The impact of polymer structure on the adsorption of ionic polyamino acid homopolymers and their diblock copolymers on colloidal chromium(III) oxide

Abstract

The aim of the presented study was to investigate the influence of the structure and ionic nature of polymers on the adsorption layer architecture. Chromium(III) oxide was used as an adsorbent. The surface behaviour with the addition of different polyamino acids or polyethylene glycol diblock copolymers was analyzed as a function of the solution pH. The analysis of the data obtained from the adsorption and electrokinetic measurements allowed for the proposal of the most probable structures of the polymer adsorption layers found at the solid particle–aqueous polymer solution interface. Moreover, the application of stability measurements enabled the determination of the interactions between the system constituents. Additionally, investigating the architecture of the adsorbed polymer chains on the solid particles is essential for the further applications of the studied macromolecular compounds.

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Introduction

Polymer adsorption at the solid–liquid interface is a very sophisticated process that is influenced by various factors, and is also a phenomenon that finds numerous applications in many branches of industry as well as in everyday activities.^1–4 Macromolecular structure is one of the factors that has a dominant influence on the type and strength of interactions with the adsorbent surface. A slight polymer chain modification contributes to changes in a studied system’s interfacial properties on account of differences in the nature of the adsorbent–adsorbate interactions. In this way, solid particles can be modified. Moreover, it is possible to obtain suspensions exhibiting specific properties (among others, the desired stability). The adsorption process is also strongly affected by the solution pH as well as the adsorbent surface properties.^5,6

Interactions between various metal oxides and polymers containing only one type of functional group in the chain structure have been tested repeatedly.^7–10 In the search for new stabilizers, suspension destabilizing agents or surface modifiers have led to increasing interest in copolymers – compounds consisting of two (or more) types of structural units of specified character connected via a covalent bond. These substances show considerably different properties compared to those of proper homopolymers, which are their building blocks.¹¹

Due to the differences in the ionic nature, polarity or structure of the macromolecular block, copolymers exhibit diverse interactions with the solid surface. In relation to the applied solution pH conditions, the ionic blocks may achieve a variety of conformations at the solid–liquid interface depending on the electrostatic forces occurring in the studied sample. Under appropriate conditions, one of the polymer structural units usually adsorbs strongly (an anchor), whereas the other extends into the bulk solution (a buoy).¹² The adsorption of various copolymers at the solid–liquid interface has become one of the most interesting subjects for researchers of dispersed systems.^13–21

Chromium(III) oxide (Cr₂O₃), used as an adsorbent, is a crystalline solid with an intense green colour. Due to its properties, it has a wide range of applications. Cr₂O₃ is used as a stable green pigment, a heterogeneous catalyst, as well as a coating for providing mechanical and thermal protection. In relation to the potential environmental risks associated with the presence of chromium(III) oxide in industrial sewage (especially for water ecosystems), it is necessary to develop effective methods for its disposal. The adsorption of polymers, resulting in aqueous suspension destabilization, could be one of them. Several papers concerning the usage of natural and synthetic substances have been published.^22–30 But on the other hand, stable Cr₂O₃ suspensions are desirable in the building and paint industries. In relation to the adsorbed layer architecture, it is possible to obtain specific stabilization/destabilization properties of metal oxide particles. In addition, the polymer chain architecture at the solid–liquid interface can be easily controlled by changing the solution pH.

In the presented paper, the analysis of the data obtained from the adsorption and electrokinetic (potentiometric titration and zeta potential) measurements of systems containing colloidal chromium(III) oxide and suitable homopolymers (or PEG-copolymers) is shown. The results were used to determine the most probable structure of the polymer layers formed on the solid particles’ surfaces. Because each type of polyamino acid unit exhibits a different ionic nature (ASP is derived from the anionic poly(L-aspartic acid), whereas the cationic LYS unit originates from poly(L-lysine)), the impact of the solution pH on the process of polymer adsorption on colloidal chromium(III) oxide was investigated. Additionally, the application of a modern turbidimetric method enables determination of the system’s stability. The hydrodynamic values (the TSI coefficient, the average aggregate diameter, as well as the average floc sedimentation velocity) calculated on the basis of the multiple light scattering theory were used to better describe the overall processes occurring between the suspension particles (especially the formation of hydrogen bonds and hydrophobic interactions). As a result, the polymer adsorption surface layer structure under different solution pH conditions was determined, which is a highly innovative element of this work. It is important to note that the polymeric substances used in the measurements are totally biodegradable and non-toxic. These features mean that the selected macromolecules can be safely used in environmental applications, as well as in many branches of industry (for example, in the agrochemistry, cosmetic and food industries).

Experimental

Materials

Chromium(III) oxide (Cr₂O₃) from POCh, Poland was used as an adsorbent. The specific surface area of Cr₂O₃ determined by the BET method was found to be 7.12 m² g⁻¹. The point of zero charge (pH_pzc) of Cr₂O₃ was 7.6 and its isoelectric point (pH_iep) was about 6.²² The average Cr₂O₃ particle size measured using dynamic light scattering was equal to 265 nm, with a polydispersity index value below 0.25.

Two classes of polymers were applied (both from Alamanda Polymers, USA) were applied in the macromolecular compounds. The first class comprises simple polyamino acids:

• poly(L-aspartic acid sodium salt), denoted as ASP, and

• poly(L-lysine hydrochloride) – LYS.

The second one consists of asymmetric diblock copolymers:

• poly(L-aspartic acid sodium salt)-block-poly(ethylene glycol) – ASP-block-PEG, and

• poly(L-lysine hydrochloride)-block-poly(ethylene glycol) – LYS-block-PEG.

The polydispersity index (PDI), which is a measure of the molecular weight distribution in the polymer is in the range of 1.02 to 1.2. Therefore, the polymers exhibit a uniform chain length. The polydispersity index is measured using gel permeation chromatography (GPC) in DMF with 0.1 M LiBr at 60 °C, and a calibration curve constructed from narrow polydispersity PEG standards was used. The average molecular weight is provided by proton NMR spectroscopy using the amino acid repeating unit to the incorporated initiator peaks integration ratio. The average molecular weights of the polyamino acid units (belonging to both the homopolymer and copolymer macromolecules) were 27 000 Da (for ASP) and 33 000 Da (for LYS), whereas the PEG block was characterized by an average molecular weight at the level of 1000 Da. The studied homopolymer and copolymer structures are presented in Fig. 1 and 2. The dissociation constant values (pK_a) determined by potentiometric titration were 3.73 and 10.55 for ASP and LYS, respectively.²³ All of the measurements were performed in the pH range of 3–10 at room temperature (≈25 °C). NaCl at a concentration of 0.01 mol dm⁻³ was used as a supported electrolyte.

Fig. 1 Structure of ASP and the ASP-block-PEG diblock copolymer.

Fig. 2 Structure of LYS and the LYS-block-PEG diblock copolymer.

Methods

Adsorption measurements were conducted using the static method. The polymer concentration after the adsorption process was estimated by using a UV-Vis spectrometer (Cary 100, Varian) at a wavelength of 210 nm. This wavelength was chosen after the UV-Vis absorption tests. The peptide bonds began to absorb at 280 nm and were most sensitive in the range of 210–230 nm. However, in that region, a severe interference from the absorption of OH⁻ ions was observed. Thus, 210 nm was chosen and the absorption of ‘blank’ samples without polymers at the same pH was subtracted. A linear calibration curve was obtained from the standard solution, and it was used to determine the unknown polymer concentrations in the supernatants.⁸ In order to eliminate the interference originating from the hydroxyl ions, the calibration curves were made as a function of the solution pH. A suitable amount of Cr₂O₃ was added into Erlenmeyer flasks containing 10 cm³ of polymer solution with fixed concentrations. After adjusting them to the appropriate pH values, the samples were shaken in water bath (OLS 200, Grant) for 24 h. Then the sediments were centrifuged twice (MPW-223e, Centrifuge) and 5 cm³ of the polymer solution was taken for the spectrophotometric analysis.

The surface charge density of chromium(III) oxide was determined using potentiometric titration. The surface charge density was calculated based on the volume of base that needed to be added to the suspension in order to obtain the desired pH value:

where: ΔV – the difference in the volume of base added to the suspension to obtain the desired pH of the solution, c – the molar concentration of the base, F – the Faraday constant (9.648 × 10⁴ C mol⁻¹), m – the mass of the metal oxide, S – the specific surface area of the applied metal oxide.

In order to obtain the potentiometric curves in the absence of the polymer, 50 cm³ of the supported electrolyte and 0.2 cm³ of 0.1 mol dm⁻³ HCl were placed into the thermostated Teflon vessel containing 50 cm³ of supporting electrolyte solution or polymer solution with a fixed concentration. A thermostated Teflon vessel with a stirrer, an automatic burette (Dosimat 765, Methrom), glass and calomel electrodes (Beckman Instruments), and a pH meter PHM 240 (Radiometer) were the parts of the measurements set. The process was controlled by a computer. The initial pH value was in the range of 3–3.5. After reaching an equilibrium, 1.5 g of Cr₂O₃ was added. The obtained suspension was titrated by NaOH at a concentration of 0.1 mol dm⁻³. The surface charge density of the adsorbent was calculated using the “Titr_v3” program written by W. Janusz.

The zeta potential measurements were taken using a Zetasizer Nano ZS (Malvern). To obtain a solid suspension in the background electrolyte solution, 0.003 g of Cr₂O₃ was added to a 50 cm³ beaker containing a suitable amount of NaCl. The suspension was further sonicated for 3 minutes and the test pH was fixed. The zeta potential measurements were carried out up to pH 10. The samples with polymers were prepared in the same way, and the polymer concentration range was 0.01–1 ppm.

Stability measurements were conducted using a turbidimeter (Turbiscan Lab^Expert) connected to the cooling module TLab Cooler and specialized computer software. The obtained results are presented in the form of transmissions and backscattering curves as a function of time. The analysis of the turbidimetric data allows the dynamics occurring in the sample during the measurement to be assessed. Moreover, due to the specialized computer software associated with the Turbiscan equipment, it was possible to calculate the TSI parameter (Turbiscan Stability Index), which is very useful in the evaluation of colloidal system stability. The TSI coefficient was calculated from the following formula:

where x_i denotes the average backscattering for each minute of measurement, x_BS is the average value of x_i, and n is the number of scans.

The coefficient ranges from 0 (for highly stable systems) to 100 (in the case of very unstable suspensions). Based on the transmission and backscattering data, it was possible to determine the stability parameters, such as the diameters of formed aggregates (particles, flocs) [μm] and the rates of particle (aggregates, flocs) migration [μm min⁻¹]. These data were calculated using the programs TLab EXPERT 1.13 and Turbiscan Easy Soft. The measurement lasted 15 hours, during which data were collected every 15 minutes.

The particle velocity rate was calculated on the basis of the multiple light scattering theory. The particle diameters were determined using the general law of sedimentation, which is Stokes’ law extended to concentrated dispersions:³¹

where: V – the particle migration velocity [μm min⁻¹], ρ_c – the continuous phase density [kg m⁻³], ρ_p – the particle density [kg m⁻³], d – the particle mean diameter [μm], v – the continuous phase viscosity [cP], φ – the volume of the dispersed solid fraction [%].

The studies involve the determination of changes in the aqueous suspensions’ stability in the absence and presence of the studied macromolecular compounds. The samples without the polymer were prepared by adding 0.02 g of Cr₂O₃ to 20 cm³ of the supporting electrolyte at a concentration of 0.01 mol dm⁻³, and then the suspensions were sonicated for 3 min. The next step was adjusting the required pH value of the samples. The suspensions containing polymers were prepared in an analogous way. The polymeric substance at a concentration of 100 ppm was added to the solid suspension after the sonication process. In order to investigate the effects of solution pH on the suspension stability, the measurements were performed at a pH equal to 3 (or 4 for LYS and LYS-block-PEG), 7.6 and 10.

Results and discussion

As can be seen from the chromium(III) oxide TSI parameter changes analysis (Tables 1 and 2), both the solution pH and the ionic nature of the polymer functional groups have an essential influence on the polymer adsorption layer structure found on the adsorbent surface. As a consequence, the abovementioned factors affect the metal oxide aqueous suspensions’ stability. Cr₂O₃ in the supporting electrolyte forms stable suspensions at pH 3 – the positively charged solid particles repel electrostatically. At the remaining solution pH values, the mineral oxide samples exhibit a considerably lower stability (the lowest value is reached at the point of zero charge, pH_pzc = 7.6). This is related to the attraction forces between the oppositely charged surface active groups located on the adjacent adsorbent particles.

Table 1 TSI values for the Cr₂O₃ suspensions in the absence and presence of ASP or the ASP-block-PEG block copolymer (c = 100 ppm)

System	Cr2O3	ASP c = 100 ppm	ASP-block-PEG c = 100 ppm
pH		TSI value
3	12.76	48.71	47.19
7.6	62.91	13.22	33.11
10	49.82	11.94	35.55

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Table 2 TSI values for the Cr₂O₃ suspensions in the absence and presence of LYS or the LYS-block-PEG block copolymer (c = 100 ppm)

System	Cr2O3	LYS c = 100 ppm	LYS-block-PEG c = 100 ppm
pH		TSI value
4	12.76	17.24	26.21
7.6	62.91	14.18	35.21
10	49.82	23.24	44.85

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Mechanism of polymer adsorption on the chromium(III) oxide surface

The poly(L-aspartic acid) and ASP-block-PEG copolymer adsorption isotherms on the chromium(III) oxide particle surfaces are presented in Fig. 3(a–c). The changes in the amount of the polymers that is adsorbed come from the differences in the adsorption mechanism. At pH 3, the Cr₂O₃ surface is positively charged. Under these conditions, a larger number of the diblock copolymer macromolecules is adsorbed at the solid interface compared to the homopolymer ASP. This is associated with the mixed adsorption layer formation. Under these conditions, both copolymer blocks undergo the adsorption process on the Cr₂O₃ surface. Moreover, the nonionic PEG segments can create hydrogen bonds with the nondissociated carboxylic groups (the dissociation degree α_ASP = 16%) belonging to the adjacent ASP chains, which additionally stabilize the structure. The presence of the PEG chains in the metal oxide surface layer provides the screening of charges originating from the neighbouring adsorbed polyamino acid units. Under the same conditions, the poly(L-aspartic acid) macromolecules can interact with the solid surface only due to the hydrogen bridges and the electrostatic attraction forces.

Fig. 3 Adsorption isotherms of ASP and the ASP-block-PEG copolymer on the Cr₂O₃ surface: (a) at pH 3, (b) at pH 7.6, and (c) at pH 10.

With an increase in the solution pH, the order of the isotherms is changed. The diblock copolymer adsorption undergoes a gradual reduction related to ASP. The reason for such behaviour is the mutual repulsion forces between the ASP block segments and the Cr₂O₃ particles’ surfaces, as well as between the adjacent strongly extended polymer chains. At pH 10 the nonionic copolymer structural unit acts as a buoy; it is directed toward the bulk solution. Due to a lack of affinity for the mineral oxide surface, it does not participate in the adsorption process at the solid–liquid interface. Additionally, the PEG block contributes to blocking the adsorbent active sites resulting in an apparent decrease in the adsorption compared to ASP.

From the analysis of the adsorption data obtained for poly(L-lysine) and the poly(L-lysine)-block-polyethylene glycol copolymer (Fig. 4(a–c)), it is clearly visible that in both cases the adsorption maximum is reached in the alkaline environment. At pH 4 the quantity of the adsorbed polymer macromolecules is considerably lower. Besides the presence of PEG fragment in the copolymer chains, this compound does not undergo binding to the Cr₂O₃ surface at pH 3. The order of the curves is also important. The presence of the PEG block in the macromolecular structure provides increased copolymer adsorption compared to the homopolymer in the alkaline or neutral medium. The order of the adsorption isotherms is reversed at pH 4, which is directly related to the changes in both polymers’ binding mechanisms on the Cr₂O₃ surface.

Fig. 4 Adsorption isotherms of LYS and the LYS-block-PEG copolymer on the Cr₂O₃ surface: (a) at pH 3, (b) at pH 7.6, (c) at pH 10.

The increase in the adsorption of LYS-block-PEG at pH 10 and 7.6 (compared to the homopolymer with a similar molecular weight) could be explained by the influence of the PEG unit on the copolymer chains structure. As was mentioned before, under alkaline conditions the nonionic block does not exhibit affinity for the adsorbent surface and it can only interact with the lysine segments forming the hydrogen bonds. At the pH_pzc (equal to 7.6), the copolymer adsorption means that at the solid–aqueous solution interface there can appear a mixed polymer layer (both structural units may be bound on the Cr₂O₃ surface). In this case, the PEG block can act in two ways. Firstly, the nonionic polymer segments participate in the LYS-block-PEG adsorption by forming the hydrogen bonds with the positively charged or amphoteric Cr₂O₃ surface groups. Moreover, these connections can be created between the LYS and PEG chains belonging to the other macromolecules.³² At pH 4, the LYS homopolymer is strongly adsorbed on the Cr₂O₃ surface. Under these conditions, the fully ionized poly(L-lysine) chains adopt a highly stretched conformation and the electrostatic repulsion forces between the system constituents play the crucial role. Placing in the structure a short nonionic PEG block, which would constitute an “anchor”, should significantly increase the amount of the adsorbed copolymer macromolecules. The reduction of the LYS-block-PEG linked with the Cr₂O₃ surface can be related to the active sites blocked by the substantially longer, expanded LYS fragment.

Surface charge density changes in the presence of the analyzed polymers

The changes in the Cr₂O₃ particles’ surface charge density (σ₀) as a function of the solution pH and the analyzed polymer contents are presented in Fig. 5(a and b). From the data analysis, it can be established that the concentration of compounds consisting of the anionic polyamino acid unit has an essential impact on the potentiometric titration curves. It is worth noting that, due to the lack of functional groups capable of electrolytic dissociation, the PEG block does not affect the changes in the σ₀ value. In the system without the macromolecular compounds, the solid particles’ point of zero charge (pH_pzc) is equal to 7.6.²²

Fig. 5 Comparison of the Cr₂O₃ surface charge density in the absence and presence of ASP and ASP-block-PEG; (a) at a concentration of 10 ppm, and (b) at a concentration of 100 ppm.

Polymer adsorption results in the reduction of the chromium(III) oxide particles’ surface charge density across all of the solution pH values (except for ASP at a concentration of 10 ppm, where the curves overlap). The σ₀ differences observed for the systems with a lower macromolecular compound concentration come from various chain conformations on the adsorbent surface. The absence of any apparent alteration of the surface charge values in the presence of ASP suggests a “flatter” conformation compared to that of the diblock copolymer. In the acidic solution the coiled anionic polyamino acid chains interact with a limited number of the Cr₂O₃ surface groups. Only in the alkaline environment does the increase in the dissociation degree of the carboxylic groups (and the associated development of the adsorbed polymer coils) ensure a slight change in the σ₀ value. The addition of the ASP-block-PEG copolymer to the studied system significantly decreased the solid particles’ surface charge density in the pH range of 3–10. This is related to the insertion of a larger number of compound macromolecules on the phase boundary on account of the presence of the nonionic PEG unit. As a consequence, more ASP block dissociated functional groups are assembled in the metal oxide particles surface layer, which contributes to the effective reduction of σ₀ values. Overlapping of the curves measured for Cr₂O₃ in the background electrolyte solution and in the presence of ASP-block-PEG above pH 10 is connected with a considerable drop in the analyzed substance adsorption amount.

The increase in the anionic polymer concentration in the solution leads to an increased reduction of the σ₀ values with respect to the Cr₂O₃/NaCl system. Simultaneously, the reverse of the obtained dependencies is observed. Under the acidic pH conditions (in the range of 3–5.5) the potentiometric curves obtained for both polymers coincide. This comes from the mutual cancellation of two effects associated with the interactions between the negatively charged polymer functional groups and the solid particle surface active species. First of all, the occurrence of numerous carboxylic groups near the metal oxide surface due to the adsorption process leads to the σ₀ values decreasing. However, on the other hand, the groups directly linked to the Cr₂O₃ surface contribute to the induction of positive charges according to eqn (4) (where S denotes the surface active group):

≡SOH + R–COOH ↔ ≡SOH₂⁺COO⁻–R (4)

In the pH range from 3 to 5.5, the interactions between a few copolymer dissociated carboxylic groups with the Cr₂O₃ particles are responsible for the appearance of the positively charged surface groups. The remaining anionic polyamino acid segments are located at a certain distance surrounding Cr₂O₃. In turn, in the case of ASP a greater number of –COO⁻ groups is present on the solid surface, leading to the less effective reduction of the surface charge. As the solution pH rises, the amount of copolymer macromolecules adsorbed on the Cr₂O₃ surface gradually drops on account of active sites blocking. Therefore, the surface charge density changes follow from the lower number of the negatively charged polymer functional groups assembled in the mineral oxide surface layer.

The impact of the homopolymer and copolymer containing the cationic poly(L-lysine) in their chains structure on the chromium(III) oxide surface charge density is presented in Fig. 6(a and b). The addition of any of the tested polymers leads to the point of zero charge (pH_pzc) shifting toward more acidic pH values. Moreover, the adsorption of both polymers results in the reduction of the surface charge density across the whole measured pH range (except for LYS-block-PEG after exceeding pH 10). It is worth noting that, regardless of the compound concentration, a larger drop in the σ₀ values is observed in the system involving the poly(L-lysine) macromolecules.

Fig. 6 Comparison of the Cr₂O₃ surface charge density in the absence and presence of LYS and LYS-block-PEG; (a) at a concentration of 10 ppm, and (b) at a concentration of 100 ppm.

The metal oxide particles’ surface charge values, in the presence of the polymers whose macromolecules contain cationic functional groups, are affected by two competitive effects. The decrease in the surface charge density, compared to the system without a polycation, is a consequence of the induction of negatively charged surface groups as a result of the electrostatic interactions between the ionized polymer amino groups directly bound to the solid surface according to eqn (5).³³

≡SOH + NH₃⁺–R ↔ ≡SO⁻NH₃⁺–R + H⁺ (5)

When the number of positive charges accumulating near to the particle’s surface exceeds that of the induced surface active groups, the total surface charge rises.

The reduction of the Cr₂O₃ particles’ surface charge density in the presence of LYS comes from the accumulation of a large number of amino groups in the immediate vicinity of the adsorbent surface. The occurrence of the short PEG fragment in the macromolecular structure results in a lesser drop in the σ₀ values. In the solution pH range from 4 to 7.6, the nonionic block segments act as the polymer anchor, therefore a few NH₃⁺ groups are located at a certain distance from the solid surface. Some of these are bound to the Cr₂O₃ surface due to the formation of hydrogen bonds (causing the σ₀ to decrease). Above pH 10 the PEG chains practically lose the affinity for the adsorbent particles but the positive charges number in the metal oxide surface layer is larger enough to predominate the induction effect, and hence, the overall surface charge density increases compared to Cr₂O₃/NaCl. In the system including the poly(L-lysine), the number of the induced charges at the solid–polymer aqueous solution interface is considerably higher as a result of the appearance of mutual attraction forces. In the case of the cationic macromolecular compounds, the increase in the polymer concentration does not influence the surface charge density curves.

Zeta potential measurements of Cr₂O₃ particles in the presence of polymeric substances with different ionic nature

The changes in the zeta potential in the absence and presence of the studied compounds containing anionic poly(L-aspartic acid) are shown in Fig. 7(a–c). It should be noted that the presence of PEG chains in the macromolecules can impact the electrokinetic potential values as a consequence of the slipping plane offset. The adsorption of this block may also result in the shifting of the counter ions from the Stern layer.

Fig. 7 Zeta potential dependencies obtained for the Cr₂O₃ systems without and with ASP (or the ASP-block-PEG diblock copolymer); when (a) c = 0.01 ppm, (b) c = 0.1 ppm, and c = 1 ppm.

For the systems characterized by the lowest polymer concentration (0.01 ppm), the electrokinetic potential in the presence of ASP does not differ from the values obtained for Cr₂O₃ in the supporting electrolyte. A very slight reduction in the ζ potential was observed throughout the pH range for the system with the ASP-block-PEG macromolecules.

Similar to the case of the data originating from potentiometric titration, for the low polymer concentrations this behaviour results from the increase in the number of negatively charged copolymer functional groups (in relation to ASP) in the solid particles’ diffusion layer. The increase in the concentrations of the studied anionic compounds contributes to the drop in the zeta potential values of the Cr₂O₃ particles. A reversal in the order of the curves is observed – the ASP homopolymer more effectively decreases the zeta potential compared to the ASP-block-PEG copolymer. Initially in the acidic medium, the presence of the nonionic PEG block causes an increase in the adsorption of the ASP-block-PEG macromolecules on the adsorbent surface. This leads to an increase in the number of counter ions removed from the Stern layer and, as a result, the zeta potential reaches higher values than those for ASP. Moreover, the formation of the densely packed mixed adsorption layer contributes to a weaker slip plane repulsion. As the solution pH rises, the polyamino acid chains in both substances adopt a more stretched conformation. Simultaneously, beyond the point of zero charge (pH_pzc = 7.6), the amount of adsorbed copolymer decreases in relation to the anionic homopolymer, mainly through the blocking of the adsorbent’s active centres. Under these conditions, a greater number of the dissociated carboxylic groups in the diffusion layer of the solid particles are covered by ASP, which is a direct reason for the electrokinetic potential reduction. The adsorption of the stretched anionic homopolymer macromolecules leads to the formation of the polymer layer consisting of numerous loops and tails at the solid–liquid interface. This results in the displacement of a larger number of positively charged counter ions out of the slipping plane (decreasing the ζ potential).

The changes in the zeta Cr₂O₃ potential in the absence and presence of the cationic polymers (c = 0.01, 0.1 and 1 ppm) are presented in Fig. 8(a–c). The zeta dependencies analysis as a function of the solution pH allows to determine the diffusion part structure of the mineral oxide particles covered by the polymer film. From the listed data, one can easily draw the conclusion that the structure of the adsorbed polymer macromolecule substantially affects the solid particles’ electrokinetic potential. The addition of poly(L-lysine) leads to a sharp increase in the ζ potential values over the entire solution pH range. As was mentioned before, the diblock copolymer binding mechanism changes depending on the presence of the short, nonionic PEG unit in the chains. Regardless of the LYS-block-PEG concentration, at low pH values the zeta potential is decreased relative to that of the Cr₂O₃–NaCl system.

Fig. 8 Zeta potential dependencies obtained for the Cr₂O₃ systems without and with LYS (or the LYS-block-PEG diblock copolymer); with (a) c = 0.01 ppm, (b) c = 0.1 ppm, and c = 1 ppm.

The electrokinetic potential differences, resulting from the presence of compounds containing the cationic poly(L-lysine), can be explained on the basis of the two opposite effects. The zeta potential reduction is induced by the slipping plane offset by the adsorbed macromolecules. The more extended the chains, the stronger the influence is. On the contrary, numerous positively charged functional groups introduced to the diffusion layer of the solid particles provide the increase in the zeta potential.

The reasons for the significant changes in the zeta potential obtained at pH 4 for both cationic polymers are the differences in the adsorption layers formed at the solid–liquid interface. The block copolymer adsorption is almost half that of LYS, though fewer amino groups are located in the Cr₂O₃ particle’s diffusion layer. In addition, the ζ potential reduction in relation to the adsorbent in the supporting electrolyte solution indicates the large contribution of the slipping plane shift effect. The strongly stretched adsorbed chains can successfully screen the access to the other adsorbent active centres for the succeeding macromolecules. At pH ≥ pH_pzc, the Cr₂O₃ surface charge density drop favours the increase in the copolymer adsorption amount on account of a larger number of electrostatic interactions. Under these conditions, both polymers contribute to the rise in the zeta potential values, but this tendency is less distinct in the case of LYS-block-PEG. This is related to the removal of positive counter ions from the Stern plane by the closely packed copolymer macromolecules.

Cr₂O₃ suspension stability in the presence and absence of the analyzed polymers

On the basis of the hydrodynamic parameters analysis (the TSI values, the average Cr₂O₃ aggregate diameter and the average sedimentation velocity) as well as the adsorption and electrokinetic data, the appropriate solid suspension stabilization (or destabilization) mechanism can be determined. The values obtained from the turbidimetric measurements carried out in the presence of suitable polyamino acids or their PEG diblock copolymers (Tables 1–4) can be very useful to describe the interactions existing between the adjacent adsorbent particles coated with the polymeric film.

A comparison of both the TSI values and the average aggregate diameter, affected by the ASP (or ASP-block-PEG) adsorption at the Cr₂O₃–aqueous polymer solution interface, exhibits the distinct influence of the polymer adsorption layer on the system’s stability. The average floc size at pH 3 in the presence of the anionic diblock copolymer sharply increases in relation to those in the presence of the homopolymers. Simultaneously, the stability of both suspensions is deteriorated. The reason for such behaviour is the Cr₂O₃ surface charge neutralization by the adsorbed polymer chains. Moreover, the low dissociation constant value of the carboxylic groups promotes the formation of hydrogen bonds between the adjacent adsorbent particles, which is responsible for increasing the average aggregate diameter as well as their sedimentation velocity (Table 3). In the system containing the diblock copolymer, the ability of the nonionic PEG unit to form hydrogen bridges is responsible for the sample instability.

Table 3 Average size of aggregates and their average velocity in the Cr₂O₃ suspension formed in the absence and presence of ASP or ASP-block-PEG

System	Cr2O3		ASP c = 100 ppm		ASP-block-PEG c = 100 ppm
pH	d [μm]	V [μm min^−1]	d [μm]	V [μm min^−1]	d [μm]	V [μm min^−1]
3	0.077	0.830	0.073	0.659	1.70	41.55
7.6	0.051	0.325	0.085	0.906	0.459	3.01
10	0.576	41.52	0.074	0.696	1.68	40.46

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At the point of zero charge (pH = 7.6) an improvement in the stability of the studied system is observed (compared to Cr₂O₃ in the supporting electrolyte). At the same time, the stability varies depending on the macromolecular structure, which is particularly evident given that the adsorption level of both polymers is very similar (TSI = 13.22 for ASP and TSI = 33.11 for ASP-block-PEG respectively). It is important to note that the average diameters of the aggregates are the smallest. This is because the PEG block can act as an anchor, while repulsive forces occur between the ASP chains and the adsorbent. Moreover, numerous hydrogen bridges formed between the ASP and PEG segments of the adjacent particles are responsible for the Cr₂O₃ particles’ aggregation. In turn, in the case of ASP, the extension of long polymer chains toward the bulk solution contributes to more effective suspension particle repulsion.

In the alkaline medium, the average floc size in the ASP-block-PEG/Cr₂O₃ system rises. The opposite tendency is observed in the solid suspension containing the homopolymer ASP. Under these conditions, the PEG fragment does not adsorb on the solid surface and is directed toward the solution. Therefore it can interact in hydrophobic ways or form hydrogen bonds with both blocks of the copolymer macromolecules adsorbed on the other solid particles. Additionally, the decrease in the amount of the adsorbed copolymer results in a lesser number of –COO⁻ groups occurring in the solid surface layer. Likewise, at pH 7.6, the existence of one type of monomer unit in the homopolymer macromolecule favours a higher Cr₂O₃ suspension stability, because of electrosteric stabilization.

The size changes of the aggregates formed in the presence of the cationic polymers and their sedimentation velocities are listed in Table 4. At pH 4, the addition of both LYS and LYS-block-PEG results in a slight decrease in the Cr₂O₃ suspension’s stability compared to the system which does not contain any polymeric substances. For the two other pH values, the Cr₂O₃ particles are stabilized, in that their TSI values are higher for LYS-block-PEG, indicating the occurrence of additional hydrogen bonding interactions.

Table 4 Average size of aggregates and their average velocity in the Cr₂O₃ suspension formed in the absence and presence of LYS or LYS-block-PEG

System	Cr2O3		LYS c = 100 ppm		LYS-block-PEG c = 100 ppm
pH	d [μm]	V [μm min^−1]	d [μm]	V [μm min^−1]	d [μm]	V [μm min^−1]
4	0.077	0.83	0.106	1.41	0.359	1.85
7.6	0.051	0.32	0.125	1.97	0.215	0.667
10	0.576	41.5	0.135	2.27	0.239	0.816

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In the acidic medium, the aggregate size (and the migration velocity) in the LYS-block-PEG/Cr₂O₃ system is significantly greater than in the homopolymer system despite the lower diblock copolymer adsorption. The LYS-block-PEG chains bound to the solid surface exhibit a spatially extended conformation (ζ = 6.4 mV in relation to 54.9 mV for LYS). In addition, the amino groups localized mainly in the particles’ diffusion layer are completely ionized, and hydrogen bonds can be formed between the segments belonging to different blocks, which explains the higher TSI value compared to LYS/Cr₂O₃ (Table 2). In the case of the suspension including the poly(L-lysine), the repulsive forces between various loops and tails contained in the different adsorbent particles’ adsorption layers contribute to a greater extent.

At pH_pzc and above, both cationic polymers contribute to the increase in the Cr₂O₃ suspensions’ stability. It should be noted that this improvement is considerably less evident after the application of the block copolymer. Additionally, under these conditions the average aggregate size is smaller than that at pH 4, in the basic solutions these values rise inconsiderably. Above the point of zero charge, the adsorption of both polymers increases rapidly, mainly through the electrostatic attraction forces. Simultaneously, the value for the degree of the amino groups’ dissociation is gradually reduced, thus the adsorbed polyamino acid chains adopt a more compact conformation. In the case of LYS, the larger number of positively charged loops and tails that accumulate in the diffusion layer (ζ = 49.7 mV at pH 7.6) results in the increasing repulsive forces between the adjacent solid particles. The slight deterioration of stability, observed in this system with an increase in the solution pH, results from the appearance of single polymer bridges (d = 0.135 μm). Despite higher copolymer adsorption and the presence of poly(L-lysine) units of the same length in the macromolecular structure, the numerous hydrogen bonds formed between both polymer structural units adsorbed on the different particles are responsible for the drop in the suspension stability.

Conclusions

The solution pH has an essential influence on the analyzed macromolecular compounds’ binding mechanism. The presence of two blocks with different ionic nature in the diblock copolymer macromolecules strongly affects the polymer film structure formed at the solid–liquid interface compared to those built up of one kind of monomeric unit. The differences in the polymer chain arrangements are translated into changes in the stability of the Cr₂O₃ suspensions.

The highest adsorption level for both anionic polymers is found at pH 3. Under these conditions, the particles’ surface charge becomes neutralized by the adsorbed oppositely charged chains. As a result, the solid suspensions in the presence of ASP and ASP-block-PEG are relatively unstable. With an increase in the solution pH, the number of polymer macromolecules linked to the Cr₂O₃ surface gradually drops. However, the impact of the polymers’ structures on the stability of systems containing both polymeric substances is more distinct. The stability reduction observed in the presence of the diblock copolymer is related to the formation of hydrogen bridges between the different polymer blocks.

In the case of compounds containing the cationic poly(L-lysine) part in the macromolecule, the adsorption isotherms have the reverse order. The adsorption maximum is reached at pH 10, however, both polymers do not undergo binding at pH 3. Under acidic pH conditions, the Cr₂O₃ suspensions in the presence of the cationic compound (or PEG copolymer) exhibit a lower stability than that of the Cr₂O₃/NaCl system. The solution pH increase results in the gradual stabilization of the solid samples. Due to possible interactions between the copolymer chains adsorbed on different adsorbent particles, this polymeric material is a better flocculating agent than poly(L-lysine) at pH ≥ pH_pzc.

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