Dynamics of Confined Short-Chain alkanol in MCM-41 by Dielectric Spectroscopy: Effects of matrix and system Treatments and Filling Factor

Abstract

The dynamics of n-propanol confined in regular MCM-41 matrix with the pore size D pore = 40 Å, under various matrix conditioning and sample confining conditions, using broadband dielectric spectroscopy (BDS), is reported. First, various drying procedures with the capacitor filling under air or N 2 influence the BDS spectra of the empty MCM-41 and the confined n-PrOH/MCM-41 systems, but have a little effect on the maximum relaxation time of the main process. Finally, various filling factors of n-PrOH medium in the optimally treated MCM-41 system lead to unimodal or bimodal spectra interpreted in terms of the two distinct dynamic phases in the confined states.Polymers 2020, 12, 610 2 of 17 atmosphere, as well as degree of filling the pores (filling factor), of the previous specifically treated matrix with a given medium. One of the most often used model inorganic matrices based on silica such as irregular mesoporous silica gels (SG) and regular periodic mesoporous silicas (PMS), e.g., and 14], has the hydrophilic character due to the presence of three basic kinds of polar silanol groups: isolated (free) silanol [O≡Si-OH], geminal silanol or silanediol [O=Si=(OH) 2 ] and vicinal silanol [O≡Si-OH...HO-Si≡O], with their different adsorption ability to the environmental moisture (H 2 O from air) or other agents [15][16][17][18] and the capability of having various modification treatments [17,18]. Consequently, many BDS works addressed this important water-adsorption phenomenon in various silicas [19][20][21][22]. However, in spite of the importance of these external parameters, a relatively small amount of attention was devoted to systematic studies of their variations from the viewpoint of their impact into the dynamic response of the various confined organics [23,24]. This is also evident from the frequent absence of detailed descriptions of the empty matrix and confined system conditioning, as well as of the filling conditions during the preparation of the confined systems.The knowledge situation is further complicated for the special group of H-bonded compounds, a very important class of organic media. Although the simpler poly alcohols, such as 1,2-ethanediol, ethylene glycol (EG), 1,2-propanediol, propylene glycol (PG) and 1,2,3-propanetriol, glycerol (GL) were among the first model organic media investigated after insertion into various inorganic silica matrices by BDS [25][26][27][28][29][30], BDS works on confined monohydroxy alcohols are rather limited [24,[31][32][33]. This family of alcohols exhibits basically two peak features, where the main peak consists of two strongly superimposed subpeaks from the dominating Debye relaxation and the essentially less intense primary α one together with the well-separated secondary β process [34]. First, branched longer monohydroxy alkanols were studied where well-separated Debye-and α-processes were observed [31,32]. Later the first simplest members of the n-alkanol family, i.e., methanol and ethanol, were measured under confinement in MCM-41, showing a single global ...

Molecular probe dynamics and free volume heterogeneities in n-propanol confined in a regular MCM-41 matrix by ESR and PALS

“…Note that this schematic model of polar medium imbedded into polar matrix is also consistent with and supported by our recent finding of two relaxation peaks stemming from two distinct dynamic phases in the same n -PrOH/MCM-41 saturated system by BDS. 43 …”

Confined Effects on Structural Isomers in the MCM-41-SIL Matrix as Seen by Extrinsic Probes via PALS and ESR: n-Butanol vs tert-Butanol

“…First of all, we recall the well-known facts from standard intrinsic structural and dynamical studies − and molecular dynamic simulations of the confined lower n -alkanols, such as methanol, , ethanol, , and n -propanol, that their spatial restriction in the variously sized pores of silicas leads to dramatic changes in their structures , and closely related dynamics. The former type of change includes a formation of new phases across the pores, such as the surface region, i.e., the first and closest adjacent more or less layered region and the central “core” region of the medium’s molecules.…”

“…In the last three compound types, supermolecular assemblies result from the subtle balance between hydrophobic and hydrophilic interactions, which control the microscopic segregation and supermolecular self-assembling of a variety of organic and biological systems . Although the confinement behaviors of short-sized − and medium-sized alcohols , in various inorganic confiners have quite often been studied by several research groups, relatively little is known about the isomeric effect on potential alterations in the structural–dynamic properties of various alcohols under geometric restriction.…”

“…According to our best knowledge, unlike the shorter-chain alkanols, such as MeOH, , EtOH, and n -PrOH in the confined state, n -BuOH has not been investigated by standard techniques so far. On the other hand, neutron scattering (NS) on t -BuOH embedded in the regular silica matrix of mobile composition matter-41 (MCM-41) type , with D pore = 24 and 35 Å revealed the absence of the prepeak in the static structure factor, suggesting that this micellelike clustering does not survive under geometric restriction.…”

Dielectric Relaxation in Nanocrystals: A Scale Effect

“…The scale effect in the melting process of nanocrystals has attracted the attention of many researchers; − several models have been proposed to explain how the size of the crystal affects both the melting temperature and the enthalpy of fusion. − However, the research is somewhat lacking as to the impact of spatial restrictions in nanocrystals on dielectric relaxation, − even though the relaxation processes can provide useful insight into the mechanisms of thermal motion of molecules in the nanosized inclusions in porous matrices. In this motion, the matrix can both hinder ,− and activate certain mobility of embedded molecules .…”