Interactions between stearic acid and calcite surfaces: Experimental and computer simulation studies

Abstract

Calcite surface was modified with stearic acids, and the interaction between them was investigated both experimentally and theoretically. Stearic‐acid‐modified calcite powders were investigated with Fourier transform infrared spectroscopy, X‐ray diffraction, zeta potential analyser and contact angle measurement. Then, the density functional theory calculations were performed to explore the interaction at the atomic scale. The experimental results and simulation indicated that stearic acids interact with calcite surface via chelation between the double‐bond O atom of –COOH and the Ca atom on calcite surface. The terminal methyl group of the stearic acids decreases the hydrophilicity of the calcite surface, and the interaction between different crystal faces of calcite is different.

INTRODUCTION

Polymer composites are a large class of materials made from polymer matrices and numerous additives. Combination of the materials often leads to many desirable properties. For instance, mineral particles are added into polymer matrices to adjust the mechanical, electrical and thermal properties []. Among mineral additives, calcium carbonate is well recognised as an important industrial additive and its application potential in plastics and rubbers has been extensively explored []. Calcium carbonate is typically obtained by grinding natural minerals or by the carbonation of calcium hydroxide. Calcite, the most stable polymorph of calcium carbonate, has drawn much attention due to its broad application potential []. It is mainly used as fillers to control the mechanical properties of polymer matrices. Liu et al. mixed calcite with poly acrylic acid (PAA) to investigate particle dispersion and interfacial interactions between calcite particles and PAA molecules []. Nano-CaCO₃ particles were added in polypropylene to explore the variation of the rheological and mechanical property []. The uniformly dispersed nano-CaCO₃ particles helped polymer matrix sustain a high modulus at high temperatures. Calcite was also added into high-density polyethylene (HDPE), low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) []. The tensile stress and the complex melt viscosity of polyethylene improved with the filling of calcite particles (50%).

However, the poor dispersion of high energetic hydrophilic calcite in low energetic hydrophobic polymers is an issue that must be solved before calcite can be used as fillers. To improve the dispersion of calcite in polymer matrices, its surface is often functionalised with modifiers such as fatty acids, titanate and aluminate to obtain an organophilic surface. Stearic acid, one of the most common saturated fatty acids, has been widely used as a surface modification agent of calcite []. Wang et al. demonstrated that stearic acid improves the dispersion of calcite nanoparticles in an acid solution [].

Arrangement modes and configurations of hydrophobic organic films attached on the calcite surface greatly influence the dispersion of calcite through influencing the interaction between the two heterogeneous materials.

It is difficult to quantitatively understand the interactions between surface modifiers and calcite only by experimental investigations, which are largely influenced by factors such as particle sizes, morphology and particle dispersion. To overcome this issue, density functional theory (DFT) calculations has been employed to investigate the interactions between surface modification agents and calcite surfaces []. DFT calculations reveal microscopic interfacial interactions and therefore provides comprehensive understanding of the experimental results [].

In the present study, we adopted a green method, in which organic solvent usage was avoided, to graft stearic acid onto calcium carbonate. Stearic acids were successfully grafted onto the CaCO₃ particle surfaces. DFT calculations were then used to further investigate the specific interaction between CaCO₃ and fatty acids. We investigated the interactions between stearic acid and (012), $\left(10\overline{1}\right)$and (104) calcite surfaces. These surfaces were chosen for their universality []. The interaction energy and specific locations of stearic acids were carefully analysed in order to understand the interaction mechanisms. It will hopefully guide further investigations of calcite modifications with organic compounds.

EXPERIMENTAL PROCEDURES

Analytically pure calcium carbonate, stearic acid, chloroform (Kelong Chemical Reagent Company) and deionised water were used as purchased. Excessive amount of stearic acid was dissolved in deionised water at room temperature. Then, a certain amount of dilute ammonia was added into the stearic acid solution. The solution was then mechanically stirred, and a certain amount of calcium carbonate powder with a certain diameter was then added into the solution in a few minutes under mechanical stirring at 80°C. Acid modified calcite powder was collected by filtration, washed several times with chloroform, and air-dried overnight at room temperature.

The effects of the stearic acid modification were explored with Fourier transform infrared spectroscopy (FTIR, Nicolet 6700 spectrometer), X-ray diffraction (XRD, X′ Pert PRO), water contact angle measurement, and Zeta potential analysis (Brookhaven Instruments Corp.). We added as-purchased calcite and stearic acid modified calcite into high-density polyethylene to investigate the cross-section morphology with scanning electron microscopy (SEM, Zeiss, EVO-18), high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) with TEM (JEOL, JEM-2100F).

COMPUTER SIMULATION

Model construction

Calcite surface model

A calcite film (hexagonal, space group R-3c, with six formula units) with a thickness of approximately 1.70 nm was cleaved along the (012), ($10\overline{1}$) and (104) planes to set up the calcite surface. We separately built ($10\overline{1}$) and (104) calcite planes to compare the effect of crystal orientation on the stearic acid–calcite interactions. The dimensions of the calcite surfaces used in the simulations were as follows: calcite (104) a = 10.10 Å, b = 8.20 Å, c = 45.81 Å, α = β = γ = 90°, ($10\overline{1}$) a = 10.10 Å, b = 18.03 Å, c = 46.37 Å, α = β = 90°, γ = 98.05° (012) a = 10.10 Å, b = 12.93 Å, c = 45.76 Å, α = β = γ = 90°, a vacuum slab of 35 Å was added onto each calcite surface.

Computational details

DFT methods were used to perform all the electronic structure optimisations and calculations in this study. The Perdew–Burke–Ernzerhof (PBE) generalised gradient approximation (GGA) exchange–correlation functional using a projector augmented-wave method was applied by using the VASP code []. A 520 eV kinetic energy cut-off was set according to the different cut-off energies testing with the energy error of 0.01 eV for the plane-wave expansion. The convergence criteria for the geometric optimisation and energy calculation were set as follows: (1) self-consistent field energy tolerance of 1.0 × 10⁻⁴ eV, (2) all the atoms in the systems were fully relaxed and maximum force tolerance on each atom is smaller than 0.05 eV/Å. The smearing value was set as 0.2 eV during the geometry optimisation and the total energy calculations. The K-points were set to be 5 × 5 × 1 for the different stearic acid–calcite interaction systems. The approximate method developed by Grimme et al. was applied during all calculations to account for the contribution of the van der Waals interactions between atoms to the energy [].

RESULTS AND DISCUSSION

Experimental results

The IR absorption spectrum of acid modified calcite exhibits characteristic absorption peaks of carboxylic groups at 2800–3100 cm⁻¹ (Figure ). This indicates that stearic acid was adsorbed on calcite surfaces, probably chemically, according to literature []. We further probed the structure-dependent interaction between calcite and stearic acid with XRD.

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Powder XRD results shown in Figure  reveal that the diffraction intensity of the calcite (104) plane decreased after the stearic acid modification. This suggests that the stearic acid solution preferentially dissolved calcite from the (104) plane. This might be due to the anisotropic structure of the calcite particles. Further, full width at half maximum (FWHM) of the (104) peak increased after the stearic acid modification indicating that the dimension of calcite particles in the (104) direction decreased upon stearic acid modification.

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We further probed the effect of acid modification on the hydrophilicity of calcite by measuring the contact angle of calcite surfaces before and after the acid modification. Figure  shows that the contact angle of stearic acid modified calcite powders increased by about 25°, compared with the averaged contact angle of unmodified calcite powders (Figure ). The average increase in contact angle is calculated according to Equation ): 1 ${\theta }_{\text{average}}=\frac{({\theta }_{\text{modified}}-{\theta }_{\text{side}1})+({\theta }_{\text{modified}}-{\theta }_{\text{side}2})}{2}$ where ${\theta }_{\text{average}}$ represents the average increased contact angle, ${\theta }_{\text{modified}}$ is the contact angle of calcium carbonate modified by stearic acid, ${\theta }_{\text{side}1}$ and ${\theta }_{\text{side}2}$ are the upper and lower contact angles of calcium carbonate before modification. This means that the amphiphilic surface of calcite turned hydrophobic after the stearic acid modification.

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Zeta potentials of pure and stearic acid modified calcite were measured to be −18.05 and −21.44 mV, respectively. We may infer that stearic acid stabilises calcite in a colloid slightly.

Cross-section SEM images of the HDPE/calcite composites show an apparent difference between pure and stearic acid modified calcite (Figure ). The modified calcite particles dispersed uniformly in the polymer matrix (Figure ) while aggregation occurred to the pure calcite particles (see Figure ). The results indicate that stearic acid could be helpful for improving the dispersibility of the calcite particles in polymers.

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Figure  depicts the HR-TEM and SAED pattern of unmodified (a and c) and modified (b and d) calcite. A decrease in image resolution of the modified sample was observed. In addition, only electron diffraction patterns for plane (104) remain intensive after the stearic acid modification, as shown in Figure , suggesting that the surface morphology of the calcite particles was modified upon stearic acid modification. This result seems inconsistent with the XRD results in which the (104) plane of calcite was preferentially dissolved by acid. To explain this divergence, further investigation will be needed.

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Computer simulation

Effect of the initial configuration of fatty acid

Interaction energy could be used to illustrate the correlation between the calcite crystallographic surfaces and the interactions between the adsorbed molecules and the calcite surfaces. The interaction energy of the system is calculated according to Equation ), 2 $E_{\text{int}}=E_{{\text{CaCO}}_3-\text{molecule}}-E_{{\text{CaCO}}_3}-E_{\text{molecule}}$ where $E_{{\text{CaCO}}_3-\text{molecule}}$ is the total potential energy of the calcite surface together with the adsorbed stearic acid molecules, and $E_{{\text{CaCO}}_3}$ and $E_{\text{molecule}}$ are the total potential energy of the calcite surface and the stearic acid molecule, respectively.

Effect of calcite surfaces

To study the influence of different calcite surfaces on the interaction between calcite and stearic acid, the interaction energy between the (012) calcite surface and fatty acid was calculated. Other typical ($10\overline{1}$) and (104) calcite surfaces were also investigated. Figure  shows the ball and stick models for the three different CaCO₃ surfaces.

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According to Figure , the following conclusions can be drawn:

• (1)

  Stearic acid can be adsorbed on (104), ($10\overline{1}$) and (012) surfaces of CaCO₃, and then hydrophobic modification of CaCO₃ surface can be carried out.

• (2)

  There is a strong interaction between stearic acid and surfaces ($10\overline{1}$) and (012) of CaCO₃.

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DFT calculation was finally carried out to illustrate detailed interaction mechanisms between the calcite (104), ($10\overline{1}$) and (012) surfaces and stearic acid at the electronic level.

The isosurface of the electron density difference between stearic acid and CaCO₃ surfaces was shown in Figure . There is a strong interaction between the carbonyl or hydroxyl O of -COOH in stearic acid and the Ca²⁺ of calcite. In addition, electron transfer also takes place between H of –COOH and O of CO₃²⁻ on stearic acid. These may be related to the decrease of the strength of calcite in XRD.

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CONCLUSION

Stearic acid was employed to modify calcite powders. DFT calculations were used to investigate the detailed interaction between fatty acid molecules and the calcite surface. Both experimental and computer simulation results demonstrated that stearic acid interacts with calcite surface through the chelation between the carbonyl or hydroxyl O of –COOH and the Ca²⁺ of calcite as well as the hydrogen bonding between the H of –COOH and the O of CO₃²⁻ on the calcite surface.

ACKNOWLEDGEMENT

The authors are grateful to the National Natural Science Foundation of China (No. 51808464) for supporting this research.

DATA AVAILABILITY STATEMENT

No data.

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