On-Demand Release of Protective Agents Triggered by Environmental Stimuli

Abstract

The aim of this study was to develop smart materials with stimuli-responsive properties for the long-term protection of steel. The idea was to obtain a tailored and controlled release of protective agents in response to the environment stimuli. First, the protective efficacy of three inhibitors containing a carboxylic moiety, such as p-aminobenzoic (pAB), succinic (SA), and caffeic (CA) acids, was investigated in alkaline chloride solutions. The results revealed that pAB is the most effective protective agent, significantly better than SA and CA. It is surprising that the steel surface in the pAB solution remains unchanged even after 5 months of corrosion treatment, whereas the formation of degradation products in the SA and CA solutions was observed after only 6 days. Based on these findings, pAB was selected and used for the functionalization of silica nanoparticles and layered double hydroxides (LDHs) that can act as delivery vehicles and as an inhibitor reservoir. Specifically, pAB was chemisorbed on silica amino groups via an amide bond, and this makes possible a gradual inhibitor release induced by an alkaline environment. The intercalation of pAB in its anionic form into the LDHs structure is responsible for a completely different behavior since the release is induced by chloride ions and occurs by an anionic exchange reaction. Thus, these materials play a dual role by acting as an inhibitor reservoir and by capturing chlorides. These findings reveal that it is possible to create a reservoir of corrosion inhibitors gradually released on demand based on the chemical environment. The stimuli-responsive properties and the complementary protective action of inhibitor-loaded silica and LDHs make them attractive for the long-term protection of steel and open the way for innovative solutions in the preservation of concrete cultural heritage.

Formulation of Mesoporous Silica Nanoparticles for Controlled Release of Antimicrobials for Stone Preventive Conservation

“…The release efficiency was in line to that observed for similar silica based CRS developed for the preventive conservation of artifacts (Sorensen et al, 2010 ; Borisova et al, 2011 ). On the other hand, it is well-known that the release could be triggered by external stimuli by proper functionalization of silica surface or that the use of layered double hydroxides (LDH) could give a dual role also acting in the capture of specific ions (Giuliani et al, 2020 ). It has to keep in mind that for applicative purposes, the balance among costs and benefits have to be accounted and, in some cases, low cost materials as natural clays (Cavallaro et al, 2018 ) could be advantageous.…”

Effectiveness of Mesoporous Silica Nanoparticles Functionalized with Benzoyl Chloride in pH-Responsive Anticorrosion Polymer Coatings

“…Comparing MSN-BC and MSN-NH 2 -E residual weights, the amount of BC grafted is estimated at about 8.5 ± 1.0 wt % (Figure 2d), more than 4 times higher than the amount of BC grafted onto nonporous silica nanoparticles. 18 This corresponds to about 8.02 × 10 −4 mol per gram of MSN-BC or, when expressed with respect to the precursor MSN-NH 2 -E, to 8.76 × 10 −4 mol per gram of MSN-NH 2 -E. This result is very interesting because despite the high hindrance due to the porous structure of MSN and the progressive decrease of the accessibility of the inner pores during the reaction with BC, demonstrated by the low SSA of MSN-BC with respect to MSN-NH 2 -E, a high yield of the reaction of BC (about 50%) with the amino groups is achieved. Moreover, considering the weight loss of the two-step synthesized MSN-NH 2 -2 (Figure S6), which is about 4.7 wt % (corresponding to 8.09 × 10 −4 mol of amino groups per Exploiting the susceptibility of the amide bonds to hydrolysis, with the designed release of benzoic acid, 38 MSN-BC were analyzed by performing release tests in acid and basic conditions.…”

Advanced Materials in Cultural Heritage Conservation

“…Environmentally friendly organic corrosion inhibitors containing at least a carboxylic moiety, such as p-aminobenzoic acid, have been used for the functionalization of both silica and layered double hydroxide nanoparticles, to create a reservoir of corrosion inhibitor on steel, released upon stimuli based on the chemical environment [ 111 ]. Active protective coatings have also been obtained using amorphous PVA loaded with 2-mercaptobenzothiazole incapsulated in layered double hydroxide nanocarriers.…”

“…Scanning electron microscopy image of layered double hydroxide/2-mercaptobenzothiazole system and the corresponding structure ( C ). Adapted from ref [ 111 ].…”