Changes in resident layer mass with increasing UCIO 4 Concentration

In Section 2.3.5, the influence o f LiC104 electrolyte concentration on the rigidity of O sPVPioo and O sP V P33 layers was discussed. The rigidity o f O sPV Pioo and thin O SPV P33 layers is essentially independent of the electrolyte concentration up to 2.0M LiC104. In 4.0M LiC104 these layers swell slightly. It is considered, however, that this expansion process is insufficient to prevent the use o f the Sauerbrey equation. The accurate evaluation o f changes in the layer mass is therefore anticipated. The level of polymer expansion is considerably greater for thicker

O SP V P33 coatings in this electrolyte and reflects the greater influx of mobile species to establish equilibrium solvent and ions levels within this thicker layer. This

behaviour is particularly prevalent in 4.0M LiC104.

The total change in layer mass on increasing the electrolyte concentration from Milli-Q water to 4.0M LiC104, is quoted in Table 2.13. These data are taken from Tables 2.10 - 2.12 and are calculated from the observed shift in resonant frequency at these electrolyte concentrations. Columns A and B o f Table 2.13 illustrate the C104‘ content of these layers, assuming the influx o f unhydrated LiC104 and hydrated LiC104 respectively. The hydration numbers used in this calculation were Li+ = 7.4 and C104‘ = 2.6 [27], A total mass change o f 602 g pol equiv'1 is observed for the OsPVPioo layer. This corresponds to the influx of 5.7 molecules of unhydrated LiC104 or 2 .1 molecules o f hydrated LiC104 per polymer equivalent.

If it is assumed that egress o f initially imbibed solvent occurs at high LiC104 concentrations (see Section 2.3.3), then these values may be viewed as the lower limit for electrolyte influx. The influx o f electrolyte is in response to activity constraints and represents a breakdown in the permselectivity o f the layer.

The level of electrolyte/ salt ingress into the thin OsPVP33 polymer coating is 3.3 (unhydrated LiC104) or 1.2 (hydrated LiC104) molecules per polymer equivalent.

These values reflect the lower limit of salt influx for this polymer and are

Table 2.13. Change in layer mass of OsPVPioo and OsPVP33 polymers on immersion in 4.0M LiC1 0 4 (following voltammetric cycling o f the layer).

Polymer p 00

(b) Estimated from the total shift in resonant frequency, following voltammetry, on increasing the electrolyte concentration from Milli-Q H20 to 4.0M LiC104

(c) Column A : Calculated assuming the insertion o f unhydrated Li+ and C104‘.

Column B : Calculated assuming the insertion of hydrated Li+ (7.4 H20 ) and C104' (2.6 H20).

approximately a factor o f 1.7 smaller than the corresponding values for the OsPVPioo polymer. This lower level o f salt influx for the OSPVP33 polymer is likely a

consequence of the more hydrophobic and compact nature of this copolymer. The level o f electrolyte influx for the thicker OSPVP³³ polymer coating is almost identical to that observed for the thinner coating. This observation suggests that, although substantial broadening of the resonance was observed, the layer is sufficiently rigid to allow for accurate mass measurements. For the thicker OsPVP33 layer, half the

observed mass change occurs following voltammetry, whilst for the thinner OSPVP33

coating, most of the electrolyte influx occurs before voltammetry. This behaviour illustrates the greater difficulty in obtaining equilibrium solute levels in thicker polymer films. The movement o f ions and solvent, that accompanies voltammetry, greatly assists the equilibration process.

The polymer layer morphology o f OsPVPioo and O SPVP33 polymer coatings is dependant on the concentration of LiClCV However, for OsPVPioo and thin OSPVP33

coatings, these morphology changes are small and the Sauerbrey equation is valid.

For thick OSPVP33 layers, although the degree of polymer swelling is greater in high

LiC^{1 0 4} electrolyte concentrations, it appears that the Sauerbrey equation is still valid.

Permselective layer behaviour is observed for all layers within the concentration range o f 0.01M to 0 .1M LiC104. Breakdown in layer permselectivity and the accompanying ingress o f electrolyte occurs in 0.5M UCIO4. The amount of salt uptake by the OsPVPioo polymer is nearly twice that observed for the OSPVP33

polymer. It is considered that this behaviour reflects the more compact and

hydrophobic nature of the styrene based OsPVP33 polymer. This behaviour highlights the different polymer properties of these polymers.

2.4. Conclusions.

In HCIO4and LiC10⁴ electrolytes, [Os(bipy)²(PVPx)ioCl]+ layers are compact and rigid. This behaviour is a consequence of the crosslinking and dehydrating properties of the perchlorate anion. Furthermore, as anticipated, there is an increase in polymer rigidity with increasing styrene content of the polymer backbone. This reflects the reduced ion and solvent content o f the higher styrene content polymers, which is a consequence of the more hydrophobic nature o f the styrene moieties. This behaviour highlights the different chemical properties o f these polymers. The

contacting electrolyte type has a profound influence on the resident layer mass of these metallopolymers. In perchloric acid electrolytes, protonation of the pyridine units of the polymer backbone provides the driving force for anion ingress. For the OSPVP⁷⁵ - OSPVP³³ polymers investigated here, there is a good correlation between the number of free protonable pyridine units and the quantity o f anion ingress, assuming the uptake o f a single unhydrated anion per protonated site.

In lithium perchlorate electrolytes, for the OsPVPioo and OsPVP^{3 3} polymers, there is no apparent change in the resident layer mass within the concentration range of 0.01M to 0 .1M LiC104. On increasing the electrolyte concentration above 0 .1M U C IO4, these layers are no longer permselective and electrolyte ingress is in

evidence. The level of electrolyte ingress is dependent on the styrene content of the polymer backbone, with the ⁰ sPVP^{3 3} polymer exhibiting a reduced layer content compared with the OsPVPioo polymer. This behaviour is again likely a consequence of the more compact nature o f the lower PVP content polymer.

2.5. References.

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27. Y. Marcus, Ion Solvation, Wiley-Interscience, New York, (1985).

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Chapter 3.