An amine-oxide surfactant-based microemulsion for the cleaning of works of art

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An amine-oxide surfactant-based microemulsion for the cleaning

of works of art

Michele Baglioni

a,1

_{, Yareli Jàidar Benavides}

a

_{, Debora Berti}

a

_{, Rodorico Giorgi}

a

_{, Uwe Keiderling}

b

_,

Piero Baglioni

a,⇑,1

a

Department of Chemistry and CSGI, University of Florence, via della Lastruccia 3 – Sesto Fiorentino, 50019 Florence, Italy

b_{Helmholtz Zentrum Berlin, D-14109 Berlin, Germany}

a r t i c l e

i n f o

Article history: Received 27 June 2014 Accepted 3 October 2014 Available online 4 November 2014

Keywords: Microemulsion Cleaning Cultural heritage N,N-Dimethyldodecan-1-amine oxide DDAO Alkyl carbonate SANS Tulum–Mexico

a b s t r a c t

Surfactant-based aqueous fluids, such as micellar solutions and microemulsions, are effective, safe and selective media for cleaning operations in conservation of cultural heritage. The search for better-performing systems and eco-friendly cleaning systems is currently a major goal in conservation science. We report here on a ternary o/w microemulsion, composed of diethyl carbonate (DC) as the oil phase and N,N-Dimethyldodecan-1-amine oxide (DDAO) as the surfactant. DDAO is a well known and widely used detergent and solubilizing agent, selected here for its degradability and eco-compatibility. Due to its nonionic/cationic nature, it can be used also when nonionic-based formulations become ineffective because of clouding and phase separation. Moreover, DDAO is insensitive to the presence of divalent metal ions, usually abundant in wall paintings substrates. Small-Angle Neutron Scattering (SANS) provided detailed information about the nanostructure of the surfactant aggregates. Finally, the cleaning effectiveness of the nanofluid was assessed both on fresco mock-ups and on real wall paintings conserved in the archeological site of Tulum, Mexico. Here, conservators successfully used the microemulsion to remove naturally aged films of complex polymer mixtures from the works of art surface.

1. Introduction

Synthetic ﬁxatives and adhesives (usually synthetic polymers) have been widely used in conservation since the 1960s[1,2]. These cheap and versatile products were applied with different purposes on a variety of works of art, including wall and easel paintings, archeological and waterlogged wood, paper, parchment, metal and glass artifacts. The easy applicability and the apparent stability of these new synthetic materials (mainly acrylic and vinyl homo-polymers and cohomo-polymers) found a fertile ground among conserva-tors that quickly replaced in a very fast and uncritical way the natural organic substances (e.g. animal glues, polysaccharides, plants extracts, milk, egg white, siccative oils, natural resins and wax[1]) that were used for centuries for the protection and consol-idation of wall paintings. A multitude of wall paintings was treated with polymers during the second half of 20th century, but,

unexpectedly, the vast majority of them showed over time a very alarming conservation state, where the usual degradation processes were accelerated or even induced by the presence of those polymeric coatings that should have guaranteed their durability.

The causes of this behavior have been now clarified in details [3–5]. The presence of a polymer film over the painting surface drastically modifies the surface properties, such as permeability to gases and liquids, leading to mechanical stresses and to the degradation of the paint layer, which can be further damaged by the crystallization of soluble salts, usually contained in the masonry[4,5].

Moreover, the degradation of the polymer itself gives rise to further negative long-term effects. Though polymers used in resto-ration show a good resistance to degradability during laboratory tests, as a matter of fact, when applied on real wall paintings and exposed to weathering, they degrade in a few decades and often become discolored and brittle, jeopardizing the readability and the integrity of the works of art.

For the above reasons, the removal of the polymeric coatings applied in previous restoration treatments is currently one of the main challenges for conservators. The use of organic solvents

http://dx.doi.org/10.1016/j.jcis.2014.10.003

Abbreviations: DDAO, N,N-Dimethyldodecan-1-amine oxide; DC, diethyl carbonate; SANS, Small-Angle Neutron Scattering.

⇑Corresponding author. Fax: +39 055 457 3036. E-mail address:baglioni@csgi.uniﬁ.it(P. Baglioni).

1 _{No kinship exists among the authors.}

Contents lists available atScienceDirect

Journal of Colloid and Interface Science

w w w . e l s e v i e r . c o m / l o c a t e / j c i s

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should be discouraged due to several drawbacks. The action of a generic organic solvent is often poorly controllable, for the usually high volatility and low surface tension. Moreover, organic solvents solubilize the polymeric coating, spreading the dissolved material into the porous matrix of the mural painting, where it remains conﬁned after solvent evaporation. Even more important, the toxicity of most solvents traditionally used in the conservation of cultural heritage represents a serious threat for the operators’ health.

The conﬁnement of organic liquids in nanosized droplets dispersed in an aqueous phase, as in micellar and microemulsive systems, is the most effective alternative to the use of neat or mixed organic solvents, because the content of volatile organic phase is reduced to few percent, making them much safer for the operators and environmentally friendly.

During the last decade, several amphiphilic formulations have been proved to be effective in safely and selectively remove organic coatings from porous substrates, such as the surface of wall paintings[5–11]. Nowadays, the search for eco-friendly nanoﬂuids formulations tailored for the removal, in a safe and controlled way, of different classes of polymeric coatings is a major goal in conservation science. Most of previous formulations were based on anionic surfactants, such as sodium dodecylsulfate [9–11]. These systems have been showed to be highly effective for the removal of polymeric coatings from wall paintings. However, concerns about the nature of the surfactant still remain. Anionic surfactants are known to be sensitive to the presence of divalent cations (such as the Ca2+_{highly available in the carbonatic matrix}

of mural paintings) and they may precipitate as insoluble salts, which are then hardly removable from the treated surface. More recently, nonionic CiEj surfactants have been proposed in a

water/2-butanone mixture as an effective and safe cleaning tool for this purpose [12]. CiEj surfactants are insensitive to the

presence of ions and they possess good biodegradability. However their cloud point, i.e. a liquid phase separation with a lower conso-lute boundary[13], limits the temperature range of application. It was shown that working as close as possible to the cloud point provides the best cleaning performances; nevertheless, these operational conditions are not feasible in some conservative con-texts, where temperature might exceeds 20–25 °C (i.e. tropical areas, desert).

In this work we propose a system based on N,N-Dimethyldod-ecan-1-amine oxide (DDAO) as the surfactant. Amine oxides repre-sent a particular class of nonionic surfactants, which turn cationic in acidic solutions, due to the protonation of the NAO amino group. Some authors also refer to them as zwitterionic surfactants[14,15]. Amine oxides are widely used in detergents, cosmetics and toiletry, they are skin compatible[16]and have found application for drug delivery in the recent past[17], since their aggregates possess a high dispersing power toward several water-insoluble organic sub-stances, even in the absence of a cosurfactant. Moreover, studies about their degradability show that amine oxide surfactants are easily converted into carbon dioxide, water and biomass under aerobic conditions in a period of time of a few days (typically, less than a week)[16].

As pointed out, DDAO is a cationic surfactant at low pH (pKa = 5 [18]), almost nonionic at high pH, while around neutrality only a fraction (up to 20%) is protonated[18–20]. This is a particularly interesting behavior because in mild pH conditions DDAO does not have a cloud point[21,22]and this implies that it can be safely used, as an alternative to the temperature sensitive CiEj

formula-tions. Moreover, differently from anionic surfactants, the solubility and aggregation of the nonionic/cationic DDAO is not affected by the presence of divalent metal ions, such as Cu2+_{, Pb}2+_{, and Fe}2+_,

that can be found in most of the pigments used for mural paintings. Finally, the pH of aqueous solutions that come into contact with

wall paintings is locally very high due to the carbonatic nature of the substrate; therefore DDAO is deprotonated when it approaches the wall/water interface. The adsorption on the negatively charged surface is not favored from an electrostatic point of view, and, as a result, the surfactant can be easily washed away after the cleaning, leaving no residues. This statement is in good agreement with the ﬁndings of Poptoshev and Claesson [23], who have shown that the adsorption of DDAO on mica is maximum around neutral pH, while it drops for higher and lower pH.

Here we describe a ternary system, hereafter named DDAO–DC, composed of water, DDAO and diethyl carbonate (immiscible with water), as the dispersed oil phase. This formulation is completely non-toxic and environmentally-friendly. Diethyl carbonate, is a good solvent for medium to high polarity polymers. Alkyl carbon-ates are generally non-toxic and low-impact organic solvents, easily produced through trans-esteriﬁcation reactions of alcohols and polyols, which are in turn relatively safe and eco-friendly. Over the last decades, alkyl carbonates have been increasingly employed as industrial solvents, in view of their excellent and versatile properties[24].

Diethyl carbonate/water microemulsions stabilized by DDAO have been never reported in the literature; besides the fundamen-tal interest in its investigation, the knowledge of the physico-chemical and structural features of the microemulsion are also important for the applicative purposes.

The removal efﬁciency toward acrylic polymer ﬁlms deposited onto solid substrates was tested in the laboratory on fresco painting model specimens. Primal AC33Ò _{(ethyl acrylate/methyl}

methacrylate, also known commercially as Rhoplex) was chosen as the polymer to be removed from the mock-ups because of its worldwide usage as a coating and preserving agent for stone and mural paintings.

Finally, the DDAO–DC was applied for the removal of an aged coating composed of mixtures of different polymers applied over time on Mesoamerican paintings conserved in the Tulum archeo-logical site, Quintana Roo (Mexico). These paintings are located in the building XVI of the Mayan archeological site, which is situ-ated in the eastern coast of the Yucatan Peninsula, in Southern Mexico. High temperatures and relative humidity characterize the local climate, which is particularly challenging for the conser-vation of the several mural paintings that decorate the building. The ﬁrst documented interventions on mural paintings, made between 1938 and 1940, was carried out using DucoÒ_{and Dulux}Ò_,

two automotive coatings developed by the DuPont Company in the 1920–30s, which are mainly composed of nitrocellulose-based pyroxylin lacquers and an alkyl resin enamel respectively. Further treatments were carried out during the 1970s, with highly reactive biocide products, and synthetic polymers such as Primal AC33Ò

and Paraloid B72Ò_{, as well as Portland and white cement. The}

paintings suffered from severe esthetical and structural degrada-tion due to the combined presence of the thick polymeric coating and soluble salts (mainly sulfates, chlorides and nitrates) that, besides altering the original aspect of color and ﬁgures, led to the detachment and ﬂaking of the paint layer. Considering the excel-lent cleaning achieved with DDAO microemulsion, the prompt removal of the polymeric coating and subsequent consolidation with more proper and compatible materials [25] was recom-mended to the Mexican authorities.

2. Materials and methods 2.1. Chemicals

N,N-Dimethyldodecan-1-amine oxide (DDAO) was purchased from Sigma-Aldrich as a 30% (w/w) aqueous solution and used

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without any further puriﬁcation. Diethyl carbonate (DC; Sigma Aldrich, purity 99%), and D2O (EurisoTop, 98%) were used as

received. Primal AC33Ò _{aqueous emulsion was purchased from}

Phase Restauro, Florence. Water was puriﬁed with a Millipore MilliRO-6 and MilliQ (Organex Systems) apparatus (resistiv-ity > 18 MXcm). Artiﬁcial ultramarine blue pigment was obtained from Zecchi, Florence, Sand (50–70 mesh particle size) was pur-chased from Sigma–Aldrich, aged slaked lime was purpur-chased from La Banca della Calce s.r.l., Bologna, Italy. Japanese paper (9.6 g/m2₎

and cellulose powder (ArbocelÒ _{BC200, J. Rettenmaier & Sohne,}

Gmbh) were purchased from Zecchi, Florence. 2.2. Samples preparation

The microemulsion was prepared by mixing the three compo-nents with the following proportions water, 90%; DDAO, 5%; DC 5%. The samples used for Small-Angle Neutron Scattering analysis were prepared with D2O to maximize the contrast between the

aggregates and the bulk phase. Two samples were analyzed: micelles of DDAO in D2O, where the ratio (w/w) between heavy

water and the surfactant was kept identical to the complete formulation; and the microemulsion, where also DC was included. DDAO was freeze-dried prior the sample preparation in D2O.

2.3. Small-Angle Neutron Scattering

Small-Angle Neutron Scattering experiments were performed on the spectrometer V4 (Bensc-Helmholtz Zentrum Berlin). Two different conﬁgurations were employed (i.e., sample-to-detector distances (SD = 2, 8 m) to cover a range of wave vectors Q (Q = (4

p

/k)sin(h/2), where k is the wavelength of the incident neutron beam and h is the scattering angle) from 0.007 to 0.28 Å1_{. For each conﬁguration a 6 Å neutron wavelength was}

used and the wavelength resolution, Dk/k, was less than 10%. Samples were contained in 1 mm thick quartz cells and kept at 22 °C during the measurements. The scattering intensity was corrected for the empty cell contribution, transmission, and detector efﬁciency and was normalized to the absolute scale by direct measurement of the intensity of the incident neutron beam. The integration of the normalized 2D intensity distribution with respect to the azimuthal angle yielded the 1D scattering intensity distribution, I(Q), in cm1_{. The reduction of the data was performed}

using standard BENSC procedures [26] for small angle isotropic scattering. The background from the incoherent scattering coming from each sample was determined from the analysis of the Porod asymptotic limit and subtracted from the normalized spectra. Experimental data normalized to absolute scale were ﬁtted using an Igor routine (NCNR_SANS_ package_6.011)[27]available from NIST, National Institute for Standard and Technology, Gaithersburg, MD, and modiﬁed by us, using Igor Pro (Version 6.22).

2.3.1. SANS ﬁtting model

Two different models were adopted: DDAO micelles in D2O

were modeled as monodisperse prolate ellipsoids, in agreement with the published literature on this surfactant [17,21,28,29], while the ternary system was modeled as composed of polydis-perse spheres that usually account well for microemulsions. In both models, core–shell aggregates were considered, thus deﬁning two contrasts: bulk/shell and shell/core. Each region (i.e. bulk, shell and core) is characterized by a scattering length density (SLD),

q

bulk,

q

shelland

q

core(reported inTable S1). A repulsive structure

factor was included and the aggregates were considered as particles interacting with each other according to a Hayter & Penfold screened Coulomb potential described by the NAR-MMSA (non-additive radius multi-component mean sphere approxima-tion)[30–35].

For globular micelles of homogeneous scattering length density, the total scattered intensity I(Q) (cm1_{) is given by the following}

equation[34,36]:

IðQÞ ¼ NpV2p

D

q

2PðQÞSðQÞ þ bkginc ð1Þ

where Npis the number density of the scattering objects (cm3),

Vpis the their volume (cm3),D

q

is the contrast term (cm2), P(Q)

is the form factor and S(Q) is the structure factor. More details about the ﬁtting model are reported in the Supplementary Information ﬁle.

2.4. Cleaning tests

2.4.1. Wall painting model samples

The fresco samples (5 5 1 cm3) were prepared by mixing three parts of sand and one part of slaked lime (vol/vol). The specimens were then painted with artiﬁcial ultramarine blue pigment. After the setting of the mortar (about 1 month), Primal AC33Ò _{was applied by brushing on the surface of the samples.}

Attention was paid in order to create uniform layers of constant thickness. Cleaning was performed using the traditional compress technique. In brief, cellulose pulp poultice soaked with the microemulsion was applied on a Japanese paper sheet, interposed between the polymer coating and the cleaning system. The detergent system/cellulose pulp was set to 15 g:3 g. The specimens were then covered with a polyethylene ﬁlm in order to avoid fast evaporation of the detergent system. The same cleaning test was repeated on three different mock-ups, in order to check for the reproducibility of the results. Application time was set to 3 h. The results of the cleaning were analyzed through visual and pho-tographic observation and by means of FT-IR micro-reﬂectance investigation.

2.4.2. In situ

The same protocol used for the laboratory tests was applied on the ‘‘real’’ paintings conserved in Tulum, Mexico. The compresses were applied on the polymeric coatings for 2–3 h, depending on polymer thickness and environmental conditions. The results of the cleaning are presented through a photographic reportage of the test.

2.5. FT-IR micro-reﬂectance

Micro-reﬂectance FT-IR spectra were obtained with a Nexus Fourier transform infrared spectrometer from Nicolet interfaced with OMNIC software and equipped with a microscope for microanalysis. A MCT detector was used to collect the signal in the 4000–650 cm1 _{range. A gilded surface was used to collect}

the background signal. The spectra were collected as single beam ﬁles as the sum of 128 scans with a resolution of 4 cm1_{. Then they}

were divided by the background signal and transformed using the Kubelka–Munk algorithm, which is commonly used to display reﬂectance spectra, as it applies a scaling factor to the curves in order to obtain data more easily comparable to absorption spectra. Finally the spectra were processed using the Kramers–Kronig correction, in order to eliminate the deformation due to the ‘‘restrahlen’’ effect[37].

3. Results and discussion 3.1. Structural characterization

DDAO–DC ternary microemulsions have never been reported in the literature. A structural characterization is useful to understand the microstructure, particularly concerning the localization of DC

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with respect to micelles, and to tune the microemulsion to the specific required application. SANS data on DDAO micelles and on the DDAO–DC microemulsion are reported inFig. 1, together with the best fitting curves, reported as continuous lines. The structural parameters obtained from the fitting are reported in Table 1.

3.1.1. DDAO micelles

Previous structural studies performed on DDAO aqueous micelles at neutral pH[17,21,28,29], report on a prolate ellipsoidal shape for the aggregates. Our SANS data for the binary systems are also consistent with the presence of elongated ellipsoidal micelles. Barlow et al.[17], by means of a concentration scan, evidenced the presence of a repulsive intermicellar interaction, which was accounted for with a screened-Coulomb Hayter–Penfold structure factor. Our samples have a lower DDAO concentration than those investigated in that report; however the agreement of the model ﬁtting with the experimental data was considerably improved by the introduction of either a hard-sphere interaction or a screened-Coulomb potential. Considering this previous work and the zwitterionic nature of the head group, we included the electro-static interaction potential. The output values for the micellar charge were indeed low (seeTable 1).

It is worth noting that the shell thickness t is particularly high, accounting for a consistent water penetration between the polar head groups and the neighboring methylene groups (seeTable 1 and Fig. 2). Nevertheless, the exact location of the core–shell interface has only a qualitative meaning, due to the fact that the SLD values for the head and the tail of DDAO molecules are very similar; the highest contrast is between the whole micelle and

the surrounding bulk phase, while the core/shell contrast is mainly driven by D2O molecules included in the aggregate.

A conﬁrmation of the ﬁtting reliability comes from the fact that the sum of the minor semi-axis b and the shell thickness t (b + t = 20.1 Å) almost perfectly matches the length of the fully-extended DDAO molecule. Finally, an aggregation number of 89 was found, and this value agrees with previously published data[17,29,38,39].

3.1.2. The DDAO–DC microemulsion

After DC addition to the DDAO micellar solution, the SANS curve (seeFig. 1) shows a signiﬁcant intensity increase, with respect to the binary system. Diethyl carbonate and DDAO scattering length densities do not signiﬁcantly differ from each other, but overall SLD of the micelle is decreased by the replacement of D2O

hydra-tion molecules with DC molecules. Despite the core/shell contrast is almost negligible, a core–shell model provided the best agree-ment with the experiagree-mental data. For the above agree-mentioned rea-sons, the shell thickness value must be interpreted with due caution. The inclusion of DC within the micelles highly affects the value of Nw, the average number of water molecules associated

with one surfactant molecule, which is decreased to zero. More-over, about 10% of total DC included in each micelle forms a droplet of 9.2 Å radius, while the remaining penetrates between DDAO Fig. 1. Top: SANS curves for DDAO micelles in D2O (red circles) and the DDAO–DC

microemulsion (white circles). Fitting curves (blue continuous lines) are reported together with the experimental points. The error bars are not visible because smaller than the symbols for experimental data. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Table 1

Main ﬁtting results (associated errors represent the uncertainties on ﬁtting values, as calculated by the software.). Nwis the

number of D2O molecule included in the micelle, for each molecule of surfactant (for a detailed description of hydration number

calculation,see the Supporting Information ﬁle). Naggis the aggregation number (for a detailed description of aggregation number

calculation, see the Supporting Information file). For each parameter it is reported whether it was fixed or constrained during the fitting, or if it was calculated from fitting results (see the Supporting Information file).

Parameter DDAO micelles DDAO–DC microemulsion

Constrained [0, 1) a (Å) 24.9 ± 0.1 – Constrained [0, 1) b (Å) 7.2 ± 0.1 – Constrained [0, 1) t (Å) 12.9 ± 0.1 17.9 ± 0.1 Constrained [0, 1) r (Å) – 9.2 ± 0.2 Constrained [0, 1] Polydispersity (r/hrci) – 0.5 ± 0.1 Constrained [0, 1) Charge (1.60 1019_C) _{0.42 ± 0.1} _{0.60 ± 0.1} Calculated Nw 10 0 Calculated Nagg 89 96

Fig. 2. Cleaning test performed on fresco model samples. (a) Reference area coated with Primal AC33Ò _{and left untreated; (b) area treated with the DDAO–DC}

microemulsion for 3 h. The Primal AC33Ò_{ﬁlm swelled and was disrupted and}

removed by a gentle mechanical action performed with a dry cotton swab, as can be noticed from the hole in the polymer coating; (c) area left uncoated and untreated as a reference for the evaluation of the cleaning results.

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tails in the surfactant layer. This finding is supported by the work of Barlow et al., which reports that both ethyl caprylate and ethyl hexanoate (i.e. solvents having chemical structure similar to diethyl carbonate) penetrate the interfacial surfactant film in a DDAO-based microemulsion[17]. Also in this case, the inclusion in the fitting model of a hard-sphere potential, instead of a Hayter–Penfold potential, significantly decreased the agreement with the experimental data, especially in the low-Q region of the scattering curve.

The shape of the micelles changes upon DC addition, undergoing a transition from ellipsoidal to spherical. Also this result is in good agreement with the observation of Barlow et al., which described the formation of spherical aggregates even after the addition of

small amounts of oil[17]. The micelle core is highly polydisperse. Finally, the aggregation number is slightly increased from 89 to 96, upon diethyl carbonate addition to the micellar system. Overall, DDAO shows its good solving power toward water-immiscible solvents, even in the absence of a co-surfactant. This feature is particularly appreciated for conservation purposes, because it limits the number of components to the minimum necessary to obtain a good cleaning system, keeping the formulation as simple as possible.

3.2. Laboratory cleaning tests

The DDAO–DC microemulsion was tested in the removal Primal AC33Ò_{from laboratory fresco mock-ups (}_{Fig. 2}_{). Primal AC33}Ò_is

commercially available as an aqueous polymer dispersion, and includes in its formulation a variety of unknown substances, such as surfactants, stabilizer, anti-foaming agents and plasticizer, which make its removal particularly challenging. The microemul-sion caused the coating to signiﬁcantly swell.

A subsequent gentle mechanical action performed with a dry cotton swab was sufﬁcient to disrupt the polymeric ﬁlm, which could be completely removed with minimum stress for the paint layer. By the comparison ofFig. 2b and c, it can be noticed that the paint layer was in good conditions after the cleaning and the polymer removal was safe and complete.

InFig. 3three FT-IR microreﬂectance spectra collected on the fresco specimens cleaned with the DDAO–DC microemulsion are reported. The wide absorption at ca. 1419 cm1_{due to the CO}

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group of the mortar substrate is clearly visible in the area left untreated as a reference (black line) (the effect of the reststrahlen deformation is still partly noticeable, though the spectra were processed using the Kramers–Kronig correction), while it is com-pletely hidden by signals of Primal AC33 in the area coated with Fig. 3. FT-IR microreﬂectance spectra collected on the fresco mock-up cleaned with

the DDAO–DC microemulsion. Black line: untreated fresco surface; red line: area coated with Primal AC33Ò_{and left uncleaned; green line: cleaned area. (For}

interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 4. Cleaning test performed with the DDAO–DC microemulsion on the mural paintings conserved in the archeological site of Tulum, Quintana Roo, Mexico. (a) A detail of the painting before the cleaning test. The discolored and aged polymeric coating hides almost completely the original color of the painting. (b) The painting during the treatment. In the red dashed box, the DDAO–DC microemulsion-soaked poultice is highlighted. (c) The painting after the cleaning test. In the red dashed box the cleaned area is visible, evidently clearer than the surrounding areas. (d) In the inset, a close-up image is shown, whose contrast was enhanced to highlight the satisfactory result of the cleaning. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

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the polymer (red line). In this case, the main signal (1733 cm1₎

comes from the C@O stretching, and it can be used as a marker for the polymer. Finally, the spectrum collected on the region cleaned with the DDAO–DC microemulsion (green line) is almost identical to the untreated reference one. In particular, it can be noticed that the marker signals coming from the polymer are no more detectable and the carbonate wide band is again visible, indicating that the cleaning was complete and the surface is reverted to its original composition.

3.3. In situ cleaning tests

Cleaning tests performed in situ on real wall paintings represent the key step of new cleaning systems’ assessment. In everyday restoration practice, one has to deal with (often unpredictable) polymer degradation, dirt, salts, hostile climatic conditions and other factors that can compromise the good outcome of the test. Prior to the cleaning test, small samples coming from the paintings were analyzed, in order to gather information about the chemical nature of the polymeric coating that was applied on the works of art. A preliminary observation with an optical microscope evidenced the presence of diffuse salt efflorescence and hard concretions in some areas of the samples’ surface. After the mechanical removal of salts and grime, the organic material was extracted with chloroform and investigated through FT-IR analysis. The polymeric coating, applied over 6 decades ago and continu-ously exposed to severe weathering conditions, was significantly altered and this made its identification particularly challenging. However, according to the main peaks identified in the FT-IR spectrum of the extracts, the organic coating most likely included also cellulose nitrate and alkyd resins.

As reported inFig. 4, the tests performed on the Tulum paint-ings provided very encouraging results, as the treated area, after the cleaning, was lighter than adjacent unclean zones, giving new life to the original color, completely altered by the presence of the discolored polymeric coating. In this case no mechanical action was needed and the coating was directly removed by the applica-tion of the microemulsion-soaked compress.

This result is particularly satisfactory due to the fact that the exact chemical nature of the coating was only partly known and, as already said, the original coating applied by conservators was probably strongly degraded and mixed to salts and grime. More-over, it has to be pointed out that DDAO–DC was the only cleaning system, among the ones tested, which was effective in removing the aged polymeric coating. Other tests included anionic surfac-tants-based microemulsions, CiEj/2-butanone/water mixtures and

pure solvents were ineffective. 4. Conclusions

We propose a new microemulsion for the removal of polymer coatings, applied for protective purposes on mural paintings some decades ago. The physico-chemical incompatibility of the polymer ﬁlm with the substrate, together with its ageing, represent a major threat for the integrity of the pictorial layer, that can be irreparably lost, if the coating is not promptly and safely removed.

Since the ﬁrst application of microemulsions to the cleaning of wall paintings[5–7], the vast majority of formulations were based on anionic surfactants, such as sodium dodecylsulfate[9–11].

The three components microemulsion system proposed in this paper is based on a different class of surfactants, i.e. DDAO, a non-ionic/cationic (depending on the pH) surfactant that possesses good bio-degradability and eco-compatibility, is benign to humans and animals, environmentally-friendly and particularly suitable to be employed in the context of cultural heritage conservation.

The system was characterized from a structural point of view by means of Small-Angle Neutron Scattering measurements, which provided detailed information about the nature of the microemul-sion’s aggregates. DDAO in water is mainly nonionic (dissociation constant about 0.2 or less[18–20]) and forms prolate ellipsoidal particles, with consistent water penetration in the micelle. The addition of diethyl carbonate to the DDAO–DC micelles was found to form a small solvent core of diethyl carbonate, while at larger amounts diethyl carbonate penetrates also the surfactant layer, thus replacing water hydration molecules at the surfactant polar head groups.

Laboratory tests performed on the removal of acrylic polymeric dispersions from fresco model samples, showed excellent cleaning effectiveness. The system was ﬁnally assessed in the Tulum arche-ological site, where Maya paintings hidden under an aged coating made of a complex polymers’ mixture have been successfully cleaned. The results of the cleaning tests, both in laboratory and in situ, showed that this system can be successfully used as a wide spectrum tool to intervene in the removal of different classes of organic coatings from porous substrates.

With respect to previous systems [5–11], DDAO–DC has the advantage of a simpler formulation, which includes non-toxic, eco-friendly chemicals. DDAO properties are particularly interest-ing for these specific applicative purposes, in view of its bio-degradability and high solubilization power, eco-compatibility, and the pH-dependent physico-chemical nature. DDAO does not present a cloud point even at the temperatures typical of tropical or desertic areas, [21,22] and can be efficiently used also when nonionic-based formulations become ineffective. Moreover, due to its nonionic/cationic nature, DDAO is insensitive to the presence of divalent metal ions, usually abundant in wall paintings sub-strates. Finally, it is poorly adsorbed on negatively charged surfaces (such as wall paintings’ interface) due to the locally high pH, leaving no residues after the cleaning treatment. In view of these features, the DDAO–DC system can be seen as a new important cleaning tool with respect to the previously formulated nanofluid for conservation of cultural heritage.

In conclusion, the present contribution further illustrates the excellent properties of microemulsions as innovative, effective, safe and eco-friendly cleaning tools, enlarging the ‘‘palette’’ tools available to conservators for their continuing rush against time for the preservation of our cultural heritage.

Acknowledgments

The authors would like to thank Lilia Rivero Weber (National Co-ordinator for Conservation of Cultural Heritage, INAH-CNCPC, 2010-March 2013), Patricia Meehan Hermanson and Valerie Magar Meurs (heads of conservation project of Tulum INAH-CNCPC) for the facilities and the application in the archeo-logical site of Tulum.

This work has been realized with the ﬁnancial support of ‘‘NANOFORART – Nano-materials for the conservation and preservation of movable and immovable artworks’’ FP7-NMP European project,http://www.nanoforart.eu. CSGI is also gratefully acknowledged for partial ﬁnancial support.

The access to SANS facilities has been supported by the European Commission under the 7th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract n°: 226507 (NMI3). Appendix A. Supplementary material

Supplementary data associated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/j.jcis.2014.10.003.

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