Otto V. M. Buenoa,
J. J. Benítez
b and
Miguel A. San-Miguel
*a
aDepartment of Physical Chemistry, Institute of Chemistry, University of Campinas – UNICAMP, P. O. Box 6154, 13083-970 Campinas, SP, Brazil. E-mail: smiguel@unicamp.br
bInstituto de Ciencias de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Avda, Americo Vespuccio, 49 (41092)-Sevilla, Spain
First published on 29th November 2019
In this paper, we focus on the segregation processes emerging when preparing mixtures with different compositions of aleuritic (9,10,16 trihydroxyhexadecanoic) (ALE) and palmitic (hexadecanoic) (PAL) acids. The combination of atomic force microscopy (AFM) and molecular dynamics (MD) simulations enabled us to prove the role of the functional groups in the formation of self-assembled monolayers (SAMs) on muscovite mica surfaces. MD simulations indicate that segregation processes are favored in high ALE composition mixtures in agreement with the experimental evidence, whereas low ALE compositions promote the co-existence between segregated and dispersed systems. The secondary hydroxyl groups play a central role in the self-assembling mechanism because they control the formation of hydrogen bonding networks guarantying system stability.
Most of the SAMs are constituted by a metal or semiconductor as support and a molecule bearing a functional group with a high chemical affinity for the substrate. The strong chemical bonding is the driving force for the molecules to pack in a competitive mechanism and therefore, conditions the structure of the monolayer. This way, robust and highly ordered monolayers are obtained but the contribution of the intermolecular interactions plays a minor role.
When support-to-molecule and molecule-to-molecule interactions are comparable, intermolecular energy becomes a relevant factor in the process of packing within the confined space of a self-assembled monolayer.14 For this reason, SAMs of amphiphilic functionalized alkyl molecules on chemical inert flat substrates have been proposed as models to study the role of secondary functional groups in the molecule-to-molecule associations. To address this issue, atomic force microscopy (AFM) has been used and topographic and frictional data have been interpreted in terms of molecular in-plane (lateral) and end-group interactions.15
Structural molecular resolution in standard AFM imaging is obtained from the lateral force signal when operating in contact mode. In weakly adsorbed SAMs, the dragging force exerted by the scanning probe may overcome the magnitude of the adhesion and cohesion of molecules and the structure of the SAMs may result distorted if not destroyed while being imaged. Furthermore, in the absence of a strong bonding with the support univocally defining the energy state of the SAM, other secondary interactions like hydrogen bonding, electrostatic and van de Waals forces can originate the coexistence of a series of arrangements close in energy and cause packing defects due to structural mismatch between domains. For instance, fatty alkylamine SAMs on mica have been found to contain a significant amount of void defects though looking uniform to AFM.16 In these situations, the support from molecular dynamics calculations is a very valuable tool to characterize the structure of such “soft” SAMs17,18 or organizational processes in biomineralization.19,20
In previous studies, our group has been able to resolve the structure of a series of hydroxy fatty acids SAMs on mica showing topographic, friction and adhesion contrast in AFM using molecular dynamics simulations.21–23 It was found that the number and position of hydroxyls groups conditions the final molecular arrangement.21 In this paper we have addressed the analysis of segregation processes in SAMs formed by a mixture of hydroxylated and non-hydroxylated fatty acids using the already tested methodology combining AFM analysis and molecular dynamics simulations.
Mica (muscovite) substrates were cut in ∼1 × 1 cm2 pieces and doubly cleaved using an adhesive tape. A 20 μl drop of prepared solutions was immediately placed on top of the cleaved surface and spun at 40 rps for 20 seconds using a spin coater (LOT, Germany). Samples were stabilized for 3–4 hours inside a covered glass crystallizing dish before being transferred to the AFM microscope. For reproducibility, each preparation was repeated at least three times.
A total of 16 systems were defined to model ALE–PAL mixtures of compositions 25:
75 (25% aleuritic acid and 75% palmitic acid) and 75
:
25 (75% aleuritic acid and 25% palmitic acid). Based on experimental results, these systems were chosen as representative cases for ideal dispersed and segregated SAMs. Thus, initial configurations were constructed by placing a total of 96 molecules of ALE and PAL vertically on a mica surface, with the carboxylic group near the surface.
For each of these two compositions, two network categories (hexagonal, H, and rectangular, R) with dispersed (D) and segregated (S) configurations, were defined. For example, for a composition 75:
25 in a hexagonal arrangement, two dispersed configurations, named as H75D1 and H75D2, and two segregated configurations, referred as H75S1 and H75S2, were established. Similarly, for a composition 25
:
75, there are two dispersed (H25D1 and H25D2) and two segregated (H25S1 and H25S2) systems. The nomenclature and details for each type of system are compiled in Table 1, and top views of each system are depicted for the H and R networks in Fig. 1 and in the ESI (Fig. S1†), respectively.
ALE composition (%) | System type | System name | Characteristics | |
---|---|---|---|---|
75 | Dispersed | H75D1 | H75D2 | Hexagonal network. (1) and (2) for different ALE and PAL locations. |
R75D1 | R75D2 | Square network. (1) and (2) for different ALE and PAL locations. | ||
Segregated | H75S1 | H75S2 | Hexagonal network. (1) For column type and (2) for island type segregations. | |
R75S1 | R75S2 | Square network. (1) For column type and (2) for island type segregations. | ||
25 | Dispersed | H25D1 | H25D2 | Hexagonal network. (1) and (2) for different ALE and PAL locations. |
R25D1 | R25D2 | Square network. (1) and (2) for different ALE and PAL locations. | ||
Segregated | H25S1 | H25S2 | Hexagonal network. (1) For column type and (2) for island type segregations. | |
R25S1 | R25S2 | Square network. (1) For column type and (2) for island type segregations. |
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Fig. 1 Top views of the initial layouts for a hexagonal network of 75![]() ![]() ![]() ![]() ![]() ![]() |
The initial configurations, created with high structural tension, were submitted to a relaxation process using the ‘zero’ technique implemented in the DL_POLY code for 1 ns. Next, the equilibration process was carried out gradually at 100, 200 and 300 K, during 1 ns for each temperature and verifying that equilibrium was reached at each stage. Finally, all systems were simulated for 10 ns, and in specific cases, simulations were extended up to 20 ns. The configurations were saved every 1000 steps (1 ps), and the statistical data analysis was done in the last 0.5 ns (9.5–10.0 ns).
On the other side, the pattern for ALE SAMs is quite different (Fig. 2, bottom). Island height is 24.0–26.0 Å indicating a quite vertical arrangement of ALE molecules.21 Furthermore, the surface coverage linearly grows with the concentration of the ALE solution used to almost complete a monolayer. In this case, no saturation limit is observed because the structure of ALE SAMs is not only conditioned by the molecule-support interaction as PAL, but also by strong lateral molecule–molecule association between secondary hydroxyls.39 Phase shift images reveal no contrast between ALE SAMs and bare mica, which indicates a similar stiffness for the two phases. Compared to PAL, the lateral cohesion between ALE molecules significantly reinforces the self-assembled structure and imposes the vertical arrangement. Thus, individual PAL and ALE SAMs can be differentiated by both topographic and phase AFM imaging.
When SAMs are prepared from ALE:
PAL mixtures, two phases (I and II), as well as the mica background, can be distinguished (Fig. 3). Phase I is characterized as high height and high phase-shift domains and phase II as low height and low phase-shift ones. According to the previous paragraph, phases I and II can be associated with ALE and PAL SAMs, respectively. Indeed, the phase I coverage vs. ALE concentration in the ALE
:
PAL mixtures (symbols in Fig. 3, top) matches the adsorption isotherm (dashed line) obtained from pure ALE solutions (Fig. 2, bottom). Thus, the most plausible hypothesis is that ALE molecules pack into SAMs (phase I) independently of the presence of PAL units. When the analogous analysis is carried out for phase II (Fig. 3, bottom) it is observed that at low PAL concentration the coverage fits the one expected for pure PAL (dashed line). However, at higher PAL content, the development of phase II is favored with respect to the one predicted. Such deviation in maximum at the 3
:
7 ALE
:
PAL ratio. According to these results and to gain insight into the nature of phases I and II, molecular dynamics simulations were carried out in singular mixtures: 75
:
25 and 25
:
75 (ALE
:
PAL) considered as ideal and non-ideal systems, respectively. The ideal and non-ideal concept arises from whether or not the surface coverage in ALE
:
PAL mixtures can be described as the sum of the individual components (ALE and PAL).
For 25:
75 (ALE
:
PAL) composition, the mean configurational energy difference between the segregated and dispersed systems decreases to −0.45 MJ mol−1 (see Fig. 4B), making the appearance of dispersed systems feasible. In fact, on analyzing each case, it is observed that, unlike the previous case, there are dispersed systems more stable than segregated ones, such as H25D2 and R25S2.
Therefore, based on the configurational energy, we can conclude that systems with a higher amount of ALE (75:
25) favor the formation of segregated phases (ideal behavior), while systems with low ALE composition (25
:
75) lead to the coexistence of segregated and dispersed phases (non-ideal behavior). Accordingly, phase I can be ascribed to ALE SAMs in both ALE and PAL rich preparations while phase II corresponds to PAL SAMs in low PAL mixtures and to a dispersed ALE-PAL system at high PAL content. In the next sections, a detailed structural description of these systems is provided.
Concerning PAL molecules, in segregated systems (H75S1, H75S2, R75S1, and R75S2), PAL molecules are not as disorganized as in pure PAL SAMs (ESI, Fig. S2†), due to the restriction that ALE molecules impose, mainly through the interactions between –COOH groups. Therefore, it is expected that PAL molecules that do not interact with ALE molecules tend to behave similarly to the pure PAL, which is a characteristic behavior for non-midchain functionalized amphiphilic alkyl molecules.
Height distribution profile obtained from AFM images for ALE rich (8:
2 ALE
:
PAL) mixtures (Fig. 6) confirms the quasi-vertical packing of ALE molecules into monolayers 24.0 Å high (phase I). Regarding the PAL component, configurational energy values indicate that segregated islands are the most feasible situation and isolated PAL moieties ∼9.0 Å high are detected by AFM (phase II). Such value is slightly lower than the one obtained for pure PAL SAMs (10.0–12.0 Å), but in both cases, they correspond to a tilt of ∼60–70°, in agreement with the tilting susceptibility of PAL predicted by molecular dynamics simulations (∼50–70° and ∼110–130° in Fig. S3†).
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Fig. 6 (Top) Height and (bottom) phase-shift histograms from AFM images of SAMs obtained from pure ALE and PAL as well as 8![]() ![]() ![]() ![]() ![]() ![]() |
The weakness of the cohesion between PAL molecules within the SAM results in a low stiffness, which is also reflected in the low phase-shift value observed (phase II). Smaller isolated PAL islands in the 8:
2 ALE
:
PAL (phase II) are more easily compressed than larger PAL SAMs prepared from pure PAL solutions and therefore the height measurement is slightly lower (9.0 Å vs. 11.0 Å). This result also illustrates the structural weakness of PAL SAMs and how the pressure exerted by the AFM probe may cause higher experimental tilt angles that those expected from simulations.
In this sense, AFM height profiles of 2:
8 ALE
:
PAL detect an slight increase of phase II height with respect to the 8
:
2 ALE
:
PAL system (Fig. 6, top). The sizes of phase II islands in 8
:
2 and 2
:
8 ALE
:
PAL mixtures are very similar (Fig. 3), and consequently, such height growth cannot be ascribed to larger and less compressible SAMs. Besides, surface coverage data show that the presence of ALE molecules favors PAL adsorption. Therefore, it is presumable that phase II in 2
:
8 ALE
:
PAL is a dispersed mixed system with few ALE molecules acting as anchorage points and reinforcing lateral interactions within the SAM structure. Such reinforcement would lead to a more vertical distribution of PAL units, as indicated by calculated tilt angle distributions and would explain the higher phase-shift (lower compressibility) values observed for phase II in 2
:
8 ALE
:
PAL.
On the other side, the height of phase I (∼24.0 Å) measured by AFM is not consistent with the height reduction predicted for most of the configurations of ALE monolayers in the 25:
75 ALE
:
PAL system. In this regard, it is interesting to notice that the size of phase I is quite small and exhibits straight edges (Fig. 3, right). Our hypothesis is that phase I is mostly a highly ordered (crystalline) ALE domain with a minimal PAL mixing, low enough to not affect island topography but responsible for the slight phase shift reduction with respect to pure ALE monolayers (Fig. 6).
In next section, we discuss how the active groups (–COOH, primary and secondary –OH) are distributed for both compositions, and how this distribution determines the stability, compactness and the degree of inclination of the molecules.
In 75:
25, the active groups maintain a separation distance between them, minimizing the cross-interactions that favor the disorganization of the systems. In 25
:
75, there is no well-defined separation between the active groups, and it is expected that the cross-interactions might gain relevance leading the systems to lose their ability to form an ordered structure (Fig. 7A).
For 25:
75, the ALE molecules are disorganized because the cross-interactions are predominant, that is, –COOH⋯(primary OH), –COOH⋯(secondary OH) and primary⋯secondary OH interactions. Based on results of tilt angle distributions and density profiles, we propose that in low ALE content SAMs, the molecular disorganization would be a consequence of favoring ALE cross-interactions. In this scenario, ALE molecules would act as anchoring of PAL molecules avoiding their disorganization as indicated in the previous section.
For 75:
25, the amount of H-bonds of the primary and secondary –OH groups is higher in the segregated system than in the dispersed system and the amount of H-bonds of the –COOH groups is the same in both systems (H75S1 and R25D2, Fig. 8A and B). Therefore, segregated systems are more stable mainly due to the H-bonds formed by the –OH primary and secondary groups.
In addition, it is observed that in all systems the number of H-bonds formed by the active groups always has this behavior: secondary –OH > primary –OH ≫ –COOH. Therefore, the secondary –OH groups are primarily responsible for forming self-assembling systems. On the contrary, the –COOH groups have a minor ability to form H-bonds, and therefore, they have little control over the packing process. This fact confirms that pure PAL systems (see Fig. S3 in ESI†) do not lead to the formation of organized systems. The primary –OH groups have the intermediate capacity to form H-bonds when compared to secondary –OH groups, and therefore, they have a limited control in the organization process.
The next step is to understand why the systems, despite having the same number of secondary –OH groups, have different behavior respect to the ability to form H-bonds. Combined distribution functions (CDF) were used to monitor the variables that define an H-bond, using the TRAVIS code.40 In particular, we use radial and angular distribution functions (RDF and ADF) to monitor the rO–O distance and α angle, respectively (Fig. 8C, D and G). It was observed that in the H75S1 system, the region that favors formation of H-bonds (rO–O = 2.4–3.2 Å and α = 0.0–30.0°) is dominant. It has a relative intensity of 1.42 times than the R75D2 system. In addition, in the less stable system (R75D2) the rO–O and α parameters define zones that have no relation to an H-bond zone (therefore it is less stable), mainly those zones comprising rO–O = 2.4–3.2 Å, α = 30.0–180.0° and rO–O = 3.5–3.8 Å, α = 70.0–150.0° (Fig. 8D). By contrast, in the most stable system (H75S1), the rO–O and α parameters have values concentrated in the zone that favors the H-bond (therefore it is most stable, Fig. 8C).
For 25:
75 composition, the behavior of the active groups is different for each type of system. For the less stable system (Fig. 8F, R25D1), the –COOH group has the highest number of H-bonds, and there is no H-bond formation between secondary –OH groups. Therefore, the primary and secondary –OH groups have less influence on stability than the –COOH groups. These results indicate that the ALE molecules are not organized, according to the previous results from density profiles. In addition, the predominance of the H-bond between the –COOH groups suggests that ALE molecules are somewhat anchored at their base.
In the case of the most stable system (Fig. 8E, H25S2), the secondary –OH groups form a higher number of H-bonds; that is, they are the dominant groups. However, its predominance decreases considerably in the first 5 ns of simulation, after that, all groups have practically the same predominance. As a consequence of this, it is expected that in segregated systems the ALE molecules are a little more organized than in the dispersed systems.
On the other side, rich PAL systems are more susceptible to form dispersed structures. Classical molecular dynamics simulations have confirmed that, in systems with a high ALE composition, segregation processes are favored and the presence of dispersed phases is unusual. On the contrary, in low ALE content mixtures, a cooperative effect between ALE and PAL molecules leads to a more extended and better packing of PAL units when compared to the pure PAL system. Carboxylic acid and hydroxyl functional groups have different influence on the self-assembly process. Thus, hydrogen bonding between secondary –OH groups is the main interaction and their preponderance favors the formation of self-assembled systems. On the contrary, –COOH groups, have less influence and they do not guarantee by themselves the formation of self-assembled systems.
Footnote |
† Electronic supplementary information (ESI) available: Support figures. See DOI: 10.1039/c9ra06799j |
This journal is © The Royal Society of Chemistry 2019 |