Zeta potential titration / Particle size analysis
To avoid the short range but strong Van-der Waals attraction between particles is an art in formulating stable dispersions. An interface coated with sterically hindering macromolecules or with electrostatically reacting ions is used to avoid that attraction. Ionic interface attributes are measurable as “zeta potential”. As ions of the interface react with ions of the liquid surrounding, their influence on the zeta potential has to be analyzed. A behavior of a colloid or dispersion is well predictable, when the potential is titrated against pH, salt or polyelectrolyte concentration. Most zeta potential determining methods, in particular optical techniques, are not suited for fast titrations. The Stabino® electrical particle charge titration system is tailored for this task.
The NANO-flex particle size analyzer adds valuable complementary information to charge results. The concentration of specific size fractions may point to a reason for instability problems. The 180° DLS dynamic light scattering applied here, offers an astonishingly high sensitivity below 100 nm. Its external probe suits well into the measurement cell of the Stabino®
EDL Electric Double Layer – how to measure zeta potential
Figure 1 left: Particle interface with surrounding screening counter ions. The zeta potential ζ is defined as the potential at the shear layer. The only measurable and in real processes effective potential is ζ.
right: Spatial progression of a repulsive electrostatic and attractive Van-der Waals force. The red curve is the sum of both.
An electrostatically charged surface attracts as many oppositely charged ions from the liquid environment, until the surface appears neutral to the outside. This ion cloud is called the “electric double layer”. The inner ions are strongly bound, the outer ions in a less extend. The charge distribution can be illustrated as a decaying potential function, whereby the potential is repulsive to other equally charged surfaces. The repulsive electrostatic and the attractive Van-der-Waals force are opposed in Fig. 1.
For the measurement of the particle interface zeta potential it is necessary to shear the outer mobile ions away from the inner ions. In the following section, the oscillating streaming potential measurement principle is explained, with which a streaming fluid is applied as the shearing force. It is implemented in the Stabino® with practical orientation.
The Streaming Potential principle of the Stabino®
In case a liquid is displaced in the neighborhood of a surface, the cloud of excess counter ions is shifted in direction of the flow. With two electrodes along the direction of the streaming fluid, an electrical potential difference DU is depicted as „streaming potential“. The signal is proportional to the velocity Dv of the liquid and to the zeta potential z of the surface. Hereby the zeta potential of plain interfaces can be determined, but also from interfaces which are coated with particles. Polarity and ionic strength of the liquid, but also the Dv defining geometry are pooled together in one single instrumental constant k (1).
SP = k.Δv.ζ (1)
The natural adhesion of macromolecules and fine particles to a wall is used to immobilize particles against a fluid stream. With a low ionic occupancy at the wall as it is the case with PTFE, the influence of the particle carrying wall is neglectable compared to the charge signal contributed by the particles sticking to the wall of the measurement arrangement.
Layout of Stabino®
Figure 2: Measurement unit consisting of a cylinder (5-10 ml sample volume), piston and 2 electrodes, at which the oscillating streaming is picked up. The corresponding titrant (pH-, salt, polyelectrolyte) solution is dropped into the sample, the smallest step being 10 µL. Measurement and through-mixing occur at once and in seconds.
By moving a piston up and down in a measurement cylinder of the same material, an oscillating streaming potential SP is produced in the narrow gap between piston and cylinder at the 2 electrodes (Fig.2).
The movement of the piston creates the oscillating signal and homogenizes the sample in a second. No convection or sedimentation problems appear. In comparison to optical methods titrations are performed 30 times faster. The electrical signal neither depends on color, transparency or shape of the sample particles. The principle applies to macromolecular solutions as well as to particle dispersions and emulsions, covering a range from 0.3nm to 300 µm. The optimum sample concentration is between 0.1 and 5% v/v. However without titration through the isoelectric point reliable results can be derived up to 40% v/v.
That principle has the genes to tailor a system for efficient charge titrations. For convenience, two titration paths are integrated. With the tablet PC, operation is made simple. On the whole, Stabino® qualifies as a „charge mapping master” in formulation work. In the past, because of time consumption titrations used to be avoided and valuable information left unexploited. Stabino® bridges this gap.
On choice, the measurement signal can be calibrated to a suspension of known zeta potential or to the „streaming potential” produced by a polyelectrolyte solution of known charge concentration. Due to the large spectrum of applications (macromolecular solutions, powder suspensions, bio-organisms, emulsions) a calibration with a standard of similar size and composition is recommended.
Application Charge Titration
Besides comparing potential measurements the most common applications are:
- Polyelectrolyte titrations
- pH – titrations
- Determination of the isoelectric point
- Occupancy with functional ionic end groups
- Kinetics of the particle interface potential
- Search for stable and unstable zones
Applications are manifold from chitosan, proteins, beverages, nano- and micro-coating, ceramic slurries, carbon nano tubes through to algae and geological samples.
The total charge or polyelectrolyte titration
This kind of titrations is less known all the more useful for formulations and stability prognosis.
Interestingly, stability problems of beverages often rest on properties and quantity of macromolecules. Right here the strength of Stabino® is apparent. It reacts to macromolecules in a superb way differing in that respect from many other instruments.
Optimizing the dispersibility of carbon nano tubes
Figure 3: CNT - samples are titrated to 0 mV with charge calibrated cationic poly-DADMAC solution. The consumption (in meq) gives the occupancy of CNTs with ionic end groups.
The interface of CNTs is neutral “by nature”. Therefore CNTs are prone to cluster. To mix CNTs into composite materials in a homogeneous way depends strongly on how well they can be dispersed. One way is to make them electrostatically repulsive. How efficient this modification was, can be attested by means of a polyelectrolyte titration (Fig. 3). 0.1 mg of an anionic CNT sample is dispersed in 10 mL of water and titrated with cationic poly-DADMAC solution of known elementary charge concentration. The volume consumption to the isoelectric point gives the charge density per weight in Coulomb per gram [C/g]. If the specific surface is known, the specific charge density can be calculated [C/m²].
Determination of the isoelectric point and of stability parameters
To measure the potential versus pH is another typical fast titration application. From the position of the isoelectric point of protein solutions the solubility of proteins can be obtained. Many industrial applications like coating require material which is independent from the pH over a wide range. Again here, Stabino® is a quick assistant to customize material properties.
Influence of salt in the fluid
With rising salt concentration, the absolute value of the zeta potential decreases making the dispersion less stable. The conductivity can be monitored during titration. The particle interface potential was measured with Stabino® up to 100 mmol KCl concentration. A conductivity monitor is built into the cell assembly.
Optional size determination 0.8 nm to 6.5 μm
Figure 4: 180° DLS heterodyne size measurement probe.
As the dip-in probe of the NANO-flex module fits into the measurement cell of the Stabino®, it is useful to have charge and size measured at the same conditions.
Via an optic fiber and the entrance window a laser is focused onto the sample. Only light which is scattered back from the sample at an angle of 180° is conducted via the same fiber and a Y-piece to the detector. A small fraction of the laser beam is reflected from the sapphire entrance window and reaches the detector via the same way. By interference with the scattered light at the detector, it acts like an optical amplifier for the scattered light. Compared to layouts, where the scattering angle is less than 180°, the 180° design protrudes with its outranging sensitivity below 100 nm.
180° DLS stands for:
- Shortest light path in the sample, therefore not multiple scattering.
- Constant size results over orders of concentration.
- Highest concentration 40% v/v from 0.8 nm to 6.5 µm, material dependent.
- Outranging sensitivity for particles < 100 nm in the presence of bigger ones.
Measurement results with the Duo Stabino® / NANO-flex
pH-titrations on OVE- and BOVE albumins with 1 and 5 nm particle size last 5 Minutes. The titration curves are quasi-continuous.
In many materials not only the isoelectric point as an ultimate point of instability is interesting but also the critical coagulation point at which a system gently starts to agglomerate. In a charge titration, this is often indicated as an inflexion point, in the corresponding size distribution agglomerates appear. In Fig. 5 this effect is demonstrated on Al2O3 - suspensions.
pH – titration and critical coagulation point
Figure 5: pH-titrations on Al2O3-suspensions of Evonik Degussa (left). The D50 and D90 values of the W630 sample (right) show a drastic change at pH=7, where the charge curve has an inflexion.
As the size range of the Stabino® ranges from macromolecules up to micrometer scaled particles, the way to a lot of new applications is open. The speed of the measurement is inviting to perform a titration, rather than a classical optical method. For this reason, the Stabino® qualifies as a fingerprint – method for formulation and characterization work. Complementary size information from the 180° DLS method is particularly useful as it can be gathered in most cases at the same concentration.