1. Introduction

As one of the most abundant chemical compounds in biological systems, water has been at the center of scientific interest for decades [1–3]. It is understood that the extraordinary ability of water to form a network of hydrogen bonds (H-bond) with itself and with organic solutes is at the heart of biochemistry. However, the H-bond network is complex. It appears to have the right balance of stability and dynamics-a relatively rigid covalently part (O-H) and an extremely dynamic van der Waals interactions (O:H), which makes it a tough task to detail how water interact with other organic substances [3–5]. Therefore, we urgently need a system accounting the effect of organic matter on the H-bond network structures of water. Since hydroxyl is one of the chemical groups that forms H-bond most easily, alcohol molecules are considered as good models. More importantly, alcohols can be miscible with water in any ratio [6,7]. In fact, there have been many experimental and computational studies on the mixed solution of alcohol and water [8–14]. But the interaction between water and alcohol remains controversial, especially for the interaction between the hydrophobic structure and water. The classical but still leading explanation is the so-called iceberg formation interpretation by Frank and Evans [8], which describes that hydrogen bonding between water molecules is strengthened near the hydrophobic groups of organic matter. There are many other scholars who also support the existence of this structure but disagree on the reasons for its creation [9–11]. Although the concept of ice-like structures is widely used, whether such structure exist is still a big debate so far [12–14].

Optical methods, especially spontaneous and stimulated Raman scattering (SRS) spectroscopy, are classical methods that have shown great power in analyzing the molecular composition and structure of various compounds [15–17]. Since OH stretching wavenumbers are very sensitive to the local molecular environments, the last decade has witnessed a rapid emergence of the Raman method being applied to probing the H-bond network of water and their solutions [15,17–19]. But unfortunately, the O-H stretching vibration peaks of both water and alcohol are very broad (from 3000 cm^-1 to 3700 cm^-1), which leads to difficulties in detecting the H-bond dynamics of the mixed solutions. Using the isotope of water molecules-heavy water can easily solve this problem. Raman shift of O-D vibration is different from hydrophobic groups of alcohol, but it has a hydrogen bonding network structure similar to that of water. So, using heavy water-alcohol system can greatly simplify this research project.

In this work, we studied heavy water-alcohol (methanol and ethanol) binary solutions with varying mixing volume ratios by Raman methods. It was found the H-bond in heavy water first strengthened but then weakened with the increased volume ratios of alcohol in the mixed solutions (V_A) under strong pulsed laser. When V_A < 20%, the hydrophilic group (-OH) and hydrophobic group (-CH₃, -C₂H₅) are equally capable of reacting with heavy water molecules, thus complementing the originally incomplete tetraheral H-bond structure in heavy water molecules and forming the strong H-bonds structure. But at V_A > 20%, the self-association of the alcohol molecules caused H-bond structure of heavy water on the shell distorted or even destroyed. The experiment not only fully demonstrated the behavior of the H-bond network in alcohol-heavy water solutions, but also provided an important reference for analyzing the H-bond structure in water-alcohol solutions.

2. Experiment

Fig. 1 schematically illustrated the SRS experimental setup. A neodymium-doped yttrium aluminum garnet (Nd: YAG) laser source of 1064 nm, was used as the fundamental light source. A KTiOPO₄ (KTP) crystal was used for the second harmonic-generation to output a 532-nm pump laser. The pulse duration and the repetition rate were 7 ns and 5 Hz, respectively. Remaining fundamental laser was filtered out by two mirrors TM₁ and TM₂, which were transparent at the wavelength of 1064 nm output and highly reflective at 532 nm. The diameter of the laser was 5 mm. To improve the beam quality, the pump beam first travelled through a small-aperture diaphragm (F). Then the beam was focused into the sample by a lens (focal length=150 mm). Samples were filed in a cell whose width, height and length were 10, 50 and 100 mm, respectively. The output light was analyzed by an Ocean Optics HR4000CG-UV-NIR spectrometer with a resolution of 1 cm^-1 through a detector D₁. All experiments were carried out under normal conditions.

 

Fig. 1. Schematic diagram of the experimental setup to measure SRS spectra.

Download Full Size | PDF

The Raman spectra were obtained using a Renishaw InVia Raman spectrometer. Argon laser at wavelength of 514.5 nm was used as an excitation laser for the spontaneous Raman measurements. The output power of laser was 10 mW. The Raman spectra in backscattering configuration were obtained using a 50× long working distance objective lens located in sample solutions, detecting by a CCD detector (Princeton Instruments SPEC-10:100B). The scanning speed is 10 cm^-1/s. A 1200 lines/mm grating was used, which resulted in a spectral resolution of 1 cm^-1.

The purity of heavy water was 99.8% (purchased from Sigma-Aldrich). The pure alcohol has been used high performance liquid chromatography grade (more than 99.5 v/v%; Wako Pure Chemical Industries). The varying mixing volume ratios (V_A) are 0, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5. (The volume of alcohol divided by the volume of the mixture solutions).

3. Results and discussion

Methanol and ethanol molecules consist of each one :Ö:H^- group with three lone electron pairs on the oxygen atom, and linear alkyl chain with different length [6]. It can be found that although there are slight differences in the intensity and shift of the Raman peaks among the two molecules, the Raman mode of C-H vibration is similar, as shown in Fig. 2(b) and Fig. S1(Supplement 1). Therefore, we can speculate that the interaction between alcohol and heavy water may also have a generally consistent trend. Focusing on the O-D vibration (Fig. 2(a) has important reference meaning when we studied how alcohol molecules functionalize the H-bond networks of D₂O. We mainly discussed ethanol-D₂O system, experimental findings of methanol-D₂O are given in SI document.

 

Fig. 2. Spontaneous Raman spectra of D₂O and ethanol.

Download Full Size | PDF

The SRS spectra of pure D₂O and ethanol have been obtained in the pumping beam forward direction as plotted in Fig. 3. Only the Raman band with the largest Raman scattering cross section can be selectively enhanced, and the others are suppressed [20]. In addition, SRS is the signal of coherent light. Therefore, the peaks of SRS are generally narrow and sharp. SRS peak of D₂O appeared at approximately at 2487 cm^-1, attributing to O-D symmetric stretching modes [21]. The Stokes SRS component at 2911 cm⁻¹ is C-H vibrational mode of ethanol (Fig. 3(b)), which agrees well with the spontaneous Raman spectrum of ethanol [20].

 

Fig. 3. SRS spectra of (a) pure D₂O at pump energy of 3.5 mJ and (b) ethanol at pump energy of 2 mJ.

Download Full Size | PDF

Figure 4 showed the SRS spectra of different concentrations ethanol-D₂O mixed solutions at the same pump laser energy of 3.5 mJ. When V_A was 10%, the Raman shift of O-D vibration moved to low-wavenumber compared with pure D₂O (Fig. 4(a)), indicating the elongation of O-D bond. It is well known hydrogen bonds consist of two parts-a relatively rigid covalently part (O-D) and an extremely dynamic van der Waals interactions (O:D). The stiffer D–O bond elongates less than the O:D bond contracts. Therefore, a net O-O length loss and an enhancement of H-bond took place. However, when V_A was up to 20%, the SRS intensity of O-D vibrational peak sharply decreased (Fig. 4(b)), and the Raman shift of O-D vibration is higher than that of pure D₂O. These phenomena meant that the H-bond structure in heavy water was weakened and softened compared to pure D₂O. The same phenomenon was also found when V_A=30% (Fig. 4(c)), which indicated that the H-bond structure of D₂O was also weaker compared to pure D₂O at this condition. Since the Raman scattering effect of ethanol is stronger than heavy water, further increasing the concentration of ethanol (V_A>30%), the SRS peak of heavy water disappeared while SRS peak of ethanol appears in the spectrum (Fig. 4(d) and 4(e)).

 

Fig. 4. SRS spectra of ethanol-D₂O system with different mixing volume ratios at pump energy of 3.5 mJ.

Download Full Size | PDF

In order to clarify the inflection point between enhancement and attenuation, we also added the SRS measurement of V_A=15%, as shown in Fig. 4(f). The SRS peak position is lower than pure D₂O. So, the H-bond also be enhanced at this concentration. Further subdividing the mixing ratio between V_A=15% to V_A=20%, we cannot get a noticeable Raman shift compared with pure D₂O. In addition, we also measured the methanol-D₂O system. The Raman shift trend of methanol-heavy water system is similar as ethanol-D₂O system (Supplement 1, Fig. S2). The changes of O-D vibration indicated that the H-bond structure of D₂O was strengthened when V_A < 20% but destroyed at V_A > 20% at strong intense laser.

We proposed that the H-bond dynamic mechanism of heavy water in mixed solutions is as follows. At the normal condition, an ethanol molecule consists of one hydroxyl (-OH) group with three lone electron pairs on the oxygen atom, and an ethyl group (C₂H₅-), which are equally capable of reacting with heavy water molecules respectively. For low-concentration mixed solutions (V_A < 20%), -OH groups of ethanol molecules can participate in and rearrange the original network structure of heavy molecules formed by H-bonds, thus complementing the originally incomplete tetraheral H-bond structure in heavy water molecules [22–24]. At the same time, the C₂H₅- groups can adsorb surrounding heavy water molecules with the generation of electronic exchange interactions, forming strong H-bonds at the interface as shown in Fig. 5 [12,25–27]. Since the self-association between the ethanol molecules is extremely weak at low concentrations, the heavy water molecules will form a shell around the single ethanol molecule, as shown in the left of Fig. 5(a). With the increase of V_A, this structure is gradually destroyed. Although the heavy water molecules still formed a shell structure around the ethanol molecule, the self-association of the ethanol molecules will cause the interaction between two or more ethanol molecules and the formation of cluster structure. So, the H-bond structure of heavy water molecules on this side of the shell are distorted or even destroyed, as plotted in Fig. 5(b) [24,28,29]. The self-association can weaken the H-bond structure of heavy water molecules in the mixed solutions.

 

Fig. 5. (a) Reactions in the solutions at the low V_A < 20% and V_A > 20% under the strong pulsed laser. (The light blue ring represents the heavy water shell structure. HB means H-bond.)

Download Full Size | PDF

The hydrated shell is easily affected by the external environment (such as temperature and pressure), especially the heavy water molecules in the first layer and 1/2 shell [30–32]. Therefore, our work aims to show the H-bond dynamics under the extreme conditions by using a strong intense pulsed laser focused on samples. Intense pulsed laser focused on samples can cause the laser-induced breakdown (LIB), accompanied by the generation of shock waves [33]. So, in this process, heavy water molecules are decomposed into ion pairs due to LIB, the reaction formula is shown as Eq. (1) [34]. Then, under the action of shock waves compression, these two ions will form a D₂O structure with a stronger H-bond. Ethanol has a large impact resistance, and it requires more energy to breakdown than heavy water molecules. For the low concentration mixed solutions, the ethanol molecules exist as a single molecule [35], they will still be decomposed. The final products are C₂H₄ and H₂O as shown in Eq. (2) [36,37]. Since heavy water contains some incomplete tetrahedral H-bond structure, one of the final products-H₂O can interact with adjacent heavy water molecules in the incomplete tetrahedral H-bond structure, thereby, complete the originally missing H-bond network structure of heavy water. In theory, HOD vibration Raman peaks can also appear in the spectrum, but SRS only amplifies the peak with the largest Raman gain, these weak vibration modes will not be observed in the SRS spectrum. At the same time, :ÖC₂H₆↔D₂Ö: compression will lengthen and soften the O-D bond, while shorten and strengthen the O:D mode, which is equivalent to the effect of pressure [24,28,29]. In addition, with the introduction of -OH groups, dangling O-D bonds in heavy water will form O:D-O bonds with their neighboring -OH groups. The effect of C₂H₄ was similar with -C₂H₅, they can adsorb surrounding heavy water molecules and form the strong H-bonds at the interface. Therefore, at the low concentrations (V_A < 20%), ethanol will enhance the H-bond structure of heavy water (Fig. 5(a)).

When V_A > 20%, the ethanol molecules in the solutions will self-associate to form chain structure [38], leading to a higher breakdown threshold of ethanol molecules in the solutions. At this condition, reaction Eq. (3) occupied a dominant position in the mixed solutions, the ethanol molecules interacted with OD^- to generate C₂H₄OH^- and HOD. C₂H₄OH^- has a similar effect to ethanol molecule. They will form cluster structure with each other through self-association, which will also distort or destroy the H-bond structure in heavy water. In addition, the OD^- in heavy water can be absorbed by ethanol and react with it, resulting in the formation of D₃O⁺ in the solutions. Therefore, at high concentrations (V_A>20%) the H-bond structure of heavy will be destroyed (Fig. 5(b)).

4. Conclusion

In summary, we used SRS method to detect how alcohol molecules functionalize the H-bond networks of D₂O under intense pulsed laser. It was found when V_A < 20%, hydrophilic group/hydrophobic group of alcohol interacted with heavy water equally, enhancing the H-bond structure. However, when V_A > 20%, the self-association of the alcohol molecules distorted or even destroyed H-bond structure of heavy water in hydrated shell. It is hoped that our results can provide meaningful references for analyzing the H-bond network dynamics of other complex organic substance-water systems.