Phase Behavior Of Aqueous Suspensions Of Mg₂Al Layered Double Hydroxide: Competition Among Nematic Ordering, Sedimentation And Gelation

The liquid-crystalline phase is a state between liquid and crystal phases.Though liquid-crystalline materials are extremely diverse,the most of them are organic.Up to date,there are thousands upon thousands of organic liquid-crystalline compounds,in contrast with decades of mineral liquid crystals that have been characterized.The mineral liquid-crystalline compounds can be electron rich and have enhanced electrical,optical,and magnetic properties.What's more;mineral materials are thermally stable,so they can be employed in extreme conditions.These entire virtues give them enormous application potential in future.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 receive extensive attention as they are considered to be used as catalysts,catalyst precursors,catalyst supports,adsorbents,anion exchangers,and promising materials for nanocomposites. Presently most of studies concerning these systems are related to synthesis,properties, performance,and so on,while little has been done on the ordered structure of their suspensions.Recently,we have studied the rheological properties,stability of Mg-Al LDHs suspensions.And we discussed application of LDHs in the field of emulsion and drilling fluid.Mg-Al LDHs suspensions have ordered phases.The ratio of Mg to Al,the Size and polydispersity of particles,the electrical properties of LDHs compounds,particle concentration and ionic strength of suspensions play important roles to the phase behavior of the suspensions.Here we study the disordered-ordered and sol-gel phase transitions of Mg₂Al LDH(Mg;Al=2;1)suspensions,and discuss the relationship between osmotic pressure and phase transitions.Gravity and particle size have effects on the phase behavior of the Mg₂Al LDH suspensions.The Mg₂Al LDH suspension was prepared by nonsteady coprecipitation method. The morphology and size of Mg₂Al LDH particles is determined from transmission electron microscopy(TEM)and atomic force microscopy(AFM)technology.Mg₂Al LDH particles are hexagonal platelets.The number-average diameter of the particles is larger than 100nm,and the number-average thickness is around 7nm.The XRD pattern provides strong evidence for the hydrotalcite-like structure of the Mg₂Al LDH sample.Birefringence observation and rheological measurement were used to monitor the phase behaviors of Mg₂Al LDH suspensions.The suspensions undergo an isotropic-nematic(I-N)transition before the sol-gel(S-G)transition.The suspensions of concentration lower than 16wt%appear isotropic(â… )between crossed polarizers.In contrast,the suspensions of concentration between 16wt%and 30wt%have I-N biphasic coexistence.In the suspension of 30wt%,a critical sol-gel transition appeared.Increasing the concentration,the gel network hindered a complete I-N separation in the suspensions.Upon raising the NaCl concentration,the liquid-crystalline phase transition and the sol-gel transition shifted to higher particle concentrations.The dimensionless density for the coexisting isotropic phaseρ_iSOD³ in this experiment was lower than that in computer simulation,and the coexistence region was wider than what simulations predicted.The deviation of the experimentalρ_iSOD³ value from the simulation could be explained by the following factors.First, Onsager's theory neglected interaction among three(or more)particles which are important in the concentrated suspensions.If the interactions among many particles are taken into account,the theoretical value ofρ_iSOD³ should be reduced.Second, the morphology of particles is important to the phase transitions of colloidal suspensions.Many of the previous simulation studies were performed on monodisperse hard disks,while the Mg₂Al LDH particles are polydisperse hexagonal platelets.For monodisperse systems,the I-N coexistence density for the hexagonal particle system would shift to a lower value than that for a circular particle system, because the effective diameter of the hexagonal platelet is larger than that of the circular platelet with equivalent area.Third,semi-grand simulations revealed that polydispersity in the size of the particles led to the widening of the coexistence region. Fourth,the existence of clouds of counterions increases the effective volume fraction of Mg₂Al LDH particles,which helps attain the phase transition threshold.Last but not least,the gravity field plays a crucial role in the phase behavior of Mg₂Al LDH suspensions.The sol-gel transition was determined using the rheological methods.Mg₂Al LDH suspensions underwent a sol-gel transition at 30wt%.The suspensions with concentration lower than 30%(w/w)were sols,and the storage modulus(G')and the loss modulus(G")were both weak,indicating a slightly viscous suspension.At 30wt%,G' was almost equal to G" in the viscoelastic linear region.In contrast,above 30wt%,the suspensions were gels;G' was significantly higher than G",and G' and G" does not vary much with frequency in the viscoelastic linear region.Increasing the particle concentration and decreasing the NaCl concentration stabilized the gelphase. The rheological behavior of the gels can be interpreted as corresponding to repulsive gels.Gravity enables the particles to span a large density range in a test tube,and increases the particle concentration at the bottom.All of these facilitate the phase transition of colloidal suspensions and broaden the region of I-N coexistence.In addition to the effect of gravity on individual particles,the dynamical stability of the colloid also depends on the particles' Brownian motion and the electrostatic interaction among the particles.In dilute suspensions,Mg₂Al LDH particles repel each other,and have time to explore configurational space before settling to the bottom.On the other hand,the Brownian motion causes particles in suspensions to collide with each other.As particle concentrations rise,the number of collisions increases,which increases the probability of particles aggregating.When the distance between two particles is close enough,some more or less extended micro-domains appeared in the colloidal suspensions.The transient birefringence in sols and the permanent birefringent textures in gels suggest that large anisotropic particle associations form in these systems.The diameter and volume of the associated structure is larger than of the individual Mg₂Al LDH particle,so the Peclet number of the associated structure is larger.Hence in the concentrated suspensions,these associated structures settled to the bottom before having time to explore configurational space.Sedimentation occurred before I-N transition in the suspensions of 25wt%to 27wt%.Mg₂Al LDH suspensions were introduced in the dialysis tubes and placed in dextran solutions.When the osmotic equilibrium was reached in those conditions,the osmotic pressure of Mg₂Al LDH suspensions was yielded according to the final dextran concentrations.Whatever the concentration and the ionic strength of Mg₂Al LDH suspensions,a net repulsion existed between the particles.For all the samples, the shape of the curve is similar,and a pseudoplateau was found in the osmotic pressures curves of Mg₂Al LDH suspensions.The extension of the plateau is more horizontal and more extended with increasing ionic strength;And upon raising the ionic strength,the osmotic pressures curves first decrease to a minimum,then increase again.The Mg₂Al LDH suspensions had an I-N transition before gel formation.The I-N transition coincides with the change of slope revealed in osmotic pressure measurements,while the appearance of permanent birefringence appears at LDH concentrations lower than that corresponding to the end of the pseudoplateau.The size and polydisperse of the Mg₂Al LDH particles affected the phase behavior of the suspensions.The location of the first break in the osmotic pressure curves increased with the particle size;and the extension of the plateau increased with the polydisperse of particle size.Whatever the particle size,the I-N transition coincides with the position of the first break in the osmotic pressure curves.The concentration at which permanent birefringence appears is higher than the end of the pseudoplateau of the osmotic pressure.These diagrams exhibit I-N transition lines with slightly positive slopes.SAXS experiments provide strong proof for the lamellar signature of the dense liquid crystal phase after longtime rest,but not nematic phase as we showed before. We ascribed the difference to the waiting time of Mg₂Al LDH samples before SAXS.Liquid crystal phases of LDHs dispersions belong to a special class of colloidal soft condensed matters,which are especially predominant for liquid crystal phase transition models or for applications.Firstly the permanent positive surface charge of the particles can be tuned by varying di- and tri-valent metal ions ratio,and variable surface charge of the particles can be controlled by electrolytes and pH value,which is convenient for studying the influence of surface charge density on liquid-crystalline phase transition.Secondly one can prepare LDHs with particular properties by selecting appropriate metal ions to form the brucite-like sheets and intercalating functional molecules into the gallery between the sheets.Furthermore,LDHs have developed into a large family of materials.This would give lyotropic liquid crystals based on LDHs platelets promising scientific and technological applications.