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Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 139–146

AFM study of paraffin wax surfaces

Marek Zbik a,∗, Roger G. Horn a, Neil Shaw b

a Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095, Australiab Unilever R&D Port Sunlight, Quarry Road, East Bebington, Wirral CH63 3JW, UK

Received 19 December 2005; received in revised form 14 March 2006; accepted 15 March 2006Available online 4 May 2006

bstract

Paraffin wax surfaces cast against NaCl crystals, silicon wafers and a silicone elastomer used for microcontact printing, Sylgard®, were theubject of AFM morphology and surface roughness investigations. A distinctive stepped texture has been found on the paraffin wax surface,uggestive of layered crystals. Step heights measured across paraffin flat surface were in the range between 3 and 30 nm, with the most frequentccurrence being around 4–8 nm. A paraffin wax cross-section measured by AFM shows lamellae of individual sheets 7.6 ± 0.15 nm. The surfaceoughness of the paraffin samples shows a clear correlation with the roughness of the substrate, but the paraffin roughness is an order of magnitudereater than that of the mould roughness. Evidently the paraffin does not produce a faithful replica of the mould in the way that Sylgard does. Thelight flexibility of Sylgard after curing allows it to be peeled off a rigid mould, and again to be peeled away from paraffin after the wax has set. It is

emonstrated, however, that separation does not occur exactly at the interface between the two materials; rather it occurs through cohesive failuref the paraffin, close to the interface, and images reveal a laminar contour-like stepped morphology texture characteristic of paraffin crystals onoth the separated surfaces. 2006 Elsevier B.V. All rights reserved.

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eywords: Paraffin wax; Paraffin morphology; Paraffin surfaces; AFM morpho

. Introduction

Paraffin wax is produced from crude mineral oil and contains-alkanes where n is the number of carbon atoms in a hydro-arbon chain. Paraffin wax has many industrial applications. Its also known to cause problems when it deposits as scale iniesel fuel lines. Paraffin crystallizes in layers which consist ofolecules having planar zigzag conformations and its molecu-

ar length depends monotonically on n. The boundaries of suchayered crystals are determined by the terminal methyl groups ofhe chains. Conventionally, the z crystal axis is chosen parallelo the molecular chains except for single alkanes where the tri-linic crystal system is applicable. The crystal structure of theseaxes can be inferred from X-ray diffraction. Paraffin waxesave been extensively investigated and show only two types of

-ray diffraction pattern [1–3]. They belong to the orthorhombic

pace groups Bb21m and Fmmm; the former is of low disorder,hile the latter has greater disorder. The crystal structure of the

∗ Corresponding author. Tel.: +61 8 8302 3688; fax: +61 8 8302 3683.E-mail address: Marek.Zbik@unisa.edu.au (M. Zbik).

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927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2006.03.043

araffin wax is a function of both average chain length and tem-erature with the more disordered form becoming prevalent ashe chain length goes down or the temperature goes up. In the-alkanes with an odd number of carbon atoms above n = 36, thetructures are bilayered, i.e. the d(0 0 1) long spacing is equal tohe unit-cell dimension c and spans a pair of layers. Below n = 36,rystals belongs to the monoclinic crystallographic system, butn paraffins there is a mixture of alkanes having different num-ers of carbon atoms, so the crystal structure is less predictable.

The paraffin surface is hydrophobic and frequently used as aater repellent. Such surfaces, when flat and smooth, can be used

s a model to study forces (using atomic force microscopy, AFM)cting between a hydrophobic surface and a probe in aque-us solutions of different electrolytes or surfactants. Amongstther applications, these studies will be useful for investigat-ng the action of personal care products. In this report an AFM

orphological study was undertaken as a precursor to forceeasurements between a hydrophobic surface and silicon oil

PDMS). Different methods of paraffin wax preparation wereested to find the effect of various surfaces against which paraf-n was cast, and evaluate which preparation method gives aaraffin surface most suitable for further AFM force measure-

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40 M. Zbik et al. / Colloids and Surfaces A: P

ents. In spite of the use of waxes in many industries only a feworphologic studies have been published so far [4,5] and this

aper is intended to contribute to our knowledge in this area.

. Experimental details

A Nanoscope III AFM (Digital Instruments) was used inapping mode with scan heads J (150 �m × 150 �m) and E15 �m × 15 �m), with scan rates between 0.5 and 1 Hz gen-rating height and frequency images. The AFM was calibratedn a gold grid with 5 �m pits separated by 5 �m. The stan-ard non-contact “Golgen” Silicon cantilevers (NSG 10) weresed in experiments. The tip height was 10–20 �m with typi-al tip curvature radius of 10 nm and cone angle less than 22◦.he standard lateral and vertical deviation of the AFM measure-ents is ±0.15 nm due, at least in part, to the uneven nature of

he paraffin surface and the relatively low aspect ratio of theFM tip. The mean roughness values (Ra), which represent the

rithmetic average of the deviation from the center plane, wereeasured from AFM images in height mode over 5 �m × 5 �m

mage dimensions.Commercially available paraffin wax (Aldrich) was studied

y X-ray diffraction (not shown here) and found to belong torthorhombic space group Bb21m. Various methods were usedo deposit the wax on top of the 10 mm stainless steel AFMoken (which is later held on top of the AFM head by magneticorce). The simplest way is to melt a small fragment of paraffinnd let it quench slowly in air. Other ways involved meltingaraffin and quenching it while in contact with a smooth surfacef mica, silicon wafer, salt crystal or polymer. Most of theseurfaces inflicted significant stress on paraffin when detachinghem, resulting in very rough surfaces with numerous cracks.odium chloride was purchased from Sigma–Aldrich in the formf an unpolished 11 mm × 30 mm × 7 mm IR crystal window.his was cleaved to give an atomically smooth surface whichas placed on top of molten wax, then allowed to cool slowly

o room temperature. After this the salt crystal was removed byissolution in water.

Several tests were conducted with paraffin cast against ailicon wafer. Separation was achieved by differential ther-al expansion during rapid quenching to low temperatures, by

lunging a sample with melted paraffin in between the steel tokennd silicon wafer into liquid nitrogen or ethane. Such detach-ents leave a surface that looks smooth, shiny and clean, in

ontrast to the result of peeling the silicon wafer away fromuenched paraffin which leaves a visibly rough surface. Similaresults (not presented here) were obtained when freshly-cleavedica surfaces were used in place of the silicon wafer.Paraffin wax was also cast against a silicone elastomer,

ylgard®184 (Dow Corning). Sylgard was designed for tem-late replication and micro-contact printing applications. It hasotential use either as a mould for paraffin wax, or as a sub-trate material itself for AFM force measurements. For all of

he experiments described below, Sylgard was used in a ratio 10arts resin base to 1 part hardener, with a period of at least 72 hllowed for curing. It was noted that shorter curing times resultedn a softer elastomer material. Sylgard was cured in contact with

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chem. Eng. Aspects 287 (2006) 139–146

hree different templates: (i) the smooth front side of a siliconafer; (ii) the rougher rear side of a silicon wafer; and (iii) air.or the last of these, a thick horizontal film was allowed to set

n air, and the upper surface was used. In the first two cases theylgard film was also formed in air, but the surface used was thene that had been in contact with silicon. Sylgard was separatedrom the silicon wafer by peeling it away after the appropriateure time (three days for the best results).

Surface hydrophobicity was characterized by measuring theontact angle of a water drop on the flat substrate using a sessilerop apparatus. Both the advancing and receding contact anglesere recorded. Contact angles were determined by fitting curves

o photographic images of the water drops.

. Results and discussion

.1. Paraffin cast in air and under a salt crystal

Paraffin cast on top of the steel token and quenched in airt room temperature shows a very uneven, hummocky surfacender the AFM (Fig. 1 left). This surface looks matt to the nakedye and the measured Ra value was 39 nm. The surface of paraffinast under the salt crystal was shiny to the naked eye and verymooth in AFM images (Fig. 1 right) with Ra = 2 nm.

These paraffin surface images show a distinctive stepped tex-ure, suggestive of layered crystals. In fact it is known that higherlkanes crystallize in lamellar crystals, even when they are mixedogether as they are in a wax [1]. Atomic force micrographs showery fine growth step patterns which looks like contours on topo-raphic maps. In a higher magnification AFM image (Fig. 2 left)here the frame dimension is 5 �m, distinctive steps are visi-le. These steps measured on paraffin surface are of differenteights ranging from 3 to 30 nm with the most frequent occur-ence being around 4–8 nm. This discrepancy on measurementsay be because stepped crystals observed in paraffin wax come

rom different angles relative to the observable surface, and/orhe occurrence of multiple steps. Similar features in paraffinrystal surfaces have previously been observed and described4,5] as post-growth steps.

In the higher magnification AFM micrograph (Fig. 2 right)here the frame size is 1 �m, the lamellar structure of a paraffin

rystal is visible. This crystal is positioned almost perpendicularo the scanning plane and individual lamellae can be measuredith reasonable accuracy. From this section one may calculate

hat each lamella is about 7.6 ± 0.2 nm thick, corresponding tomolecular bilayer. This result is in good agreement with our-ray diffraction investigations and those of [1] which find thearaffin unit cell c-parameter to be equal to 7.82 ± 0.04 nm. The-parameter for the Bb21m wax structure is the equivalent ofwo molecular lengths.

.2. Paraffin cast against silicon wafer

A different morphology of paraffin wax can be obtained byasting paraffin against silicon wafers. Silicon wafers are oftensed in AFM experiments and are greatly valued as a substrateecause of their flat and smooth surfaces. The silicon wafer that

M. Zbik et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 139–146 141

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ig. 1. AFM images in amplitude mode of the paraffin surface resolidified in air (lefoth images show a 15 �m × 15 �m area.

ig. 2. AFM images in amplitude and height modes of a paraffin surface resolidifiedf paraffin crystal showing lamellae of individual sheets 7.6 ± 0.15 nm (right image &

t image) and the paraffin surface resolidified under a salt crystal (right image).

under a salt crystal showing contour-like features (left image) and cross sectionsection below).

142 M. Zbik et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 139–146

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Fig. 3. AFM images in amplitude mode of a 5 �m × 5 �m area of the front

as used in this study has two distinctive sides, one polished andne rough, matt in appearance (Fig. 3). We refer to these as theront and rear sides, respectively. The roughness measurementsver a 5 �m × 5 �m area gave Ra = 0.12 nm (front) and 23 nmrear).

The results of casting molten wax against the front and rearurface of a silicon wafer with subsequent rapid quenching iniquid nitrogen are shown in Fig. 5. The left-hand picture showsaraffin cast against the front surface of a silicon wafer. This sur-ace looks glossy to the naked eye. In AFM images sharp shapedlongated paraffin crystals of dimension 10–30 �m appear tornpen, exposing smooth cross sections. Individual sheets andheet assemblages display bending. In many places deep cav-ties are present. These cavities are zones where pre-existingaraffin crystals were extracted by detachment of silicon wafer.he missing crystals were found later, adhered to the detached

ront surface of silicon wafer (to be shown below in Fig. 9, leftmage). The Ra roughness on this surface is ∼9 nm measuredver the 5 �m × 5 �m frame dimension.

A surface of different morphology was obtained by castingaraffin against the rear surface of a silicon wafer (Fig. 5 right).orphology in this AFM micrograph is very rugged and shows

arger crystal assemblies up to 40 �m in dimension with rougharginal surfaces caused probably by stress during detachment

rom the wafer surface.In comparison to the images of a silicon wafer itself pre-

ented in Fig. 3, paraffin cast against a silicon wafer did noteplicate morphological features but rather exhibited its ownnternal texture packed by twisted and elongated paraffin crys-als. Measurements of Ra over 5 �m × 5 �m on this paraffinurface give a much higher figure for roughness – about 33 nm.

owever, neither of these measurements reflects the roughness

hat is present on a larger scale, and visible to the naked eye. Likehe appearance of the wafer itself, the paraffin cast against theront surface is glossy, while that cast against the back surface

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th (left image) and the rear rough (right image) surface of a silicon wafer.

as a matt appearance. Macroscopic surface roughness has noteen measured (the AFM is not suitable for this). Despite theisible change in texture, the contact angles measured on bothf these paraffin surfaces, glossy and matt, were similar, around35◦ advancing and 95◦ receding (Fig. 4, left image).

The paraffin surface with the rugged morphology seen inig. 5 (right) can be cleaned by ethanol and observed with bet-

er resolution (Fig. 6 left). In this micrograph large and twistedaraffin crystals display discrete laminar texture. A differenturface texture was obtained by plunging the paraffin/Si wafernto liquid ethane instead of liquid nitrogen. The difference isttributable to a higher cooling rate, and the fact that ethanes a solvent for paraffin. An AFM image of the resultant sur-ace is shown in (Fig. 6 right). In this micrograph paraffinrystals appear very small and rather granular with diametersbout 5 �m, frequently ordered into longer and twisted chains.uch small size of paraffin crystals can be attributed to the rapidooling rate which significantly increases the number of nuclei,esulting in small crystallite sizes.

In other experiments whose results are not shown, the siliconafer surface was also modified from hydrophilic to hydropho-ic by gold coating, UV treatment, and plasma treatment, witho effects on paraffin morphology. As we will discuss below, inll cases when a silicon wafer was detached from the paraffinurface by differential thermal expansion, both the morphologyattern and roughness measurements as well as the contact anglehecked on the both detached surfaces suggests cohesive failuref the paraffin close to the interface rather than a clean separationetween the materials.

.3. Paraffin cast against Sylgard

Sylgard’s reasonably faithful reproduction of surface topog-aphy is illustrated in Fig. 7, which shows Sylgard after it hadeen cast against the front (left image) and rear (right image) of

M. Zbik et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 139–146 143

Fig. 4. Receding contact angles of water measured on the paraffin surface (95◦, left image), on the Sylgard surface (51◦, center image) and on the Sylgard afterpeeling it away from paraffin that had been cast against it (103◦, right image).

Fig. 5. AFM images in amplitude mode of a 50 �m × 50 �m area of paraffin surface prepared by casting against the front (left image) and the rear (right image)surface of a silicon wafer, then detached by rapid quenching in liquid N2.

Fig. 6. AFM images in amplitude mode of a 50 �m × 50 �m area of paraffin surface prepared by casting against the front surface of a silicon wafer, then detachedby rapid quench in liquid N2 and cleaned by rinsing in ethanol (left image); or detached by rapid quench in liquid ethane (right image).

144 M. Zbik et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 139–146

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ig. 7. The amplitude mode AFM images of a 5 �m × 5 �m area of Sylgard® 1urface of a silicon wafer, then peeled away from it after curing.

he silicon wafer. By comparing Fig. 7 with the previous imagesf the two sides of the original silicon wafer (Fig. 3) it can beeen that replication is reasonably good. The Ra roughness mea-ured over 5 �m × 5 �m area was 0.4 nm (Fig. 7 left image)nd 15 nm (Fig. 7 right image) in values. While the match isot perfect between Sylgard parameters and those of the waferace against which it was formed (0.12 and 23 nm, respectively,rom Fig. 3) the variation is no more than the typical sample-to-ample variation of such measurements, particularly for rougherurfaces.

The contact angles of water measured on Sylgard show thathe material is strongly hydrophobic, with advancing contactngles of around 135◦, which is comparable to those measuredn paraffin. The receding angles are in the range 50–65◦ (Fig. 4,enter image), compared to typical values of 80–95◦ on paraffin.

The Sylgard elastomer can also be used as a mould for paraffinax. The reason why this may be preferable to casting the waxirectly against the original templating surface is that in generalt is difficult to separate the paraffin from a rigid mould withoutohesive failure within the paraffin, whereas it may be possible toeparate the Sylgard by peeling. The slight flexibility of Sylgardfter curing allows it to be peeled off a rigid mould, and againo be peeled away from paraffin after the wax has set. This isn contrast to the other methods required to separate paraffinax from its mould. An attempt was made to detach Sylgardy plunging into liquid nitrogen but the deformation of Sylgarduring quenching was so severe that the paraffin substrate waseeply cracked.

An AFM image of paraffin wax cast against Sylgard is shownn Fig. 8. In this case the Sylgard was separated from the paraffin

y peeling it off at room temperature. In the AFM image afteremplating against the front wafer surface (Fig. 8 left) a distinc-ive stepped morphology has been observed. The step heights inhis structure range from 3.0 to 5.8 nm for some crystals and up to

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at has been cast in contact with the front (left image) and the rear (right image)

.0 for others. Such step heights may reflect some fractionationf the paraffin allowing shorter chain molecules to solidify onhe top of the sample. These step heights may be due to multipleax layers being present in different steps.The surface roughness of the paraffin samples shown in Fig. 8

ndicated Ra = 15 nm (left image) and 30 nm (right image). Theres a clear correlation, but the paraffin Ra roughness is an order of

agnitude greater than that of the mould in the case of paraffinast against smooth Sylgard and subsequently separated and twoimes larger when paraffin is cast against rough Sylgard andubsequently separated. Evidently the paraffin does not producefaithful replica of the mould in the way that Sylgard does

Fig. 7).The facts that paraffin does not form a perfect replica of the

ylgard against which it was cast, and that the images revealtexture characteristic of paraffin crystals, raise the question

f what happens when the Sylgard mould is peeled away fromhe paraffin. One possibility is that separation does not occurxactly at the interface between the two materials, but rathert occurs through cohesive failure of the paraffin close to thenterface. If this were the case, it should be possible to detectesidual paraffin on the Sylgard. Evidence for this is shown inig. 9 (right image) where the morphological pattern indicates

hat residual paraffin is present on the Sylgard surface after sep-ration by peeling. Further evidence comes from water contactngle measurements on this surface (Fig. 4 right image). This fig-re shows a higher receding angle, about 100◦, observed on theylgard surface after it had been separated from paraffin by peel-

ng, compared to the 51◦ observed on bare Sylgard (Fig. 4 centermage). This receding angle is comparable to previous measure-

ents on paraffin surfaces (80–95◦ receding contact angle), sot suggests that cohesive failure occurs within the paraffin whenylgard is peeled from it, leaving some of the paraffin film stuck

o the Sylgard surface. Contact angles measured on the paraffin

M. Zbik et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 139–146 145

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ig. 8. AFM images in amplitude mode of a 5 �m × 5 �m area of paraffin surgainst the front side of a silicon wafer (left image) and against the rear side of

urface formed by casting against Sylgard lie within the sameange as reported previously for other paraffin preparations.

Similarly, remnants of paraffin are to be seen on a siliconafer surface after casting and separation by quenching in liq-id N2. However the morphology of these paraffin fragmentss completely different to what is observed on Sylgard. This iseen in (Fig. 9 left image), where regions of flat lamellae andarge lumps of material are discernible on the smooth surface of

silicon wafer against which paraffin had been cast. Large holes

n the facing paraffin surface were seen in Fig. 5 (left image).The mean roughness (Ra) values for the various paraffin sur-

aces investigated are summarized in Fig. 10. From this figure

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ig. 9. AFM images in amplitude mode of a 14.7 �m × 14.7 �m area of the front sidas been cast and subsequently separated.

repared by casting against a Sylgard mould that itself was formed by castingon wafer (right image).

t can be seen that paraffin cast against a salt crystal that isubsequently removed by dissolving in water guarantees a flaturface with low roughness suitable for use as a substrate inFM force measurements. Paraffin cast against Sylgard or the

ront side of a silicon wafer displays moderate Ra values, andaraffin cast against the rear side of a silicon wafer or againstylgard which has been formed from the rear side of a siliconafer both display high mean roughness values. This significant

ncrease in Ra values in the paraffin wax surfaces cast againstubstrates other than a salt crystal seems to be due to cohesiveailure of the paraffin close to the interface during cast/mouldeparation.

e of silicon wafer (left image) and Sylgard (right image) against which paraffin

146 M. Zbik et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 139–146

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. Conclusions

Paraffin wax quenched under halite (NaCl) crystal which isubsequently dissolved in water has a fairly flat surface with lowean roughness. The surface shows a contour-like stepped mor-

hology. Steps measured on the paraffin surface are of differenteights ranging from 3 to 30 nm with the most frequent occur-ence being around 4–8 nm. AFM measurements from a paraffinrystal section indicate that each lamella is about 7.6 ± 0.2 nmhick, corresponding to a molecular bilayer.

Because of weak cohesion within paraffin, a thin paraffinlm adheres to silicon wafer or to Sylgard when paraffin is castgainst either of these materials. Although not presented here,imilar observations were made with paraffin cast against siliconafers modified to be hydrophilic or hydrophobic, and againstydrophilic mica used in place of silicon. This indicates that theydrophilicity of the mould has little or no effect on the modef separation or the final paraffin surface that is produced. Theesulting paraffin substrate is equally hydrophobic regardless ofow hydrophilic or hydrophobic was the casting surface andegardless of the separation method.

The paraffin surface roughness has been found to depend onhe mould roughness, even though it does not reproduce it faith-ully. The lowest roughness values were obtained when casting

araffin wax against a salt crystal which was removed by dis-olution in water. Slightly higher values were obtained whenaraffin was cast against Sylgard cast in air, then against themooth side of a silicon wafer and against Sylgard which had

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een cast against the smooth wafer. Maximum roughness val-es were obtained when paraffin was cast against Sylgard thatas previously formed against the rough side of the wafer and

gainst the matt surface of the silicon wafer itself.The contact angle measured on all paraffin substrates was

onstant and did not indicate any relationship between paraf-n substrate roughness and mould surface hydrophilicity. Theharacteristic stepped morphology remains visible regardless ofifferences in mould material used and paraffin film preparationethod. The paraffin crystal size depends on quenching rate,

nd is minimum for paraffin quenched in liquid ethane.

cknowledgments

We thank Andrea Gerson, Paul Jenkins and Stephen Wire forelpful discussions.

This work was funded by Unilever R&D Port Sunlight (UK).

eferences

1] A.R. Gerson, S.C. Nyburg, A. McAleer, J. Appl. Cryst. 32 (1999)296.

2] S.T. Craig, G.P. Hastie, K.J. Roberts, A.R. Gerson, J.N. Sherwood, R.D.Tack, J. Mater. Chem. 8 (4) (1998) 859.

3] A.R. Gerson, S.C. Nyburg, Acta Cryst. B50 (1994) 252.4] J.R. Fryer, C.H. McConnell, D.L. Dorset, F. Zemlin, E. Zeitler, Proc. R.

Soc. Lond. Series A 453 (1997) 1929.5] M. Plomp, W.J.P. van Enckevort, P.J.C.M. van Hoof, C.J. van de Streek,

J. Cryst. Growth 249 (3–4) (2003) 600.