anion and zwitterion composite surfactant. The method of the present invention not only effectively solves the problem that the conversion rate of amino acid anion surfactant synthesized by fatty acid process is not high, and the fatty acid residue is high, but also solves the problem that the current amino acid surfactant product has poor low temperature stability and is inconvenient to use.

Description

Technical Field

[0001] The invention belongs to the technical field of daily chemical materials, and particularly relates to a method for synthesizing an anionic and zwitterionic composite surfactant. Background Art

[0002] At present, the large-scale production of amino acid surfactants mostly adopts the following two process routes: 1. The Schotten-Baumann condensation process with acyl chloride as the main raw material: fatty acyl chloride and amino acid salt are subjected to amidation reaction in water / organic solvent under alkaline conditions. This process route uses acyl chloride as the raw material, introduces more chloride ions, and the subsequent process of removing chloride ions will produce a large amount of chlorine-containing wastewater, which is environmentally poor, and acyl chloride and organic solvents are hazardous materials, but because of its extremely high reaction efficiency, which can reach more than 95%, the degree of industrialization is relatively high. Patent EP1156033A describes the synthesis of acidic amino acid surfactants of the Schotten-Baumann process, and conducts in-depth research on the solvent phase ratio. The product needs to be desalted and uses organic solvents, which is environmentally poor and has certain safety risks in the process; Patent EP1314717B1 describes the synthesis of neutral amino acid surfactants of the Schotten-Baumann process, compares the influence of the reaction solvent on the reaction efficiency, introduces chloride ions into the product, and the post-treatment desalination produces a large amount of wastewater, which also has the same environmental protection problems and raw material safety problems. 2. Direct amidation condensation process with fatty acid as main raw material: fatty acid and amino acid salt are dehydrated and condensed under high temperature conditions to synthesize the target product in one step. This process has simple reaction, high atom economy, and safe and harmless raw materials, but due to its low reaction efficiency, the residual fatty acid in the product is high, which leads to difficulties in industrialization. For example, patents US5496959 / US 2880219 / JP2002-234868 describe the direct condensation process of fatty acid and sodium methyl taurine, respectively screening and optimizing parameters such as catalyst, reaction temperature, feed ratio, etc., and the yield of the obtained product is only 50-80%, and the remaining fatty acid is as high as 20-50%. Other alternative methods such as CN102266743A disclose a method for preparing N-fatty acyl amino acid surfactants using oil esters. This inventive method uses oil to replace acyl chloride to synthesize N-fatty acyl amino acid surfactants, and uses oil and amino acid sodium to react under catalyst conditions to generate N-fatty acyl amino acid sodium. This patented technology requires the use of a supported metal oxide catalyst, and the product yield cannot be guaranteed. Patent CN111004156A discloses a method for synthesizing fatty acyl amino acid surfactants by direct method, the steps of which are as follows: (1) adding fatty acid, methylamine and isethionate to a reactor; (2) sealing the reactor and slowly heating it to 150-300°C; (3) keeping the temperature for reaction for 2-10 hours; (4) removing unreacted methylamine by distillation; (5) treating to obtain the finished product. However, this method also has the problems of low product yield and high fatty acid content. Patent CN111072524A discloses a method for preparing fatty acyl amino acid surfactants by high temperature crystallization, the steps of which are as follows: adding fatty acid, catalyst and amino acid to a reactor; sealing the reactor and slowly heating it to 200-300°C; keeping the temperature for reaction for 2-10 hours; slowly stirring and cooling it down to below 200°C for crystallization; cooling it down to 80-100°C and filtering it while hot, recycling the filtrate for reuse; treating the filter cake to obtain the finished product.This patent removes excess fatty acids by centrifugation during the cooling process. Although it can reduce the fatty acid content in the product to a certain extent, the product yield is difficult to guarantee.

[0003] In addition, the above processes also have the common defects that the products are solid at low temperatures, have poor low-temperature stability in the formulation application system, and some products have serious low-temperature precipitation, which seriously affects the ease of use and formulation stability. Summary of the invention

[0004] In view of the shortcomings and deficiencies of the above prior art, the primary purpose of the present invention is to provide a method for synthesizing an anionic and zwitterionic composite surfactant. The method of the present invention synthesizes an anionic and zwitterionic composite surfactant, which not only solves the safety and environmental protection problems caused by the acyl chloride process, but also solves the problem of high fatty acid residues in the fatty acid process, and at the same time improves the low temperature stability of the amino acid surfactant by compounding with the zwitterionic surfactant.

[0005] Another object of the present invention is to provide an anionic and zwitterionic composite surfactant obtained by the above method.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A method for synthesizing an anionic and zwitterionic composite surfactant comprises the following steps:

[0008] (1) adding fatty acid and amino acid salt into a reactor and mixing them evenly, and then heating to 100-250° C. to carry out amidation reaction to obtain an anionic surfactant with residual fatty acid;

[0009] (2) adding N,N-dimethyl-1,3-propylenediamine to the anionic surfactant containing residual fatty acid in step (1) and continuing the reaction to obtain an anionic surfactant containing fatty amide propyl dimethyl tertiary amine intermediate;

[0010] (3) Adding a quaternizing agent to the system of step (2) to carry out a quaternization reaction or adding an oxidizing agent to carry out an oxidation reaction to obtain the anionic and zwitterionic composite surfactant.

[0011] Furthermore, the fatty acid in step (1) is selected from fatty acids with carbon atoms of 6 to 22. The hydrophobic long chain in the fatty acid is a straight chain or a branched hydrophobic long chain, and the hydrophobic long chain is a saturated alkyl group or an alkyl group having one or more unsaturated groups. Preferably, the fatty acid can be selected from at least one of oleic acid, linoleic acid, linolenic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, and coconut acid, and more preferably at least one of lauric acid and coconut acid.

[0012] Furthermore, the amino acid salt in step (1) is an alkali metal salt or ammonium salt of at least one of sarcosine, alanine, glycine, glutamic acid, methyltaurine, aspartic acid, proline, serine, and histidine. Preferably, it is an amino acid sodium salt, and more preferably, it is at least one sodium salt of sarcosine, alanine, glycine, glutamic acid, and methyltaurine.

[0013] Furthermore, the molar ratio of the fatty acid and the amino acid salt added in step (1) is 1 to 10:1, preferably 1.5 to 10:1, and more preferably 1.5 to 5:1. When the molar ratio is lower than 1.5, the viscosity of the reaction process is too high and the reaction efficiency is low; when the molar ratio is greater than 10:1, the fatty acid residue is too high, resulting in a high residue of subsequent by-products.

[0014] Furthermore, the temperature of the amidation reaction in step (1) is preferably 150-250°C, more preferably 150-220°C; the reaction time is 3-10h, preferably 5-10h, more preferably 5-8h. When the reaction temperature is lower than 150°C, the reaction efficiency is low and the viscosity of the reaction system is high; when the reaction temperature is higher than 250°C, the product has the risk of yellowing or decomposition.

[0015] Furthermore, in step (1), the active matter content of the anionic surfactant containing the residual fatty acid is 30-70%, and the residual fatty acid content is 30-70%. The contents are all in mass percentage.

[0016] Furthermore, in step (2), the molar ratio of the amount of N,N-dimethyl-1,3-propylenediamine added to the residual fatty acid is 1 to 3:1, preferably 1.05 to 2.50:1, and more preferably 1.05 to 2:1. When the molar ratio is lower than 1:1, the reaction efficiency is poor; when the molar ratio is higher than 3, the reaction economy is poor.

[0017] Furthermore, the reaction temperature in step (2) is 150-250°C, preferably 170-250°C, more preferably 170-220°C; the reaction pressure is 0-2Mpa, preferably 0-1Mpa, more preferably 0.5-1Mpa; the reaction time is 3-8h, preferably 4-8h, more preferably 5-7h. When the reaction temperature is lower than 150°C, the reaction efficiency is poor, and when it is higher than 250°C, the product has the risk of yellowing or decomposition.

[0018] Furthermore, in the anionic surfactant containing the fatty amide propyl dimethyl tertiary amine intermediate in step (2), the fatty acid content is 0-5%, and the residual N,N-dimethyl-1,3-propylenediamine is 0-1%.

[0019] Furthermore, in step (3), the quaternizing agent is sodium chloroacetate, acrylic acid, sodium chlorohydroxypropyl sulfonate, 2-hydroxy-1,3-propane sultone, 1,3-propane sultone, etc., preferably sodium chloroacetate; the oxidant is hydrogen peroxide, sodium peroxide, peracetic acid, potassium peroxide, etc., preferably hydrogen peroxide.

[0020] Furthermore, in step (3), the molar ratio of the quaternizing agent to the fatty amide propyl dimethyl tertiary amine intermediate is 1 to 2:1, preferably 1.05 to 2:1, and more preferably 1.05 to 1.50:1; the molar ratio of the oxidant to the fatty amide propyl dimethyl tertiary amine intermediate is 1 to 2:1, preferably 1.05 to 2:1, and more preferably 1.05 to 1.50:1. When the molar ratio of the quaternizing agent is lower than 1:1, the residual intermediate of the product is high, and when the molar ratio is higher than 2:1, the residual quaternizing agent is high. When the molar ratio of the oxidant is lower than 1:1, the reaction effect is poor, and when it is higher than 2:1, the residual oxidant is high.

[0021] Furthermore, in step (3), the quaternization reaction temperature is 60-95°C, preferably 65-95°C, most preferably 80-95°C; the reaction time is 3-8h, preferably 3-7h, more preferably 4-7h; the oxidation reaction temperature is 50-95°C, preferably 60-95°C, most preferably 60-80°C; the reaction time is 3-8h, preferably 3-7h, more preferably 4-7h. If the oxidation reaction temperature is lower than 50°C or higher than 95°C, the reaction effect is not good.

[0022] Furthermore, the quaternization reaction or oxidation reaction in step (3) is carried out under water solvent conditions.

[0023] An anionic and zwitterionic composite surfactant is prepared by the method.

[0024] Furthermore, the solid content of the anionic and zwitterionic composite surfactant is 25% to 50%, the residual fatty acid content is less than 3%, and the product is stable at 0 to 5°C without precipitation.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention innovatively synthesizes a surfactant composite of anions and zwitterions. First, the amino acid anion surfactant synthesized by the fatty acid process has obvious cost advantages over the products of the acyl chloride process, and effectively avoids the problem of a large amount of wastewater caused by desalination in the acyl chloride process. Secondly, the unreacted fatty acid in the obtained amino acid anion surfactant is reacted with N,N-dimethyl-1,3-propylenediamine, and further quaternized or oxidized to obtain an anion and zwitterion composite surfactant. This not only effectively solves the problems of low conversion rate and high fatty acid residue in the synthesis of amino acid anion surfactants by the fatty acid process, but also solves the problem of poor low-temperature stability and inconvenience in use of current amino acid surfactant products.

[0027] (2) In the composite surfactant synthesized by the present invention, the amino acid anionic surfactant is green and environmentally friendly due to the direct acylation process of fatty acids and amino acid salts, which meets the current demand for green and sustainable development. At the same time, the residual fatty acids in the amino acid anionic surfactant are further quaternized or oxidized by reacting with N,N-dimethyl-1,3-propylenediamine, which not only solves the problem of high fatty acid content in the product obtained by the direct acylation process of fatty acids and amino acid salts, but also improves the low-temperature stability of the amino acid surfactant by compounding the amino acid anionic surfactant with the amphoteric surfactant.

[0028] (3) The present invention has no adverse effect on the subsequent synthesis of zwitterionic surfactants by pre-synthesizing amino acid anionic surfactants, and can improve the synthesis conversion rate of zwitterionic surfactants to a certain extent. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0030] Example 1

[0031] A method for preparing a sodium cocoyl glycinate and cocamidopropyl betaine composite surfactant of the present embodiment comprises the following steps:

[0032] (1) 100 g of coconut oil and 24 g of sodium glycinate were added to a four-necked flask equipped with a polytetrafluoroethylene stirring blade, a thermometer, a condenser and a nitrogen inlet, and the flask was placed in an oil bath and heated to 150° C. for 3 h. The residual coconut oil was measured by HPLC to be 38.3%.

[0033] (2) Add 28 g of N,N-dimethyl-1,3-propylenediamine to the product obtained in the above step (1), control the reaction temperature at 190° C., and react for 7 h. After the reaction is completed, remove the remaining N,N-dimethyl-1,3-propylenediamine.

[0034] (3) Add 310 g of deionized water and 28.8 g of sodium chloroacetate to the product obtained in the above step (2) to carry out quaternization reaction at a reaction temperature of 90° C. for a reaction time of 5 h. After the reaction is completed, a composite surfactant of sodium cocoyl glycinate and cocamidopropyl betaine is obtained.

[0035] The coconut acid residue of the composite surfactant obtained in this example was measured to be 1.52%, and the product (about 35% solid content) was stable at 5°C without precipitation.

[0036] Example 2

[0037] The preparation method of a composite surfactant of sodium lauroyl glycinate and lauroyl propyl betaine in this embodiment comprises the following steps:

[0038] (1) 100 g of lauric acid and 24 g of sodium glycinate were added to a four-necked flask equipped with a polytetrafluoroethylene stirring blade, a thermometer, a condenser and a nitrogen inlet, and the flask was placed in an oil bath and heated to 150° C. for 3 h. The residual coconut acid was measured by HPLC to be 40.5%.

[0039] (2) Add 30 g of N,N-dimethyl-1,3-propylenediamine to the product obtained in the above step (1), control the reaction temperature at 200° C., and react for 6 h. After the reaction is completed, remove the remaining N,N-dimethyl-1,3-propylenediamine.

[0040] (3) Add 315 g of deionized water and 30.5 g of sodium chloroacetate to the product obtained in the above step (2) to carry out quaternization reaction at a reaction temperature of 95° C. for a reaction time of 6 h. After the reaction is completed, a composite surfactant of sodium lauroyl glycinate and lauroylpropyl betaine is obtained.

[0041] The lauric acid residue of the composite surfactant obtained in this example was measured to be 1.66%, and the product (about 35% solid content) was stable at 5°C without precipitation.

[0042] Example 3

[0043] The preparation method of a composite surfactant of sodium methyl lauroyl taurate and lauramide propyl betaine in this embodiment comprises the following steps:

[0044] (1) 100 g of lauric acid and 100.6 g of 40% sodium methyl taurate were added to a four-necked flask equipped with a polytetrafluoroethylene stirring blade, a thermometer, a condenser and a nitrogen inlet, and the flask was placed in an oil bath and heated to 180° C. for 5 h. The residual lauric acid was measured by HPLC to be 44.1%.

[0045] (2) Add 35 g of N,N-dimethyl-1,3-propylenediamine to the product obtained in the above step (1), control the reaction temperature at 190° C., and react for 5 h. After the reaction is completed, remove the remaining N,N-dimethyl-1,3-propylenediamine.

[0046] (3) Add 450 g of deionized water and 36.5 g of sodium chloroacetate to the product obtained in the above step (2) to carry out quaternization reaction at a temperature of 85° C. for a reaction time of 6 h. After the reaction is completed, a composite surfactant of sodium methyl lauroyl taurate and lauroyl propyl betaine is obtained.

[0047] The lauric acid residue of the composite surfactant obtained in this example was measured to be 1.85%, and the product (about 30% solid content) was stable at 0°C without precipitation.

[0048] Example 4

[0049] The preparation method of a composite surfactant of sodium methyl lauroyl taurate and lauramide propylamine oxide of the present embodiment comprises the following preparation steps:

[0050] (1) 100 g of lauric acid and 100.6 g of 40% sodium methyl taurate were added to a four-necked flask equipped with a polytetrafluoroethylene stirring blade, a thermometer, a condenser and a nitrogen inlet, and the flask was placed in an oil bath and heated to 200° C. for 6 h. The residual lauric acid was measured by HPLC to be 42.8%.

[0051] (2) Add 33.3 g of N,N-dimethyl-1,3-propylenediamine to the product obtained in the above step (1), control the reaction temperature at 180° C., and react for 5 h. After the reaction is completed, remove the remaining N,N-dimethyl-1,3-propylenediamine.

[0052] (3) Add 390 g of deionized water and 10.6 g of hydrogen peroxide to the product obtained in the above step (2) for oxidation reaction at a temperature of 65° C. for 5 h. Upon completion of the reaction, a composite surfactant of sodium methyl lauroyl taurate and lauroyl propylamine oxide is obtained.

[0053] The lauric acid residue of the composite surfactant obtained in this example was measured to be 2.11%, and the product (about 30% solid content) was stable at 0°C without precipitation.

[0054] Example 5

[0055] The preparation method of a composite surfactant of sodium laurylaminopropionate and laurylamidopropyl betaine in this embodiment comprises the following steps:

[0056] (1) 100 g of lauric acid and 18.5 g of sodium alanine were added to a four-necked flask equipped with a polytetrafluoroethylene stirring blade, a thermometer, a condenser and a nitrogen inlet, and the flask was placed in an oil bath and heated to 200° C. for 4 h. The residual lauric acid was measured by HPLC to be 45.2%.

[0057] (2) 32.0 g of N,N-dimethyl-1,3-propylenediamine was added to the above product, the reaction temperature was controlled at 200° C., the reaction time was 6 h, and after the reaction was completed, the remaining N,N-dimethyl-1,3-propylenediamine was removed.

[0058] (3) Add 400 g of deionized water and 35.8 g of sodium chloroacetate to the product obtained in the above step (2) to carry out quaternization reaction at a reaction temperature of 90° C. for a reaction time of 6 h. After the reaction is completed, a composite surfactant of sodium laurylaminopropionate and laurylamidopropyl betaine is obtained.

[0059] The lauric acid residue of the composite surfactant obtained in this example was measured to be 2.55%, and the product (about 30% solid content) was stable at 5°C without precipitation.

[0060] Example 6

[0061] A method for preparing a composite surfactant of sodium cocoylaminopropionate and cocamidopropylamine oxide of the present embodiment comprises the following steps:

[0062] (1) 100 g of coconut oil and 22.1 g of sodium alanine were added to a four-necked flask equipped with a polytetrafluoroethylene stirring blade, a thermometer, a condenser and a nitrogen inlet, and the flask was placed in an oil bath and heated to 180° C. for 6 h. The residual lauric acid was measured by HPLC to be 46.8%.

[0063] (2) Add 32.3 g of N,N-dimethyl-1,3-propylenediamine to the product obtained in the above step (1), control the reaction temperature at 180° C., and react for 7 h. After the reaction is completed, remove the remaining N,N-dimethyl-1,3-propylenediamine.

[0064] (3) Add 350 g of deionized water and 10.1 g of hydrogen peroxide to the product obtained in the above step (2) for oxidation reaction at a temperature of 60° C. for a reaction time of 5 h. After the reaction is completed, a composite surfactant of sodium cocoylaminopropionate and cocoylaminopropylamine oxide is obtained.

[0065] The coconut acid residue of the composite surfactant obtained in this example was measured to be 2.02%, and the product (about 30% solid content) was stable at 0°C without precipitation.

[0066] Comparative Example 1

[0067] This comparative example is a method for synthesizing sodium methyl lauroyl taurate using acyl chloride as a raw material, comprising the following steps:

[0068] 100.6g of 40% sodium methyl taurine and 70g of deionized water were added to a five-necked flask equipped with a polytetrafluoroethylene stirring paddle, a thermometer, a pH meter probe and a nitrogen inlet, and the mixture was stirred and dispersed uniformly. Then 49.6g of lauroyl chloride and 27.5g of 32% sodium hydroxide solution were added dropwise to the five-necked flask at the same time. The five-necked flask was placed in a 55°C thermostat for reaction for 3h, and the residual lauric acid was measured by HPLC to be 1.83%. The obtained product was a paste at room temperature, and a large amount of wastewater was generated by post-treatment and desalination. The ratio of the obtained product to the saline wastewater was close to 1:1, and the environmental friendliness was poor. The product was adjusted to about 30% solid content with deionized water, and the product precipitated seriously at 0°C, and the low-temperature stability was poor and inconvenient to use.

[0069] Comparative Example 2

[0070] This comparative example is a method for preparing sodium lauroyl sarcosinate using acyl chloride as a raw material, comprising the following steps:

[0071] Add 100g 35% sodium sarcosinate and 150g deionized water to a five-necked flask equipped with a polytetrafluoroethylene stirring blade, a thermometer, a pH meter probe and a nitrogen inlet, stir and disperse evenly, then drop 65.6g lauroyl chloride and 26g 32% sodium hydroxide solution into the five-necked flask at the same time, place the five-necked flask in a 15°C constant temperature bath for reaction for 3h, heat to 80-90°C after the reaction, adjust the pH of the reaction solution to about 2, remove the water layer, neutralize the oil layer with 35.7g 32% sodium hydroxide solution, and adjust the solid content to about 30% with 160g deionized water. HPLC measured the residual lauric acid to be 0.7%, the ratio of the obtained product to the saline wastewater was close to 1:1, and the environmental friendliness was poor.

[0072] From the results of the above examples and comparative examples 1 to 2, it can be found that the composite surfactant obtained by the method of the present invention has a low fatty acid content and excellent low-temperature stability, and avoids the use of acyl chloride substances in the synthesis of amino acid surfactants, which is environmentally friendly.

[0073] Comparative Example 3

[0074] This comparative example is the synthesis of a single cocamidopropyl betaine zwitterionic surfactant, and the specific steps are as follows:

[0075] (1) 38.3 g of coconut oil and 28 g of N,N-dimethyl-1,3-propylenediamine were added to a four-necked flask equipped with a polytetrafluoroethylene stirring blade, a thermometer, a condenser and a nitrogen inlet. The reaction temperature was controlled at 190° C. and the reaction time was 7 h. After the reaction was completed, the remaining N,N-dimethyl-1,3-propylenediamine was removed.

[0076] (2) Add 310 g of deionized water and 28.8 g of sodium chloroacetate to the product obtained in the above step (1) to carry out quaternization reaction at a reaction temperature of 90° C. for a reaction time of 5 h. After the reaction is completed, a cocamidopropyl betaine zwitterionic surfactant is obtained.

[0077] The residual coconut acid in the surfactant obtained in this comparative example was measured to be 4.66%.

[0078] It can be seen from the results of Example 1 and this comparative example that the present invention has no adverse effect on the subsequent synthesis of zwitterionic surfactants by pre-synthesizing amino acid anionic surfactants, and can improve the synthesis conversion rate of zwitterionic surfactants to a certain extent.

[0079] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for synthesizing an anionic and zwitterionic composite surfactant, characterized in that: The specific synthesis steps are as follows: (1) Add fatty acid and amino acid salt into a reactor and mix them evenly, then heat to 100-250°C for amidation reaction to obtain an anionic surfactant with residual fatty acid; (2) adding N,N-dimethyl-1,3-propylenediamine to the anionic surfactant containing residual fatty acid in step (1) and continuing the reaction to obtain an anionic surfactant containing fatty amide propyl dimethyl tertiary amine intermediate; (3) adding a quaternizing agent to the system of step (2) to carry out a quaternization reaction or adding an oxidizing agent to carry out an oxidation reaction to obtain the anionic and zwitterionic composite surfactant; The fatty acid in step (1) is at least one of oleic acid, linoleic acid, linolenic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, and coconut acid; the amino acid salt is at least one sodium salt of sarcosine, alanine, glycine, glutamic acid, and methyl taurine; the molar ratio of the fatty acid to the amino acid salt is 1.5-10:1; The active matter content of the anionic surfactant containing residual fatty acids in step (1) is 30-70%, and the residual fatty acid content is 30-70%; The molar ratio of the amount of N,N-dimethyl-1,3-propylenediamine added to the residual fatty acid in step (2) is 1 to 3:1; The solid content of the anionic and zwitterionic composite surfactant in step (3) is 25% to 50%, the residual fatty acid content is less than 3%, and the product is stable at 0 to 5°C without precipitation.

2. The method for synthesizing an anionic and zwitterionic composite surfactant according to claim 1, characterized in that: The temperature of the amidation reaction in step (1) is 150-250° C., and the reaction time is 3-10 h.

3. The method for synthesizing an anionic and zwitterionic composite surfactant according to claim 1, characterized in that: The reaction temperature in step (2) is 150-250° C.; the reaction pressure is 0-2 MPa; and the reaction time is 3-8 h.

4. The method for synthesizing an anionic and zwitterionic composite surfactant according to claim 1, characterized in that: In the anionic surfactant containing the fatty amide propyl dimethyl tertiary amine intermediate described in step (2), the fatty acid content is 0-5%, and the residual N,N-dimethyl-1,3-propylenediamine is 0-1%.

5. The method for synthesizing an anionic and zwitterionic composite surfactant according to claim 1, characterized in that: In step (3), the quaternizing agent is sodium chloroacetate, acrylic acid, sodium chlorohydroxypropyl sulfonate, 2-hydroxy-1,3-propane sultone or 1,3-propane sultone; the oxidizing agent is hydrogen peroxide, sodium peroxide, peracetic acid or potassium peroxide; the molar ratio of the quaternizing agent to the fatty amide propyl dimethyl tertiary amine intermediate is 1-2: 1; the molar ratio of the oxidizing agent to the fatty amide propyl dimethyl tertiary amine intermediate is 1-2: 1; the quaternization reaction temperature is 60-95° C., and the reaction time is 3-8 hours; the oxidation reaction temperature is 50-95° C., and the reaction time is 3-8 hours; the quaternization reaction or oxidation reaction is carried out in an aqueous solvent.

6. An anionic and zwitterionic composite surfactant, characterized in that: It is prepared by the method according to any one of claims 1 to 5.