NMR and Dielectric Studies of Nano-Confined Nematic Liquid Crystals

Cramer, Ch.; Cramer, Th.; Arndt, M. B.; Kremer, Friedrich; Naji, Lama; Stannarius, Ralf

doi:10.1080/10587259708039426

Cited by 30 publications

(5 citation statements)

References 14 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…So it is argued that this relaxation process has to be assigned to the constrained rotational dynamics of water molecules confined in the interlayer galleries of NiAlMo for the following reasons: It is well-known that in the experimental frequency and temperature window no relaxation process due to silica (a material related to LDHs) itself or to bulk liquid water molecules is expected. , Therefore, a similar behavior is supposed for NiAlMo. Moreover, it was already shown for AlMCM-41, porous glass, and Anopore membranes 32 that the contribution of the pore wall material (motions of the silica/alumina tetrahedra as building groups) to the dielectric loss is negligibly small (3 orders of magnitude lower) compared to that observed for the (liquid) loaded samples. In addition, the exchanged cations generally contribute mostly to the dc conductivity .…”

Section: Resultsmentioning

confidence: 94%

Rotational Fluctuations of Water Confined to Layered Oxide Materials: Nonmonotonous Temperature Dependence of Relaxation Times

et al. 2007

View full text Add to dashboard Cite

The rotational molecular dynamics of water confined to layered oxide materials with brucite structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(7) Hz and in a broad temperature interval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuations of water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence of the relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov et al. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules in porous glasses is employed also for the layered materials. This model assumes two competing effects: rotational fluctuations of water molecules that take place simultaneously with defect formation, allowing the creation of free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy of defect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters from a fit of this model to the data. The values of these parameters were compared with those found for water confined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed in detail, such as the lower the value of the energy of defect formation, the higher the number of defects. The pre-exponential factor increases with increasing activation energy, as an expression of the compensation law, and indicates the cooperative nature of the motional process. The involvement of the surface OH groups and of the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. For the birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution, which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessite obeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawa relationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity sigma0 in detail gives some hint that sigma0(T) has also an unusual saddlelike form.

show abstract

Section: Resultsmentioning

confidence: 94%

Rotational Fluctuations of Water Confined to Layered Oxide Materials: Nonmonotonous Temperature Dependence of Relaxation Times

et al. 2007

View full text Add to dashboard Cite

show abstract

“…But, one has to note that both materials have a negligible cation content: AlSBA-15 is in hydrogen form, whereas NMS-F is a pure silica. In fact, SBA-type molecular sieves behave like other related confining materials such as Anopore membranes and porous glass. − …”

Section: Resultsmentioning

confidence: 99%

Rotational Fluctuations of Water inside the Nanopores of SBA-Type Molecular Sieves

et al. 2005

View full text Add to dashboard Cite

The rotational molecular dynamics of water confined to nanoporous molecular sieves of a regular hexagonal (SBA-15) and of a foamlike pore structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(9) Hz and in a broad temperature interval. Two relaxation processes were observed: the process at lower frequencies is related to water molecules forming a layer, which is strongly adsorbed at the pore surface, whereas the relaxation process at higher frequencies is assigned to fluctuations of water molecules situated close to the center of the pore. The relaxation times of the low-frequency process for both materials and of the high-frequency process for the SBA-15 material have an unusual saddlelike temperature dependence, reported here for the first time. To describe this temperature dependence, a model developed for water confined to nanoporous glasses by Ryabov et al. [J. Phys. Chem. B 2001, 105, 1845] was applied, which considers two competing effects. The characteristic features of these two competing processes were compared with those reported for other porous systems.

show abstract

“…These results are in agreement with different experiments that pointed out differences in the behavior of molecules at the surface and in the center of the pores. In a set of experiments on the dynamical behavior of confined fluids Jonas and co-worker studied dynamical behavior of confined fluids. − Their results as well as dielectric spectroscopy experiments performed by Kremer and co-workers − showed that surface interactions strongly affect the dynamics and consequently the phase changes in the confined fluids. Takei et al studies on the evolution of the melting point temperature of n -hexane and benzene as a function of the number and the type of OH at the surface of silica-walled porous materials .…”

Section: Discussionmentioning

confidence: 99%

Cyclohexane and Benzene Confined in MCM-41 and SBA-15: Confinement Effects on Freezing and Melting

2003

View full text Add to dashboard Cite

Using DSC scans and NMR line-shape analysis we have investigated the thermal properties of orientationally disordered crystal forming cyclohexane and benzene confined in highly ordered mesoporous materials MCM-41 and SBA-15. Phase transition temperatures and nature of the phases as a function of temperature were determined. In pores with diameters corresponding roughly to 10-30 molecular sizes (4.7 e d e 14.0 nm), important depressions of the melting point were observed and the confined material crystallizes only partially. The progressive melting of confined crystals detected by NMR is comparable to premelting effects usually observed in bulk molecular crystals. Here the surface of the pores plays the same role as the extended defects at the origin of premelting effects in bulk crystals. Unless in bulk crystals where premelting begins only a few degrees below the melting point, these effects spread out over a very large temperature range in confined crystals. The systematic analysis of various topology and surface interaction clearly showed that, besides the pore size, the other driving parameters for the phase transitions are the interactions between the confined fluids and the pore surface and the commensurability between the crystal symmetry and the surface structure. The melting point depression is larger in cyclohexane than in benzene due to the interaction between the π electrons of the benzene ring and the OH at the pore surface. In cyclohexane, the depression of the monoclinic to cubic transition temperature is always smaller than the melting point depression, inducing a decrease of the cubic phase temperature range with decreasing pore diameter. In 4.7 nm or narrower pores, both transitions disappear and liquid cyclohexane undergoes a glass transition even at cooling rate as small as 1 K/min. Benzene does crystallize in 4.7 nm pores but vitrifies in narrower pores. A phase diagram, presenting solidsolid-phase transition, melting and glass transition temperatures as a function of pore diameter was derived from experimental results.

show abstract

NMR and Dielectric Studies of Nano-Confined Nematic Liquid Crystals

Cited by 30 publications

References 14 publications

Rotational Fluctuations of Water Confined to Layered Oxide Materials: Nonmonotonous Temperature Dependence of Relaxation Times

Rotational Fluctuations of Water Confined to Layered Oxide Materials: Nonmonotonous Temperature Dependence of Relaxation Times

Rotational Fluctuations of Water inside the Nanopores of SBA-Type Molecular Sieves

Cyclohexane and Benzene Confined in MCM-41 and SBA-15: Confinement Effects on Freezing and Melting

Contact Info

Product

Resources

About