Study On Afm Determination Of Point Of Zero Charge Of Gibbsite

Gibbsite is widely used in flame retardants,electrolytic aluminum,building materials,medicine and other fields.Point of zero charge(PZC)is an important parameter to characterize the charging behavior of the gibbsite surface.In theory,PZC values were determined by the proton reactions of different hydroxyl groups on the surface.However,the measured PZC values of gibbsite reported in the previous studies were not constant and varied from p H=7 to p H=10.Particularly,there was still a controversy on the proton reaction activity of the double coordination hydroxyl group(Al2OH)of(001)surface.The mainly problem lies in that the sample size adopted by the researchers limited the application of the PZC determination methods for single crystal surface.And the influence of purity of gibbsite and dissolved CO2 on PZC were ignored in the experiment,which caused the inaccuracy of PZC determination.In addition,there is a gap in the PZC measurement of the gibbsite(100)surface,and the theoretical model of the proton reaction of single coordination hydroxyl group(Al OH)has not been verified by experiment.Here,we prepared gibbsite crystals using supersaturated potassium aluminate solution.The effects of relative supersaturation,growth temperature and alkali concentration in solution on crystal growth were systematically studied,and then suitable conditions for crystal growth were obtained.When the ratio of alkali to aluminum in the solution is fixed,the change of alkali concentration will affect the morphology of gibbsite crystals.At this time,the relative supersaturation decreases with increasing temperature,regardless of the change in alkali concentration.The results showed that high temperature and high alkali concentration were favorable for the growth of large-size,prismatic gibbsite crystals.Under the conditions of potassium hydroxide concentration of 4 mol/L,relative supersaturation of 0.5 and temperature of 90 ℃,the gibbsite crystals with 100～150 μm and higher purity than industrial crystals were obtained.The AFM colloidal probe technique was used to measure the PZC of the(001)surface of gibbsite before and after degassing.The effect of CO2 in solution on PZC was systematically studied and compared with the experimental results in the literature.It is found that after degassing treatment,the PZC value of the(001)surface measured by the experiment is lower than that before the degassing treatment.Compared with the experimental results of the same sample in the literature without degassing,the difference between the research results and the literature results fits to the rule of the influence of CO2 on the PZC value.The effect of sample purity on the PZC measurement results was studied,by measuring the PZC of(001)and(100)surfaces of industrial crystal and the gibbsite prepared in this study.The shortcomings of the experimental measurement process in the literatures were pointed out and optimized by degassing and improving the purity of the sample.The PZC value of the gibbsite(001)surface was p H=6.1 under the optimized conditions.Due to the small number of data points,the experimental results of PZC are uncertain.In the subsequent studies,the number of measured data points should be further increased to improve the accuracy of the PZC of gibbsite.The results were in accordance with the view of the Bickmore model,in which the Al2 OH groups have protonation activity in acidic solutions.Combined the results of this study with the Bickmore model,the proton reaction constant of the Al2 OH group was improved from p Ka=5.2 to p Ka=5.2～6.1.In addition,it is pointed out that the(100)surface cannot be cleaved,so the contaminants or other crystal planes on the(100)surface affect the accuracy of PZC.The AFM contact,tapping mode probes and diamond colloidal probe were used to flatten the(100)surface,which provided a reference for the subsequent(100)surface smoothing and PZC measurement.