1. Introduction

Water has long been considered as one of the most important substances in chemical and biological processes, which has complex structural characteristics and abnormal properties [1–4]. The ability of water to donate and accept hydrogen bonds (HBs) can participate in specific and nonspecific interactions with dipolar solutes, and lead to unique and complex dynamic phenomena related to its HB network [5,6]. Acetone is an important prototype molecule to form HB network. As an important organic solvent, acetone is a trigonal planar molecule with the higher symmetry of the C_2v point group and the carbonyl oxygen can participate in the composition of traditional HB as HB acceptor [7,8]. Thus, understanding the solvation of water-acetone solution is ubiquitously important in the physics and chemistry industries.

Some successful investigations on the structure of water-acetone mixtures have been reported by the experiment and theoretical methods [9–11]. Zhang et al. [12] indicated that C = O group interacted with water forming the strong HB, and its nonpolar site CH group acted as the proton donor as well. Because the vibrational transitions are highly sensitive to the local environment, the vibrational modes are explored to extract the molecule-level information about the structure transformation, such as modification of HB, coupling between vibrational modes, and dynamics of water [13–15]. Spontaneous Raman spectroscopy is a useful method to probe molecular structure [16,17]. For example, the solvation dynamics of aldehydes and formic acids studied by Raman spectroscopy indicated the molecular dipolar interaction playing an important role in disrupting the solution network and surface stress [18]. Besides, the low-frequency Raman spectra of water-acetone mixtures showed that the low-frequency Raman band was associated with the oscillation of a water molecule within the cage formed by its neighbors [19]. However, a detailed analysis of the HBs interaction between water and acetone mixtures, is still lacking. Especially, the effect of acetone in the dilute solution on the HB structure of water has not been systematically measured. Compared with spontaneous Raman, SRS from the strong interaction between strong laser and matters (gaseous, liquid, and solid media), which belongs to the third-order nonlinear optical effect. SRS spectra can sensitively reflect slight changes in the HB network structure and kinetics under different conditions [20,21].

In this work, the spontaneous Raman and SRS were combined, which was used to insight into the effects of hydrocarbons and carbonyls on the structure of the aqueous HB network, and further determine the transitional concentration of the HB network in the acetone-water mixed solution. The HB network of water molecules was enhanced at the low acetone concentrations. A new SRS peak of water was obtained when the volume concentration was lower than 0.25. Then the main peak of water at 3380 cm⁻¹ disappeared as the volume concentration exceeded 0.25. Besides, spontaneous Raman spectroscopy indicated that almost HB among water molecules was destroyed as the concentration of acetone was higher than 0.8.

2. Experimental setup

Acetone purchased from Wako Pure Chemical Industries was used as a sample. The liquid water has been deionized from triple distilled water. All experiments were carried out under the room temperature. The volume fractions of Acetone were 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1 (the volume of acetone divided by the volume of the mixture solutions). The spontaneous Raman spectra of acetone-water binary solutions were obtained by Renishaw-Invia Raman spectrometer. The sample was excited by an Ar+ laser at 5145Å and the output power was 10 mW. Raman spectra were obtained by Olympus objective lens (focal length: 26.5 mm) and CCD detector (Princeton Instruments SPEC-10:100B) in backscattering configuration. The scanning range was from 100 to 3800 cm⁻¹. SRS was excited by a frequency-doubled Nd: YAG laser system, providing a wavelength of 532 nm and delivering a pulse duration of 7 ns. The repetition rate and beam diameter were 10 Hz and 4 mm, respectively. The aperture diaphragm (AD) was used to improve the beam quality by cutting off the edge of the laser facula. The samples were filed in a cell (width, height, and length: 10 ×50 ×100 mm) and the focal length of the focusing lens was 150 mm. The experiment has been performed in the breakdown regime (the obvious self-focusing and filament phenomenon were shown in the illustration). We placed a white screen (THORLABS 150 mm Square Plastic Viewing Screen) at the distance of∼30 cm after the sample cell to control visually the Stokes beams divergence and intensity distribution. The 532 nm notch filter was used to filter out the remaining pump laser and pass the Stokes light. Then it was analyzed by an Ocean Optics HR4000CG-UV-NIR spectrometer with a resolution of 1 cm⁻¹. The experimental setup was shown in Fig. 1.

 

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

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3. Results and discussion

The typical Raman band of water can be identified as 3000 cm⁻¹−3800 cm⁻¹ (Fig. 2(a)), which has a shoulder peak at 3230 cm⁻¹ and the main peak at 3445cm⁻¹, representing the strong and weak HB of water structures, respectively [22]. Both two peaks are also identified as fully and partially HB structures of water [23]. In order to examine the effect of acetone on the HB kinetics of water molecules, the fitting curve and derived differential Raman spectra (DRS) were used (Fig. S1 and S2 in SI). Among them, the DRS was obtained by subtracting the referential spectrum (pure water) from the corresponding acetone-water mixtures. All measured spectra were corrected by the background of elastic scattering effect [24]. Raman shifts of strong and weak HB of the water were irregular with the increase of acetone concentration. As shown in Fig. 2(b), the strong and weak HB of water first experienced red shifts, and then reached the maximum as the concentration of acetone was 0.1. It indicated the HB strength of the water was the strongest at this point. Then the weak and strong HB began blue shifts, since the HB strength of the water was weakened. The strong HB structure of water disappeared as the concentration of acetone exceeds to 0.8 due to the weakening of the HB structure of water. These phenomena indicated that the HB kinetics of water molecules in the mixed solution has changed.

 

Fig. 2. (a) Spontaneous Raman spectra of OH stretching band of water in acetone-water mixtures with different volume fractions, the DRS in the top inset. (b) The relationship between Raman shifts of OH with the strong and weak HB and the different volume fractions.

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To determine acetone could whether enhance the HB network structure of water and to further understand the hydration environment of acetone aqueous solution, SRS was used. The Raman intensity and shift of SRS peaks of the acetone-water mixtures were shown in Fig. 3(a). The SRS intensity of OH stretching vibration first strengthened and then weakened until it disappeared. SRS of OH stretching vibration only had one characteristic peak whose wavenumber appeared at 3380 cm⁻¹ (in Fig. 3(b)), corresponding to the weak HB structure of water. With the volume fraction of acetone increasing to 0.05, a new peak begins to appear at 3272 cm⁻¹, which represented a strong HB structure of water [25]. As the volume fraction of acetone increased to 0.15, SRS spectra of acetone only have one characteristic peak at 2906 cm⁻¹ which corresponded to the CH stretching vibration mode of spontaneous Raman spectra. In addition, the strong HB structure of water disappeared when the volume fraction of acetone was 0.25 and the intensity of weak HB decreased.

 

Fig. 3. SRS spectra of acetone-water mixtures at the concentration of acetone ranging from 0 to 1. (a) pseudo-color two-dimension spectrum; (b) SRS spectra of acetone-water mixtures with different volume fractions. (Normalized intensity: the intensity of the pump peak is classified as 1, and other peaks are calculated in proportion);

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In order to better confirm the changes of peaks, SRS shifts of OH with the strong and weak HB as a function of acetone concentration were depicted in Fig. 4(a). The Raman shift was inversely proportional to the strength of the HB. Besides the strong HB of water starting to appear at 0.05, the strong HB presented red shifts from 3272 cm⁻¹ to 3228 cm⁻¹, and the weak HB moved from 3380 cm⁻¹ to 3352 cm⁻¹ in the range from 0.05 to 0.1. Meanwhile, the SRS intensity of OH with the weak HB at 0.1 was about twice that of pure water (Fig. 4(b)). These results are consistent with that of spontaneous Raman spectra in Fig. 2, which shows the enhancement effect of the HB structure of water was the strongest at 0.1. In addition, it indicated that water molecules presented a strong vibrational coupling effect, affecting the OH stretching vibration mode in the mixed solution. That was, acetone formed HB only with some water molecules in the tetrahedral structure, and the tetrahedral structure in the mixed solution was more than that in bulk water. The peak at 3228cm⁻¹ was the same as the SRS peak of ice Ih [26,27], which represented the ice-like structure was formed in dilute acetone solutions (Fig. S3 in SI).

 

Fig. 4. (a) The relationship between the concentration V(x) of acetone and the SRS shifts of OH with the strong and weak HB and CH stretching vibration; (b) The relationship between the concentration V(x) of acetone and the normalized Raman intensity of OH with strong and weak HB and CH stretching vibration; (c) Spontaneous Raman spectra of the C = O stretching vibration with different volume fractions ;(d) Raman shift and FWHM of C = O stretching vibration versus different volume fractions.

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Importantly, with the increase of acetone concentration, the vibrational coupling between water molecules decreased, while the vibrational coupling between corresponding acetone molecules would strengthen. Therefore, combined with Fig. 4(b), the intensity of weak HB (at 0.25) is lower than that of bulk water. These results indicated that the HB structure of water was indeed weakened as the volume fraction of acetone was greater than 0.25. The changes of HB of water molecules will also lead to the corresponding changes of HB dynamics of acetone, which can be reflected by the CH and C = O stretching mode in acetone [24,28]. It should be noted that the Raman shift of CH stretching vibration appeared the red shift in Fig. 4(a), meanwhile, the intensity of the CH was enhanced greatly as the volume fraction increased to 0.8 (Fig. 4(b)), which was consistent with the disappearance of the strong HB of water at 0.8 in Fig. 2(b). From Fig. 4(c) and 4(d), the C = O stretching vibration begins to present an obvious blue shift after the volume fraction exceeded 0.25, and the blue shifts range enhanced as the volume fraction of acetone exceeded 0.8. It confirmed that the HB of OH:O = C was formed between the O atom of acetone and the H atom of water. Hence, when more acetone was added, the Raman shift of C = O stretching mode presented the length of C = O stretching decreased, and the corresponding force constant increased. Figure 4(d) depicted the relationship between the concentration of acetone and FWHM of the C = O stretching vibration, which was consistent with the Raman shift, and the FWHM begins to decrease at 0.8. Thus, these analyses indicated that the HB in the acetone-water mixed solutions changed dramatically at 0.8 and the coupling effect between acetone molecules became stronger.

The evolution of the spatial distribution of SRS (in Fig. 5) showed some correlation with the intensity diagram of Fig. 3(a). Four volume fractions of samples, pure water, 0.05, 0.1, and 0.15 were selected. As the volume fraction of acetone increased from 0 to 0.1, the center spot and outer large ring became brighter. And the faculae size of the center and the outer large ring was gradually increased. SRS beams from OH stretching vibration at 3380 cm⁻¹ (weak HB) were mainly concentrated in the center. In contrast, the outer large ring mainly contained a part of the contribution from 3380 cm⁻¹and the radiation of the new Stokes SRS at 3272 cm⁻¹ (strong HB) [29]. Under the interference of the characteristic SRS peak of acetone, the outer ring-shaped pattern disappeared at 0.15, and the central brightness spot remained stable. Therefore, the trend of SRS faculae demonstrated that acetone can enhance the HB structure of water, it can be clear that the enhancement effect of acetone reached the maximum at 0.1.

 

Fig. 5. Spatial profiles of SRS taken with a digital camera in acetone-water mixtures;(a) pure water and the volume fractions of (b) 0.05, (c) 0.1, (d) 0.15.

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The mechanism of enhancement and weakening the HB network of water by acetone is discussed as follows. As the acetone increased, the original stable microstructure in aqueous solutions was changed [11]. Particularly, the C = O with two lone electron pairs can replace the H₂O molecule, which also has two lone electron pairs. The results indicated that in dilute solutions (from 0 to 0.25), the carbonyl oxygen interacted with one of its four neighboring water molecules through H₂$\mathop {\ddot {\textrm{O}}}\limits : \leftrightarrow $ $: \mathop {\ddot {\textrm{O}}}\limits $=C compression to form a fully tetrahedral HB structure, and partially HB structure water with one or more surrounding water molecules [30,31]. As a result, $\mathop {\ddot {\textrm{O}}}\limits : \leftrightarrow :\mathop {\ddot{\textrm{O}}}\limits $ point compression effect will in turn elongate and soften the OH bond, as shown in Fig. 6(a).

 

Fig. 6. Schematic diagram of (a) the interaction of C = O group with water molecules (form OH⋯O = C type HB) (b) the ice-like structure around CH₃ groups and (c) the deformation of HB network structure in water because of the self-association.

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In addition to hydrophilic interaction, the interaction between non-polar groups (such as the methyl of acetone) and water molecules also played a key role in the anomalous properties of the acetone-water system. Since most methyl groups could exist in the cavities of hydrogen bond networks, resulting in the intense hydrophobic interaction leading to the rearrangement of HB configuration in overlapping solvation regions [32,33]. It was noteworthy that the presence of positive charge on the hydrophobic methyl group reduced the tendentiousness of dangling OH around them. In dilute acetone solutions, the HB strength and viscosity of water in the hydration layer around methyl were stronger than in bulk water [33,34]. Moreover, this interaction results in the formation of ice-like structure around the methyl group were confirmed in Fig. 6(b). Therefore, the HB network strength of water reached the maximum at 0.1.

Previous researches have shown that the hydration layer was not stable, especially since the water network in the second coordination shell was slightly interrupted under the influence of concentration [7,8,35]. Therefore, as the volume fraction of acetone exceeded 0.25, some acetone molecules tended to self-associate through dipole-dipole interaction. The increase of acetone aggregates and the reduction of single acetone resulted in a decrease in the total active area of methyl group and water molecules in the solutions. It can deform the HB and further destroyed the hydration layer structure of acetone, as shown in Fig. 6(c). Therefore, with the concentration increase of acetone, the fully HB tetrahedral structure was gradually broken and transformed into partially HB water [36,37], and the HB network structure of water was weakened at 0.25. This result was consistent with the nonlinear change of excess enthalpy of acetone aqueous solution, which also presented that acetone had the role of destroying the HB structure. Meanwhile, the acetone molecule aggregates formed the self-association process enhanced the HB network strength between acetone molecules, leading to the enhancement of CH stretching vibration. Then multiple water molecules were surrounded by acetone molecules to form cluster structures with the increase of acetone concentration. During this process, the combination form gradually was changed, from one acetone molecule with multiple water molecules to one acetone molecule and one water molecule. Thus, it resulted in the destruction of local HB structure and the fully HB tetrahedral structure of water over 0.8.

4. Conclusion

The spontaneous Raman and SRS spectra of acetone-water binary solutions with different volume fractions were studied. The results indicated that the HB structure of water was enhanced with the volume fraction of acetone ranging from 0 to 0.25, due to the carbonyl oxygen interacting with one of four neighboring water molecules to form a fully tetrahedral HB structure. At the same time, the methyl also enhanced the HB structure of surrounding water molecules, an ice-like structure was formed around methyl groups. With the increase of acetone, a large number of acetone molecules self-associate through dipole-dipole alignment, resulting in HB structure of water being destroyed, which can also be confirmed by the nonlinear variation of the Raman peaks of acetone. The combination of spontaneous Raman with SRS leads to a better knowledge of the HB structure in aqueous solutions, it can be extended to study of HB kinetics of other small molecules-water system.