Dynamics of propylene glycol and its oligomers confined in clay

Dynamics of aqueous binary glass-formers confined in MCM-41

Dielectric permittivity measurements were performed on water solutions of propylene glycol (PG) and propylene glycol monomethyl ether (PGME) confined in 21 Å pores of the silica matrix MCM-41 C10 in wide frequency (10(-2)-10(6) Hz) and temperature (130-250 K) ranges. The aim was to elucidate how the formation of large hydrogen bonded structural entities, found in bulk solutions of PGME, was affected by the confined geometry, and to make comparisons with the dynamic behavior of the PG-water system. For all solutions the measurements revealed four almost concentration independent relaxation processes. The intensity of the fastest process is low compared to the other relaxation processes and might be caused by both hydroxyl groups of the pore surfaces and by local motions of water and solute molecules. The second fastest process contains contributions from both the main water relaxation as well as the intrinsic β-relaxation of the solute molecules. The third fastest process is the viscosity related α-relaxation. Its concentration independency is very different compared to the findings for the corresponding bulk systems, particularly for the PGME-water system. The experimental data suggests that the surface interactions induce a micro-phase separation of the two liquids, resulting in a full molecular layer of water molecules coordinating to the hydrophilic hydroxyl groups on the surfaces of the silica pores. This, in turn, increases the geometrical confinement effect for the remaining solution even more and prevents the building up of the same type of larger structural entities in the PGME-water system as in the corresponding bulk solutions. The slowest process is mainly hidden in the high conductivity contribution at low frequencies, but its temperature dependence can be extracted for the PGME-water system. However, its origin is not fully clear, as will be discussed.

The molecular dynamics of different relaxation modes in asymmetric chlorinated butyl rubber/petroleum resin blends

The PR plays a role like an anti-plasticizer in decreasing the free volume fraction of the CIIR/PR blend. The mobility of Rouse modes is confined significantly more than that of local segmental motion.

Relaxation processes in supercooled confined water and implications for protein dynamics

We show that the viscosity-related main (alpha) relaxation of confined water vanishes at a temperature where the volume required for the cooperative alpha relaxation becomes larger than the size of the geometrically confined water cluster. This occurs typically around 200 K, implying that above this temperature we observe a merged alpha-beta relaxation, whereas below it only a local (beta) relaxation remains. This also means that such confined supercooled water does not exhibit any true glass transition, in contrast to other liquids in similar confinements. Furthermore, it implies that deeply supercooled water in biological systems, such as membranes and proteins, generally shows only a local beta relaxation, a finding of importance for low temperature properties of biological materials.

Water dynamics in n-propylene glycol aqueous solutions

The relaxation dynamics of dipropylene glycol and tripropylene glycol (nPG-n=2,3) water solutions on the nPG-rich side has been studied by broadband dielectric spectroscopy and differential scanning calorimetry in the temperature range of 130-280 K. Two relaxation processes are observed for all the hydration levels; the slower process (I) is related to the alpha relaxation of the solution whereas the faster one (II) is associated with the reorientation of water molecules in the mixture. Dielectric data for process (II) at temperatures between 150 and 200 K indicate the existence of a critical water concentration (x(c)) below which water mobility is highly restricted. Below x(c), nPG-water domains drive the dielectric signal whereas above x(c), water-water domains dominate the dielectric response at low temperatures. The results also show that process (II) at low temperatures is due to local motions of water molecules in the glassy frozen matrix. Additionally, we will show that the glass transition temperatures (T(g)) for aqueous PG, 2PG, and 3PG solutions do not extrapolate to approximately 136 K, regardless of the extrapolation method. Instead, we find that the extrapolated T(g) value for water from these solutions lies in the neighborhood of 165 K.

Dynamics of propylene glycol and its oligomers confined to a single molecular layer

The dynamics of propylene glycol (PG) and its oligomers 7-PG and poly-propylene glycol (PPG), with M(w) = 4000 (approximately 70 monomers), confined in a Na-vermiculite clay have been investigated by quasielastic neutron scattering. The liquids are confined to single molecular layers between clay platelets, giving a true two-dimensional liquid. Data from three different spectrometers of different resolutions were Fourier transformed to S(Q,t) and combined to give an extended dynamical time range of 0.3-2000 ps. An attempt was made to distinguish the diffusive motion from the methyl group rotation and a fast local motion of hydrogen in the polymer backbone. The results show that the average relaxation time tau(d) of this diffusive process is, as expected, larger than the relaxation time tau averaged over all dynamical processes observed in the experimental time window. More interesting, it is evident that the severe confinement has a relatively small effect on tau(d) at T = 300 K, this holds particularly for the longest oligomer, PPG. The most significant difference is that the chain-length dependence of tau(d) is weaker for the confined liquids, although the slowing down in bulk PG due to the formation of a three-dimensional network of OH-bonded end groups reduces this difference. The estimated average relaxation time tau at Q = 0.92 Angstroms(-1) for all the observed processes is in excellent agreement with the previously reported dielectric alpha relaxation time in the studied temperature range of 260-380 K. The average relaxation time tau (as well as the dielectric alpha relaxation time) is also almost unaffected by the confinement to a single molecular layer, suggesting that the interaction with the clay surfaces is weak and that the reduced dimensionality has only a weak influence on the time scale of all the dynamical processes observed in this study.

Dynamics of water in a molecular sieve by quasielastic neutron scattering

We have investigated the dynamics of water confined in a molecular sieve, with a cylindrical pore diameter of 10 A, by means of quasielastic neutron scattering (QENS). Both the incoherent and coherent intermediate scattering functions I(Q,t) were determined by time-of-flight QENS and the neutron spin-echo technique, respectively. The results show that I(Q,t) is considerably more stretched in time with a slightly larger average relaxation time in the case of coherent scattering. From the Q dependence of I(Q,t) it is clear that the observed dynamics is almost of an ordinary translational nature. A comparison with previous dielectric measurements suggests a possible merging of the alpha and beta relaxations of the confined water at T=185 K, although the alpha relaxation cannot be directly observed at lower temperatures due to the severe confinement. The present results are discussed in relation to previous results for water confined in a Na-vermiculite clay, where the average relaxation time from spin-echo measurements was found to be slower than in the present system (particularly at low temperatures).