Ice nucleation in the atmosphere influences cloud properties, altering precipitation and the radiative balance, ultimately regulating Earth’s climate. An accepted ice nucleation pathway, known as deposition nucleation, assumes a direct transition of water from the vapor to the ice phase, without an intermediate liquid phase. However, studies have shown that nucleation occurs through a liquid phase in porous particles with narrow cracks or surface imperfections where the condensation of liquid below water saturation can occur, questioning the validity of deposition nucleation. We show that deposition nucleation cannot explain the strongly enhanced ice nucleation efficiency of porous compared with nonporous particles at temperatures below −40 °C and the absence of ice nucleation below water saturation at −35 °C. Using classical nucleation theory (CNT) and molecular dynamics simulations (MDS), we show that a network of closely spaced pores is necessary to overcome the barrier for macroscopic ice-crystal growth from narrow cylindrical pores. In the absence of pores, CNT predicts that the nucleation barrier is insurmountable, consistent with the absence of ice formation in MDS. Our results confirm that pore condensation and freezing (PCF), i.e., a mechanism of ice formation that proceeds via liquid water condensation in pores, is a dominant pathway for atmospheric ice nucleation below water saturation. We conclude that the ice nucleation activity of particles in the cirrus regime is determined by the porosity and wettability of pores. PCF represents a mechanism by which porous particles like dust could impact cloud radiative forcing and, thus, the climate via ice cloud formation.

Functionalized mesoporous silica offers promising possibilities for numerous applications, including drug delivery, catalysis, and adsorption. This minireview focuses on recent developments related to the postsynthetic positioning of functional groups on mesoporous silica. After briefly introducing the reagents that are commonly used for this purpose, methods to control and to analyze the distribution of the grafted functional groups are discussed, with particular emphasis on concepts that allow the placement of the groups at specific distances from each other, as well as on approaches towards the selective functionalization of the external particle surface.

Zeolites and mesoporous silica are versatile host materials for the supramolecular organization of a large variety of guests. The inclusion of luminescent molecules, complexes, or nanoclusters into ordered one-dimensional channel systems is particularly intriguing, as the resulting host–guest compounds may exhibit unique properties such as optical anisotropy, efficient energy transfer, and enhanced stability. Several levels of organization have been realized for zeolite L, extending from the interior of a given crystal to the channel entrances and the external surface as well as from the microscopic to the macroscopic scale. The diverse chemistry that is involved in the development of a highly organized and functional host–guest material is illustrated by the design of molecules which are able to selectively adsorb at the channel entrances and establish communication between included guests and external objects. The importance of phthalocyanines as chromophores for this particular concept is exemplified. Based on the extensive research on zeolite L, possibilities and recent developments in the field of mesoporous silica hosts are discussed, revealing that despite the obvious differences between microporous and mesoporous host–guest materials, many similarities and analogies exist.