Synthesis And Properties Of A PH-Responsive Precipitation-Dissolution Surfactant

Surfactants are widely used in industrial and agricultural production and daily life.However,most of the surfactants after using are discarded or even dispersed directly into the natural environment,which not only increases the environmental burden but also wastes valuable petroleum-based carbon resources.Therefore,highly efficient recovery of surfactants is one of the common problems in the field of surfactants.Switching surfactant provides the possibility to solve the above problems.In the case of aqueous solution system,if the“on”and“off”states of some switching surfactant correspond to the forms of“dissolution”and“solid precipitation”,respectively,called precipitation-solution-type switching surfactant（PDSS）,which will certainly contribute to the recovery and reuse of surfactants.Therefore,in this paper,sodium 3-laurylaminopropanesulphonic acid（LMPS）with secondary amine group was selected as the object of investigation,and the pH switching property of secondary amine group was fully utilized to make it reversible between anionic surfactant LMPS（open state,dissolve state）and amphoteric internal salt 3-laurylaminopropanesulphonic acid（LMP,closed state,precipitate state）.Using quartz sand contaminated with tetradecane and polycyclic aromatic hydrocarbons（phenanthrene）as models,the removal of tetradecane or phenanthrene（Phe）by LMPS water solution from the surface of quartz sand was investigated.Furthermore,the possibility of recovery of LMPS,tetradecane and Phe and reuse of LMPS were investigated.Within the experimental research scope of this paper,the main results and conclusions are summarized as follows:（1）Synthesis of LMPS and pH switching propertiesLMPS was synthesized by dodecylamine and 1,3-propyl sulfonolactone（yield 76.3%）.The molecular structure of LMPS was verified by ¹H NMR,ESI-MS and FT-IR.The purity of LMPS was determined to be 98.3%by two phase titration.By investigating the Krafft temperature（K_T）,surface tension（γ）and interfacial tension（IFT）of LMPS as a function of pH,the pH switching window of LMPS was determined to be 7.5-12.5.When pH=12.5,K_T of LMPS was about-0.5°C,the critical micellar concentration（cmc）was about 8.7×10^-4 mol·L^-1（surface tension method）,γ_cmc was about 35.8m N·m^-1,and IFT_cmc was about 4.9m N·m^-1,which indicated that LMPS is one of the typical water-soluble anionic surfactants at pH=12.5 with the properties of micelles formation,good foaming and emulsification.When pH=7.5,LMPS was converted to LMP.At this time,the K_T of LMP was higher than 95°C,and the solubility in aqueous solution was 4.89×10^-5 mol·L^-1.γwas about 56.5 m N·m^-1,IFT was about 23.8 m N·m^-1.And LMP aqueous hasn’t micellar formation,foaming and emulsifying ability were poor,couldn’t reduce the surface tension value of water to 20 m N·m^-1,which did not have the characteristics of typical surfactants.（2）Emulsification performance of LMPS and pH response of emulsionWhen pH=12.5,2 mmol·L^-1 LMPS could emulsify the water-n-hepthane mixed system into O/W emulsion.The particle size of the emulsion showed Gaussian normal distribution,and the central value was about 17.4μm.The emulsion has good stability and could be stable at room temperature for at least 30 days.When the pH of emulsion was reduced from 12.5 to 7.5,the emulsion was rapidly demulsified to oil-water separation.LMPS were transformed into LMP,which were insoluble in the water phase,n-heptane,tetradecane,vegetable oil and other oil phases,and suspended in the oil-water interface.When pH increased from 7.5 to 12.5,LMP changed into water-soluble LMPS again,and homogenized emulsion was formed again.After nine cycles,the emulsion still maintained good stability and particle size increased slightly（about 20.6μm）.After the emulsion was demulsified（pH=7.5）,about 94.4%of LMP was suspended in solid precipitation at the oil-water interface,and about 3.1%of LMP was dissolved in the water phase,and the content of LMP in the oil phase was lower than 2.5%.If the pH of the water phase was adjusted back to 12.5 after demulsification without homogenization,the content of LMP in the oil phase was further reduced to under 0.7%.（3）LMPS enhanced soil washing and recovery from eluentsThe quartz sand contaminated with n-tetradecane（15 g/30 g sand）and Phe（3.5 mg/5 g sand）was washed with 150 g(1.0×10^-2 mol·L^-1)and 100 g（0.5wt%）LMPS solution,respectively.The removal rates of n-tetradecane and Phe on the surface of quartz sand were higher than 99.5%,indicating that LMPS has good washing ability.When the pH of the eluent was reduced from 12.5 to 7.5,the recoveries of tetradecane and LMPS were about 94.8%and 99.5%,respectively.After four reuses of the recovered LMPS,the recoveries of tetradecane and LMPS were 93.6%and 98.7%.Phe and LMP can be precipitated from the eluent at the same time by co-precipitation.Phe-LMP co-precipitation can be separated by Soxhlet extraction.The recovery rate of LMPS is about 96.5%.By adding a small amount of tetradecane（1 g/100 g eluent）to the above Phe-containing LMPS eluent,the operation was further simplified and the recovery rate of LMPS was increased to approximately 99.3%.After the treatment with anion exchange resin and activated carbon（0.25g/100g wastewater）,the chemical oxygen demand（COD）of the wastewater after recovering LMPS and organic pollutants significantly decreased from 500-1700 mg·L^-1 to less than 30 mg·L^-1.Total organic carbon（TOC）decreased from 100-210 mg·L^-1 to less than 15 mg·L^-1.