Fourier Transform Infrared Spectroscopy Investigation of Water Microenvironments in Polyelectrolyte Multilayers at Varying Temperatures
Abstract
Polyelectrolyte multilayers (PEMs) are thin films formed by the alternating deposition of positively and negatively charged polyelectrolytes. They have become materials of great interest over the past few decades due to their highly adaptive properties. Water plays an important role in influencing the physical properties of PEMs, as it can act both as a plasticizer and swelling agent. However, the way in which water molecules distribute around and hydrate ion pairs has not been fully studied. In this thesis, the influence of ionic strength and temperature on PEM water microenvironments is studied. Here, we examine the effects of temperature and ionic strength on the hydration microenvironments of fully immersed poly(diallyldimethylammonium)/ polystyrene sulfonate (PDADMA/PSS) PEMs. This is accomplished by tracking the OD stretch peak using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy at 0.25 M – 1.5 M NaCl and 35 – 70 °C. The OD stretch peak is deconvoluted into three peaks: 1) high frequency water represents tightly bound water, 2) low frequency water represents loosely bound water, and 3) bulk water represents water with little or no binding to ion pairs. In general, the majority of water absorbed into the PEM exists in a bound state, with little-to-no bulk water observed. As temperature increases, PDADMA/PSS PEM loses a small amount of water; meanwhile, the high frequency water maintains its peak area while the peak center shifts to the right. This indicates that tightly bound water becomes even more tightly bound with no change in amount. The effect of ionic strength on the PDADMA/PSS PEM is more nuanced and follows a trend influenced by competing effects of electrostatic screening and repulsion. A van’t Hoff plot obtained from the ratio of the amount of high frequency water to low frequency water at each temperature shows that the amount of energy required to exchange between water states is from 11-22 kJ/mol across the studied range of salt concentration. The entropy gain associated with this transition is 48-79 kJ/molK. These results provide quantitative information on the association of water molecules within PEMs and serves as a precursor to understanding these materials’ ion-water microenvironments with respect to temperature and ionic strength.
Subject
Polyelectrolyte multilayersPolyelectrolyte complexes
FTIR
spectroscopy
layer by layer assembly
Citation
Eneh, Chikaodinaka Ifunanya (2020). Fourier Transform Infrared Spectroscopy Investigation of Water Microenvironments in Polyelectrolyte Multilayers at Varying Temperatures. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /192263.