TY - JOUR A1 - Frunza, L. A1 - Frunza, S. A1 - Zgura, I. A1 - Beica, T. A1 - Gheorghe, N. A1 - Ganea, P. A1 - Stoenescu, D. A1 - Dinescu, A. A1 - Schönhals, Andreas T1 - Involvement of cyan and ester groups in surface interactions of aerosil-cyanophenyl alkyl benzoate systems wih high silica density: infrared investigations JF - Spectrochimica acta A N2 - Composites prepared from aerosil A380 and liquid crystals (LCs) of 4-n-alkyl-4'-cyanophenyl benzoate type, with four to six carbon atoms in the alkyl chain were investigated by infrared spectroscopy. Their high silica content (of 2-7 g aerosil/1 g of LC) was given by thermogravimetric investigations and allows the observation of a rather thin LC layer on the silica particles. Several surface species onto the external surface of the grains were demonstrated. Arguments are given that monomer and dimer species are present in the bulk cyanophenyl benzoate materials while bulk-like species along with hydrogen-bonded ones coexist in the so-called surface layer of the composites. The main interaction of LC molecules with the aerosil surface is by hydrogen bonding taking place with the involvement of the cyan group. There is a contribution of ester carbonyl group to these surface interactions but this cannot be well quantified. KW - Cyanophenyl alkyl benzoate KW - Aerosil composites KW - Infrared spectra KW - Band shape analysis PY - 2010 DO - https://doi.org/10.1016/j.saa.2009.12.029 SN - 1386-1425 SN - 0370-8322 SN - 1873-3557 SN - 0584-8539 VL - 75 IS - 4 SP - 1228 EP - 1235 PB - Elsevier CY - Amsterdam AN - OPUS4-20997 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Frunza, S. A1 - Schönhals, Andreas A1 - Frunza, L. A1 - Ganea, P. A1 - Kosslick, H. A1 - Harloff, J. A1 - Schulz, A. T1 - Molecular relaxation processes in a MOF-5 structure revealed by broadband dielectric spectroscopy: signature of phenylene ring fluctuations JF - The journal of physical chemistry / B N2 - The molecular mobility of a MOF-5 metal–organic framework was investigated by broadband dielectric spectroscopy. Three relaxation processes were revealed. The temperature dependence of their relaxation rates follows an Arrhenius law. The process observed at lower temperatures is attributed to bending fluctuations of the edges of the cages involving the Zn–O clusters. The processes ('region II') at higher temperatures were assigned to fluctuations of phenyl rings in agreement with the NMR data found by Gould et al. (J. Am. Chem. Soc. 2008, 130, 3246). The carboxylate groups might also be involved. The rotational fluctuations of the phenyl rings leading to the low frequency part of relaxation region II might be hindered either by some solvent molecules entrapped in the cages or by an interpenetrated structure and have a broad distribution of activation energies. The high frequency part of region II corresponds nearly to a Debye-like process: This is explained by a well-defined structure of empty pores. PY - 2010 DO - https://doi.org/10.1021/jp1071617 SN - 1520-6106 SN - 1520-5207 SN - 1089-5647 VL - 114 IS - 40 SP - 12840 EP - 12846 PB - Soc. CY - Washington, DC AN - OPUS4-22255 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Frunza, L. A1 - Zgura, I A1 - Ganea, C. P. A1 - Schönhals, Andreas T1 - Molecular dynamics of alkyl benzoate liquid crystals in the bulk state and in the surface layer of their composites with oxide nanopowders JF - Journal of Molecular Liquids N2 - This paper presents the results concerning monotropic nematic liquid crystals 4-pentylphenyl 40-alkyl benzoate (5PnB) (n = 3 or 5 carbon atoms in the alkyl chain). Their mesophase properties were supported by images of the polarized optical microscopy. Molecular dynamics in the bulk samples or in the composites prepared with aerosil A380 was investigated by broadband dielectric spectroscopy in a large temperature range, appropriately chosen. Thermo gravimetric and infrared investigations were additionally performed. The data were compared with those of structurally related nematics like cyanophenyl pentyl benzoates, which have a cyan group instead of the pentyl chain. The dielectric spectra of the bulk 3P5B and 5P5B demonstrate a dielectric behavior with several relaxation processes as expected for nematic liquid crystals. The temperature dependence of the relaxation rates (and of the dielectric strength) seems to have two distinguished regimes. Thus, in the isotropic state, at higher temperatures the data obey the Vogel–Fulcher–Tammann law, whereas an Arrhenius law is fitted at lower temperature, in a close similarity to the behavior of a constrained dynamic glass transition. Samples with a high density of silica (larger than 7 g aerosil/1 g of 5PnB) were prepared to observe a thin layer adsorbed on the particle surface; it was estimated that almost each guest 5PnB molecule interacts with the aerosil surface. For the composites only one main relaxation process is observed at frequencies much lower than those for the corresponding bulk, which was assigned to the dynamics of the molecules in the surface layer. Infrared spectroscopy shows that these molecules interact with the surface by the ester carbonyl group leading to the monolayer self-assembly at liquid–solid interface. We note once more the importance of the functional unit(s) for the interaction with the hydroxyl groups on the aerosil surface. KW - Liquid crystals PY - 2022 DO - https://doi.org/10.1016/j.molliq.2022.119374 SN - 0167-7322 VL - 359 SP - 1 EP - 11 PB - Elsevier B.V. AN - OPUS4-54867 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Frunza, S. A1 - Schönhals, Andreas A1 - Frunza, L. A1 - Beica, T. A1 - Zgura, I. A1 - Ganea, P. A1 - Stoenescu, D. T1 - Dynamics of cyanophenyl alkylbenzoate molecules in the bulk and in a surface layer adsorbed onto aerosil. Variation of the lengths of the alkyl chain JF - Chemical physics N2 - The molecular mobility of 4-butyl- and 4-pentyl-4'-cyanophenyl benzoate (CP4B, CP5B) and their composites prepared from aerosil A380 was investigated by broadband dielectric spectroscopy in a large temperature range. Thermogravimetric and infrared investigations were additionally performed. High silica density (larger than 7 g aerosil/1 g of liquid crystal) was selected to observe a thin layer adsorbed on the surface of the silica particles. The data were compared with those of the member of the series with six carbon atoms in the alkyl tail. Bulk CP4B and CP5B show the dielectric behaviour expected for liquid crystals. For the composites one relaxation process is observed at frequencies much lower than those for the corresponding bulk, which was assigned to the dynamics of the molecules in a surface layer. The temperature dependence of the relaxation rates (and of the dielectric strength) shows a crossover behaviour with two distinguished regimes. At higher temperatures the data obey the Vogel-Fulcher-Tammann law, whereas an Arrhenius law is observed at lower temperature, in a close similarity to the behaviour of a constrained dynamic glass transition. The estimated Vogel and crossover temperature is independent on the tail length, while the activation energy for the low temperature branch increases weakly with increasing the alkyl tail. KW - Confined liquid crystals KW - Molecules at surfaces KW - Dielectric properties KW - Relaxation processes KW - Glassy dynamics KW - Crossover phenomenon PY - 2010 DO - https://doi.org/10.1016/j.chemphys.2010.04.031 SN - 0301-0104 SN - 1873-4421 VL - 372 IS - 1-3 SP - 51 EP - 60 PB - Elsevier CY - Amsterdam AN - OPUS4-21588 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Frunza, S. A1 - Frunza, L. A1 - Ganea, C. P. A1 - Zgura, I. A1 - Bras, A. R. A1 - Schönhals, Andreas T1 - Rod-like cyanophenyl probe molecules nanoconfined to oxide particles: Density of adsorbed surface species JF - The European Physical Journal Plus N2 - Surface layers have already been observed by broadband dielectric spectroscopy for Composite systems formed by adsorption of rod-like cyanophenyl derivates as probe molecules on the surface of oxide particles. In this work, features of the surface layer are reported; samples with different amounts of the probe molecules adsorbed onto oxide (nano) particles were prepared in order to study their interactions with the surface. Thermogravimetric analysis (TGA) was applied to analyze the amount of loaded probe molecules. The density of the surface species ns was introduced and its values were estimated from quantitative Fourier transform infrared spectroscopy (FTIR) coupled with TGA. This parameter allows discriminating the composites into several groups assuming a similar interaction of the probe molecules with the hosts of a given group. An influence factor H is further proposed as the ratio of the number of molecules in the surface layer showing a glassy dynamics and the number of molecules adsorbed tightly on the surface of the support: It was found for aerosil composites and used for calculating the maximum filling degree of partially filled silica MCM-41 composites showing only one dielectric process characteristic for glass-forming liquids and a bulk behavior for higher filling degrees. KW - Cyanophenyl molecules KW - Nanoconfinement KW - Surface Species PY - 2016 DO - https://doi.org/10.1140/epjp/i2016-16027-5 SN - 2190-5444 VL - 131:27 IS - 2 SP - 1 EP - 9 PB - Springer AN - OPUS4-35364 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Frunza, S. A1 - Ganea, C. P. A1 - Zgura, I. A1 - Frunza, L. A1 - Schönhals, Andreas T1 - Molecular dynamics in bulk and surface species of cyanophenyl alkyl benzoates with 2, 3 and 7 carbon atoms in the alkyl chain: comparison in the whole homologous series JF - Liquid Crystals N2 - Molecular mobility of cyanophenyl alkylbenzoates (CPnBs) (n = 2, 3, 7 – number of carbon atoms in the alkyl chain) in the bulk and in composites with aerosil A380 is investigated by broadband dielectric spectroscopy, while thermal analysis and infrared spectroscopy were applied to characterise the molecular species. The work completes preliminary results obtained for the members with n = 4 … 6. An interaction by hydrogen bonding, between aerosil surface – OH groups and – CN or ester groups of the CPnB molecules takes place. It slows down the relaxation process as observed for related composites in comparison to the pure materials. The existence of two types of bonding might be the reason that Vogel temperature for the relaxation process in the surface layer does not show the odd-even effect. Temperature dependence of the relaxation rates for composites shows a crossover behaviour from a high to a low temperature regime. Moreover, the temperature dependence of the dielectric strength is unusual. As the loading degree is similar, comparison of the dielectric, spectroscopic and thermal data obtained here and with the results obtained for the composites with n = 4 … 6 can be directly done. Increasing the number of the members of the homologous series confirms and hardens the preliminary conclusions. KW - Liquid crystals PY - 2019 DO - https://doi.org/10.1080/02678292.2019.1687768 SP - 1 EP - 10 PB - Taylor & Francis AN - OPUS4-49924 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Frunza, S. A1 - Frunza, L. A1 - Ganea, C. P. A1 - Zgura, I. A1 - Schönhals, Andreas T1 - Density of adsorbed surface species for cyanophemyl benzoates confined to Aerosil 380: Development of the evaluating algorithm for Attachment by two types of bonds JF - UPB Scientific Bulletin, Series A N2 - We found previously how to estimate the density of the adsorbed surface species in the case of the molecules interacting to the oxide support surface by one type of bond. Here this algorithm is developed for the case of the molecules which can be bonded to the support surface by two types of bonds. The adsorption assumptions are similar to those considered in the case of only one type of bond. The calculation is exemplified for some composites of cyanophenyl alkylbenzoates (CPnBs) (n is the number of carbon atoms in the alkyl chain) interacting with Aerosil A380. The interaction takes place by hydrogen bonding between the –OH groups or the support and the functional groups of the CPnB molecules. The estimated values of the total surface density of CPnBs agree well with those found for the composites containing related but simpler molecules. KW - Surface species PY - 2019 SN - 1223-7027 VL - 81 IS - 4 SP - 232 EP - 236 PB - University Politehnica of Bucharest CY - Bucharest AN - OPUS4-49615 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ungureanu, F. A1 - Manea, A.S. A1 - Frunza, L. A1 - Frunza, S. A1 - Ganea, C.P. A1 - Cotorobai, F. A1 - Diamandescu, L. A1 - Schönhals, Andreas T1 - Pentylcyanobiphenyl as test molecule for the acid sites of powdered titanium(IV) oxides: sensitivity of core levels to the local structure JF - Molecular crystals and liquid crystals: an internat. journal N2 - Pentylcyanobiphenyl was used successfully as a test molecule to show the acid sites upon some commercial nanoscaled titanium dioxide materials. The surface interactions were investigated in detail using methods sensitive to the surface or to the bulk material, e.g., X-ray photoelectron spectroscopy (XPS), infrared spectroscopy, and thermal analysis. Different oxidation states of surface Ti ions were revealed. Several species were found: the majority were bonded through the nitrogen lone pair of the cyano group, but bonding to the surface OH groups by benzene π electrons might also appear. The N 1s lines of adsorbed 5CB were able to distinguish between Lewis acid and relatively weak Brønsted acid sites. KW - Infrared spectra KW - Pentylcyanobiphenyl KW - Size dependence KW - Titanium dioxide KW - XPS PY - 2012 DO - https://doi.org/10.1080/15421406.2012.697416 SN - 1542-1406 SN - 0026-8941 VL - 562 IS - 1 SP - 200 EP - 217 PB - Taylor & Francis CY - Philadelphia, PA, USA AN - OPUS4-26397 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -