PL223812B1 - Method for dispersing carbon nanotubes in a hydrophilic ionic liquid - Google Patents

Abstract

A method for dispersing carbon nanotubes in a hydrophilic ionic liquid is disclosed which comprises applying ultrasound to a mixture of carbon nanotubes and hydrophilic ionic liquid, characterised in that the said stage is carried out in an ultrasonic bath. The ionic liquid may be a room temperature ionic liquid (RTIL) such as 1-ethyl-3-methylimidazolium diethyl phosphate (EMIMDEP). The ionic liquid may contain an anionic surfactant such as sodium dodecyl sulphate (SDS) or sodium 1-dodecylsulphonate (DSS).

Description

Description of the invention

The subject of the invention is a method for dispersing carbon nanotubes in a low-temperature hydrophilic ionic liquid, possibly with the use of anionic surfactants.

Ionic liquids are chemicals consisting of a large organic cation and an organic or inorganic anion. They are characterized by an asymmetrical structure and a low melting point - usually below 100 ° C. Ionic liquids with a melting point lower than room temperature are called low temperature ionic liquids (RTlLs).

By appropriately modifying the cation and anion structure and changing various properties (melting point, density, viscosity, etc.), it is possible to obtain ionic liquids with specific applications.

There are hydrophobic and hydrophilic ionic liquids, as well as those that are partially miscible with water. The nature of a given ionic liquid depends mainly on the anion present in it. Typical ionic liquids have a wide range of liquid temperatures, good solubility of organic and inorganic compounds, thermal stability, poor ion coordination properties, high boiling point and non-flammability. In addition, they moisten the surface of metals, polymers and minerals, and conduct electricity. Their viscosity is twice as high as that of traditional solvents.

Carbon nanotubes are one of the allotropic forms of carbon that create cylindrical structures. They are made of a hexagonal network of carbon atoms creating crystalline graphite. They can be represented as cylindrically folded layers of graphite (graphene). We distinguish single-wall nanotubes (SWCNT - single wall carbon nanotubes), made of a single graphene plane folded into a cylinder, and multi-wall nanotubes (MWCNT - multiwall carbon nanotubes), consisting of successive layers of nanotubes with common axes.

Depending on the way the graphite plane is folded, two types of carbon nanotubes are distinguished: chiral (torsion structure) and noniral (armchair and zigzag structure).

Carbon nanotubes were discovered in 1991 by Sumoi Jim (A. Huczko, Carbon nanotubes. 21st century black diamonds, Bel Studio, 2004) and thanks to their special properties they were called "21st century black diamonds". They show very high mechanical strength. This is due to the strong sp bonds between the carbon atoms. They are characterized by extraordinary tensile and tear strength. This property is due to the relatively large Young's modulus. Due to the empty interior, they have a very low density. Depending on their structure, they are either current conductors or semiconductors. They have a very high thermal conductivity.

In order to be able to fully use the properties of carbon nanotubes in solvents, i.e. ionic liquids, a dispersed solution of individual tubes or small coils should be obtained. Unfortunately, as a result of van der Waals interactions, carbon nanotubes spontaneously combine to form aggregates. The main reason for this behavior is their hydrophobic nature.

To our knowledge, the optimal method of dispersing carbon nanotubes in hydrophilic ionic liquids with anionic surfactants using an ultrasonic bath and a centrifuge has not been determined yet.

So far, only a few research groups have dispersed carbon nanotubes in hydrophilic ionic liquids. However, the dispersion was carried out without the use of surfactants and with completely different methods.

It is known from the literature that a dispersion of carbon nanotubes in solvents, i.e. ionic liquids, can be obtained in many ways. The most common are mechanical methods such as grinding or grinding carbon nanotubes or mixtures of carbon nanotubes and ionic liquids for a specified amount of time without the presence of a surfactant. In Trevor Simmons' group ("Preparation of synthetic wood composites using ionic liquids", Wood Science and Technology, DOI 10.1007 / s00226-010-0395-6, Coverage Volume 1/1967 - Volume 45/2011), a suspension of polyhedral carbon nanotubes in a hydrophyte ionic liquid (EMIM Ac, 1-ethyl-3-methylimidazolium acetate) by grinding them and grinding them in a mortar for 20 minutes. The nanotubes prepared in this way remained dispersed in the ionic liquid for over a month. In the work by Sameer Rahatekar ("Solution spinning of cellulose carbon nanotube composites using room temperature ionic liquids", Polymer 50 (2009) 4577-4583), MWCNTs placed in EMIM Ac were also ground in a mortar to obtain a homogeneous paste. Another research group (Zhang, H .; Wang, Z. G .; "Regenerated-Cellulose / Multiwalled-Carbon-Nanotube Composite Fibers

PL 223 812 B1 with Enhanced Mechanical Properties Prepared with the lonic Liquid 1-Allyl-3-methylimidazolium Chloride ”, Adv, Mater. 2007, 19 (5) 698-704), tried to disperse multi-wall carbon nanotubes using an ionic liquid - 1-allyl-3-methylimidazolium chloride (AMIMCI), while the carbon nanotubes were also grinded in a mortar with pure AMIMCI, without any additives. This mechanical treatment aims to improve the dispersion of nanotubes. During grinding or abrasion, nanotubes are partially damaged by shortening their length (breaking).

Another method of obtaining a dispersed suspension of carbon nanotubes in hydrophilic ionic liquids is the use of ultrasound. An ultrasound-generating probe is used to disperse nanotubes in hydrophilic ionic liquids with surfactant added. Jun Wan and colleagues ("A simple method for preparing biocompatible composite of cellulose and carbon nanotubes for the cell sensor", Sensors and Actuators B 146 (2010) 221-225) prepared a suspension of SWCNTs in an ionic liquid using a homogenizer, Sonication lasted 4 hours, until a homogeneous suspension is obtained. However, the use of the probe also affects the hydrophilic ionic liquid, often decomposing it. Well dispersed carbon nanotubes can be obtained by this method, but the hydrophilic ionic liquid used is destroyed. In summary, in the method described above, the necessity to place an ultrasonic probe in a liquid causes a highly undesirable effect of destroying both the nanotubes and the ionic liquid itself.

In turn, there is known from the patent description CN101469141 a method of creating a composite by means of the surface identification of carbon nanotubes in order to improve their compatibility with organic solvents. The process involves a series of chemical reactions, for example with concentrated nitric acid and concentrated sulfuric acid.

Therefore, the object of the present invention is to propose a method for dispersing carbon nanotubes in hydrophilic ionic liquids, free of the above drawback.

According to the invention, a method for dispersing carbon nanotubes in a hydrophilic ionic liquid, comprising the step of treating a mixture of carbon nanotubes and a hydrophilic ionic liquid with ultrasound, is characterized in that said step is performed in an ultrasonic bath, said hydrophilic ionic liquid containing an anionic surfactant added, which is an anionic surfactant, preferably sodium dodecyl sulfate, SDS or sodium 1-dodecanesulfonate, DSS, and the hydrophilic ionic liquid is diethyl 1-ethyl-3-methylimidazolium phosphate, EMIMDEP, said step of subjecting the mixture of carbon nanotubes and the hydrophilic ionic liquid the operation of ultrasound in an ultrasonic bath lasts from 2h to 14h, preferably from 3h to 8h, and most preferably from 4h to 5h, and is carried out at a temperature of 25 ° C to 65 ° C, preferably 45 ° C to 65 ° C, and most preferably from 60 ° C to 65 ° C. Exposing the mixture of carbon nanotubes and the hydrophilic ionic liquid to ultrasound in an ultrasonic bath prevents the destruction of the nanotubes and the ionic liquid itself.

Preferably, the suspension of carbon nanotubes in a hydrophilic ionic liquid obtained in an ultrasonic bath is subjected to centrifugation in a centrifuge for a period of 2 min to 60 min, more preferably 2 min to 15 min, and most preferably 3 min to 5 min.

In such a case, preferably the speed of rotation of the centrifuge is from 1000 rpm to 9000 rpm, more preferably from 2000 rpm to 4000 rpm, most preferably from 2000 rpm to 2500 rpm. Centrifugation of the sample obtained in the above-mentioned conditions makes it possible to remove non-dispersed aggregates and impurities present in the sample.

Preferably, in said mixture of carbon nanotubes and the hydrophilic ionic liquid, the concentration of the carbon nanotubes is from 0.05% to 3% by weight.

Preferably, said carbon nanotubes are a mixture of single-wall carbon nanotubes, SWCNTs, and multi-wall carbon nanotubes, MWCNTs,

To the knowledge of the present inventors, this is the first example of such a method for dispersing carbon nanotubes in hydrophilic ionic liquids with added surfactant.

Preferred embodiments of the invention are described below with reference to the accompanying drawings in which:

Fig. 1 shows a diagram of the procedure performed to obtain a suspension of dispersed carbon nanotubes, Fig. 2 shows a suspension of dispersed carbon nanotubes after the operation of an ultrasonic bath, Fig. 3 shows a suspension of dispersed carbon nanotubes after the operation of a centrifuge,

Fig. 4 shows the suspension of carbon nanotubes in a hydrophilic ionic liquid (EMIM DEP) with the addition of anionic surfactant (SDSu) after the separation of undispersed aggregates and impurities, Fig. 5 is an optical microscope image juxtaposition showing the suspension of nanotubes in an ionic liquid with SDS - em at different residence times of the sample in the ultrasonic bath, Fig. 6 is a juxtaposition of microscopic images showing the suspension of nanotubes in an ionic liquid with SDS at different spinning parameters of the sample.

Materials

Carbon nanotubes: CNTs-1 sample, purchased from Shenzhen Nanotech Port Co. Ltd .; obtained by chemical vapor deposition (CVD). The sample contained ~ 60% of single-walled carbon nanotubes (SWCNTs) and 35% of multi-walled carbon nanotubes (MWCNTs) of 2-15 pm length, The remaining 5% was amorphous carbon and catalyst.

Carbon nanotubes: sample CNTs-2, purchased from SkySpring Nanomaterials, Inc .; obtained by chemical vapor deposition (CVD). The sample contained ~ 90% single-wall carbon nanotubes (SWCNTs) and 5% multi-wall carbon nanotubes (MWCNTs) 5-20 µm in length. The remaining 5% is amorphous carbon and catalyst.

Hydrophilic ionic liquid, 1-ethyl-3-methylimidazolium diethyl phosphate; EMIM DEP (C ₁₀ H ₂ and N ₂ O ₄ P) with a purity> 98% purchased from Sigma-Aldrich (CAS Number 848641-69-0).

Anionic Surfactant, sodium dodecyl sulfonate; SDS (C12H25SO4Na) purchased from Sigma-Aldrich with 99% purity.

Apparatus

The following equipment was used in the conducted research:

• an ultrasonic bath with a power of 2x480 W and a frequency of 40 kHz Polsonic was used to prepare the dispersed carbon nanotube suspensions in the hydrophilic ionic liquid EMIM DEP. This device enables the temperature variation in the range from 30 ° C to 80 ° C and time control;

• Universal 32 centrifuge - manufactured by Hetiich Zentrifugen was used to slide down non-dispersed aggregates and pollutants. This device enables the change of the spin speed in the range of 500-9000 rpm and the change of the sample centrifugation time in the range of 1-99 min;

• the obtained materials were examined with the use of a NIKON ECLIPSE E 400 optical microscope, controlled by a computer with the LUCIA G program, which enables temperature control in the range of 20 ° C - 60 ° C.

Preparation of samples

The suspension of dispersed carbon nanotubes in a hydrophilic ionic liquid was obtained by adding a certain amount of carbon nanotubes, a hydrophilic ionic liquid (EMIM DEP) and a surfactant (SDSu). The whole was initially placed in an ultrasonic bath and then in a centrifuge. Schematic of the procedure performed with the use of EMIM DEP and SDS is shown in Fig. 1.

Carbon nanotubes (concentration from 0.05% to 0.15% by weight) with EMIM DEP and the appropriate amount of SDS (below cmc, concentration from 0.1% to 0.5% by weight) were placed in an ultrasonic bath on about 4-5 hours. After this time, a dark suspension of carbon nanotubes in a hydrophilic ionic liquid was obtained (Fig. 2). Dispersion was carried out at a temperature of ~ 60 ° C in an ultrasonic bath. The temperature does not affect the formation of aggregates, but it affects the ionic liquid - the increase in temperature causes EMIM DEP to become a less viscous liquid, which facilitates the diffusion of CNTs detached by ultrasonics.

The mixture was then placed in a centrifuge for 5 minutes at 2000 rpm so that most of the non-dispersed aggregates and the contaminants present would sink to the bottom of the bottle (Figure 3). After separation of undispersed aggregates and impurities, a well-dispersed suspension of carbon nanotubes in a hydrophilic ionic liquid (EMIM DEP) was obtained with the addition of anionic surfactant (SDSu) (Fig. 4).

Surprisingly, it turned out that both the residence time of the samples in the ultrasonic bath and the centrifugation parameters affect the dispersion of the nanotubes in the ionic liquid. The addition of a surfactant allows for optimal dispersion of nanotubes without the need for mechanical rubbing.

Confirmation of the beneficial effects of ultrasound time and power on the reduction of aggregation is shown in Fig. 5. Optical microscope pictures are compiled showing a comparison of the CNTs dispersion degree for samples under ultrasound at 60 ° C for 2, 4, 8 and 12 hours, respectively. They clearly show that it is favorable

Place the sample in the ultrasonic bath for not less than 4 hours and not more than 5 hours. It was found that too short operating time of the ultrasonic bath (2 hours) did not result in satisfactory separation of carbon nanotubes. In the solution, the residence time of which in the bath was 2 hours, a considerable number of aggregates with quite large dimensions, reaching even 60 µm, is visible. The optimal residence time of the sample in the ultrasonic bath was 4-5 hours. Moreover, it was found that the long stay of the sample in the ultrasound (more than 10 hours) causes re-aggregation of previously dispersed carbon nanotubes. The photo shows numerous aggregates of considerable size.

The confirmation of obtaining the optimal centrifugation parameters is shown in Fig. 6. Photographs obtained from an optical microscope showing suspensions of carbon nanotubes in a hydrophilic ionic liquid with an anionic surfactant added in the centrifuge at different times (from 5 to 50 minutes) and with different centrifugation speeds (from 2000 rpm) are summarized. ./min to 8000 rpm). It was found that both the time and the speed of centrifugation influenced the degree of dispersion of carbon nanotubes. It was observed that increasing the time and speed of centrifugation reduces the number of carbon nanotubes in the obtained solutions. As a result of increasing the centrifugation parameters, both undispersed and dispersed carbon nanotubes mostly fell to the bottom of the bottle, so the actual concentration of carbon nanotubes in the solution is much lower. The optimal centrifugation parameters were determined, they were 5min @ 200Q rpm.

Obtained suspensions

P r z k ł a d 1

Obtaining a suspension of dispersed carbon nanotubes in a hydrophilic ionic liquid with the addition of an anionic surfactant. In this experiment, a 0.1 wt% solution of a hydrophilic CNTs ionic liquid. was subjected to an ultrasonic bath for about 5 hours at 60 ° C. We get a slightly tinted yellow dispersed nanotube suspension, which is then placed in a centrifuge for 5 minutes at a speed of 2000 / min. During this time, the black sediment containing larger nanotube aggregates and bundles falls to the bottom of the bottle. After separating non-dispersed aggregates and impurities, we obtain a well-dispersed suspension of carbon nanotubes in a hydrophilic ionic liquid (EMIM DEP) with the addition of an anionic surfactant (SDSu). The obtained suspension containing carbon nanotubes was observed using a polarizing optical microscope. These studies were carried out to check how carbon nanotubes are distributed in a hydrophilic ionic liquid. For this purpose, a small sample of the composite was placed on a round glass slide with a diameter of 15 mm. The whole was covered with a second slide with a diameter of 12 mm. To protect the samples against changes in their properties during microscopic observations, the slides were sealed around the perimeter with epoxy glue ("Poxipol").

P r z k ł a d 2

Obtaining a suspension of dispersed carbon nanotubes in a hydrophilic ionic liquid with the addition of an anionic surfactant. In this experiment, a 0.1 wt% solution of a hydrophilic CNTs ionic liquid. was subjected to an ultrasonic bath for about 8 hours at a temperature of 60 ° C. We get a slightly tinted yellow dispersed nanotube suspension, which is then placed in a centrifuge for 5 minutes at a speed of 2000 / min. During this time, the black sediment containing larger nanotube aggregates and bundles falls to the bottom of the bottle. After separating undispersed aggregates and impurities, we obtain a suspension of carbon nanotubes in a hydrophilic ionic liquid (EMIM DEP) with the addition of an anionic surfactant (SDSu). The obtained suspension containing carbon nanotubes was observed using a polarizing optical microscope. These studies were carried out to check how the carbon nanotubes are distributed in the hydrophilic ionic liquid. For this purpose, a small sample of the composite was placed on a round glass slide with a diameter of 15 mm. The whole was covered with a second slide with a diameter of 12 mm. To protect the samples against changes in their properties during microscopic observations, the slides were sealed around the perimeter with epoxy glue ("Poxipol").

Example 3

Obtaining a suspension of dispersed carbon nanotubes in a hydrophilic ionic liquid with the addition of an anionic surfactant. In this experiment, a 0.1 wt% solution of a hydrophilic CNTs ionic liquid. was subjected to an ultrasonic bath for about 4 hours at a temperature of 60 ° C. We get a slightly tinted yellow dispersed nanotube suspension, which is then placed in a centrifuge for 45 minutes at a speed of 8000 / min. At the time

The black sediment containing larger nanotube aggregates and bundles falls to the bottom of the bottle. After separating undispersed aggregates and impurities, we obtain a suspension of carbon nanotubes in a hydrophilic ionic liquid (EMIM DEP) with the addition of an anionic surfactant (SDSu). The obtained suspension containing carbon nanotubes was observed using a polarizing optical microscope. These studies were carried out to check how the carbon nanotubes are distributed in the hydrophilic ionic liquid. For this purpose, a small sample of the composite was placed on a round glass slide with a diameter of 15 mm. The whole was covered with a second slide with a diameter of 12 mm. To protect the samples against changes in their properties during microscopic observations, the slides were sealed around the perimeter with epoxy glue ("Poxipol").

Claims (5)

A method for dispersing carbon nanotubes in a hydrophilic ionic liquid comprising the step of treating a mixture of carbon nanotubes and a hydrophilic ionic liquid with ultrasound, wherein said step is performed in an ultrasonic bath, said hydrophilic ionic liquid containing an anionic surfactant which is an anionic surfactant. a surfactant, preferably sodium dodecyl sulfate, SDS or sodium 1-dodecane sulfonate, DS5, and the hydrophilic ionic liquid is 1-ethyl-3-methylimidazolium diethyl phosphate, EMIMDEP, said step of treating the mixture of carbon nanotubes and the hydrophilic ionic liquid with of ultrasound in an ultrasonic bath lasts from 2h to 14h, preferably from 3h to 8h, and most preferably from 4h to 5h, and is carried out at a temperature of 25 ° C to 65 ° C, preferably 45 ° C to 65 ° C, and most preferably from 60 ° C to 65 ° C. 2. The method according to p. The method of claim 1 or 2, characterized in that the suspension of carbon nanotubes in a hydrophilic ionic liquid obtained in an ultrasonic bath is subjected to centrifugation in a centrifuge for a period of 2 min to 60 min, preferably 2 min to 15 min, and most preferably 3 min to 5 min. Method according to claim 2, characterized in that the speed of rotation of the centrifuge is from 1000 rpm to 9000 rpm, preferably from 2000 rpm to 4000 rpm, most preferably from 2000 rpm to 2500 rpm. min. 4. The method according to any one of the preceding claims 1 to 3, characterized in that in said mixture of carbon nanotubes and hydrophilic ionic liquid the concentration of carbon nanotubes is from 0.05% to 3% by weight. 5. The method according to any one of the preceding claims 1 to 4, characterized in that said carbon nanotubes are a mixture of single-wall carbon nanotubes, SWCNTs, and multi-wall carbon nanotubes, MWCNTs.