3.1 Characterization
3.1.1. FT-IR Characterization
A Perkin–Elmer FT-IR spectrophotometer (Spectrum BX-II) was used to record FT-IR spectra on KBr disks with a range of 4000–400 cm−1 as shown in Fig. 2.
To study the synthesis of new materials that has been synthesized for TMP removal (SiNP-Cu) it is necessary to study the functional groups. Si-O-Si stretching vibrations peak appear at 1,100 cm−1. The presence of vibration peaks at 3446 and 1620 cm−1 confirmed presence of adsorption water. The presence of SiNP may be determined by analyzing the appeared bond about 1,4900 cm−1, which reflects the bonds of C=N and C=N vibrations. This validates the molecule's functioning via the silica surface.
3.1.2. Scanning Electron Micrographs
Scanning electron micrographs (SEM) of SiNP and SiNP-Cu were obtained using a scanning electron microscope (Jeol JSM 60) and an accelerating voltage of 20 kV.
The SEM is shown in Fig. 3 were obtained at 800 X magnification. From the SEM we can see the rough surface for the new synthesized functionalized structures were present on the surface equally. This demonstrated the unalienability of the silica gel species after curing to demonstrate the presence of functional group distribution on the entire surface. In addition, we can see in portion (a) that the copper ions were spread equally.
3.1.3. TGA Analysis and Thermal Stability
For mass loss and stability studies, a dry sample was heated in a nitrogen gas at a rate of 10°C/min (flow rate: 50 mL/min) as shown in Fig. 4.
This study will help us to have an idea about the surface stability and at the same time to be sure there is mobility on the surface and how much. According to the profiles in a and c, the degradation process occurred between 120 and 800°C, demonstrating that both SiNP and SiNP-Cu produced materials had high thermal stability.
In the temperature range of ambient temperature to 105°C, both samples revealed a first mass loss stage of 3.20 percent, which was attributed to the water loss that the samples had absorbed. The second loss was 5.95 percent between 105 and 800°C which represents the condensation of free silanol groups (Si-O-Si) [32]. The significant rise in mass loss demonstrates the presence of a large number of anchoring organic groups. Again two distinct mass loss steps were detected for the SiNH2 sample. The first one, a small mass loss of 1.56% in the room temperature to 100°C range is attributed to the remaining silanol hydration water, as a consequence of the use of these groups in the immobilization process. On the other hand, a pronounced mass loss increase of 9.77% was observed for the second step, between 208 and 800°C, which corresponds to the organic matter added onto the surface during immobilization. The final SiNP material presented two distinct mass loss stages.
3.1.4. Surface Properties
The nitrogen adsorption isotherm was used to characterize the produced compounds' surface area, pore sizes, and volume. The pore diameter was calculated using the Barrett-Joyner-Halenda (BJH) method [33]. The pore volume was 0.77 cm3/g and the surface area was 310 m2/g.
3.1.5 Batch method
To study the effects of pH, contact time, effect of dosage and temperatures, A 10 mg of the synthesized material (SiNP-Cu) was added to mL of the TMP solution and shaking for a period of time. The contact time was studied up to 150 min. The range of pH was from 2 to 12, the dosage was from 0.01 t0 0.3 g. Three different temperatures were used during this study (310, 315 and 320 K). The concentration of TMP was determined spectrophotometry measurements. The amount of TMP removed by the prepared material SiNP-Cu from aqueous medium was calculated using the following equations [33]:
\(\begin{gathered} {Q_M}=\frac{{\left( {{C_0} - {C_e}} \right)}}{W}{\text{ (1)}} \hfill \\ {Q_W}={Q_M} \times M{\text{ (2)}} \hfill \\ \end{gathered}\)
QM denotes the quantity of TMP on the adsorbent (mmol/g), QW denotes the amount of TMP on the adsorbent (mg/g), V denotes the volume of the aqueous solution (L), W denotes the weight of the adsorbent (g), and C0 denotes the initial concentration of TMP (mmol/L), The equilibrium concentration of TMP in the prepared solution (mmol/L) is represented by Ce, while the atomic weight of TMP (g/mol) is represented by M. Analyses were carried out in duplicate for each sample, and the mean data is what is provided.
3.2. Sorption experiments
3.2.1. Effect of adsorbent dose
Several parameters were studied to determine the capacity of an adsorbent. One of them is the effect of adsorbent dose on adsorption of TMP by SiNP-Cu which showed higher removal capacity of TMP during the increasing of the dose amount as shown in Fig. 5. This rise is due to an increase in the number of available reaction sites for the adsorbent. With 300 mg of SiNP-Cu, about % of TMP was removed from a 50 mL solution.
3.2.2. Effect of pH
Previous research has shown that adsorption of medicines to functionalized compounds in solution or placed on solid supports is usually influenced by numerous parameters such as pharmaceutical compound size, charge, shape of the donor atom [34], as well as their binding properties and buffering conditions [35]. These characteristics were examined utilizing solution chemistry and solid-phase extraction of various materials based on the coordination of the immobilized on the surface of solid supports, such as silica gel, nanomaterials, and polymeric compounds. To investigate the suitability of synthesized SiNP-Cu for TMP removal, the effect of pH was investigated as one of the critical parameters.
The adsorption properties of SiNP-Cu were investigated between 2.5 and 11.0 is represented in Fig. 6.
As seen in Figure 6, the TMP uptake of the adsorbent changes as the pH changes were studed using 50 mL of 50 mg/L solution of TMP at room temperature and 100 mg of SiNP-Cu. The retention of TMP by the functionalized silica SiNP-Cu is not excessive at low pH levels, which is due to the ligand, which must be in its protonated form. When the pH rises, protonation weakens, which improves chelation and adsorption of pollutants such as TMP.
Figure 6 clearly shows that an increase in pH had an effect on the adsorbent surface and TMP. At pH =12, the elimination percentage nearly reached 90%. The explanation for this is that TMP is a weak base with a pKa of 7.3. As a result, all TMP take the form of \(T{H^+} \leftrightarrow T:+{H^+}\).
Another fact is that the protonated form of TMP does not favor the adsorbent surface of the produced SiNP-Cu, which has a positive charge. At acidic pH values, both the TMP and the adsorbent surface are positively charged, which may explain why TMP adsorption on SiNP-Cu surfaces is low. On the other hand, as pH rises, the proportion of adsorption rises as well. In general, the removal percentage reached about 91.5% at pH =8.
3.2.3. Effect of contact time
At different temperatures, (308, 315 and 320 K), the influence of contact time on TMP adsorption by SiNP-Cu was investigated as seen in Fig. 7. As seen it took 90 min to start reaching the adsorption equilibrium. This time used for the rest of batch studies including effect of dosage and temperatures. The equilibrium times were found to be the same for all temperatures, and they increased as the temperature increased, as seen in the graph.
3.2.4. Adsorption kinetics
To study the kinetics for the adsorption and to understand the change in adsorption with time, three models have been applied [36]:
Pseudo-first-order, pseudo-second-order, & intraparticle diffusion models are among them.
Lagergren's pseudo-first-order kinetic model has the following equation:
\(\frac{1}{{{q_t}}}=\left( {\frac{{{k_1}}}{{{q_1}}}} \right)\left( {\frac{1}{t}} \right)+\frac{1}{{{q_1}}}{\text{ (3)}}\)
The parameters in the equation are: qt represents the quantity of TMP adsorbed (mg/g) on SiNP-Cu at different times t; q1 represents the maximal adsorption capacity (mg/g); and k1 represents the pseudo-first-order adsorption rate constant (min−1). The parameters (q1 and k1) were calculated using the intercept and slope of a simlated first-order straight line. Table 1 summarizes these parameters and correlation coefficients (R2).
Table 1
TMP adsorption onto SiNP-Cu kinetic parameters at various temperatures.
Kinetic Model
|
Temperature (K)
|
308
|
315
|
320
|
Pseudo-1st Order
|
|
R12
|
0.75
|
0.81
|
0.91
|
K1 (min−1)
|
3.91
|
6.35
|
7.32
|
q1 (mg g−1)
|
19.38
|
19.82
|
20.3
|
qe (Calculated)
|
9.32
|
11.23
|
13.62
|
Qe(exp)
|
41.21
|
23.17
|
48.23
|
Pseudo-2nd Order
|
|
R22
|
0.992
|
0.981
|
0.999
|
K2 (g mg−1 min−1)
|
0.003
|
0.004
|
0.005
|
q2 (mg g−1)
|
19.32
|
20.38
|
20.46
|
qe (Calculated)
|
42.86
|
22.23
|
47.85
|
Intra-particle diffusion
|
|
Rp2
|
0.999
|
0.964
|
0.999
|
Ki (mg s−1\2 g−1)
|
0.18
|
0.26
|
0.19
|
C
|
12.32
|
14.8
|
15.7
|
From Table 1, the values for (R12) were between 0.75 and 0.91 for the TMP at 308, 315 and 320 K, respectively.
The equation below was used to calculate the parameters for the pseudo-second-order adsorption kinetic rate [36]:
\(\frac{{d{q_t}}}{{dt}}={k_2}{\left( {{q_2} - {q_t}} \right)^2}{\text{ (4)}}\)
For the pseudo-second-order adsorption kinetic model, K2 is the rate constant (g mg−1min−1) and q2 is the maximal adsorption capacity (mg g−1). The equation below was generated by integrating the equation and using a boundary condition (qt = 0 at t = 0 and qt = qt at t = t), the equation below was obtained:
\(\frac{1}{{{q_t}}}=\frac{1}{{{k_2}d_{2}^{2}}}{\text{+}}\frac{t}{{{q_2}}}{\text{ (5)}}\)
A plot of (t/qt) versus t was obtained to calculate the parameters that are listed in Table 1. We found that the correlation coefficients (R2) of the first-order and pseudo-second-order kinetic models are substantially lower in the first order model than in the second order model by comparing the correlation coefficients (R2) of the two models. In addition, using different plots for both kinetic models to compare both calculated and experimental qe, we found that the results for the pseudo second order model correspond better, as shown in Table 1. The second-order kinetic model can better characterize the sorption process for TMP on SiNP-Cu based on the data and results in the Table. Another finding showed that when the temperature rose, the maximum adsorption capabilities of TMP adsorption onto SiNP-Cu increased. This leads to the conclusion that TMP promotes adsorption onto SiNP-Cu. Another finding is that the adsorption process can be regarded as chemical, and TMP adherence occurs from the bulk phase to the solid phase (SiNP-Cu) as the temperature of the solution rises. This finding was also discovered by investigating the isotherm adsorption section.
The study of the intraparticle diffusion model of Weber and Morris can be presented by the mathematical equations [35, 36]:
\({q_t}={k_t}{{\text{t}}^{1/2}}{\text{+C (6)}}\)
In this equation, qt is the quantity of TMP adsorbed (mol/g) at time t, and C is the intercept, which is used to define the thickness of the boundary layer; the larger the intercept, the stronger the boundary layer effect. The intraparticle diffusion rate constant is denoted by ki (mg s−1 g−1). The slope of qt versus t1/2 was used to calculate ki. The plot may show multilinearity, indicating that a few steps occur. For the two temperatures (308 and 315), the first portion (14 to 20.5) describes the diffusion of adsorbate from the solution to the adsorbent's external surface or, in some cases, displays the boundary layer diffusion of dissolved molecules. The second portion (20.5 to 23) typically reflects the progressive rise of layer adsorption stage where intraparticle diffusion is the rate limiting phase. The third portion (22 to 26) is attributed to the ultimate equilibrium stage, as seen in Fig. 8 and Table 1.
Figure 8 shows that the figure for qt vs t0.5 at 320 K is almost straight line, which can demonstrate and establish intraparticle diffusion effects.
As previously stated, the intercept (C) values often describe the thickness of the boundary layer. The bigger the intercept, the stronger the boundary layer effect. In our experiment, the values for C increased with increasing temperature. This demonstrated that the boundary layer effect played no significant influence in the adsorption of TMP onto SiNP-Cu.
Furthermore, the linear sections of the intraparticle diffusion curves in the figure did not pass through the origin, implying that additional effects and processes control the entire adsorption process.
3.2.5. Adsorption isotherms
The results of the change in the adsorbed amount of TMP with equilibrium concentrations on the surface of SiNP-Cu was given in Fig. 9. The symbol Cs is representing the solid phase concentration (mmol g−1) and the Ce symbol is representing the final concentration (mmol L−1) in the supernatant during equilibrium for each single initial concentration.
Plotting Ce/Cs vs Ce, the slope and the shape of the initial portion of these isotherm curves for the three temperatures plot is very close to S-type and C-type as in Giles classification, respectively (Giles, MacEwan et al. 1960). This type of isotherms is relatively rare and indicative of weak adsorbent–adsorbate interactions [37].
Several isotherm equations have been developed and used to study the equilibrium nature of adsorption processes. In our study we used two models to describe the isotherm equilibrium: The Langmuir and the Freundlich sorption isotherm.
According to Langmuir isotherm principle, it assumes the presence of monolayer coverage of adsorbate over a homogenous adsorbent surface like TMP on SiNP-Cu in our case the adsorption data were represented in Table 2 and were obtained using the linear form of Langmuir adsorption model (Eq. (7)).
\(\frac{{{C_e}}}{{{C_s}}}{\text{=}}\frac{1}{{{C_m}{a_L}}}{\text{+}}\frac{{{C_e}}}{{{C_m}}}{\text{ (7)}}\)
The variables Cs and Ce were mentioned above while Cm is the amount of drug that is required for making monolayer (adsorption capacity). The variable aL is a Langmuir constant and representing the intensity of the adsorption and adsorption energy.
The dimensionless constant separation factor RL was used to study the essential characteristics of the Langmuir isotherm :
\({R_L}{\text{=}}\frac{1}{{\left( {1+{a_L}.{C_0}} \right)}}{\text{ (8)}}\)
C0 is the initial TMP concentration (mmol L−1) and aL is Langmuir constant. All the variables values are summarized in Table 2. Usually, the RL values between 0 and 1 indicate favorable adsorption of TMP onto SiNP-Cu at the concentration studied.
From Table 2 all the RL values at different temperatures were higher than 1. Which indicates that unfavorable adsorption was occurred and also all the adsorption capacity (Cm) values were negative for the SiNP-Cu. These results agree with previous study by Molu and Yurdakoc, 2010 [38].
Table 2
Isotherm coefficients for TMP adsorption onto SiNP-Cu at various temperatures.
Isotherm Model
|
Temperature (K)
|
308
|
315
|
320
|
Langmuir model
|
|
|
|
R2
|
0.71
|
0.85
|
0.73
|
Cm (mmol g−1)
|
-0.02
|
-0.03
|
-0.18
|
L (g L−1)
|
-6.32
|
-4.67
|
-4.13
|
RL
|
4.28
|
2.16
|
2.32
|
Freundlich model
|
|
|
|
R2
|
0.95
|
0.98
|
0.95
|
Kf (mg(1−1/n) g−1 L1/n)
|
15.82
|
8.75
|
4.75
|
nf
|
2.11
|
2.32
|
1.97
|
From Table 2, the correlation coefficients (R2) obtained for Langmuir equation were smaller than the one obtained by Freundlich equation (around 0.75) which suggested that the boundary layer thickness that has been studied by intraparticle diffusion was also increased (Özcan and Özcan 2004). The mathematical Freundlich equation (Eq. (9)) is an exponential equation which is usually used for multilayer adsorption with a heterogeneous energy distribution of active sites. The equation can be presented as below [36] :
\({C_s}{\text{=}}{k_f}{\text{C}}_{e}^{{nf}}{\text{ (9)}}\)
The linearized form of the equation can be written as:
\(\ln {C_s}{\text{=ln}}{k_f}{\text{+}}nf\ln {{\text{C}}_e}{\text{ (10)}}\)
The coefficient nf is a characteristic constant that describes sorption intensity, while the parameters Cs, Ce and kf represent the sorbed amount (mmol g−1), concentration (residual) at equilibrium (mmol L−1) and the sorption capacity of sorbent (mmol g−1), respectively. All the kf values decreased when temperature is increasing.
3.2.6. Adsorption thermodynamics
The study of thermodynamic parameters like equilibrium constants (Kc), the standard enthalpy (∆H°), the standard Gibbs free energy (∆G°) and the standard entropy (∆S°) of the adsorption process of TMP onto SiNP-Cu were studied using the following equations [39]:
ΔGº = -RTlnKc (11)
Kc = (12)
lnKc = (13)
The parameter Cs is representing the concentration of TMP adsorbed (mol L−1); Ce is describing the equilibrium concentration of TMP in solution (mol L−1) at a given specific temperature; while T is the studied solution temperature (K); and R is the ideal gas constant (8.314 J K−1 mol−1). To find the standard enthalpies (∆H°) of the TMP-SiNP-Cu adsorption, the Van’t Hoff equation was used and the plots of ln Kc versus 1/T were obtained. The list of thermodynamic parameters are shown in Table 3. From the Table, the value for the enthalpy is positive (1.63 KJ.mol−1). At the same time, the plot for Van’t Hoff which represents the interactions of the drug with the surface of the adsorbents usually require energy and as seen this interaction is endothermic in nature. The obtained low adsorption enthalpy values caused due to a number of physical interactions which can be described by the electrostatic attractions and nonpolar characteristics interactions, the hydrogen bonding and bridging.
Table 3
The standard thermodynamic parameters for TMP adsorption onto SiNP-Cu.
Thermodynamic
|
Temperature (K)
|
Kc
|
∆G° (kJ mol−1)
|
∆S° (J mol−1)
|
∆H° (kJ mol−1)
|
308
|
0.318
|
2.53
|
3.73
|
1.63
|
315
|
0.321
|
2.62
|
320
|
0.336
|
2.71
|
Besides that, other parameters like ion-exchange reactions between TMP molecules and the adsorbent structure (SiNP-Cu) [39]. The main possibility factor for the low enthalpy is the H-bond and the formation of Water Bridge between the groups of N or O in the TMP organic structure (Fig. 1) and the group of -NH that is present during the synthesis of SiNP-Cu helped to lower the value. For the standard entropy (∆S°) which was determined from the intercept of the Van’t Hoof plot (∆S°/R) is positive (3.73 Jmol−1K−1). This shows that the degrees of freedom of adsorbed species are increasing. At the same time, all the values for ∆G° at various temperatures are also positive. The values of the ∆G° of the process for SiNP-Cu decreased with increases the temperature, which lead to assume that the process may be spontaneous at high temperatures.
3.3. Theoretical results
As mentioned above, the main aim of this research study is the removal of the TMP from wastewater using SiNP (Fig. 10a). So that our discussion focuses on the results obtained in aqueous solution. The optimized geometrical structures, the molecular electrostatic potential maps (ESP) and the distribution of HOMO orbital and LUMO orbital of the TMP compound are shown in Fig. 10 (b and c). We found that the distribution of the electron cloud in these two orbitals was mainly concentrated on the most of the entire moiety of the molecule. In particular, we found that the HOMO orbital was more distributed on the N atom and the delocalized π-electrons of the pyrimidine and the aromatic benzene ring. This finding suggests that the N atoms and the delocalized π-electrons are responsible to donate the electrons to interact with the SiNP surface. On the other hand, we also found that the LUMO was found more distributed on the C atoms, suggesting that the C atoms are the centers that responsible to accept electron from the SiNP. These findings were also confirmed by monitoring the ESP map (see Figure 10 (d)). The small value of the energy gap indicates the high reactivity and the ease of the adsorption process of the TMP on the SiNP surface (Table 4). As in known, in the process of adsorption of TMP by SiNP, the electron charge is transferred from the TMP toward the SiNP surface, and this result is in agreement with positive values charge transfer maximum parameter (∆N) [40]. The positive value of ∆N proves that the TMP has donor electron effect and is a donor electron (Table 4). Furthermore, the chemical electronic potential of the TMP is negative and it means that the TMP is stable and it is not decomposing spontaneously into the elements and compounds are made up of them. The energy gap of TMP is quite high. The dipole moment of TMP (4.97 Debye) is higher than that of water (1.82 Debye) and it means that TMP able to expel water from the SiNP surface (Table 4).
Table 4
Quantum global reactivity descriptors of the TMP molecule
|
Gas phase
|
aqueous solution
|
Etotal (hartree)
|
-989.054
|
-989.071
|
Volume (bohr3/mol)
|
2174.001
|
2561.223
|
Dipole moment µ* (Debye)
|
4.05
|
4.97
|
EHUMO (eV)
|
-5.883
|
-6.154
|
ELUMO (eV)
|
-0.637
|
-0.897
|
EHOMO−1 (eV)
|
-6.346
|
-6.522
|
EHOMO−2 (eV)
|
-6.590
|
-6.803
|
Energy gap ΔE (eV)
|
5.245
|
5.258
|
Hardness (η) (eV)
|
2.623
|
2.629
|
hyper-hardness (γ)
|
4.782
|
4.890
|
Softness S (eV−1)
|
0.381
|
0.380
|
Chemical potential (\(\)) (eV)
|
-3.260
|
-3.525
|
Electrophilicity (ω) (eV)
|
2.026
|
2.364
|
Maximum charge transfer (ΔN)
|
1.243
|
1.341
|
Full set of the NBO charges, LRDs and DDs obtained at B3LYP/6-31+G (d,p) level of theory in aqueous solution are listed in Table S1 of the electronic supplementary information (ESI). Figure 11 (a-c) shows the graphical representation of the LRDs (\({f}_{k}^{\pm }, {\sigma }_{k}^{\pm }\) and \({\omega \sigma }_{k}^{\pm },\text{r}\text{e}\text{p}\text{e}\text{c}\text{t}\text{i}\text{v}\text{e}\text{l}\text{y}\)) of TMP compound. Our computed results suggest that the highest nucleophilic attack \(\left({f}_{k}^{+}\right)\)are found on C17, N4 and C16, whereas, the highest electrophilic attack \(\left({f}_{k}^{-}\right)\) are found on C10, N5, N7 and N6, see (Figure S1 (a)). As known, the most nucleophilic sites in investigated compounds have the highest value of \({\sigma }_{k}^{-} \text{a}\text{n}\text{d} {\omega }_{k}^{-}\), while the highest value of \({\sigma }_{k}^{+} \text{a}\text{n}\text{d} {\omega }_{k}^{+}\) reveals the most electrophilic site in TMP compound (Pathak, Srivastava et al. 2015), see panels (b) and (c) of Figure 11. For numerical results, see Table S1 of the ESI. Consequently, similar conclusions can also be marked by following the results of the local softness and the local electrophilicity
Figure 11 (d) shows the graphical representations of the dual reactivity descriptors of TMP compound obtained using B3LYP/6-31+G(d,p) level of theory in aqueous solution. Full set of numerical results of the DDs can be followedin Table S1 of ESI. A close inspection of the figure and Table S1 reveals that the most active sites, with and Δωk < 0, that are responsible to donate electron to SiNP surface are C10, N7, N5 and N6, whereas the most active sites, with and Δωk > 0 , that are responsible to accept electron from SiNP surface are C17, C16 and N4. These results agree with the results obtained by HOMO, LUMO and ESP maps.
3.3.2. Monte Carlo (MC) and Molecular Dynamic (MD) simulation
The interaction between the modified silica surface and the TMP molecule was investigated using a large number of randomized Monte Carlo steps (configurations). The MD computations continue to employ the lowest energy geometry as provided by the MC. The simulation is performed under Periodic Boundary Conditions using the cell with dimensions presented in Figure 12 (a). In the simulations the modified silica surface box is filled with 1 TMP and 650 water molecules. Prior to the MD stage, the geometry was optimized using the Forcite module built into the Biovia software (tolerance for energy convergence of 1*10-5 kcal/mol; atom-based summation method for both electrostatic and van der Waals interactions with a cutoff distance of 15.5, a spline width of 1, and a 0.5 buffer; atom-based summation method for both electrostatic and van der Waals interactions with a cutoff distance of 15.5, a s MD was done at 25°C with a 1 ns simulation duration using the Constant volume/constant temperature (NVT) canonical ensemble (using a 1fs time step) [41]. The Berendsen thermostat maintains the T control. Calculations for MC and MD are performed using the Universal forcefield [42].
The lowest energy configurations for the silica surface and the TMP molecule are shown in Figure 12 (b). The measurable verdict of the interaction between TMP molecule and the modified silica surface is calculated using the following equation:
$${E}_{ads}={E}_{total}-\left[{E}_{\text{T}\text{M}\text{P}}+{E}_{\text{M}\text{o}\text{d}\text{i}\text{f}\text{i}\text{e}\text{d} \text{s}\text{i}\text{l}\text{i}\text{c}\text{a} }\right]$$
Where: Etotal is the total energy of the system as a result of Modified Silica surface and the TMP molecule interaction; \({E}_{\text{C}\text{u}\left(\text{I}\text{I}\right)or Pb\left(II\right)ions}\) and \({E}_{\text{M}\text{o}\text{d}\text{i}\text{f}\text{i}\text{e}\text{d} \text{s}\text{i}\text{l}\text{i}\text{c}\text{a}} is\)system energy in the absence and presence of TMP molecule. MC calculations yield the lowest energy pose after a considerable number of randomized configurations. The Monte Carlo simulations (Figure 12 (b)) show that the TMP molecule adsorbs extensively on the modified silica surface, which is consistent with the experimental findings. The adsorption's negative value indicates the adsorption process' spontaneity on this adsorbent [43]. Figure S1 (a-d) in the ESI depicts the energies during the attendance of the lowest energy position, The graph shows temperature control from MD during the interaction of the TMP molecule onto modified silica surface, and the interaction energy of the TMP molecule onto modified silica surface during MD, respectively.
The distribution of the adsorption energies as shown in the Figure 12 (b) are in range -5 to -95 kcal/mol depending on the contact configurations among TMP and modified silica surface. MD is primarily concerned with monitoring the overall dynamics of the process (Amrhar, Berisha et al. 2021). The small temperature drift on the graph in Figure S3 (c) shows that the equilibrium configuration has been reached. Figure 12 (c) shows the lowest equilibrium energy structure for the TMP molecule interaction with modified silica surface obtained from MD. The interaction (adsorption) energy during the MD is assessed at each time elapse and is presented in the Figure 12 (c) and descriptive statistics in Table 5. Relative high adsorption energies are consistent with experimental findings. The negative values of the adsorption energies indicate the spontaneity of the adsorption process.
Table 5
Statistics of the interaction energy of the TMP molecule onto modified silica surface during MD (energy values are in kcal/mol).
Molecule
|
Mean
|
Minimum
|
Median
|
Maximum
|
TMP
|
-15.48
|
-3.78
|
-16.51
|
-29.58
|