The Microscopic Structure And Dynamics Of Liquid Alcohols, Water And Their Mixtures

In this dissertation we employ spontaneous Raman spectroscopy to study the microstructures and dynamics of the solutions including hydrogen bonds. The following topics are included: the local structure of liquid methanol; the local structure of liquid water; the quantum effect in liquid water; the microstructures in the water mixtures with methanol, ethanol, n-propanol, i-propanol and tert-butanol; the segregation in molecular level of those aqueous alcohols; the relaxation dynamics of those alcohols in their water solutions; the cononsolvency of macromolecular poly(N-isopropylacrylamide) in aqueous methanol. And it is the final object that we can know the microstructures at molecular leverl with the help of Raman spectroscopy, and then understand the abnormal properties of these liquids.Microscopic structure of liquid methanolWe discuss the microscopic structure of liquid methanol with the help of the polarized Raman spectra of methanol in the -OH stretching region. The Raman peak in this region is a broad band without fine characters. We measure the Raman spectra of liquid methanol at some temperatures, we observe two spectra component with different response to temperature, and another two spectra components were also observed in the depolarization ratio curves in different temperature. And to understand these four components, employing density functional theory, we calculate the Raman spectra of many methanol clusters with different size and shape. Finally we interpret successfully the four components in our measured spectra. United the observed and theoretical spectra, we suggest that the cluster with 3⁵ methanol molecules is the dominant local structure in liquid methanol.The microscopic structure of liquid waterWe discuss the microscopic structure of water with the help of the Raman spectra in -OH stretching region. Employing the dilution with the isotopic water, we avoid the coupling of molecule vibration among and between water molecules, and we measured the Raman spectra in the OD and OH stretching region respectively for HOD in water and heavy water. Same with the decomposition of the spectra of liquid methanol, changing the temperature of liquid water, we successfully decomposed the spectra of water with five peaks. And we also use the density functional theory to calculated the Raman spectra of a large amount of water molecules, according the theoretical spectra, we assign the five peaks from lowest wavenumber to highest region into DDA, DDAA, DDAA, DDA and free OH. We got the distribution of some water molecules with different hydrogen bond patterns, with the help of the distribution, we think about 60⁸0% water molecules is four coordinated and 20⁴0% water molecules is three coordinated in liquid water, and in the temperature 10 ⁷0 oC, the average hydrogen bonds number was suggest to be 3.55³.8.Quantum effect in liquid waterThe quantum effect is thought the reason of the difference of physical and chemical properties of water and heavy water, and we firstly observed the exhibition of the quantum effect on the Raman spectra. We measured the Raman spectroscopy in the–OH stretching region of water and heavy water at different temperatures, and we found the quantum effect can induce heavy water more structured than water, the quantum effect is decreased when the temperature is increased. From our Raman spectra, we observed a turning point occurred at 39⁴5 oC, below and upon the temperature, the shape of water and heave water, and also the difference of the shape of these two kinds of water have different relationships with temperature, we suggest a weak continuous phase change occurred at the temperature, which might can be used to explain the macroscopic properties and the differences of water and heavy water, many of the properties have turning point at 40⁵0 oC.Iceberg in aqueous alcoholIceberg are the water molecules around the hydrophobic groups, and the structure of those molecules is similar with ice because of hydrophobic effect. It is debate whether the iceberg indeed exist in the aqueous solution. We measure the Raman spectra of various aqueous alcohols in the–OH stretching region, and employ the excess Raman spectra to analysis the observed Raman spectra, and a positive peak located around the vibration region of ice is pronouncedly observed, then we think the iceberg is indeed introduced while adding alcohol into water, we also observed two peaks located at the free OH stretching region, according the relationship of the intensities of the two peaks with the composition of alcohol, those two bands are assigned to the water molecules that located around the hydrophobic and hydrophilic environments, and we think those water molecules mentioned above are located respectively at the micro-surface of iceberg with hydrophobic groups and bulk water. The lager of the hydrophobic, the more the water molecules with free OH at the micro-surface are.Microheterogeneity of aqueous methanolThe aqueous methanol is homogeneous at macroscopic level, however they are heterogeneous at molecular level. We employ Raman spectroscopy to discuss the microheterogeneity in the aqueous alcohols with the help of noncoincidence effect. Because of the local order in liquids, the effect can cause the difference of the positions of the anisotropic and isotropic Raman peaks that are assigned to the same vibration modes. We record the effect with the concentrations, and it is found the effect dose not response like ideal mixtures to the change of the composition. When the methanol is diluted gradually with water, the effect decreased quickly at the high concentration region, however it increase at the 20³0% concentration region, we think there are stable ring-shape clusters composed two methanol and one water molecules in this concentration region, this structure induced the increase of the local order. And we affirm the stable clusters according the effect at different temperatures.Relaxation dynamics of alcohol in pure state and their water mixturesReorientation motion and collision induce translation are the basic motions in condensed phase. These motions of alcohol molecules have been studied in liquid alcohols and aqueous alcohols, however they are still under debate. We use polarized Raman spectra to record the relaxation time referring the translation of alcohols in pure state and their water solutions. Our result in the pure state demonstrate that those times are on the sub-picosecond and a few picosecond, the larger of the alkyl, the longer the times are and the slower the motions are, and they follow the sequence: methanol CH3OH=1 - 0.9, PNIPAM absorbs methanol molecules preferentially, and atχ_CH3OH = 0.8-0.5, PNIPAM absorbs water molecules preferentially, and atχ_CH3OH = 0.5-0.2, PNIPAM disrupt the stable methanol-methanol-water ring-shape cluster. And we compare the effect of NCE by PNIPAM and its monomer NIPPA, the comparison demonstrate PNIPAM absorb methanol molecules preferentially through the cooperative hydrophobic interactions. We think at the diluted region of aqueous methanol these cooperative interactions destroy the stable ring structure that is composed by one water and two methanol molecules, and this structure reforms while the temperature is increase, that induce the noncosolvency of PNIPAM in aqueous methanol.