gorel, f. agar gel micro emulsion for cleaning porous surface. 2010

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CeROArtNuméro 6 (2010)Horizons

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Florence Gorel

Assessment of agar gel loaded withmicro-emulsion for the cleaning of porous surfaces

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Référence électroniqueFlorence Gorel, « Assessment of agar gel loaded with micro-emulsion for the cleaning of porous surfaces », CeROArt [En ligne], 6 | 2010, mis en ligne le 17 novembre 2010. URL : http://ceroart.revues.org/index1827.htmlDOI : en cours d'attribution

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Assessment of agar gel loaded with micro-emulsion for the cleaning of porous surfaces 2

CeROArt, 6 | 2010

Florence Gorel

Assessment of agar gel loaded withmicro-emulsion for the cleaning of porous

surfacesIntroduction

1 During my training at the Wereldmuseum of Rotterdam, I had the opportunity to experimentwith some mixtures made with agar and micro-emulsion. These experiments are reported inthis paper.

2 For the cleaning purposes, the nature of porous surfaces forced the conservators to look at solubility, capillary action and evaporation rate of their materials and tools. The use of pure organic solvents can have the undesirable effect, typical of the surface considered, of redistributing the dissolved material further within the porous matrix.

3 This research focuses on the removal of hydrophobic materials on a porous and hydrophilicsurface. We studied an agar gel loaded with a micro-emulsion. The agar gel was chosen for

its common use in cleaning and its safety, and the micro-emulsion for the good results in thecleaning of porous paint layers. The purpose of the research was to improve the strength, thecapability to retain the solvents and the efficacy of the agar gel loaded with micro-emulsionto remove wax, as well as to prevent the redistribution of this hydrophobic material into theporous layer.

Materials and methods

Micro-emulsion4 Materials: Reverse Osmosis water, sodium dodecylsulphate (SDS, Fluka, purum ≥ 96 %),

pentan-1-ol (Fluka, ≥ 99 %), Petroleum ether (Fishersci, 100/140 °).5 Micro-emulsions are microheterogeneous liquid systems, which means structured systems

of colloidal dimensions formed when amphipathic substances are dissolved in solvents,

in appropriate concentrations1. Among these microheterogeneous systems we may list theemulsions, the micro-emulsions and the micelles.

6 The first use of a micro-emulsion in the conservation field is mentioned by L. Borgioli for the

cleaning of the Masaccio’s frescoes in the Brancacci Chapel2. The cleaning of these paintingsincludes the removal of hydrophobic impurities (wax) using micro-emulsions. The Micro-emulsions were used later by E. Carretti for the removal of deteriorated organic materials, oil-

and acrylic-based materials from mural paintings3.7 The most innovative aspects of the used of micro-emulsion are:

• A higher extracting efficiency of hydrophobic materials than emulsions.•

The redistribution of dissolved hydrophobic materials into the porous surface is avoidedby the hydrophilic barrier of the continuous phase since dissolution takes place withinthe micelles aggregation.

• Micro-emulsion systems are thermodynamically stable throughout a wide range of environmental conditions.

8 Oil-in-water micro-emulsions contain a continuous phase (water) and a dispersed phase

(aliphatic hydrocarbon) presents within the core of micelle aggregates of surfactant4. Thecontent of surfactant is much higher than the Critical Micelle Concentration. It causes thespontaneous aggregation of these molecules and reduces the interfacial tension between oil andwater. A micro-emulsion can contain a co-surfactant in order to maximize the concentration




CeROArt, 6 | 2010

of the dispersed phase in the system and lower the interfacial tension between water and oildroplets

Fig. 1 Oil-in-water micro-emulsion

Schematic representation of the micellar configuration into oil-in-water micro-emulsion.

Crédits: Holmberg, 1999

9 The dispersed phase forms micro-droplets in the continuous phase, coated by a mixed filmof surfactant and co-surfactant. The size and shape of the micelles are a function of theconcentration and structure (length and volume of the hydrophobic tail) of the surfactant;

the droplets diameters are typically from 5 to 50 nm5. In the case of SDS as the surfactant,the micelles are spherical. The micelle size of SDS/Pentanol micro-emulsion, obtained from

small-angle X-ray scattering, is about 4 nm6.10 Addition of co-surfactant is not needed to form micro-emulsion, although it is used to simplify

the work. The solvent Pentanol, a medium-chain length alcohol, changes the size of the

micelles: smaller with a low concentration, bigger with a higher one7. The Pentanol moleculesare located at the head at the surface of the micelle whereas the tail is penetrating the micellecore in the case of an oil-in-water micro-emulsion.

11 In the case of oil-in-water micro-emulsions, the dissolution sites may be identified in several

regions8: between the hydrophilic groups and the first carbon atoms of the alkyl chains of surfactants – in this region large aromatic molecules and long chain alcohol can be dissolved–, within the core of the droplets – dissolution of aliphatic hydrocarbons in the case of oil-in-water micro-emulsion –, and in the large hydrocarbon volume. The type of hydrophobicsites exhibited by micro-emulsion explaines the high dissolving capacity of these systems.Moreover micro-emulsion systems are strongly dynamic which means that the componentscan show different kinds of exchange processes. The dynamic behavior of micro-emulsionscontrols the exchange of solubility between droplets, and it has a strong impact on the chemical

reactivity of such systems9.




CeROArt, 6 | 2010

12 For conservation purposes, micro-emulsion with SDS (NaC12H25SO4), as an anionic surfactant,

and Pentanol (C5H12O), as the co-surfactant, were used. The micro-emulsion is prepared using

the amount of materials indicated as in Table 1, the compositions are given in % of weight10.The micro-emulsion is stable and transparent.

Table 1 Composition of the oil-in-water micro-emulsion

Continuous phase

RO waterSurfactant (SDS)

Co-surfactant(1-

Pentanol)

Dispersed phase

(Petroleum ether)

µE1 85 4 6 5

Preparation of the gel13 The name “agar” refers to a complex mixture of polysaccharide components, which may be

derived from certain genera of the Rhodophyceae group of red sea weeds11. The principalgelling component is Agarose based on a disaccharide repeat unit

Fig. 2 Disaccharide repeat unit of Agarose

Credits: Clark, 1987

14 The thermoreversible gelation of Agarose occurs when hot Agarose solutions are cooled belowabout 40°C. The Agarose network structure involves a double-helix formation

Fig. 3 Model for Agarose network formation

Credits: Clark, 1987

15 The agar gel is known to present large pore sizes which may allow the microemulsion tomigrate to the surface of the gel that is in contact with the porous surface.




CeROArt, 6 | 2010

16 In this research, in order to test the faculty of agar gel to be loaded with micro-emulsion, fiveagar gels were be loaded with oil-in-water micro-emulsion and their properties were comparedwith these of an agar reference gel. Agar gels of concentration 2 % (w/v) were prepared bydissolving agar in Reverse Osmosis water. Each solution was heated over a hot plate in aPyrex beaker placed in a bain-marie at 90 °C during 5 minutes. Then the agar dispersion wasplaced in an ice bath to rapidly cool down until a temperature of 40°C was reached. Afterthat, the micro-emulsion was gradually added to the agar gel and the whole preparation was

placed in the fridge. The agar reference gel without micro-emulsion was prepared using thesame procedure. The composition of the gels is given in Table 2. To evaluate the maximumamount of micro-emulsion that could be loaded in the gel structure before detecting any phaseseparation, several agar gels were prepared (by mixing) with different concentration of micro-emulsion from 10 to 40 % w/v.

Table 2 Composition of agar gels (% weight)

Agar RO water μE

Gelref 2 98 0

Gel1 2 88 10

Gel2 2 78 20

Gel

3 2 68 30

Gel4 2 58 40

Preparation of the samples

17 The agar-micro-emulsion gels were used to clean the surface of wood samples (1,5 x 5 x 0,5

cm3). This material was selected because of its porosity, allowing the trapping of materialsinto the structure. Bees wax was chosen due to its common use in conservation treatments andits solubility in hydrocarbon solvents. The wax was heated and applied on the samples with abrush and then with a spatula in order to increase its penetration into the porous matrix. Theexcess of wax was removed with only a thin layer of wax remaining on the samples. Thecleaning test was performed by direct application of the reference gel and gels 1, 2, and 3

onto the area to be cleaned. The gels were stored during 24 hours before being applied on thesamples. Each gel was applied during 90 minutes on the samples.

Results

18 After adding the first drops of micro-emulsions in agar, the gels become milky and show ahigh viscosity




CeROArt, 6 | 2010

Fig. 4 Partial opacity in the agar-micro-emulsion gel with respect to the agar-geltransparency

Credits: Fl. Gorel

19 For the agar-micro-emulsion gels, the gelation process occurs few minutes after the addition

of micro-emulsion. Gels 1 and 2 are homogeneous. Gel 3 shows a small quantity of solvent inexcess while the agar gel and the micro-emulsion are fully separated in gel 4. After 1 day of storage, the gels 1, 2 and 3 exhibit a small amount of solvent in excess, a change of volumeand a good strength. Agar-micro-emulsion gels, unlike pure one, are partially opaque.

20 The capability of the gel to retain the micro-emulsion in its structure was studied usingthe dehydratation curves. Three agar-oil-in-water micro-emulsion gels (1, 2 and 3), the agarreference gel and the micro-emulsion were stored at ambient conditions (temperature: 25°C,

± 1°C, and relative humidity: 46%, ± 2%), and the evolution of their weights was measured12.The Figure 5 shows no differences between the reference and the agar-oil-in-water micro-emulsion gels, suggesting that the presence of the micro-emulsion does not affect the retentionproperties of the agar




CeROArt, 6 | 2010

Fig. 5 Dehydratation curves of agar-micro-emulsion gel and reference gel

Crédits: Fl. Gorel

21 For cleaning, after 90 minutes of time application of gels 1, 2 and 3, the surface seems to be

partially free of wax. This observation was confirmed by the UV inspection

Fig. 6 Observation under visible light and under UV light of the samples without wax, with

wax and after application of Gel2 and Gel3

Credits: G. Vanneste




CeROArt, 6 | 2010

Conclusion

22 Introduction of oil-in-water micro-emulsion in agar gel when the gel is already formed do notprevent the gelation process of agar. The gel network is able to maintain its structure until 1,5% weight of Petroleum ether. This is sufficient to dissolve bees wax due to the high extractingefficiency of micro-emulsion.

23 The physical properties of the Agar gels are good enough for a conservation work when the

concentrations of micro-emulsion are between 10 and 30% but they should be applied onthe object during a long time. Moreover, agar gel allows the micro-emulsions to flow on theporous surface and to wet it but maintains the micro-emulsion in its structure and preventsthe formation of rings. Evaporation of the solvents is slowed down and the gels can be usedfor a long period.

Notes

1 HOLMBERG, JÖNSSON, KRONBERG AND LINDMAN, Surfactants and Polymers in Aqueous Solution, JohnWiley and Sons, New York, 1999, pp. 365-380.

2 BORGIOLI L., CAMINATI G., GABRIELLI G., FERRONI E., “Removal of hydrophobic impurities frompictorial surfaces by means of heterogeneous systems”; Science and Technology for Cultural Heritage,

4 (2), 1995, pp 67-74.3 CARRETTI E., SALVADORI B., BAGLIONI P. and DEI L., “Microemulsions and micellar solutions forcleaning wall painting surfaces”, Studies in Conservation, 50 (2), 2005, pp 128-136.

4 CARRETTI E., SALVADORI B., BAGLIONI P. AND DEI L., Op. cit., p. 129.

5 TONDRE C., “Dynamic Processes in Microemulsions”, Dynamics of Surfactant Self-Assemblies.

Micelles, Microemulsions, Vesicles and Lyotropic Phases , vol. 125, Boca raton, New York, 2005.

6 BONINI M., LENZ S., GIORGI R., AND BAGLIONI P., “Nanomagnetic Sponges for the Cleaning of Works of Art”, Langmuir, 23, 2007, p. 8683, the droplet size of micro-emulsion of Nitrodiluente and Xylene inwater with SDS and Pentanol was obtained by small-angle X-ray scattering.

7 ZANA R., Dynamics of Surfactants self-assemblies. Micelles, Microemulsions, Vesicles and Lyotropic

Phases, vol. 125, Boca raton, New York, 2005.

8 BORGIOLI L., CAMINATI G., GABRIELLI G., FERRONI E.,Op. cit., p. 70.

9 TONDRE C.,Op. cit.

10 Correspondence with K. Holmberg.

11 CLARK A. H. AND ROSS-MURPHY S. B., “Structural and Mechanical Properties of Biopolymer Gels”, Advances in Polymer Science, 83, 57, Springer-Verlag, Berlin, 1987, pp. 122-125.

12 BONINI M., LENZ S., GIORGI R. AND BAGLIONI P.,Op. cit., p. 8684.

Pour citer cet article

Référence électroniqueFlorence Gorel, « Assessment of agar gel loaded with micro-emulsion for the cleaning of poroussurfaces », CeROArt [En ligne], 6 | 2010, mis en ligne le 17 novembre 2010. URL : http://

ceroart.revues.org/index1827.html

Florence Gorel

Diplômée de l’Institut national du patrimoine en restauration de peinture, Florence Gorel sepassionne depuis quelques années pour la peinture asiatique et le nettoyage des surfaces poreuses.Ses recherches l’ont amenée à étudier les microémulsions, les gels et les éponges nanomagné[email protected]

Droits d'auteur




CeROArt, 6 | 2010

© Tous droits réservés

Résumé / Abstract

Le système composé d’un gel d’agar-agar et d’une microémulsion présente plusieurs qualités

pour extraire des matériaux hydrophobes de couches poreuses. Les propriétés rhéologiques dece système sont adaptées à un usage en restauration et sont stables pendant plusieurs jours. Lesgels permettent la solubilisation du matériau à l’aide de faible quantité de solvant, l’empêchentde créer des auréoles, permettent le contrôle de l’évaporation des solvants et ne laissent pasde résidus de gel dans les pores. Mots clés : agar-agar, auréole, résidu, gel, surface poreuse, nettoyage, microémulsion

Agar gel loaded with micro-emulsion could be used to extract lipophilic materials from poroussurfaces. The physical properties of the gels are good enough for a conservation work. Theyallow the micro-emulsion to flow on the porous surface and to wet it but maintain the micro-emulsion in its structure and prevent the formation of rings. The evaporation of the solvents

is slowed down and the gels can be used during a long period. Keywords : ring, gel, residue, porous surface, cleaning, micro-emulsion, agar

ndlr : Institut National du Patrimoine – Contact : Patricia Vergez

gorel, f. agar gel micro emulsion for cleaning porous surface. 2010

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