Deryaguin Landau Verwey Overbeek

Published Date: 02 Nov 2017

1Department of Chemical Engineering, Faculty of Natural Sciences and Technology

Norwegian University of Science and Technology (NTNU), NO-7491, Trondheim, Norway.

2Polymer Nanocomposite Laboratory, Material Physics Division, School of Advanced Sciences, VIT University, Vellore - 632014, TN, India.

Corresponding author: Kalim Deshmukh

Email: [email protected]

Abstract:

In the present investigation, TiO2 nanoparticles were dispersed and stabilized in polyallylamine hydrochloride (PAAm.HCl) and polyvinylalcohol (PVA) aqueous solution by ultrasonication. The electrokinetic properties were studied by varying pH and temperature of nanodispersion with an emphasis on dispersion stability. Electrophoresis method was used to determine the zeta potential of nanodispersion in presence of polymers and their isoelectric points (IEPs) were determined. The measurements were performed over a wide range of pH from 2-12 and temperature from 25-65oC. The concentration of TiO2 was varied from 2-12 wt %. The dispersions were found to be quite stable (no sedimentation) over the period of two months. It was observed that polymer addition can affect the surface properties of nanoparticles to a great extent. The zeta potential of nanodispersions decreases as the pH increases. When temperature increases, more distinct decrease in the zeta potential was observed. Such behavior is may be due to the changes occurred in the linear dimension of adsorbing macromolecules with increasing temperature. As temperatures increases, the adsorption of polymer causes decrease in the diffuse layer charge, which lead to decrease in zeta potential. Also the increase in the ionic strength leads to a compression of the diffuse layer and reduction in the zeta potential. The conductivity of nanodispersions increases for all the pH and temperature studied.

Key words: Dispersion Stability, Zeta Potential, Isoelectric Point, Nanodispersion

Introduction:

The stability of colloidal dispersion against aggregation has been the subject of extensive research because of its importance in many industrial applications [1, 2]. The most important forces that determine the stability of the colloidal dispersions are attractive and repulsive van der Waals forces, electrostatic forces and forces resulting from adsorbed polymers [3, 4]. In various industries such as pharmaceutical, ceramic, paints and pigments the long-term colloidal stability of dispersion is of great importance. The repulsive forces must be dominant in order to maintain the stability of colloidal system,. The particles will aggregate, if the repulsive forces present in the sample are weak and this can also influence the final properties of the solution [5]. There are two fundamental mechanisms to stabilize colloidal particles in an aqueous solution, namely electrostatic and/or steric stabilization [6, 7]. A schematic representation of the two mechanisms is shown in Figure 1. Electrostatic stabilization takes place when particles bear the same electrical surface charge and as a result, repulsion takes place. This mechanism relies on the separation of ionic charges; therefore, it is mainly relevant in systems of high polarity like aqueous paints [5]. In the system, electrostatic stabilization takes place due to the distribution of charged species and its effect on particle interaction. The charge around the particle is arranged into a double layer in which each layer possesses an equal charge. When two particles approach one another, their charged double layers overlap and repulsion takes place. At the same time, London-Van der Waals forces cause the particles to attract [8]. If the attractive forces are stronger than the repulsive forces the dispersion will be unstable. However, if repulsive forces predominate, the system will be non-flocculating. In electrostatic stabilization the dispersion can be stabilized or flocculated by changing the concentration of ions in the system. In steric repulsion polymers are added to the system adsorbing onto the particle surface which prevents the particle surfaces coming into close contact [9]. The particles can be separated if the thickness of the coating is sufficient and if enough polymers are adsorbed. This stabilization is dependent on the structure and dimensions of the polymer layer adsorbed. The polymer can adsorb onto a particle through anchoring groups which have strong affinity for the chemistry of the surface [10].

Due to their effective photo catalytic activity, TiO2 nanoparticles have attracted significant interest of researchers all over the world due to their special properties which are of great importance in several technological applications such as self cleaning [11], water treatment [12] and air purification [13]. TiO2 has three naturally occurring polymorphs namely, anatase, rutile and brookite [14]. The rutile TiO2 is widely used in different industries such as plastics, foods, pharmaceuticals, inks, paper and especially paints and surface coatings [15]. Stabilization studies of colloidal dispersions of various commercial and synthesized metal oxides nanoparticles by electrostatic, steric or electrosteric interactions has been documented [9, 16-19]. The TiO2 nanoparticles have to be properly dispersed in liquid medium and remain stable for certain period of time in order to achieve maximum efficiency in a wide range of its applications. To exploit the advantages of TiO2 nanoparticles in various industrial applications, controlling the dispersion and aggregation of TiO2 nanoparticles is crucial. Most colloidal particles dispersed in a polar liquid medium like water possess a negative charge, as a result of differences in electrical potential between water and the particle. This change is because of an unequal distribution of ions over the particle surface and the surrounding medium. The double layer theory explains the ionic environment surrounding a charged colloid and explains how the repulsive forces are set up around a colloid. A schematic representation of electrical double layer theory is presented in Figure 2.

Deryaguin-Landau-Verwey-Overbeek (DLVO) theory [20] gives quantitative description of the stability of a colloidal dispersion, which is determined by the balance between the repulsive and attractive forces. The electrostatic repulsive force is dependent on the degree of double layer overlap and van der Waals interaction results in attractive force. The magnitude of both, attractive and repulsive forces is a function of the separation between the particles. The measurement of zeta potential has been widely used to assess the stability of colloidal systems [21, 22] because it gives information about the magnitude of the repulsive interaction between colloidal particles. In dispersions the particles tend to agglomerate where the value of the ξ potential is close to zero (IEP) whereas the particles tend to repel each other and no agglomeration occurs at highly negative or positive values of ξ potential (more than 30mV or less than -30mV) [23]. The knowledge of zeta potential brings information on several properties of dispersed system. Significantly, the zeta potential depends on the type of particles, type of medium in which they are dispersed and the type of ion present in the medium [24]. The zeta potential responds to changes in the pH, temperature and concentration of particular components [25, 26] as well as in the ionic strength of the electrolyte used. Zeta potential measurements have been widely used in advanced industrial technologies like in water treatment, biomedicine or in paper production [27]. Determination of potential has also been used for the evaluation of the effect of substrate in the colloidal phase which is of great importance in pharmaceutical, cosmetic and food industries to get stable colloidal systems [28]. Because of their great technological importance and significance, polymers are often used to control the stability and flocculation behavior of colloidal dispersions and suspensions. To control the stability of the dispersion with respect to sedimentation is very important in vast areas of applications such as treatment of waste water, in paint production, processing of mineral and construction materials such as joint treatment compounds [29].

The present study deals with the investigation of electrokinetic properties of aqueous TiO2 nanoparticles dispersion in polyallylamine hydrochloride (PAAm.HCl) and polyvinylalcohol (PVA) with the emphasis on dispersion stability. The effect of pH and temperature on dispersion stability of aqueous suspension was studied. The knowledge about the effect of polymeric dispersants on the electrokinetic properties has great importance in paints and coating industry [30]. In aqueous systems, polymers are increasingly being used in many technological applications where stability is required at high dispersed phase loading level and high electrolyte concentrations as well as under extreme conditions of temperature and flow [31]. We have chosen PAAm and PVA as dispersant because they could also provide well dispersed slurries with improved dispersion stability.

Experimental:

1.1 Materials:

Polyallylamine hydrochloride (PAAm.HCl) of molecular weight 60,000 g/mol was purchased from Polysciences; Inc. and polyvinyl alcohol (PVA) of molecular weight 72,000 (90+% hydrolyzed powder) was purchased from Merck Schuchart. AEROXIDE hydrophilic fumed TiO2 P25, having specific surface area of 50 ± 15 m2/ g, supplied by Degussa, Germany was used in this study. The average particle size of TiO2 nanoparticle was 21nm and the crystallographic composition were 80% anatase and 20% rutile. Water was purified through a Millipore Milli- Q academic system with a resistivity of 18.2 MΩ.cm and used as a solvent in all the experiments.

Methods:

2.1 Sample Preparations:

PVA was dissolved in distilled water by heated at 90oC for two hours and further added to aqueous solution of PAAm. PAAm/PVA/TiO2 dispersion was prepared with different concentration of nanoparticles by adding ultra pure water (Millipore Milli- Q system, Ω = 18.2M Ω.cm). Dispersion was prepared by means of minishaker at 2500 rpm for 10 min. In order to form homogenous dispersions before any tests sonication was applied for the duration of 30 min. Adjustment of pH was accomplished by auto titration using separate solutions of 0.1 M NaCl and 0.1 M HCl. The pH values were read on a 420A plus pH meter (Thermo Orion) which has been calibrated with standard buffers. A protocol for the preparation of PAAm/PVA/TiO2 nanodispersion is shown in Figure 3. A schematic representation of the bonding interaction between PAAm, PVA and TiO2 nanoparticle is shown in Figure 4. The samples were prepared by varying different concentrations of polymers and TiO2 nanoparticles and the details are given in Table 1.

2.2 Principle of Zeta Potential Measurements:

Zeta potential is an important tool for understanding the state of nanoparticle surface and predicting the long term stability of the nanoparticles dispersion. The most important forces governing the behavior and physical stability of colloidal systems is an electrostatic repulsion between the particles and zeta potential gives the quantitative information about such forces. In order to measure zeta potential, the sample is placed in the path of laser light and a pair of electrode is introduced into the sample [32]. An electric field is applied across the sample and the movement of the particle (electrophoretic mobility) is measured by the light scattering of the particles. Under the influence of an electric field applied across the electrodes, the charged particles in the nanodispersion will move around. The direction of the motion will indicate the sign of the charge on the particles, i.e. negatively charged particles will move towards the positive electrode (due to gravity) and vice versa [32].

The velocity of particles, per unit electric field, is referred to as its electrophoretic mobility (µ). Thus,

µ = v / E (1)

Where v is the particle velocity in the colloidal state and ‘E’ is applied field strength. The velocity is dependent on the strength of electric field or voltage gradient, the dielectric constant of the medium, the viscosity of the medium and the zeta potential. Zetapotential is related to the elctrophoretic mobility by the Henry equation:

µ = 2 ε z f (ka) / 3η (2)

Where µ= electrophoretic mobility, z = zeta potential, ε= dielectric constant, η=viscocity and f (ka) = Henry’s function.

The Zeta potentials of nanodispersions were using a Zetasizer 2000 Nano-sizer instrument (Malvern Instrument Ltd, Malvern UK) which follow Laser Doppler Electrophoresis technique. The optic unit contains a 4 mW He-Ne laser (λ = 633 nm). The measurements were performed in the temperature range 25- 65o C and applied voltage of 80V. The pH of the dispersion was varied from 2-12. Smoluchowski equation [33] was used to calculate zeta potential from the electrophoretic mobility of the particles. For each sample, the electrophoretic mobility was obtained by performing an electrophoresis experiment. Each zeta potential value is the average value of ten measurements.

The stability test was performed to determine the ability of TiO2 nanoparticle suspensions to remain dispersed over a period of time. Photos of the suspensions prepared under various experimental conditions inside clear glass vials were captured at certain interval over the course of two months.

Results and discussions:

3.1 Dispersion Stability:

Physical stability and shelf life are key parameters in the development of nanodispersions. Colloidal dispersions are often referred to as being either stable or unstable. In colloid science, a colloidal dispersion is said to be stable if the dispersion (dispersed phase) remains same over a long time period (e.g., months or even years) [8]. For a typical colloidal dispersion, it is easy to distinguish between the flocculated and non flocculated states as indicated by phase separation at macro level. However for very concentrated or extremely dilute systems and/or systems with large particles this distinction may be difficult to make [34]. In this study, the term stability refers to stability against sedimentation indicating the uniformity of the system. The dispersion stability was evaluated by allowing the dispersion to stand for atleast two months to visually examine the sedimentation behavior. Figure 5 illustrates the results of dispersion stability test on various samples. It can be seen from Figure 5 that the dispersion was quite stable for longer period of time and no sedimentation or phase separation was observed for a period of around two months. With different wt% of TiO2 different degrees of stability was observed. The nanoparticle dispersion can be called as stable dispersion when nanoparticles prevent the London-Vander Waals interactions with the help of static electricity repellence and steric hindrance repulsion [14].

Zeta Potential of TiO2 Nanoparticles:

Zeta potential measurements have been a valuable tool in monitoring and evaluating the influence of various electrolytes on dispersion stability. Other than physical stability zeta potential measures the difference in the electrical charge between the dense layer of ions surrounding the particle and the charge of the bulk of the suspended fluid surrounding the particle [35]. For TiO2 dispersed in an aqueous medium, equilibrium reaction will occur which causes charges on the surface and the formation of double layer at the solid /liquid interface. In highly dispersed systems, properties of the double layer have great influence on the interactions of the particles with each other as well as with surrounding medium. Because of surface protonation, the surface charge of metal oxides particles in water may vary from positive to negative as pH increases [9]. The surface charge of the TiO2 used was determined by measuring the zeta potential over the pH range of 2 – 12 for and three different concentrations of NaCl and KCl solutions. In addition, the zeta potential of TiO2 nanoparticles was measured in deionized water over the same pH range. The results are shown in Figure 6. The isoelectric point (IEP) of TiO2 in water was observed at pH 6.7. At pH values lower than the IEP, zeta potential is positive, reaching 30 to 38 mV in the pH range of 2 to 4, while at pH above IEP, the zeta potential is negative reaching – 40 mV at pH 12. The IEP of TiO2 in three different concentrations of NaCl and KCl solution is shifted to lower pH values. For all the three different concentrations of NaCl, the IEP was observed in the pH range of 4 to 5 and for all three different concentrations of KCl, the IEP was observed in the range 5 to 6. At pH values above IEP the zeta potential is positive and becomes negative as the pH and ionic strength increases, which is a characteristic of oxides. Zeta potential becomes smaller as the concentration of electrolyte increases which leads to the compression of double layer with increasing ionic strength. Generally, metal oxides in aqueous solution possesses electrical charge due to dissociation of hydroxyl groups attached to the surface, adsorption of H+ or OH- ions or due to hydrolysis of surface groups. Therefore, the resultant surface charge is pH dependent.

Effect of pH and Temperature on Zeta Potential of PAAm/PVA/TiO2:

pH is very important parameter for controlling the colloid stability because it affects the surface charge and interactions among the particles. The effect of pH on the zeta potential of TiO2 in PAAm/PVA/TiO2 dispersion with different wt% of TiO2 is shown in Figure 7-11 and their iso electric points were investigated. By increasing the pH of the solution, the zeta potential starts decreasing, reaching the lowest isoelectric point at pH 7.2 for 40/50/10 of PAAm/PVA/TiO2 dispersion. Further increase of pH leads to more negative values of zeta potential. The highest IEP was observed around pH 8.8 (Figure 7). The maximum zeta potential value was 30 mV at pH 2 at 25oC. Similarly at 35oC the zeta potential starts decreasing, reaching the lowest IEP at pH 7.1 for 40/50/10 of PAAm/PVA/TiO2 dispersion. Further increase of pH leads to negative values of zeta potential. The highest IEP was observed around 7.8. The shift of IEP is depends on the polymer concentration. The changes in the zeta potential are may be due to blockage of active sites on the surface of TiO2 and presence of charged groups in macromolecules of adsorbed polymers.

Effect of pH and Temperature on Conductivity of PAAm/PVA/TiO2:

When the temperature of nanodispersion increases, its viscosity decreases and the mobility of ions increases. An increase in temperature may also cause an increase in the number of ions in the nanodispersion due to dissociation of molecules. As the temperature increases, the polymer can expand easily which results in free volume [36]. The resulting conductivity is represented by the overall mobility of nanoparticles and polymer which can be determined by the free volume around the polymer chain. Therefore, an increase in the temperature of nanodispersion will lead to an increase in its free volume and eventually an increase in the conductivity. The conductivity of PAAm/PVA/TiO2 nanodispersions increases with increase in temperature as shown in Figure 12-16. This is may be due to the fact that any of these materials has a negative thermal coefficient of resistance. TiO2 nanoparticles and polymer chains acts as a trap between charge carriers which transits by hopping process. The segments of polymer try to release the trapped charges on increasing the temperature and this release of trapped charges is associated with molecular motion. The increase in conductivity with temperature is attributed to charge carriers and charge mobility. The number of charge carriers will increase when temperature increases and the mobility depends on the structure of the materials.

Conclusions:

In the present investigation, the quality of the nanodispesion was investigated by zeta potential measurements at various pH and temperature. The experimental results show that surface properties of TiO2 nanoparticles in aqueous suspensions are greatly influenced by the addition of polymers. The polymer addition can affect the zeta potential of nanoparticles to a great extent. Zeta potential gives the quantitative information about one of the most important forces (electrostatic repulsion) governing the behavior and physical stability of colloidal systems. The IEP values of PAAm/PVA/TiO2 dispersions were shifted to higher values and the zeta potentials values were much smaller than 40 mV, which indicate the degree of the stability of nanodispersion (ASTM1985). The applied temperature also has significant effect on zeta potential and conductivity measurements. The polymer adsorption causes decrease of the diffuse layer charge at all investigated temperatures. As the temperature increases, the zeta potential becomes lower. The surface charge at the interface of nanodispersion is affected by the temperature which results in an increase in IEP values as the temperature increases. Hence, these nanodispersions can be applicable in making water borne paints where the degree of the stability is very important for further development of these nanodispersions to make a new product.

Acknowledgements:

Authors would like to thanks Ugelstad Laboratory for availing various analytical instruments facilities at Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU) and Degussa, Germany for providing AEROXIDE hydrophilic fumed TiO2 P25 nanoparticles.