Additive chemical structure and its effect on the wetting behaviour of oil at 100 °C

Applied Surface Science 506 (2020) 145020

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Additive chemical structure and its effect on the wetting behaviour of oil at 100 °C M. Kus, M. Kalin

T

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University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana, Slovenia

A R T I C LE I N FO

A B S T R A C T

Keywords: Advancing contact angle Receding contact angle Contact-angle hysteresis Additives Oil Steel Polarity Chain length Saturation Temperature

In lubrication, additives are added to base oils to form surface layers that crucially improve its performance, particularly at high temperature. However, very little is known about their influence on the wetting behaviour of oil, even at room temperature, while for high-temperature, where the effects of the additives are the most intense, is absent. Accordingly, this work focuses on the additives’ effects on the wetting of oil on steel at 100 °C, which is relevant for the activity of most oil additives. Simple organic friction modifiers with different numbers of polar head groups, non-polar tail chain length, head-group polarity and saturation were investigated. The results show that additive-films change the steel surface, making it more oleophobic. Additives with one polar head group decrease the wettability by 98% more than the additives with two polar groups, the most-polar fatty acid by 75% more than the least-polar amine, the longest chain length of 18C atoms by 55% more than the shortest chain of 11 C atoms, and the unsaturated additive by 12% more than the saturated additive.

1. Introduction Wetting, as a liquid-solid interfacial phenomenon, has been the focus of research for a long time. In spite of this, there is still a lack of understanding about how various physico-chemical liquid and surface parameters effect the wetting. It is, however, clear that temperature notably affects the wetting behaviour [1–4]. The change in temperature affects several liquid properties, at most surface tension, viscosity, density, as well as those of the solid surface, like oxidation, surface energy, reactivity, etc., which consequently affects their interactions with liquids [1,5]. Even though this was realised decades ago, there are a surprisingly small number of studies that experimentally deal with wetting at different temperatures [6–11], and even fewer that investigate the influence of temperature on dynamic wetting [12,13]. It is obvious that wettability has a paramount role in lubrication tribology, which is a field dedicated to the interactions between the liquid, in this case the oil and additives, and the solid surfaces. Additives that are added to the oil to improve the tribological performance form surface layers that are crucially dependent on the temperature [14–17]. Since these layers adsorb and at least partially cover the original surface, the surface energy and so the wettability of the original surface changes with these adsorbed additive layers. However, the effect of additives on wetting is scarcely investigated and therefore poorly understood. Even less is known about the effect of temperature

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on the formation of additive-based layers and their consequent influence on the wetting. However, it was reported in recent years that poor wetting can dramatically reduce the EHD (elasto-hydrodynamic) friction with base oils [18,19]. Moreover, just recently, it was also demonstrated that when additives were used in the base oil, the EHD friction is reduced [20,21]. This makes understanding the wetting of oil in the presence of additives of immense importance, since most of the frictional losses in lubricated engineering components are due to EHD full-film lubrication [22], and thus energy savings can be at the forefront of this technology. The effect of additives on wetting has already been systematically studied at room temperature [23]; however, the influence of elevated temperature, which is common in lubrication and is a key parameter for every additive’s reactivity and the formation of films, has not yet been investigated in any detail. Since the surface tension and liquid viscosity decrease with the temperature, the wettability should improve with an increase in the temperature [1]. This has been shown for water droplets on aluminium surfaces [9] and some polymers [8]. However, the opposite effects have also been reported [10], showing that the contact angles of water increase with temperature on polyamide and a decrease in the contact angles of several organic liquids on siliconated glass. A decrease of the surface energy with an increasing temperature [11,24,25] has also been argued to cause better wettability at higher temperature. However, this relation has not proved to be universal,

Corresponding author. E-mail address: [email protected] (M. Kalin).

https://doi.org/10.1016/j.apsusc.2019.145020 Received 14 September 2019; Received in revised form 6 November 2019; Accepted 8 December 2019 Available online 16 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 506 (2020) 145020

M. Kus and M. Kalin

tribology. The samples’ surface roughness was measured with a stylustip type profilometer (T8000, Hommelwerke GmbH, Schwenningen, Germany). Samples were prepared to a low roughness value of 0.05 μm ± 0.02 μm to avoid any roughness effect in wetting measurements. Nine simple organic friction modifiers were chosen based on their differences in the chemical structure. The chosen additives differ in the polar head-groups number, the length of the non-polar tail, the polarity of the head-group, and the saturation. The number of the polar headgroups was investigated by comparing undecanoic, undecanedioic, hexadecanoic and hexadecanedioic acids. These are additives with either one (-oic) or two (-dioic) polar carboxyl (COOH) heads. It has been demonstrated that the longer chains of organic friction modifiers improve their tribogical behaviour [31,32]. For this reason, we have compared undecanoic, hexadecanoic and octadecanoic acid – saturated fatty acids with eleven, sixteen and eighteen C-atoms, respectively. Organic friction modifiers with different polarity of head-group [33] with the same chain (18 C-atoms with one double bond) and were tested in order to elucidate the influence of the additive polarity on the wetting properties. We have chosen octadecenylamine, octadecenol, octadecenamide, and octadecenoic acid: additives that have amine (NH2), hydroxide (OH), amide (CONH2), and carboxyl (COOH) headgroups, respectively. Monounsaturated additives were used because they are more easily mixed in oil than saturated additives. The effect of the saturation was evaluated by comparing saturated (octadecanoic) and unsaturated (octadecenoic) fatty acid. In the presented study, the tested additives are denoted like so:

since the surface energy is not the only parameter that is temperaturedependent and can affect the static contact-angle values [1,3,26,27,28]. It has been shown, for example, that at higher temperatures glycerol, ethylene glycol and diethylene glycol on elastomers give larger static contact angles [7]. It has also been reported that for n-alkanes the temperature effect on the wetting is very much related to the intermolecular forces, as well as the thickness of the film that is adsorbed in the region adjacent to the three-phase line of the contact [6]. It has also been reported that a temperature increase results in severe oxidation for most metals, and the oxide layers change the properties of the interface and so can cause poorer wetting [29]. Therefore, as seen from the above studies, apart from some general effects of temperature on the liquid and solids, the increase in temperature influences different types of solids and liquids in different ways, in particular their interactions, and so the synergetic effect leads to differences in the wetting at various temperatures [1,3]. It is important to state that all the above-mentioned studies were carried out mainly with water as a test liquid [8–10,29], or in some cases with model organic liquids such as glycerol, formamide or ethylene glycol [7,10,30], which are hardly relevant for lubricating oils and additives, as well as with regards to tribological aspects. There also exists a study of the temperature dependence on the wetting of squalene on polymer [12] between 10 °C and 55 °C, where it was reported that the higher temperature improved the wetting [12]. Although this work relates to tribology more closely, no specific details or the effects of additives on temperature have been studied. Accordingly, in this work we present the findings about the wetting behaviour of poly-alpha olefin oil on bearing steel at high temperature in the presence of additives with different chemical structures. Both static (θst ) and dynamic parameters (θa , θr , CAH), were employed to evaluate the wetting behaviour, Fig. 1. The contact-angle hysteresis (CAH) is derived from the measurements as defined in Eq. (1), where θa denotes the advancing and θr the receding contact angle.

CAH = θa − θr

“number of C-atoms – / head-group”, for saturated additives, “number of C-atoms = / head-group”, for unsaturated additives. Additives’ chemical structures along with their abbreviations are also shown in Table 1. We have used poly-alpha-olefin oil PAO 65 as the base oil. This high-viscosity oil was used to obtain the same (as far as possible) viscosity and surface tension during the measurements as for the very common engineering oil PAO 6, which is widely used in automotive tribological applications, and we also studied it previously in a companion study at 25 °C [23]. This also enables a direct comparison of the temperature influence on the changes in wetting compared to 25 °C. The most relevant properties of PAO 65 are shown in Table 2, including the properties of PAO 6 for comparison purposes only.

(1)

Nine well-known simple organic additives, known as friction modifiers, i.e., fatty acids, amide, alcohol and amine (see Table 1) were tested. Their chemical structure was also studied, since the chosen organic friction modifiers differ in the number of the polar head-groups, the length of the non-polar tail, the polarity and the saturation. These are common additive parameters that play an important role in many lubrication applications and it was already shown that they do affect the wetting of steel at room temperature [23].

2.2. Preparation of additive mixtures and surfaces 2. Experimental For every chosen additive, we have prepared a 4 wt% mixture in the base oil. This concentration was chosen based on our experiences in the stability of various oil-additive mixtures. Furthermore, the percentage of additives in commercial oils is up to 4% [34–36]. The additives were mixed in the base oil at ambient temperature.

2.1. Materials As an engineering-relevant surface we chose steel (AISI 52100/ DIN 100Cr6), which is a commonly used material in the field of

Fig. 1. The principle of measuring (a) static wetting parameter, where θst is the static contact angle; and (b) dynamic wetting parameters, where θa is the advancing contact angle, θr is the receding contact angle, and α is the sliding angle. 2

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Table 1 The tested additives’ chemical structures. Additive Name

Denotation in this study

Number of Ceatoms

Polar group

Chemical structure

Undecanedioic acid Hexadecanedioic acid Undecanoic acid Hexadecanoic acid Octadecanoic acid Octadecenylamine Octadecenol Octadecenamide Octadecenoic acid

11Ce/2xCOOH 16Ce/2xCOOH 11Ce/COOH 16Ce/COOH 18Ce/COOH 18C]/NH2 18C]/OH 18C]/CONH2 18C]/COOH

11 16 11 16 18 18 18 18 18

COOH/ COOH COOH/ COOH COOH COOH COOH NH2 OH CONH2 COOH

HOOCe(CH2)9eCOOH HOOCe(CH2)14eCOOH CH3e(CH2)9eCOOH CH3e(CH2)14eCOOH CH3e(CH2)16eCOOH CH3e(CH2)7CH]CHe(CH2)8eNH2 CH3e(CH2)7CH]CH(CH2)8OH CH3e (CH2)7eCH]CH(CH2)7eCONH2 CH3e(CH2)7eCH]CHe (CH2)7eCOOH

The mixtures were than stirred with 300 rpm for five hours at 100 °C to ensure their uniformity. The stability check of the mixtures was performed visually for two days by observing possible precipitation. Before further preparation of the solid samples, the stable mixtures were again agitated for 30 min. The steel discs were first cleaned in an ultrasonic bath of n-heptane for two minutes, dried at 35 °C and cooled to ambient temperature in a stream of cool air. Cleaned and cooled discs were then poured with 0.4 ml of additive-oil mixture as schematically shown in Fig. 2. Afterwards, the samples were exposed to 100 °C for two hours (ST-10, Kambič Laboratorijska oprema d.o.o., Slovenia). This allowed for the formation of the additive-film from the mixtures on the steel discs. The samples were then cooled to ambient temperature, rinsed with n-heptane and ultrasonicated in n-heptane for five minutes to clean-off the remaining lubricant. After drying in a stream of dry cold air, the samples were prepared for static and dynamic wetting measurements (see Fig. 3).

Saturated

Unsaturated

chemisorbed. After both cleaning variations and drying the contactangle measurements with demineralised water and diiodomethane were performed to obtain the required data for calculating the surface energies. 2.4. Chemical characterization using ATR-FTIR For chemical analysis of the formed additive-film on the surfaces attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) was employed. The spectra were recorded on the formed films, after being rinsed with n-heptane and subsequently dried in air (Fig. 2). The spectra were taken on Spectrum One FTIR spectrometer (Perkin Elmer, USA) that contains 4000–400 cm−1 range high-performance diamond ATR accessory (Harrick Scientific Products Inc., USA). The spectra between 2500 and 1800 cm−1 is, however, not presented in the results, since this is the range in which the diamond crystal produces noise. Every spectra was collected from 256 scans, the resolution was 4 cm−1. To obtain the best possible spectra, the measured surface was put into the contact with the ATR diamond with sufficient pressure. Before the start of every measurement, the background of clean ATR diamond was recorded. The cleaning procedure of the diamond crystal always consisted of rinsing it with isopropanol and drying it with cellulose fibres.

2.3. Determination of the surface-energy To identify the possible surface alterations, we calculated the surface energies of the discs before and after the formation of the additive film. Calculation of the surface energy was performed using the OWRK method, which is detailed in [37,38]. To calculate the surface energy using the OWRK theoretical model, the input parameters for the equations (contact angles with model liquid on tested surface and surface tension of the used model liquid) need to be measured experimentally. The used model liquids were demi-water and diiodomethane. To obtain the information about the type of additive adsorption on the steel, we varied the cleaning process of the samples after the additive-film was formed. Firstly, the surface energies were determined after rinsing the surfaces using n-heptane (Fig. 2), which is a mild cleaning agent that also leaves some physisorbed additive species on the surface. Secondly, the surface energy was calculated after 30 min cleaning in ultrasonic ethanol bath, which is a much more severe procedure and known to remove these additives if they are not

2.5. Static wetting with oil The static contact-angle wetting was measured at 100 °C ± 1 °C with a goniometer (CAM 101, KSV Instruments, Helsinki, Finland). This optical goniometer was modified to enable high-temperature wetting measurements up to 150 °C. The device is equipped with a temperatureregulated and software-supported system that enables the separate heating of a solid sample as well as of a sample liquid at the set temperature, which in our case was 100 °C. To ensure full control of the liquid temperature, the temperature was additionally monitored on the solid surface, as well as on the solid–liquid interface, using a thermal camera (Optris® PI 160 Infrared Camera, Optris GmbH, USA). This

Table 2 xxx. Additive Name

Denotation in this study

Number of Ceatoms

Polar group

Chemical structure

Undecanedioic acid Hexadecanedioic acid Undecanoic acid Hexadecanoic acid Octadecanoic acid Octadecenylamine Octadecenol Octadecenamide Octadecenoic acid

11Ce/2xCOOH 16Ce/2xCOOH 11Ce/COOH 16Ce/COOH 18Ce/COOH 18C]/NH2 18C]/OH 18C]/CONH2 18C]/COOH

11 16 11 16 18 18 18 18 18

COOH/ COOH COOH/ COOH COOH COOH COOH NH2 OH CONH2 COOH

HOOCe(CH2)9eCOOH HOOCe(CH2)14eCOOH CH3e(CH2)9eCOOH CH3e(CH2)14eCOOH CH3e(CH2)16eCOOH CH3e(CH2)7CH]CHe(CH2)8eNH2 CH3e(CH2)7CH]CH(CH2)8OH CH3e (CH2)7eCH]CH(CH2)7eCONH2 CH3e(CH2)7eCH]CHe (CH2)7eCOOH

3

Saturated

Unsaturated

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Fig. 2. Process of preparing the samples for the static and dynamic contact-angle measurements.

being tilted from 10° to 50°. This angle of inclination is referred to in this work as “sliding angle”, since at every measured angle of inclination the oil drop slid over the surface. The movement of the deposited oil drop over the solid sample at every measured sliding angle was captured by employing a high-speed camera that takes measurements in sequences of 16 ms. For every measurement, 1250 frames were recorded and then analysed to determine the point of the first movement of the three-phase line. The image taken at said point was again put through the dedicated goniometer software that calculated the advancing (θa ) and receding contact (θr ) angles. From these results for every sliding angle, we calculated the contact-angle hysteresis (CAH), Eq. (1). 3. Results 3.1. Surface energy after additive-film formation Surface energies of the samples after the additive-film was formed at 100 °C are presented in Fig. 4. Fig. 4(a) shows the results after the surfaces were rinsed with n-heptane and Fig. 4(b) the results after ultrasonically cleaning the samples in ethanol bath. The sample that was treated with base oil only (with no presence of additives) has significantly higher surface energy (50.9 mJ/m2) than samples that were treated with mixtures of additive-oil (from 37.2 mJ/ m2 to 43.1 mJ/m2), Fig. 4. This indicates that an additive-film was formed at 100 °C for all the tested additives. The results show that the film formed from additives alters the steel surface and reduces its surface energy by up to 27%. A comparison of the results after cleaning the samples with the formed additive-layer by rinsing them with n-heptane (Fig. 4a) or ultrasonically cleaning them in ethanol bath (Fig. 4b) shows similar surface energies, since they differ only up to 2 mJ/m2. This shows that neither the mild nor the severe cleaning procedure removed the additive-film from the surface, indicating that at 100 °C the additives chemisorbed to the steel surface.

Fig. 3. The setup for dynamic contact-angle measurements at high temperature.

allows us to monitor the temperature and simultaneously record a video or a snapshot of the monitored areas, which ensures the temperature control of the measured solid–liquid system. Static contact-angle measurement results were captured during 15 s after the oil drop was put on the solid surface. It has been shown that most oils wet high-energy surfaces (like metals) via the so-called spreading [18,19,38], which is why it takes several seconds until the three-phase line reaches the final equilibrium value where liquid-solidgas phase are in equilibrium. The oil drop volume was monitored using a microliter syringe (Hamilton, USA) and the drops were between 5 ( ± 1) μl in size. This has been reported to be the size that allows us to neglect the impact due to the drop’s weight [22]. Twenty contact-angle measurements were repeated for each sample, the presented values are the calculated averages and their standard deviations.

2.6. Dynamic wetting with oil 3.2. Chemical characterization using ATR-FTIR The advancing and receding angles were measured by employing the same contact-angle goniometer as for static contact angle measurements that was in addition to being upgraded with the heating module, upgraded with the tilting cradle that is controlled with dedicated software. The cradle enables continuous tilting of the goniometer from 0° to 110° ( ± 0.05°) with tilting speeds from 1°/s to 3°/s. The cradle also has a feature that retains the contact-angle goniometer in the position of the desired tilting angle, which was used in this work. The temperature changes within the liquid, on the solid surface and at the solid-liquid interface were monitored with a thermal camera (Optris® PI 160 Infrared Camera, Optris GmbH, USA), as described previously. The dynamic wetting measurements were performed at goniometer

The ATR-FTIR spectra of the surfaces after the additive-film was formed are presented in Fig. 5. The characteristic peaks show the presence of additive-film on all samples except for the sample that was treated with base oil only. These ATR-FTIR results again confirm that the studied additives adsorbed to the steel, forming an additive-film. This adsorbed film changes the wetting behaviour of oil, which is shown in the following results and discussion. For additives with two COOH groups, we see characteristic peaks in the ranges 1420–1190 cm−1 and 945–880 cm−1 that correspond to dicarbonyl vibrations (HOOC–R–COOH) [39]. The intensity of these peaks is higher for 16C-/2xCOOH than for 11C-/2xCOOH. Accordingly, these peaks are not observed for additives with one COOH group. For 4

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Fig. 4. Surface energies after the additive-film formation at 100 °C (a) and cleaning the samples with n-heptane, (b) and ultrasonically cleaning the samples in ethanol bath.

The ATR-FTIR spectra of the samples that were prepared at 100 °C show the same peaks as the ATR-FTIR spectra of the samples treated with the same additives at 25 °C [23]. However, the intensity of the peaks of samples prepared at 100 °C is significantly stronger (∼0.26 A.u.) and the peaks are more distinct than at 25 °C (∼0.08 A.u.). This again shows that the additive adsorption at 100 °C is stronger (chemisorption) and more effective than at 25 °C (physisorption). 3.3. Static wetting with oil The static contact-angles with oil measured at 100 °C are shown in Fig. 6. The results for additives with different non-polar tail chain length and different number of COOH head groups are shown in columns 1–5. Columns 1–3 show results for additives that have only one COOH and columns numbered with 4 and 5 those with 2xCOOH polar heads. The influence of the head-groups polarity is presented in columns 6–9. The effect of saturation is given in columns 3 and 9. The reference result without additives is presented in column 10. The static contact angles increased up to 48% when additive-film was present compared to the sample with no additive-film. This shows that all the tested additives form the steel surface more oleophobic. Additives that have only one COOH group increase the static

Fig. 5. The spectra of steel discs after the additive-film formation at 100 °C, recorded with ATR-FTIR.

additives with one and two COOH groups we observe bands at ∼2920 cm−1 and ∼2855 cm−1 that are assigned to the stretching and bending of CeC sp3 hybridisation (methylene (ν CH2) group) [28,39,40], and the strongest intensity is observed for additives with one COOH group. Two IR bands at ∼1700 cm−1 and ∼1460 cm−1 are assigned to the vibrations mode of (ν C]O) bonds [28,39,40]. The spectra of the surfaces that were treated with additives that have amine functional head-group have characteristic peaks at ∼2855 cm−1 and ∼1470–1365 cm−1 that correspond to eCH2eNH2 [39]. Additives with alcohol functional group show peaks corresponding to alcohols (ReOH) at 3500–3200 cm−1, ∼1470–1365 cm−1 and ∼1265–1140 cm−1 [39]. The only tested additive with amide functional group shows additional peaks at ∼3500–3200 cm−1 and ∼ 1300–1000 cm−1, corresponding to eCH2eCOeNH2 [39]. Carbonyl acids with eCOOH functional group show peaks at 1820–1670 cm−1 that correspond to (ν C]O) stretching and at 1320–1210 cm−1 that corresponds to (ν CeO) stretching [28,39,40]. The samples that were poured over with only base oil (without additives), put into an oven at 100 °C for duration of two hours and were cleaned as the samples with additives shows no characteristics peaks, confirming the surface energy results that no adsorbed film was formed.

Fig. 6. Static angles at 100 °C for oil on steel with and without additive-film. Note: The column number on the bottom is for easier following of the sample type. 5

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Additives having only one COOH head-group result in larger advancing angles by up to 45% compared to additives with 2xCOOH (Fig. 7a), indicating that with increasing number of additive polar headgroups the oleophobicity decreases. The wettability for additives with one COOH and 2xCOOH is comparable only at the lowest sliding angle (α = 10°). Increasing the non-polar tail length increases the oleophobicity up to 63% for additives having one COOH group (see Fig. 7b). When the non-polar tail increases from 11 C-atoms to 16 C-atoms, the advancing angles increase up to 18%, and from 16 C-atoms to 18 C-atoms up to 45% larger angles. The same trend is observed for additives with 2xCOOH (Fig. 7a), where longer chains increase advancing contact angles up to 28%. Increased head-group polarity results in larger advancing contact angles (Fig. 7c). The additive having COOH functional group that is the most polar (No. 9) increases the advancing angles up to 14% compared to CONH2 (No. 8), up to 18% compared to OH (No. 7), and up to 30% in comparison to the least-polar NH2 (No. 6). The effect of saturation was evaluated by comparing the saturated and monounsaturated fatty acid with 18 C-atoms (Fig. 7d). The saturated fatty acid gives up to 12% lower advancing angles than the unsaturated. Among all the tested additives at 100 °C, the additive-film formed from the monounsaturated long-chain fatty acid with 18 Catoms shows the most oleophobic behaviour (Fig. 7).

contact-angles up to 9% in comparison to additives with 2xCOOH groups. Increasing the non-polar tail length does not show a trend of a monotonic increase in the static angle. Additive with 16C-atoms provides up to 5% increase in contact angles compared to additive with 11 C-atoms, while additive with 18 C-atoms reduces the angles by 3% compared to the shorter 16C non-polar tail. On the other hand, chain length does not affect the wetting for the additives with 2xCOOH groups, since the static angles changed by less than 0.5%. Additives with different head-group polarity [33] and the same chain (18 C-atoms with one double bond) and were tested to evaluate the effect of the additive polarity on the wetting behaviour. The static angle results show that with increasing polarity of the additive headgroup the oleophobicity increases. Comparing to the least-polar tested additive with amine head-group, alcohol increases the static contactangles for 10%, amide for 13% and fatty acid for 16%. The effect of saturation on the wettability with oil was evaluated by comparing the saturated and monounsaturated fatty acid with 18Catoms. The results show that saturated fatty acid gives 9% lower static angles than the unsaturated. 3.4. Dynamic wetting with oil 3.4.1. Advancing contact angles Additives’ chemical structure influence on the advancing contact angles at 100 °C is shown in Fig. 7. Results show that the formed additive-films give up to 4 times larger advancing angles compared to the reference steel without additives. This indicates that the adsorbed film layer reduces the wettability with oil, making the surface more oleophobic. The reference sample with no adsorbed additive-film, however, shows oleophilic behaviour, with all advancing contact angles between 11° and 32°.

3.4.2. Receding contact angles Additives’ chemical structure influence on the receding contact angles at 100 °C is shown in Fig. 8. The formed additive-film increase the receding angles on steel by as much as 8.1 times compared to the reference sample without additives. This again shows that the presence of the additive-film significantly increases the oleophobicity. The reference sample with no adsorbed additive-film shows oleophilic

Fig. 7. Effect of (a) number of COOH groups; (b) length of non-polar tail; (c) polarity of the head-groups; (d) saturation of additive on advancing contact angle at high temperature for steel with and without additives. Note: A number by every curve is for easier following of the sample type. 6

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Fig. 8. Effect of (a) number of COOH groups; (b) length of non-polar tail; (c) polarity of the head-groups; (d) saturation of additive on the receding contact angle at high temperature for steel with and without additives. Note: A number by every curve is for easier following of the sample type.

48% in comparison to those with 2xCOOH (Fig. 9a). Amid all the tested additives, the films from additives with 2xCOOH show the highest CAH. Increasing the non-polar tail length decreases CAH by up to 23% for one COOH group additives (see Fig. 9b). Elongating the non-polar tail from 11 C-atoms to 16 C-atoms lowers CAH up to 15%, and from 16 Catoms to 18 C-atoms up to 8%. This trend, however, is not observed for the tested additives with 2xCOOH (Fig. 9a; No. 4, 5), since longer chain increases CAH up to 9%. Increasing the head-group polarity lowers the CAH (Fig. 9c). The most-polar additive with COOH functional group (No. 9) lowers the CAH by up to 5% compared to CONH2 (No. 8), by up to 15% compared to OH (No. 7), and by up to 27% compared to the least-polar NH2 (No. 6). Saturated and monounsaturated fatty acids with 18 C-atoms were compared for the effect of saturation (Fig. 9d). The saturated fatty acid (No. 3) lowers the CAH up to 17% in comparison to fatty acid that is unsaturated (No. 9).

behaviour, with all receding contact angles bellow 10°. Additives having one COOH head-group results in larger advancing angles by up to 98% compared to additives with 2xCOOH (Fig. 8a), indicating that with increasing number of additive polar head-groups the oleophobicity decreases. Among the tested additives, the films from additives with 2xCOOH are the least oleophobic. Increasing the tail chain length gives more oleophobic surfaces, namely the receding angles are larger up to 55% for additives with one COOH group (see Fig. 8b). When the non-polar tail chain length increases from 11 C-atoms to 16 C-atoms, this leads to up to 34% larger receding angles, and from 16 C-atoms to 18 C-atoms up to 21% larger receding angles. The same trend is found for additives having 2xCOOH (Fig. 8a; No. 4, 5), where longer chain gives increase in receding angles up to 32%. Increasing the polarity of the head-group leads to increased receding contact angles (Fig. 8c). The additive having carbonyl group, which is the most-polar (No. 9) increases the receding angles up to 32% in comparison to CONH2 (No. 8), up to 41% in comparison to OH (No. 7), and up to 75% in comparison to the least-polar NH2 (No. 6). Saturated and monounsaturated fatty acids with 18 C-atoms were compared for the effect of saturation (Fig. 8d). The saturated fatty acid gives up to 12% lower advancing angles than the unsaturated. Among all the tested additives at 100 °C, the additive-film formed from the monounsaturated long-chain fatty acid with 18 C-atoms shows the most oleophobic behaviour, evaluated with the receding angles as the wetting parameter (Fig. 8).

4. Discussion 4.1. Static vs. Dynamic wetting parameters Additive-film formation’s effect on the wetting of steel with oil at 100 °C was measured with static (θst ) and dynamic (θa , θr , CAH) wetting parameters. The ATR-FTIR and surface-energy results confirm that the studied additives adsorb to the steel at 100 °C (Fig. 2). The static and dynamic wetting parameters show that this formed additive-film changes the steel surface to affect the wetting behaviour with oil, namely, making the surfaces more oleophopbic. Nevertheless, all the selected parameters do not evaluate this change in wetting behaviour equally well, which was also noted for wetting at room temperature

3.4.3. Contact-angle hysteresis CAH Additives’ chemical structure influence on the contact-angle hysteresis (CAH) is shown in Fig. 9. Additives having only one COOH group give lower CAH for up to 7

Applied Surface Science 506 (2020) 145020

M. Kus and M. Kalin

Fig. 9. Effect of (a) number of COOH groups; (b) length of non-polar tail; (c) polarity of the head-groups; (d) saturation of additive on CAH at high temperature for steel with and without additives. Note: A number by every curve is for easier following of the sample type.

with dynamic, and not static, parameters. Furthermore, comparing the dynamic parameters we see that although they all show the same trends in the wetting behaviour, the receding contact-angles are the most differentiating among the three.

(25 °C) [23]. The static contact-angles indicate that oil exhibits good wetting behaviour on steel with and without the adsorbed additive-film. The measured static contact-angles are between 8.3° and 12.2°, indicates very oleophilic behaviour. Materials with high surface energy (metals like steel) are known to exhibit the so-called complete wetting behaviour when interacted with liquids having low surface tensions (common lubricating oils) [3,26,38,40]. Our static wetting results seem to comply with this literature and could be misinterpreted to indicate that the formed additive-film is practically insignificant for any change in wetting behaviour of oils on steel. Nevertheless, the relative increase of the static contact-angles for samples with adsorbed additives compared to the reference sample without the additive-film is up to 48%, which is quite significant. The results therefore indicate that the formed additive-layer alters the steel surface to make it more oleophobic, meaning that the interactions between the oil that wets the surface and the additive-film weaken. Still, the differences between the static contact angles are less than 3.9° (Fig. 6), which is certainly quite small (absolute) variation. This indicates that the static contact-angle is not the most fitting and sensitive wetting parameter to evaluate the changes in the wetting behaviour of oil. Similar suggestions about the suitability of static contact-angles for describing the wetting phenomena have already been made in some previous studies [18,19,38,41,42]. Contrary to static angles, when analysing the dynamic wetting parameters, the influence of the presence of the additive-film on the wetting with oil becomes very noticeable. Compared to reference samples without additives, the adsorbed additive-films give up to 4 times larger advancing contact-angles (Fig. 7), up to 8.1 times larger receding contact-angles (Fig. 8), and lower the CAH up to 52% (Fig. 9). The dynamic parameters therefore show that additives greatly reduce the interactions between oil and the surface it wets. The changes in the wetting behaviour are hence incomparably larger when we evaluate it

4.2. Additives’ chemical structure influence on the wetting with oil Fig. 10 summarises the empirical effects (in percent) of all the additives with different chemical structures, by showing the largest difference between the studied additives for wetting at 100 °C. The influence of the additives’ different number of head-groups, different length of the non-polar tail, different head-group polarity and different saturation, is presented by using the four wetting parameters. It is again 75

80

63

Difference, %

40

Static angle

55 30

26

16 5

Advancing angle

0 -9

-9 -23

-40

-27

-12 -12

-17

Receding angle

-45

-80

CAH

-98

-120 Nr. of polar Chain length heads

Polarity

Satura on

Fig. 10. The greatest variations (in %) in wetting at 100 °C for the additives with different number of polar heads, chain length of non-polar tail, polarity of the head-group, and saturation when evaluating the wetting with static and dynamic parameters. 8

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The saturated additive increases the wettability with oil at high temperature compared to the unsaturated additive (Figs. 6, 7d, 8d). This is the opposite to our previous results at room temperature, where we showed that the saturated additive reduced the wettability and the unsaturated additive improved the wettability with oil [23]. Studies performed at room temperature [14,54,55] have shown that (mono) unsaturated additives can form more disordered and heterogeneous films than compounds that are fully saturated. This would make us expect that at elevated temperature, the unsaturated fatty acid also forms less homogeneous and therefore less oleophobic films than the saturated fatty acid. However, our results show that at 100 °C the unsaturated fatty acid forms slightly more oleophobic films than the saturated one, although the receding angles differ by only up to 12% (Fig. 10). Unsaturated additives with one double bond in the alkyl chain are thermally less stable than saturated ones [34,49,54]. The oxidation stability of the adsorbed film of the unsaturated additive therefore decreases with increasing temperature [54], which can have an impact on the wetting behaviour.

shown that the differences in wetting with oil is the most obvious when evaluating it with dynamic parameters. As seen in Fig. 10, the number of polar head-groups has the largest relative effect of all the chemical parameters. Additives with 2xCOOH form the surface less oleophobic compared to additives having one COOH group, which therefore improve the oil wetting behaviour, Figs. 6–8. Literature reports [43] that at room temperature compounds with more than one functional group form less oleophobic films than compounds with one polar functional group, which is in agreement with our results. In fact, it has been reported that acids with two carbonyl groups can form a reacted layer at the steel, both with one or two COOH groups, as well as with other molecules from the additives [32]. This leaves the adsorbed film heterogeneous and consequently less oleophobic [32]. The possible free (not adsorbed) carbonyl group [44] at the surface promotes the interactions with the oil in surrounding, thus improving the wetting, which is again in agreement with our results at elevated temperature. The length of the non-polar tail does not monotonically increase the static contact-angle (Fig. 6); however, increasing the length gives significantly larger advancing angles (up to 63%) and receding contact angles (up to 55%) for both additives with one COOH and 2xCOOH groups in this work. As seen from Fig. 10, the length of the non-polar tail has significant chemical-structure influence at high temperature, although the significance with respect to changes in wetting are not equally strongly evaluated by the advancing and receding angles. In agreement with the main findings in this work, the non-polar tail length of various amphiphiles have been shown to increase the contact-angles at room temperature [45,46], but the friction modifiers have not been tested in these works. It has been reported that fatty acids with a longer non-polar tail form films that are more stable and strongly adsorbed compared to those of fatty acids having shorter alkyl chain [31,32,47], which affects the wetting behaviour; however, these studies were only performed at room temperature. Simple alkanes have been studied at temperatures above 50 °C and it was shown that the contact angles decrease with temperature and chain length [6]. It was suggested [6] that the temperature increases the thickness of the adsorbed film, which consequently affects the oil/additive-film interactions. We have shown that at 100 °C the longer non-polar chains of the friction modifiers increase the oleophobicity of the surface, meaning that the oil contactangles are larger. We have also shown that at high temperature the CAH decreases with the increasing chain length of the tested friction modifiers, which indicates that longer alkyl chains provide less resistance to motion for the three-phase line of the oil drop. With increased polarity of the head-group of the additive the wettability decreases (Fig. 10). It has been reported [48] that polar hydrocarbon molecules on solid substrates form layers that are more oleophobic than the original surface; however, the effect of temperature on this phenomenon has not been reported. Fig. 10 shows that polarity can decrease the wetting by up to 75%, measured with the receding angle, which shows the polarity does have an important effect, also at high temperature. The governing factor for the additive’s ability to form a strong adsorbed film is the polarity of the functional group [14,34,49,50]. It has been reported that stronger additive adsorption leads to a more closely packed film with oriented methyl groups, which decreases the interactions between the oil and the steel surface [43]. Furthermore, because more-polar additives form more closely packed films on steel, the surfaces are less heterogenic [43], and the oil slides over the non-polar layer of hydrocarbon tails more easily. Studies have shown that surfaces that are more heterogenic results in stronger resistance to the motion of the three-phase-line of the liquid droplets (the so-called pinning occurs), which results in large CAH [41,51–53]. In agreement with these low-temperature findings, this study also shows that increasing polarity lowers the CAH at high temperatures, which again indicates that the additive polarity decreases the interactions between the oil and the steel, and therefore decreases the wetting with oil.

4.3. Effect of an increase from room temperature to 100 °C on the wetting of steel with oil In this paper, we present the effect of presence of additives and their chemical structure on the wetting at 100 °C. In a companion study we have previously investigated these influences at 25 °C using the same additives [23], so we can directly assess and compare the effect of the higher temperature used in this work on the above-mentioned phenomena, as follows. At 100 °C the presence of additives reduces the interactions between the oil and steel, making the surface more oleophobic, the same as at 25 °C [23], and this was best observed with a dynamic wetting parameter, i.e., the receding contact angles. The ATR-FTIR results of the steel discs after additive film formation (Fig. 2) confirm the tested additives adsorbed to the steel surface at both 25 °C and 100 °C; however, the intensity of the characteristic peaks is greater at 100 °C. This shows that the additive adsorption is stronger at high than at low temperature, which is also supported by other reports [14,27,34,56,57]. The surfaceenergy results confirm that at 100 °C the additives chemically adsorb to the steel surface, while at 25 °C they adsorb via physical adsorption [23]. This is also the reason for the greater intensity of the ATR-FTIR spectra of additive films formed at 100 °C compared to the ones formed at 25 °C. At 100 °C the surface energies of the steel and the steel with additives are slightly lower than at 25 °C, which is in agreement with other literature reporting that with increasing temperature the surface energy of the metals decreases [11,24,25]. At 100 °C the oleophobicity increases with a decreasing number of COOH groups, with increasing chain length and with the increasing polarity of the functional head group. This is the same molecular effect as was found with 25 °C [23]. However, we observed that at 100 °C the saturated fatty acid showed slightly lower contact angles than the unsaturated fatty acid, which is opposite to the case at 25 °C. Thus, at 100 °C the oleophobicity slightly decreases with the saturation of the additive. The possible reason for the opposite saturation effect at low and high temperatures is the thermal stability of the unsaturated fatty acid. At 100 °C the film that was formed from unsaturated fatty acid is not completely stable since thermal oxidation occurs [34,49,54], so the chemical structure of the film is not exactly the same as the one at 25 °C, which might be the reason for differences in the high- and low-temperature results. We should emphasise, however, that at 100 °C the differences between the unsaturated and the saturated fatty acids were only up to 10° (Fig. 8d), i.e., 12%. Fig. 11 summarizes how the chemical structure of the additive changes the wetting behaviour at 25 °C [23] and 100 °C (receding contact angles are shown as the most sensitive wetting parameter). We see that the additives’ chemical structure, apart from the saturation, has 9

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No. of COOH groups

Chain length

Change in receding angles, %

100

98

76

80

Polarity

additive layer compared to 25 °C, i.e., the number of polar groups by 32.4%, the chain length by 27.6%, and the polarity by 23.5%. High temperatures were also reported earlier to decrease the static contact angles (improve wettability), because at higher temperatures the surface energy of the samples decreases [11,21,22], which is thus in agreement with our results. The effect of the additive chemical structure on the wetting of oil on steel at 25 °C and 100 °C is, for convenience, also schematically presented in Table 3.

Satura on

75

55

60

39

40 20 0 -20

-12

5. Conclusions

-40 -60 -80 -100

-74

1. The ATR-FTIR spectra and the calculated surface energies show that at 100 °C, all the studied additives chemisorb to the steel surface. This chemisorbed additive-films decrease the wetting with oil on steel at 100 °C, making the surfaces more oleophobic. 2. The dynamic parameters for wetting evaluation show, contrary to the static, a huge effect of the additives on the wettability with oil. Compare to reference sample without additives, the additive-films provide up to 4 times increase in advancing contact angles, up to 8.1 times larger receding angles, and up to 52% lower CAH. 3. At 100 °C the one COOH group additives, the additives with longer alkyl chain length, and those with more polar head-groups chemisorb to steel, and form layers, which reduce the wetting with oil

-98

T= 25 °C

T = 100 °C

Fig. 11. The most pronounced differences on wetting for various additive molecular structures on the receding angles at 25 °C (adapted from [23]) and 100 °C.

a similar effect at 100 °C and 25 °C; however, the measured angles with oil are generally lower at 100 °C than at 25 °C, indicating that a high temperature of 100 °C slightly decreases the oleophobicity of the

Table 3 Summary of how chemical structure of simple organic additives influence the wetting between the oil and steel at 25 °C (adapted from [23]) and 100 °C.

Additive film formation Surface energy Wetting of steel with oil Wetting of steel with additive film with oil Poor wetting with oil (oleophobic behaviour)

25 °C

100 °C

Physisorption Low Oleophilic Oleophobic

Chemisorption Lower than at 25 °C More Oleophilic than at 25 °C Less Oleophobic than at 25 °C

One COOH group

One COOH group

Long non-polar tail

Long non-polar tail

More polar head-group

More polar head- group

Saturated

Unsaturated

10

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more than the additives having two COOH groups, short non-polar tails, and less polar head-groups, which is the same behaviour as at 25 °C. The reduced oleophobicity at 100 °C compared to room temperature due to the molecular structure is 32.4%, 27.6% and 23.5% for the number of polar head-groups, the non-polar tail length and the polarity, respectively. 4. The saturated additive at 100 °C shows better wetting, i.e., lower oleophobicity, than the unsaturated, which is the opposite behaviour than at 25 °C. The difference in the saturation effect at the two temperatures is up to 12%, which is up to 10° if evaluated using receding contact angles as the wetting parameter.

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CRediT authorship contribution statement M. Kus: Conceptualization, Methodology, Software, Data curation, Writing - original draft. M. Kalin: Visualization, Supervision, Writing review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors acknowledge the financial support from the Slovenian Research Agency, Slovenia (research core funding No. P2-0231). References [1] G. Kumar, K.N. Prabhu, Review of non-reactive and reactive wetting of liquids on surfaces, Adv. Colloid Interface Sci. 133 (2007) 61–89. [2] L. Gao, T.J. McCarthy, Wetting 101°, Langmuir 25 (2009) 14105–14115. [3] E.Y. Bormashenko, Wetting of real surfaces, Walter de Gruyter GmbH, Berlin/ Boston, 2013. [4] W.D. Kaplan, et al., A review of wetting versus adsorption, complexions, and related phenomena: the rosetta stone of wetting, J. Mater. Sci. 48 (2013) 5681–5717. [5] A.W. Adamson, Physical chemistry of surfaces, fifth ed., Wiley, New York, 1990. [6] M.E. Diaz, M.D. Savage, R.L. Cerro, The effect of temperature on contact angles and wetting transition for n-alkanes on PTFE, J. Colloid Interface Sci. 503 (2017) 159–167. [7] C.J. Budziak, E.I. Vargha-Butler, A.W. Neumann, Temperature dependence of contact angles on elastomers, J. Appl. Polym. Sci. 42 (1991) 1959–1964. [8] F.D. Petke, B.R. Ray, Temperature dependence of contact angles of liquids on polymeric solids, J. Colloid Interface Sci. 31 (2) (1969) 216–227. [9] J.D. Bernardin, et al., Contact angle temperature dependence for water droplets on practical aluminum surfaces, Int. J. Heat Mass Transf. 40 (5) (1997) 1017–1033. [10] A.W. Neumann, Contact angles and their temperature dependence: thermodynamic status, measurement, interpretation and application, Adv. Colloid Interface Sci. 4 (2) (1974) 105–191. [11] C. Rulison, Effect of Temperature on the Surface Energy of Solids. KRÜSS Application Report, 2005. [12] M.D. Ruijter, et al., Effect of temperature on the dynamic contact angle, Colloids Surf. A: Physicochem. Eng. Aspects 144 (1988) 235–243. [13] E.V. Gribanova, Dynamic contact angles: Temperature dependence and the influence of the state of adsorption film, Adv. Colloid Interface Sci. 39 (1992) 235–255. [14] H. Spikes, Friction Modifier Additives, Tribol. Lett. 60 (2015) 5. [15] H.A. Spikes, Film-forming additives - direct and indirect ways to reduce friction, Lubr. Sci. 14 (2) (2002) 147–167. [16] M. Ratoi, et al., Mechanisms of oiliness additives, Tribol. Int. 33 (2000) 241–247. [17] H.A. Spikes, Additive-additive and additive-surface interactions in lubrication, Lubr. Sci. 2 (1) (1989) 3–23. [18] M. Kalin, M. Polajnar, The effect of wetting and surface energy on the friction and slip in oil-lubricated contacts, Tribol. Lett. 52 (2013) 185–194. [19] M. Polajnar, M. Kalin, Effect of the slide-to-roll ratio and the contact kinematics on the elastohydrodynamic friction in diamond-like-carbon contacts with different wetting behaviours, Tribol. Lett. 60 (8) (2015). [20] M. Polajnar, et al. Role of lubricant and test temperature for EHD friction reduction by employing DLC coatings, in: ECOTRIB 2019, 2019. [21] M. Polajnar, et al., The Effect of different additives on interfacial properties and ehd

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