Mesoporous layers, adsorbance

For an initial concentration of 2.5 pg/ml of GFP solution, the maximum amount of GFP adsorbed on Aerosil was 120.8 jLtg/g, whilst using calcined SBA-15 was 166.5 pg/g, corresponding to 72.5% and 99.9% of the initial concentration, respectively. This result is an indication that the inner surface of the mesoporous material plays a key role in the immobilisation of guest molecules. The relatively small difference in adsorption between the two materials suggests also that the GFP molecules might form multiple layers on the Aerosil nanoparticle surface. 

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

A typical N2 adsorption measurement versus relative pressure over a solid that has both micropores and mesopores first involves essentially a mono-layer coverage of the surface up to point B shown in isotherm IV (lUPAC classification) in Figure 13.1. Up to and near point B the isotherm is similar to a Langmuir isotherm for which equilibrium is established between molecules adsorbing from the gas phase onto the bare surface and molecules desorbing from the adsorbed layer. The volume of adsorbed N2 that covers a monolayer volume, hence the surface area of N2 can then be determined from the slope of the linearized Langmuir plot when P/V is plotted against P ... 

The smaller the radius of curvature, the lower the vapor pressure. Here is the radius of curvature, which is equal to the pore radius minus the thickness of the adsorbed N2 layer, 7 is the surface tension and Vi is the molar volume. This capillary condensation as a function of pres sure helps establish the pore size distribution when the volume of adsorbed N2 is plotted against P/P . A sharp increase in the N2 uptake is then observed at the pressure corresponding to the filling of mesopores. This type of isotherm is known as type IV, as illustrated in Figure 13.1. 

The model is extended here for adsorption in mesoporous materials, thickness t of the adsorbed layer at pressure P is given by... 

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

The t-curve was originally developed to evaluate the thickness of the adsorbed layer on the walls of the pores of mesoporous solids. 

Mesoporous materials of the M41S family with their regular arrays of uniform pore openings and high surface areas have attracted much attention since their first synthesis in 1992 (61), because their properties were expected to open new applications as catalysts and/or adsorbents. These materials are formed by condensation of an amorphous silicate phase in the presence of surfactant molecules (usually ammonium salts with long alkyl chains). However, the chemistry of the steps of the synthesis process is still not fully clear. Ideas put forward so far include (a) condensation of a silicate phase on the surface of a liquid crystalline phase preformed by the surfactant molecules (62) (b) assembly of layers of silicate species in solution followed by puckering of those layers to form hexagonal channels (63) and (c) formation of randomly disordered rod-like micelles with the silicate species... 

Recently, there has been an increasing interest in the synthesis of ordered mesoporous carbons, since such materials are very promising as adsorbents, catalyst supports, and electrochemical double-layer capacitors. Ordered mesoporous silicas have been shown as suitable templates to prepare periodic mesoporous carbons with various pore shapes and connectivity. The synthesis procedure involves impregnation of the mesoporous silica with an appropriate carbon precursor, carbonization of carbon source, and subsequent removal of silica using an aqueous solution of HF or NaOH. ... 

Base material provides mechanically stable rigid porous particles (mostly spherical) for reversed-phase HPLC adsorbents. Particle porosity on the mesoporous level (30 to 500-A diameter) is necessary to provide high specific surface area for the analyte retention. Surface of the base material should have specific chemical reactivity for further modification with selected ligands to form the reversed-phase bonded layer. Base material determines the mechanical and chemical stability—the most important parameters of future (modified) reversed-phase adsorbent. 

These limits are somewhat arbitrary pore filling mechanisms also depend on the shapes of the pores and on the size of the adsorptive molecule. Despite this Inherent vagueness, the classification has its use as a first means of discrimination because it points to different pore filling mechanisms macropores are so wide that they behave as "virtually flat" surfaces, mesopores are mainly responsible for capillary condensation, whereas micropores are so narrow that one cannot speak of a macroscopic fluid in them. Because in micropore Jilling adsorbates are only a few layers thick, an adsorption plateau is found suggesting monolayer filling and applicability of the Langmuir or Volmer premises. This mechanism Is distinct from that in meso- and macropores. 

Consider a sorption experiment, where a mesoporous solid, denoted hereafter as S, is progressively loaded by a condensable gas or vapour. Initially, a layer of adsorbate L is building up on the walls of the pores. When condensation occurs, all the pores with radii smaller than a critical value, given by the Kelvin equation, are progressively blocked, and the adsorbate is in equilibrium with its vapour, V. The distribution of the condensed phase in a reconstructed Vycor structure for a given degree of pore filling (saturation), is determined by... 

BJH and Dollimore-Heal methods are based on the assumption that the statistical thickness of the adsorbed layer is independent of the surface curvature and assume that the meniscus between vapor and condensed phase is hemispherical. This kind of meniscus is met in the case of a cylindrical pore during desorption. The hypothesis of constant thickness, independently of surface curvature, is justified for large mesopores, but generally leads to underestimation of pore size [7]. 

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