| Spray drying is widely used in daily chemicals,pharmaceuticals,and food industries due to its advantages of high productivity,low cost and a high level of automation.The literature shows that spray drying of homogeneous precursor solution containing multi-components can sometimes produce dried particles with segregating ingredients,forming distinct coreshell structure.The core-shell structure has a profound effect on the stability,solubility and flowability of dried particles.At present,several hypotheses have been proposed to explain the mechanisms underlying the drying-induced ingredients segregation,such as the Marangoni flow,different surface activity of solutes and different diffusivity of solutes.These hypotheses suggested that mixed components could show relative migration during the process of droplet drying,and factors influencing the migration behavior may include the diffusivity,solubility,surface activity and hydrophobicity of each solute.In order to examine the influence of solid solubility on droplet drying kinetics and particle formation process,the present study investigates the droplet drying kinetics of sucrose which has a very high solubility,compares the migration behavior of solids with different solubility,and monitors the distribution of solids with different solubility in dried composite particles.The objective is to explore how the migration behavior of solutes with different solubility influence the particle formation process.Specific research content is shown below:(1)The single droplet drying(SDD)equipment was used to dry sucrose droplets with 10 wt.%,30 wt.% and 50 wt.% initial concentration,and the shrinkage kinetics and drying kinetics of sucrose droplets were calculated.Sucrose showed similar droplet shrinkage kinetics to the Ideal Shrinkage,and it was used as a reference material in the Differential Shrinkage Model(DSA)to study the shrinkage behavior of lactose,mannitol and skim milk with various initial solids contents.The resulting differential shrinkage kinetics of these materials clearly captured the stages of free water evaporation and crust formation.The results showed that solid solubility affects droplet shrinkage to a certain extent,but it is not the sole factor governing the shrinkage kinetics.The drying kinetics of sucrose droplets with three initial concentrations was correlated with the Reaction Engineering Approach(REA)model.When the water content was reduced to lower than 1 kg/kg,the normalized activation energy curves of sucrose droplets with different initial concentrations collapsed into a general trend,indicating that initial concentration minimally affects the drying behavior of sucrose droplets at the later drying stage.The result is in agreement with the observed shrinkage behavior of sucrose,which shows nearly ideal shrinkage.(2)Sucrose was used as a control sample to study the influence of Rhodamine B,which was a common model drug in control delivery studies,on the particle structure and particle formation process of core-shell-structured carrier particles.The experimental results showed that the addition of either model drug hardly influenced the core-shell structure of the composite carrier particle.Rhodamine B was mainly enriched in the shell region of the particle.When 1.5 mg/m L Rhodamine B was added to the precursor,the property of initial surface shell was changed from insoluble to soluble in water,and particle surface morphology was changed from smooth to wrinkling.Adding sucrose did not alter the surface morphology of the carrier particle,nor the particle formation process.These differences may indicate that the different material property of sucrose and Rhodamine B likely results in different migration behavior during droplet drying,which further leads to the discrepancy in the property of dried carrier particles.(3)The effect of solid solubility on its migration behavior was quantitatively investigated.Three types of binary sugar solution with a solid content of 20 wt.% were prepared using sucrose,lactose and mannitol.The mass ratio of sugars in each type of solution was 1:1.Each type of binary sugar solution was subjected to SDD experiments,and the surface layer,middle layer and core layer were dissolved in water,consecutively,after drying of each particle.Quantitative analysis of the sugar composition at each layer was performed using HPLC.As shown by the experimental results,in sucrose/mannitol(S/M)particles,the S/M ratios of the surface,middle and core layers were 0.9732,1.1442 and 0.9929,respectively.The three layers of lactose/mannitol(L/M)particles showed L/M ratios of 1.2493,1.2414 and 0.9830,respectively,and the sugar ratios in sucrose/lactose(S/L)particles were 0.9851,0.9895 and 0.9824,respectively.Comparing with values obtained with standard samples,which were 1.0156 for S/M solution,1.0565 for L/M solution and 0.9725 for S/L solution,the results showed that the binary sugar particles of sucrose/mannitol and lactose/mannitol demonstrated ingredients segregation,whereas the sucrose/lactose particles did not,albeit the largely different solubility of the two sugars.The trends suggested that the solubility of solid has little effect on the segregation of components during droplet drying.(4)The drying kinetics of liquid marbles comprising either sucrose solution or lactose solution was also studied.Sucrose and lactose solutions with 10 wt.% concentration were prepared,and a 2 ?L droplet was gently coated with polytetrafluoroethylene(PTFE)powder to prepare liquid marbles.The particle sizes of PTFE were 1 ?m,35 ?m,and mixed 1 & 35 ?m.The experimental results showed that the particle size of PTFE does not affect the drying kinetics of liquid marble droplets.The mass and diameter profiles of liquid marbles prepared from both sucrose and lactose solutions well follow the drying kinetics of pure sugar droplets. |
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