pH-Responsive Semi-Interpenetrated Polymer Networks of pHEMA/PAA for the Capture of Copper Ions and Corrosion Removal

Bronze artifacts constitute a fundamental portion of Cultural Heritage, but effective methodologies for the removal of corrosion layers, such as those produced by the “bronze disease”, are currently missing. We propose the formulation and application of novel poly(2-hydroxyethyl methacrylate) (pHEMA) networks semi-interpenetrated (SIPN) with poly(acrylic acid) (PAA) to achieve enhanced capture of copper ions and removal of corrosion products. The pHEMA/PAA SIPNs were designed to improve previous pHEMA/poly(vinylpyrrolidone) (PVP) networks, taking advantage of the chelating ability of pH-responsive carboxylic groups in PAA. Increasing the pH ionizes carboxyls, increases the porosity in pHEMA/PAA, and leads to the co-presence of enol and enolate forms of vinylpyrrolidone (VP), changing the macroporosity and decreasing the mesh size in pHEMA/PVP. The ion–matrix interaction is stronger in pHEMA/PAA, where the process occurs through an initial diffusion-limited step followed by diffusion in smaller pores or adsorption by less available sites. In pHEMA/PVP, the uptake is probably controlled by adsorption as expected, considering the porogen role of PVP in the network. Upon application of the SIPNs loaded with tetraethylenpentamine (TEPA) onto corroded bronze, copper oxychlorides dissolve and migrate inside the gels, where Cu(II) ions form ternary complexes with TEPA and carboxylates in PAA or carbonyls in PVP. The removal of oxychlorides is more effective and faster for pHEMA/PAA than its /PVP counterpart. The selective action of the gels preserved the cuprite layers that are needed to passivate bronze against corrosion, and the pH-responsive behavior of pHEMA/PAA allows full control of the uptake and release of the Cu(II)–TEPA complex, making these systems appealing in several fields even beyond Cultural Heritage conservation (e.g., drug delivery, wastewater treatment, agricultural industry, and food chemistry).


Chord length distribution analysis
. Binarized SEM images of pHEMA/PAA and pHEMA/PVP gels, used to implement the chord length distribution. Top row, from left to right: pHEMA/PAA at pH 8; pHEMA/PAA at pH 12.
Magnification and scale are the same as Figure 1 in the main text. Figure S2. Cumulative frequencies of chords, obtained from chord analysis on the pores (left) and the walls (right) of the binarized SEM images (figure S1). The black lines indicate the R value at 50% of the total population. Table S1. Slopes (1/λ) and corresponding persistence length (λ) of pores size distributions of pHEMA/PAA and pHEMA/PVP gels at different pH values, fitted to equation 3 (see main text). Uncertainties on slopes account for the 10% at least of λ values. In some cases, two trends were evident in the distributions, at low and high ranges of the chord length (R). The last columns show: the average diameter D with standard deviation, calculated with the software Image J on the same images and on three images at different magnification (values in brackets); the value of R at 50% of the total population, obtained from data in figure S2.  Table S2. Slopes (1/λ) and corresponding persistence length (λ) of gels' walls size distributions of pHEMA/PAA and pHEMA/PVP gels at different pH values, fitted to equation 4 (see main text). Uncertainties on slopes account for the 10% at least of λ values. The last column shows the value of R at 50% of the total population, obtained from data in figure S2.

Sample
Slope (     The study of the amount and types of water loaded in the hydrogels provided information on the absorption and permeation properties of these systems. DSC curves of all the gels are reported in Figure S3-6. The EWC in pHEMA/PAA SIPNs, swollen at different pH, is mainly due to the gels' porosity and the hydrophilic character of polymer matrix (mostly PAA). At pH 6, where the carboxylic group are completely protonated, the EWC is about 43% (see Table S3), only slightly higher than in the case of classical pHEMA chemical gels (i.e. 38%) [1] [2], and it increases up to 57 and 79% at pH 8 and 12, owing to the ionization of the carboxyl groups and consequent pores enlargement. When the SIPN is loaded with TEPA (pH 12), the solvent content is ca. 76%. After the absorption of Cu(II) this value decreases to 71%, indicating that some free water is lost through evaporation during the application of the gel on the bronze coin, despite having covered the gel with parafilm. The FWI values increase in passing from pH 6 to 12 (see Table S3), consistently with the presence of more hydrophilic moieties in the network (carboxylate groups in PAA), and with a higher meso-and macroporosity as shown by SAXS measurements and by the chord analysis implemented on SEM images. ions, demonstrating the high hydrophilicity of these systems.
It must be noticed that for both systems there is a significant decrease in the heat of the melting transition (ΔH tr , see Table S3-4) when the gels are uploaded with the TEPA solution, and when Cu(II) ions are absorbed in the gels. The FWI decreases accordingly. In the case of TEPA-loaded gels, this was ascribed to strong hydrogen bonding taking place between the amine and water molecules, which also explains the lower critical solution temperature (as previously observed in amine-water solutions). When the gels absorb the copper ions, the further decrease was explained considering that part of the bulk water molecules coordinate with the metal ions, participating in the formation of complexes.

Kinetic Models
Pseudo-first order rate law, K1 The pseudo-first order kinetic equation, initially introduced by Lagergren [3], is generally used in the form proposed by Ho and McKay [4]: where q is the amount of adsorbed solute, q e its value at equilibrium, k 1 is the pseudo-first order rate constant, and t is the time.
For fitting the experimental data, we used the alternative expression: .
The pseudo-first order kinetic equation differs in principle from a true first order equation in two aspects [5] : q e can, in principle, differ from the theoretical maximum adsorption capacity of the surface, since reacting surfaces can be inhomogeneous, and effects of transport phenomena and chemical reactions are often experimentally inseparable. Therefore, the parameter k 1 (q e -q t ) does not represent the number of available sites, as opposed to true first order equations.
-The coefficient of the exponential term exp(-k 1 t) can be adjusted to values greater than 1, whereas it has to be strictly 1 in true first order equations.

Pseudo-second order rate law, K2
The pseudo-second order model assumes that the rate of adsorption of solute is proportional to the available sites of the adsorbent. The kinetics equation is normally used as indicated by Ho and McKay [4]: where k 2 is the pseudo-second order kinetic rate constant. q e can, in principle, differ from the theoretical maximum adsorption capacity of the surface (see above), thus the model equation differs from a true second order equation.
To obtain statistically relevant comparisons with the other models (i.e. which one provides better fits), we used the following expression that considers the original scale (y = q(t)): where k * 2 = k 2 q e .

Intraparticle diffusion model, IPD
The intraparticle diffusion model employs a power law where q ∝ t 1/2 [6]: where k ipd is a rate constant and c is the thickness of a boundary layer on the adsorbent's surface.

Weibull function
The Weibull function is an empirical equation frequently applied to the analysis of dissolution and release studies: where k w defines the time scale of the process (i.e., the time required for the absorbate to cross the free path in the sorbent), and nw is used to indicate the transport mechanism: nw ≤ 0.75 corresponds to a Fickian transport, 0.75 < nw < 1 indicates a combination of a Fickian diffusion and a Case-II transport, nw>1 indicates a complex transport mechanism [7]. The Weibull function is commonly used as an alternative to the semi-empirical Korsmeyer-Peppas equation (q(t) = kt n ), which is in turn an extension of the Higuchi law (q(t) = kt 1/2 ) [7].

Double exponential model, DE
This is an empirical model that is not based on any assumption regarding the chemistry of the process but rather describes sorption from a mathematical point of view.
The model uses a double exponential function to correlate the two-step kinetics of the adsorption of a metal ion onto a matrix [8]: where D1 and D2 are sorption rate parameters (mmol L -1 ) of the rapid and the slow step, respectively, and K D1 and K D2 are parameters (min -1 ) controlling the mechanism; m is the adsorbent amount in the solution (g L -1 ). If the exponential term corresponding to the rapid process is assumed to be negligible on the overall kinetics (K D1 >>K D2 ), the model equation can be simplified to a single exponential (SE): .57 x 10 -4 ± 0.11 x 10 -4 3.04 x 10 -4 ± 0.14 x 10 -4 -