Hydrothermal Synthesis, Self-assembly On Solid/Water Interface And Phase Behavior Of Suspensions Of LDH Nanoparticles

Layered double hydroxides(LDHs) are a class of inorganic compounds which are structurally related to the mineral brucite Mg(OH)₂ with certain Mg²⁺ ions replaced by trivalent cations of similar size.This replacement results in a net positive charge on the layers,which is compensated by exchangeable anions placed between interlayer.Because of their unique properties and structure,LDHs can be used in catalysis,adsorption,nanocomposites,drug carriers and biosensors.Recently,LDHs nanoparticles self-assembled on the solid-liquid interface to form oriented LDH films have received extensive attention.In our previous work,Mg/Al LDH suspensions have been investigated for rheological properties,their stability,the formation of mineral liquid crystals and the ability to stabilize Picketing emulsions or foams.These applications require well-defined colloidal LDHs suspensions.A typical and widely used method for preparing colloidal LDH suspensions is co-precipitaion.It has been reported that when using non-steady coprecipitaion method to prepare LDH,the crystalline structure of LDH has formed at an early stage of the coprecipitaion and the following process of peptization only was used to make the particles grow up and develop their crystallinity.For a long time,crystal growth has been considered a process involving deposition of individual atoms or ions onto an existing crystal. However,a different picture about the growth of crystalline structures has emerged,in which nanoparticles serve as building blocks for construction of single crystals.This is referred as non-classical crystallization.Non-classical crystallization in organism has recently been found in crystallization of inorganic materials,which promotes us anew realizing the crystallization mechanism of LDH and provides a new method for us to obtain stable suspensions of LDHs particles with controlled sizes and shapes.In our group,mineral liquid crystalline phases have been found in colloidal suspensions of Mg-Al LDH platelets,which has largely contributed to the field of mineral lyotropic liquid crystals.Any researches for liquid crystalline properties of colloidal LDH suspensions can contribute to the typical Onsager's theory of liquid crystalline phase transition,and eventually it can well validate and enrich the Onsager's theory.As we all known that the phase behavior of colloidal suspensions can be well controlled through adjusting the interactions between colloidal particles in the suspensions by the addition of electrolytes or polymers.In this thesis,we have investigated the introduction of PVP on the phase behavior of suspensions of positively charged Mg₂Al LDH platelets by birefringence observations and rheological measurements.In terms of the AFM,XRD and FTIR analysis,we can conclude that PVP does not affect the crystal structure of LDH,nor adsorb onto the LDH particle surface,but induces a depletion attraction among the LDH particles. Compared to pure Mg₂Al LDH,the addition of nonadsorbing PVP enriches the phase behavior of such suspensions.At intermediate LDH and polymer concentration,two isotropic phases(I₁-I₂) coexistence,nematic-nematic(N₁-N₂) demixing and a sediment phase are observed,with the appearance of two-,three- four- and even six-phase coexistence.Multiphase coexistence in a binary system seems to conflict with the Gibbs' phase rule.The particles polydispersity and gravity reconcile the contradiction.We explain the richness of phase behavior in such LDH-PVP mixtures by discussing the interactions between PVP-induced depletion attraction,particle polydispersity and particle sedimentation.When the PVP and PEO with higher molecular weight were added,the LDH-PVP and LDH-PEO mixtures do not show the appearance of multiphase coexistence,in spite of the combined interactions of depletion attraction,particle polydispersity and particle sedimentation is still existent.Surprisingly,the addition of PEO makes the 23 wt%Mg₂Al LDH suspension show complicated rheological behavior.With the increasing of PEO concentrations,a continuous phase transition of sol-gel-sol has been found,which has never been reported.Our investigation has systemically explored the effect of nonadsorbing polymer induced depletion attraction on the phase behavior of Mg₂Al LDH suspensions.We hope that it can provide an effective theoretical fundamental for the potential applications of mineral lyotropic liquid crystals and artificial adjustment of liquid crystalline phase transition.Inspired by the non-classical crystallization,the synthesis of amorphous LDH precursors was carried out by a non-steady co-precipitation method under lower temperature of 0℃.The XRD and SEM characterization results demonstrated that amorphous nanoparticles instead of crystalline LDH particles formed in the coprecipitation process.The amorphous precursors were diluted and dripped on substrates,and then the dried gels were formed through the sol-gel transition.The following hot water and vapor phase transport treatments both resulted in crystalline LDH platelets,but the arrangement of the hexagonal platelets were parallel to and vertical to the substrates,respectively.The crystal growth mechanism of LDH nanoparticles is based on Ostwald ripening,while the formation of LDH films via dissolution-reprecipitation process.The size of LDH particles can be effectively adjusted.Increasing hydrothermal treatment time and temperature resulted in increasing of LDH particle diameters,but the transmittance and smoothness of the LDH films were reduced.The formation of oriented LDH films can be regarded as the growth of the LDH nanocrystals immobilized on the substrates,which is significant for the investigation of the crystallization process from amorphous precursors.In this thesis,according to our experience of the investigation of Mg-Al LDH, we also prepared aqueous suspensions of Ni/Al LDH nanoparticles via the non-steady co-precipitation followed by peptization.The peptization temperature and peptization time have a significant effect on the stability and dispersibility of the obtained colloidal suspensions.By choosing suitable peptization temperature and time,well dispersed suspensions were obtained by peptization at 130℃for 24 h.Meanwhile, the particle size,shape and size polydispersity can be efficiently controlled.Nematic ordering is observed in concentrated Ni/Al LDH suspensions and confirmed by birefringence observations and SAXS measurements.We also showed that the sol-gel transition takes place after a liquid crystalline phase transition in concentrated Ni/Al LDH suspensions.The absence of isotropic-nematic phase separation can be attributed to the fact that the nematic phase droplets are too small to settle to the bottom of the cuvette.The observation of nematic phase in Ni/Al LDH suspensions largely enriches the field of mineral lyotropic liquid crystals.In summary,stable LDH suspensions synthesized through hydrothermal treatment and oriented LDH films prepared by hot water and vapor phase transport treatments have been successfully carried out from the amorphous precursors.The addition of nonadsorbing polymer induces a depletion attraction among the LDH particles,and the combination of particles polydispersity and particle sedimentation can effectively control the liquid crystalline phase behavior of the LDH suspensions.