Dynamic light scattering has been extensively investigated as a method to characterize micelles, both for their size and stability, and also to define the critical micellization concentration (CMC) and temperature (CMT) 1-3.
Following the unimer aggregation, as a function of temperature in the case of this study, a rise in the scattered intensity is anticipated as a result of bigger particles scattering a significantly higher number of photons.
Reports have claimed that long correlation times are required1, 4, 5, 6 in order to allow light-scattering techniques to be reliably employed as CMC or CMT determination methods. This makes for a time-consuming procedure. This article will describe how DLS can be employed to determine the CMT in a way that is efficient and strongly reproducible.
Each measurement outlined in this article was carried out in a Malvern Panalytical’s Zetasizer Ultra. The surfactant employed was Pluronics F-68 (poloxamer 188, Sigma); a triblock co-polymer made up of polyethylene oxide (PEO) and polypropylene oxide (PPO), with a general structure of PEO-PPO-PEO.
This particular polymer was chosen as it demonstrated a transition from unimer to micelle at relatively high temperatures, which therefore enables study of its CMT through DLS.
The amphiphilic structures of these polymers make them outstanding surfactants, allowing them to be employed in a number of industrial applications, such as raising the water solubility of hydrophobic substances, or increasing the miscibility of two or more substances in a mixture with varying hydrophobicities.
They are frequently employed in industrial settings and within the synthesis of mesoporous materials.
To determine CMT, the derived count rate was tracked at a backscattering angle of detection, at 1 °C increments from 40 °C – 70 °C. In order to lessen the impact of temporary events (contaminants, dust particles and aggregates) on the final result and to enable a more sensitive determination of the unimer/micelle transition point, adaptive correlation was employed.
To determine the impact of concentration levels on the CMT, this process was carried out for five differing concentrations, from 1.25 mg/mL to 20 mg/mL. A multi-angle dynamic light scattering measurement (back, side and forward scatter detectors) was also engaged to resolve the transition stage, as a higher resolution size analysis is required here.
Intensity-weighted distribution plots are used to display the findings. Three sets of measurements were carried out at each temperature.
Initially, a 10 mg/mL solution of F-68 in water was examined on the Zetasizer Ultra at a range of temperatures. Bearing in mind F-68’s previously identified behavior of developing micelles at higher temperatures, it was anticipated that the derived count rate (the count rate determined by taking into account the attenuation factor employed), would indicate an inflection point at the temperature where micelles began to develop.
As a result of larger particles scattering a significantly greater number of photons, this would be accentuated, as according to the Rayleigh theory, intensity proportional to the 6th power of diameter for Rayleigh scatterers (dh < 1/10 λlaser).
As can be seen in Figure 1, the impact on the derived count was significant and it was easy to identify the inflection point (or the CMT) at the meeting point of the two lines. There was little variation in the count rate within the temperature range from 40 °C – 51 °C, thus a straight line was applied to the data points.
Figure 1. Derived count rates for the sample F-68 (10 mg/mL) at different temperatures, showing an inflection point around 52 °C – the temperature at which micelles start to be formed (CMT). This analysis was performed in triplicates.
In contrast, a sudden increase in the intensity of scattering could easily be discerned from 52 °C upwards. A 3rd degree polynomial was therefore applied to this section of the data points, since it demonstrated the highest correlation coefficient value (R2 = 0.9989). The CMT can then be recognized as the temperature at which the two lines meet, i.e., 52 °C.
As seen in Figure 2, it was identified that the CMT decreased by several degrees Celsius as concentration increased. This has been attributed to the lowered hydration at higher concentrations and therefore, the greater hydrophobicity of the polymer molecules4, 6.
Figure 2. Concentration dependence of the F-68’s CMT.
A differential scanning calorimetry (DSC) instrument was also engaged to run these samples in parallel. The CMT points gathered using DLS varied by an average of 0.5 °C to those acquired in the DSC, demonstrating the viability of the technique outlined in this note. As a process, dynamic light scattering is known to generate low resolution results, especially around similarly sized modes.
With the multi-angle dynamic light scattering (MADLS) feature in the Zetasizer Ultra, a higher resolution size determination of multimodal samples is possible, through the use of the three differing angles (back, side and forward scattering detection) and the consolidation of the information gathered into one size distribution plot.
As can be seen in Figure 3, this enabled a better resolved size distribution at the CMT, with the presence of both unimers and micelles. In addition, on individual study of each angle, the modes for the unimer and micelle could not be distinguished, substantiating the fact that the MADLS analysis offers a higher resolution result (Figure 4).
Figure 3. Multi-angle dynamic light (MADLS) scattering result for the sample F-68 (10 mg/mL) at different temperatures, showing the transition from unimers to micelles.
Figure 4. Comparison between the MADLS result (a) and the 3 individual angle measurements (b) for the 10 mg/mL F-68 at 53 °C (above CMT; red, green and blue colors represent back, side and forward detection angles, respectively).
This article has demonstrated the Zetasizer Ultra’s abilities to pinpoint the critical micellization temperature of Pluronic surfactants. Unlike earlier studies on light-scattering, the Zetasizer Ultra has the power to drastically lower the measurement time and render the experiment process far simpler.
Multiple sample filtrations and lengthy data collection periods are unnecessary. In addition, the MADLS measurements enable the creation of a higher resolution size distribution plot. This allows for improved monitoring of the transition stage between micelles and unimers.
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This information has been sourced, reviewed and adapted from materials provided by Particulate Systems.
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