Self‐assembly studies in water

Chapter 5: Amphiphilic calix[6]arene wheels

The scattering process experienced by electrons during their passage through the specimen determine the kind of information obtained. The higher the operating voltage of a TEM instruments, the greater its lateral spatial resolution is. High voltage TEM instruments (400 kV) have point-to-point resolution better than 0.2 nm, but an electrons beam characterized by high voltage can destroy the sample, in particular if it is of organic matter. For analyses on receptors CA8 and CA18 an 85kV electrons beam was used.

The samples analyzed with this technique were four different aqueous solutions containing: i) CA8 (8 ^. 10^-6 M), ii) CA18 (8 ^. 10^-6 M), iii) CA8 + di-octylviologen ditosylate (8

. 10^-6 M), iv) CA18 + di-octylviologen ditosylate (8 ^. 10^-6 M).

The solubility of these samples in water is very low and the aqueous solution requests a preliminary filtration on Millipore HVLP (0.45 µm) to separate the undissolved solid particles. What appears from the TEM images is the presence, in all samples, of small aggregates characterized by a spherical shape whose size is about 100 nm. However, also the presence of larger aggregates, not characterized by regular in shape, was evidenced, and some of which appear to be formed by several small clusters joined together (Figure 5.9).

Figure 5.9: TEM images of CA8 self-assembled structure in water.

Chapter 5: Amphiphilic calix[6]arene wheels

These TEM analyses did not show particular differences in the dimensions and morphology of the aggregates into the aqueous solutions in presence or not of the organic guest. An increase in the dimensions of the aggregates was expected in the passage from the free receptors to the complexes with the viologen salts, because of the different conformation that the calix[6]arene assumes in these two situations. It has a trigonal prismatic conformation when it is free, while it assumes a truncated cone conformation complexing viologen salts. Probably during the evaporation of the solvent on the TEM grid the sample is subjected to a variation with respect to the behavior it has in the solution.

For this reason these systems have been studied with the DLS technique that permitted to evaluate the dimension of the aggregates directly in solution.

DLS represents a powerful technique to determine the size distribution profile of small particles in solution.^[6] When light hits small particles or aggregates, the light scatters in all directions (Rayleigh scattering) as long as the particles are small compared to the wavelength (<250 nm). If the light source is a laser, and thus is monochromatic and coherent, then it observes a time-dependent fluctuation in the scattering intensity. These fluctuations are due to the fact that small molecules in solution are undergoing Brownian motion. Therefore the distance between either constructive or destructive interference by the surrounding particles and within this intensity fluctuation information is contained about the time scale of movement of the scatterers. The dynamic information of the particles is then derived from an autocorrelation of the intensity trace recorded during the experiment.

The samples analyzed with this technique were four different aqueous solutions containing: i) CA8 (8 ^. 10^-6 M), ii) CA18 (7.4 ^. 10^-6 M), iii) CA8 + di-octylviologen ditosylate (8 ^. 10^-6 M), iv) CA18 + di-octylviologen ditosylate (1.8 ^. 10^-6 M).

For the calculation of the autocorrelation functions, two software were used, NNLS and Contin, whose results were in agreement for all experiments done.

The analyses on the sample containing receptor CA8 revealed the presence of a predominant distribution of aggregates whose average dimension was 122 ± 32 nm (Figure 5.10). The dimensions measured for this self-assembled structures in solution were in substantial agreement with those obtained by TEM analyses. In this sample another distribution of entities with average dimension of about 1,5 nm was present. This second distribution could be referred to the presence in solution of not assembled

Figure 5.10: DLS analysis output of sample containing receptor CA8.

The analyses on the sample containing receptor CA8 + di-octylviologen ditosylate revealed the presence of two predominant distribution of aggregates. The smaller is centered around 100 nm, and could be assigned to the presence of the same self-assembled system present in the former sample. The principal distribution of aggregates, instead, is centered at 420 nm (Figure 5.11). Even in this sample it was possible to revealed the presence of a small distribution at 1.5 nm probably referred to the presence of the free receptor CA8 in solution.

Chapter 5: Amphiphilic calix[6]arene wheels

Figure 5.11: DLS analysis output of sample containing receptor CA8 + di-octylviologen ditosylate salt.

From these preliminary analyses it seemed that receptor CA8 is able to self-assemble, in aqueous solution, generating 3D structures that could be tentatively identified as vesicles or micelles (Figura 5.12).Moreover it seemed that the complexation of the viologen salt from the calix[6]arene derivative could have the effect to increase the dimension of these self-assembled structures. The hypothesis proposed was that the changing in the conformational structure of calix[6]arene CA8, passing from the guest-free conformation to the complexed one, affected the 3D self-assembly mode of the molecules causing a variation in the dimensions of the aggregates.

Figure 5.12: schematic representation of a vesicle-like structure (left) or a micelle-like structure (right) constituted by receptor CA8.

The DLS analyses performed on the aqueous solution containing the receptor CA18 (Figure 5.13) revealed the presence of entities with an average dimension of 215 ± 56 nm.

A second distribution centered at 1.4 nm was also present, probably referred to the non-aggregated receptor. In this case this distribution is significantly larger than for receptor

CA8 indicating, probably, that the receptor CA18 forms self-assembled structures in aqueous solution more hardly.

Figure 5.13: DLS analysis output of the sample containing receptor CA18.

The analyses on the sample containing receptor CA18 + di-octylviologen ditosylate revealed that the addition of the organic salt did not modify the dimension of the aggregates present in solution (Figure 5.14).

Chapter 5: Amphiphilic calix[6]arene wheels

Figure 5.14: DLS analysis output of sample containing receptor CA18 + di-octylviologen ditosylate salt.

In order to demonstrate that the capability of these receptors to form aggregate structures in aqueous solution originates from the presence, at the upper rim of the cavity, of the six polar chains, similar DLS analysis were carried out on the calix[6]arene derivative C8 (Figure 5.15).^[8] This receptor bears three apolar octyl chains at the lower rim, but lacks of the di-ethylene-glycol-monomethyl ether chains at the upper.

Figure 5.15: the calix[6]arene derivative C8.

The DLS analyses of the 7,7 ∙ 10^-6 M aqueous solution of C8 revealed the total lack of self-assembled 3D structure, and the only distribution present is centered at an average dimension of 1.2 nm ca. that was attributed, as in the previous cases, to the free molecule in solution.

Figure 5.16: DLS analysis output of sample containing receptor CA8.

Chapter 5: Amphiphilic calix[6]arene wheels