A Model for the Adsorption Equilibria of Solutes with Multiple Adsorption Orientations

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• 作者：Xuezhi Jin ; Zidu Ma ; Julian Talbot ; and Nien-Hwa Linda Wang
• 刊名：Langmuir
• 出版年：1999
• 出版时间：April 27, 1999
• 年：1999
• 卷：15
• 期：9
• 页码：3321 - 3333
• 全文大小：212K
• 年卷期：v.15,no.9(April 27, 1999)
• ISSN：1520-5827

文摘

Both direct and indirect experimental evidence indicates that nonspherical adsorbates adopt multipleout-of-plane orientations on solid substrates. Few existing models account explicity for this effect on theadsorption equilibrium and kinetics, particularly when steric blocking effects are significant. In this work,a quantitative model is developed to account explicitly for the effects of steric blocking, solute shape,affinities of the different adsorption orientations, and solute concentration on the intrinsic equilibriumbehavior. The model accurately correlates experimental isotherms in both gas/solid and liquid/solid systems.The fitted adsorption capacity and aspect ratio of the solutes are consistent with the solute size, shape,and specific surface area of the adsorbent. The proposed model describes quantitatively the tendency atlow surface coverage for a solute to adsorb in an orientation with the largest surface contact area (energeticallyfavored), while at a high coverage, most adsorbed molecules are orientated with a smaller contact area(sterically favored). The transition from side-on to end-on average orientation occurs over a large rangeof coverage if the aspect ratio is small and the affinity of the side-on orientation is much higher than thatof the end-on orientation. The model also predicts that the apparent linear isotherm parameter estimatedexperimentally can increase with decreasing surface coverage, resulting in a strongly nonlinear Scatchardplot. This model could be valuable both in fundamental mechanistic interpretation of adsorption processesand for accurate experimental data correlation.