Topics CoveredBackgroundIntroductionThe ExperimentMicellar Size, Structure and Phase BehaviorResultsSummary
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Micelles are a classical example of microstructures formed through the self-assembly of amphiphilic molecules in aqueous solvents. Micellar systems have a wide range of applications, such as detergency, drug microencapsulation, purification of proteins, and recovery processes in the petroleum industry, to name just a few. In view of these important applications, the various physico-chemical properties of micellar solutions as well as their interactions with additive agents have been widely studied. The behavior of surfactant systems may depend critically on solution composition.
Triton® X-100 is a non-ionic (neutral) surfactant and is one of the most frequently studied and exploited micellar systems. Studies have been carried out to elucidate the phase diagram and the micellar size and shape under various conditions such as the effect of elevated temperatures. It has been shown that Triton® X-100 is able to create H-H bonding with water molecules or additive particles. Moreover, despite the fact that Triton® X-100 is non-ionic, the structure of the Triton® X-100 micelles is influenced by ions.
Figure 1. Schematic representation of Triton® X-100. The number of oxyethylene groups per chain is between n=9 and n=10.
The influence of the presence of macro- ions such as small nanoparticles (size below 50 nm), whose charge can be tuned as a function of pH, is thus an interesting question that can be ideally investigated using small-angle X-ray scattering.
In this report, we describe the influence of silica particles on the structure of Triton® X-100 solutions using the SAXSess small-angle X-ray scattering instrument from Anton Paar GmbH.
The study was performed using commercially available Triton® X-100 from Sigma-Aldrich. This surfactant is composed of a short aliphatic chain and a hydrophilic moiety with an average number of 9.5 oxyethylene units per molecule (Fig 1).
The colloids were the silica spheres Ludox® TM-50 and LS-30 from Dupont®. The average diameter is s = 22 nm with a specific surface area of 140 m2/g for Ludox® TM-50 and σ = 12 nm with a specific surface area of 215 m2/g for LS-30. Both particles are negatively charged. The pH of the stock silica solutions at T = 25 °C is around 9 and 8 for Ludox® TM-50 and LS-30, respectively. Both particles were used as received. For convenience, Triton® X-100 and Ludox® TM-50 and LS-30 will be referred to in the report as TX-100, TM and LS, respectively.
For all samples, the preparation was as follows: The stock silica solutions were dissolved in pure TX-100. Then deionized water (Milli-Q Millipore system, 18 MΩ.cm) was added to obtain the final concentrations of both components. The composition of the samples was 50 wt% of TX-100 (H1 crystalline phase, see below) mixed with TM or LS particles at concentrations in the range from 0.5 wt% to 4 wt%. The samples were vigorously stirred for 5 to 10 minutes to ensure complete mixing and equilibrated for 2 days. Since the samples at these surfactant concentrations of 50 wt% were solid-like at room temperature, they were then heated to 50 °C to decrease the viscosity and filled into 1 mm quartz capillaries. The capillaries were again equilibrated for 1 day and mounted in the sample holder of the SAXSess instrument.
The SAXSess small-angle X-ray scattering instrument from Anton Paar GmbH was used in the slit-collimation configur- ation using the PW3830 X-ray generator (40 kV, 50mA) from PANalytical with a long fine-focus X-ray tube (CuKa wavelength λ = 0.1542 nm).
Detection of the scattered radiation was performed with a 2D image plate reader (Cyclone® by Perkin Elmer). The investigations were focused on a range of scattering vectors from 0.1 nm-1 = q = 4 nm-1, where q = (4π/λ) sin (θ/2) and q is the scattering angle.
The TCS 120 temperature control unit supplied by Anton Paar GmbH was used for temperature-dependent studies.
Micellar Size, Structure and Phase Behavior
TX-100 forms micelles at concentrations above the so-called critical micellar concentration (cmc) of ~0.3 mM. These micelles are believed to have an oblate structure, corresponding to a two-shell ellipsoid of revolution, with semi-axes a1=b1=3.5 nm, c1=1 nm for the core and a2=b2 = 4.75 nm, c2=2.25 nm for the outer shell. Since the ratio of major to minor axis is about 2, the ellipsoid could be approximated reasonably well by an effective sphere of diameter σ1 = 7.5 nm. There exists some general consensus that the micellar size does not vary with increasing temperature and that the increase in scattering intensity mainly arises from changes in intermicellar interactions. Goyal et al have shown that the corresponding structure factor S(q) is well described using a sticky Hard Sphere model assuming that micelles interact via a thin attractive square well potential of depth U0 (< 0) and width Δ (~ 3 Å-1). At high temperatures above the cloud point temperature (TCP), the micellar solutions undergo phase separation into a dilute and concentrate phase.
In contrast to the situation at low to intermediate surfactant concentrations, TX-100 forms a hexagonal liquid crystalline phase (H1 phase) with a measured cloud temperature of around 89 °C at a concentration of 50 wt% in water. SAXS now allows us to investigate the effect of temperature on the structure of the hexagonal phase. Figure 2 shows the desmeared patterns obtained with the SAXSess instrument for temperatures from 25 °C to 50 °C, i.e., well below TCP.
The relative order of the structure at T = 25 °C can be deduced by the experimentally observed ratios between the peak positions:
These values are in full agreement with the theoretically expected values for a hexa- gonal liquid crystalline phase:
√3, 2, √7,…
Figure 2. Desmeared SAXS scattering curves  (log-lin representation) obtained for pure TX-100 at 50 wt% in H2O for temperatures from 25 °C to 50 °C. The positions of the Bragg peaks are indicated by the arrows. In the inset, the original slit-smeared scattering curves are shown.
The lattice parameter (nearest-neighbor distance) can then be obtained using:
where (h, k, l) are the Miller indices. This leads to a lattice parameter for pure TX-100 at 50 wt% of a ⋍ 5.80 nm. Based on this value, we are now able to follow the influence of the added silica nanoparticles on the resulting structure of the surfactant- colloid mixture.
Figure 2 also clearly reveals the strong influence of temperature on the degree of correlation in the liquid crystalline phase, which abruptly decreases between 25 °C and 35 °C. Indeed, the peaks q and q disappear; the peak q is still visible but its shape is broader and its intensity lower, and the scattering pattern becomes similar to a strongly correlated liquid-like micellar system. The peak position is also shifted slightly, yielding a lattice parameter of about a35 ⋍ 6.10 nm (a35 / a = 1.05). Above T = 35 °C no clear changes can be observed on the scattering curves; the structure remains the same.
Having been able to quantitatively reproduce the known phase behavior of TX-100 at high surfactant concentrations and quantitatively determine the lattice parameter of the hexagonal phase, we are now in a position to investigate the influence of the addition of small colloidal particles to the structure of the surfactant phase. This is shown in Figure 3, which gives the scattering curves obtained for TX-100 at 50 wt% in water with different LS particle concentr- ations at T = 25 °C. It reveals that the scattering signal below q = 1 nm-1 is essentially due to the partial structure factor of the particles (form factor Pcc(q) and structure factor Scc(q)). Above q=1 nm-1 a hexagonal structure is still clearly visible (inset of Fig. 3).
Figure 3. Slit-smeared SAXS scattering curves (log-log representation) obtained for TX-100 at 50 wt% in H2O with different LS particle concentrations at a temperature of T = 25 °C. In the inset, details on the Bragg peaks for the liquid crystal structure are given for the same scattering curves (lin-lin representation). The image plate picture obtained with 0.5 wt% of LS particles is also shown.
However, when compared to pure TX-100 solutions the peak positions are systematically shifted to lower q values, yielding a lattice parameter of aLS25 ≈ 6.0 nm. This shift is also found with TM particles (results not shown) where we find aTM25 ≈ 6.0 nm. Such results are compatible with the fact that the particles induce defects inside the crystalline phase of the TX-100 micelles. Interestingly, the particle size (σTM/σLS = 1.83) seems unrelated to the resulting lattice parameter.
Figure 3 also shows that the peak positions are quite independent of the LS particle concentrations in the investigated range from 0.5 to 3.2 wt% (similar results have been found with TM particles from 0.8 to 2.4 wt%). This could indicate that the particle-induced defects correspond to the creation of TX-100 crystalline domains, possibly at slightly lower surfactant concentrations, which coexist with areas where a colloidal suspension is present. It is clear that additional experiments at higher particle concentrations will be needed to obtain a conclusive picture.
The behavior of TX-100 micelles in a liquid-crystalline area of the phase diagram was investigated using the SAXSess small- angle X-ray scattering instrument.
We have shown that the instrument allows the study of the liquid-crystalline structure as a function of the temperature. Moreover, the large q-range and the high sensitivity of the instrument makes it ideally suited to study the influence of the addition of charged colloidal nanoparticles. Here we have particularly focused on the lattice parameter of the crystalline phase as a function of particle concentration.
Source: Anton Paar GmbH.
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