Model and mechanism of carbon nanotube stabilization with plasticizer

Mechanism of stabilization of carbon nanotube (CNT) water suspension with plasticizer based on sulfated naphthalene formaldehyde resins is proposed in this article. CNT stabilization is achieved at the expense of fixing plasticizer group on the nanoparticle surface. Nonpolar part of plasticizer provides the formation of high-viscosity streak between CNTs particles and dispersion medium, and polar part provides the formation of double electrical layer, that supports formation of CNT micelle. Model of carbon nanotube micelle is described. It is determined that the conducting of ultrasonic dispersion of CNTs water suspensions with plasticizer based on sulfated naphthalene formaldehyde resins provides stability of CNTs suspensions for 7 and more days. Optimal options of ultrasonic effect are identified. It is shown that CNT suspension stabilized with sulfated naphthalene formaldehyde evenly apportion in volume of cement system in conclusion of the adding them to composition of cement paste. This distribution helps with obtaining cement stone with high exploitative properties.


Introduction
Nowadays the world scientific community pays a lot of attention to the applying of carbon nanotubes (CNTs) in the compositions of building composite materials.We can find many articles in different magazines and discussions from forums [1][2][3][4][5][6][7][8].However, CNT does not get wide usage on an industrial scale because of the complexity of entering them into cement system and evenly allocation in its volume [9,10].
One of the perspective method of CNTs adding to cement composition is the applying of stabilized CNTs suspensions both in water [11] and dispersion water-polymeric [12][13][14] mediums by acoustic dispersion [9].Efficiency of influence on dispersions systems stabilization with ultrasonic [15,16] and different physicals options [17] are paid a lot of attention from the industrial scale point of view.
Usage nanosized modifiers like suspension for making advanced building materials with given functional properties needs to research physicals and chemicals properties in details.
The main aim of this article is study aggregative and sedimentary stability CNTs suspensions with plasticizer based on sulfated naphthalene formaldehyde resins at ultrasonic effect, determination of kinetic addictions of coagulation, description of Cement samples for researches have been made with the applying of stabilized CNTs suspensions in water and dispersion water-polymeric mediums to Portland cement.There is used both as fresh and stored for 30 days suspensions.Cement strength have been determined by national standard GOST 30744-2001.Porosity of cement stone have been determined by the method of saturation samples with inert liquid.
The produce of hydration, structure formation and properties of cement stone have been determined by X-ray analyze and electron microscopy.X-ray analysis is done with diffractometer with copper anticathode (Cu anode λKα1 = 1.54056Å; 40 mA and 40 kV).The acquisition range was from 5° to 60° 2θ in 0.02° 2θ steps with integration at the rate of 50 s per step.The identification of minerals was realized by using data base JCPDS.
Content of hydrated phases has been determined by quantitative phase analysis with using of internal standard method.When use this method, it is added the accurately known amount of reference substance to test sample.Fluorite (CaF2) has been used as reference substance because of his intensive lines with d = 0.316 nm (2θ = 28°10') are close to analytical lines of cement minerals.

Stabilization of CNT suspension
The plasticizer is used as a stabilizer of CNT water suspension.It is added to water suspension at the amount of following ratio: from 0.1 to 1.0 % with step of 0.2%; from 1.0 to 2.5% with step of 0.5%; from 3 to 5% with step of 1%.Index of stabilizer protective action is considered with minimal amount of substance, which needs to stabilize volume sol unit.It calls protective number.It can be count by this formula: where Sprotective number, g/l; Cctconcentration of stabilizer, g/l; Vzvolume of stabilizer solution, ml; Vvolume of suspension, ml.The table 2 shows the results of counting.Stabilized action of superplasticizer can be evaluated according to the information from the table.For having stability after 7 days it is needed to add plasticizer at an amount of 3-5 g/l.Suspension stays stability after 30 and more days on conclusion of the adding amount of 5 g/l.

Aggregative and sedimentary stability of CNTs suspensions after acoustic impact
Separation of fine-grained powders in water and water-polymeric medium has been done with ultrasonic dispersion.Abrupt changes of pressure in dispersion medium, that entail disconnection of particle aggregates, arise because of fluid-passing acoustic wave and cavitation (formation of gas bubbles in a liquid).However, in this case dispersion medium warms up.The temperature increases to 50 о С and subsidence rate of CNT increases (fig.1), but aggregative and sedimentary stability of CNT suspension decreases.https://doi.org/10.1051/matecconf/201819303050ESCI 2018 Fig. 1.Ratio of subsidence rate of CNT and the temperature of dispersion.
In dispersion medium at the increase of temperature to 50 о С two competing processes occur at the same time.On the first side, Brownian motion is intensified, that provides homogeneity of CNT suspension and, as a result, sedimentary stability of nanoparticles.On the other side, frequent collision of CNT particles provides the process of coagulation, that leads to enlargement of particle aggregates and gains sedimentation.There is an imbalance in the system, that has a negative effect on the CNT division with ultrasonic impact and on their stabilization both in water (fig.2a) and water-polymeric (fig.2b) mediums, that is consistent with researches by this authors [16,17].a. Thermostating of the suspensions at the temperature of 25 ± 2 о С has been done for prevention influence of high temperature on the process of dispersion.Stationary temperature allows to get aggregative and sedimentary stabilized CNT suspensions (fig.3) Fig. 3. Schedule of CNT distribution at the stationary temperature 25 оС in water-polymeric medium.

Model of carbon nanotube micelle
Stabilized action of plasticizer based on sulfated naphthalene formaldehyde resins can be explained with formation of micelles.Molecules of plasticizer, that has non-polar and polar parts, lyophilize CNT surface, adsorb on it and form oriented layer, where hydrocarbon radicals of the main chain are inward (to CNT) and polar groups of sulfated naphthalene formaldehyde (R-SO3 -)to water dipoles.In this case surface tension is minimal and double electrical layer is formed [18,19], that in couple with dispersion phase is CNT micelle (fig.4).Potential-determining ions nR-SO3¯ (2) are located on the surface of this aggregate, that in couple compose the core of micelle.The core with counter ions (n-x) Н+, which are in a dense part of a double electric layer (3), form the granule, that has negative charge.The granule is surrounded with remaining counter ions x Н+ of diffuse layer (4), that are components of electrically neutral micelle.

Mechanism of carbon nanotube stabilization with plasticizer
CNT stabilization is achieved at the expense of fixing plasticizer group on the nanoparticle surface.Nonpolar part of plasticizer provides the formation of high-viscosity streak between CNTs particles and dispersion medium, and polar part provides the formation of double electrical layer, that supports formation of CNT micelle.
Molecules of sulfated naphthalene formaldehyde form the gel-like pellicle of the main chain radicals (6) on the surface layer of CNTs.This pellicle has high structural-mechanical and hydrodynamic properties, that provides stabilization of CNT.They have mechanical strength, elastic properties and block coalescence and fusion of particles.That explains mechanical factor of aggregative stability [20,21].
Stabilizing effect of the plasticizer on the CNT suspension is reached due to electrostatic, adsorption-solvate and structural-mechanical stability factors.Adsorptionsolvate and electrostatic stability factors are components of the structural-mechanical factor and contribute to the formation of elastic surface layers, thus, supplementing the action of the structural-mechanical factor.In this regard, we can assume that the stabilization of the system "CNT-water" with sulfated naphthalene formaldehyde is provided due to the structural-mechanical factor of aggregative stability.

Study of the suspensions effect on the cement properties
In first time, increase mobility of cement paste are shown on conclusion of adding the stabilized CNTs suspensions with lyophilized surface, and the water requirement of cement decreases and cement setting processes slow down (Table 3).The higher the concentration of CNTs in a cement system, the more it grasps more slowly.23,0 225 PC with CNT (0,25 g/l) and with SP (5 g/l) 22,0 235 PC with CNT (0,5 g/l) and with SP (5 g/l) 22,0 238 PC with CNT (1 g/l) and with SP (5 g/l) 22,0 245 Such properties of cement paste can be explained by mutual repulsion of negatively charged particles of stabilized CNTs and hydrated cement grains (fig.5a).The particles of stabilized CNTs get negative charge because of formation oriented layer of anionic plasticizer on their surface.Particles of cement have a negative charge due to the hydrolysis process on the surface of cement grains.
While passing the hydrolysis process, the capsule of hydrated neoplasms on the surface of cement particles is charged positively.It is happened because of the accumulation of ions Са 2+ in it.Stabilized CNTs with their anionic part are attracted to hydrated cement particles, thereby reinforcing the cement array (fig.5b).
With further hydration, the reinforcing carcass overgrown with crystalline hydrates, which determines the formation of a dense and durable cement stone.The table 4 shows the results of determining the strength and porosity of Portland cement.This information shows that strength of cement stone increase at 1 day on 37%, at 28 dayson 15%, and porosity decrease at 1 day on 13%, at 28 dayson 15%.https://doi.org/10.1051/matecconf/201819303050ESCI 2018 a. b.Fig. 5.The mechanism of interaction of stabilized CNTs with cement: astabilized CNT particle and particle of cement have a negative charge; bstabilized CNT particle and particle of cement have a positive charge.

Study of the cement stone structure
The quantitative composition of the cement stone with CNTs was determined by X-ray analysis.The results are shown at the table 5.This results help to estimate the dynamic growth of neoplasms and changes in the crystalline structure of cement stone.For 1 day of hardening, the content of calcium hydrosilicates (C-S-H) and the amount of C-S-H in cement stone when water and waterpolymer suspensions of CNTs are added to the cement composition increase compared to the zero-additive cement.Such trend persists to 28 days of hardening.Formation of this structure is due to the fact that calcium hydroxide, adsorbed on the surface of stabilized CNTs, does not crystallize into individual portlandite fields, but interacts with the crystalline hydrates of the solidifying system and binds to calcium hydrosilicates, which envelop the CNTs and bind the individual phases in the composition.

Conclusion
The study of the mechanism of CNT stabilization with sulfated naphthalene formaldehyde from the position of colloid chemistry makes it possible to explain the aggregative stability of water-polymeric CNT suspensions due to the formation of micelles {(CNT)m nR-SO3¯ (n-x) H+}¯• xH+.Ultrasonic impact on CNTs suspensions provides their aggregative stability due to the structural-mechanical factor.Stabilization of CNT suspension with sulfated naphthalene formaldehyde resins evenly apportion in volume of cement paste.The influence of stabilized CNTs on formation the cement stone structure allows to consider them like primary nanomaterials, that contain primary hydrated phases.Further formation of C-S-H around CNTs forms a cement stone with improved physical-mechanical and

Fig. 4 .
Fig. 4. The model of structure of micelle of carbon nanotubes, which stabilized sulfated naphthalene formaldehyde, where: 1aggregative of CNT; 2layer of potential-determining ions; 3counter ions of a dense part of a double electric layer; 4counter ions of diffuse layer; 5 -CNT micelle; 6gel-like pellicle of the main chain radicals R-SO3¯.Internal part of the micelle (5) consists of aggregate having m particles of CNTs (1).Potential-determining ions nR-SO3¯ (2) are located on the surface of this aggregate, that in couple compose the core of micelle.The core with counter ions (n-x) Н+, which are in a dense part of a double electric layer (3), form the granule, that has negative charge.The granule is surrounded with remaining counter ions x Н+ of diffuse layer (4), that are components of electrically neutral micelle.

MATECFig. 6 .
Fig. 6.SEM images of hydrated samples after 28 days, where: a -PC; b -PC with CNT; c -PC with CNT and with SP.

Table 1 .
Portland cement and its major constituent phases.

Table 3 .
The properties of cement paste of nanomodified samples

Table 4 .
Strength and porosity of Portland cement.

Table 5 .
The quantitative composition of the cement stone with CNTs.