Characterization of Water Solubility in n-Octacosane Using Raman

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Characterization of Water Solubility in nOctacosane Using Raman Spectroscopy Cédric Giraudet, Konstantinos D. Papavasileiou, Michael Heinrich Rausch, Jiaqi Chen, Ahmad Kalantar, Gerard P. van der Laan, Ioannis George Economou, and Andreas Paul Fröba J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b07580 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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The Journal of Physical Chemistry

Characterization of Water Solubility in n-Octacosane Using Raman Spectroscopy Cédric Giraudet,a Konstantinos D. Papavasileiou,b Michael H. Rausch,a Jiaqi Chen,c Ahmad Kalantar,c Gerard P. van der Laan,c Ioannis G. Economou,b,d and Andreas P. Fröba*,a a

Institute of Advanced Optical Technologies – Thermophysical Properties (AOT-TP), Department of Chemical and Biological Engineering (CBI) and Erlangen Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander-University ErlangenNürnberg (FAU), Paul-Gordan-Straβe 6, 91052 Erlangen, Germany b

National Centre for Scientific Research “Demokritos”, Institute of Nanoscience and Nanotechnology, Molecular Thermodynamics and Modelling of Materials Laboratory, GR15310 Aghia Paraskevi Attikis, Greece

c

d

Shell Global Solutions International B.V., Grasweg 31, 1031 HW Amsterdam, The Netherlands

Chemical Engineering Program, Texas A&M University at Qatar, Education City, PO Box 23874, Doha, Qatar

__________________________ * Author to whom correspondence should be addressed. Tel. +49-9131-85-29789, Fax +499131-85-25851, E-mail [email protected]

ACS Paragon Plus Environment

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Abstract In this study, we demonstrate the ability of polarization-difference Raman spectroscopy (PDRS) to detect dissolved free water molecules in a n-octacosane (n-C28H58) liquid-rich phase, and thus to determine its solubility, at temperatures and pressures relevant to the Fischer-Tropsch synthesis. Our results for the pure alkane reveal thermal decomposition above a temperature of 500 K as well as an increase of gauche conformers of the alkane chains with an increase in temperature. For binary homogeneous mixtures, raw spectra obtained from two different polarization scattering geometries did not show a relevant signal in the OH stretching frequency range. In contrast, isotropic spectra obtained from the PDRS technique reveal a narrow and tiny peak associated with the dangling OH bonds. Over all the complete range of temperatures and pressures, no signature of hydrogen-bonded water molecules was observed in the isotropic Raman scattering intensities. A thorough investigation covering a large range of temperatures and pressures using PDRS signals showed that the higher the fraction of gauche conformers of hydrocarbon, the higher the solubility of water. The proportion of gauche and trans conformers was found to be water concentration-independent, and the intensity of the OH-dangling peak increased linearly with increasing the vapour partial pressure of water. Therefore, we established a relation between a relevant intensity ratio and the concentration of water obtained from SAFT calculations. In opposite to the results from relevant literature, the calibration factor was found to be temperature independent between 424 K and 572 K. The isotropic Raman scattering intensities are corrected in order to provide a better representation of the vibrational density of states. The influence of correction of the isotropic scattering intensities on the solubility measurements as well as on the analysis of the molecular arrangement is discussed.


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Introduction Water and oil mixtures are the typical example of solvophobicity. This phenomenon is limited and essentially depends on the molecular structure of the oil in terms of, e.g., chain length, and its polarity. The molecular arrangement in the bulk of homogeneous and heterogeneous water and oil mixtures affects their thermophysical properties, which are of major interest in process and energy engineering1 as well as in pharmaceutical2 and environmental sciences.3 However, despite an improved understanding of interactions between water and hydrophobic molecules since the early 1990s

4–8

and the development of

reliable predictive models for respective solubilities,9–16 there is still a clear lack of knowledge on the solvation of water in n-alkanes where the chain length is higher than in nC16H34. In this paper, the temperature and pressure dependency of revealed OH dangling peaks4,17–19 are assessed using polarization difference Raman spectroscopy (PDRS) to determine the solubility of homogeneous mixtures composed of dissolved water (H2O) in noctacosane (n-C28H58). The solubility of hydrocarbon in water is governed by the collective ability of water to form a network by means of hydrogen bonds5,20–22 and to form dangling OH (OHd) bonds.23–27 As a solute, water molecules are exclusively present in the in the alkane liquid-rich phase in form of monomers.28 The inability of water molecules to form hydrogen bonds in when water acts as a solute is explained by the breaking of its hydrogen bonds before its spreading in a hydrophobic solvent.2 Nevertheless, the exclusive presence of free OH groups in the alkane liquid-rich phase has only been shown for small hydrophobic solvents28 in which water solubility is low. In solvents consisting of large molecules, the water solubility increases significantly.29,30 Hence, one can expect some similarities with large hydrophobic solutes


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dissolved in water. Water molecule arrangement around alkane chains and water solubility are mainly governed by van der Waals interactions. These interactions can be quantified by the number of perturbed water molecules at the molecular interface or, in other words, by the formation of OHd bonds in the bulk phase. OHd bonds are the signature of free OH groups. A quantitative and in situ measurement of the relative concentration of a component solution can be achieved using Raman spectroscopy from the ratio of the heights, or areas, of two respective peaks characteristic for each mixture component.31–37 Standard Raman scattering techniques, however, can be limited by the presence of fluorescence backgrounds, which can mask the pure Raman signal.38 They are also often restricted regarding the study of components with a small Raman scattering cross-section, such as water. Therefore, the determination of the concentration of a mixture containing one component with a low Raman cross section and in presence of a large fluorescence background remains challenging. Many techniques have been devoted to solving the overwhelming fluorescence background problem, from data processing techniques39–42 to new experimental methods based on timeresolved setups43 or on spectra subtraction such as shifted-excitation Raman difference spectroscopy.44 These experimental techniques have been widely investigated and improved over time. All of them, however, require substantial modifications to the standard Raman spectroscopy technique, as explained in the reviews by Kiefer45 and of De Luca et al.46 as well as in the references cited therein. Recently, Le Ru et al.47 proposed the polarization difference resonance Raman spectroscopy (PD-RRS) technique, which can overcome the fluorescence background problem as well as “extract” tiny peaks of polarization-dependent systems with a resolution which cannot be achieved by the majority of time-resolved setups. Le Ru et al. demonstrated on Nile Blue and rhodamine 6G that the resolution of resonance


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Raman spectra is larger than that of standard Raman spectra. This method can be applied with continuous waveform excitation and can be integrated in a conventional Raman setup, but it is limited to the resonance. The present paper discusses Raman spectroscopy measurements based on the polarizationdifference technique used to determine the relative amount of water dissolved in liquid nC28H58 at temperatures and pressures that are relevant to Fischer-Tropsch synthesis.48,49 After a description of the spectra acquisition and analysis strategy, the Results and Discussion section is divided in two subsections. The first subsection reports the increase in gauche alkane conformers resulting from the increased temperature favouring the formation of aggregates in pure n-C28H58, while the second subsection describes the results for water dissolved in liquid n-C28H58. The PDRS spectra show a narrow peak associated with the dangling OH bonds, whose temperature and pressure dependency on its height can be analysed. Finally, the results show that the height of the OHd peak can be directly correlated to the relative concentration of water in the n-octacosane liquid-rich phase. Experimental results are compared to vapor-liquid equilibria (VLE) calculations based on the statistical associating fluid theory (SAFT) equation of state.50–52 Specifically, the model proposed by Huang and Radosz for pure components and mixtures was used.52,53

EXPERIMENTAL METHODOLOGY Materials and sample preparation. The investigated n-octacosane was purchased from Alfa Aesar GmbH & Co. KG with a purity of 99% by mass. Liquefied n-C28H58 was filtered using a filter with 200 nm pore size in order to obtain particle free samples. Deionized water was used without further purification. As represented in Figure 1 (A), after filling the sample cell


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with liquefied and particle-free n-C28H58, the cell is placed in the setup and connected to the water vessel by a set of high pressure and high temperature valves and tubes. Before injection of water in the cell, remaining air in the pipes and in the cell was removed by a vacuum pump. The pressure in the sample cell, corresponding to the vapour pressure of water, was controlled by adjusting the temperature of the water vessel. In average, the temperature stability was of 0.6 mK per hour and the pressure stability was of 0.004 MPa per hour. More details about the temperature control, the sample cell and the manifold system can be found in Ref. 54.

Raman spectra acquisition and post-treatment. The Raman setup used here represents an extension of the optical arrangement previously developed for the measurement of diffusivities of gases and water dissolved in heavy n-alkanes by means of dynamic light scattering (DLS).54,55 A drawing of the extension of our DLS setup for Raman spectroscopy is represented in Figure 1 (A). Raman spectroscopy was conducted using a QE65Pro spectrometer (from Ocean Optics), a frequency-doubled Nd:YVO4 laser source (λ0 = 532 nm, from Coherent) operated at 1.75 W, and a collection of optical components including lenses (L), mirrors (M) and a long pass filter (LF). In order to modulate the Raman signal,47 the polarization of the laser excitation was controlled using a half-wave plate mounted on a rotary support (WP), and the polarization detection was fixed to a vertical (V) polarization by means of a sheet polarizing filter (SPF).


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Figure 1. Experimental setup. (A) Setup. The excitation polarization is modulated by a rotated half-wave plate (WP). The scattered light is collected to the spectrometer (QE65Pro) by a set of lenses (L) and a mirror (M). (B) Excitation polarization dependency of the Raman signal of a peak associated to a stretching mode of n-alkanes.

The laser power measured at the sample was of 1.3 W. The large difference between the output laser power and the laser power at the sample can be explained by the beam splitter as well as different optical element including a polarization beam splitter and several mirrors used for dynamic light scattering. The laser power, which was about five times larger than that often used in Raman spectroscopy of liquids, was chosen to provide the best resolution in the spectra. The relative power fluctuations, i.e. the ratio between the power fluctuations and the power of the laser, are small enough to neglect their influence in our spectra. Moreover, even by reducing the output laser power to 1 W, we always observed a beam expansion in our


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study either caused by absorption or diffraction rising from particles. Therefore, parasite effects linked to the high laser power should be discussed. First, as mentioned in more detail in the first part of the Results and Discussions, laser-induced molecular cracking at high temperatures seems to be negligible. Secondly, assuming that the laser induced a thermal gradient, a radial concentration gradient will result due to the Soret effect.56 For leading to a significant local concentration gradient, the Soret coefficient has to be large which is not common in liquid mixtures.57 Thus, such a phenomenon can be neglected most probably. Finally, thermal polarization of water can be observed, leading to a reorientation of water molecules along the thermal gradient.58 While this phenomenon can highly influence the structure of a hydration shell in an aqueous solution, the thermopolarization coefficient remains small far from a critical point.59

Because our systems of investigation showed intense fluorescence, which can hide information on the molecular structure, we used a polarization-difference method to overcome this problem. For each thermodynamic state, i.e. each pressure and temperature, the measurement strategy consists of acquiring the spectra for each horizontal (H) polarization and vertical (V) polarization laser excitation over the 360° of the rotary support. Figure 1 (B) depicts the evolution of the intensity of the first CH3 stretching mode from Raman spectra of pure n-octacosane as a function of the rotation angle of the half-wave plate which was used to determine the position of each V and H polarization excitation.

Figure 2 (A) and (B) show the post-treatment methodology. Isotropic Raman scattering intensities I iso are obtained by subtracting the vertical-vertical polarization scattering intensities I VV by the horizontal-vertical polarization scattering intensities I HV multiplied by


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a geometry factor γ equal to 4/3.60,61 In order to extract quantitative information from tiny peaks, we use a simple statistical approach which consists into acquiring each spectrum acquired under V polarization excitation and H polarization excitation and averaging the 16 resulting isotropic spectra, I iso (ν , T ) =

4 1 × 16 i =1

4

∑∑ I ( ) (ν , T ) − γ × I ( ) (ν ) , i VV

j HV

(1)

j =1

where ν is the Raman shift and T the temperature. The superscripts i and j refer to the number of each maximum and minimum shown in Figure 1 (B). The statistic limits the intensity of the visible Fabry-Perot interferences and is needed to extract quantitative information from narrow and tiny peaks. For the best representation of the vibrational density of states, the isotropic spectra are corrected following the expression proposed by Paolantoni et al.62 * (ν , T ) = ν 1 − exp − hν I iso  kT 

 −3 (ν L − ν ) × I iso (ν , T ) , 

(2)

where ν L is the frequency of the excitation, h is the Planck constant, and k is the Boltzmann constant. The influence of this correction on the analysis of the molecular arrangement as well as on the determination of the concentration is discussed in the Results and Discussions section.


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Figure 2. Methodology example. Raman spectra of pure n-C28H58 at atmospheric pressure and T = 548.45 K. (A) Raman spectra obtained for the two different polarization scattering geometries. (B) Final normalized iso*-spectrum. * are normalized using standard normal variate (SNV) The resulting isotropic spectra I iso

normalization,63 see Figure 2 (B). In the following, the notation iso*-spectrum refers to the isotropic averaged, corrected, and normalized scattering intensities. Finally, relevant bands of the iso*-spectra, i.e. referring to the CH and OH stretching regions, were adjusted using a sum of Gaussians and a linear baseline correction. In the manuscript, the superscript * always refers to the corrected isotropic intensities, see Eq.(2).

RESULTS AND DISCUSSIONS Pure n-octacosane. Figure 2 shows representative spectra of the pure n-octacosane, which was investigated under atmospheric pressure for temperatures between 398 and 548 K. In the VV and HV spectra, a similar fluorescence background could be found irrespective of the temperature, see Figure 2 (A). An emerging fluorescence background was observed above


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500 K due to the thermal decomposition of n-C28H58.64,65 In Figure 2 (B), it can be seen that the overwhelming background resulting from the thermal decomposition of n-C28H58 can be compensated by the PDRS technique. This is true as long as the fluorescence intensity is polarization-independent. Furthermore, a gas chromatographic analysis of n-octacosane samples investigated at temperatures above 500 K showed that their purity is only reduced from a mass percentage of 99.5% at T = 398 K to 98.3% at T = 548 K. The chromatograms revealed a slight increase of the amount of shorter n-paraffins, olefins, carbonyls, branched ketones and branched alcohols. While the presence of branched ketones and alcohols in the bulk will increase the solubility of water, shorter n-paraffins will decrease its solubility. Considering the low concentration of impurities formed at high temperatures, it seems to be reasonable to neglect their influence on the solubility. Figure 3 (A) shows two representative iso*-spectra in the CH stretching frequency domain, i.e. between 2800 cm-1 and 3000 cm-1, at temperatures of 398 K and 548 K. In agreement with relevant literature, the CH stretching domain can be decomposed in five fundamental ss ss contributions, which are the CH2 symmetric mode ν CH , the CH3 symmetric mode ν CH , the 2 3 as as,out CH2 asymmetric mode ν CH , the CH3 asymmetric out-skeletal-plane mode ν CH , and the CH3 2 3 as,in asymmetric in-skeletal-plane mode ν CH . All the CH stretching bands were fitted to within 3

0.5% of error after linear baseline subtraction, where this value is calculated from the root mean square deviation.


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Figure 3. Conformation changes of n-octacosane as a function of temperature. (A) iso*spectra of pure n-C28H58 at T = 398.35 K (

) and at T = 548.45 K (

) after baseline

subtraction, and their five respective CH stretching modes. The symbols represent the raw data. The thick lines represent the global fit after baseline subtraction, while the thin lines represent the different contributions of the CH stretching. (B) Evolution of the ratio *

ss as, out R = I CH / I CH 2 3

*

as a function of temperature for pure n-C28H58 ( ) and for the

homogeneous water and n-C28H58 mixtures ( ).

The measurements performed on pure n-C28H58 using the PDRS technique showed an ss as,out intensity switch between ν CH and ν CH which is highly sensitive to intramolecular Fermi 2 3

resonance interactions.66,67 As suggested by Wunder et al.,68 we studied the evolution of the ss ratio between the peak heights R = I CH

* 2

*

as, out , see green circles in Figure 3 (B). This ratio / I CH 3

is highly sensitive to the proportion of trans and gauche conformers. Similar to the results observed for n-C16H34,68 R linearly decreases with the temperature, which reflects an increase in gauche conformers. This results from the effect of higher thermal energy that allows energetically less favourable conformations, such as gauche, to increase their probability. An increase of gauche conformers favours alkane-alkane contacts per unit of


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chain length which increases the van der Waals interactions. By comparing the results obtained with and without the correction method proposed by Paolantoni et al.,62 see Eqs. (2) and (1), it can be seen that the ratio R remains the same over the complete temperature range.

Homogeneous n-octacosane/water mixtures. The temperature-dependency of the CH stretching bands of the homogeneous n-C28H58/H2O mixtures follows the very same behaviour as for pure n-C28H58. As shown by the red squares in Figure 3 (B), the relative quantity of gauche conformers rises with increasing temperature. Since this increase in gauche conformers was found to be pressure-independent, we can conclude that water molecules do not play a significant role in the molecular arrangement of the alkane chains in the bulk. Similar studies on ethylenediamine69 as well as on dimethoxyethane and dimethoxymethane70 have shown an opposite effect. By applying Raman spectroscopy, both Cáceres et al.69 and Wada et al.70 revealed an increase of gauche conformers by increasing the water concentration. The absent influence of water molecules on the observed alkane-alkane contacts can be explained by the water spreading in the alkane liquid-rich phase during the solvation process combined with its low solubility,30 which hinders the formation of hydrogen bonds. Hence, although the solubility of water in n-C28H58 is considerable compared to that in small n-alkanes,30,71 water molecules do not influence the alkane conformations and, subsequently, the solubility, regardless of the temperature and pressure of the binary mixture. As the solubility of water in long alkane chains increases with temperature,30,71 the only conclusion which can be drawn here is that the presence of gauche conformers favours the solubility of water.


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Figure 4 shows representative measurement examples for the raw Raman spectra and the averaged iso*-spectra. At frequencies greater than those characteristic for CH stretching, the raw VV-spectra and the HV-spectra do not clearly show a signal between (3150 and 3800) cm-1. This frequency range is relevant to OH stretching of water.72 In opposite, over the complete range of temperatures and pressures, the iso*-spectra showed a narrow peak at a frequency of (3627 ± 1) cm-1, see Figures 4 and 5, whose intensity is pressure and temperature dependent.

Figure 4. Raman spectra of the homogeneous n-C28H58/H2O mixtures at different temperatures and pressures. On the top row, from the left to the right side: raw spectra for the VV-polarization scattering geometry, raw spectra for the HV-polarization scattering geometry, and the associated iso*-spectra. On the bottom row, enlarged views of the same signals as the ones in the upper row at frequencies corresponding to the OH stretching region are shown. For clarity of the graph, spectra have been rescaled.


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In contrast to results on pure n-C28H58, the PDRS technique does not allow for a complete suppression of the fluorescence background. This is most probably due to the anisotropy of the system in form of particles which could be observed by the presence of particle scattering. The set of spectra shown in Figure 4 are representative for measurements at low and moderate water concentrations. One can clearly observe a well-defined Gaussian peak rising from water molecules dissolved in the n-octacosane liquid-rich phase. Although the signal-to-noise ratio is obviously low, no systematic could be observed in the residual plots. The peak, which is centered at (3627 ± 1) cm-1, can be attributed to the OH dangling bonds (OHd), i.e. to the signature of OH groups which are not altered by hydrogen bonds. The reported frequency and the related uncertainty correspond to the average value and its standard deviation determined over the complete set of measurements. By the present spectra, it can be conclude that, in opposite to our expectation developed in the Introduction, there is no similarities between water dissolved in large alkane chains and large alkanes dissolved in water. In fact, with the actual resolution of our spectra, no signal rising from hydrogen-bonded water molecules could be observed. The correction applied for the calculation of the isotropic scattering intensities, see Eq. (2), does not affect noticeably the results. It has been seen that it only allows for a better resolution of the OHd peak. This is particularly true for measurements at high temperatures, where the ratio between the intensity of the fluorescence background and the intensity of the CH stretching mode can reach 1500:1. Also considering the uncertainty, the frequency of the observed OH dangling peak in the nC28H58/H2O mixture is significantly lower than the frequency determined experimentally on alcohol/water mixtures27,73 or aqueous solutions of sodium decanoate.74 This, however, can


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be explained by the low water concentration75 as well as by the nature of the hydrophobic molecule. Perera et al.27 observed a slight decrease of the OH dangling peak frequency with increasing the chain length of the solute dissolved in water. Similarly, Tassaing76 observed a frequency red-shift by increasing the temperature at constant pressure and concentration of a water and benzene solution. In our investigations, however, no significant temperature or pressure dependency was observed. The given frequency of the OH dangling peak is the average calculated over all the investigated thermodynamic states. Within this representative set of spectra, it clearly appears that the dangling bond observed is attributed to a signal from water, and that this signal is characteristic for water molecules dissolved in the alkane liquid-rich phase. For all temperatures, the intensity of the OHd peak increases linearly with pressure. This increase is purely due to the increase in the concentration of water in the alkane liquid-rich phase according to Henry’s law. Thus, the probability of finding molecules of water in the vicinity of an alkane chain in the sampling volume increases linearly with increasing pressure. This implies that the restricted analysis of the OHd bond allows for the determination of the relative amount of water in the solution. Uncertainties on the height of the OHd peak were estimated by calculating the ratio between the standard deviation of the measured data from the fit and the amplitude of the peak. The average uncertainty was found to be 15%. In Figure 5 (A), for calibration purposes, the solubility of water in n-octacosane at the temperature and pressure range in question was determined by VLE solubility calculations for calibration purposes. Symbols represent the calculation results and the lines the linear fit. For the calculations, the SAFT52,53 equation of state model was used, which is a powerful method for studying thermodynamic properties of such mixtures.77 SAFT was originally


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developed in the early 1990s. Since then, the model has been extended to account for polar and electrostatic interactions, for mixtures of molecules that vary widely in size and shape, etc. Several review papers summarize nicely such models.77–79 In this work, the model proposed by Huang and Radosz52,78,79 was used. The model was shown to be very accurate for the correlation of water solubility in hydrocarbons from methane to heavy nalkanes.15,80,81 According to our literature survey, only Breman et al.30 have measured the solubility of water in n-octacosane. While their experimental uncertainties are smaller than 5% for the solubility of water in n-decane and in n-hexadecane, the uncertainty is 14% for noctacosane.30 Using a temperature-independent binary interaction parameter k ij , Economou and co-workers showed that the model values are within less than 5% compared to experimental data over a broad temperature range for n-alkanes up to hexadecane.15,80 This is why we prefer to use the SAFT data for our purposes. Furthermore, Economou and coworkers found that as the n-alkane size increases, k ij reaches a constant value which is very advantageous for mixtures where no experimental data exist.15 The number of segments per molecules m, the temperature-independent segment volume parameter υ 00 , and the temperature-independent energy parameter u 0 / k B used in SAFT for water and n-octacosane are shown in Table 1. k B is the Boltzmann’s constant. Water was modeled as a 4-site associating component with an energy of association ε HB / k B = 1634.7 K and a volume of association κ = 0.3374 .


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Table 1. SAFT pure component parameters for water and n-octacosane.

Component

m

υ 00 (cm3⋅mol-1)

u 0 / k B (K)

Ref.

water

2.853

3.3041

167.10

Voutsas et al.80

n-octacosane

19.287

12

209.96

Huang et al.52

VLE TP-Flash phase equilibria calculations were performed by means of the SciTherm module in the Scienomics MAPS software package82 by setting k ij equal to 0.33, a typical value used in the study of water/n-alkane mixtures.15 In line with Henry’s law and the experimental results, VLE SAFT calculations display a linear increase in the concentration of water with increasing vapour pressure. Although it is well known that a slowing down of the concentration increase with increasing pressure occurs in the vicinity of the three-phase line, it is not relevant here due to the limited pressure range.83 All the measurements and calculations which were performed in this study, are sufficiently far from the three phase line. In the present study, the analysis of the intensity of the OHd peak only seems to be sufficient to calculate the concentration in a water/oil mixture. At a fixed temperature, the intensity of the OH dangling peak is directly proportional to the concentration of water in the bulk, * I OHd = K * (T ) × x H 2O , where K (T ) can be effectively adjusted by a first-order polynomial,

similarly to the ratio R . For calibration purposes, the concentration of water from the VLE calculations can be correlated to a peak intensity ratio, I r* = K * (T ) × xH 2O .33 From the collected spectra, it clearly appeared that the OH dangling peak is relevant for water, and that both *

ss as, out and I CH I CH 2 3

*

are relevant for n-octacosane due to their temperature sensitivity. Thus, we *

*

ss as,out + I CH ) for the calibration. The results are used the peak intensity ratio I r * = I OHd* ( I CH 2 3


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reported in Figure 5 (B) by the empty symbols. For the investigated pressure and temperature ranges, it was found that the calibration function can be assimilated to a constant K * (T ) = (0.22 ± 0.04 ) . The fit is represented in Figure 5 (B) by the dashed line. The

uncertainty of the calibration factor K * was determined by calculating the maximum absolute deviation.

Figure 5: Calibration curves of water dissolved in n-C28H58. Results from four different temperatures T : 424.11 K ( (

), 448.13 K (

), 547.45 K (

), and 571.90 K

). (A) Vapour-liquid equilibrium calculations using SAFT equation of state. The

lines represent a linear fit. (B) Calibration curve of the intensity ratio as a function of the * molar fraction of water. Empty symbols correspond to the results from I iso , the dashed line

corresponds to the linear fit I r* = f (x H2O ) . Filled symbols corresponds to the results from the uncorrected isotropic scattering intensities I iso , the continuous line represents the linear fit

(

)

I r = f x H 2O .


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In their manuscript,62 Paolantoni et al. proposed to correct the isotropic scattering intensities by Eq.(2) in order to have a better representation of the vibrational density of states. While it was used to determine the tetrahedral ordering of water by means of hydrogen bonds at different temperatures, this correction was never applied for the determination of the relative concentration of a mixture by Raman spectroscopy. In Figure 5 (B), we have reported the variation of the intensity ratio I r , i.e. the uncorrected isotropic intensities as a function of the concentration of water estimated from SAFT. Similar to the calibration curve I r* = f (x H2O ) , the calibration factor obtained from I r = f (x H2O ) was found to be temperature independent. Nevertheless, it can be seen from the coefficient of determination R2 that the intensity ratio of * the corrected isotropic intensities I iso allows for a better evaluation of the solubility of water

in n-octacosane. Due to the low signal-to-noise ratio of our measurements, it can be estimated that the PDRS technique combined with our statistical approach cannot detect water concentration below 2 mol%.

To the best of our knowledge, these results are the first to show evidence of free water molecules in the bulk of mixtures where water is dissolved in liquid n-octacosane. As mentioned above, the presence of water in the bulk does not affect the conformation of the alkane chains. Within the studied conditions, only the temperature has a major impact on the molecular arrangement of n-C28H58 by increasing the proportion of gauche conformers in the bulk. It explains the increase in solubility resulting from an increase in temperature at saturation. Moreover, the presence of only free water molecules dissolved in the bulk confirms that results from dynamic light scattering under similar conditions54 are associated with pure molecular diffusion and not with that of clathrate-like structures.


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CONCLUSION The present study shows the ability of the PDRS technique combined with a statistical approach to extract quantitative concentration information from the spectra of homogeneous water/n-octacosane binary mixtures under conditions relevant to Fischer-Tropsch synthesis. With respect to pure n-C28H58, its thermal decomposition above 500 K induces an increase in the fluorescence background which can be satisfactorily removed using the PDRS technique. Moreover, the results indicate an increase in gauche conformers with increasing temperature, which favours alkane-alkane contacts per unit of chain length. For the binary mixtures, the PDRS technique allowed us to verify the presence of a tiny narrow peak associated to dangling OH bonds which are highly sensitive to pressure and temperature. Our spectra show only evidence of free OH groups in the bulk rising from water molecules. Thus, although the concentration of water is high, in contrast to results on alkanes dissolved in water, no liquidvapour-like interface was observed. The intensity of the OH dangling peaks was found to correlate to Henry’s law. The findings in this study concur with previous observations and predictions on the temperature dependence as well as the packing effect on the percentage of accessible wetting sites and solubility. For calibration purposes, the ratio between the height of the OHd peak and the sum of the two peak heights relevant for the alkane packing was chosen. Over the investigated pressure and temperature range, it was found that the amount of water dissolved in the alkane liquid-rich phase is directly proportional to the intensity ratio. A thorough investigation on the influence of the correction on the isotropic Raman scattering intensities proposed by


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Page 22 of 27

Paolantoni et al., see Ref. 62, allows for a better estimation of the vibrational density of states, and thus of the concentration of water dissolved in the n-octacosane liquid-rich phase.

ACKNOWLEDGMENTS This work was financially supported by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) by funding the Erlangen Graduate School in Advanced Optical Technologies (SAOT) within the German Excellence Initiative. Financial support from Shell Global Solutions International BV in the form of a contracted research agreement is gratefully acknowledged.

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Characterization of Water Solubility in n-Octacosane Using Raman

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