Decoupling of Dynamic Processes in Surfactant-Based Liquid Mixtures

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Decoupling of dynamic processes in surfactant-based liquid mixtures: the case of lithium-containing Bis(2ethylhexyl)phosphoric acid/bis(2-ethylhexyl)amine systems Isabella Nicotera, Cesare Oliviero Rossi, V. Turco Liveri, and Pietro Calandra Langmuir, Just Accepted Manuscript • DOI: 10.1021/la501744u • Publication Date (Web): 30 Jun 2014 Downloaded from http://pubs.acs.org on July 9, 2014

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Decoupling of dynamic processes in surfactant-based liquid mixtures: the case of lithium-containing bis(2ethylhexyl)phosphoric acid/bis(2-ethylhexyl)amine systems Isabella Nicoteraa, Cesare Oliviero Rossia, Vincenzo Turco Liverib, Pietro Calandra*,c a

Department of Chemistry and Chemical Technologies, University of Calabria, 87036 Rende,

Cosenza, Italy b

Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche “STEBICEF”,

Università degli Studi di Palermo, Viale delle Scienze

I-90128 Palermo – Italy; Tel: +39 091

6459844 c

CNR-ISMN, Consiglio Nazionale delle Ricerche – U.O.S di Montelibretti – Via Salaria km 29.300

Monterotondo Stazione - Roma (Italy) Tel: +39 0690672409

*corresponding author: [email protected], tel +39 06 90672409, fax +39 06 90672445

Abstract Pure surfactant liquids and their binary mixtures, owing to the amphiphilic nature of the molecules involved, can exhibit nano-segregation and peculiar transport properties. The idea inspiring this work is that the possibility of including in such media salts currently used for technological applications should lead to a synergy between the properties of the salt and those of the medium. So the dynamic features of Bis(2-ethylhexyl)amine (BEEA) and bis(2-ethylhexyl)phosphoric acid (HDEHP) liquid mixtures were investigated as a function of composition and temperature by 1H NMR spectroscopy and rheometry. Inclusion of Litium Trifluoromethanesulfonate (LiT) has been investigated by IR spectroscopy, Pulsed Field Gradient NMR and conductimetry methods to highlight the solubilizing and confining properties of these mixtures as well as the Lithium conductivity. It was found that BEEA/HDEHP binary liquid mixtures show zero-threshold percolating self-assembly with a maximum in viscosity and a minimum in molecular diffusionat 1:1 composition. Dissolution of LiT in such system can occur via confinement in the locally selfassembled polar domains. Despite this confinement, Li+ conduction is scarcely dependent on the medium composition thanks the possibility of a field-induced hopping decoupled by the medium structural and dynamical features.

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Keywords: self-assembly; HDEHP; BEEA; dynamics; NMR spectroscopy; self-diffusion coefficients; rheometry; LitiumTriflate


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INTRODUCTION Surfactant-based liquid mixtures form a fascinating class of nanostructured materials. Such systems are characterized by local and spatially separated nanodomains with different polarity as a result of the peculiar interactions between the molecules involved (dipolar interactions between the polar heads, apolar interactions between the apolar tails, eventual H-bonds, etc.) These structural properties can be tailored by the appropriate selection of the mixture composition and nature of components [1, 2] and, consequently, the dynamical properties constitute a wide and complex scenario which deserves to be carefully investigated. [3] So, these water-free liquid mixtures show several promising characteristics, which can be exploited for specialized applications. For instance, they can be used as “green” solvents and reaction media where solutes can be selectively localized within the spatially separated polar and apolar nanodomains or as conductive liquid phases [4-6]. Recently, we reported on the conductometric behaviour of liquid mixtures composed of bis(2ethylhexyl)amine (BEEA) and bis(2-ethylhexyl)phosphoric acid (HDEHP): these mixtures are characterized by a marked conductivity enhancement with respect to that of pure components which has been interpreted in terms of the acidic and basic nature of the molecules involved [7]. In fact, when HDEHP and BEEA are mixed together, the definite proton transfer from the acidic HDEHP to the basic BEEA drives the system to the equilibrium where i) charged species (DEHP-, BEEA-H+ and free H+) are present and are able to migrate under the effect of an applied electric field and (ii) supra-molecular structures can offer preferential pathways for charge migration. The formation of supramolecular structures was revealed by XRD suggesting that their abundance reaches a maximum at XBEEA=0.5. This involves the appearance of the local minimum in conductivity around XBEEA=0.5 and the corresponding maximum in the Activation Energy at the same composition. This state-of-art knowledge leaves, however, some points which deserve to be clarified, such as the diffusive dynamics of surfactant molecules and the rheological features of mixtures which can surely give further insight on the system behaviour with possible implications from the applicative point of view. For example, in view of application in the lithium-ion batteries, the use of such peculiar liquid systems as solvents for Lithium trifluoromethanesulfonate (Lithium Triflate, LiT) salt, seems quite promising given their thermal and chemical stability and absence of water [8,9]. Therefore, in this work, the dynamic features of bis(2-ethylhexyl)amine (BEEA) and bis(2ethylhexyl)phosphoric acid (HDEHP) liquid mixtures were investigated as a function of composition and temperature by 1H NMR spectroscopy and rheometry. Inclusion of Litium


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Trifluoromethanesulfonate (lithium triflate, LiT) has also been investigated by IR spectroscopy, Pulsed Field Gradient NMR and conductimetry methods to highlight the solubilizing and confining properties of these mixtures as well as the lithium conductivity

EXPERIMENTAL PART Bis(2-ethylhexyl)amine (BEEA, Sigma 97%) and Bis(2-ethylhexyl)phosphoric acid (HDEHP, Aldrich 97%) were used as received. Their molecular structures are shown in Scheme 1. BEEA/HEDHP mixtures were prepared by weight. The moderate heating observed during mixing, also reported in ref [7], confirms the occurrence of the acid base reaction between the two components leading to a definite proton transfer from the acid HDEHP to the basic BEEA. The proton transfer is known to be among the fastest chemical reactions since it takes place in a timescale of ns. For this reason, acid-base reactions are generally diffusion-limited, i.e. their rate is limited by the diffusion of the reacting molecules. In our case, since the reactants are liquid, the reaction can be considered complete in a period of seconds (stirring + diffusion). Mixture composition hereafter is expressed as BEEA molar fraction (XBEEA). Mixtures at the same compositions were also prepared in presence of LiT (lithium triflate, Sigma Aldrich 99.995% trace metals basis) salt in concentration of 0.4 M. They were prepared by adding the opportune amount of salt to a weighed quantity of HDEHP/BEEA mixture. In this case the addition of salt took place after the acid-base reaction went to completion so it could not have any influence on this reaction rate. Litium triflate dissolution took some hours, especially for the most viscous samples, and was sped up by gentle sonication. However all samples were stored in sealed vials and kept overnight prior measurements.

O

O P

HO

HN

O

HDEHP

BEEA

lithium triflate

Scheme 1. Schematic representation of HDEHP, BEEA and lithium triflate molecular structures


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Rheological measurements were performed using a shear strain controlled rheometer RFS III (Rheometrics, USA) equipped with concentric cylinder geometry (external and inner radius 18 and 17 mm, respectively). The temperature was controlled by a water circulation apparatus (± 0.2 °C). In the steady flow experiments, the viscosity is measured as a function of the shear rate, which is tuned by controlling the velocity of the moving cylinder of the rheometer. The force exerted on the fluid is measured and the macroscopic shear rate dγ/dt is obtained as the ratio between the velocity over the gap, and the shear stress σ is defined as the macroscopic force divided by the surface. These experiments were performed in the shear rate range 1–1000 s-1. To ensure steady flow conditions, the required equilibration time was determined by transient experiments, according to step-rate tests. Ten seconds perturbations ensured steady flow conditions in the system for the whole shear rate range. NMR measurements were performed on a Bruker NMR spectrometer AVANCE 300 Wide Bore working at 300 MHz on 1H. The employed probe was a Diff30 Z-diffusion 30 G/cm/A multinuclear with substitutable RF inserts. Spectra were obtained by applying the Fourier transform to the resulting free induction decay (FID) of a single π/2 pulse sequence. The π/2 pulse width was about 8 µs. All the spectra were acquired with the same number of scans and were referenced against pure water set at 4.79 ppm, i.e. its chemical shift with respect to Tetramethylsilane (TMS). Pulsed field gradient spin-echo (PFG-SE) method was used to measure the self-diffusion coefficients (D). This technique, first proposed by Stejskal and Tanner [10] and recently employed to study surfactant-based liquid mixtures [11], consists of a Hahn-Echo pulse sequence

(π/2−τ −π) with two identical magnetic field gradient pulses, the first applied between the 90° and 180° rf pulse (during the dephasing) and the second after the 180° rf pulse (during the rephasing) but before the echo. Following the usual notation, the magnetic field pulses have magnitude g, duration δ, and time delay ∆. The attenuation of the echo amplitude is represented by the StejskalTanner equation:

A(g) = exp[-γ2g2Dδ2(∆ - (δ/3)]

(1)

where D is the self-diffusion coefficient and γ is the nuclear gyromagnetic ratio. Note that the exponent in the equation is proportional to the mean-squared displacement of the molecules over an effective time scale (∆ - (δ/3)). For the investigated samples, the experimental parameters, ∆ and δ, ranged between 20-25 ms and 1-4 ms, respectively. The gradient amplitude, g, varied from 10 to 850 G cm-1. In this condition the uncertainty in the self-diffusion measurements is ∼3%. FT-IR spectra were recorded in the wavelength range 900–4000 cm−1 by a Perkin-Elmer (Spectrum BX) spectrometerusing a fixed-path cell equipped with CaF2 windows. All measurementswere performed at 25 °C with a spectral resolution of 0.5 cm−1. ACS Paragon Plus Environment

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Static conductivity was measured with an LCR meter (Hewlett Packard HP4263A). The cell consisted of a homemade parallel plate capacitor able to host liquid samples, constituted by two mutually isolated stainless steelcircular plates connected by coaxial cables to the LCR meter. The inter-space was 1.0 mm and the diameter of each plate was 31.0 mm. The cell was inserted in a thermostatted oven at 25°C.

RESULTS AND DISCUSSION In order to understand the solvent/solute interactions we have first studied the dynamical features of the bare solvent, specifically the bare HDEHP/BEEA binary mixtures, by viscosimetry and 1H-NMR and then we have explored the peculiar properties of the lithium triflate containing HDEHP/BEEA mixtures by Pulsed Field Gradient NMR Methods, IR, and conductimetry. Bare HDEHP/BEEA mixtures In steady shear experiments, the viscosity is measured as a function of the shear rate under steady flow conditions. It has been found that pure components are Newtonian whereas mixtures show a slight shear thinning at higher shear rates, an effect whichbecomes more marked when the composition get closer to XBEEA=0.5 and which tends to vanish with increasing temperature. The zero-shear viscosity (η) is plotted in Fig. 1 as a function of composition for all the investigated temperatures.

100

30

η (Pa s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

20°C 30°C 40°C 50°C 60°C 70°C

1 0.01 1E-4 0.15

XBEEA 0.5

1

10

0 0.0

0.2

0.4

0.6 XBEEA

0.8


1.0

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Fig. 1: zero-shear viscosity (η) as a function of XBEEA for all the investigated temperature (lines are only a guide for the eye). The inset shows the same data in a log-log scale plot. It is evident the steep increase in viscosity when the composition approaches the XBEEA=0.5 value, wherethe viscosity reaches a maximum. The V-shaped linear trend in the log η vs.log XBEEA plot, shown in the insert of Fig.1, suggests that the viscosity can be described in terms of power law in the form η∝XBEEAα in the range 0≤XBEEA ≤0.5 and η∝(1−XBEEA)β in the range 0.5≤XBEEA ≤1. This behavior, typical of zero-threshold percolating systems [12], can be rationalized in terms of a network of interacting HDEHP-BEEA mixed reverse micelles dispersed in HDEHP (in the range 0≤ XBEEA ≤0.5) or in BEEA (in the range 0.5≤ XBEEA ≤1) whose concentration increases when the composition of XBEEA =0.5 is approached [13]. The critical exponents (α and β) obtained by power law fitting of viscosity versus XBEEA plots are collected in Table 1 for the various temperatures.

Table 1: Fitting parameters (α and β) derived by power law fitting of viscosity versus XBEEA plots at various temperatures. α (±0.1)

β (±0.1)

(range 0≤XBEEA ≤0.5)

(range 0.5≤XBEEA ≤1)

20

4.6

7.6

30

3.9

7.2

40

4.0

7.1

50

3.8

6.9

60

3.4

6.7

70

3.5

6.8

Temperature (°C)

It should be noted that the critical exponent α is comprised between 3.4 and 4.6 whereas β between 6.7 and 7.6, both decreasing quite smoothly with temperature. The difference in the α and β values and their temperature dependence reflect changes of the dynamic regime of the micellar network, and suggest the dynamical nature of percolation [14-16]. We also measured the self-diffusion coefficients (D) by pulsed field gradient spin-echo 1HNMR method (PFG-SE) as a function of XBEEA and temperature. The 1H-NMR spectra collected at ACS Paragon Plus Environment

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the various compositions, together with the appropriate comments are reported in supporting information (Fig. S1). We underline that, for a given temperature and composition, analysis to calculate the self-diffusion coefficients (D) has been carried out on each of the proton signals present in the spectrum (including the less intense signals assigned to the polar head groups) and it was not found any difference. This implies that, within the NMR timescale (ms), all the species present in each mixture show the same mean self-diffusion coefficient and, as expected from the consideration that the acid-base proton transfer takes place in timescales typically in the range of ns, the proton exchange rate between ionic species, as well as their breaking down and reformation, are much faster respect to the measuring times. The self-diffusion coefficients measured at 20°C are plotted against XBEEA in Fig. 2. The V-shaped trend clearly indicates inhibited diffusion for the mixtures, as a consequence of the tendency of the acidic HDEHP and the basic BEEA to form acid-base stable complexes. It is interesting to note that this reduction of the mobility has its strongest effect around the 1:1 composition suggesting that HDEHP-BEEA adducts are the building blocks of the supramolecular structures formed. It is also interesting to note that whereas the molecular self-diffusion is suppressed, the proton conductivity is enhanced [7]. This suggests that proton conduction is due to "free" protons jumping among basic sites in polar domains of the mixtures: their hop occurs in a timescale shorter than that of NMR so that it cannot be detected by PFG-SE NMR.

-1

self-diffusion coefficient (cm s )

DLi

2

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-6

10

D

-8

10

-10

10

0.0

0.2

0.4

0.6

0.8

1.0

XBEEA

Fig. 2: Molecular self-diffusion coefficient (D) and lithium self-diffusion coefficient (DLi) as a function of composition at 20°C


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The temperature effect on both the self-diffusion coefficient and viscosity has been investigated through the Arrhenius plots. Some representative plots are reported in supporting information (Fig. S2). It has been observed that all Arrhenius plots show linear trends in the investigated temperature range for both processes. This confirms the validity of the two wells potential model and allows safe determination of the activation energy values (EA) and pre-exponential factors, which are related to the energy barrier between the two potential wells and the frequency of oscillations within each well. These values are reported in Fig. 3 as a function of XBEEA. First of all it can be seen that the activation energy and pre-exponential factor values for self-diffusion and viscosity are different suggesting that the molecular processes governing diffusion and momentum transfer are decoupled. Besides, the maximum around XBEEA=0.5 shown by the activation energy for the viscosity confirms the building up of better and better interconnected molecules to form local networks when BEEA is added to HDEHP which, in turn, makes the activation energy required for momentum transport higher. The intra-network diffusion is then facilitated justifying the corresponding minimum in the activation energy for the molecular self-diffusion. On the other hand, the lower pre-exponential factor in the mixtures with respect to that of the two pure components for both viscosity and molecular diffusion is due to a lower oscillation frequency within each potential well and interpreted as the effects of the percolation-induced freezing of dynamics arising from the substitution of loosely H-bonded HDEHP-HDEHP aggregates with strongly interacting HDEHP-BEEA ones.

D η

EA (kJ / mol)

60

40

20

1000 1 pre-exp factor

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1E-3 1E-6 1E-9

1E-12 0.0

0.2

0.4

0.6

0.8

1.0

XBEEA


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Fig. 3: Activation energy (EA, upper panel) and pre-exponential factor (lower panel) for viscosity and diffusion coefficient as a function of XBEEA (lines are only a guide for the eye).

Lithium Triflate containing HDEHP/BEEA mixtures To investigate how the structural and dynamic evolution of the mixtures with composition and temperature impacts on the diffusional dynamics of lithium ions, we have measured the selfdiffusion coefficient of lithium ions (DLi) in the same mixtures containing lithium triflate at a fixed concentration ([LiT]=0.4M) and compared to the molecular (1H) self-diffusion (D). The comparison is shown in Fig. 2. It is worth to note that the self-diffusion coefficient of lithium ions (DLi) is always somewhat lower than the molecular self-diffusion D, and the two quantities follows the same trend as a function of composition. This indicates that lithium ions are located in close proximity to DEHP- anions and confined within the closed structure of polar domains formed by the opportunely oriented HDEHP-BEEA adducts. This confinement suppresses Lithium ion selfdiffusion. This picture is also consistent with the presence of different populations of HDEHP highlighted by IR spectroscopy: one formed by bare species and another one constituted by molecules interacting with Lithium ions in rapid exchange. IR spectroscopy was used to probe the interactions between Lithium ion and surfactant molecules in order to establish the localization of this ion within the complex self-segregating surfactant-based structure. The study has been carried out on both bare and Lithium- containing HDEHP/BEEA mixtures as a function of XBEEA. Some representative spectra are reported in supporting information (Fig. S3). Given the high number of peaks present in all the spectra, only the final conclusion will be given here. No marked changes have been observed in the IR profile of the HDEHP/BEEA mixtures when Lithium salt is dissolved. This rules out the occurrence of any structural change in the surfactant-based mixture induced by the presence of the salt. The mixture can be considered as a solvent for the salt, even if salt concentration is quite high (0.4M). The only change deserving to be underlined involves the band centered around 1230 cm-1: this band is a combination band (ν(PO) + δ(POH)) [17,18] characteristic of the phosphate functional group of HDEHP, and turns out to be very sensitive to the interactions between the HDEHP polar head and hosted species [3]. The change occurring in our systems is shown in Fig 4 where the band of bare HDEHP/BEEA mixture is compared to that of the lithium-containing mixture at the same XBEEA value for some different compositions chosen as representative. The position of this band reflects the intensity of H-bonds established by the phosphate group. In the mixtures this group is involved in HDEHP-HDEHP H-bonds as well as HDEHP-BEEA ones, ACS Paragon Plus Environment

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which are stronger than the former. This tendency of establishing HDEHP-BEEA H-bond has been found to be the driving force for intermolecular self-assembly in such systems [7]. A perusal of Fig. 4 reveals that this combination band is generally slightly shifted to higher frequencies when the salt is hosted. This is equivalent to say that the phosphate group is involved in weaker H-interactions in presence of salt and implies that HDEHP-Li interactions are weaker than HDEHP-HDEHP or HDEHP-BEEA ones. This unveils the competition between Lithium ion and BEEA to bond the HDEHP polar head. Although lithium ion cannot form H-bond, there is, on the other hand, a compensating effect due to its high charge density which makes it well accommodated close to the polar HDEHP (phosphate) group. Another interesting observation is a band shape change: when the salt is dissolved the band becomes more structured and this can be taken as a hint for the presence of different populations of surfactant molecules: a population of molecules interacting with the lithium ion and another population of unperturbed molecules. The presence of different populations of molecules can be highlighted thanks to the shorter timescale typical of IR probe (∼10-11 s). XBEEA=0.33

XBEEA=0

XBEEA=0.5

Absorbance

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1300 1200 1100 -1 wavenumber (cm )

1300 1200 1100 -1 wavenumber (cm )

1300 1200 1100 -1 wavenumber (cm )

Fig 4: comparison between the combination band (ν(PO4) + δ(POH)) of bare HDEHP/BEEA mixture (black line) compared to that of the lithium-containing mixture at the same XBEEA value (blue line) for some different compositions chosen as representative.

Finally, the conductivity of BEEA-HDEHP mixtures in the presence of lithium triflate (0.4M) is shown in Fig. 5.


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conductivity @ 25°C (S/cm)

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-5

10

-7

10

-9

10 10

-11

10

-13

0.0

0.2

0.4 0.6 XBEEA

0.8

1.0

Fig. 5: conductivity (log scale) as a function of XBEEA for HDEHP/BEEA and HDEHP/BEEA/Li systems at 25°C. Comparison with the conductivity of bare BEEA-HDEHP mixtures taken from the literature [7] shows that addition of LiT generally enhances the mixture conductivity of more than one order of magnitude. So the measured conductivity can be essentially regarded as Lithium ion conductivity, being the proton contribution coming from the bare HDEHP/BEEA mixtures much lower. The observed lithium ion conductivity can be attributed to the presence of lithium ions able to jump among DEHP- sites under the applied electric field, a peculiar behavior which is emphasized by the small size of the lithium ion. Another interesting observation is that the proton conductivity in bare HDEHP/BEEA mixtures depends on XBEEA, whereas the Lithium conductivity does not. This shows that the proton conductivity is sensitive to the structure of the system and a comparison with the data of viscosity and molecular diffusion suggests that it is molecule-assisted; on the contrary the lithium conductivity, being constant in a wide XBEEA range, is not dependent on the structure, so that a field-induced charge transport due to relatively free Lithium ions can be claimed.

Conclusions Information on the dynamic features of bis(2-ethylhexyl)-phosphoric acid (HDEHP)/bis(2ethylhexyl)amine (BEEA) mixtures have been gained by viscosimetry and NMR methods. Data analysis shows a continuous variation of the dynamic properties (diffusion coefficient and viscosity) as a function of BEEA mole fraction (XBEEA) as a consequence of the occurring of a zero-threshold percolating self-assembly showing a maximum in viscosity and a minimum in molecular diffusionat 1:1 composition. The fact that whereas the molecular self-diffusion is suppressed the proton ACS Paragon Plus Environment

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conductivity is enhanced [7] indicates that proton conduction is due to "free" protons jumping among basic sites in polar domains of the mixtures, their hop occurring in a timescale shorter than that of NMR. This study has also highlighted, by IR spectroscopy and Pulsed Field Gradient NMR, the solubilizing and confining properties of these mixtures toward Lithium trifluoromethanesulfonate: its encapsulation within the local polar domains formed by opportunely oriented HDEHP/BEEA self-assembled structures has potential application in the lithium ion batteries. In fact, the conductometric investigation has highlighted enhanced conductivity of the lithium-containing HDEHP/BEEA mixtures which is not dependent to the HDEHP:BEEA ratio, indicating a fieldinduced lithium ion hopping which is decoupled by the dynamical features of the bare HDEHP/BEEA mixtures. Interpretation of all experimental data at a molecular level allowed us to rationalize the observed behavior in terms of peculiar dynamics of self-assembled HDEHP/BEEA structures. This report presents novel and promising candidate systems for lithium-ion batteries [19,20] and furnishes new knowledge to be used in the design of novel materials with planned physicochemical properties: water-free conductive fluids, specialized solvents and reaction media.


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TOC Graphic

DLi

-5

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-1

self-diffusion coefficient (cm s )

D

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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XBEEA


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Decoupling of Dynamic Processes in Surfactant-Based Liquid Mixtures

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