Study On Synthesis And Properties Of N-lauroyl Methylalanine Sodium

As a natural mild surfactant with excellent surface chemical properties,N-fatty acyl amino acid salt has been widely used in various fields such as daily chemicals,food,medicine and so on.Generally the Schotten-Bauman condensation reaction is used for its industrial production,where acetone and water are used as the solvents.Thus,the reaction conditions are mild,and the raw materials are cheap and easy to get.However,acetone has poor safety in use because of its low boiling point,low flash point,high volatility and toxicity.And it is readily to condense to form impurities in an alkaline environment,resulting in products with yellowish color.In addition,the presence of organic solvents requires much increased demand for processes and equipment,and the trace organic solvents remaining in the products are irritating,which limit the application range of the products.Therefore,finding suitable reaction solvents is beneficial to improve the quality of products and expand their application range.In this work,an anionic amino acid surfactant,N-lauroyl methylalanine sodium（SLMA）,was synthesized in a greener and more environmentally friendly way through the Schotten-Bauman condensation reaction,where ethanol-water mixed solution and water were used as solvents,respectively.The content of the fatty acid salt was determined by high performance liquid chromatography,which the yield was calculated by the two-phase titration method.The optimal synthesis conditions of SLMA were explored through single-factor and orthogonal experiments,and high-purity products were obtained after purification.The results showed that the optimal synthesis condition can be obtained by using ethanol-water mixed solution as the solvent,the volume ratio of ethanol to water was 1:1,the feeding molar ratio of methylalanine to lauroyl chloride was 1.2:1,the reaction temperature was 35°C,and the p H of the reaction system was 10～11.After all the feeds were completed,the reaction was carried out for 2 h,the content of SLMA was 30wt%,and the yield was as high as 90.78%.When water was used as the solvent,the feeding molar ratio of methylalanine to lauroyl chloride was 1.3:1,and the reaction temperature was 35°C.The p H of the system was 9～10,and the reaction was carried out for 2.5 h after all the feeds was completed.The content of SLMA was 25wt%,and the yield could reach 54.21%.The characterization results of surface chemical and application properties showed that the Krafft point was lower than 0°C,which indicated that SLMA had broad applicability.The critical micelle concentration（cmc）of SLMA at 25°C was 8.23×10^-3 mol·L^-1,and the lowest surface tension(γ_cmc)was 33.00m N·m^-1.As the temperature was increased,its cmc was increased,γ_cmc was decreased,and the adsorption capacity and efficiency of surface tension or interface tension reduction was increased.The SLMA micellization process was an entropy-driven exothermic spontaneous process.The micelle aggregation number of SLMA at cmc was 11 at 25°C.At different concentrations,there were two kinds of micelles.One was spherical and the other was non-spherical.As the concentration was increased,the proportion of spherical micelles was gradually increased.The adsorption of SLMA was changed from diffusion controlled adsorption to mixed dynamically controlled adsorption from the initial stage to the late stage.It had excellent emulsification and wettability,while possessed inferior foaming stability and detergency compared to the N-lauroyl sarcosinate sodium（LS）.SLMA was binary and ternary compounded with lauryl amidopropyl betaine（LAB）and alkyl glycoside（APG1214）in sequence,it was found that both the SLMA/LAB binary compound system and the SLMA/LAB/APG ternary compound system showed a comprehensive synergistic effect,and the best ratios were that the molar ratio of（SLMA:LAB）was 3:7 and the molar ratio of（SLMA/LAB:APG1214）was 3:7,corresponding to1.054×10^-3 mol·L^-1 and 1.595×10^-4 mol·L^-1 of cmc.The binary compound system tended to form concentrated single-form aggregates,and the overall size was relatively small.The micelle size of the ternary compound system was wider than that of the single system,and its micelle volume was obviously larger than that of the binary compound system.The micelle aggregate number formed by the two compound systems was smaller than that of the single system,and both formed a tighter,stable and smaller micelle structure.The application performance such as foaming,emulsification,wetting and decontamination was better than that of the single system.SLMA could not achieve satisfactory viscosity due to its intrinsic structure,even with the simple methods.In this work,SLMA was mixed with LAB and APG1214 to form the SLMA self-thickening system to increase the viscosity of SLMA.The results showed that the optimal preparation condition for SLMA self-thickening system was that the total amount of SLMA and LAB accounted for 12wt%of the sample,and their mass ratio was 8:4.The amount of APG1214 and Na Cl was both 3wt%of the sample,and the p H was 6.When the shear rate was 1 s^-1,the viscosity could reach 3663.40 m Pa·s,and its viscosity was decreased with the increase of temperature.Under the optimal preparation condition,the structure recovery rate of the SLMA self-thickening system exceeded 95%within 250 s,and exhibited the properties of shear thinning.At a fixed frequency,the viscosity of the self-thickening system was dominant,and the structural strength of the system was weak.At a fixed strain,the viscous structure and the elastic structure of the self-thickening system tended to be consistent.When the angular frequency reached to a certain degree,the system turned to the dominant position of elasticity,and the structural strength of the system was increased.SLMA was further applied to a silicone-free amino acid shampoo as an ingredient,which had a transparent and uniform appearance,with high stability and normal smell.The p H of the sample was 6.14,viscosity was 10136.99 m Pa·s,and it had good centrifugation,heat resistance and cold resistance stability and foaming.Volunteer sensory evaluation from various aspects showed that the sample had achieved satisfactory results.