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Applied Physics AMaterials Science & Processing ISSN 0947-8396Volume 104Number 1 Appl. Phys. A (2011)104:401-406DOI 10.1007/s00339-010-6170-6

The study of cyclododecane as atemporary coating for marble by NMRprofilometry and FTIR reflectancespectroscopies

C. Anselmi, F. Presciutti, B. Doherty,B. G. Brunetti, A. Sgamellotti & C. Miliani

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Appl Phys A (2011) 104:401–406DOI 10.1007/s00339-010-6170-6

The study of cyclododecane as a temporary coating for marbleby NMR profilometry and FTIR reflectance spectroscopies

C. Anselmi · F. Presciutti · B. Doherty · B.G. Brunetti ·A. Sgamellotti · C. Miliani

Received: 5 March 2010 / Accepted: 22 November 2010 / Published online: 4 January 2011© Springer-Verlag 2011

Abstract This contribution focuses on an analytical eval-uation of the use of cyclododecane (CDD) (C12H24) as atemporary protective coating for non-porous stone materialsof cultural heritage interest.

A facile solvent spray application technique for the pro-duction of an adherent continuous film has been assessed.The criterion for monitoring the sublimation of the cyclodo-decane film on marble has been established through the useof non-invasive analytical techniques so as to avoid any in-teraction with the process under study, where results serveto integrate and enhance knowledge into the use of cyclodo-decane in this discipline. Research is directed towards test-ing the applicability of portable infrared reflectance spec-troscopy and nuclear magnetic resonance profilometry sys-tems to follow the in-situ behavior of temporary consoli-dants. In particular, the coupling of two spectroscopic tech-niques such as IR and NMR has been possible, enabling thedescriptions of both the formation of the film and its kineticsof sublimation.

C. Anselmi · B.G. Brunetti · A. Sgamellotti · C. MilianiSMAArt c/o Dipartimento di Chimica, Università degli studidi Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy

F. Presciutti (�) · B. DohertyINSTM, SMAArt c/o Dipartimento di Chimica, Università deglistudi di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italye-mail: federica@thch.unipg.itFax: +39-075-5855638

B.G. Brunetti · A. Sgamellotti · C. MilianiCNR-ISTM (Istituto di Scienze e Tecnologie Molecolari) c/oDipartimento di Chimica, Università degli studi di Perugia,Via Elce di Sotto, 8, 06123 Perugia, Italy

1 Introduction

Cyclododecane (CDD) is a volatile cyclic alkane (C12H24)

that observes a waxy appearance at room temperature. Itis characterized by a low melting point (60°C) [1], and alow enthalpy of sublimation (�H ≈ 76 kJ/mol at 25°C) [2],which permits its complete sublimation at room temper-ature. Its physico-chemical properties include good film-forming capabilities, insolubility in water, solubility in or-ganic solvents, and a low toxicity [3], which render thecompound particularly useful in the field of cultural her-itage. Literature describes the ample use of cyclododecaneas a temporary consolidant, sealant and hydrophobic protec-tive coating, playing a role in the temporary consolidationof weak or friable materials (paintings, ceramics, papers,stones, textiles) [4–7], which in cases of emergency requireimmediate intervention or transportation to undergo furtherrestoration steps [8, 9]. In such applications, much attentionhas to be paid to the fact that CDD must have completelysublimed before further treatment so as to avoid any stressor cracking of the original materials. This is the principalreason that prompted the authors over the years to measurethe sublimation rate of CDD through a range of techniquesand laboratory conditions, namely by gravimetric, gas chro-matographic and cryo-SEM measurements [7, 10, 11].

This research has been particularly oriented towards thesetting-up of a new methodology for investigating CDD be-havior from its application method and resulting film for-mation to subsequent sublimation process. The employ-ment of non-invasive and non-contact analytical techniques,namely fibre-optic infrared reflectance spectroscopy and nu-clear magnetic resonance profilometry for analyses on Car-rara marble samples, ensures that no interaction occurs withthe CDD film or influence on its rate of sublimation. Theinfrared spectroscopic study by fibre optic mid-FTIR spec-

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trometer extending to the NIR range is utilized to character-ize the fundamental modes, as well as overtone and combi-nation bands of the resulting cyclododecane film, thus fol-lowing solvent evaporation and the sublimation kinetic ofthe film. Analyses by Profile NMR-MOUSE [12], consist-ing of a 1H NMR relaxometer, made it possible to estimatethe thickness of formed CDD layer, and its decrease as afunction of time due to its sublimation.

The value of non-invasive on-site monitoring of conser-vation treatments has already been demonstrated in the casesof cleaning easel and mural paintings [13–15]. In fact it isgenerally observed that the behavior of a conservation treat-ment may be unique for every artwork and hardly repro-ducible in the laboratory. Both fibre-optic mid-FTIR spec-trometer and 1H NMR relaxometer here used are portableequipment, thus they can allow the sublimation process tobe followed directly on the object to be treated giving infor-mation on the actual behavior of CDD in the specific con-servation condition of an artwork.

2 Experimental

2.1 Instrumentation

2.1.1 Infrared spectroscopy

Micro-FTIR analyses were carried out by a JASCO FTIR470-plus spectrophotometer, equipped with a MCT detectorand an IRT-30 optical microscope. Measurements were per-formed in reflectance geometry using a Cassegrain 16× ob-jective. The total reflectivity is measured using the spectrumfrom an aluminum mirror plate for background correction.The spectra of the pure solvents and of their solution withstandard cyclododecane are primarily characterized using ahigh reflective sample holder in order to take advantage ofthe transflection effect [16].

Non-invasive infrared investigations have been carriedout on CDD treated Carrara marble samples with a com-pact portable FT-IR JASCO VIR 9500 spectrophotometer(50 × 50 × 50 cm3;35 kg weight), equipped with a fibre-optic extension. The spectral range considered is 6000–900 cm−1 with a resolution of 4 cm−1. The fibre-optic probewas kept perpendicular to the marble surface (0◦/0◦ geome-try) at a distance of about 2–4 mm. The investigated samplearea, as determined by the probe diameter, is about 12 mm2,where four measurements within a central quadrant in eachmarble sample were considered. The total reflectivity, R, dueto the combined diffuse and specular components, is mea-sured using the spectrum from an aluminum mirror platefor background correction. The spectrum intensity was de-fined as the pseudo absorbance A′ as A′ = log(1/R). Whilediffuse reflection can cause only slight shifts in absorption

band position, as compared to transmission spectra, spec-ular diffusion may cause derivative or inverted band shape(reststrahlen effect) [17].

All data have been treated with a constant baseline. Thearea of the pseudo-absorbance peaks given by the C–H over-tone bands at 5680 and 6810 cm−1, which have proved re-liable for monitoring sublimation, have been calculated andthen plotted as a function of time for each CDD film. Eachpoint in the resulting plot stems from the average of fourmeasurements over an area of 20 × 20 mm2, correspondingto the same criteria as the effectuated NMR measurements.The error is given by the calculated standard deviation.

2.1.2 Profile NMR-MOUSE

Profile NMR-MOUSE® [12] is a portable and non-invasive1H NMR sensor equipped with a single-sided permanentmagnet arrangement generating a uniform static magneticfield gradient suitable for high-resolution sample profilingup to depths of about 5 mm from objects placed nearby. Theworking frequency is 17.6 MHz. The NMR signal derivesfrom a thin slice centered 5 mm above the device parallel toits surface, where the field strength is 0.411 T and the gradi-ent is 20 T/m along the depth direction. The slice volume ex-tends about 10 × 10 mm2 along the lateral directions, a sizethat is determined by the geometry of the radio-frequencycoil of the sensor, while the thickness in the depth directiondepends on the setting of the measurement parameters andcan range from 180 to 2.3 µm.

The sensor is fixed on a lift, which can be positionedwith a precision of 10 µm to scan the profile by shifting thesensitive slice through the object. For each slice the trans-verse magnetization decay is collected with a multi-echotrain. The measured signal is reduced to a T2-weighted pro-ton density within the sensitive slice by partial integrationof the echo envelope. By default, the recycle delay is chosenlarge enough to avoid T1 weighting. The parameters fixedfor each profiles are the following: echo time = 0.052 ms,relaxation delay = 0.2 s, and 256 scans. The depth resolu-tion used was of 40 µm. The sensitive slice is moved throughthe object by steps of 30 µm guaranteeing a small overlap-ping between the successive slices. The thickness of the lay-ers was determined by measuring the distance from the po-sition of the half maximum amplitude of the first edge tothe equivalent point of the second edge. The error in the de-termination of the thickness was measured as the standarddeviation calculated from five measurements performed onthe same spot [18].

2.1.3 Optical microscope

A Leica DMR Optical Microscope equipped with a Digi-tal Camera Leica DC 300 has been used. Images have been

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The study of cyclododecane as a temporary coating for marble by NMR profilometry and FTIR reflectance 403

Fig. 1 Optical micrographs ofapplicative methods for CDD:(a) spray and (b) brush,respective magnification of400×

captured using polarized visible light to observe superfi-cial morphology changes from film formation through tocomplete sublimation. Relevant magnification is highlightedwhere necessary.

2.1.4 Sample preparation

All tests were performed on Carrara marble (porosity ∼0%).Stone samples with dimensions 5 × 5 × 2 cm3 were previ-ously polished using a graded silicon carbide paper. Non-polar solvents were used as purchased for the CDD solu-tion: cyclohexane (b.p. 86°C) and n-heptane (b.p. 98°C).The chosen method for the application solutions on the stonesurface consists in utilizing 1/1 weight/volume CDD in sol-vent, with gentle heating and accurate spraying utilizing anatomizer over the entire available surface, in order to specifyan easily reproducible method.

3 Results and discussion

3.1 Spectroscopic and microscopic characterization ofCDD and solvents employed for film formation

Cyclododecane, also termed a “volatile binder” [3], can beapplied as a coating on stone surfaces in a molten stateor dissolved in non-polar solvents such as saturated, aro-matic, or halogenated hydrocarbons or petroleum ethers. Ithas been observed that the film conforms to the topographyof the substrate to which it is applied [4], while its charac-teristics, in general, may vary depending on the applicationmethods chosen, the porosity of the substrate, and the rate offilm formation [19]. The quality of applications can be mod-ulated by the selection of opportune low-toxic non-polar sol-vents. In this work, the preferred solvents of use were cyclo-hexane and n-heptane, whilst the modality of a spray atom-ization of the cyclododecane-solvent solution was based onsimple hydraulic pressure without using any carrier gas.

The CDD application technique has been reviewed bycomparison of feasible techniques through spray and brush

Fig. 2 Micro-FTIR spectra in transflection mode of standard: (a) cy-clododecane, (b) cyclohexane, and (c) n-heptane

applications. The packing density of the dried film as it ap-pears at the microscope is related to its application modeover the surface. It may be observed that the film producedby the spraying method resembled a glass amorphous struc-ture with a high packing density, which provided completecoverage of the surface. This is in strong contrast to the filmproduced by brush application, as normally used by conser-vators, which showed a lower packing density with needlesand plate-like crystals, with the formation of holes that couldultimately favor an easier penetration by water with an ulti-mately scarce protective action (Fig. 1).

As a preface to following the formation of the CDD filmand its kinetics of sublimation, it has been important to sepa-rately characterize the CDD raw material and the employedsolvents since each of them observes a similar alkane na-ture. To aid in this, micro-infrared was utilized in transflec-tion mode to record the characteristic modes of vibrationin the fingerprint range (Fig. 2). The spectrum of cyclodo-

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Table 1 Infrared band assignments of pure CCD

Vibrational frequenciesof cyclododecane (cm−1)

Tentative assignments

1472 δ(CH2)

1442 δ(CH2)

1344 CH2 wag

1309 –

1243 CH2 wag

1041 ν(CC) or ρ(CH2)

963 –

821 –

784 ν(CC) or ρ(CH2)

718 C–H or ring deformation

decane in Fig. 2a and tentative assignments (Table 1) canbe utilized for comparison with the solvents. In fact, at-tributable vibrational frequencies of cyclohexane (Fig. 2b)were noted at 1452, 1348 and 1257 cm−1 given by the δ,π and τ(CH2), respectively. Further bands are noticed witha doublet at 1036 and 1016 cm−1, attributed to the ν(CC)

or ρ(CH2), at 902 cm−1 and 862 cm−1 due to the ρ(CH2)

and ν(CC) [20]. It may be observed that n-heptane (Fig. 2c)shows bands at 1378 and 1460 cm−1, related to the C–Hbond deformation vibrations of –CH2 and –CH3 [21].

In order to examine any interaction between the chosensolvents and CDD during the film formation, further micro-infrared measurements of sprayed CDD solution were exe-cuted to follow each solvent as it evaporates at room temper-ature leaving behind the formed CDD film (Fig. 3). Bandsof cyclohexane and n-heptane can be detected over a periodof hours together with CDD. For cyclohexane, despite itslower boiling point (higher volatility) (Fig. 3a), the bands at862 and 902 cm−1 are observed to persist to 7 hours, afterwhich time the solvent is considered to have evaporated. The–CH2 band deformations of n-heptane instead (Fig. 3b) at1378 cm−1 are noted to decrease within 2 hours. Consider-ing the aforementioned results, measurements commencedafter 24 hours so as to deal with a completely dry, solvent-free film.

3.2 Sublimation behavior of CDD film

It has been possible to confirm the cyclododecane film for-mation on a Carrara marble substrate enabling the subse-quent sublimation to be followed with fibre-optic infraredspectroscopy through the characteristic first overtone of C–H stretching of CDD noted at 5680–5810 cm−1. These CDDbands were chosen on the basis that they are free from over-lap from the marble substrate as observed in Fig. 4. Theadvantage of using overtone bands is their low absorptiv-ity, which allows energy to pass easily through the samples

Fig. 3 Micro-FTIR spectra in transflection mode of thin layer of CDDin: (a) cyclohexane (b) n-heptane

Fig. 4 Comparison between reflection FTIR spectra of: (a) marble and(b) CDD. The CH overtone bands of CDD are highlighted

without rapid attenuation and low reflectivity, which has as aconsequence that a greater penetration in this region beneaththe surface of the sample is obtained when compared to thefundamental bands in the mid-infrared region. It can be fur-ther noted too, that the overtone bands are free from distor-tions due to the interactions that arise between the roughnessof the surface, as a result of crystal packing, and the irradia-tion wavelength [17]. Therefore they can reliably aid a semi-quantitative monitoring of the CDD sublimation process.Therefore, it is possible, in Fig. 5, to compare the kineticdata obtained for the integrated area of the C–H overtonebands versus time.

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The study of cyclododecane as a temporary coating for marble by NMR profilometry and FTIR reflectance 405

Fig. 5 Plot of area of Infrared bands at 5680 and 5810 cm−1 againsttime (hours) of marble treated with CDD/solvent mix of cyclohexaneand n-heptane

Complementary behavior of the CDD films producedwith cyclohexane, and n-heptane, respectively, on the mar-ble surface at constant temperature, humidity and pressureis highlighted. The films sublimate in a rather similar way inthe two aforementioned cases, and the duration of the film isless than 700 hours. The CDD overtone signals in the caseof application with cyclohexane increase in intensity in thefirst 70 hours, even though this may appear as anomalousbehavior; it has been confirmed by three other sets of exper-iments. Moreover, it may be noted that the calculated errorsin the 1 cm2 area analyzed increase with time, indicatingheterogeneous sublimation.

From Fig. 6a–b it is possible to note the resulting NMRprofiles obtained for CDD application on marble in cyclo-hexane and n-heptane, respectively. By means of this tech-nique it has been possible to reconstruct a 2D image of thefilm plotting the averaged 1H signal of a volume of 1 cm2

on the xy plane and, in this case, 40 µm on the z directionin function of the depth. A similar technique was applied tostudy the drying process dynamics of alkyd coatings overporous substrates [22]. The profiles obtained made it pos-sible to estimate the thickness of each resulting CDD film,which reached 400 µm in both the examined cases. The dif-ferent shape of those obtained from the cyclohexane solu-tion (Fig. 6a) with respect to those relative to the n-heptane(Fig. 6b) can be ascribed to a different degree of flatness ofthe investigated area.

Due to the sublimation process, the CDD films becomeincreasingly thinner with time until they completely dis-appear in 696 hours for cyclohexane and in 624 hours forn-heptane. As a qualitative consideration, if the daily rateof sublimation could be considered constant, it may be cal-culated as the ratio of the entire thickness of the initial layer

Fig. 6 NMR profiles of CDD sprayed on Carrara marble in (a) cy-clohexane (initial thickness 420 µm), (b) n-heptane (initial thickness400 µm)

analyzed by the time employed to the completely sublimatedcase. This suggests that the daily sublimation for cyclo-hexane and n-heptane is about 15 µm/day. However, plottingthe thickness of the layer of the different films as a func-tion of time, as shown in Fig. 7, it is possible to observethat the sublimation kinetics of the two films is very simi-lar. Moreover a quasi-linear behavior is noted and this maybe compared with that obtained by infrared spectroscopy. Infact, after normalization, the linear fitting of the kinetics, forboth the techniques and the solvents shows similar values forthe slopes (circa −0.00130) and the regression coefficient ishigher than 0.94.

A further consideration can be extrapolated by the NMRprofiles. As expected, being a non-porous substrate, theCDD does not penetrate the marble matrix and remains inthe surface layers, consistent with the level of abrasion ofthe marble, yet this behavior could likely change when ap-plied to more porous substrates. The applicative method ob-served a gentle heating of the CDD in the solvent produc-ing an oversaturated solution, which, when sprayed onto thestone, also produced a degree of thermal shock inducing a

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Fig. 7 Sublimation of CDD film with each employed solvent as afunction of coating thickness against time

rapid crystallization of CDD on the surface followed by abuild-up of successive crystal layers. It follows then that thispreferential application method can be selected when CDDis to be used as a temporary coating on stable matrices suchas stones following careful evaluation of porosity and ther-mal shock induction to prevent penetration within the ma-trix.

4 Conclusions

Temporary cyclododecane film coatings and their sublima-tion rates on a marble substrate have been evaluated bystudying their film formation with two solvents namely cy-clohexane and n-heptane. The solutions have been sprayedonto the surface, a method that could be routinely employedin the field of cultural heritage conservation. The study wasfocused on the spectroscopic characterization of the CDD,the solvents employed and the evaporation process of thesolvents in the formation of the CDD film. The resultingfilms appeared to observe an amorphous structure, and theirduration on a marble substrate was monitored by followingthe sublimation process. Due to the intrinsic peculiarity ofthe process, the application of non-invasive techniques is re-quired in order to avoid any kind of interference and accel-eration of the rate of sublimation. The sublimation of CDDwas followed through characteristic overtone bands by fibre-optic infrared spectroscopy determining the efficiency andduration of the film formed with each different solvent. TheNMR profiling technique has been able to further ascertainthe non-penetration of the CDD within the marble matrix as

well as to estimate the thickness of each resulting film and tofollow each separate sublimation process that correspondswell with data collected by infrared. The kinetics of subli-mation show a quasi-linear trend and similar behavior inde-pendent of the solvent employed to create the film. Throughthis work it has been possible to develop a methodology thatmay be applied directly in situ on non-movable artworks,highlighting the advantages of portable instrumentation forthe on-line monitoring of conservation treatments.

Acknowledgements This work was carried out within Eu-ARTECH,an Integrated Infrastructures Initiative supported by the 6th FrameworkProgramme of the European Union (Contract RII3-CT-2004-506171)and A3S (APQ della Regione dell’Umbria).

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