Ionic strength and zeta potential effects on colloid transport and retention processes

Previous studies speculate that hydroxo species present in flotation pulps at pH > 9, particularly those of polyvalent cations, selectively adsorb onto gangue minerals. Such species supposedly enhance the depressive action of carboxymethyl cellulose (CMC) onto gangue via an acid-base interaction between the positively charged mineral surface and the negatively charged CMC molecule. Thus, the hydrophilicity of gangue minerals is enhanced, preventing the dilution of the concentrate. However, as there is little evidence to support these claims for complex process waters of increasing ionic strength, it is important to investigate. Adsorption data and mineral surface charge analyses provide a fundamental understanding of how electrolytes and their ionic strengths affect gangue mineral-depressant adsorption. It is strongly anticipated that decoupling these effects will allow process operators to tailor their process water quality needs towards best flotation operating regimes and, in the long run, effect closed water circuits. Thus, using talc as a proxy for naturally floatable gangue common in sulfidic Cu–Ni–PGM ores, this work investigates the influence of the ionic strength of process water on the adsorption of CMC onto talc for a perspective on how saline water in sulfidic ores would affect the behavior and therefore management of floatable gangue. In the presence of CMC, the microflotation results showed that the rate of talc recovery decreased with increasing ionic strength of process water. Increases in ionic strength resulted in an increase in the adsorption of CMC onto talc. Talc particles proved to have been more coagulated at higher ionic strength since the settling time decreased with increasing ionic strength. Furthermore, the zeta potential of talc particles became less negative at higher ionic strengths of process water. It is thus proposed that increases in the ionic strength of process water increased the zeta potential of talc particles, enhancing the adsorption of CMC onto talc. This in turn created a more coagulated nature on talc particles, increasing their hydrophilicity and thereby retarding floatability.

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Colloid transport with wetting fronts: Interactive effects of solution surface tension and ionic strength

Water Research ◽

10.1016/j.watres.2009.12.012 ◽

2010 ◽

Vol 44 (4) ◽

pp. 1270-1278 ◽

Cited By ~ 23

Author(s):

Jie Zhuang ◽

Nadine Goeppert ◽

Ching Tu ◽

John McCarthy ◽

Edmund Perfect ◽

...

Keyword(s):

Surface Tension ◽

Ionic Strength ◽

Interactive Effects ◽

Colloid Transport ◽

Solution Surface

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Influence of concentration, ionic strength and pH on zeta potential and mean hydrodynamic diameter of edible polysaccharide solutions envisaged for multinanolayered films production

Carbohydrate Polymers ◽

10.1016/j.carbpol.2011.03.001 ◽

2011 ◽

Vol 85 (3) ◽

pp. 522-528 ◽

Cited By ~ 133

Author(s):

Maria G. Carneiro-da-Cunha ◽

Miguel A. Cerqueira ◽

Bartolomeu W.S. Souza ◽

José A. Teixeira ◽

António A. Vicente

Keyword(s):

Ionic Strength ◽

Zeta Potential ◽

Hydrodynamic Diameter

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Understanding how the properties of whey protein stabilized emulsions depend on pH, ionic strength and calcium concentration, by mapping environmental conditions to zeta potential

Food Hydrocolloids ◽

10.1016/j.foodhyd.2017.12.003 ◽

2018 ◽

Vol 79 ◽

pp. 572-578 ◽

Cited By ~ 16

Author(s):

S. Ravindran ◽

M.A.K. Williams ◽

R.L. Ward ◽

G. Gillies

Keyword(s):

Ionic Strength ◽

Zeta Potential ◽

Whey Protein ◽

Environmental Conditions ◽

Calcium Concentration

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Surface charge characteristics of Bacillus subtilis NRS-762 cells

10.7287/peerj.preprints.26626v1 ◽

2018 ◽

Author(s):

Wenfa Ng

Keyword(s):

Sodium Chloride ◽

Ionic Strength ◽

Zeta Potential ◽

Cell Surface ◽

Surface Charge ◽

Sodium Nitrate ◽

Actual Surface ◽

Wash Buffer ◽

Ph Range ◽

Ph Profiles

Bacterial cell surface carries an electrical charge due to the myriad functional groups present, as well as assortment of ions and molecules nonspecifically adsorbed to the cell surface. Thus, solution in contact with the bacterial cell surface play a critical role in influencing the overall surface charge characteristics through conferring nonspecifically adsorbed ions and molecules. Various wash buffers are commonly used in removing nonspecifically adsorbed ions and molecules for revealing the real surface charge of the bacterium. Using electrophoretic mobility measurement of zeta potential, this study attempted to understand the surface charge characteristics of Bacillus subtilis NRS-762 (ATCC 8473) with the help of three wash buffers: deionized (DI) water, 0.1M sodium nitrate, and 9 g/L sodium chloride. Experiment results revealed that B. subtilis NRS-762 was negatively charged over the entire pH range from 1.5 to 12. Specifically, with deionized water as wash buffer, the point-of-zero-charge (pHzpc) was at pH 1.5, which indicated that large amount of negatively charged functional groups were present on the cell surface. Comparison between the zeta potential-pH profiles of B. subtilis NRS-762 cultivated at 30 oC and 37 oC revealed that the profile for growth at 37 oC was more negatively charged over the entire pH range compared to that for growth at 30 oC. This highlighted that physiological adaptation might had occurred on the cell surface for coping with growth at a higher temperature. Zeta potential-pH profiles obtained revealed that DI water could not remove significant quantities of the nonspecifically adsorbed ions and molecules. On the other hand, the zeta potential-pH profiles of cells washed with 0.1M sodium nitrate and 9 g/L sodium chloride overlapped each other substantially, and were more negatively charged over the pH range from 2 to 11, compared to that of cells washed with DI water. This revealed substantial removal of nonspecifically adsorbed ions and molecules with the use of 0.1M sodium nitrate (0.1M ionic strength) and 9 g/L sodium chloride (0.15M ionic strength), which helped reveal the actual surface charge of B. subtilis NRS-762 cells. Collectively, actual surface charge of B. subtilis NRS-762 was masked by nonspecifically adsorbed ions and molecules, which could be removed by 0.1M sodium nitrate and 9 g/L sodium chloride wash buffer. Thus, in the case of B. subtilis NRS-762, 0.1M ionic strength wash buffer was the threshold at which there was complete removal of nonspecifically adsorbed ions and molecules from the cell surface.

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Zeta potential of bacterial cells: Effect of wash buffers

10.7287/peerj.preprints.110v1 ◽

2013 ◽

Author(s):

Wenfa Ng

Keyword(s):

Sodium Chloride ◽

Ionic Strength ◽

Zeta Potential ◽

Cell Surface ◽

Phosphate Buffer ◽

Bacterial Cells ◽

Ph Profile ◽

Wash Buffer ◽

Ph Range ◽

Zeta Potential Analysis

Zeta potential - defined as the electrical charge at the shear plane - is widely used as a proxy for cell surface charge. Consequent of its definition, nonspecific adsorption of ions on the cell surface may alter the value - and polarity - of the measured zeta potential, thereby, leading to erroneous results. Multiple wash and centrifugation steps are commonly used in preparing cells for zeta potential analysis – where various wash buffers (such as 9 g/L sodium chloride and 0.1M sodium nitrate) help remove ions and charged molecules nonspecifically bound to the cell surface. Nevertheless, little information on the wash buffers’ relative efficacies in removing nonspecifically bound ions hamper the comparison of zeta potential results across laboratories even for the same bacterial strain cultured under identical conditions. Thus, the present study sought to evaluate the effect of various wash buffers on zeta potential of bacterial cells grown in two culture media differing in salt content – thereby, allowing potential differential efficacy of buffers in removing nonspecifically adsorbed ions and metabolites to be discerned. Preliminary data revealed that for Escherichia coli DH5α (ATCC 53868) grown in LB Lennox (supplemented with 2 g/L glucose), the zeta potential-pH profile was not significantly different over the pH range from 2 to 12 for deionized water, 9 g/L sodium chloride, and phosphate buffer saline (PBS) wash buffers. As the glucose supplemented LB medium was a low salt medium without a phosphate buffer, it was unlikely that nonspecific adsorption of ions on the cell surface was extensive – thus, supporting the observation that the various wash buffers used did not have differential effect on zeta potential measurement. On the other hand, the zeta potential-pH profile of E. coli grown in a semi-defined medium with a high capacity phosphate buffer system, was significantly different over the pH range from 1 to 12 for deionized water, 9 g/L sodium chloride, 0.1M sodium nitrate, 0.1M sodium acetate, and 0.1M sodium citrate with the extent of difference positively correlated with wash buffers’ ionic strength. A similar relationship was also observed between the measured point of zero charge (pHzpc) and ionic strength of wash buffer, which, taken together, suggested that charge screening might be an important mechanism for removing the adsorbed ions. Collectively, although the experimental data suggests possible use of high ionic strength wash buffer in removing nonspecifically adsorbed ions from bacterial cell surface prior to zeta potential analysis, possible structural damage to the surface from removing intrinsic ions - necessary for stabilizing the bacterial cell wall - could not be discounted.

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Zeta potential of bacterial cells: Effect of wash buffers

10.7287/peerj.preprints.110 ◽

2017 ◽

Author(s):

Wenfa Ng ◽

Yen-Peng Ting

Keyword(s):

Ionic Strength ◽

Zeta Potential ◽

Cell Surface ◽

Bacterial Cell ◽

Phosphate Buffer ◽

Defined Medium ◽

Deionized Water ◽

Nonspecific Adsorption ◽

E Coli ◽

Ph Range

Zeta potential, defined as the electric charge at the shear plane, is widely used as a proxy parameter for bacterial cell surface charge. Nonspecific adsorption of ions or polyelectrolytes onto the cell surface, however, alters the value and polarity of the measured zeta potential, leading to erroneous results. Multiple wash and centrifugation steps are commonly used in preparing cells for zeta potential analysis, where various wash buffers (such as 9 g/L NaCl, 0.001M KCl, and 0.1M NaNO3) are routinely used for removing (by charge screening) ions and charged molecules that bind nonspecifically to the cell surface. Using Escherichia coli DH5α grown in LB Lennox (with 2 g/L glucose), experiment data showed that the zeta potential-pH profile was not significantly different over the pH range from 2 to 12 for deionized water, 9 g/L NaCl, and phosphate buffer saline (PBS) wash buffers. As LB Lennox is a low salt medium without a phosphate buffer, it was likely that the extent of nonspecific adsorption of ions on the cell surface was not severe, and the different wash buffers would correspondingly not exert much effect on measured zeta potential. Zeta potential-pH profiles for E. coli grown in a semi-defined medium (with a high capacity phosphate buffer system), on the other hand, was significantly different over the pH range from 1 to 12 for deionized water, 9 g/L NaCl, 0.1M NaNO3, 0.1M sodium acetate, and 0.1M sodium citrate wash buffers with the deviation positively correlated with wash buffer’s ionic strength. Furthermore, the point of zero charge (pHzpc) for E. coli grown in the semi-defined medium varies between 1.5 and 3, in an ionic strength dependent manner, for the various wash buffers tested. Collectively, this preliminary study highlights the importance of wash buffer ionic strength in affecting removal efficiency of non-specifically absorbed ions on bacterial cell surface, where a threshold exists (0.15M) for charge screening to be effective. At the upper bound, 0.6M ionic strength might remove cations intrinsic to the cell envelope, leading to possible cell surface damage and erroneous measurements.

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Bacterial surface charge in “layers”: revealed by wash buffers of different ionic strength

10.7287/peerj.preprints.2086 ◽

2018 ◽

Author(s):

Wenfa Ng ◽

Yen-Peng Ting

Keyword(s):

Ionic Strength ◽

Zeta Potential ◽

Surface Charge ◽

Cell Envelope ◽

Cell Mass ◽

Culture Broth ◽

Point Of Zero Charge ◽

Bacterial Surface ◽

Incomplete Removal ◽

Wash Buffer

Bacterial surface charge derives its meaning from the cell’s environment such as the solution in contact with the cell. Determining the surface charge of bacteria in its native environment requires measuring the proxy variable, zeta potential, using cells obtained from field studies. However, lack of adequate cell mass and concerns over measurement of a mixed species consortia rather than a specific species meant that bacterial surface charge measurement require biomass obtained from pure culture. Often grown in rich medium where myriad proteins and ions nonspecifically adsorbed onto the cell envelope or peptidoglycan layer, standard procedures for preparing the cell mass incorporated repeated steps of washing and centrifugation with various wash buffers, the efficacies of which are poorly understood. This report describes the results of a systematic study on how wash buffers of different composition and ionic strength affect the efficiency of removing nonspecifically adsorbed biomolecules and ions from Escherichia coli DH5α (ATCC 53868) cultured aerobically (shake flask, 37 oC and 230 rpm) in LB Lennox medium with 2 g/L glucose and a formulated medium. Using zeta potential-pH profiles over pH 1 to 12 as readout, the results showed that efficiency of removing nonspecifically adsorbed ions and metabolites positively correlated with wash buffer ionic strength. More importantly, 0.15M ionic strength (i.e., 9 g/L NaCl) seemed to be the minimum below which there was incomplete removal of nonspecifically adsorbed biomolecules. On the other hand, high ionic strength of 0.6M (e.g., 0.1M sodium citrate) significantly changed the point of zero charge (pHzpc), a reference marker for removal of ions intrinsic to the cell envelope. Collectively, results obtained inform wash buffer choice with regards to preserving cell envelope integrity, and avoidance of adsorption of buffer ions such as citrate. But, is there a true cell surface charge? Yes, but how do we define it in number of “layers” of adsorbed biomolecules? Philosophically, cells in culture broth are coated with layers of metabolites, proteins and ions. Hence, desire to reveal the true surface charge is essentially a decoating process, where wash buffers of increasing ionic strength remove each layer via charge screening. However, where is the endpoint? This research offers a different perspective and answer. Imagine a single bacterium suspended in LB medium, where there is constant adsorption and desorption of biomolecules as the cell grows: what is its relevant surface charge? It is the one where the loosely associated ions and metabolites are removed while retaining the nonspecifically adsorbed ions and biomolecules. Thus, deionized water wash provides a good estimate of the bacterial surface charge as grown in specific medium.

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Microparticle Deposition on Human Serum Albumin Layers: Unraveling Anomalous Adsorption Mechanism

Colloids and Interfaces ◽

10.3390/colloids4040051 ◽

2020 ◽

Vol 4 (4) ◽

pp. 51

Author(s):

Małgorzata Nattich-Rak ◽

Maria Dąbkowska ◽

Zbigniew Adamczyk

Keyword(s):

Ionic Strength ◽

Human Serum Albumin ◽

Zeta Potential ◽

Serum Albumin ◽

Human Serum ◽

Electrostatic Interactions ◽

Surface Concentration ◽

Dlvo Theory ◽

Adsorption Model ◽

Negative Zeta Potential

Human serum albumin (HSA) layers are adsorbed on mica under controlled diffusion transport at pH 3.5 and various ionic strengths. The surface concentration of HSA is directly determined by AFM imaging of single molecules. It is shown that the adsorption kinetics derived in this way is quantitatively described using the random sequential (RSA) adsorption model. The electrokinetic characteristics of the HSA layers at various pHs comprising their zeta potential are acquired in situ while using the streaming potential method. It is shown that at pH 3.5 the zeta potential of mica becomes positive for HSA concentrations above 3000 μm−2. At larger pHs, HSA layers exhibit negative zeta potential for the entire range of coverage. Thorough characteristics of these monolayers at various pHs were performed applying the colloid deposition method involving negatively charged polystyrene microparticles. The kinetics of their deposition and their maximum coverage are determined as a function of the HSA layer surface concentration, pH, and ionic strength. An anomalous deposition of microparticles on substrates also exhibiting a negative zeta potential is observed, which contradicts the Derjaguin, Landau, Vervey, Overbeek (DLVO) theory. This effect is interpreted in terms of heterogeneous charge distribution that results from molecule concentration fluctuations. It is also shown that the maximum concentration of microparticles abruptly decreases with the electric double-layer thickness that is regulated by changing ionic strength, which indicates that their deposition is governed by electrostatic interactions. One can argue that the results obtained in this work can be exploited as useful reference data for the analysis of deposition phenomena of bioparticles on protein layers.

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