The adsorption kinetics and thermodynamics of cationic surfactants on graphite oxide

2.1 Reagents and materials

Natural flake graphite, obtained from Qingdao Shenshu Graphite Manufacturing Plant, Qingdao, Shandong, China, was sieved through a 200-mesh sieve. The cationic surfactants used in this study were dodecyl trimethyl ammonium bromide (C₁₂ TAB), tetradecyl trimethyl ammonium bromide (C₁₄ TAB) and hexadecyl trimethyl ammonium bromide (C₁₆ TAB), and were obtained from Chengdu Kelong Chem. Co., Ltd. (Sichuan, China). Other materials and reagents, such as potassium permanganate (KMnO₄) and concentrated sulfuric acid (H₂ SO₄), were chemically pure and purchased from Kelong Chem, Co., Ltd., China.

2.2 Preparation of C_n TAB/GO composites

GO was synthesized with natural flake graphite according to modified Hummers’ method [19]. GO (0.2 g) was dissolved in 50 ml of 0.05 m NaOH and the mixture was evenly stirred in order to produce a GO suspension. Then, surfactants such as C₁₂ TAB, C₁₄ TAB and C₁₆ TAB (dissolved in 50 ml of deionized water to obtain an aqueous solution) were added into the GO suspension and stirred for 1 h at different temperatures. The concentrations of added C_n TAB were 2.2, 3.3, 5.5, 11.0, 16.5, 22.0, 33.0 and 44.0 mm. The precipitate was filtered and then washed with deionized water to remove the residual C_n TAB and NaOH. After drying at 333 K overnight, C_n TAB/GO intercalation composite samples were obtained, marking C_n-GO-t (n=12, 14, 16; t=2.2, 3.3, 5.5, 11, 16.5, 22, 33, 44).

2.3 Instrumentation

Nitrogen content was measured by a Vario EL CUBE elemental analyzer (Elementar Analysensysteme GmbH, Germany) with 2 mg of sulfanilic acid as a reference. The amount adsorbed and the equilibrium concentration of surfactants were then calculated. XRD patterns were recorded on an X’pert Pro diffractometer (PANalytical B.V., The Netherlands) with Cu K_a radiation (40 kV and 40 mA in the continuous scanning range from 0.5° to 60°). FTIR spectra were recorded by a Nicolet 5700 infrared absorption spectrometer (Nicolet, USA) using KBr pellets. Raman spectra were recorded by an InVia spectrometer (Renishaw, UK) using a He-Ne laser operating at 514 nm.

3.1 Adsorption kinetics

In order to evaluate the adsorption process of C_n TAB on GO, adsorption isotherms were plotted through measuring the nitrogen content of C_n-GO-t via elemental analyzer. Figure 1 shows the adsorption isotherms of C₁₂ TAB, C₁₄ TAB and C₁₆ TAB on GO at different temperatures. It is found that the adsorption process of C₁₂ TAB on GO is similar to that of C₁₄ TAB and C₁₆ TAB and can be divided into three stages. First, when the C_n TAB concentration is less than 5.5 mm, the adsorption amount of C_n TAB on GO is increased sharply with increasing equilibrium concentration. Second, the adsorption amount of C_n TAB on GO is increased sluggishly when the C_n TAB concentration is increased from 5.5 to 16.5 mm. Finally, the saturated adsorption amount of C_n TAB is reached when the C_n TAB concentration is higher than 22.0 mm.

Figure 1

Adsorption isotherms of C₁₂ TAB (A), C₁₄ TAB (B) and C₁₆ TAB (C) on GO at different temperatures.

Experimental data of C₁₂ TAB, C₁₄ TAB and C₁₆ TAB were fitted into the Freundlich, Langmuir, modified Langmuir (Langmuir-Freundlich) and BET isotherm equations. It was found that the experimental data fitted well into the nonlinear modified Langmuir isotherm equation, with correlation coefficients higher than 0.91 (Table 1). Furthermore, the experimental data of C₁₂ TAB, C₁₄ TAB and C₁₆ TAB are in line with the linear regression equation Γ=ac at low C_n TAB concentration (<5.5 mm), but are in line with the linear regression equation Γ=bc+d at high C_n TAB concentration (from 5 to 16.5 mm). The parameter a is the parameter related to the ionic bonding force, and the parameters b and d are the hydrophobic bonding force and saturated adsorption amount via ionic bonding, respectively [12]. This indicated that the C_n TAB molecules are adsorbed onto the interlayer spacing of GO via ionic bonding under a low C_n TAB concentration, and the main driving force for the adsorption under a high C_n TAB concentration is hydrophobic bonding, which is weaker than the ionic bonding.

It was also found that the adsorption amount of C_n TAB on GO has little change while the temperature changed at a low C_n TAB concentration but has obvious reduction along with the rise in temperature at a high C_n TAB concentration. In pace with the temperature rising from 293 to 353 K, the saturated adsorption amounts of C₁₂ TAB, C₁₄ TAB and C₁₆ TAB on GO are reduced from 4.065 to 3.705, 4.277 to 3757 and 3.879 to 3.493 mmol·g^-1, respectively (Table 1). This can be attributed to the solubility and desorption of C_n TAB. (1) Owing to the powerful force of ionic bonding, temperature has little influence on the adsorption amount of C_n TAB at low C_n TAB concentration. (2) At high C_n TAB concentration, the driving force changed to a powerless one (hydrophobic bonding); the Brownian motion of C_n TAB molecules was enhanced with the rise in temperature and thus increased the kinetic energy of C_n TAB molecules, consequently leading to a desorption of C_n TAB from GO. (3) Due to the rise in temperature, the interaction between C_n TAB molecules and water molecules intensified and led to a weaker adsorption tendency of C_n TAB onto GO [15, 20]. All of these led to the decrease in the saturated adsorption amount of C_n TAB on GO.

3.2 Adsorption thermodynamics

The thermodynamic parameters can be obtained by using the thermodynamic equations (1)–(3) [13, 14].

(1)ln Kd=ΔS/R-ΔH/RT (1)

(2)ΔG=ΔH-TΔS (2)

(3)Kd=Γ/c. (3)

ΔG is Gibbs free energy change of the adsorption process, and ΔH and ΔS are the enthalpy change and entropy change of the adsorption process, respectively. The thermodynamic parameters of the adsorption process of C₁₂ TAB, C₁₄ TAB and C₁₆ TAB onto GO at various concentrations and different temperatures are shown in Table 2.

The negative values of ΔH in different C_n TAB concentrations are an indication of an exothermic adsorption process. It is obvious that the rise in temperature will hinder the adsorption. The value of ΔH is also appropriate to determine the molecules’ interactions in the adsorption process. As introduced by Oepen et al. [21], the measured energies of each interaction are as follows: van der Waals interaction, 4–9 kJ·mol^-1; hydrophobic bonding, 4 kJ·mol^-1; hydrogen bonding, 2–40 kJ·mol^-1; and chemisorptions, 62–84 kJ·mol^-1. The absolute value of ΔH in the adsorption of C₁₂ TAB amounts to 1.76–6.96 kJ·mol^-1, and those of C₁₄ TAB and C₁₆ TAB amount to 2.15–28.54 and 2.18–18.83 kJ·mol^-1, respectively. This indicated that the driving forces of the adsorption process include ionic bonding, hydrophobic bonding and hydrogen bonding.

In addition, ΔG reflects the driving force of the adsorption process. As shown in Table 2, the negative value of ΔG indicates that C_n TAB molecules are adsorbed onto GO spontaneously, and the values of ΔS, which indicate the degree of confusion in the random motion of microscopic particles, are decreased along with the increase in C_n TAB concentration. This can be interpreted by the further filling and agglomeration of C_n TAB in the interlayer spacing of GO, thus decreasing the degrees of freedom of motion of the C_n TAB molecules.

Furthermore, increasing the C_n TAB concentration will also lead to fewer negative values of ΔH. This indicated that the driving forces of adsorption changed from a strong one (ionic bonding) to a weak one (hydrophobic bonding). In addition, decreasing the force will weaken the spontaneity of the adsorption process, consequently leading to a less negative value of ΔG.

3.3 Formation of C_n TAB/GO composites

In order to confirm the adsorption kinetics and thermodynamics of C_n TAB absorbed onto GO, XRD and FTIR spectroscopy were used to investigate the formation of C_n TAB/GO composites. Figure 2 shows the XRD patterns of GO together with C_n TAB/GO prepared in different concentrations of C₁₂ TAB, C₁₄ TAB and C₁₆ TAB at 293 K. The diffraction peak of GO gradually shifts toward the lower angle with the increase in C_n TAB concentration, indicating that the C_n TAB molecules are intercalated into the interlayer spacing of GO, leading to an expansion of GO layer spacing. However, the diffraction peaks of intercalation composites become more diffuse than that of GO under a low C_n TAB concentration but become sharper with increasing C_n TAB concentration. This indicated that at low C_n TAB concentration, the low packing density of C_n TAB on GO will lead to the random distribution of C_n TAB into the interlayer spacing of GO, representing a disordered stacking of GO layers. The packing density of C_n TAB into GO was increased with the increase in C_n TAB concentration, thus leading to regular distribution and aggregation of C_n TAB into the interlayer spacing of GO. It is noteworthy that the two diffraction peaks of the C₁₄ TAB-GO-22 sample can be assigned to different arrangement models of C₁₄ TAB into the GO layer.

Figure 2

XRD patterns of GO and series of C₁₂-GO-t (A), C₁₄-GO-t (B) and C₁₆-GO-t (C).

Figure 3 shows the FTIR spectra of GO and series of C₁₆-GO-t. The peaks at 3390, 1722, and 1400–1095 cm^-1 for GO are attributed to the OH, C=O in COOH and C-O in C-OH/C-O-C (epoxy) functional groups, respectively [22]. Compared with the peaks of GO, the peaks at 1722 and 1400–1095 cm^-1 for C₁₆-GO-t disappeared along with the increase in C₁₆ TAB concentration. These indicate that the cationic surfactants with positive charge are ionically bonded to COO^- and CO^- groups of GO. Those at 2917, 2850 and 1473 cm^-1 assigned to intercalated surfactant appeared when C₁₆ TAB molecules were inserted into the interlayer spacing of GO. Moreover, the peak at 2917 cm^-1 of C₁₆ TAB is blue-shifted to 2920 cm^-1, and the peak at 1473 cm^-1 red-shifted to 1466 cm^-1. These prove that in pace with the increase in C₁₆ TAB concentration, the conformation of C₁₆ TAB molecules is becoming more ordered and aggregated in the interlayer spacing of GO [23].

Figure 3

FTIR spectra of GO and series of C₁₆-GO-t.