Cosolvent Gels: Viscoelastic


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Poly(vinyl alcohol)-Borate Hydro/Cosolvent Gels: Viscoelastic Properties, Solubilizing Power, and Application to Art Conservation† Emiliano Carretti,‡ Scilla Grassi,‡ Manuela Cossalter,‡ Irene Natali,‡ Gabriella Caminati,‡ Richard G. Weiss,§ Piero Baglioni,‡ and Luigi Dei*,‡ ‡

Department of Chemistry & CSGI Consortium, University of Florence, via della Lastruccia, 3 I-50019 Sesto Fiorentino, Florence, Italy, and §Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227 Received December 30, 2008. Revised Manuscript Received March 25, 2009

We report the development of a new type of hydrogel in which a cosolvent has been added to the water component. The gel networks are based on the well-known poly(vinyl alcohol)-borate systems (PVA-borate). However, it is shown that the rheological and solubilizing properties of the hydrogels can be modified drastically by the addition of a cosolvent. The studies have focused on 1-propanol as the added liquid, although it is shown that others (propylene carbonate, 1-pentanol, cyclohexanone, and 2-butanol) are amenable to making modified hydrogels as well. In addition to the rheological measurements, the gels have been investigated by differential scanning calorimetry (free water index) and determination of their solubilizing power. Finally, the gels have been applied to clean and oxidized varnish (patina) from the surface of a XVI-XVII century oil-on-wood painting by Ludovico Cardi detto il Cigoli. The mode of cleaning by and removal of the PVA-borate water/1-propanol gel from the painted surface demonstrate several advantages over other gels used in art conservation.

Introduction 1

Polymeric hydrogels have been studied extensively in the past because they have many realized and potential applications, especially in the field of biomaterials2,3 such as contact lenses, real-time immunoassay,4 tissue engineering,5 drug delivery systems,6,7 and the entrapment of bioactive substances.8 Poly (vinyl alcohol) (PVA)-based hydrogels have also been studied extensively in the last few decades.9,10 They are useful in the same applications as other polymeric hydrogels11-13 as well as in other fields such as catalysis14 and the generation of chemically multifunctional gels.12 For these systems, the formation of a 3D PVA network is modulated by the addition of a cross-linking agent,15,16 frequently † Part of the Molecular and Polymer Gels; Materials with Self-Assembled Fibrillar Networks special issue. *To whom correspondence should be addressed. E-mail: [email protected] Fax + 39 0554573036.

(1) Hoffman, A. S. Adv. Drug Delivery Rev. 2002, 54, 3–12. (2) Myung, D.; Duhamel, P. E.; Cochran, J. R.; Noolandi, J.; Ta, C. N.; Frank, C. W. Biotechnol. Prog. 2008, 24, 735–741. (3) Krysmann, M. J.; Castelletto, V.; Kelarakis, A.; Hamley, I. W.; Hule, R. A.; Pochan, D. J. Biochemistry 2008, 47, 4597–4605. (4) Carrigan, S. D.; Scott, G.; Tabrizian, M. Langmuir 2005, 21, 5966–5973. (5) Ferruti, P.; Bianchi, S.; Ranucci, E.; Chiellini, F.; Piras, A. M. Biomacromolecules 2005, 6, 2229–2235. (6) Brazel, C. S.; Peppas, N. A. Biomaterials 1999, 20, 721–732. (7) Korsmeyer, R. W.; Peppas, N. A. J. Membr. Sci. 1981, 9, 211–227. (8) Shome, A.; Debnath, S.; Das, P. K. Langmuir 2008, 24, 4280–4288. (9) Shibayama, M.; Yoshizawa, H.; Kurokawa, H.; Fujiwara, H.; Nomura, S. Polymer 1988, 29, 2066–2071. (10) Shibayama, M.; Takeushi, T.; Nomura, S. Macromolecules 1994, 27, 5350–5358. (11) Doria-Serrano, M. C.; Ruiz-Trevino, F. A.; Rios-Arciga, C.; Hernandez-Esparza, M.; Palma, S. Biomacromolecules 2001, 2, 568–574. (12) Bryant, S. J.; Davis-Arehart, K. A.; Luo, N.; Shoemaker, R. K.; Arthur J. A.; Anseth, K. S. Macromolecules 2004, 37, 6726–6733. (13) Millon, L. E.; Nieh, M.-P.; Hutter, J. L.; Wan, W. Macromolecules 2007, 40, 3655–3662. :: (14) Prusse, U.; Morawsky, V.; Dierich, A.; Vaccaro, A.; Vorlop, K.-D. Stud. Surf. Sci. Catal. 1998, 118, 137–146. (15) Juntanon, K.; Niamlang, S.; Rujiravanit, R.; Sirivat, A. Int. J. Pharm. 2008, 356, 1–11. (16) Wu, L.; Brazel, C. Int. J. Pharm. 2008, 349, 144–151.

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borate, that leads to the formation of esters with hydroxy groups on the polymer chains (Scheme 1).17 Both the mechanism and the specific nature of the cross-link have been found to depend on the PVA and borate concentrations,18 pH, temperature,19 and chemical composition of the system.20 Although many studies have been carried out in recent years to investigate the structural,21 mechanical,22 and thermal properties23 of these gels, we are unaware of any reports on the gelation properties of PVA-borate for water/organic liquid mixtures. Here, we present such a study and examine, in particular, the changes in the viscoelastic properties induced by the addition of 1-propanol as well as an application of the so-formed gels. We report also some results on gels containing other cosolvents: propylene carbonate (PC), 1-pentanol (1-PeOH), cyclohexanone (CY), and 2-butanol (2-BuOH). An important objective of this project was to develop a new family of polymeric hydrogels containing variable amounts of an organic liquid that is easily removed from a surface and thus lowimpact tools for cleaning easel paintings and sculptures.24 In that regard, 1-propanol is used for the solubilization of a large number of polymeric materials that have been used in the past as coatings or fixatives on painted surfaces.25 On the basis of the unified concept of solubilization in water by hydrotropes, solvosurfactants, (17) Chen, C. Y.; Yu, T.-L. Polymer 1997, 38, 2019–2025. (18) Koike, A.; Nemoto, N.; Inoue, T.; Osaki, K. Macromolecules 1995, 28, 2339–2344. (19) Wu, W.; Shibayama, M.; Roy, S.; Kurokawa, H.; Coyne, L. D.; Nomura, S.; Stein, R. S. Macromolecules 1990, 23, 2245–2251. (20) Keita, G.; Ricard, A.; Audebert, R.; Pezron, E.; Leibler, L. Polymer 1995, 36, 49. (21) Horkay, F.; Geissler, E.; Hecht, A. Macromolecules 1994, 27, 1795–1798. (22) Robb, I. D.; Smeulders, J. B. A. F. Polymer 1997, 38, 2165–2169. (23) Horkay, F.; Burchard, W.; Geissler, E.; Hecht, A. Macromolecules 1993, 26, 1296–1303. (24) Stulik, S.; Miller, D.; Khanjian, H.; Khandekar, N.; Wolbers, R.; Carlson, J.; Petersen, W. C. In Solvent Gels for the Cleaning of Works of Art: The Residue Question; Dorge, V., Ed.; The Getty Conservation Institute: Los Angeles, 2004; pp 18-83. (25) Horie, C. V. In Materials for Conservation; Butterworths: London, 1987.

Published on Web 04/30/2009

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Carretti et al. Scheme 1. Cross-Linking Reaction Leading to the Formation of a Gelator Network

and cosolvents reported recently,26 we reasoned that the water/ 1-propanol mixtures would be effective in the solubilization of many water-insoluble substances present as foreign patinas on easel paintings. Previously, we developed and applied other gel systems for art conservation.27-30 The gels, in those cases,27,28 were rheoreversible at room temperature. They can be applied to a surface and, by the addition of a second component (usually a drop of very dilute acetic acid), can be transformed within seconds into nonviscoelastic, freely flowing liquids that are removed easily without causing mechanical damage to the surface being cleaned. Additional advantages of these gels compared to neat liquids are that they inhibit surface spreading (so that only a designated area is exposed to the cleaning agents). Furthermore, they can reduce solvent penetration into the original painting layers, mainly via capillary action, where solvent penetration often leads to swelling and leaching of varnishes and binders constituting the work of art,31 and favor the uptake of the solubilized materials into the liquid part within the porous matrix. However, as noted above, they do require the addition of a second component to remove them efficiently from a surface. The impetus for this project was stimulated by two major potential advantages of the PVA-borate 1-propanol/hydrogels over gels previously studied by others24 and us27,28 for cleaning purposes. (1) The elastic properties of the PVA-borate 1-propanol/hydrogels should permit them to be removed safely and easily without the addition of a second component. (2) The aforementioned study to develop a unified concept of solubilization demonstrated that the ability of water-alcohol mixtures to dissolve nonhydrophilic materials increases exponentially with increasing alcohol concentration.26 We report here that the PVA-borate 1-propanol/hydrogels do provide selective, surface-controlled cleaning action as well as facile and benign removal from a painting surface. Their very high elasticity allows them to be peeled from a surface without introducing a strong lateral force. By so doing, residues left on the painted surface from the patina and from the gel are expected to be minimized and the mechanical action and repeated washings usually necessary for the complete removal of traditional (26) Bauduin, P.; Renoncourt, A.; Kopf, A.; Touraud, D.; Kunz, W. Langmuir 2005, 21, 6769–6775. (27) Carretti, E.; Macherelli, A.; Dei, L.; Weiss, R. G. Langmuir 2004, 20, 8414–8418. (28) Carretti, E.; Dei, L.; Weiss, R. G. Soft Matter 2005, 1, 17–22. (29) Carretti, E.; Dei, L.; Weiss, R. G.; Baglioni, P. J. Cult. Herit. 2008, 9, 386–393. (30) Carretti, E.; Dei, L.; Baglioni, P.; Weiss, R. G. J. Am. Chem. Soc. 2003, 125, 5121–5129. (31) Michalski, S. In Preprints of the Contributions to the Congress Cleaning Retouching, Coatings, Bruxelles, September 3-7, 1990; pp 85-92.

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gels can be avoided. Moreover, this elasticity can be easily modulated by changing the ratio of PVA to borate or their total concentration.32,33 Detailed experiments characterize the viscoelastic properties of the different compositions of the gels by rheology (N. B., storage modulus (G0 ), loss modulus (G00 ), and complex viscosity (η*)), their free water indices (FWI)34 by DSC calorimetry, and their solubilization capacities by UV-vis spectroscopy using disperse red 13 dye as a model hydrophobic substance. The ability of the gels to clean the surfaces of works of art is demonstrated by results from a test with the PVA-borate hydrogel containing 20 wt % 1-propanol to remove the brown patina from layers of oxidized varnishes used in past conservation treatments of the wood panel “Santo Stefano” painted by Ludovico Cardi detto il Cigoli (1559-1613).

Materials and Methods Poly(vinyl alcohol) (PVA) (99+ % hydrolyzed, avg Mw 124 000-186 000, Aldrich), sodium tetraborate decahydrate (99.5%, Merck), 1-propanol (99+ %, SAFC), propylene carbonate (PC) (99.5%, Merck), 1-pentanol (1-PeOH) (99.5%, Merck), cyclohexanone (99.5%, Merck), 2-butanol (99.5%, Merck), and disperse red 13 (DR13, 2-[4-(2-chloro-4-nitrophenylazo)-N-ethylphenylamino]-ethanol) (95%, Sigma-Aldrich) were used as received. Water was purified by a Millipore Organex system (R g 18 MΩ cm). Poly(acrylic acid) Carbopol Ultrez 10, which is commonly used in easel painting conservation, was supplied by Goodrich (Cleveland, OH) and used without further purification.

Oscillatory shear measurements were carried out on a Paar Physica UDS 200 rheometer working under controlled shear stress equipped with a 1° cone and plate geometry of 25 mm diameter. The dependencies of the storage modulus (G0 ) and the loss modulus (G00 ) on the oscillation frequency were obtained from the phase lag between the applied shear stress and the related flow and from the ratio between the amplitudes of the imposed oscillation and the response of the gel. G0 and G00 were measured over the frequency range of 0.001-50 s-1. The values of the stress amplitude were checked by means of an amplitude sweep test in order to ensure that all measurements were performed within the linear viscoelastic region. After their loading, all of the samples were equilibrated for 1 h at 20 °C prior to conducting the experiments. All measurements were performed at a temperature of 20.0 ( 0.1 °C (Peltier temperature control system). DSC experiments on the prepared gels were carried out with a Q1000 TA Instruments apparatus using sealed stainless steel pans. The heating rate was 0.5 °C/min unless otherwise specified. The phase-transition temperatures were taken as the temperatures of the maximum heat flow of the endothermic peaks, and the enthalpy changes were determined by integrating the heat-flow curves. The free water index (FWI) parameter was calculated from eq 1 FWI ¼ ΔHexp =AΔHinit

ð1Þ

where ΔHexp is the enthalpy change (in J g-1 of gel) of the water melting determined by the DSC experimental curve, A is the (32) Lin, H.-L.; Yu, T. L.; Cheng, C.-H. Colloid Polym. Sci. 2000, 278, 187-194. (33) Lin, H.-L.; Liu, Y.-F.; Yu, T. L.; Liu, W.-H.; Rwei, S.-P. Polymer 2005, 46, 5541–5549. (34) Grassi, S.; Dei, L. J. Phys. Chem. B 2006, 110, 12191–12197.

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average weight fraction of water in the gel, and ΔHinit is the theoretical value of the specific enthalpy of fusion of water at 0 °C in the gel determined by assuming that all water in a gel can be frozen. The accuracy in the determination of ΔHexp was (0.5 J g-1. To verify the solubility capacity of the water/1-propanol mixtures, 200 μL aliquots of a solution of 10-3 M DR13 in acetone were transferred to open vials, and the acetone was allowed to evaporate, leaving a ca. 400-500-μm-thick DR13 film. Then 5 mL of a water/1-propanol mixture was added, and the sample was kept in the dark without stirring for 48 h at 25 ( 1 °C; the choice of 48 h is based upon the empirical observation that no additional dye was dissolved after this period in experiments where excess dye was present. The liquid was removed from the vial and diluted by a factor of 10 with the same water/1-propanol composition, and UV-vis spectra were recorded on a PerkinElmer Lambda 900 spectrometer in 1 cm optical path length cells in order to determine the solubilization capacity of the different liquid mixtures. Gels with 2 wt % PVA and 0.4 wt % borax were prepared by first dissolving the sodium borate decahydrate in water and then adding the desired amount of cosolvent. Then, PVA was added and dissolved by stirring and heating the mixture to 90 °C for 3 h in hermetically closed vials (to prevent the evaporation of the volatile components). A concentration of 2 wt % PVA was selected in order to be sure that it was above overlap concentration C*, 1.54 wt %.17 Gels with buffered poly(acrylic acid) (PAA) were prepared by dispersing 1 wt % PAA in 20/80 (w/w) 1-propanol/ water and neutralizing to pH 7 with 1 M NH4OH(aq); hydrogels of 1 wt % PAA were selected according to the best formulation for cleaning easel paintings.35

Results and Discussion Rheological Measurements. The frequency sweep measurements showed that all of the gels, at least qualitatively, are similar rheologically to the sample containing 20 wt % 1-propanol (Figure 1). The crossover between the G0 and G00 curves occurred at a value typical of structured fluids characterized by a 3D network of both entangled chains and reversible or transient cross-links. At low frequencies, the polymer chains can release stress by disentanglement or molecular rearrangement during the period of oscillation, whereas they cannot disentangle during the short periods of oscillation at higher frequencies. In agreement with earlier results on PVA-borate-based hydrogel systems,22 these curves cannot be fitted to a single Maxwell element. Nevertheless, as indicated by Piculell et al.,36 it is possible to use the crossover frequency ωc of the G0 and G00 curves to define an apparent relaxation time, τc (= 1/ωc), and the crossover modulus Gc (which provides information about the degree of cross-linking). These two parameters are diagnostics for relative comparisons among samples containing different amounts of cosolvent. The trend of G0 versus frequency is asymptotic for many complex fluids, approaching a value that is almost independent of the applied perturbation that corresponds to the intrinsic elastic shear modulus G.37 This modulus is associated with the entanglement density Fe: G = FekBT.38 For the PVA-borate gel containing 20 wt % 1-propanol, the asymptotic value is 595 ( 5 Pa at 25 °C (Figure 1A). For the PAA gel containing the same amount of 1-propanol, G0 is nearly constant but G00 increases slightly at high frequencies (Figure 1B); therefore, the G0 value (235 ( 8 Pa) is almost equal to the intrinsic elastic shear modulus. This value (35) Cremonesi, P.; Curti, A.; Fallarini, L.; Raio, S. Progetto Restauro 2000, 7, 25–33. (36) Piculell, L.; Egermayer, M.; Sjostrom, J. Langmuir 2003, 19, 3643–3649. (37) Srinivasa, R.; Raghavan, S. R.; Kaler, E. W. Langmuir 2001, 17, 300–306. (38) Schubert, B. A.; Kaler, E. W.; Wagner, N. J. Langmuir 2003, 19, 4079–4089.

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is ca. one-half that obtained for the analogous PVA based gel, indicating a smaller value of Fe and thus a much less rigid network formed by the PAA acid chains. On the basis of G0 and G00 , the PVA hydrogels clearly have stronger elastic character than hydrogels of carboxymethylcellulose,39 another gel commonly used in the conservation of easel paintings, even after the addition of large amounts of 1-propanol. Moreover, the value of the storage modulus above the G0 /G00 crossover frequency is almost 1 order of magnitude higher than that of carboxymethylcellulose gels.39 It is useful in this regard to compare the rheological properties of our PVA-borate hydrogels containing 1-propanol to those of gels commonly used in art conservation that are based on PAA or carboxymethylcellulose, in relationship to the ease of gel removal by peeling off. In practice, peeling a gel from a surface at the end of its application is usually accomplished with the aid of a spatula that applies a soft mechanical perturbation in the form of a sum of waves that propagates within the gel. This perturbation can be visualized as a complex mechanical vibration similar, from a physical point of view, to usual ground vibrations for which the Fourier spectral analysis gives frequency components in the range from a few to hundreds of hertz. Therefore, the part of the frequency sweep curves to taken into account with respect to using these (and other) gels for art conservation lies above the crossover point. In this region, the rheological properties of the PVA-borate hydrogels, even with 20 wt % added 1-propanol, are more elastic than PAA- or carboxymethylcellulose-based hydrogels. For that reason, the PVA-borate gels should be more easily removed from a surface and therefore superior mechanically as vehicles for cleaning works of art. This prediction was validated experimentally by peeling the three gels from a glass surface (Figure 2). The rigidity of the two PVA-borate-based hydrogels examined allowed their facile removal whereas the PAA hydrogel was less rigid and more difficult to remove with the spatula. Rigidity increased in the hydrogels when the PVA concentration was increased from 2 to 3 wt % and the borate concentration was increased from 0.4 to 0.6 wt % (compare the left and middle panels in Figure 2); more quantitatively, the asymptotic values of the measured G0 parameter were ca. 1650 ( 20 Pa for the sample with 3 wt % PVA and 0.6 wt % borate and 595 ( 5 Pa for that with 2 wt % PVA and 0.4 wt % borate. These data demonstrate that G0 values in the region above the crossover are a reasonable diagnostic of gel rigidity and ease of removal from a surface. Figure 3 shows the trend in the crossover values between the moduli, Gc (i.e., where the loss modulus prevails to a state where the system is driven mainly by the storage modulus), and the apparent relaxation times for PVA-based gels as a function of the amount of added 1-propanol. The data indicate that as the elastic character of the gels increases, Gc increases almost linearly with 1-propanol content: increasing 1-propanol content enhances the elasticity of the hydrogels over the frequency range investigated. The relaxation time of the hydrogels begins to increase precipitously above ca. 15 wt % 1-propanol, indicating that the rate of the shear-induced rearrangement of the 3D network is much slower than at 0-15 wt % 1-propanol. From the trend in the parameters in Figure 3 and in agreement with the entanglement density Fe data discussed above, we conclude that 1-propanol strengthens the 3D entangled networks of the gels. The driving forces causing the changes in Gc and τc can be understood by considering (1) hydrogen bonding (39) Bayarri, S.; Gonzalez-Tomas, L.; Costell, E. Food Hydrocolloids 2009, 23, 441–450.

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Figure 1. Frequency sweep curves for hydrogels containing 20 wt % 1-propanol: (A) PVA-borate system and (B) buffered PAA system. Storage modulus G0 (9), loss modulus G00 (b), and complex viscosity (0).

Figure 2. Removal of hydrogels by peeling them off of a glass surface by means of a spatula. (Left to right): 3 wt % PVA/0.6 wt % borax/20 wt % 1-propanol, 2 wt % PVA/0.4 wt % borax/20 wt % 1-propanol, and PAA gel (20 wt % 1-propanol).

Figure 3. Crossover parameters Gc (b) and τc (9) as a function of 1-propanol (1-PrOH) content. The vertical bars are standard deviations of six measurements.

(the hydrogen bonding index (HBI) is about 18 for 1-propanol and ca. 40 for water40), (2) hydrophobic interactions,41 (3) alcohol clustering,42 and (4) complex ion-water-alcohol structuring42 (boron-centered anions and sodium cations are present in the PVA-borate hydrogel networks). Separating the role of each of these in determining the strength of the PVA-borate hydrogels when 1-propanol is added will require additional studies. When additional 5 wt % 1-propanol was added dropwise to a sol containing 25 wt % 1-propanol and then the sol was cooled to room temperature, an unstable gel formed that became increasingly opaque and continued to expel clear liquid (syneresis) with time. The composition of the expelled liquid was determined from (40) Beerbower, A.; Kaye, L. A.; Pattison, D. A. Chem. Eng. 1967, 74, 118–128. (41) Franks, F. Water: A Comprehensive Treatise; Plenum Press: New York, 1975; Vol 4.  (42) Odriozola, G.; Schmitt, A.; Callejas-Fernandez, J.; Hidalgo-Alvarez, R. J. Colloid Interface Sci. 2007, 310, 471–480.

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its density assuming that no PVA or borate salts were present. Using solutions of water/1-propanol of known compositions as standards, it was concluded that the expelled liquid is much richer in water than the 70 wt % in the sol; 1-propanol was retained selectively by the gel network. Therefore, we can conclude that when up to 25 wt % 1-propanol is present it is not released and the gels are stable over periods of months; 1-propanol is within the PVA-borate hydrogel network. When the alcohol concentration is increased to above 25 wt %, both 1-propanol and water are released from the PVA-borate network, but more water is released proportionate to their concentrations. It appears that 1-propanol is held by the gel network more strongly than water. This observation is in agreement with the free water index data43 calculated from DSC measurements (Figure 4). The amount of freezable water decreases with increasing 1-propanol content. Together, these findings imply that 1-propanol acts as a waterstructure maker (a cosmotropic effect)44 and reinforces the water/ polymer gel network. The large decrease in the FWI (from 0.95 to 0.45) is a consequence of the large increase in bound water that is unable to freeze under the experimental conditions and therefore is retained strongly within the PVA-borate gel network. This effect is very desirable for gels used to clean the surfaces of easel paintings because more water is expected to reside at the gelpainting interface and the swelling of underlayers should be reduced. The mechanical spectra of the various gels containing different amounts of 1-propanol are plotted in Figure 5. To simplify the analyses of this information and to gain insight into the dynamics (43) Damasceni, A.; Dei, L.; Fratini, E.; Ridi, F.; Chen, S.-H.; Baglioni, P. J. Phys. Chem. B. 2002, 106, 11572–11578. (44) Jeruzalmi, D.; Steitz, T. A. J. Mol. Biol. 1997, 274, 748–756.

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Figure 4. Free water index (FWI) as a function of 1-propanol (1-PrOH) content.

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Figure 6. Normalized mechanical spectra at various 1-PrOH concentrations. G0 /Gc (closed symbols) and G’’/Gc (open symbols) data at 5 (squares), 10 (triangles), 15 (circles), 20 (diamonds), and 25 (hexagons) wt % 1-propanol. Gc is the crossover modulus, and ωc is the crossover frequency.

Figure 7. UV-vis absorption spectra of DR13 in water/ 1-PrOH mixtures. The numbers in the graph refer to the wt % of 1-PrOH. Figure 5. Mechanical spectra at various 1-PrOH concentrations. 0

G (closed symbols) and G’’ (open symbols) data at 5 (squares), 10 (triangles), 15 (circles), 20 (diamonds), and 25 (hexagons) wt % 1-propanol.

of the gels containing different amounts of 1-propanol, the mechanical spectra have been plotted in a frequency-normalized fashion36 (Figure 6). The observation that the normalized curves overlap at low 1-propanol concentrations indicates that the global time scale of the relaxation changes are nearly the same at all frequencies;36 see τc in Figure 3. At higher 1-propanol contents above 15 wt %, the overlap of the normalized curves is not complete; both the overall timescale of the relaxation process and the distribution of relaxation times change with 1-propanol concentration. Interestingly, there is good correspondence between the region where both the overall timescale and distribution of relaxation times change and the point at which the variation of the τc versus 1-propanol concentration becomes acute (Figure 3). These observations can be attributed to an increase in the degree of cross-linking in the PVA-borate polymer network induced by the alcohol molecules (Gc in Figure 3). Solubilization of a Hydrophobic Dye by Water/1-Propanol Dyes. The ability of 1-propanol/H2O mixtures to extract DR13, a hydrophobic red dye that is soluble in 1-propanol but not in H2O, has been determined to be a model for removing 8660 DOI: 10.1021/la804306w

hydrophobic materials with the PVA dyes from the surfaces of works of art. The ca. 400-500 μm films of DR13 on the interiors of glass tubes are comparable to the average thicknesses of patinas on easel paintings and served as a crude model for them. The saturation limit for DR13 in 25.5 wt % (ca. 4 M) 1-propanol in water is ca. 9.1 ppm.26 From the data in Figure 7 (and shown graphically in Figure 8), it is clear (and as expected) from the intensities of the absorption peak at 505-520 nm that the dye solubility increases with increasing 1-propanol content. Below 15 wt % 1-propanol, very little solubilization of DR13 is detected; between 20 and 30 wt %, a large increase in the solubilization is observed. No further increase is noted at alcohol concentrations above 25.5 wt % (ca. 4 M) because none of the solid DR13 remains. The results found here match those found by Bauduin et al. previously at ca. 4 M 1-propanol in water.26 On the basis of the results of Bauduin et al.,26 the solubility power of 1-propanol/water mixtures is linear as a function of concentration up to ca. 6 M 1-propanol (ca. 40% by weight), and it becomes exponential above this concentration. We have chosen to work near the linear region (our highest alcohol concentration was 8.6 M) because no more than 20 wt % 1-propanol in water can be used in the PVA-borate gels without syneresis. Whereas Bauduin et al. were interested in determining the maximum amount of dye that can be solubilized at different water/cosolvent compositions, we have designed our experiments to maintain the total amount of dye constant both below and Langmuir 2009, 25(15), 8656–8662

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Figure 8. Absorbances at λmax (9) and λmax values in nm (b) versus the 1-propanol content in water.

Figure 10. Wood panel by Ludovico Cardi detto il Cigoli (1559-1613). The box indicates the region where a very small cleaning test was carried out. This picture was collected after the cleaning test.

Figure 9. Crossover parameters (A) Gc and (B) τc as a function of cosolvent concentration in PVA-borate hydrogels: 1-propanol ((), 1-pentanol (2), 2-butanol (b), cyclohexanone (9), and propylene carbonate (f).

above the saturation value. The wavelengths of maximum absorbance are not affected by the alcohol content up to ca. 30 wt % (Figure 7), whereas a hypsochromic shift is observed for concentrations above this value (Figure 8). The wavelength shift is a result of strong interactions between 1-propanol and the hydrophobic dye that begin to manifest themselves spectroscopically above a threshold alcohol content. Langmuir 2009, 25(15), 8656–8662

Gels with Other Organic Liquid/Water Mixtures. The influence of adding liquids of different polarities to the hydrogels was also explored to determine their influence on the solubilization capacity. Figure 9 reports the rheological parameters for gels with all of the cosolvents studied. The choice of the five cosolvents is based on their known abilities to act as cleaning agents in painting conservation.25,27-29,45 For all of the cosolvents studied (apart from propylene carbonate), Gc increases with the amount of added cosolvent (Figure 9); the order of change induced is cyclohexanone > 1-pentanol > 1-butanol > 1-propanol. This trend is in agreement with the expected increase in the degree of cross-linking;36 these cosolvents make the PVA-borate gel network more robust. However, the addition of the very polar molecule PC leads to a slight decrease (45) Carretti, E.; Dei, L; Baglioni, P. Langmuir 2003, 19, 7867–7872.

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Article

Carretti et al.

(Figures 10 and 11). The patina was present only on some pale parts of the painting (Figure 11A): the other portions of the painting that were affected had been cleaned previously by traditional methods. The gel was applied with a spatula, allowed to remain in contact with the surface for 4 min, and then removed in one piece by exploiting the elasticity of the gel film. While the surface was still wet after the removal of the gel, some of the brown patina that remained but had lost its adhesion to the painting surface could be removed with cotton-wool flock (i.e., the common procedure employed with common gels or other cleaning agents). The remaining brown patina was removed after a second application of the gel. Thus, the gels allow selective, controlled cleaning of the surface. To the best of our knowledge, this test is the first in which an easel painting has been cleaned without using any additional solvent as an after-treatment. Figure 11. Magnified view of the boxed region in Figure 10 (A) before application of the gel and (B) after two applications of the gel as described in the text. The area where the gel was applied is indicated by the red dashed line.

in Gc (i.e., it breaks cross-links in the PVA-borate matrix). The trends in τc in the same range of cosolvent concentrations (Figure 7B) indicate a more complex behavior, although as expected the addition of cyclohexanone, 1-butanol, or 1-propanol causes an increase in the global time scale of the relaxation and the addition of propylene carbonate leads to almost no change. Unexpectedly, τc does not change when 1-pentanol is added. Further studies are needed to relate this complex behavior to molecular interactions between the cosolvents and the PVA-borate networks. Application to Cleaning an Easel Painting. On the basis of the DSC and rheological results obtained with the PVA-borate hydro/1-propanol gels and the solubilizing power of water/1-propanol mixtures, the elastic PVA-borate hydro/ 1-propanol gels should be useful cleaning agents for easel paintings and sculptures, with advantages over other gels in terms of their ease of removal. To this end, initial experiments were conducted with a PVA-borate hydrogel containing 20 wt % 1-propanol to remove varnishes (Dammar, Mastic, etc.) that are traditionally used in easel painting conservation. When glass was the test substrate, the gel succeeded in removing the varnishes completely. The first test on a surface with oil-based paintings was performed in cooperation with the International University of Art Foundation of Florence (Fondazione Universita Internazionale dell’Arte (UIA)) on a portion of a wood panel by Ludovico Cardi detto il Cigoli (1559-1613) that is currently a part of the collection of the Curia Museum and on exhibition at the Santo Stefano al Ponte Church in Florence. The painted surface had been altered by a thin brown patina from layers of oxidized varnishes used in past conservation treatments

8662 DOI: 10.1021/la804306w

Conclusions A new type of PVA-borate hydrogel with added organic liquids has been made and characterized. These materials, especially those with added 1-propanol, have been shown to have viscoelastic and polarity properties that are very different from those of the pure hydrogels. The effect of adding 1-propanol to the gelator network of PVA-borate is striking in that strong, elastic films can be made with small concentrations of the polymer. The ability of the PVA-borate water/1-propanol gels to dissolve a hydrophobic dye has been demonstrated and exploited to remove the oxidized varnish from the surface of a XVI-XVII century oil painting. The advantages of these gels over several others employed to clean works of art are clear: they require no after-washing to remove residues, their cleaning action is easily controlled, and they can be removed in one piece by peeling. The determination of the ability of related gels to clean the surfaces of a variety of other art objects is ongoing.46 Acknowledgment. We express our gratitude to Mrs. Paola Bracco, Kyoko Nakahara, and Dr. Caterina Matarrese for their assistance and for stimulating discussions and comments during the tests on the Cigoli’s panel. Thanks are due to the Soprintendenza Speciale per il Polo Museale Fiorentino (Dr. Maria Matilde Simari) and to Fondazione Universita’ Internazionale dell’Arte (Florence, Italy) for their cooperation in the application of the gels to the Cigoli’s panel. Financial support from the Ministero dell’ Universita e della Ricerca Scientifica (MIUR) Italy Fondi PRIN 2007-2008 and from the Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (CSGI, Florence, Italy) is gratefully acknowledged. R.G.W. and S.G. thank the U.S. National Science Foundation for its support of the portion of this research performed at Georgetown. (46) Berrie, B.; Weiss, R. G. Symposium on Productive Affinities: Successful Collaborations Between Museums and Academia; Art Institute of Chicago and Northwestern University, Chicago, IL, Oct 29-31, 2008.

Langmuir 2009, 25(15), 8656–8662