Poly(ethylene glycol), also known as PEG, is a nonionic surfactant which is extensively used in a wide range of applications such as drug delivery, lubrication, cosmetics, and antifouling surfaces.
PEG has both polar and non-polar groups in its structure, which contribute to its solubility and properties in organic and aqueous solvents.
In addition, the PEG conformation responds to its environment, and this conformational inconsistency is a critical feature of its properties including solubility.
Infrared Spectroscopy
Infrared (IR) spectroscopy has been utilized to explore the effect of solvent temperature and composition on conformation of PEG with polyether backbone modes (1180-1000cm-1), CH2 wagging and twisting modes (1400- 1180cm-1), and CH2 rocking modes (800-1000cm-1) being those mainly affected. Figure 1 shows three possible configurations of a PEG chain segment.
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Figure 1. Three possible poly(ethylene glycol) conformations.
The conformation of TGT relating to a helical polymer with a gauche conformation about the C-C bond and a trans conformation about C-O bonds lead to infrared marker absorptions at approximately 1350 and 1288cm-1. Elevated temperatures result in less helical ordering, thus affecting solubility and associated properties.
An Horizon multiple reflection ATR accessory (Figure 2) was used with a ZnSe trough sample plate in order to capture infrared spectra of PEG aqueous solutions at 18, 26, and 40°C temperatures.
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Figure 2. The Horizon ATR accessory.
One major benefit of using the Horizon accessory is that it is possible to achieve spectra across a broad concentration range with excellent sensitivity. The temperature of the sample was regulated with a flow of thermostated water via a metal block placed under the prism. Under this experimental arrangement, it was possible to observe the variations in PEG conformation in aqueous solution in infrared difference spectra.
Experimental Framework
For the experiment, the PEG in the dimethyl ether form with a number average molecular weight of 500 was used. A range of PEO-water mixtures were made by diluting PEG with deionized water; compositions are specified as % by volume. Here, the IR spectrometer used was a Digilab FTS 4000 equipped with Win IR Pro 3.4 software.
Then, IR spectra were recorded using a Horizon ATR accessory fitted with a ZnSe trough having 13 reflections, and spectra were recorded at 4cm-1 resolution and rectified for wavelength-dependent variation in path length. With the heating/cooling block, temperature control of ±0.5°C was achieved.
Results and Discussion
Figure 3(A) shows the 900-1400cm-1 region of the infrared spectrum of pure PEG at temperatures of 18, 26, and 40°C. This region includes the most critical bands related to conformation.
A strong CO stretch-related absorption proximal to 1100 cm-1 dominates the spectrum. The difference in the spectrum with temperature can be vaguely seen in these spectra; however, the spectral variations are more clearly observed in the difference spectra: (a) between 26 and 18°C and (b) between 40 and 18°C temperatures.
The slight amount of noise observed in the peak is because of the near total absorption of the IR beam from this band. The elevated temperature leads to several spectral changes of which the most common is a widening of peaks, which denote an increased disorder.
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Figure 3. ATR-IR spectra of (A) poly(ethylene glycol) dimethyl ether (PEG) and (B) a 60% aqueous PEG solution at 18, 26 and 40°C. Also shown are ATR-IR difference spectra indicating the influence of temperature increase from 18 to 26°C (a and c) and from 18 to 40°C (b and d).
Figure 3(B) shows the matching spectral data for a 60% PEG aqueous solution. This composition relates to approximately two water molecules for each (-CH2-CH2-O-) chain segment, rendering a minimum in the gauche/trans absorbance ratio (A1288/A1305) of the C-O bond. In the solution spectra, only slight variations with temperature are seen, but the difference spectra clearly reveal the effect of temperature variations.
The decrease in absorbance of the 1080 and 1350cm- 1 bands and the increase in absorbance of the 1140cm-1 band denote that a gauche conformation regarding the C-C bond is preferred at higher temperature.
Likewise, a decrease in absorbance at 950 and 1288cm-1 with increase in temperature signifies that the trans conformation regarding the C-O bond becomes highly stable. On the whole, the information demonstrates that with increasing temperature, the helical TGT configuration of PEG becomes less stable.
Conclusion
This article shows the key capabilities of ATR-IR techniques to achieve in situ vibrational spectra from species in aqueous solutions.
The inclusion of temperature control in the Horizon accessory further expands its range of applications to thermally-affected structural variations in bulk media as well as at surfaces.
About Harrick Scientific Products, Inc.
Since its beginnings in 1969, Harrick Scientific has advanced the frontiers of optical spectroscopy through its innovations to transmission, internal reflection, external reflection, diffuse reflection, and emission spectroscopy. The president and founder of the corporation, Dr. N. J. Harrick, pioneered internal reflection spectroscopy and became the principal developer of this technique.
Harrick Scientific offers a large selection of standard and custom-built accessories for IR and UV-VIS spectrometers. Many of these attachments were originally forerunners in their field and their contemporary versions are considered industry standards. Harrick Scientific continues to introduce innovative new products. In addition to these state-of-the-art accessories, Harrick Scientific supplies a complete line of optical elements, including windows, ATR plates, prisms, and hemispheres.
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This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.
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