Method and Apparatus for Determining the Zeta Potential of Colloidal Particles

The present invention relates to a method and apparatus for determining the zeta potential of particles in a suspension comprising, a cell containing at least a portion of the suspension of particles; a first electrode and a second electrode disposed in the cell; means for producing an electric field between the first and second electrodes, the electric field causing the particles to accelerate; means for converting a force, the force being generated by the acceleration of the particles, into electrical energy; and means for measuring the electrical energy. In particular, the present invention provides a novel method and apparatus for determining the zeta potential of colloidal particles by means of measuring the force created when an unsteady driving voltage is applied to the electrodes of a cell, the mechanical force produced by acceleration of the particles is converted to an electrical energy or signal by a piezoelectric mechanical resonator and the electrical energy being detected by a synchronous demodulator.

Determination of the zeta potential of particles in a suspension is very helpful in controlling the addition of auxiliary agents to influence the flocculation and retention characteristics of particles. The addition of auxiliary agents can substantially influence the zeta potential and it is for this reason that much time has been devoted to methods and apparatuses to be used in determining the zeta potential of colloidal particles.

Many attempts have been made at measuring the zeta potential among them the use of ultrasonic sound to measure the vibration potential between two electrodes, and application of a laser beam for optical measurements thereof to determine the electrophoretic mobility of the migrating particles. The use of ultrasonic sound to measure the vibration potential of particles in suspension is disclosed in U.S. Pat. Nos. 4,294,656, 4,381,674 and 4,497,208.

U.S. Pat. No. 4,294,656 provides for a process for measuring zeta potential wherein a portion of the suspension is exposed to an ultrasonic field in a measuring cell, the measuring cell having two electrodes which extend into the suspension and are spaced from each other by an odd multiple of half ultrasonic wave length of the ultrasonic field in the suspension, and generating a signal from the voltage thereby formed between the electrodes which corresponds to the state of charge and determines the addition of auxiliary agent.

U.S. Pat. No. 4,381,674 discloses a method of detecting and identifying particulates in the recycling fluid flow of an oil recovery system by counting the number of ultrasonic pulses reflected from the particulates and comparing the number counted with the amount of ultrasonic energy across the flow.

U.S. Pat. No. 4,497,208 discloses a method and apparatus for measuring electro-kinetic properties of charged particles dispersed in a liquid medium which comprises the step of positioning two electrodes to contact the liquid medium, energizing the electrodes with an alternating electrical potential to cause a charged separation between the surfaces of the dispersed particles and the charged layers which surround the particles in the liquid medium and thereby to generate an acoustic signal, spacing an acoustic transducer from the electrodes for detecting an acoustic signal, and measuring the amplitude of the detected signal, the amplitude of the detected signal apparently being a function of the electro-kinetic properties of the particles present in the liquid medium, the number of particles per unit volume and the amplitude of the excitation potential on the electrodes.

Another means of measuring the zeta potential of colloidal particles is described in U.S. Pat. No. 4,046,667 which provides for a microelectrophoresis apparatus for measuring the zeta potential or electrophoretic mobility of particles suspended in a bulk medium, e.g. colloids suspended in a liquid. It further provides for the use of a light beam, microscope and objective lens system for physically determining the zeta potential of colloidal particles. The use of the microscope and light beam according to U.S. Pat. No. 4,046,667, however, requires a highly trained technician to physically determine the zeta potential. This system is inherently subject to human error and also requires prolonged analysis prior to each measurement.

The aforementioned patents relate either to the measuring of a vibration potential by use of ultrasonic sound in a frequency range of above 100 khz or to the measurement of electrophoretic mobility by the use of optical methods. Ultrasonic methods which measure a so-called "vibration potential" suffer from a lack of adequate theory linking vibration potential measurements to familiar colloidal properties such as zeta potential. Optical methods suffer from complexity and human error in the measurement of electrophoretic mobility.

The present inventor proposes a novel means and apparatus which avoids the inaccuracies and disadvantages of the prior art. Moreover, the present invention provides a method and apparatus for detecting an electrical signal which is directly proportional to the zeta potential of colloidal particles. The present invention also overcomes the disadvantage of the prior art which requires periodic testing by trained technicians. The advantages of a method and apparatus according to the present invention will be further described below.

It is the primary object of the present invention to provide a method and apparatus for determining the zeta potential (electro-kinetic potential) of charged colloidal particles in a suspension. In particular, it is an object of the present invention to provide a method and apparatus for determining the zeta potential of particles in a suspension comprising: inserting at least a portion of a suspension of particles in a cell, the cell having a first electrode and a second electrode; connecting the first and second electrodes to a power source for generating an unsteady electric field; accelerating the particles by the unsteady electric field; converting the force generated by the acceleration of the particles to electrical energy; and measuring the electrical energy by a detector.

Moreover, it is an object of the present invention to provide an apparatus for determining the zeta potential comprising: a cell containing at least a portion of the suspension of particles; a first electrode and a second electrode disposed in the cell; means for producing an unsteady electric field between the first and second electrodes, the unsteady electric field causing particles to accelerate; means for converting a force, the force being generated by the acceleration of the particles, into electrical energy; and means for measuring electrical energy.

It is also an object of the present invention that the cell can be inserted directly into a container or feed stream of the suspension of particles to be measured. The cell having first and second electrodes at opposite ends thereof.

It is a further object of the present invention to provide a power oscillator as the means for producing an unsteady electric field between the first and second electrodes, this field causing the particles to accelerate which in turn produces the force. According to the present invention the force produced by the acceleration of the particles in the cell is converted into electrical energy by means of a mechanical resonator. The mechanical resonator operates at the same frequency as the driving voltage produced by the power oscillator and is a piezoelectric force transducer.

Furthermore, it is an object of the present invention that the electrical energy converted from the mechanical resonator is measured by a synchronous demodulator. It is preferable according to the present invention to insert a FET input operational amplifier between the mechanical resonator and the synchronous demodulator.

Additionally, it is an object of the present invention that the voltage applied to the first and second electrodes from the power oscillator has a frequency in the range between 0-50 khz.

The present invention may also include many additional features which shall be further described below.

FIG. 1 is a block diagram detailing the present invention; and

FIG. 2 is a sectional view of another embodiment according to the present invention.

The present invention provides a novel method and apparatus which detects the unsteady force caused by the acceleration of the particles in suspension between a first and second electrode disposed within a cell. Accordingly, the unsteady force measured by the present invention is directly proportional to the zeta potential of the colloidal particles of suspension. The unsteady force can be easily detected (demodulated) using extremely accurate A-C signal detection techniques, e.g. synchronous demodulators. It has been found by the present inventor that it is important to limit the frequency of operation to a range between 0-50 khz, so that the unsteady acceleration of the cell will result in a rigid body translation of the entire cell contents. If frequencies in excess of 50 khz are attempted, the motion can no longer be described as a homogeneous acceleration of the liquid in the cell. Rather, the system must be described using an acoustic wave model similar to that used in measuring the so-called "vibration potential" which is treated in the prior art. This distinction, between a slowly accelerated cell undergoing a rigid body translational acceleration, and the acoustic wave system used by others serves to separate this invention from the prior art.

The zeta potential is particularly important when solids, such as kaolin clay particles, are placed into an electrolyte solution (sodium sulfate or other salts), a surface of the solids often become charged up relative to the bulk salt solution. Typically, this charging up of the solid surface is due to adsorption of one or several ions of a particular charge. Kaolin clay, for instance, tends to be an anion exchange material, that is, the clay particles tend to adsorb anions on its surface to neutralize immobile cationic exchange sites. The adsorption of anions on to the clay particles tend to make the particles become negatively charged. The charging of the small particles, i.e. less than 10 microns in diameter, is very important in determining the macroscopic behavior of a slurry of such particles. Such slurries occur in a wide variety of systems including paints, pigments, coal slurries, pulp and paper manufacturer, and many others.

The region of liquid near a charged solid surface in contact with electrolytes is called a diffuse double layer. In this region, two counterposed effects are in balance. On the one hand, ions, for example, cations, want to move towards the solid surface to join the adsorbed anions and thus produce a neutral space-charge distribution. On the other hand, cations mutually repel one another, and spontaneous aggregation of cations in one region of space near the particle constitutes a concentration gradient which tends to be reduced by diffusion away from the solid surface. The equilibrium distribution of cations outside the region of adsorbed anions is the result of a superposition of all these effects. The potential distribution which exists inside the double layer is roughly exponential in shape.

Thus, when electric fields are applied to the charged bodies, forces act on the charged bodies. The relationship being:

where F is the force, E is the electric field strength and q is the charge.

A particle in an electrolyte within an applied electric field will result in having a force couple acting to move the minus charges attached to the particle in one direction, and there will be opposing forces acting to move the plus charged liquid surrounding the particle and double layer in the opposite direction. Thus, there is a net migration of particles in one direction and liquid in the opposite direction. This in effect causes the particle to accelerate.

To define the zeta potential of a colloidal particle in suspension one must first determine the distribution of charge near the particle by the following equations: (1) Poisson's equations which relates charge density to electric potential; (2) the Poisson-Boltzman's equation which balances the desire of cations to join with anions against diffusion forces which resist the aggregation of cations into one region of space; (3) Ohm's law which relates the electric field strength to the conductivity of the electrolyte; and (4) the momentum equation where F=ma as applied to the liquid surrounding the particle and applied to the particle itself. These equations determine how colloidal particles will react to the application of electric field, e.g. by passing currents through a slurry.

To solve the system of coupled partial differential equations, one must apply boundary conditions. Thus, equations 1 and 2 above require the specification of a potential some place near the solid surface. Equation 4 requires that a no-slip condition be applied at the "shear-plane", where the "liquid" outer region of the ion cloud surrounding a particle becomes "solid" in nature, and shear is no longer allowed. The potential is often specified at a location coincident with the place where the no-slip condition is imposed. This potential is then defined as zeta potential.

Zeta potential is used in practice as a measure of the amount of charge adsorbed on to the particles in a suspension. Consider the case where a suspension of colloidal particles is settling or sedimenting out under the action of gravity. In such a case, charged colloidal particles are seen to migrate with a constant velocity toward the bottom of the vessel which contains the slurry. This movement of charge associated with the migrating particles constitutes a small electric current which in turn produces a small electric field. This electric field can be detected by inserting two electrodes spaced along the direction of migration of the particles. The potential detected by the electrodes is called the sedimentation potential. (E.sub.sed) ##EQU1## where .epsilon.=.epsilon..sub.o D

.epsilon..sub.o =permittivity of free space

D=dielectric constant of liquid

.mu.=fluid viscosity

.lambda.=fluid conductivity

R=particle radius

.rho..sub.p =particle density

.rho..sub.e =liquid density

C=particles concentration - #particles/unit volume

g=acceleration due to gravity

Z=zeta potential

E.sub.sed =sedimentation

The steady acceleration of gravity produces a steady sedimentation potential which is extremely hard to detect because other steady potentials also exist due to non-sedimentation related effects. To make the sedimentation potential more easily detected, the present invention applies an unsteady acceleration to the entire system by vibrating the entire cell at a convenient frequency, e.g. 1 khz. The theory for sedimentation potential is then modified by substituting g=A sin .omega.t in the equation for sedimentation potential. Where A is the amplitude of the externally applied unsteady acceleration, .omega. is the frequency of the unsteady acceleration, and t is time. Thus, ##EQU2## where E.sub.used is the unsteady sedimentation potential.

Conversely, the present inventor has discovered that when an unsteady electric field is applied, the result is a corresponding acceleration of the entire slurry which can be detected by a force transducer. This force can be calculated from a knowledge of known physical constants and material properties of the system. Thus, the force (F) is proportion to: ##EQU3## where A is applied electric field amplitude, and F is the measured unsteady force. By averaging, the unsteady force amplitude can be accurately determined, e.g. by means of synchronous demodulation of the force signal.

The mechanical force is thus converted to electrical energy by means of a force transducer 6, such as a piezoelectric mechanical resonator. The piezoelectric element being, for instance, a lead-zirconium-titanate supported by a brass disc. The output voltage from the force transducer will be proportional to the zeta potential. The output voltage is measured by a detector 7, which may be a synchronous demodulator. An amplifier such as a FET input operational amplifier may be inserted between force transducer 6 and detector 7 to enhance the detecting of the small forces and energy signals achieved from force transducer 6.

FIG. 2 describes the preferred embodiment of the apparatus according to the present invention. Thus, FIG. 2 has a cell 9 containing particles in suspension 10 and electrodes 11 and 12 being disposed at opposite ends of cell 9. The electrodes 11 and 12 are made of electrically conductive material, such as platinum or stainless steel sheet metal. Electrode 11 is of a ring shape and adhered to the inner wall of cell 9 and electrode 12 is attached to cell 9 at one end thereof to seal one end of cell 9. Electrodes 11 and 12 are electrically connected to amplifier 13 via electrical wires 14 and 15, respectively. Amplifier 13 being electrically connected to a power oscillator 16 via wire 17.

Cell 9 is disposed on a plate 18, plate 18 being supported by stabilizing mass 19. On the opposite side of plate 18 from cell 9 is a cone 20 mounted on a base 21 and positioned such that the pinnacle of cone 20 is in contact with a force transducer 22. Force transducer 22 being a mechanical resonator formed from a piezoelectric crystal, e.g., a lead-zirconium-titanate piezoelectric crystal. A conductive disc 23 supports the force transducer 22 and transfers the electric signal which is converted by force transducer 22 from mechanical energy generated by the vibration of cell 9. The vibration of cell 9 being caused by the acceleration of particles 10 due to an electric field caused when a voltage is displaced across electrodes 11 and 12 via power oscillator 16. Disc 23 may be brass or the like and is electrically connected to an amplifier 24 via wire 25. Amplifier 24 may be a FET input operational amplifier which enhances the electrical signal received from the force transducer 22 which is thereafter detected by a synchronous demodulator 26 via wire 27.

Thus, the present invention operates such that either a portion of particles in suspension may be inserted into cell 9 or cell 9 may itself be inserted directly into a container or feed stream of particles in suspension for testing thereof. After cell 9 has been filled such that both electrodes 11 and 12 are simultaneously submerged in a suspension of particle 10, cell 9 is induced to vibrate by the application of an unsteady electric field caused by the application of an unsteady driving voltage of a frequency range between 0-50 khz generated from oscillator 16 and amplifier 13. The unsteady driving voltage according to the present invention between the first and second electrodes is approximately 10 to 100 volts.

An unsteady electric field being applied across electrodes 11 and 12 accelerates particles 10 in suspension thus resulting in vibration of cell 9. The vibration (Force) of cell 9 is coupled or transferred to a force transducer, such as a piezoelectric mechanical resonator 22, which is to operate at the same frequency as the unsteady driving voltage. The mechanical resonator 22 contains a piezoelectric force transducer which extracts mechanical energy, i.e. vibration of cell 9 caused by acceleration of particles 10 in the electric field, and converts the mechanical energy to an electrical signal. The electrical signal is then amplified via amplifier 24 and detected by a synchronous demodulator 26. Stabilizing solid 19 may be made of aluminum or the such and is of such a mass as to permit up to 70% conversion of mechanical energy into electrical energy.

Amplifier 24 is usually a FET input operational amplifier which is capable of providing extremely high power amplification. The electrical signal power available from the system as mechanical energy is 10.sup.-6 watts to 10.sup.-12 watts, depending on the contents of cell 9.