Dynamic light scattering has been greatly examined as a tool for micelles characterization, for their size and stability as well as to determine the critical micellization temperature (CMT) and concentration (CMC).1-3 Due to the aggregation of unimers, as a function of the temperature for the case of this study, a rise in the scattered intensity is expected as a result of larger particles scattering significantly more photons.
It has been reported that in order to reliably use light-scattering techniques as CMC or CMT determination techniques, long correlation times are required,1,4,5 which make this a time-consuming process and thus it is not generally regarded as a primary technique. This article suggests that DLS can be used in conjugation with other techniques to determine the CMT in a highly reproducible and time-efficient manner.
All measurements described in this article were carried out using Malvern Panalytical’s Zetasizer Ultra. The surfactant used was Pluronics F-68 (poloxamer 188, Sigma), a triblock co-polymer consisting of polypropylene oxide (PPO) and polyethylene oxide (PEO) with a general structure of PEO-PPO-PEO. This polymer was chosen because it shows a transition from unimer to micelle at comparatively high temperatures, and hence its CMT could be examined by DLS. In order to determine the CMT, the derived count rate was monitored at a back-scattering angle of detection, at 1 °C increments from 40 to 70 °C.
Adaptive Correlation was used to minimize the effect of transient events (dust particles, contaminants, and aggregates) on the final result and enables a more sensitive determination of the unimer/micelle transition point. This was carried out for five different concentrations, ranging between 1.25 and 20 mg/mL, to evaluate the effect of concentration on the CMT. Additionally, a multi-angle dynamic light scattering measurement (back, side, and forward scatter detectors) was performed to try to resolve the transition stage, which needs a higher resolution size analysis, and these were reported as intensity-weighted distribution plots. Each temperature point was performed in triplicates.
First, a 10 mg/mL solution of F-68 in water was analyzed on the Zetasizer Ultra at various temperature points. Taking into account the known behavior of F-68 to form micelles at increased temperatures, it was anticipated that the derived count rate (the count rate obtained by considering the attenuation factor used) would exhibit an inflection point at the temperature where micelles begin to form. This would be intensified because of larger particles scattering significantly more photons, because according to Rayleigh theory, intensity = diameter6, for Rayleigh scatterers (dh < 1/10 λlaser).
The effect in the derived count was considerable and the inflection point (or the CMT) was easily noticeable at the intersect between the two lines, as represented in Figure 1. For the initial temperature range, that is, from 40 to 51 °C, the count rate did not change considerably, and thus a straight line was applied to the data points. On the other hand, for temperatures above 52 °C, there was a rapid increase in the number of photons detected, and to this part of the data points, a third-degree polynomial was applied, because it exhibited the highest correlation coefficient value (R2 = 0.9989). The CMT can then be identified as the temperature at which the two lines intersect, 52 °C.
Figure 1. Derived count rates of 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.
As the concentration increased, it was noted that the CMT decreased by several degrees Celsius (see Figure 2). This has been attributed to the decreased hydration at higher concentrations and therefore the increased hydrophobicity of the polymer molecules at high temperatures.4
Figure 2. Concentration dependence of F-68’s CMT.
These samples were also run simultaneously in a differential scanning calorimetry (DSC) instrument. Each CMT point obtained in DLS differed from those obtained in the DSC by an average of 0.5 °C, proving the feasibility of the method described here.
Dynamic light scattering as a method is known to produce low-resolution results, specifically in the presence of similarly sized modes. The multi-angle dynamic light scattering (MADLS) feature in the Zetasizer Ultra enables a higher resolution size determination of multimodal samples, by using the three different angles (back, side, and forward scattering detection) and combining the information obtained into one size distribution plot. This enabled a better resolved size distribution at the CMT, where both micelles and unimers are present, as illustrated in Figure 3. Moreover, when each angle was separately analyzed, it was difficult to differentiate the modes for the unimer and micelle, showing that the MADLS analysis gives a higher resolution result (see 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 three 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 shows that the Zetasizer Ultra can be used to determine the critical micellization temperature of Pluronic surfactants. Contrary to previous light-scattering studies, the Zetasizer Ultra has the ability to considerably reduce the measurement time and simplify the experimental procedure. It eliminates the need for several sample filtrations and long data collection times. Furthermore, the MADLS measurements allow for a higher resolution size distribution plot, which allows for a better monitoring of the transition stage between unimers and micelles.
References and Further Reading
- Alexandridis, P. & Alan Hatton, T. Poly(ethylene oxide) poly(propylene oxide) poly(ethylene oxide) block copolymer surfactants in aqueous solutions and at interfaces: thermodynamics, structure, dynamics, and modeling. Colloids Surfaces A Physicochem. Eng. Asp. 96, 1–46 (1995).
- Gaucher, G. et al. Block copolymer micelles: Preparation, characterization and application in drug delivery. J. Control. Release 109, 169–188 (2005).
- Yeon, C., Lee, I., Kim, G. H. & Yun, S. J. Unimer-Assisted Exfoliation for Highly Concentrated Aqueous Dispersion Solutions of Single- and Few-Layered van der Waals Materials. Langmuir 33, 1217–1226 (2017).
- Zhou, Z. K. & Chu, B. Light-Scattering Study on the Association Behavior of Triblock Polymers of Ethylene-Oxide and Propylene-Oxide in Aqueous-Solution. J. Colloid Interface Sci. 126, 171–180 (1988).
- Matsuoka, H., Moriya, S. & Yusa, S.-I. Fundamental properties, self-assembling behavior, and their temperature and salt responsivity of ionic amphiphilic diblock copolymer having poly(N-isopropylacrylamide) in aqueous solution. Colloid Polym. Sci. 296, 77–88 (2018).
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
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