Adsorption And Rheological Properties Of Aqueous Suspensions Containing Hydrotalcite-like Compounds And Cationic Starch

Adsorption and rheological properties of suspensions are interesting subjects in colloidal chemistry.But some basic problems in adsorption and rheological phenomena of suspensions have been incompletely understood.For instance,in our former works, we discovered a new thixotropical behavior,i.e.,complex thixotropy in HTlc/clay suspensions.But we lack of comprehensive understanding of the phenomenon and mechanism of complex thixotropy.In this paper,the adsorption and rheological properties of suspension containing hydrotalcite-like compound(HTlc)which has permanent positive charge and cationic starch were studied.The main purpose of this paper is to investigate whether the complex thixotropy exists objectively and understand the factors which influence thixotropy of system and the mechanism of thixotropy.Additionally,both hydrotalcite-like compounds(HTlc)and cationic starch(CS)are materials which have broad application foreground.So,the suspensions containing HTlc and CS have extensive appliance and wide foreground in many fields,such as the treatment of wastewater,drug delivery,cosmetics,recovery of oil and so on.It is a theoretical prerequisite for enhancing and broadening application of HTlc and CS to investigate whether there are synergistic effects between their properties.The adsorption measurement is a valid method to investigate the interaction between polymer and particles.And the theology of colloidal suspension containing charged particles and water-soluble polymer is helpful to understand that the adsorption of the polymer on charged particles,the stability and flocculation of the suspension.On the other hand,we can definite the rules of their interaction and expect to control the microstructures of suspensions.The main contentents of this paper are as follows.1.The synthesis and structure properties of samplesMg-Al HTlc was synthesized by using the co-precipitation method.HTlc particles were plate-like and the average diameter was about 159.8 nm.The isoelectric point(IEP) of HTlc suspension was about pH_IEP=11.2.CS-1 and CS-2 solution were prepared.The intrinsic viscosity,weight-average molecular weight and the degree of substitution of CS-1(or CS-2)were 33.92(or 39.88)mL/g,5.01×10⁵(or 6.44×10⁵)g/mol and 0.21(or 0.03),respectively.2.The adsorption of CS on Mg-Al HTlc particlesThe objective of this part is to investigate the adsorption behaviors of CS on Mg-Al HTlc and establish the adsorption mechanism.The main results are as follows.â‘ The adsorption isotherms for CS on Mg-Al HTlc were Langmuir types.The adsorption amount of CS increased evidently with increasing of the concentration of electrolyte and pH.The adsorption amount of CS-1 increased but that of CS-2 decreased with increasing of the temperature.â‘¡The adsorption kinetics curve of CS-1 showed an overshoot,and that of CS-2 increased gradually and then reached a steady state.â‘¢CS molecules were only adsorbed on the external surface and not intercalated into the gallery of HTlc.3.The linear viscoelastic behaviors of Mg-Al HTlc/CS suspensionsDynamic theology under oscillation shear was the preferred technique to monitor the microstructure of suspensions.Low-frequency oscillation in the linear viscoelastic region is a sensitive tool to determine the microstructural change and respond the interaction between particle and particle and/or particle and polymer.In this section,the linear viscoelastic behaviors of Mg-Al HTlc/CS suspensions were studied.The effects of the degree of substitution of CS,the content of HTlc(R),pH,electrolytes and the temperature on the linear viscoelasticity of Mg-Al HTlc/CS suspensions were investigated,in order to definite the interaction between CS and HTlc.The main results are as follows.â‘ G′andσ_c of CS solution increased with increasing of the concentration of CS,decreased with increasing of the concentration of electrolyte and temperature,and increased and then decreased with increasing of pH.â‘¡For HTlc/CS suspensions,G′increased initially and then decreased with increasing of R because the 'briding effect' and 'screening effect' cxsited in the suspension.G′of suspension decreased with increasing of NaCl and CaCl₂ concentrations,but increased with the addition of AlCl₃.G′increased initially and then decreased with increasing of the pH value;G′decreased with increasing of the temperature.For Mg-Al HTlc/CS-1(R =0.08)suspension with the addition of 3 mmol/L AlCl₃,G′increased with increasing of pH;G′increased initially and then decreased with increasing of the temperature and had the maximum value at 55℃. 4.The thixotropic behaviors of Mg-Al HTlc/CS suspensionsThe thixotropy of suspension is a complicated rheological phenomenon.In this part, the effects of the degree of substitution of CS,the content of HTlc(R),pH,electrolytes and the temperature on the thixotropic properties of Mg-Al HTlc/CS suspensions were investigated,in order to find the thixotropic behaviors of suspensions under above conditions and whether suspensions could display complex thixotropy.Steady shear experiment,hysteresis loop experiment and linear oscillatory shear experiment were used to study on the thixortopy of suspensions.The results can lead us to understand the microstructure change within suspensions under shear and at rest,and it is helpful to apprehend the thixotropic mechanism and phenomena more deeply.The main results are as follows.â‘ The results that Mg-Al HTlc/CS suspensions showed complex thixotropy indicated that the complex thixotropy was a natural phenomenon,not an apparent one.â‘¡For pure CS solution,the negative thixotropy is contributed to the formation of shear-induced aggregation or supermolecular structures in CS solution.The strength of negative thixotropy of CS solution decreased with the addition of electrolytes,with increasing of temperature and in acidic or alkali conditions. The appearance of complex thixotropy of HTlc/CS suspension is due to the formation of shear-induced supermolecular structures among CS molecules and shear-induced destruction of the bridge structures between CS molecules and HTlc particles.If the strength of the former is bigger than that of the latter,suspension displays neagative thixotropy.Contrarily,suspension displays positive thixotropy.When the strength of the former is similar to that of the latter,suspension can diaplay complex thixotropy.â‘¢The thixotropic type of HTlc/CS suspensions transformed from negative thixotropy to complex thixotropy to positive thixotropy to complex thixotropy and then to negative thixotropy with increasing of R.The thixotropic type of HTlc/CS suspensions transformed from original type to positive thixotropy with increasing of the concentration of electrolyte,temperature and pH.â‘£It was demonstrated again that the hysteresis loop of complex thixotropy had a crossover point that divided the loop into two sections.⑤The viscosity decreased firstly and then increased in complex viscosity of HTlc/CS-1 suspension but the viscosity increased firstly and then decreased in complex viscosity of HTlc/CS-2 suspension,which is due to the interaction strength between HTlc and CS-2 is stronger than that between HTlc and CS-1.