Interface Electrochemical Properties Of Colloids With Structural Charge

The interface electrochemical(IEC)properties of colloids are the basic research contents of the colloid and interfacial chemistry.Many models such as surface-charging models,electrostatic models,and the point of zero net charge(PZNC)models have been proposed to describe or predict the IEC properties of solid-water interfaces.The structural charge(?st)was not taken into account when establishing the model equations.Therefore,these models are not suitable for colloids with structural charge.Studying the effect of structural charges on IEC properties can provid a better understanding for the IEC phenomena.In our previous work,the structural charge density as a model parameter was introduced into the basic 1-pK and 2-pK models,and the so-obtained models may be called "general" models,which are suitable for colloids with or without structural charge.However,it has been not performed to combine the "general" surface-charging models with an electrostatic model to describe the surface-charging behavior of amphoteric solids.It is essential to examine the practicability of the general surface-charging models.Recently,Li-Al layered double hydroxides(LDHs)with positive structural charge and ammonium aluminum carbonate hydroxide(AACH)with negative structural charge have received great attention.However,no attention has focused on their IEC properties.Especially,the main chemical components(or basic structural unit)of Li-Al LDHs and AACH both are Al(OH)3,and thus they can be regarded as the derivatives of Al(OH)3(called Al(OH)3-based materials).Li-Al LDHs and AACH are the ideal models to study the IEC properties of colloids with structural charge.Hydrothermal method is the most common method to synthsize Li-Al LDHs and AACH,but the formation mechanism is not clear.In this work,the IEC properties of colloids with structural charge were investigated.A general PZNC model was established to reveal the effect of structural charge on PZNC of colloids.The hydrothermal mechanisms of Li-Al LDHs and AACH were studied,providing a better understanding of the formation process and the samples for the IEC studies.The IEC properties of Li-Al LDHs and AACH were investigated by experiments and model fitting.Especally,the general surface-charging models were used to describe the surface-charging behavior of Li-Al LDHs and AACH,revealing the influence of structural charge on IEC properties and verify the rationality of the established general surface-charging models.Main research contents and conclusions:(1)Establishment of PZNC models:(1)Genereal crystallochemical modelThe PZNC is one of the most important parameters involved in the IEC properties of amphoteric solids.Many models such as electrostatic,crystallochemical,and electronegativity models have been developed to predict the PZNC of(hydr)oxides without structural charge.Based on general 1-pK model and the Pauling's electrostatic valence principle,a general crystallochemical model was proposed,which correlates the PZNC(or pHPZNC)of amphoteric solids with ?st,Pauling bond valence,and metal-oxygen(M-O)bond length,indicating that the PZNC is proportional to the ?st.The positive structural charge(?st>0)leads to an increase in pHPZNC,while the negative structural charge(?st<0)to a decrease.The rationality of the general PZNC model was confirmed by the experimental data of LDHs with positive structural charge and clay minerals with negative structural charge.The model can be used to describe or predict the PZNC of colloids with and without structural charge.(2)Establishment of PZNC models:(?)Genereal electrostatic-electronegativity modelThe application of the genereal crystallochemical model model islimited,because the crystal parameters,including the mean metal-oxygen(M-O)bond length and Pauling bond valence,exist in the model equations.The values of the crystal parameters are not available in the literature for some solids,especially for new materials.It is of great practical value to establish the correlations between the PZNC and some easily available characteristic physical constants of constituent elements for structurally charged amphoteric solids.Therefore,we developed a new model,called the general"electrostatic-electronegativity"(ESEN)model,by combining the classical electrostatic and electronegativity models.The general ESEN model correlates the PZNC with ?st and characteristic physical constants.The characteristic physical constants involved include ionic valence,ionic radius,and electronegativity,which are easily available in the literature or chemistry handbooks.The general ESEN model can describe or predict the PZNC of(hydr)oxides(without structural charges),LDHs,and clay minerals with acceptable feasibility and accuracy.The prediction accuracy of this model for PZNC is close to that of the genereal crystallochemical model.The characteristic physical constants in the model are easily available in the literature or chemistry handbooks,which is important for the design synthesis and application of new materials.(3)Formation mechanisms of Li-Al-CO3 LDHs and AACHLi-Al-CO3 LDHs were synthesized by a hydrothermal method using LiCl,AlCl3,and urea as raw materials.The effects of LiCl/AlCl3 molar ratio(RLi/Al)and hydrothermal time(tHT)on the structure,morphology,and composition of products were investigated.It was found that only at RLi/Al>1.00,the pure LDHs phase is formed,which shows flower-like microspheres with a chemical formula of[LiAl2(OH)6](CO3)0.5·2H2O.At RLi/Al?0.25,the pure AACH phase is formed,which shows rod-like particles with a chemical formula of NH4Al(OH)2CO3·0.5H2O.At 0.25<RLi/Al<1.00,the product is the mixture of LDHs and AACH.The formation of the Li-A1 LDHs is essentially via a Li+-intercalation mechanism.Amorphous Al(OH)3 is first formed,and Li+ions are then intercalated into the vacancies in the Al(OH)3-layers to form crystalline Li-Al-CO3 LDHs phase.AACH was synthesized by a hydrothermal method using AlCl3 and urea as starting materials.Special emphasis was placed on the effects of tHT on the structure,morphology,and composition of products.A two-step formation mechanism was demonstrated,namely,bayerite Al(OH)3 is first formed and then transformed to AACH via a dissolution-recrystallization process in the presence of HCO3-and NH4+.(4)Interface electrochemical properties of Li-Al-CO3 LDHsThe IEC parameters of Li-Al-CO3 LDHs,including adsorbed proton charge density(?H),effective ?st(?st,eff),and PZNC,are directly obtained from the acid-base titration data.Its ?st,eff and pHPZNC are obtained to be?5.27×10-3 C/m2 and 9.22,respectively.The pHPZNC of Li-Al-CO3 LDHs is higher than that of Al(OH)3(?8.25),and consistent with the predicting value of the general PZNC models.On the basis of the general surface-charging(1-pK and 2-pK)models,the intrinsic surface reaction equilibrium constants pK,pKalint,and pKa2int were obtained to be-8.50,7.49,and 9.51,respectively.The general 2-pK model is combined with the diffuse layer model(DLM,an electrostatic model)to fit the surface-charging data at different concentration of NaNO3(c)and pH.Model predictions are in good agreement with the experimental data,indicating that the general surface-charging models are reasonable.In addition,the changes of the adsorbed proton charge density(?H),surface potential(?0),surface site(Sur-OH2+,Sur-OH,and Sur-O-)distribution,and the point of zero net proton charge(pHPZNPC)with c and pH were analyzed.The pHPZNPC is lower than the pHPZNC,and increases with the increase of c,which is consistent with the prediction of the general surface-charging model.This work provides a better understanding of the IEC properties of Li-A1 LDHs.(5)Interface electrochemical properties of AACHThe IEC parameters of AACH including ?H,?st,eff,pHPZNC,and pHPZNPC were determined by potential titration and salt titration.The ?st,eff and pHPZNPC were obtained to be-0.24 C/m2 and 5.10,respectively,which indicates that AACH possesses negative structural charge.The pHPZNC of AACH is lower than those of Al(OH)3(?8.25)and Li-Al-CO3 LDHs(9.22),consistent with the prediction of the general PZNC models.Contrast to Li-Al-CO3 LDHs,the pHPZNPC of AACH is higher than its pHPZNC,and decreases with an increase in c,consistent with the prediction of the surface-charging model.Based on the general 1-pK and 2-pK models,the pK,pKalint,and pKa2int were obtained to be ca.-6.05,4.92,and 7.18,respectively,which are all lower than those of Li-Al-CO3 LDHs(ca.-8.50,7.49,and 9.51,respectively).Based on the general 2-pK model and the Stern model,the theoretical relationship of pHPZNPC,?st and c was established.In addition,a "surface net charge site"(SNCS)model was suggested,which assumes that the formation of the Stern layer in the Stern model arises from the electrostatic adsorption of counterions on the surface net charge sites,which is characterized by the intrinsic electrostatic adsorption equilibrium constant(Keaint).The general 2-pK model was combined with the Stern model and the SCNS model to fit the surface-charging data of AACH.Model predictions are in good agreement with the experimental data,and the pKeaint was optimized to be?1.10,which reveals the main influence factors on the IEC properties and indicates that the general surface-charging model and the SCNS model are reasonable.In addition,the changes of ?0,surface charge density of the Stern layer(?Stern),?0,the potential at the head end of the diffuse layer(??),and surface site distribution with c and pH were analyzed.This work provides a better understanding of the IEC properties of AACH.