Dirk Enke

Porous glasses in the 21st century––a short review

This article reviews recent studies concerning the preparation, modification, characterization, modeling and application of porous glasses on the basis of phase-separated alkali borosilicate glasses.

Stabilization of the amorphous state of pharmaceuticals in nanopores

Confinement in nanoporous host systems with strongly interacting pore walls is shown to be a powerful approach to increase the lifetime of amorphous drugs based on changes in thermodynamics and crystallization kinetics in nano-sized systems.

Cavitation and pore blocking in nanoporous glasses.

In gas adsorption studies, porous glasses are frequently referred to as model materials for highly disordered mesopore systems. Numerous works suggest that an accurate interpretation of physisorption isotherms requires a complete understanding of network effects upon adsorption and desorption, respectively. The present article deals with nitrogen and argon adsorption at different temperatures (77 and 87 K) performed on a series of novel nanoporous glasses (NPG) with different mean pore widths. NPG samples contain smaller mesopores and significantly higher microporosity than porous Vycor glass or controlled pore glass. Since the mean pore width of NPG can be tuned sensitively, the evolution of adsorption characteristics with respect to a broadening pore network can be investigated starting from the narrowest nanopore width. With an increasing mean pore width, a H2-type hysteresis develops gradually which finally transforms into a H1-type. In this connection, a transition from a cavitation-induced desorption toward desorption controlled by pore blocking can be observed. Furthermore, we find concrete hints for a pore size dependence of the relative pressure of cavitation in highly disordered pore systems. By comparing nitrogen and argon adsorption, a comprehensive insight into adsorption mechanisms in novel disordered materials is provided.

Reversible Adhesion Switching of Porous Fibrillar Adhesive Pads by Humidity

We report reversible adhesion switching on porous fibrillar polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) adhesive pads by humidity changes. Adhesion at a relative humidity of 90% was more than nine times higher than at a relative humidity of 2%. On nonporous fibrillar adhesive pads of the same material, adhesion increased only by a factor of ~3.3. The switching performance remained unchanged in at least 10 successive high/low humidity cycles. Main origin of enhanced adhesion at high humidity is the humidity-induced decrease in the elastic modulus of the polar component P2VP rather than capillary force. The presence of spongelike continuous internal pore systems with walls consisting of P2VP significantly leveraged this effect. Fibrillar adhesive pads on which adhesion is switchable by humidity changes may be used for preconcentration of airborne particulates, pollutants, and germs combined with triggered surface cleaning.

Single‐Particle and Ensemble Diffusivities—Test of Ergodicity

Diffusion is the irregular, omnipresent motion of the elementary constituents of matter. It is prerequisite for life quite in general and key to innumerable processes in nature and technology. After one and a half centuries of diffusion measurements with large ensembles of diffusing particles [1], the option of single-particle tracking (SPT) with single molecule sensitivity [2] has recently provided us with a totally new view of diffusion. With this in mind, a central problem of matter dynamics can now be addressed by direct experimental evidence – the proof of the ergodic theorem indicating that the average value of the squared displacement r2(t) of a diffusing particle during a time interval t, if taken over many subsequent time intervals (“time average”), agrees with the average taken over many different particles (“ensemble average”) during one and the same time interval t.

Microimaging of Transient Concentration Profiles of Reactant and Product Molecules during Catalytic Conversion in Nanoporous Materials

Microimaging by IR microscopy is applied to the recording of the evolution of the concentration profiles of reactant and product molecules during catalytic reaction, notably during the hydrogenation of benzene to cyclohexane by nickel dispersed within a nanoporous glass. Being defined as the ratio between the reaction rate in the presence of and without diffusion limitation, the effectiveness factors of catalytic reactions were previously determined by deliberately varying the extent of transport limitation by changing a suitably chosen system parameter, such as the particle size and by comparison of the respective reaction rates. With the novel options of microimaging, effectiveness factors become accessible in a single measurement by simply monitoring the distribution of the reactant molecules over the catalyst particles.

Nanoporous Glass as a Model System for a Consistency Check of the Different Techniques of Diffusion Measurement

The remarkable differences in the guest diffusivities in nanoporous materials commonly found with the application of different measuring techniques are usually ascribed to the existence of a hierarchy of transport resistances in addition to the diffusional resistance of the pore system and their differing influence due to the differing diffusion path lengths covered by the different measuring techniques. We report diffusion measurements with nanoporous glasses where the existence of such resistances could be avoided. Molecular propagation over diffusion path lengths from hundreds of nanometers up to millimeters was thus found to be controlled by a uniform mechanism, appearing in coinciding results of microscopic and macroscopic diffusion measurement.

Two-phase porous silica: Mesopores inside controlled pore glasses

The structural and textural properties of “Two-Phase Porous Silica” consisting of a mesoporous pore system formed by finely dispersed silica-gel inside the original pores of a macroporous glass framework have been carefully investigated. Nitrogen adsorption at 77 K, Mercury Intrusion, Small Angle X-ray Scattering (SAXS), Temperature-Programmed Desorption (TPD) of ammonia and 27Al MAS NMR spectroscopy were used. Different mesoporous glasses investigated show BET surface areas up to 300 m2/g, a total pore volume of about 0.16 cm3/g and pore sizes of <2–10 nm, depending on the acid leaching conditions of the phase-separated sodium borosilicate initial glass. The texture of the macroporous “frame” glass was characterized after removing the mesoporous finely dispersed silica-gel phase with alkaline solution. Leaching of the phase-separated initial glass with Al(NO3)3 solution and followed by thermal treatment lead to the formation of Brönsted acid sites at the surface of the resulting mesoporous glasses by an incorporation of aluminium in tetrahedral coordination in the silica framework.

Pore Size Distribution and Chord Length Distribution of Porous VYCOR Glass (PVG)

A porous VYCOR-glass of porosity c ≈ 30% was analyzed by use of nitrogen adsorption (NA), mercury intrusion (MI) and small-angle scattering (SAS). The distribution density of the pore diameter was determined from the SAS experiment, based on the stereological information for a fixed order range L = 40 nm.A pore can be described by use of two random variables, which depend on each other: The pore diameter d and the chord length l. In a first step, an assumption free data evaluation method yields the second derivative of the SAS correlation function γ″(r). Then, based on the intimate connection between γ″(r) with random chord lengths, an interpretation of the first two mean γ″ peaks was performed. These peaks reflect the chord length distributions of pore and wall. The problem of the allocation of the peaks has been solved based on the information of the NA and MI experiments. The transformation of the distribution densities of the pore diameters VM(d) (obtained by MI a experiment) and VN(d) (obtained by a MI experiment) into chord length distribution densities AM(l) and AN(l) have allowed the clear interpretation of γ″(r). It was possible to separate the chord distributions of the pores from those of the walls. The first γ″(r) peak reflects the chord length distribution density Φ(l) of the pores (first moment l¯ = 10.6 nm) and the second one that of the walls f(m) (first moment m¯ = 21 nm). It follows c ≈ 30%. The average mean chord length is d¯lm ≈ 15 nm. The second moment of Φ(l) is 108 nm2.Finally, from the separated function Φ(l), the diameter distribution density of the pores VSAS(d) has been obtained. VSAS(d) was calculated, neither assuming a defined mathematical function type of the distribution nor a certain shape or dimension of the pore. The first and second moments of VSAS(d) are 7 nm and 74 nm2. From comparing the three distribution densities VSAS(d), VM(d) and VN(d) it can be concluded that the assumption of cylindrical pores is fulfilled.While the chord length distribution density of the walls is a highly symmetrical function, which can be approximated by a Gauss term, the pores have an unsymmetrical chord distribution density with the PVG.

Crystallization Behavior of Acetaminophen in Nanopores

The influence of nanoconfinement on the crystallization behavior of acetaminophen, a polymorphic drug occur- ring in three different crystalline forms, is investigated. Differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS) data for a series of controlled porous glasses (CPGs) filled with acetaminophen are presented. The re- sults show clearly that (i) the usually inaccessible crystalline form III of acetaminophen can be produced in pores with di- ameters between 22 and 103 nm and that (ii) the life time of amorphous acetaminophen is significantly increased in 10 nm pores. Bulk melting temperature and heat of melting of form III are estimated based on the Gibbs-Thomson equation. The experimental findings are confronted with the predictions of theoretical approaches aimed to describe thermodynamics and crystallization kinetics in nano-sized systems in order to understand the physical background of the observed changes.