Introduction

The boric acid has multiple uses. Some of industrial applications of boric acid are: manufacture of textile fiberglass [1]; combination of boric acid with denaturated alcohol contributes to the reduction of the surface oxidation in jewelry industry; uses of B(OH)3 in production of the glass in LCD flat panel displays; in neutralization of the active hydrofluoric acid (HF). Since 1948, boric acid was firstly registered in the US as an insecticide, because it acts as a stomach poison affecting the insects’ metabolism [2]; the dry powder of B(OH)3 is abrasive to the insects’ exoskeletons; in combination with an ethylene glycol is used to treat external wood against fungal and insect attack [2]; concentrates of borate-based treatments can be used to prevent slime, mycelium, and algae growth, even in marine environments [2]. Boric acid buffers against rising pH in swimming pools [2]. We mention also the remarkable lubricant properties of a colloidal suspension of nanoparticles of boric acid dissolved in petroleum or vegetable oil [3]. Boric acid is added in the reactor coolant which circulates through the reactor and due to the fact that boron-10 has a high cross-section for absorption of thermal neutrons, is contributing to effectively regulate the rate of fission taking place in the reactor. The medical uses of boric acid are explained by its antiseptic and antibacterial action, being very appreciated in acne, otitis externa treatments, in eye washing and others [2, 4]. But one of the most important researches for the medicine are the use of acid boric in combination with calcium carbonate and fructose or glucose to produce future cancer drugs. It is known that the cancer cells consume glucose in a proportion of 1,000 times more than the normal cells, and therefore, by some mechanism, boron is also transported into the cell in large amounts being toxic to cancer cells [5, 6]. In this paper, we report on the results of a study concerning the modification of the crystallization system of the boric acid (TBA) from triclinic to hexagonal, when a special preparation procedure has been applied. Some properties of the new obtained compound, named HBA, are analysed by thermal, XRD, optical, and FTIR investigations. In plus, the dielectric behavior of HBA compound in water solutions of different concentrations is also presented.

Experimental

HBA preparation

The hexagonal structure of the boric acid (HBA) was obtained from a mixture of triclinic boric acid (TBA), d-glucose, calcium carbonate, and calcium hydroxide at temperatures between 60 and 70 °C, under controlled pH conditions.

Methods and techniques

XRD measurements were performed on powder samples, with a Shimadzu XRD-6000 X-ray Diffractometer, equipped with a vertical goniometry and a scintillation detector. The functioning parameters of the X-ray tube (A40-Cu type) were established at a voltage of 40 kV and a current of 30 mA. The wavelength of CuKα type X-ray radiation has a mean value of λ = 1.54189 Å. As operation mode, we have chosen a continuous scan measurement in a geometry (θ/2θ), setting a scan 2θ rate of 1°min−1 and a scan 2θ range from 10° to 65°. Divergence slit was of 1.0000°, scattering slit was of 1.0000°, and receiving slit of 0.1500 mm.

Thermal analysis measurements (TG, DTG, DTA, and DSC) of both TBA and HBA compounds were carried out in dynamic air atmosphere (150 cm3 min−1), from room temperature (RT) to 1,000 °C, with a rate of 10 K min−1. A horizontal analyzer “DIAMOND” TG/DTA from PerkinElmer Instruments was used during the measurements. The thermogravimetric and enthalpic calculations were performed with the specialized software Pyris.

The chemical bonds of samples were investigated by FTIR with a PerkinElmer “SPECTRUM100” spectrometer in the wavenumber range of 600–4,000 cm−1. The spectrum was obtained using universal attenuated total reflectance (UATR) accessory, at a resolution of 4 cm−1, with ten scans, and CO2/H2O correction.

Optical microstructure observations have been made with an optical polarizing microscope LEICA DM 2500 P equipped with video camera for microphotographs of sample and a hot thermostatic stage TMS 94 (Linkam Scientific Instruments Ltd.) connected to the temperature programmer. The HBA mono-crystals as well as polycrystalline conglomerates, synthesized by growth from water solutions on a plate silica support, have been observed between crossed analyser and polarizer under a polarizing microscope.

Dielectric properties of HBA in water solutions and its temperature dependence were examined with a Digital Refractometer RA-520 N. We measured the refractive index of the HBA + H2O solutions by detecting the critical angle of the total reflection. The device was equipped with a temperature control system with the build in Peltier elements in order to maintain and measure the border temperature between sample and prism.

Results and discussion

XRD analysis

Since 1954, Zachariasen [7] discovered, by X-ray diffraction methods, that B(OH)3 belongs to the triclinic system (space group \( {\text{P }}\bar{1} \)), consisting of stacked layered sheets: each sheet contains H3BO3 molecular units (possessing a nearly perfect C3h symmetry) that are connected between them through hydrogen bonds (as seen from the inserts of Fig. 1).

Fig. 1
figure 1

XRD patterns of TBA and HBA in 10°–65°. 2θ range. In the insets are given the trigonal H3BO3 molecular unit (the left) as well as the structure of one sheet consisting of hydrogen-bonded B(OH)3 molecules

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Ab initio calculations show that the binding energy between stacked sheets is about one-third of the intralayer interaction energy [8]. This facilitates the mutual sliding between layers, thus explaining the lubricant properties of boric acid.

The Bragg diffraction patterns, indicate a good crystallinity for both TBA and HBA compounds, but surprisingly they are belonging to different crystallographic systems (Fig. 1).

By XRD, performed with a Shimadzu XRD 6000 Diffractometer using a CuKα source, we indexed the unit cell of TBA compound, as belonging to the triclinic system. For indexing diffraction patterns of this compound named TBA, we were guided by a similar structure given by the cards no 30-0199 [7], taken from 2001 JCPDS-international Centre for Diffraction Data (PCPDFWIN v.2.2).

The main peaks (intensity, inter-plane distance, d hkl, diffraction angles, 2θ, and (hkl) Miller indexes for diffraction planes) in the case of triclinic boric acid (TBA) are given in Table 1.

Table 1 Main XRD peaks of TBA
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In conformity with the card no. 30-0199 [7], the parameters for the triclinic lattice of TBA are the followings: a = 7.039 Å, b = 7.053 Å, c = 6.578 Å, α = 92.58°, β = 101.17°, and γ = 119.83°.

The new prepared compound, named HBA, has a different structure with a better symmetry, the unit cell being indexed as belonging to the hexagonal system (\( {\text{P }}\overline{6} \) space group), in conformity with the card no 36-1481, taken from 2001 JCPDS-international Centre for Diffraction Data (PCPDFWIN v.2.2). The calculated lattice constants (a, b, and c) of the hexagonal unit cell have been determined by means of the well known Eq. 1 giving the relationship between the inter-plane distance, d hkl, Miller indexes (hkl), and lattice constants, respectively [9, 10].

$$ \frac{1}{{d_{\text{hkl}}^{2} }} = \frac{4}{3}\frac{{h^{2} + hk + k^{2} }}{{a^{2} }} + \frac{{\ell^{2} }}{{c^{2} }}. $$
(1)

Table 2 shows the main XRD peaks of HBA compound (intensity, inter-plane distance, d hkl, diffraction angles, 2θ, and (hkl) Miller indexes for diffraction planes).

Table 2 Main XRD peaks of HBA
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The experimental lattice parameters were calculated using the least square refinement from a Shimadzu adequate program, obtaining the following cell parameters: a = b = 20.4869 Å and c = 12.1506 Å. This proves a structural organization degree at long range.

There are some approximation methods to define the breadth B in the Scherrer formula to calculate the average crystallite sizes [11–14]. In the assumption of ellipsoidal particles, in Ref. [11] it is found that the Jones more purely empirical approach will provide the best means for making allowance to the dimensions of the samples. In this last case, using the integral breadths B I, when K I = 1.333, we obtained for HBA particle dimensions a mean value of about 19.77 nm.

Thermal behavior

Thermal analysis is a method that is frequently used for determining the temperature range of the thermal stability, as well as, the thermal effects characterizing the phase transitions and decomposition processes taking place either in organic or inorganic compounds [15–23].

In Fig. 2 are shown TG, DTG, DSC, and DTA curves obtained for TBA compound. The results of thermal measurements for HBA are presented in Fig. 3.

Fig. 2
figure 2

Thermoanalytical curves for TBA

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Fig. 3
figure 3

Thermoanalytical curves for HBA

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Examination of the thermal results exhibits a good thermal stability till 98 °C for TBA and a slight increase of the thermal stability limit until 101 °C for HBA. Over those temperatures, the decomposition of both samples is taking place, the fall in mass being accompanied by endothermic processes. The endothermic process is finished at 203 °C for both samples. After 203 °C it is observed a very small rate of mass decreasing until 1,000 °C, without any resizable thermal effect. After the total decomposition, we recorded a residuum of 56.44 % for TBA specimen, and of 54.81 % for HBA sample, respectively. A simple calculation shows that after the total decomposition of the boric acid, we should obtain a residuum of B2O3 in a cuantum of 56.32 %.

The most evident differences between TBA and HBA are seen on DTG curves. From DTG curve of Fig. 2, is observed that, in the case of TBA sample, the decomposition is occured in three steps: the three peaks of the decomposition process have the maximums at 117.56, 131.32, and 160.53 °C. Each step corresponds to the breaking of the one of the three O–H bonds. The first step of the decomposition is accompanied by a major endothermic effect having the peak at the same temperature of 117.56 °C (as seen from DSC curve, given in Fig. 4). The second (at 131.32 °C) and the third stages (at 160.53 °C) of the decomposition are also endothermic processes. For the total endothermic effect, we calculated, within the specialized software Pyris, a value of the enthalpy variation equal with ΔH = 1.41 kJ g−1.

Fig. 4
figure 4

The total endothermic effect for the non-isothermal decomposition of TBA

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Concerning the HBA decomposition, it is seen from DTG curve of Fig. 3 that this is taking place in three stages too, but with a different kinetics compared with TBA: the thermal effect of the first stage is camouflaged by the second endothermic effect having the maximum at 130.75 °C (Fig. 5); the third stage of the decomposition has the maximum of the endothermic effect at 160.84 °C. The enthalpy variation of the total endothermic effect corresponding to the decomposition of HBA is ΔH = 1.51 kJ g−1. The calculations give an energy of 4.40 eV for the decomposition of a TBA molecule, while for the decomposition of a HBA molecule is needed an energy of 4.68 eV. Those values proved that the bound forces are much stronger for HBA molecules than for TBA molecules.

Fig. 5
figure 5

The total endothermic effect for the non-isothermal decomposition of HBA

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FTIR spectra of TBA and HBA

The FTIR absorbance spectra of the boric acid in the cases of the two crystalization systems are presented in Fig. 6. The two spectra are overlapped in order to evidence the small differences between the positions and intensities of the IR absorbance for TBA and HBA, respectively. For a better comparison between the two compounds, in Table 3 are given the IR wavenumbers, and the intensities for the principal IR absorption peaks obtained in the cases of TBA and HBA, respectively.

Fig. 6
figure 6

Absorbance FTIR spectra of TBA (a-curve) and HBA (b-curve)

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Table 3 Absorbance and wavenumber for the principal IR absorption peaks characteristic to TBA and HBA
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Using the cards no 511 and no 4609, from the IR spectra database of the FTIR spectrometer SPECTRUM100, we identified the IR absorbance bands having the peaks at 3,187.6, and 706.95 cm−1 (for TBA) and at 3,191.20, and 703.58 cm−1 (for HBA) as belonging to the O–H bond stretching vibration of the trigonal boric acid molecule [24].

The strong IR bands that appear at 1,406.97 cm−1 (for TBA) and 1,409.03 cm−1 (for HBA) can be assigned to the asymmetric B–O bond stretching mode of the trigonal planar boric acid [24]. The B–O–H in plane bending [24] appears for TBA at 1,190.52 cm−1, while for HBA at 1,191.08 cm−1.

Comparing the intensities of the principal IR absorbance peaks, given in the Table 3, we concluded that the IR absorption is a little much stronger in the case of HBA compared to TBA.

We recorded some small displacements in the positions of the IR absorption peaks for the HBA compared to TBA. This is probably due to the differences between the crystalization system characteristic to each compound.

Temperature dependence of the refractive index of HBA + H2O solutions

Knowledge of the refractive index of aqueous solutions of salts and biological agents is of crucial importance in many applications. Different methods have been developed to measure the refractive index of liquids [25–29].

The solution was prepared by dissolving the adequate amounts of prepared HBA powder in 10 mL of distilled water to obtain solutions of different concentrations (3, 5.12 and 7 %). For this purpose, an electronic Precisa XT 120A balance with a precision of 0.0001 g has been used. With a digital refractometer, we measured the refractive indices for the liquid samples at desired temperatures in the range from 20 to 50 °C. The light source was a LED type with the characteristic wavelength of 589.3 nm (the Na-D line). A CCD-optical sensor was used to receive the reflected light by a prism made from sapphire [30]. A precision of ±0.00002 for the refractive index in the range from 1.32 to 1.58, allows us to obtain highly reliable data from measurements results [30].

A decrease of refractive index with increasing temperature is observed in Fig. 7 for distilled water as well for the HBA solutions of 3 and 5.12 %, respectively. The experimental data of refractive index over absolute temperature indicate polynomial fits. It is observed that we have as higher value as higher is the solution concentration (under the saturation state).

Fig. 7
figure 7

The refractive index over temperature for H2O and two HBA + H2O solutions (3, and 5.12 %)

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The dielectric constant ε r = n 2 has been extracted from the experimental refractive index data. Then, the electric susceptibilities χ e = ε r−1 have been calculated and their temperature dependence have been plotted (Fig. 8).

Fig. 8
figure 8

The electric susceptivity is plotted as a function of T −2 for all HBA + H2O investigated solutions. The solid lines are the theoretical fits

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As it was explained in Ref. [29], the water’s strong polar character makes it very good at shielding charged atoms from one another. This fact contributes to the creation of hydrates shell of interactions around charged molecules or ions and thus increasing the dielectric constant of water solutions of those compounds. This behavior is also confirmed in the case of HBA water solutions as observed from Figs. 7 and 5.

The following linear functions are the theoretical fits for T −2 dependence of the electric susceptivity in the case of 0, 3, and 5.12 % of aqueous HBA solutions:

$$ \chi_{{_{\rm{{{H_{2} O}}}} }} = 0.71689 + 5208\frac{1}{{T^{2} }}. $$
(2)
$$ \chi_{{_{{ 3 {\text{\% HBA}}}} }} = 0.71569 + 5842\frac{1}{{T^{2} }}. $$
(3)
$$ \chi_{{_{{ 5. 1 2 {\text{\% HBA}}}} }} = 0.71604 + 6036\frac{1}{{T^{2} }}. $$
(4)

As observed, the second term is the polar one (≈T −2). For pure water (0 % of HBA) b 0 %  = 5,208 K2 is the contribution given by the dipole moments of water polar molecules. But the factor b takes as higher values as higher are the concentrations of HBA in water solution: b 3 %  = 5,842 K2 and b 5.12 %  = 6,036 K2.

Because of insignificant value for the ionic polarizability of water dipoles at optical frequencies, we may assume that the electric susceptibility could be expressed by the Eq. 5 [25–30]:

$$ \chi = \varepsilon_{\text{r}} - 1 = N_{1} \left( {\alpha_{1} + \frac{{p_{0}^{2} }}{{3\varepsilon_{0} kT}}} \right) + N_{2} (\alpha_{ + } + \alpha_{ - } ). $$
(5)

Here N 1 is the number of water molecules, which is temperature dependent and have the electronic polarizability, α 1, N +2 is the number of positive ions of HBA compound with the electronic polarizability, α + and N −2 is the number of negative ions of HBA compound with the electronic polarizability, α [25–29]. There is an evidence that N +2  = N −2  = N 2.

By comparing the two last expressions, we conclude that the electronic contributions of positive ions and negative ions to the polarizability are lower than in the case of pure water. Instead, it is dominant the polar behavior being as higher as the solution concentration is increased [30].

Optical properties

Crystallization from solution can be thought of as a two step process. The first step is the phase separation (or birth) of a new crystals. The second is the growth of these crystals to larger size. These two processes are known as nucleation, and crystal growth, respectively. The birth of a new crystals, which is called nucleation, refers to the beginning of the phase separation process. The solute molecules have formed the smallest sized particles possible under the conditions present. The next stage of the crystallization process is for these nuclei to grow larger by the addition of solute molecules from the supersaturated solution. This part of the crystallization process is known as crystal growth. Crystal growth, along with nucleation, controls the final particle size distribution obtained in the system. In addition, the conditions and rate of crystal growth have a significant impact on the product purity and the crystal habit.

Due to the stacked layered sheets, each sheet consisting of hydrogen-bonded of trigonal B(OH)3 molecules, and taking into account the primitive cell of hexagonal type with a = b = 20.4869 Å, higher than c = 12.1506 Å, the crystal growth from water solutions begins in the nucleation stage with the smallest sized particles in the shape of hexagonal prisms having the hexagonal base on the support, to provide a stable equilibrium.

Crystal growth, along with nucleation, the conditions and rate of crystal growth control the final particle size distribution and the product purity.

In Figs. 9 and 10 are seen the images of hexagonal crystallites of different dimensions, grown from HBA water solution (7 %) at normal pressure and room temperature (22 °C), having the c-axes perpendicular to the silica support. The photographs have been obtained with video camera for microphotographs, using the polarizing microscope arranged in reflection view (Fig. 9), and transparency view (Fig. 10) equipped.

Fig. 9
figure 9

Image of grown crystals observed with polarizing microscope, arranged for reflection view in cases of two angles between the polarizer and analyser: a 0°; b 45°

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Fig. 10
figure 10

Hexagonal shape of the base of a HBA mono-crystal, grown from water solution. The image was obtained through transparency view with an objective lens of ×20

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From Fig. 11, it is observed that we have a non-linear increase over time for the area of the base belonging to HBA hexagonal prism grown from 7 % of HBA in water solution.

Fig. 11
figure 11

Hexagonal prism base area versus time in the case of HBA grown from water solution (7 %)

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In Figs. 12 and 13, are given some examples of HBA exotic polycrystalline structures obtained for different angles between polarizer and analyser. As expected from the XRD data, the HBA compound belonging to the hexagonal system should crystallize from water solutions in mono-crystals or polycrystalline structures that exhibit the characteristic microscopic structure.

Fig. 12
figure 12

Exotic polycrystalline structures grown from HBA + H2O solution (of 7 %) obtained through transparency view for 0° (left) and 45° (right) between the polarizer and analyser

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Fig. 13
figure 13

The colors of polycrystalline structures grown from HBA + H2O solution (of 7 %) are changed when the angle from analyser and polarizer is changed from 0° (a) to 90° (b)

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These microscopic sculptures belonging to an “unknown author” have nice combination of colors (Figs. 12 and 13), that are changing in complementary colors when the angle between polarizer and analyser takes four values: 0°, 45°, 90°, and 135°. As an illustration, in Fig. 13 is seen how green color is changed into yellow when the angle between polarizer and analyser is changes from 0° to 90°.

This behavior proves a strong anisotropy of all the characteristic physical properties of the compound.

Conclusions

A special preparation procedure has leaded to the change of the crystallization system of boric acid from the triclinic to hexagonal, as confirmed by XDR analysis. The XRD diffraction maxima of HBA powder permitted the calculation of lattice parameters (a = b = 20.4869 Å and c = 12.1506 Å), and of the mean value of the particle dimensions (of about 19.77 nm). By optical microscopy, we observed the evident hexagonal prism structure of mono-crystals grown from water solution of HBA, as well as the strong anisotropy of the exotic HBA polycrystalline structures. The dielectric properties of HBA & H2O solutions proved polar character of the compound. Thermal analysis of HBA, performed under a heating rate of 10 K min−1, has shown a good stability till 101 °C, and then decomposition into three stages, all endothermic, but having a different kinetics compared to TBA. While for TBA, we have all the three endothermic processes very clearly separated between them, for HBA the first stage of decomposition is camouflaged by the second endothermic effect having its maximum at 130.75 °C; the third stage of the decomposition has the maximum of the endothermic effect at 160.84 °C. The calculations give an energy of 4.40 eV for the decomposition of a TBA molecule, while for the decomposition of a HBA molecule is needed an energy of 4.68 eV. Those values proved that the bound forces are stronger for HBA molecules than for TBA molecules.

By FTIR analysis evidenced, the presence of the absorption IR bands assigned to O–H, B–O, and B–O–H bonds, respectively, characteristic to boric acid, but small displacements of the IR peak positions and higher intensity for absorbance have been observed in the case of HBA compared to TBA.