Colloidal Stability Of Organic Particles In Nonpolar Media

Nonpolar suspensions have been widely used in numerous applications,including medicine,food,cosmetic,and petroleum industry.To improve the affinity between particles and nonpolar media,and the adsorption capacity of particles at the oil/water interface,surfactants and silane coupling agents having a long chain alkane are used to modify the particles.The modifiers adsorb on the particles by ion exchange and covalent grafting,and the adsorption strength is determined by the adsorption mode.Temperature variation can shift the adsorption-desorption equilibrium of modifiers at the solid/liquid interface,which will change the wettability of particles,and finally affect the stability of nonpolar suspensions.Meanwhile,it can also change the conformation of modifiers,which will alter the solvation of particles and the interaction between particles,and finally affect the rheological properties of nonpolar suspensions.In this dissertation,based on the stabilization mechanism of nonpolar suspension,the dispersion stability and rheological property of nonpolar suspensions were controlled by adjusting the equilibrium of adsorption-desorption of the modifiers at the solid/liquid interface and the solvation of particles.The contents of this dissertation are as follows:1.The Na-montmorillonite was modified with conventional quaternary ammonium salt(DODMAC)to prepare organoclay,which was then dispersed in Marcol 52 or n-heptane for aging.The increase of aging temperature leads to the decrease of dispersion stability and apparent viscosity,and the enhancement of particle aggregation.It may be attributed to the high temperature induced desorption of DODMAC in organoclay,which was proved by elemental analysis and XRD measurement.According to the thermal decomposition process of organoclay analyzed by TG-IR-GCMS,the adsorption types of DODMAC on Na-montmorillonite were classified:(1)DODMAC molecules physically adsorbed on particle surface;(2)DODMA+ions adsorbed on particle surface by ion exchange;(3)DODMAC molecules physically adsorbed in layers and DODMA+ ions intercalated by ion exchange.Specifically,high temperatures induced the complete desorption of physically adsorbed DODMAC molecules on particle surface,the partial desorption of ion-exchanged DODMA+ions from particle surface to nonpolar media,and the partial transfer of ion-exchanged DODMA+ions from the particle surface to the interlayer.Importantly,the desorption of ion-exchanged DODMA+ions resulted in the destabilization of organoclay suspensions at high temperatures.This work provides new insights into the investigation of temperature dependent adsorption-desorption behavior of modifiers at the solid/liquid interface.2.In order to reduce the desorption of quaternary ammonium salt at high temperatures,it is necessary to optimize their structure.Gemini quaternary ammonium salt(18-4-18)and conventional quaternary ammonium salt(OTAC)were used to prepare organoclay,respectively.The hydrophobicity of these two organoclays were lower than that of organoclay prepared by DODMAC,which was proved by contact angle test.High temperature induced desorption of 18-4-18 and OTAC in organoclay suspensions was confirmed by XRD,elemental analysis and TGA measurement.Both 18-4-18 and OTAC are less hydrophobic than DODMAC,they tend to adsorb on organoclay rather than desorb to nonpolar media.Therefore,the desorption ratios of 18-4-18 and OTAC were significantly lower than that of DODMAC.The quaternary ammonium salts desorb from the solid/liquid interface to the nonpolar media can adsorb at the oil/water interface,and the desorption amount can be calculated according to the interface tension obtained by IFT test.Due to the low desorption ratio of 18-4-18 and the moderate hydrophobicity of organoclay,the water-in-oil Pickering emulsion prepared with these organoclays has high coalescence stability and stable rheological properties at high temperatures.This work has guiding significance for the preparation of water-in-oil Pickering emulsions with stable rheological properties at high temperatures.3.In order to prevent the dramatic increase in viscosity of water-in-oil Pickering emulsions upon cooling,the alkyl chain length of modifiers on silica nanoparticles(NPs)and the surface coverage of NPs were optimized.The rheological properties and microstructure of the water-in-oil Pickering emulsions,as well as the solvation of the NPs,were studied using diffusing-wave spectroscopy microrheology measurements,confocal laser scanning microscopy,and low-field nuclear magnetic resonance measurements.The results indicated that decreasing the temperature can reduce the solvation of NPs.As a result,the attractions between NPs adsorbed on adjacent emulsion droplets or flocculated in the nonpolar media may increase with decrease in temperature.The increased attraction may increase the apparent viscosity of emulsions significantly at low temperatures.Importantly,the solvation of NPs is less dependent on the temperatures by shortening the alkyl chain length of surface groups and optimizing the contact angles of NPs.The small decrease in solvation lead to the slight increase in viscosity upon cooling.This work proved that the temperature dependent rheological properties of Pickering emulsions can be controlled by adjusting the solvation of particles,thus providing a new strategy to yield Pickering emulsions with excellent rheological properties at low temperatures.