### Effect of ion hydration on THz transmission characteristics of ammonium salt solution

The spectral intensity of distilled water injected into the chip was much lower than that of the empty chip in this study, indicating that water absorbs the vast majority of THz waves, which is mainly due to hydrogen bonding between the water molecules; and the difference in spectral intensity between distilled water and the four ammonium salt solutions indicates that the hydration of the ions affects the hydrogen bonds in the water and then changes the transmission of the THz wave to the solution. Free water is a group of water molecules that are polymerized by hydrogen bonding. Water has a coordination number of 4 in the range of 4 at 200°C, according to Liu shiyin12, indicating that most of the water cluster is in the form of five-ring water, as shown in Fig. 10. However, getting the structure of water in the strict sense is more difficult. Indeed, although the interaction of hydrogen bonding in water is relatively strong, its three-dimensional structure is dynamic, and the temperature, electric field, magnetic field and all kinds of ion hydration can cause fracture and transformation of the hydrogen bond between water molecules.

The interaction of hydrogen bonds between water molecules or clusters falls under the category of complex multi-body interactions, which are affected by many factors. In this study, when ammonium salts are dissolved in water to form an electrolyte solution, the electric field formed near the ions causes some solvent water molecules to align around the ions to form ions. hydrated, and the microstructure of water in the ion hydration layer changes, which is the influence of solution ions on the structure of water.

Kitadai et al.13 used ATR-IR spectra to explore the effects of different cationic and anionic ions in the OH stretching band (2600–3800cm−1) on the hydrogen bond between water molecules in the hydration shell. The OH stretching band of water is sensitive to the interaction between ions, hydrated water molecules, and hydrogen bonding between water molecules. Ions that move the OH stretch band to a lower frequency are generally interpreted to enhance hydrogen bonding between water molecules in the hydration shell, other ions have been interpreted to weaken hydrogen bonding, and it was found that the effect of anions on hydrogen bonding was greater than that of cations. Wang et al.14 divided these ions into two categories according to their influence on the structure of water. One is to associate the hydrogen bonds of water molecules in the hydration layer; the other is to destroy hydrogen bonds between water molecules in the hydration layer, and the effect is related to the coefficient of viscosity Bη:

$$eta /{eta }_{0}=1+A{c}^frac{1}{2}+{B}_{eta }c+D{c}^{2}$$

or η and η0 are the viscosities of electrolyte solution and pure water, respectively, and Bη is the viscosity coefficient of the electrolyte. The characteristic constants of the solution are A and D. The concentration of the solution indicated by vs. A The value can be used to characterize the interaction of solutes in a solution, which can be ignored in this study. the D The value is related to solute-solute and solute-solvent interactions in solution and can be ignored in this case. Accordingly, the viscosity coefficient of the electrolyte can be calculated by measuring the viscosity of the solution and pure water. Generally, the coefficient of viscosity of the electrolyte which weakens the hydrogen bonds between water molecules is less than zero, while the coefficient of viscosity of the electrolyte which strengthens the hydrogen bonds between water molecules is greater than zero.

In this study, the influence of anions on the hydrogen bonds of water molecules in the hydration shell was explored in the THz range, and four common solutions of ammonium salts (0.9 mol/L NH4Cl, (NH4)2SO4(NH4)2CO3CH3COONH4) at the same concentration were selected, and because the cations in the solution are the same, the anions play an important role in changing the hydrogen bond between water molecules. The spectral information in the THz frequency domain of the solutions obtained by several measurements, the order of the spectral intensity is: CH3COONH4 > (NH4) 2SO4 > (NH4) 2CO3 > NH4Cl. Since the influence of water on the absorption of THz is mainly due to the hydrogen bond between water molecules, it is found that the spectral intensity of the four ammonium salts is lower than that of distilled water. Therefore, it can be deduced that the four ammonium salt solutions promote the formation of hydrogen bonds between water molecules, thereby reducing the intensity of the THz spectrum. Based on the above theory, the viscosity coefficients Bη SO42− and co32− are calculated at 0.206 and 0.294, respectively, indicating that both anions promote the formation of hydrogen bonds between water molecules and CO32− is greater than SO42−, which is consistent with experimental results from THz spectroscopy. It can be deduced that CH3COO and Cl can also form hydrogen bonds between water molecules, and the order of the association effect of four anions on molecular hydrogen bonding in the hydration shell from strong to weak is Cl > OC32− > SO42− >CH3COO.

### Effect of external electric field on THz transmission characteristics of ammonium salt solution

It is well known that the strong absorption of THz waves by water mainly comes from the resonant absorption of hydrogen bonds around water molecules, which largely limits the application of THz technology in the field of biochemistry. However, the distribution of hydrogen bonds in water is dynamic in nature and is sensitive to the influence of external factors. As mentioned above, with the introduction of ammonium salts, ionic hydration occurs in solution and the super strong electric field of about 108 V/cm14 near the ion changes the structure of water. Inspired by this phenomenon, we realized the effect of an applied electric field on the THz transmission properties of ammonium salt solutions.

Zong et al.15 used MD simulations under a very high external electric field of 9 × 106 V/cm to probe the variation of the viscosity component of water and the anisotropy due to the redistribution of hydrogen bonds in water. Based on this, we used the chip to study the THz characteristics of water under the electric field applied at different times. The experimental results are shown in Fig. 11.

The external electric field strength in this experiment is only 2500 V/cm, which is much lower than the simulated value, so no significant change in the viscosity of water and the four ammonium salt solutions was found, nor anisotropy caused by the redistribution of hydrogen bonds. However, we can see in Fig. 11 than the THz transmission intensity of H2O gradually increases with increasing applied electric field time and reaches saturation at about 15 min. This is because water as a polar molecule has electric dipole interactions. When an electric field is applied, the dipole moment rotates at different angles, which in turn affects the vibration, rotation, and spatial structure of the entire water molecule, causing changes in the THz transmission. Under the action of a uniform electric field, the electric dipole of water molecules will be affected by the moment, and the relationship between them is:

$$M={mu }_{0}ESintheta$$

or M is the torque of the water molecule under the action of the applied electric field; µ0 is the natural dipole moment of the water molecule. θ is the angle between the electric field strength E and the natural dipole moment µ0.

With the introduction of ammonium salts, ionic hydration first occurs in solution, forming an ionic hydration layer and changing the structure of water. After the application of the electric field, the molecular orientation of the four ammonium salts undergoes reorientation, and with increasing time of the applied electric field, the orientation gradually increases, the dielectric constant of the solution decreases, and the peak of the radial OH distribution function in the electrolyte solution has a slightly lower peak than the case without the applied electric field16. The electric field is shown to weaken the hydrogen bond in the ammonium salt solution. The structure of the original clusters in the water also changes. Large clusters gradually dissociate into smaller clusters of water molecules, the number of individual water molecules increases, their activity increases, and the diffusion coefficient of water molecules increases, which hinders ionic bonding in solution. Thus, due to the applied electric field, the orientation of the four molecules of ammonium salt became larger, and the THz transmission intensity of the solution increased with time and did not show similar saturation phenomenon. to that of water, which we attribute to the introduction of solute molecules.

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