The layout of early film materials (Fig.1) include a thick, transparent and flexible Cellulose Nitrate (CN) base (c) coated with the film emulsion (a). The emulsion is the layer employed to record the image and, in already developed films, it consists of a colloidal suspension of dark silver particles and color dyes (if the film was colored) fixed in a matrix of photographic-grade gelatin(1). Sometimes, a thin intermediate adhesive or “subbing” layer (b) was applied to guarantee the adhesion between the emulsion and the polymeric base.
CN is an inorganic cellulose derivate where hydroxyl groups in the glucopyranose ring have been substituted by nitrate groups O-NO2.
Since 1889(2, 3), flexible polymeric films made of CN with a degree of substitution (DS) of around 2 were used as support for the first examples of cinematographic film.
Thanks to its low cost, CN was initially widely employed for producing film bases, but due to its high flammability, its use of was progressively reduced and then definitely abandoned in 1951(2, 3).
Cellulose nitrate photographic and cinematographic materials are known to be intrinsically unstable, mainly due to the degradation mechanisms triggered by thermal (Fig. 2), photocatalytic and hydrolytic loss of nitro substitutive groups of the CN base(4). This process occurs quickly under uncontrolled storage conditions, particularly unventilated environments showing high temperature and humidity.
The resulting degradation product, the NO2 gases, react with environmental water producing nitric and nitrous acids, which catalyze further loss of nitro groups in the CN polymer and the reduction of the molecular weight of the backbone.
Eventually, the base deforms, becomes frail and brittle, and crumbles to dust(5). To avoid the complete loss of the recorded images, their scanning and digitalization is a priority for cinematheques, libraries and other institutions safeguarding such audiovisual archives(6).
However, nitrate supports which have already underwent some degree of hydrolytic degradation of their bases can suffer from softening of their gelatin emulsions since the pH decreases to values lower than the isoelectric point of type B gelatin, where the gelatin molecule becomes positively charged, and the repulsion forces between positive charges slightly uncoil the gelatin molecule and facilitate its solubilization(7). Nguyen et al. have suggested also that NO2 species promote the hydrolysis of hardened (cross-linked) and unhardened photographic gelatins, lowering their molecular weight and their viscosity(8).
Photographic gelatin is most of the time alkaline or type-B gelatin, produced from the alkaline treatment of demineralized cattle bone, ossein(9). Ossein is mostly made up of type I collagen, an heterotrimer collagen formed by three polypeptide α-chains associated in a triple helix configuration(10). By treating parent collagen with an hydrated lime slurry, type B gelatin is produced, destroying the crosslinking between collagen(9–12).
Gelatin softening is a serious drawback, because upon becoming more fluid it can easily migrate laterally when it is pressured and adhere to any surface in contact with it. This often affects the back side of the subsequent coils of the same film (Fig. 3), causing the loss of images in the first coil and gelatin accumulation on the back of the second. The adhesion of convolutions, known as blocking, ultimately transforms the film into a solid unit which cannot be unrolled, reaching the so-called “hockey puck” state(5).
Therefore, to allow the digitalization of the film and to avoid subsequent blocking when the reel is stored, it becomes mandatory to remove gelatin accretions.
Traditional cleaning approaches to eliminate gelatin residues from the side of film rolls include mechanical removal with surgical scalpels, and the use of polar solvents, such as distilled water, Ethanol (EtOH) and Isopropyl Alcohol (IPOH). However, the use of alcohols results in a slow, ineffective cleaning, whereas water may be potentially dangerous if it accidentally leaks towards the front of the frame when cleaning a section of the base. Furthermore, the use of organic solvents presents different drawbacks, since they are flammable, and the excessive emissions of volatile solvents can harm the environment and can pose health risks to the operator upon extended unprotected exposure.
To overcome these drawbacks, we have proposed, tested and evaluated the performance of three Deep Eutectic Solvent (DES) formulations, providing green, unexpensive, easy-to-prepare and effective alternative for the cleaning of gelatin accretions from CN photographic bases.
DES have been previously employed for the dissolution of proteinaceous(13, 14) and other organic materials, but to the best of the authors’ knowledge have not been employed for the restoration of photographic negatives or cinematographic films. A different DES formulation has been previously applied in gel form to remove proteinaceous coatings in paintings(15).
Deep Eutectic Solvents, first defined by Abbot et al. in 2003(16), are mixtures of a Hydrogen Bond Acceptor (HBA), commonly a quaternary ammonium salt, with an Hydrogen Bond Donor (HBD), like an amide, amine, alcohol or carboxylic acid. Electrostatic charge delocalization (through hydrogen bonds and van der Waals interactions) between these two constituents lower the fusion point or glass transition temperature below that of the original components when both are present near a certain molar ratio(17, 18).
The precursors, such as Choline Chloride (ChCl), Betaine (B) and Urea (U), are biodegradable, environmentally friendly (being obtained from renewable sources), relatively cheap and non-toxic.
Choline chloride is regarded as a B-complex vitamin and is extracted from biomass; betaine is the trimethyl derivative of glycine and is obtained as a metabolic oxidation product of choline in different organisms (19). Betaine can be commercially retrieved by separation during sugar production from beets. Urea is the most commercialized nitrogenous fertilizer and is employed by mammals for processing nitrogen-containing compounds(20, 21).
Ethylene glycol (EG) instead, is a non-toxic compound commonly exploited as antifreeze, wetting and plasticizer agent in industrial processes(22).
By mixing choline chloride with ethylene glycol at a 1:2 molar proportion, a DES commonly called ethaline is obtained. This product has been widely studied due to its low viscosity and therefore high solubilizing power. Through computer modelling, it has been found that the HBD and HBA in this DES formulation form a supramolecular cage-like arrangement where the Cl− anion becomes the central element interacting with five hydroxyl groups, one from the choline cation and four from both EG molecules(23). Ethaline has been reported as capable of extracting collagen peptides from cod skins without destroying the peptide bonds in the process and also of being able to solubilize singular alanine, glutamic acid, lysine, glycine and hydroxyproline amino acids without creating new chemical bonds with them, so the solubilization process is probably based on intermolecular hydrogen bond formation between Cl− and the amino and carboxyl groups(13).
When urea is used as HBD, it has been observed that relatively basic DES are obtained, owing to the presence of the amino group, and to the fact that a small fraction of ammonia is released through urea decomposition during DES preparation, rising the pH of the mixture(24).
The DES formed by mixing betaine and urea in a 1:2 ratio worked well for the extraction of bovine serum albumin protein, showing a low glass transition temperature. After FTIR studies, it was suggested that in this DES formulation not only hydrogen bonds but also Coulomb interactions are formed between HBD and HBA, so its intrinsic interactions and structure differ from those of choline chloride-based DES(25).