Study On The Covalent/non-covalent Construction And Aggregation Behavior Of Rosin-based Surfactants Containing Large Hydrophobic Groups

Surfactant molecules can form various aggregates driven by molecular interactions,endowing solutions with rich physicochemical properties.Surfactant viscoelastic solutions have important applications in oil recovery,material science,drug delivery and other fields.However,the aggregation process of surfactants is delicate and complicated.To prepare advanced molecular aggregates with designed structures,the self-assembly mechanism of surfactants needs to be thoroughly investigated.It is necessary to clarify the impact of specific groups on the molecular interactions,as well as the relationship between molecular structures and aggregate morphologies.Therefore,investigation on the aggregates formed by specially synthesized surfactants is important for the development of soft materials with novel properties.Dehydroabietic acid is an important derivative of rosin.Herein,by introducing flexible alkyl groups of different lengths,two series of six rosin-based surfactants derived from dehydroabietic acid were prepared.The general feature of these surfactants is the large hydrophobic groups contained in the molecule.The first series of molecules is pseudo Gemini surfactants di（R-n-3）-MA（n=6,11,12）,which is formed by rosin-based tertiary amine and malic acid（MA）with the molar ratio of 2:1 through acid-base neutralization.The other series of molecules is surfactant R-n-3-DA（n=6,11,12）with double head group and one tail chain.Subsequently,the surface activities,solutions rheological behavior and aggregate morphologies of two series of surfactants were systematically investigated,and the unique behavior of rosin groups in the interfacial adsorption and aggregation was explored.The main results were summarized as follows:（1）The surface activities of these rosin-based surfactants.The critical micelle concentrations of the pseudo Gemini surfactants di（R-n-3）-MA（n=6,11,12）are 0.29 m M,0.037 m M,and 0.010 m M respectively.The surface tensions at critical micelle concentrations are 33.58 m N·m^-1,37.90 m N·m^-1and 37.25 m N·m^-1 respectively.The critical micelle concentrations of surfactants R-n-3-DA（n=6,11,12）with double head group and one tail chain are 6.51 m M,2.05 m M and 2.03 m M respectively.The surface tensions at critical micelle concentrations are 43.94 m N·m^-1,47.43 m N·m^-1,and 46.06 m N·m^-1 respectively.The comparative results indicates that the pseudo Gemini surfactants di（R-n-3）-MA shows better surface activity and stronger aggregation ability.Due to the strong electrostatic repulsions between molecules,R-n-3-DA has weaker aggregation ability and rather higher critical micelle concentrations.R-n-3-DA molecule also arranges loosely at the air/water interfaces(A_min>1nm²),leading to poor ability in reducing the surface tension.（2）The viscoelastic properties of rosin-based surfactant solutions.The peduo Gemini surfactants di（R-n-3）-MA solutions exhibits excellent viscoelasticity.di（R-6-3）-MA is a low molecular weight gelator at low temperatures.The critical gelation concentration of di（R-6-3）-MA solution is 10 m M,and the sol-gel transition temperature is between 10 ^oC and 12 ^oC.On the other hand,the critical overlap concentration of di（R-11-3）-MA and di（R-12-3）-MA is as low as 7.79 m M and 8.36 m M respectively.The zero-shear viscosity of solutions is highly dependent on concentrations,and the two follow a power exponential relationship ofη₀∝C^7.79andη₀∝C^12.34,which fully demonstrates the prominent effect of hydrophobic groups with large volume on the growth of aggregates.In addition,the viscosity of di（R-11-3）-MA solution is highly sensitive to inorganic salts.The addition of 0.06 wt.%Na Cl can increase the viscosity of the solution by over 100 times.The di（R-11-3）-MA/Na Cl system thus has the potential to be applied in oil recovery as the fracturing fluid.Unfortunately,the viscosity of double-headed R-n-3-DA solution is similar to that of water within a wide concentration range.However,by adding appropriate amount of carboxylate surfactant C_m-1COONa（m=10,12,14）,the solution becomes gel.The R-11-3-DA/C₁₁COONa system is optimal.When the molar ratio of R-11-3-DA and C₁₁COONa is 1:1.5,the critical gelation concentration and the critical overlapping concentration can be as low as 5 m M（0.58 wt.%）and 0.1 m M（0.01 wt.%）.And the system has a the gel-sol transition temperature,which is about 60 ^oC.（3）Aggregates morphology in the viscoelastic solutions of rosin-based surfactants.Cryo-TEM results shows that there are mainly two types of aggregates in the viscoelastic solutions.One is the rigid nanofibers and the other is flexible wormlike micelles.In the pseudo Gemini surfactant di（R-n-3）-MA solutions,the proportion of the rigid and flexible hydrophobic portions in the molecule affects the type of aggregates.When the proportion of rigid part is larger,rigid nanofibers are preferred due to the densely regular packing of molecules within the aggregates.Conversely,the surfactants tend to form wormlike micelles.In the mixed systems of R-n-3-DA and C_m-1COONa,the morphology of aggregates is affected by the relative length of alkyl chains in R-n-3-DA and C_m-1COONa.When the alkyl chain in R-n-3-DA is shorter than that in C_m-1COONa,flexible wormlike micelles are formed due to the disturbance of the hydrophobic chains of C_m-1COONa on the ordered packing of R-n-3-DA molecule.Or wormlike micelles will appear instead.The above findings provide new ideas for regulating the morphology of aggregates at the molecular level.（4）Performance comparison between wormlike micelle solutions and nanofiber solutions.There are significant differences between the two in viscosity recovery and viscosity changes with temperature.The viscosity of the wormlike micelle solution can recover within 1 min after shearing,and the flow activation energy is in the range of 70-300 k J/mol.However,the viscosity of the nanofiber solution recovers slowly and cannot reach initial value.The nanofiber solution has a gel-sol transition temperature.Before this temperature,the viscosity of the solution exhibits temperature inertia,while at which the viscosity drops promptly.In addition,nanofiber solutions can prepare gel emulsions due to their high efficiency in gelling continuous phase.For example,a new type O/W gel emulsions are successfully prepared with R-12-3-DA/C₉COONa mixture as emulsifiers and stabilizers.The volume fraction of oil phase can be adjusted between 10%and 90%,expanding the preparation method of gel emulsions.