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

Florence Gorel

Résumés

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 de ce système sont adaptées à un usage en restauration et sont stables pendant plusieurs jours. Les gels permettent la solubilisation du matériau à l’aide de faible quantité de solvant, l’empêchent de créer des auréoles, permettent le contrôle de l’évaporation des solvants et ne laissent pas de résidus de gel dans les pores.

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Notes de la rédaction

Institut National du Patrimoine – Contact : Patricia Vergez

Texte intégral

Introduction

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

2For 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.

3This research focuses on the removal of hydrophobic materials on a porous and hydrophilic surface. 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 the cleaning of porous paint layers. The purpose of the research was to improve the strength, the capability to retain the solvents and the efficacy of the agar gel loaded with micro-emulsion to remove wax, as well as to prevent the redistribution of this hydrophobic material into the porous layer.

Materials and methods

Micro-emulsion

4Materials: Reverse Osmosis water, sodium dodecylsulphate (SDS, Fluka, purum ≥ 96 %), pentan-1-ol (Fluka, ≥ 99 %), Petroleum ether (Fishersci, 100/140 °).

  • 1 Holmberg, Jönsson, Kronberg and Lindman, Surfactants and Polymers in Aqueous Solution, John Wiley a (...)

5Micro-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 the emulsions, the micro-emulsions and the micelles.

  • 2 Borgioli L., Caminati G., Gabrielli G., Ferroni E., “Removal of hydrophobic impurities from picto (...)
  • 3 Carretti E., Salvadori B., Baglioni P. and Dei L., “Microemulsions and micellar solutions for clean (...)

6The 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 paintings includes 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.

7The 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 avoided by the hydrophilic barrier of the continuous phase since dissolution takes place within the micelles aggregation.

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

  • 4 Carretti E., Salvadori B., Baglioni P. and Dei L., Op. cit., p. 129.

8Oil-in-water micro-emulsions contain a continuous phase (water) and a dispersed phase (aliphatic hydrocarbon) presents within the core of micelle aggregates of surfactant4. The content of surfactant is much higher than the Critical Micelle Concentration.  It causes the spontaneous aggregation of these molecules and reduces the interfacial tension between oil and water. A micro-emulsion can contain a co-surfactant in order to maximize the concentration of the dispersed phase in the system and lower the interfacial tension between water and oil droplets

Fig. 1 Oil-in-water micro-emulsion

Fig. 1 Oil-in-water micro-emulsion

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

Crédits: Holmberg, 1999

  • 5 Tondre C., “Dynamic Processes in Microemulsions”, Dynamics of Surfactant Self-Assemblies. Micelles, (...)
  • 6 Bonini M., Lenz S., Giorgi R., and Baglioni P., “Nanomagnetic Sponges for the Cleaning of Works of (...)

9The dispersed phase forms micro-droplets in the continuous phase, coated by a mixed film of surfactant and co-surfactant. The size and shape of the micelles are a function of the concentration 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.

  • 7 Zana R.,Dynamics of Surfactants self-assemblies. Micelles, Microemulsions, Vesicles and Lyotropic P (...)

10Addition 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 molecules are located at the head at the surface of the micelle whereas the tail is penetrating the micelle core in the case of an oil-in-water micro-emulsion.

  • 8 Borgioli L., Caminati G., Gabrielli G., Ferroni E.,Op. cit., p. 70.
  • 9 Tondre C.,Op. cit.

11In 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 hydrophobic sites exhibited by micro-emulsion explaines the high dissolving capacity of these systems. Moreover micro-emulsion systems are strongly dynamic which means that the components can show different kinds of exchange processes. The dynamic behavior of micro-emulsions controls the exchange of solubility between droplets, and it has a strong impact on the chemical reactivity of such systems9.

  • 10 Correspondence with K. Holmberg.

12For 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 water

Surfactant (SDS)

Co-surfactant            (1-Pentanol)

Dispersed phase

(Petroleum ether)

µE1

85

4

6

5

Preparation of the gel

  • 11 Clark A. H. and Ross-Murphy S. B., “Structural and Mechanical Properties of Biopolymer Gels”, Advan (...)

13The 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 principal gelling component is Agarose based on a disaccharide repeat unit

Fig. 2 Disaccharide repeat unit of Agarose

Fig. 2 Disaccharide repeat unit of Agarose

Credits: Clark, 1987

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

Fig. 3 Model for Agarose network formation

Fig. 3 Model for Agarose network formation

Credits: Clark, 1987

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

16In this research, in order to test the faculty of agar gel to be loaded with micro-emulsion, five agar gels were be loaded with oil-in-water micro-emulsion and their properties were compared with these of an agar reference gel. Agar gels of concentration 2 % (w/v) were prepared by dissolving agar in Reverse Osmosis water. Each solution was heated over a hot plate in a Pyrex beaker placed in a bain-marie at 90 °C during 5 minutes. Then the agar dispersion was placed in an ice bath to rapidly cool down until a temperature of 40°C was reached. After that, 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 the same procedure. The composition of the gels is given in Table 2. To evaluate the maximum amount of micro-emulsion that could be loaded in the gel structure before detecting any phase separation, 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

Gel3

2

68

30

Gel4

2

58

40

Preparation of the samples

17The 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 materials into the structure. Bees wax was chosen due to its common use in conservation treatments and its solubility in hydrocarbon solvents. The wax was heated and applied on the samples with a brush and then with a spatula in order to increase its penetration into the porous matrix. The excess of wax was removed with only a thin layer of wax remaining on the samples.  The cleaning 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 the samples. Each gel was applied during 90 minutes on the samples.

Results

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

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

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

Credits: Fl. Gorel

19For 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 in excess 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 volume and a good strength. Agar-micro-emulsion gels, unlike pure one, are partially opaque.

  • 12 Bonini M., Lenz S., Giorgi R. and Baglioni P.,Op. cit., p. 8684.

20The capability of the gel to retain the micro-emulsion in its structure was studied using the dehydratation curves. Three agar-oil-in-water micro-emulsion gels (1, 2 and 3), the agar reference 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 retention properties of the agar

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

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

Crédits: Fl. Gorel

21For 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

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

Conclusion

22Introduction of oil-in-water micro-emulsion in agar gel when the gel is already formed do not prevent 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 extracting efficiency of micro-emulsion.

23The 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 on the object during a long time. Moreover, agar gel allows the micro-emulsions to flow on the porous surface and to wet it but maintains the micro-emulsion in its structure and prevents the formation of rings. Evaporation of the solvents is slowed down and the gels can be used for a long period.

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Notes

1 Holmberg, Jönsson, Kronberg and Lindman, Surfactants and Polymers in Aqueous Solution, John Wiley and Sons, New York, 1999, pp. 365-380.

2 Borgioli L., Caminati G., Gabrielli G., Ferroni E., “Removal of hydrophobic impurities from pictorial 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 for cleaning 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 in water 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.

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Table des illustrations

Titre Fig. 1 Oil-in-water micro-emulsion
Légende Schematic representation of the micellar configuration into oil-in-water  micro-emulsion.
Crédits Crédits: Holmberg, 1999
URL http://ceroart.revues.org/docannexe/image/1827/img-1.jpg
Fichier image/jpeg, 108k
Titre Fig. 2 Disaccharide repeat unit of Agarose
Crédits Credits: Clark, 1987
URL http://ceroart.revues.org/docannexe/image/1827/img-2.jpg
Fichier image/jpeg, 32k
Titre Fig. 3 Model for Agarose network formation
Crédits Credits: Clark, 1987
URL http://ceroart.revues.org/docannexe/image/1827/img-3.jpg
Fichier image/jpeg, 48k
Titre Fig. 4 Partial opacity in the agar-micro-emulsion gel with respect to the agar-gel transparency
Crédits Credits: Fl. Gorel
URL http://ceroart.revues.org/docannexe/image/1827/img-4.jpg
Fichier image/jpeg, 80k
Titre Fig. 5 Dehydratation curves of agar-micro-emulsion gel and reference gel
Crédits Crédits: Fl. Gorel
URL http://ceroart.revues.org/docannexe/image/1827/img-5.jpg
Fichier image/jpeg, 32k
Titre Fig. 6 Observation under visible light and under UV light of the samples without wax, with wax and after application of Gel2 and Gel3
Crédits Credits: G. Vanneste
URL http://ceroart.revues.org/docannexe/image/1827/img-6.jpg
Fichier image/jpeg, 1,0M
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Florence Gorel, « Assessment of agar gel loaded with micro-emulsion for the cleaning of porous surfaces », CeROArt [En ligne], 1 | 2010, mis en ligne le 17 novembre 2010, consulté le 26 mai 2017. URL : http://ceroart.revues.org/1827

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Auteur

Florence Gorel

Diplômée de l’Institut national du patrimoine en restauration de peinture, Florence Gorel se passionne 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 nanomagnétiques. florence.gorel@gmail.com

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Licence Creative Commons
CeROArt – Conservation, exposition, restauration d'objets d'arts est mis à disposition selon les termes de la licence Creative Commons Attribution - Pas d'Utilisation Commerciale - Pas de Modification 4.0 International.

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