Film Doping

PEMU Hydration Equilibrium

Number of water molecules per ion pair (r) versus NaCl concentration for a PDADMA/PSS multilayer. The upper curve represents the first run of a freshly prepared multilayer and the lower curve represents second and subsequent runs. The dashed lines show how total multilayer water is broken down into intrinsic and extrinsic contributions.

The number of water molecules associated with an ion in solution depends on the method of measurement and whether one differentiates between “strongly” or “weakly” bound water (i.e. inner and outer coordination sphere). Since the method used here provides no ambiguity in the number of water molecules in the condensed PEC phase, we are able to present a quantitative and general measure of PEMU hydration by different salt ions.

Since PSS is common to both our PEMUs, r is the molar ratio of total water in the multilayer to SO3-, [H2O]/[SO3-] (standardization is performed with an aqueous solution of PSS) is the slope of r vs. [MAn] and is thus a simple way of expressing the effectiveness of a particular salt in hydrating a particular multilayer. The intrinsic water content of the multilayer is obtained by extrapolating back to [MAn] = 0.

IR active ions

ATR-IR spectra of PDADMA/PSS doped with A, NO3-; B, ClO4-; C, N3-; D, SCN-; E, undoped; F, P4VMP/PSS undoped. All spectra were recorded with 256 scans and 4 cm-1 resolution.

These ions are used to replace existing Cl- ions.  The replacement level and rates can be monitored and recored by ATR-FTIR.  This allows us to calculate a doping level.  Ions also bring in associated water molecule with their hydration levels.  These ratios can be calculated as well.

Change in film water structure

O-D stretching region from ATR-FTIR of a (PDADMA/PSS)12.5 PEMU as it is dried under a stream of N2. Upper panel: multilayer is converted from fully hydrated (A, r=11; B, r=7; C, r=4; D, r=2; E, r=0.7; F, r=0.1) scale bar is 0.07 AU. Lower panel: curve E has been normalized to curve A to illustrate the difference in shape. Scale bar is 0.07 AU for curve A and 0.014 for curve E (r=0.7).

Indications of the change in the structure of water, particularly those changes that impact the degree of hydrogen bonding, are traditionally observed from vibrational spectroscopy. We compared the IR spectra of the water O-H stretching region in bulk water, a multilayer with intrinsic water, and a multilayer doped at approximately y=0.5 with a hydrophobic ion (perchlorate) and a more hydrophilic ion (nitrate). No significant differences could be discerned in the shape of the water spectrum. For comparison, in this figure we follow the “brute force” removal of water from a multilayer by drying it. Because the signal from liquid water bands is distorted by the omnipresent water vapor absorption in the optical path, D2O was employed for this experiment. As expected, when hydrogen (deuterium-) bonds are removed the absorption shifts to higher energy, but only when the water becomes quite diluted, suggesting a robust ability of water to maintain a H-bonding network within the PEMU matrix.

Claudiu Bucur,
Feb 2, 2010, 12:21 PM