The fluidity of hydrocarbon regions in organo-gels, studied by NMR

Colloids and Surfaces A: Physicochemical and Engineering Aspects 154 (1999) 303 – 309

The fluidity of hydrocarbon regions in organo-gels, studied by NMR Basic translational and rotational diffusion measurements Mathias Hermansson 1,* Institute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden Received 11 February 1998; accepted 1 July 1998

Abstract Organogels composed of 12-hydroxystearic acid (12-HSA) and low viscosity solvents such as decane, decalin and ethyl acetate have been used as model systems for lubricating greases. The fluidity of perdeuterated probe molecules in the above mentioned model systems was studied using two NMR spectroscopy methods. First, translational diffusion was studied for the perdeuterated probes benzene, toluene, decane and dodecane by pulsed-field gradient NMR. The probes were introduced into a commercial lubricating grease and the model systems mentioned above. Second, the rotational diffusion tensor for perdeuterated trans-decalin was studied by deuterium spin relaxation measurements. All probed samples contained 5% probe (by total mass). The measurements show that the microviscosity in the gels is quite similar to the bulk viscosity in the corresponding neat solvent. This indicates that no strong interactions occur between 12-HSA, and it also indicates that the obstruction effect is responsible for the decrease in diffusion rate compared to the neat solvents in 12-HSA-thickened systems. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Diffusion; 12-Hydroxystearic acid; Lubricating grease; NMR

1. Introduction Lubricating greases are thickened oils, with incorporated additives, where the thickener holds the oil in a fibrous structure by a combination of capillary forces and interactions between oil molecules and the thickener [1]. The most com* Corresponding author. Fax: +46 8 642 8399. E-mail address: [email protected] 1 Present address: Casco Products, Analyscentrum, P.O. Box 115 38, SE-100 61 Stockholm, Sweden.

mon thickener is the lithium salt of 12-hydroxystearic acid (12-HSA). Other possible thickeners are sodium, calcium and aluminum soaps of 12HSA and other fatty acids, polyurea and colloidal silica. The oil may be a mineral oil, a synthetic oil, or a vegetable oil. Mineral oils include naphthenic oils and paraffinic oils. Synthetic oils include polyalphaolefins, polyol esters, silicone oils and organic polyphosphate esters [2]. Additives are added to give the grease certain properties, i.e. antiwear properties, extreme pressure properties,

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rust inhibition properties, etc. Compared to lubricating oils, greases show a complex rheological behaviour, with a yield stress that has to be exceeded before the grease flows. The main difference compared to lubricating oils, which, from a technical point of view, may be considered as Newtonian, is the yield stress. The yield stress is important for many applications, since it is the relevant parameter for quantifying the tendency for the grease to stay in the lubrication area. The grease industry uses a number of test methods to classify greases according to different properties, i.e. consistency, flow properties, consistency stability, apparent viscosity, dropping point, oil separation, evaporation, corrosion inhibition, load carrying ability and wear. However, these methods are of a very empirical character, for example, rheological properties are measured as a penetration value with unit a tenth of a millimetre. Also, some of the methods have bad precision and are of limited use. Thus, there is a need for characterisation methods that provide a more fundamental understanding of the properties of grease. Some other techniques, which have been used to study lubricating greases or similar systems, are scanning electron microscopy [3], transmission electron microscopy [4], rheology [5], differential thermal analysis and NMR [6 – 8]. Blum and coworkers studied self-diffusion by PGSE-NMR on thickener and on oil with different thickener concentrations. Their thickener was an organic soluble polymer. They found PGSE-NMR to be a promising technique in oil chemistry [6,7]. In a previous investigation by Hermansson et al. [8], self-diffusion coefficients were measured directly on base oil molecules in lubricating greases. It showed that diffusion was slower in thickened oil than in non-thickened oil, but it was not clear if the lowering of the diffusion coefficient was due to an obstruction effect from the thickener or intermolecular forces between thickener and oil. 12-HSA and its lithium salt are known to form thermo-reversible gels, in a variety of sol-

vents. This is due to the ability of 12-HSA (or the Li salt) to form thin fibres upon cooling from 12-hydroxystearic acid (or Li salt respectively). The shape of the fibres, and thus also the gel structure, varies with the solvent [9–11]. In this work we have further investigated the diffusion in thickened non-aqueous systems in order to elucidate which of the above mentioned factors dominates the lowering of the diffusion coefficient in thickened systems. The translational diffusion study presented in this paper is similar to the former one, with the exception that in the present case diffusion was measured on deuterium in added perdeuterated probes, thus eliminating the possibility of signal overlap. The translational diffusion was measured for both a commercial Li-12-hydroxystearate grease and for 12-HSA gels. Further, rotational diffusion was measured on perdeuterated trans-decalin added to 12-HSA gels via T1 measurements. Of course, the macroscopic viscosity is heavily dependent on the gel structure, but according to hydrodynamic theories for reorientation, the microviscosity in the gels should be similar to the viscosity in bulk provided that strong intermolecular forces do not exist between thickener and fluid phase. Lithium hydroxystearate, which is a common thickener in lubricating greases, has a high melting point ( 210°C). Therefore, it is difficult to use lithium-12-hydroxystearate in model systems. In this study, 12-hydroxystearic acid has been used as model thickener in systems with solvents such as decane, cis/trans-decalin, hexadecane and ethyl acetate. Table 1 Translational diffusion of different probes in a commercial grease. Symbol, line in figure

Probe (5 wt% of total mass)

, +, ×, 2,

,

heptane-D16 benzene-D6 toluene-D8 xylene-D10 decane-D22

--· · · · · · ––– ——

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Table 2 Translational diffusion of different probes in 12-HSA gel Symbol, line in figure

Thickener type

“, —— “, - - , —— , - - , ——

, - - , ——

, - - ", —— ", - - 2, —— 2, - - -

not thickened not thickened 12-HSA 12-HSA not thickened not thickened 12-HSA 12-HSA not thickened not thickened 12-HSA 12-HSA

Thickener concentration (wt%)

5 5

5 5

5 5

Fluid phase

Probe (5 wt% of total mass)

decane decane decane decane cis/trans-decalin cis/trans-decalin cis/trans-decalin cis/trans-decalin ethyl acetate ethyl acetate ethyl acetate ethyl acetate

heptane-D16 toluene-D8 heptane-D16 toluene-D8 heptane-D16 toluene-D8 heptane-D16 toluene-D8 heptane-D16 toluene-D8 heptane-D16 toluene-D8

Table 3 Rotational diffusion of trans-decalin-D18 in 12-HSA gel Symbol, line in figure

Thickener type

Thickener concentration (wt%)

Fluid phase

Probe (5 wt% of total mass)

, ——

, - - “, —— , - - ", —— 2, - - , —— , - - -

12-HSA 12-HSA 12-HSA 12-HSA 12-HSA 12-HSA 12-HSA 12-HSA

5 10 5 10 5 10 5 10

ethyl acetate ethyl acetate decane decane cis/trans-decalin cis/trans-decalin hexadecane hexadecane

trans-decalin-D18 trans-decalin-D18 trans-decalin-D18 trans-decalin-D18 trans-decalin-D18 trans-decalin-D18 trans-decalin-D18 trans-decalin-D18

2. Experimental

2.2. Translational diffusion by NMR

The chosen systems and sample compositions are shown in Tables 1 – 3.

The technique used was the pulsed-gradient spin-echo (PGSE) NMR [12,13]. The general principle is that the magnetic field varies along the x-axis during the magnetic gradient pulses, which causes the resonance frequencies of the nuclear spins to vary along the x-axis:

2.1. Sample preparation As a model thickener, 12-hydroxystearic acid of technical grade was used. Ethyl acetate, decane, cis/trans-decalin solvent and hexadecane were of puriss grade or better. Perdeuterated trans-decalin (denoted trans-decalin-D18) was also of puriss grade. The commercial grease Statoil Li62, which is a Li-12-hydroxystearate-thickened multi-purpose grease with additives, was purchased from Statoil Sweden. The thermo-reversible gels were prepared by dissolving 12-HSA (5 or 10 wt%) in the solvent at 75°C, after which they were allowed to cool in the NMR tubes at room temperature.

v(x)= −gB(x)

(1)

where B(x)=B0 + Gx

(2)

Fig. 1 shows the applied pulse sequence. It is assumed that the background magnetic field gradient (dB0/dx) is small compared to the applied pulses, thus G dB/dx. Z is the normal direction of the gradient, i.e. the field does not vary along the Z-axis during the pulse. The echo amplitude for Gaussian diffusion is then given by:

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M. Hermansson / Colloids and Surfaces A: Physicochem. Eng. Aspects 154 (1999) 303–309

Fig. 1. The PGSE pulse sequence.

A= A0 exp( −2t/T2) exp( − g 2G 2Dd 2D −d/3) (3) where G is the magnetic field gradient strength, g is the magnetogyric ratio, T2 is the transverse spin relaxation time and D is the translational diffusion coefficient. The procedure is repeated a number of times, with successively increasing strength of the magnetic field gradient. Finally, a non-linear fitting procedure is applied to the raw data to calculate D. These experiments were carried out on a JEOL FX-100 NMR spectrometer operating at 15.35 MHz for deuterons. The gradient strength was varied from 0.05 to 0.4 T/m. D and t were kept constant at 81 ms for all measurements.

tion in non-aqueous solutions [14]. Thus, transdecalin-D18 should be a good molecular probe for measuring microviscosity of non-aqueous solvent systems. The only significant T1 relaxation for trans-decalin-D18 is intramolecular, and of quadrupolar origin. The orientation of the rotational diffusion tensor is, due to symmetry arguments, equal to the orientation of the moment of inertia tensor of classical mechanics. Determination of the rotation diffusion tensor was made using Woessner’s equation [Eq. (4)], relating the relaxation times of the three principal axes to the respective rotational diffusion coefficients. The form of Woessner’s equation presented below [Eq. (3)] applies to analysis of intramolecular, purely dipolar–dipolar (from attached or nearby protons) carbon-13 spin relaxation under complete proton broad-band decoupling, and under extreme narrowing conditions. The sum of dipolar interactions converges rapidly, and it is therefore not necessary to consider either C–H interactions beyond 0.25 nm distance or intermolecular contributions for carbons with directly attached protons. 1

= DD

2.3. Rotational diffusion by deuterium T1 relaxation NMR In the second part of this work, trans-decalinD18 (Fig. 2) was used as a rotational probe. Trans-decalin-D18 is a good probe molecule in the sense that it is small, insoluble in water and probably does not show any preferential orienta-

T1

!

g 2Hg 2C' 2 1 C1 C2 + 6 r ij 2 4R1 + R2 + R3 4R2 + R3 +R1 +

C3 C4 + 4R3 + R1 + R2 6[R +(R 2 − L 2)1/2]

+

C5 6[R −(R 2 − L 2)1/2]

"

(4)

where R= (R1 + R2 + R3)/3 and L 2 = (R1R2 + R2R3 + R1R3)/3. The C constants are geometric

Fig. 2. Atomic numbering of trans-decalin-D18 and orientation of the rotational diffusion tensor.

M. Hermansson / Colloids and Surfaces A: Physicochem. Eng. Aspects 154 (1999) 303–309

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Fig. 3. The deuterium NMR spectrum of trans-decalin, with assignment by Stilbs et al. [14].

expressions in terms of directional cosines of the respective C – H bond vectors in the principal axis system. When applied to deuterons, the application of the Woessner treatment becomes simpler, and there is no signal overlap since the probe molecule is the only significant deuterium-containing molecule in the system. For deuterium spin relaxation, the factor g 2Hg 2C' 2/r 6ij is replaced with (3/ 8)(e 2qQ/')2 where e 2qQ/' is the quadrupole coupling constant. According to hydrodynamic theories, the following equations hold: ti = Cih + t (0) i

(5)

ti = 1/6Ri

(6)

There is a direct relationship between each of the single components of the correlation time tensor and the bulk viscosity of the neat solvent. Rotational diffusion measurements are independent of the obstruction effect, since the time scale in the experiment is short. Thus, any lowering of the rotational diffusion with increasing concentration of thickener must arise from intermolecular forces between thickener and fluid phase. The T1 measurements were carried out on an AMX-300 spectrometer operating at 46.05 MHz

for deuterons (without field-frequency lock). The inversion-recovery method in the Fourier transform mode was applied. All calculations of the moment of inertia tensor and the rotation diffusion tensor were made with the iterative FORTRAN V program WOESNR [14]. Stilbs et al. [14] assigned the spectra of transdecalin-D18 using 2D correlation between ordinary trans-decalin protons and C-13, and used the close proton–deuterium shift scale correspondence to assign the deuterium spectra of perdeuterated trans-decalin. See Fig. 3 for the assignment of spectra.

3. Results and discussion In these systems, translational diffusion was measured on the deuterium nuclei on a probe molecule. Essentially only the probes contain deuterium, therefore the possibility of signal overlap is completely eliminated. Fig. 4 summarizes the results of the diffusion study for the commercial grease with five different probes, and Fig. 5 the diffusion study of 12-HSA gels, with two different probes (experimental set-up, see Table 1).

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Fig. 4. Diffusion coefficients for commercial lubricating grease.

Fig. 5. Diffusion coefficients for gels and non-thickened solvents with different probe molecules.

The Arrhenius relation provides a very good description in the case of the probed commercial grease (correlation coefficients  0.99). The fact that the diffusions for the different probes have essentially the same activation energy ( 26.5 kJ/mol for the grease), and that the activation energy is independent of temperature, suggests that the interactions between thickener and fluid phase are small. If, instead, the interactions between thickener and fluid phase are significant, it is likely that they would affect the two types of probe very differently, which would be evident in the translational diffusion results.

Fig. 6. The rotational diffusion coefficients for the different gels.

With the exception of the diffusion results at the highest temperatures, the Arrhenius relation is a good description also for the probed 12-HSA–solvent systems (Fig. 5). The deviations from the Arrhenius relation might be due to melting of the thickener or occurrence of convection in the NMR tubes at elevated temperatures. The activation energy is equal, within experimental error, for all probes in each solvent group, but differs between the various thickened solvents. In the second investigation, trans-decalin-D18 was used as a rotational diffusion probe and

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Fig. 7. Correlation times vs. viscosity at 35°C.

introduced at 5 wt% into the 12-HSA –solvent systems (Table 3). Deuterium spin relaxation was studied, and the results were used to calculate the rotational diffusion tensor (as outlined above). The logarithm of the rotational diffusion coefficient is plotted as a function of 1/T in Fig. 6. The correlation times were plotted against the viscosities for the different solvents used, at 35°C, see Fig. 7. This plot indicates that Eq. (4) holds quite well, and that the rotational behaviour of trans-decalin-D18 in these solutions essentially depends on the macroscopic viscosity only. Another manifestation of this condition is that there is no significant difference of the rotational diffusion between 5% and 10% gels for respective solvents. Both the translational and rotational diffusion measurements show that the microviscosity in 12HSA-thickened solvents equals the macroscopic viscosity of the pure solvent. Also, this indicates that the interactions between 12-HSA and solvent are small.

Acknowledgements This project was funded by TFR (Swedish Research Council for Engineering Sciences). Niklas .

Hedin is thanked for experimental assistance, and Peter Stilbs for stimulating discussions and use of the equipment and computing facilities.

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