Article pubs.acs.org/Langmuir
Analysis of the Interaction of Surfactants Oleic Acid and Oleylamine with Iron Oxide Nanoparticles through Molecular Mechanics Modeling Richard Anthony Harris,* Poslet Morgan Shumbula, and Hendrieẗ te van der Walt DST/Mintek Nanotechnology Innovation Centre, Advanced Materials Division, Mintek, Private Bag X3015,Randburg, Johannesburg 2125, South Africa S Supporting Information *
ABSTRACT: The interface interactions between surfactants oleic acid and oleylamine and magnetic nanoparticles are studied via molecular mechanics and dynamics. Mixtures of these two surfactants are widely advocated in the chemical synthesis of nanoparticles. However, the exact dynamic mechanism remains unclear. Here we report, for the first time, a comprehensive qualitative model showing the importance of acid−base complex formation between oleic acid and oleylamine as well as the presence of free protons in the engineering of nanoparticles of specific shapes and sizes. We show why critical parameters such as surfactant concentration may modify iron oxide nanoparticle shape and size and how this can be understood in the light of acid− base complex pair formation. We report on the influence these parameters have on both the in situ nanoparticle surface charge and zeta potential. Transmission electron microscopy (TEM), FTIR, and pH studies are used to confirm the validity of the calculated binding energies and number of acid−base pairs.
1. INTRODUCTION The development of nanoparticles (NPs) has been intensively pursued not only for fundamental scientific interest but also for many technological applications.1,2 Specifically, iron oxide magnetic nanoparticles are extensively investigated because of their many potential applications in biochemistry, high-density data storage, cancer hyperthermia, in vivo drug delivery, magnetic resonance (MR) contrast reagents, and immunoassay analyzers.3−9 To apply magnetic nanoparticles in these various fields, it is important to control the size and shape and to maintain the thermal and chemical stability by surface modification. In all of these applications, the magnetic NPs are coated with surfactants and/or polymers to enhance biocompatibility, provide functionality, and prevent agglomeration.10 Many chemical routes for the synthesis of iron oxide nanoparticles rely on varying the type and concentration of surfactants to tune the size and magnetic-phase properties of the nanoparticles obtained.11,12 A popular surfactant for magnetic nanoparticles in nonaqueous ferrofluids is oleic acid (OA). Over the past decade, the combination of OA with © 2015 American Chemical Society
oleylamine (OLA) has been advocated because it affects the synthesis of nanoparticles of various other materials such as FeMo and FePt as well as magnetite (Fe3O4).13−16 Magnetite is a magnetic iron oxide that has a cubic inverse spinel structure with oxygen forming face-centered cubic (fcc) close packing and Fe cations occupying interstitial tetrahedral sites and octahedral sites.15 Over the past decades, there have been a number of routes for synthesizing Fe3O4 nanoparticles, such as coprecipitation, the reverse micelle method, the hydrothermal method, and the thermal decomposition of organometallic or coordination compounds.13,17−21 Nonetheless, despite receiving much attention, the mechanism of nanoparticle−surfactant interaction remains unclear13,14,16 as there is very little understanding of the spatiotemporal evolution of these complex systems. It is known that surfactant desorption is expected to take place,22−24 but very few studies address this problem. Ligand adsorption on nanoparticles affects the colloidal stability and the optical and magnetic properReceived: August 1, 2014 Published: March 13, 2015 3934
DOI: 10.1021/acs.langmuir.5b00671 Langmuir 2015, 31, 3934−3943
Article
Langmuir ties.6,25−27 Therefore, to engineer these properties to suit the formerly mentioned applications, a thorough study into the dynamics of the surfactant−nanoparticle interaction and adsorption as well as desorption would be the starting point. This system evolves toward equilibrium with the proper fine tuning of critical parameters such as temperature (T) and constituent concentration (C). If, however, the system is allowed to cool to room temperature after synthesis without removing any of the constituents, then the system will find a new equilibrium. This equilibrium state will be the one that is observed during microscopy, FTIR, and other characterization techniques. These parameters directly influence the bond strength and consequently the electrostatic pressure and steric hindrance among the NP, surfactants, and dispersion medium caused by changing the NP surface charge and total dispersion medium charge. Because it is practically impossible to carry out microscopy, FTIR, and Zetasizer characterization of the samples in situ without modifying the local chemical environment too much, we therefore have to rely heavily on theoretical and simulation insights to best describe how this equilibrium is reached. In this context, first-principles modeling is a useful tool that provides information about the microscopic properties of a metal surface/adsorbed molecule interface. Here we use molecular mechanics to compliment experimental observations and to investigate the surface of magnetite NPs that is exposed to solutions of OA and OLA in different concentrations. To do this, it is critical to note that lowdimensional nonequilibrium dynamical systems undergo transitions where the properties of the attractors change. In the iron oxide NPs case, the critical equilibrium point may be reached by fine tuning parameters such as T and C. This would allow for a dynamical system with extended spatial degrees of freedom in three dimensions to evolve into self-organized equilibrium states. However, this self-organized system is not robust with respect to variations of the T and C parameters during self-organization. Because the system has many metastable states that are directly related to these two critical parameters, it is of crucial importance to understand how these metastable states are reached (and more importantly why they are critical and why they exist) and therefore how to manipulate the system to reach different (and perhaps more desirable) metastable states. In this study, we will focus only on the effects of the C parameter and present findings on T at a later stage; however, it will be selfexplanatory for the knowledgeable reader to see how weak hydrogen bonds are affected by temperature. If the reader is interested, they are referred to the works of Zang et al.28 for a more comprehensive discussion on how temperature affects the nanoparticle shape and size.
negatively charged deprotonated carboxyl group of OA and the deprotonated lone pair of the amine group may form a bond with Fe3+ and Fe2+ atoms on the NP surface.22,30,31 Because the NP with a diameter of 2.6 nm has the largest positive charge per surface area,29 this NP size was selected for the simulations because it would give the best adsorption response. Three different simulation experiments were performed. In the first experiment, only deprotonated OA was used and no OLA was present (system A). In the second experiment, both OA and OLA molecules were present. Deprotonated OA molecules were used, and the amine groups of OLA molecules were not allowed to adsorb the deprotonated H+ atoms (system B). In the third experiment, deprotonated OA molecules were used. The amine groups of the OLA molecules were allowed to adsorb these deprotonated H+ atoms, thus forming NH3+ (system C). In all three experiments, the number of OA molecules was systematically varied, and in the latter two experiments, OA/OLA was varied from 1:5 to 5:1. Molecular mechanics were used to determine the optimum geometries and energetics for each of the resulting OA/OLA and OA-only systems, and the binding energy between the surfactants and the NP surface was calculated. Sterically stabilized colloidal dispersions of magnetite nanoparticles were prepared according to a well-known procedure that was previously described,15,31 and pH as well as TEM and FTIR characterization of these samples was done for ratios of OA/OLA of 1:1−1:5 and 2:1−5:1. (Details are given in the Supporting Information.)
3. RESULTS AND DISCUSSION Surfactants form a basic requirement for particle stabilization in most synthesis processes.34,36 Their steric demand controls the minimal distance between particles. The interaction between a surfactant and a NP surface can occur in many ways and is mainly based on dipole−dipole, hydrogen bond, or van der Waals interactions. A distinction can be made between surfactants such as OLA, which has an amine headgroup and can only bind in a single motif to the surface, and surfactants such as OA where different binding motifs are possible: monodentrate, bridged, or chelating.36 By only considering the three binding modes of OA compared to one binding mode for OLA and ignoring the effect of electron affinity, a comparison between carboxyl and amine groups makes it clear that (statistically) OA has a greater probability of binding to surface Fe atoms than OLA if they are competing for the same Fe2+ or Fe3+ atom on the NP surface. Figure 1 shows the resulting simulated structures where the OA/OLA concentrations were varied, ranging from 5:1−1:1 up to 1:5. A surfactant double layer is formed with OA molecules closest to the NP core and OLA molecules forming an outer layer, nonbonded to the NP surface atoms. Figure 2 shows a representative FTIR spectrum for the OA/OLA = 1:1 case. Similar spectral features are obtained for the other samples. The two sharp bands at 2922 and 2852 cm−1 are attributed to the asymmetric CH2 stretch and the symmetric CH2 stretch, respectively. No peak at 1710 cm−1 (seen for pure OA) is observed, indicative that OA molecules were chemisorbed onto the Fe3O4 NPs as a carboxylate. As such, the broad band between 1541 and 1639 cm−1 is characteristic of the asymmetric νas(COO−) and the symmetric νs(COO−) stretch. Thus, the bonding pattern of the carboxylic acids on the surface of the NP was a combination of molecules bonded symmetrically and bonded at an angle to the surface. No NH2 stretching
2. THEORY, SIMULATIONS, AND EXPERIMENTAL METHODS From theory, it is known that a magnetite NP’s surface will either be O-rich or Fe-rich depending on the NP size.29 Thus, the NP surface charge is a function of NP size and will therefore be either positive (on average) or negative (on average). For this investigation, a NP size was selected that would (i) give the largest adsorption response based on the value of the NP charge per surface area and (ii) allow for the fastest simulation time based on the number of atoms in the NP. (Table S.1 in the Supporting Information gives various calculated surfacecharge values for different NP sizes.29) It is known that the 3935
DOI: 10.1021/acs.langmuir.5b00671 Langmuir 2015, 31, 3934−3943
Article
Langmuir
The argument against the presence of OLA on the surface layer is that it is unclear how it would bind to magnetite and that OA cannot form a carboxylate with OLA and with the magnetite surface at the same time. As shall be seen later, due to acid− base complex formation with OLA, many more OA molecules have a dissociated hydroxyl group than with OA alone, and this promotes adsorption. The energetically and geometrically optimized systems are shown in Figure 3 for a ratio of OA/ OLA = 1:1 and the two extreme cases of 1:5 and 5:1.
Figure 3. Resulting configurations after molecular mechanics and dynamics optimization simulations for OA/OLA (left) 1:1, (middle) 1:5, and (right) 5:1.
Binding energies were calculated for the three different systems. Figure 4 shows the binding energies for these systems. It is observed that the binding energy of an OA-“only”-system increases, leading to a more stabilized NP, by increasing the number of surfactants present. When the electrostatic pressure and steric hindrance increase, some OA molecules are forced closer to the NP surface Fe atoms and therefore more bonds may form between more Fe atoms and OA molecules. A slight decrease in binding energy is observed when increasing the number of OA surfactants 3-fold, suggesting that the balance between electrostatic pressure and steric hindrance has shifted and some OA molecules are desorbing. An OA-only system does not have the lowest binding energy compared to the two OA/OLA systems studied. The presence of OLA molecules leads to more stable nanoparticles. Thus, the presence of nonbinding/excess OA and OLA molecules plays an important role in the formation of the final NP configuration. At an OA/OLA ratio of about 1.2, the binding energies of systems B and C cross. With increasing OA concentration, system B has a lower binding energy. This signifies an important difference in the dispersion medium
Figure 1. OA (green molecules) and OLA (blue molecules) adsorbed onto the nanoparticle in different ratios. From top left to bottom right: 1:1, 1:2, 1:3, 1:4, 1:5, 2:1, 3:1, 4:1, and 5:1.
modes were observed. This is in agreement with FTIR spectra from Klokkenburg et al.,22 who showed that no FTIR spectral features are found that are characteristic of an amine-containing species in the spectra of adsorbed surfactants. From Smolensky and a co-worker’s study,32 similar FTIR spectra are seen, and even when only pure OLA is used and no OA, He et al.33 showed that the significant broadening of peaks in the region from 1300 to 1500 cm−1 indicates a low ligand density, whereas when only OA is used the 1710 cm−1 peak (representing the CO stretching mode) disappears for NPs indicating a complete chemisorption of OA. Similarly, Zang and coworkers31 showed no characteristic −NH2 bending modes or any NH2 wagging vibrations at 787 cm−1 in their FTIR data.
Figure 2. FTIR spectrum of OA/OLA = 1:1 Fe3O4-capped nanoparticles. 3936
DOI: 10.1021/acs.langmuir.5b00671 Langmuir 2015, 31, 3934−3943
Article
Langmuir
Figure 4. Binding energies of surfactants to the NP surface for OA only (system A), COO− and NH2 − functional groups (system B), and COO− and NH3+ functional groups (system C). The line at a ratio of 1.2 shows the border between regimes I and II at the crossing-point ratio between systems B and C.
environment. When moving toward increased OA concentrations, system B (with COO− and NH2) has the lower binding energy and would therefore be the more likely description of what happens in reality. In system B, the number of deprotonated H+ atoms that are absorbed is significantly lower and predominantly NH2 functional groups remain. Therefore, many free protons are present in the dispersion medium and consequently affect the pH of the environment wherein the NP nucleus grows. In contrast, for system C, free protons are readily absorbed by OLA functional groups to become RCNH3+. For the rest of the discussion, the binding energies are divided into two regimes: regime I for OA/OLA < 1.2 and regime II for OA/OLA > 1.2. Consider the acid−base complex (ABC) formation between OA and OLA: RCOOH + RCNH 2⇋RCOO−:RCNH+3
Figure 5. Complete system geometries for OA/OLA (left) 1:1 and (right) 1:2. Green molecules are adsorbed OA, pink molecules are ABC pairs, and gray molecules are excess OA and OLA that have neither bonded to the NP nor formed ABC pairs. In regime I, nonbonded OLA outnumbers nonbonded OA.
energy value results from the lower number of ABC pairs that have formed. This suggests that a slight decrease in pH toward a more acidic dispersion medium leads to more stable NP formation in this case. The presence of free protons is therefore critical in maintaining a positive charge and electrostatic pressure among the NP surface, dispersion medium, and ligands. To test this hypothesis, pH studies were done. The simulation experiment was repeated 10 additional times for each OA/OLA ratio because the stochastic Monte Carlo adsorption process has a random probability distribution pattern that may be analyzed statistically but may not be predicted precisely. It was important to see whether the local minima at OA/OLA = 1:2 was just an outlier or indeed a true representative value. If the simulation were allowed to run for a larger number of iterations, then we would expect the average result to be reasonably accurate whereas for a small number of iterations we would expect a different result. However, because a trade-off exists between the number of simulation iterations and the increase in computational time, 10 iterations was chosen as a reasonable compromise between accuracy and computational time. Figure 7 shows the resulting binding energy curve. The extreme fluctuations between OA/OLA = 0.2 and 0.4 have smoothed out; however, the local minima at 1:2 persists,
(1)
The deprotonated OA from the above equilibrium equation can easily be adsorbed by the NP surface because only weak hydrogen bonds form these ABC pairs. This results in a free proton that will be available in the dispersion medium to be absorbed by OLA: RCOOH⇋RCOO− + H+and RCNH 2 + H+⇋RCNH+3 (2)
Figure 5 shows the system geometries for two of the five OA/OLA ratios in regime I, where OA/OLA is 1:1 and 1:2. An important observation is the fluctuating number of ABC pairs that have formed. The fluctuation is directly related to the NP surface area because only a fixed number of surfactants can adsorb to the NP surface. The binding energies for these samples in regime I are compared (in Figure 6) to the number of ABC pairs that have formed, and a correlation is observed. Thus, the dispersion medium’s pH plays an important role in the synthesis of stable NPs because the pH is directly affected by the number of ABC pairs, free H+ and H+ adsorption. A local minima for regime I exists around the ratio of OA/OLA = 1:2. This higher-binding3937
DOI: 10.1021/acs.langmuir.5b00671 Langmuir 2015, 31, 3934−3943
Article
Langmuir
Figure 6. Binding energies and number of acid−base complex pairs for regime I.
Figure 7. Comparing experimentally measured pH to the calculated binding energy curve (directly related to the number of ABC pairs). After repeating the calculation 10 times, the random events/noise is smoothed out; however, the minimum at OA/OLA = 1:2 persists.
C is presented in this study, the discussion would not be complete without the mention of T. What is clear from Klokkenberg et al.’s22 study is that OLA forms an acid−base complex with OA in the solution and that OLA does not prevent the desorption of surfactants. When considering what happens during NP formation (in situ), the influences of three other important variable parameters need to be carefully considered: (a) the nanoparticle surface charge (and resulting surface potential); (b) the electrokinetic potential in the colloidal systemthe zetapotential (ζ) which is the potential difference between the dispersion medium and the stationary layer of material attached to the dispersed nanoparticle; and (c) free protons (H+) in the dispersion medium which directly influences the electric potential in the dispersion medium and consequently the zeta potential. Because the mechanism during NP formation is being investigated and it is practically impossible to measure these three parameters during NP synthesis without changing the local chemical environment too much, we have to rely heavily on the theoretical simulations to infer our conclusions. (Supporting Information, Additional Information A). In light
suggesting that for this particular nanoparticle size this is a representative value. Figure 7 also shows a correlation between the theoretically predicted binding energy curve and the experimentally measured pH values. Therefore, some function f, representative of NP stability (also representative of size and shape), may be postulated whose values are related to the pH, which, in turn, is related to the number of ABC pairs and free protons in the dispersion medium: f (NP stability) = f [pH(no. ABC pairs)]
(3)
Note that the number of ABC pairs that form is a function of the kinetic−dynamic conditions, i.e., T as well as C, because these parameters affect the weak hydrogen bonds between ABC pairs. This means that relation 3 can be further expanded to include f (no. ABC pairs) = f (T , C)
(4)
Relation 3 should come as no surprise because it is well known that these two parameters (T and C) affect the shape, size, and ultimate stability of nanoparticles.34,35 Although only a focus on 3938
DOI: 10.1021/acs.langmuir.5b00671 Langmuir 2015, 31, 3934−3943
Article
Langmuir
Figure 8. OA only. Variation of NP surface charge, free proton concentration in the dispersion medium, and zeta potential over time as inferred from the modeling experiments. The TEM image shows agglomerated nanoparticles with a weak monodispersity and different NP shapes. (The statistical size analysis were carried out for N > 50.) This is in agreement with Gao et al.,39 Yang et al.,40 and Nguyen et al.41
the use of OLA alone produced iron oxide nanoparticles in a much lower yield than the reaction in the presence of both OA and OLA. 3.2. OA/OLA = Small/Large (Regime I, Figure 4). An initial decrease in NP surface charge is expected. The presence of OLA molecules in the dispersion medium allows for ABC pair formation, preventing some of the OA molecules from binding to the NP surface. Consequently, the NP surface charge will not decrease to an electrically neutral value but will retain a slight positive value. As more OA molecules become deprotonated during bond formation with the NP surface, the free proton concentration in the dispersion medium will gradually increase. Some of the NH2 groups will absorb free protons and form NH3+. Because there are a lot more OLA molecules available compared to OA, most of the protons will be absorbed by OLA, leading to a decrease in the free proton concentration of the dispersion medium to a value that is close to zero. The zeta potential will at first increase and then decrease to a lower but nonzero value as the NP ASC approaches a much lower, nonzero, positive value and the free proton concentration diminishes. At this point, dynamic kinetic conditions will have a direct impact on the final equilibrium state that will be reached for the amine groups and free protons: RCNH2 + H+ ⇌ RCNH3+. For example, a change in T or vigorous stirring might easily break the fragile hydrogen bond on the right side of the equilibrium equation and result in the release of free protons again. Thus, an average zeta potential value (Δ zeta) will exist that will be low but nonzero. Because the NP ASC may go down to close to electrically neutral, small, nearly “ideal” (almost perfectly capped) nanoparticles will form initially. However, there will still be some Fe atoms that are not capped, contributing to a small, positive ASC. Because Δ zeta will be small (fluctuating between
of the experimental and theoretical modeling observations, the discussion will be broken down into four sections: preparing iron oxide NPs by using OA only, OA/OLA = small/large, OA/OLA = large/small, and OA/OLA = optimized ratio. 3.1. OA Only. At time = 0, the NP will have a positive average surface charge (ASC). As more OA molecules bind to the NP surface, the ASC value decreases to reach an electrically neutral value when an ideal nanoparticle forms (Supporting Information, Additional Information B). When more OA molecules become deprotonated during bond formation, the free proton concentration in the dispersion medium increases. The zeta potential will at first increase (a decreasing, positive ASC value and increasing, positive proton concentration creates a potential difference between the NP and the dispersion medium) and then decrease to a low value for an ideal NP. When the NP ASC approaches an electrically neutral value, the potential difference between the NP and medium can no longer exist. Therefore, even though nanoparticles do form, they tend to agglomerate over time (since ζ → 0) and then aggregate and form large NPs as well as have a relatively bad monodispersity. This is predicted by the binding energy graph (Figure 4) where the nanoparticle system’s binding energy with only OA is lower than those for the other two OA/OLA systems. And in the case where limx→∞ OA(x)/OLA, with x being the number of OA molecules, the binding energy decreases when increasing the OA concentration to OA/OLA = 1:5. Therefore, the advocating of using combinations of OA and OLA to synthesize NPs has not been without merit. The hypothesized in situ changes are graphically and qualitatively presented in Figure 8. The TEM image shows the resulting NPs. This is in agreement with Sun et al.14 and Hou et al.,35 who found that the sole use of OA during synthesis resulted in a viscous red-brown product that was difficult to purify and characterize. On the other hand, 3939
DOI: 10.1021/acs.langmuir.5b00671 Langmuir 2015, 31, 3934−3943
Article
Langmuir
Figure 9. OA/OLA = small/large = 4:10 (regime I). Variations in the NP surface charge, proton concentration, and zeta potential over time are shown. The resulting TEM image shows nearly perfect spherical nanoparticles; however, agglomeration has already started to occur because of the fluctuating but small zeta potential. This agrees with Gao et al.39
At first, a limited quantity of RCNH2 groups will be able to absorb these free protons, lowering the gradient of H+ release slightly, but then, because there will be more OA molecules than OLA molecules, the free proton concentration would start to increase again at a rapid rate. This would raise the electrostatic pressure and chemical potential and consequently the desorption of some OA molecules from the NP surface would counter this effect by adsorbing free protons. As before, fluctuations in the thermodynamic conditions would shift the free proton concentration back and forth until equilibrium is reached under stabilized conditions. The zeta potential will increase at first and for a small time frame will stabilize or even reverse as some free protons are adsorbed by limited RCNH2. Thereafter, as the free proton concentration increases, the zeta potential would also increase, up to the point where the free proton concentration and the NP surface charge value fluctuate. At this point, the zeta potential would follow suit to have a medium to high average value. Depending on the exact OA/OLA ratio (more OA means better capping but also increased H+ electrostatic pressure), some nanoparticles would form but would be of “medium” size (not extremely small, i.e.,
