A model for ion-exchange behaviour of polyampholytic resins: Using polystyrene polyampholytic latex

Accepted Manuscript Title: A model for ion-exchange behaviour of polyampholytic resins; using polystyrene polyampholytic latex Author: N.P.G.N. Chandrasekara R.M. Pashley PII: DOI: Reference:

S0927-7757(16)31033-0 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.12.006 COLSUA 21213

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

12-7-2016 3-12-2016 7-12-2016

Please cite this article as: N.P.G.N.Chandrasekara, R.M.Pashley, A model for ionexchange behaviour of polyampholytic resins; using polystyrene polyampholytic latex, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.12.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A model for ion-exchange behaviour of polyampholytic resins; using polystyrene polyampholytic latex N.P.G.N. Chandrasekara and R. M. Pashley* School of Physical, Environmental and Mathematical Sciences, University of New South Wales, Canberra, ACT 2610, Australia. *Corresponding author, Tel: +61 2 6268 8482, Fax: +61 2 6268 8298 E-mail: [email protected] Abstract A polystyrene based polyampholytic (zwitterionic) latex was synthesised with an isoelectric point (IEP) of 6.9 and particle size of about 110 nm. Ion-exchange (IEX) ability of this latex with sodium chloride and ammonium bicarbonate (AB) was studied and it was found that Na+, NH4+, Cl- and HCO3- ions exchange with the surface groups following the law of mass action. The latex typically had an adsorption capacity of about 0.11 mmol/m2, for both NaCl and NH4HCO3 electrolyte solutions. Surface adsorption constants, i.e. KNH2 and KCOOH, were assumed to be similar to solution values, that is: 1×10-9 M and 1×10-6 M, respectively. Precise ionization ability of the active groups is specific for the type of polymer used to form the latex particles. Calculated surface charge densities, obtained from zeta potential studies, shows that the best pH range for IEX studies with this polyampholytic latex was in the range 6.8-8.0. The average affinity of HCO3- ions was found to be higher than Cl- during the IEX process. This latex was used to demonstrate an ion adsorption model which could be applied to the IEX properties of polyampholytic resins containing the same weak acid (WA) and weak base (WB) groups. 1

Keywords: polyampholytic latex, ion-exchange, adsorption, carboxyl, amine, charge density 1. Introduction Synthetic polyampholytic resins have been used in ion-exchange (IEX) studies, however, a mixed-bed of anion and cation exchanging beads are more frequently used, since they are easier to regenerate, and so polyampholytic resins have been largely neglected over the last few decades. Ion adsorption behaviour of such resins composed of weak-acid (WA) and weak-base (WB) have been determined in batch, equilibrium studies [1-6], but there is little evidence on their dynamic properties, that is, in fixed-bed, continuous-flow systems [6-9], probably because of their difficulty in complete regeneration. However, recent studies have shown that ammonium bicarbonate (AB) can be utilized to completely regenerate WA and WB polyampholytic resins [10, 11]. Therefore, it is useful to make an ionization model to describe the adsorption/desorption behaviour of such resins, using a similar kind of polystyrene latex colloid synthesised with the same WA/WB groups, in close proximity. Weak-acid/ weak–base polyampholytic resins are not commercially available. Therefore, the surface ion-exchange properties of such resins have not been studied previously and there is no exemplary molecule that can be used to explain the surface adsorption ability of such resins. However, since the resin could be used commercially in future, using our new regeneration method (based on AB), it is useful to study its surface adsorption ability experimentally. The ionization behaviour of carboxylic and amine groups on amino acids molecules [12, 13] in aqueous solution can be used as a basis to explain the ionization behaviour of polyampholytic resins at different pH and temperatures [10], but such simple molecules are unlikely to explain the IEX behaviour of solid resins.

2

Therefore, surface studies of

synthesised polyampholytic latices were selected here as a model system for the study of the ionization behaviour of amine and carboxylic groups in these resins. Polyampholytic latices with small particle sizes which are similar to the polyampholytic IEX resins, consisting of carboxylic and tertiary amine groups have been synthesised recently[14]. Moreover, studies have shown that polyampholytic polystyrene latex can be used for metal ion adsorption and has the ability to adsorb various divalent cations [14] and, in addition, these latices have the ability to IEX with both cations and anions [15]. This behaviour has also been found with protein molecules and for biomolecules, which have both cationic and anionic sites, to bind with multivalent ions [16-18]. A large number of factors influence the ion adsorption properties of these kinds of polyampholytic ion-exchangers, including temperature [6, 10, 14] duration of equilibration [19], ionic strength of the electrolyte solutions [3, 20] , pH [14, 19, 21], the ratio of acid to basic groups and the affinities of counter ions [3, 20, 22-25]. pH of the IEX system plays a controlling role during the ion adsorption process with WA/WB latex [14], since the ionization of the active sites on the latex surface are pH dependent. Therefore, IEX studies need to be performed considering the pH of the electrolyte solutions. This also helps to understand the IEX behaviour of counter ions with exchanging materials. Surface ion adsorption models can be used to describe the surface charging behaviour of polyampholytic latex particles, determined from zeta potential measurements taken in an appropriate range of solutions. The measured zeta potentials can be converted to the corresponding surface charge densities depending on the radius of curvature (i.e. radius ‘a’) of the latex particles compared with the Debye length (-1) of the solution. For large values of

3

the parameter a (>50) a surface flat-plate model can be used [26], whereas for lower a values models taking particle curvature into account have to be used [27, 28]. These polyampholytic latices are zwitterionic, and mostly exhibit a pH of net zero charge (pzc) or, an isoelectric point (IEP). Surface studies and ionization models can be used to calculate the IEP of such latices and the behaviour of adsorption sites present on the particles [29-31] in different solution conditions. Ions affinity towards surfaces and ionization constants of charged groups are strongly correlated with the pH [14]. The ion adsorption or ion exchange process can be very complicated and depends on both the adsorbing ion and the chemical and physical properties of the substrate. Studies have shown that hydrolysis of metal ions influences the affinity of adsorption of these ions onto the adsorption surface [32, 33] and surface complexation mechanisms can prime the metal-ion adsorption to surfaces by forming chemical bonds with the functional groups present in the adsorbent surface [34, 35]. Different adsorbent materials, such as synthetic polymers, metal oxides, active carbon, metal oxides, natural materials and biological substrates have been studied to explain the adsorption behaviour of cations and anions and the affinity of ions towards surfaces[14, 35]. Some of the studies have been performed on a qualitative basis and also, ionization behaviour of active groups have been quantitatively studied [33] to explain this adsorption behaviour. Metal-carboxyl ligand stability constants [36-38] for metal-ion adsorption onto biological substrates have also been determined. In the present study, ionization constants and binding constants were determined to help explain the nature of the adsorption of ions to combined WA/WB resins. Polyampholytic latex colloids with WA and WB surface groups were synthesised and used as a model for these resins. Ion-exchange and adsorption behaviour with sodium chloride and AB solutions were studied. Measurements of zeta potentials of the latex in a wide range of solutions were 4

also obtained and used to provide the surface charging properties of this latex surface to create a charging model to describe the IEX process for surfaces having both WA and WB groups, similar to the polyampholytic resins.

2. Materials and methods 2.1 Materials Styrene (99%), methacrylic acid (99%), diethylaminoethyl methacrylate (99%) and ammonium persulfate (98%) were used during this synthesis. All the chemicals were purchased from Sigma-Aldrich, Australia, as reagent grade. Ammonium bicarbonate (99%) was obtained from May & Baker, Dagenham, England and sodium chloride (98%) from Sigma-Aldrich, Australia, and were used as purchased. Sodium ion, chloride ion and ammonium ion standard solutions, in the range 100 ppm and 1000 ppm, were obtained from Hach Pacific Pty Ltd, Australia.

2.2 Synthesis of polyampholytic polystyrene latex colloids The polyampholytic latex samples were synthesised following the previously published procedure [14, 39]. Here, 60 g of styrene monomer, 6 g of methacrylic acid and 12.9 g of diethilaminoethyl methacrylate were mixed in a baffled flask and the reaction mixture was adjusted for pH 1.5 using nitric acid (HNO3). The total volume of the reaction mixture was maintained to 600 ml. This mixture was sparged with nitrogen gas (AR) over-night to remove any dissolved O2 from the system, prior to addition of the initiator, 0.3 g of ammonium persulfate. The reaction mixture was then heated at 70oC in the presence of N2 gas purging,

5

and the mixture was stirred continuously at 60 r.p.m for 24 hrs (see Fig. 1). The resulted suspension was kept for cooling, filtered through glass wool to remove any coagulum present in the reaction mixture. The final product obtained was stored in a glass bottle at room temperature. Fig 1 The latex samples synthesised was then purified by repeated dispersion in water followed by centrifugation [14, 40] at 4000 r.p.m using initially 4.5 ml of samples with 35.5 ml of Milli-Q water followed by several dispersions in water, until the conductivity of the supernatant remained constant less than 25 µS/cm. 2.2 Analysis The specific surface area of the latex was determined by the Bruner-Emmett-Teller (BET) method using a Micrometrics TriStar 3000. Morphology of the latex particles was studied using transmission electron microscopy (TEM, JEOL 1400) and the images were captured with a GATAN digital camera interfaced with GATAN digital micrograph software. Zeta potential measurements for the latex samples, in different electrolyte solutions, were obtained using a Malvern Nano-ZS particle analyser, with zetasizer software 6.32. Cation and anion adsorption to the latex was studied by measuring the concentrations of Na+, NH4+ and Cl- in the equilibrated solutions, through their ionic activities using ion selective electrodes (ISE), supplied by Hach (HQ440d-Hach). 2.2.1

Determination of surface are of the latex particles

The specific surface area of the latex was measured by the BET method. The samples were initially freeze-dried, and then vacuum dried at 80oC, in a vacuum compartment, and then used for adsorption/desorption studies with nitrogen gas. Finally, the BET surface area was 6

obtained. The particle size of the, assumed spherical, colloid particle can then be estimated based on the surface area values. 2.2.2 Determination of the morphology of the latex particles Latex samples was diluted for ×100000 in Milli-Q water, and applied to a formvar coated copper grid and allowed to dry. Then samples were examined using the TEM and images captured to measure the particle size of the latex.

2.2.3

Determination of particle density of the latex dispersions

The particle density of the latex was determined by drying a known volume of purified latex solution in an oven at 104oC for 18 hrs. The initial and the final weight of the latex samples were obtained and based on the particle size obtained from TEM studies, the particle density was calculated. 2.2.4

Zeta potentials of polyampholytic latex with pH

The purified latex was diluted for ×2500 times in Milli-Q water, and then used for these pH studies. pH studies with this polystyrene latex was carried out in the presence of 10 mM NaCl concentration. Here, the samples were prepared using 15 ml of latex samples, and the total volume of the mixture was 20 ml. The pH of the colloid was adjusted using 0.1M, 0.01, 0.001M NaOH and HCl solutions. The samples were prepared at pH 2.0, 4.0, 5.6, 8.0, 10.0 and 12.0 and the zeta potentials measured after 30 minutes, for equilibrium at 22oC. 2.2.5

Zeta potential measurements of latex in electrolyte solutions.

Purified latex samples of 5 ml aliquot were used and equilibrated with different salt solutions, maintaining a total volume of 10 ml. A series of NaCl solutions (0.1-0.6M), at pH 5.7-5.9,

7

NH4HCO3 solutions (0.1-0.5M) at pH 8 was used and equilibrated the colloid samples for 30 minutes and then the zeta potential for each sample was measured after diluting the samples by ×1000 times in Milli-Q water at 22oC. Further, the concentrations of Na+, NH4+, Cl- and HCO3- ions in the equilibrated colloid dispersions were measured in dispersion filtrate to determine the amount of ions adsorbed to the latex. Another set of latex samples pre-treated with 0.5M NaCl solution followed by washing with Milli-Q water and centrifugation was used for equilibrium studies in the presence of 1 mM NaCl and a series of NH4HCO3 solutions (0.1M-0.5M) at pH 8.2-8.3. The zeta measurements were taken by diluting samples by ×1000 times in Milli-Q water and the concentrations of Na+, NH4+, Cl- and HCO3- ions were measured in the dispersion filtrate, and the amount of adsorbed ions was determined using those values. A set of latex samples pre-treated with 0.5M NH4HCO3 was washed with Mili-Q water and centrifuged several times. Then the samples were equilibrated with a series of NaCl solutions (0.1M-0.5M) at pH 6. The zeta measurements also were obtained for those samples by diluting ×1000 times in Milli-Q water and the concentrations of Na+, NH4+, Cl- and HCO3ions were measured in the dispersion filtrate, to determine the amount of adsorbed ions. 3. Results and discussion 3.1 Particle size and the morphology of the polyampholytic latex TEM studies showed that the particle size of the polyampholytic latex was about 100 nm in diameter (Fig. 2), and the calculated particle size using BET values was about 110 nm (Table 1). These results agree reasonably well, and also the TEM images show the spherical shapes of the latex particles synthesised. In later calculations an average latex particle radius of 55 nm was assumed.

8

Fig. 2 and Table 1 3.2 Zeta potentials of polyampholytic latex with pH Zeta potentials as a function of pH were studied, and it was found that the latex had an IEP of 6.9, as shown in Fig. 3. Similar curves have been obtained for this kind of ampholytic latex published by other authors [14, 41]. Fig. 3 The zeta potentials of spherical latex particles in a range of electrolyte solutions and pH values were studied, at 22 C. The zeta potentials and the Debye lengths (-1) of each electrolyte solution were used to calculate the corresponding surface charge densities (0) using Equation (1), assuming that the solution Debye length was significantly less than the radius of the particles. This is because Equation (1) was derived for flat surfaces. Since the particles used in this study were about 55 nm in radius, this equation is reasonable for Debye lengths of about 5nm or less. The background electrolyte of 10mM NaCl, used in this study, corresponds to a Debye length of 3.04 nm and a value of about 20. The surface charge density on a flat isolated surface immersed in 1:1 electrolyte solution is given by the following equation, assuming that the measured zeta potential is equal to the surface potential (0): (

)

( )

(

)

(1)

Debye lengths can be readily calculated for any simple electrolyte from the simple relation: (

)

√∑

(2) ( )

9

where Ci(B), the bulk ion concentration at 25°C, is in number/m3 and zi the valency of the ion. For 1:1 electrolytes this becomes: (

)

(3)

√

where M is the salt concentration in mol /l at 25 C. However, calculation of surface charge densities (σ) for situations where a <10 requires the equation developed by White and Ohshima [28] , which for any z:z electrolyte is given by:

(

| |

| |

)[

* (

| |

(

)

)

( (

| |

| |

)

)+

⁄

]

(4)

For the case of a flat surface (a>>1) this becomes: (

| |

| |

)

(5)

The Debye length of any solution is given by:

(

)

( )

⁄

(6)

Which for z:z electrolytes becomes:

(

( )

)

⁄

(7)

where z is the valency of the ion, q is the electric charge of an electron,

is the zeta potential,

is the Boltzmann constant and T absolute temperature. ε0 is the vacuum dielectric constant (permittivity of free space) and ε r is the relative dielectric constant of the solution.

10

Table 2 shows the surface charge densities obtained for the synthesized polyampholytic latex particles according to surface flat-plate model and using the equations of White and Ohshima for smaller particles. Table 2 3.3. Development of an ion adsorption model for H+ on amphoteric latex surfaces

By using measured zeta potential (surface potential) values of the particles in different pH and electrolyte solutions we can apply the Boltzmann ion distribution model in a theoretical analysis using the law of mass action via numerical solutions, to describe an ion adsorption model (H+) for surface amines and carboxyl groups. A simple surface ionization model was developed based on the surface ion binding constants for H+ ions when immersed in aqueous NaCl solution. The ion adsorption processes and the corresponding equilibrium constants are given below. |

(

[

|

)[

] (

|

|

)

]

(8)

(9)

where S corresponds to the surface binding group and [SNR2], [SNR2H+], [SCOOH] and [SCOO−] are the surface site densities of each group. The total site densities [SCOOH] and [SNR2] are input as variables in the calculation, with the two binding or dissociation constants. [HB+] is the concentration of hydrogen ions in the bulk solution. The surface charge density of the amphoteric latex is defined below: ( ⁄

) | |

(10)

| |

11

In these ion adsorption equations, it is assumed that the pH is the same throughout the solution, that is in the bulk solution and next to the surface, since pH is defined in terms of the activity of the hydrogen ion (see equation 11) not its number density or concentration. In each case, in the ion adsorption equations given above, the ion concentrations are the bulk ion concentrations, which for the dilute solutions considered in this work, correspond closely to the ion activities. The exponential Boltzmann distribution terms arise, in each case, from the effect of the surface electrostatic potentials on the chemical potential of each of the charged surface groups (i.e. SCOO− and SNR2H+). (

)

(11)

Since, at equilibrium, the ion activity

H+

should be the same throughout the solution, so

should the pH value. Earlier models have assumed that the pH will actually be different next to charged surfaces. It is interesting, however, that removing this assumption still produces the same ion adsorption equations (see above) if we add the reasonable assumption that the chemical potentials of all charged surface groups are affected by the surface electrostatic potential. This difference in view will change the way we develop ion surface equilibria equations. In this work we have assumed that the pH is the same throughout the solution. These arguments are illustrated below using the fundamental and important case of the ionization of SCOOH groups, when immersed in aqueous solution. We can describe equilibrium in terms of the chemical potentials of solution species and surface species. Thus, at equilibrium: (12) And therefore, | |

(

12

)

(13)

where

is the fraction of ionized carboxyl groups on the surface and

is the standard

chemical potential. For dilute solutions abH+ = cbH+ , where cbH+ is the concentration of hydrogen ions in the bulk solution and hence all the constant terms can be combined to give a simple surface equilibrium equation:

(

)

(

|

|

)

(14)

This is the form of the equilibrium equation used in this work and in many earlier studies. However, here this equation was derived with the assumption that the activity and chemical potential of the hydrogen ion was the same throughout the solution, i.e. in bulk solution and at the surface. The Boltzmann term in equation (14) derives from the electrostatic energy of the ionized carboxylic groups not to the increased hydrogen ion concentration next to a negatively charged surface (which is given by:

( )

( )

*

|

|

+). Further, the

( ) is the

ion concentration at distance x from the charged surface. The form of equation (14) has led to misleading interpretations of this equation. The results obtained from analysis of the zeta potentials obviously depend on the surface adsorption model, whereas solution equilibrium analysis gives direct adsorption values, which were used for comparison. The surface site densities of the SCOO−, SCOOH, SNR2H+ and SNR2 groups were calculated using the flat surface model with the law of mass action. The best pH range for IEX applications using this polyampholytic latex was found to be about 6.8-8.0 (see Fig. 4). Further, in this model the pKNR2 and pKCOOH values used in this model were 9 and 6, respectively. By comparison, amine and carboxylic groups in simple molecules, in solution, 13

have values around 9.78 (e.g. glycine) and 4.75 (e.g. acetic acid) [10, 12, 13]. Glycine, which consists of both carboxylic and amine functionalities, shows diverse ionisation constants (i.e. KCOOH and KNH2 at 25oC, are respectively 4.47×10-3 and 1.66×10-10). Further the tertiary amine [42] and diethylaminoethyl methacrylate [43] also, have different pKa values, as shown in Table 3. Glycine was used to explain the IEX behaviour of polyampholytic resins in previous studies [10] which had a primary amine group. However, the synthesised latex should be more similar to the polyampholytic resin structure, with ionization constants of carboxyl and amine groups. Fig. 4 and Table 3

3.4 IEX behaviour of polyampholytic latex with different salts Zeta potential measurements obtained for the polyampholytic latex in a series of NaCl solutions have been used for the calculation of surface charge densities of the particles (see Fig. 5). The adsorption isotherms for Na+ and Cl- ions for latex particles was also measured and the results are shown in Fig. 6. For NaCl and NH4HCO3 solutions the IEX behaviour of the polyampholytic resins [10] had similar adsorption isotherms and this adsorption agrees with the results obtained for the isotherms of Na+ and Cl- adsorbed onto the polyampholytic latex. However, the adsorption capacity of NaCl onto the latex was lower than the values obtained for the polyampholytic resins [10, 11]. The measured zeta potentials and the Debye lengths (-1) of each electrolyte solution were used in the surface charge density (0) calculations and the Debye lengths used were always significantly less than the radius of the latex particles. According to the model at pH 5.7-5.9, 14

the surface-charge site density of the SNR2H+ groups was greater than for SCOO− groups, resulting in a greater adsorption of Cl- ions than Na+ ions. Fig. 5 and 6 NH4HCO3 solutions were used in a similar study and it was found that the latex had a negative zeta potential (see Fig. 5) at pH 8, where the surface density of the SCOO− groups was greater than the SNR2H+ groups. However, the equilibrium ion adsorption isotherms showed higher adsorption of HCO3- over NH4+ ions (see Fig.7). Adsorption of anions, such as OH- and HCO3-, at the surface of the latex particles will produce negative zeta potentials, which confirms that HCO3- ions must have been adsorbed into the resin. These results suggest that the average affinity of HCO3- ions for the latex was relatively high; and this behaviour can be further investigated using suitable mixed electrolyte solutions. Fig. 7 The sorption isotherms obtained for NaCl and NH4HCO3 show that maximum sorption density for the latex was around 0.11 mmol/m2 (see Figs. 6 and 7). Further, the adsorption and desorption of Na+ and NH4+ ions were the same as for Cl- and HCO3- ions. The sorption isotherms obtained for this latex with these ions were well fitted with both Langmuir and Freundlich isotherms, as shown in Table 4. Table 4 The results of equilibrium studies performed with a series of NH4HCO3 solutions in the presence of 1 mM NaCl for the latex samples at pH 8.2-8.3, which were pre-treated with 0.5M NaCl solution followed by several washing with Milli-Q water and centrifugation, show IEX behaviour as illustrated in Fig. 8. Negative zeta potential values were obtained with the equilibrated NH4HCO3 solutions in this study. Surface charge densities of the latex 15

particles are shown in Fig. 5. Adsorption of NH4+ and HCO3- ions to the polyampholytic latex equilibrated with NH4HCO3 solutions showed a similar adsorption level to the equilibration in NH4HCO3 and 1 mM NaCl. This confirms that NH4+ and HCO3- ions have greater affinity during the IEX process, when exchanging with Na+ and Cl- ions by mass-action. Fig. 8 Further studies, using latex samples pre-treated with 0.5M NH4HCO3 and equilibrated with NaCl solutions showed the IEX ability by adsorption of Na+ and Cl-, and desorption of NH4+ and HCO3- ions (see Fig. 9). The surface charge density of the latex was also calculated from the measured zeta potential values, as shown in Fig. 5. The positive zeta potential values obtained here show that more positive ions adsorbed to the latex particles and the adsorption isotherms (Fig. 9) for this study clearly confirms that more Na+ ions were adsorbed to the surface. All these studies show that this polyampholytic latex has the ability for exchanging the monovalent counter-ions: Na+, NH4+, Cl- and HCO3-, equally, via the Law of Mass Action. Fig. 9 Polyampholytic latex can be used as a model to examine the monovalent ion adsorption behaviour of polyampholytic resins, when the two materials have the same IEX groups. This comparison could be extended to help explain the more complex adsorption behaviour of divalent, trivalent and multivalent ions.

4. Conclusion A basic ion adsorption model was used here to accurately describe the surface charging behaviour of amphoteric latex particles with pH. This model was developed using a modified approach, which assumes that the bulk pH value remains constant throughout the solution, 16

even close to the charged particle surfaces. These observations were supported by direct adsorption isotherm measurements using ion selective electrodes. The IEX behaviour of ions to the weak acid and weak base (i.e. carboxylic and tertiary amine) groups of the latex can be well described and used as a model to explain the ion exchange properties of a recently developed regeneration method for polyampholytic resins, using ammonium bicarbonate. Acknowledgements The authors acknowledge financial support from the Australian Research Council (ARC) for funding this project (ARC Discovery Project DP120102385).

17

Figure 1 The reaction apparatus used for the preparation of latex solutions.

Figure 2 TEM images of the synthesised latex particles.

Figure 3 Zeta potential of the latex particles as a function of pH at 22oC measured in 10 mM NaCl.

18

Figure 4 Schematic diagram of sorption of Na+ and Cl− onto the resin (inset), and the calculated surface charge densities of SCOO− and SNR2H+ groups as a function of pH at 22°C in 10 mM NaCl.

(a)

(b)

(c)

(d)

19

Figure 5 Surface charge densities with concentration of different salt solutions; (a) NaCl for the new latex (b) NH4HCO3 for the new latex (c) NH4HCO3 and 1 mM NaCl for the pretreated latex with 0.5M NaCl solution (d) NaCl for the pre-treated latex with 0.5M NH4HCO3 solution, at 22oC.

Figure 6 Equilibrium sorption isotherm of polyampholytic latex with Na+ and Cl- at 22oC.

20

Figure 7 Equilibrium sorption isotherm of polyampholytic latex with NH4+ and HCO3- at 22oC.

Figure 8 Equilibrium sorption isotherm of polyampholytic latex with NH4+ and HCO3- in the presence of 1mM NaCl at 22oC, for the pre-treated latex with 0.5M NaCl solution.

21

Figure 9 Equilibrium sorption isotherm of polyampholytic latex with Na+ and Cl- at 22oC, for the pre-treated latex with 0.5M NH4HCO3 solution. Table 1 Properties of the polyampholytic latex Properties of the latex

Value

Units

Particle size

100 -110

nm

BET surface area

25.9±0.1

m2/g

4.66×1011

particles/ml

Particle density

Table 2 Surface charge densities calculated according to the surface flat-plate model and the White and Oshima model for latex particles with 55 nm radius in 10 mM NaCl at different pH (i.e. a ~20). Surface Charge Density pH

2

Theoretical Flat-plate

Calculated Flat-plate

Calculated White and

Model (C/m2)

Model (C/m2)

Oshima Model (C/m2)

7.69×10-3

8.39×10-3

8.70×10-3

22

4

7.38×10-3

7.14×10-3

7.51×10-3

6

2.04×10-3

3.40×10-3

3.59×10-3

8

−2.27×10-3

−1.67×10-3

−1.77×10-3

10

−7.63×10-3

−8.26×10-3

−8.67×10-3

12

−9.59×10-3

−1.10×10-2

−1.14×10-2

Table 3 Ionization constants of amines in aqueous solution at 25oC. Base

Ka 1.91×10-11

Triethylamine

3.63×10-9

Diethylaminoethyl methacrylate

Table 4 Langmuir and Freundlich isotherm parameters for sorption of NaCl and NH4HCO3 onto polyampholytic latex. Sorbate Isotherm

Parameter

Units NaCl

Langmuir isotherm

qmax

meq/m2 of sorbent

0.18

0.2

K

l/ meq

3.34x10-3

3.05x10-3

0.99

0.99

p

22131

13546

n

1.75

1.61

0.97

0.98

Correlation coefficient Freundlich isotherm

NH4HCO3

Correlation coefficient

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Petrochemicals (DMAEMA).

Specialty 2015,

Monomers: Available

N,N-Dimethylaminoethyl from:

Methacrylate

http://www.specialty-

monomers.basf.com/portal/streamer?fid=235729, Accessed on 28 July, 2016.

28

Research Highlights  Polyampholytic latex samples were synthesised containing weak acid and weak base groups.  These latex particles can be used as a model to explain polyampholytic resin ionexchange behaviour.  Surface charge densities at different pH values were determined from zeta potential measurements.

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