PROCESS FOR MEASURING AND DETERMINING ZETA-POTENTIAL IN A LAMINARLY FLOWING MEDIUM FOR PRACTICAL PURPOSES

Abstract: Measuring method for determining the zeta-potential of a material suspended in a liquid medium which comprises streaming the suspension in a laminar flow, then subjecting the system to the effect of D.C. voltage and determining the electrophoretic velocity from the such way induced flow velocity's change of a selected particle and calculating zeta-potential in a manner known per se.

Description: PROCESS FOR MEASURING AND DETERMINING ZETA-POTENTIAL IN A LAMINARLY FLOWING MEDIUM FOR PRACTICAL PURPOSES The invention relates to a new process for measuring the zeta-poteπtial on the surface of a material suspended in a liquid medium. The measuring is carried out in a visually controlled measuring pipe provided with two electrodes at si¬ multaneously regulated flow velocity and voltage values. According to the experiences of Ξmoluchowski , M. /Randbuch der Elektrizitat und Magnetis us, Ed. L. Gratz, (Barth Verlag, Leipzig 1921) Volume II, page 3667 if a material suspended in liquid is subjected t-o the effect of steady voltage, the particles migrate. Based on the electrophoretic velocity (migration speed) (v_ = cm.s " ) and the electric voltage den- sity applied (E = V.cm. " , wherein V = voltage in Volt and cm = the distaπce between the electrodes in cm) the zeta-poteπtial (ZP) characteristic for the material can be determined, while + or - sign thereof depends on the direction of the migration of the particles towards the cathode or the anode. Zate-poteπt'ial can be calculated on the basis of the following formulae and A = 9.10 wherein *? stands for the viscosity of the liquid medium, 4 £, represents dielectric permittivity and 9.10 is the conversion factor of the electrostatic unit into Volts. In case of water, A = 150 at a temperature of 20 °C (Martin, A. N. Swarbrick, 3 . Cammarata, A : Physical Pharmacy) (Lea and Febiger, Philadelphia 1969, page 458). Determination of ZP on the basis of electrophoretic velocity is carried out in a measuring cell provided with molybdenum or platinum electrodes placed under a special colloidal microscope. The migration speed of the particles can be determined under stable D.C. voltage of known value after a certain time, in a stationary state, by means of a stop-watch, and an ocular micrometer, whereafter the ZP value can be calculated according to the aforementioned formula. The unavoidable disadvantage of this widely used method resides in that in course of electrophoresis, in particular in a medium which also comprises electro¬ lytes, the temperature of the system continuously changes. Thermic flow considerably disturbs electropho¬ retic migration. In addition, on the electrodes gas evolves, electrochemical processes occur, in the measuring cell ions migrate, which also have a negative effect on the accuracy of the measurement. The fact that the liquid medium and the particles migrate in the opposite direction along the wall of the cell due to electrαendosmosis makes the measurement especially difficult. Along the vertical wall of the measuring cell the particles migrate a changing velocity corresponding to a parabolical velocity profile. According to said velocity profile velocity reaches its maximum at the axis of the measuring cell, then at the limit of eπdosmosis it equals to zero, while between this limit and the wall of the measuring cell the particles migrate in the opposite direction. From all what has been said it becomes obvious that measuring of ZP on basis of the electro¬ phoretic migration speed is most wearisome, complicated equipments are required, in addition, measuring errors frequently exceed more, than 10 %. In the prior art several other measuring methods are known. So e.g. the method based on measur¬ ing the electroos osis formed under the influence of electric field of force (Biefer, G.3., Mason S.G.: Colloid Sci. 9_, 20) (1954), or on measuring flow potential (among others Martin, W. McK., Gortner, R.A.: 3 . Phys. Chem. 34 1509) (1930), or on the basis of measuring volumetric flow or potential of sedimentation, ZP-value can also be determined. According to specific embodiments of these techni¬ ques, the measurement is carried out in double microcapillary tubes in an electrophoretic cell; or using the phenomenons of zone-electrophoresis , the relationship of moving phase borders and mass flow; or by electrophoretic light dispersion, laser technique (Robert 3 . Hunter: Zeta Potential in Colloid Science (Acad. Press London, 1981), pages 127-175). However, these known suitable apparatuses are either not available in commerce, and if, they are most expensive. Additionally, with these methods determina¬ tion of the ZP-value is labαursome and time consuming, furtheroπ, accuracy of measurement is not better either, than the variation coefficient of + 5 %. In several professional fields, among others in the chemical and pharmaceutical industry, when plaπt- -protectiπg agents, cosmetics and drugs are produced, or when the optimalization of filtering processes connected to chemical industry, ecology, sewage treatment, drink water production is required, the determination of the ZP-value of colloides, suspensions is indispensable. Stabilization of suspensions, prevention of the aggregation of the suspended particles is possible by regulation of the ZP-value and proper choice of the additives on basis of ZP-measurements. In technical literature it has been generally accepted that suspensions with a ZP-value of -100 mV are extraordinarily stable. In order to increase efficiency of filtering processes the aggregation of the particles i.e. the ZP-value should be reduced, while proper choice of additives or conditions requires also measuring of the ZP-value. In the course of our work our aim was to improve the technique of measuring of ZP-value, to work out a process which is easy to carry out, requires less labour and suitable for accurate measure¬ ments. Surprisingly we have found that the electro¬ phoretic-migration of suspended material particles can be produced not only in D.C. voltage field of force, but by allowing to flow the suspension laminarly and equally in time in an adaquate apparatus, and by switching on the proper + or - pole in the direction of flow and applying momentary controlled direct voltage of proper magnitude. According to the process according to the invention, the electroforetic migration speed can be calculated in an other way, than described in the prior art. It can be observed that local flow velocity (v) of the selected particle changes to the flow velocity v v due to the effect of the voltage V applied. In this case the particle does not stop but travels slower or faster depending on the direction of poles. In case, if the negative pole is switched - in the direction of flow and flow velocity of the particle decreases upon the effect of the voltage V, sign of the ZP-value will be negative, otherwise positive. On basis of the expression i V V = v e electrophoretic migration speed v can be calculated (expressed in units crn.s " ) and in such a manner ZP can be given by using the formula as described. According to a preferred embodiment of the process it can be achieved that in the invariably flowing liquid the grain intended to be observed will just stop, as the magnitude of the electrophoretic wandering velocity arising under the effect of the voltage complies with the velocity of liquid flow, but just of the opposite direction. ZP-value can be calculated by using the previously described equation, considering that velocity of flow of the grain corresponds to the value ve . According to a preferred embodiment of the method of the invention two 0.2 mm thick platinum wires in a distance of 10.0 cm are installed into a cylindrical glasspipe of an inner diameter of 3 to 6 mm, preferably 4 mm, thereafter the measuring pipe is placed under a microscope suitable for 15-30- fold magnification and provided with a calibrated ocular micrometer scale. The illumination of the suspension is solved by fibre-optics or by means of a 100 micron optical slot arranged before the microscope lamp to eliminate the disturbing (heating) effect of the light source. One end of the measuring pipe is coupled to the synthetic sprayer of 2 - ]_g cm , containing the suspension tα ~ bef ~ tested via a synthetic connection, while the other end is coupled to a vessel receiving the throughflowing suspension. When filling the suspension into the system, care is to be taken for filling the measuring pipe, the connecting synthetic pipes and the sprayer free of air bubbles. Thereafter the sprayer is fitted into the equipment and the suspension is allowed to flow at a regulated, time-permanent velocity. Meanwhile the electrodes (made of platinum) of the measuring pipe are connected to a D.C. voltage supply unit provided with a pole changer switch and with a stabilized voltmeter which works in the voltage range between 1 and 800 V. Steady flow of the particles in the microscope can be controlled by measuring 5 the travelling time of the selected particle, observ¬ ing the same through the 100-scale of the ocular micrometer, with an accuracy of 0.01 s, separately In the first and second 50-50 pitch of the scale. In case, if the particle takes the first and second 10 50 pitch within about the same time (0.01 - 0.02 s difference is allowed) , the flow in the system is considered to be time-permanent, laminar. There¬ after, an other particle is selected and the travelling time thereof is measured at an optional distance 15 (10, 20, ... 50 ocular pitch) with an accuracy of 0.01 s. Then the D.C. voltage is switched on the pole changer is positioned, and by regulating the voltage applied onto the measuring pipe the chosen particle should just stop. This voltage value is 2 Q registered. The flow velocity measured with the calibrated ocular micrometer with an accuracy of 0.01 s is calculated to Ve = crn.s " units with an accuracy of four digits. Dividing the voltage needed for stopping the particle by the distance of the -_- electrodes (in our case ten), the value E of voltage density is obtained. In such a manner for an aqueous suspension, by using the known formula ZP = 150 ZP-value of the desired material can be obtained. It is recom aπded to perform measurements in large series for particles migrating with different speeds. Thus, the average of the ZP-values, and variation coefficient or standard deviation can be calculated by using mathematic statistic methods in a manner known per se, and these values simultaneous- ly characterize the accuracy of measuring. The testing method widely variable, as by scanning among the flowing particles, ZP-value αf the selected particle can be determined by simul¬ taneously regulating flow velocity and the voltage having been applied onto the measuring pipe. According to a preferred embodiment of the measuring system suitable for carrying out the testing method according to the invention, measuring pipes with different inner diameter - within the range 3 to 6 mm - are used, into which platinum wires (0 0.2 mm) in a distance of 10.0 cm from another are installed. Measuring pipe can be fixed with the usual clamping plates to the stage of the micro¬ scope. Illumination is solved with fibre-optics instead of condenser or with a 100 micron optical slot arranged in front of the condenser.Ocular micro¬ meter is to be calibrated, path length of the particles is calculated in cm. In case of quickly sedimenting suspensions, it is considered as advantageous to position the microscope at an angle of 90 so, that the stage of the microscope should be turned from horizontal into vertical. In this case the sedimenting particle bends from the line of the ocular micrometer scale, therefore it becomes possible to examine those particles only which are travelling parallelly with the axis of the measuring pipe and consequently with the scale line of the ocular micrometer. As a sprayer containing the sample any synthetic injecting sprayer can be used. Practically if flow velocities is used, sprayer with a smaller volume, while if higher velocities are applied, sprayers with a larger volume should be used. For this purpose the so-called tuberculiπe sprayers or sprayers with a volume of 2, 5, perhaps 10 cm cm can be expediently used. Sprayers with a far larger volume can be recommanded only in case of extremeley high ZP-values. As connect¬ ing synhetic pipes, pipes made of polyethylene or any other structural material can be optionally chosen. As a sample feeder the Hungarian INFUC0NT quartz-controlled electronic apparatus was used, which is well suitable for the control of the drives INFUMAT or INFUDRIVER which serve for the steady moving of the pistons of infusion syringes. Electronics supply electric energy for the shifting-motor of the dirve with proper frequency, phase and voltage, while continuously controllable rate of feeding is displayed digitally. Digital display (D) of the feeder and flow velocity observed in the measuring pipe with an inner diameter of 5.0 mm (expressed in cm.s " ) were controlled with a 10 cm synthetic syringe. It could be stated that within the digital display range 0 - 40 the apparatus delivered the aqueous suspension with an unobjectionable linearity. In the given arrangement the linear regression coeffi¬ cient (cm.s -1 ) = 0.00141. D was found to be 0.9999. It goes without saying that numeric solving of the linear equation will yield- always different values, when using measuring pipes or sprayers with different inner diameters. Said calibration is to be performed for an arrangement of given dimensions in each case separately. It is a well known fact that in a cylindrical pipe within the range of velocity as applied here, based on the low Reynolds-number a pure laminar flow will be established, while the velocity distribu¬ tion curve has a parabolic shape. In the axis of the pipe the flow velocity amounts just to the double of the average flow velocity (v) . In the point of the optional radius r of the cross - section of the radius ro the local flaw velocity v can be calculated on basis of the Knudsen and Katz rule r 2 — = 2 1 - (Oohn H. Perry: Manual of Chemical Engineers, Techπi- cal Publishers, Budapest, 1953. volume I, page 564). From this it becomes obvious that in the microscope, in dependence of the depth of focues, at a relatively high velocity (above 100 micron.s ~ ) parabolic velocity distribution becomes flattened to such an extent that in its visual field particles travelling with different speeds can be observed. At considerably lower velocities particles travelling with nearly identical velocities can be observed. According to an improved mode of realization of the process according to the invention, measuring pipe is placed on the microscope - as detailed previous¬ ly -, the sprayer is arranged in the feeding device and connection is made with synthetic pipes. The feeder is calibrated with the suspension to be examined as specified earlier. In the microscope the axis- -line of the measuring pipe is phased, in the axis the flow velocity of the particles equals to the double of the digital signal of the feeder. By the aid of the voltage regulator of the D.C. voltage current source, the voltage needed for stopping the particles is adjusted. Care should be taken that the pole switch should be in the proper direction, in compliance with the nature of the material tested. As in this case the feeder and the D.C. voltage supply unit deliver an electric sign being proportional with the flow velocity and the voltage value needed for stopping the particles, these two signs are led into a computer and display unit, thereafter based on a pre-determiπed algorithm the ZP-value of the material tested is obtained automati- cally. Accuracy of the measuring system and the testing method are illustrated by the following examples. Example 1 2-3 mg of 17o£-hydroxy-21-acetoxy-cortico- steroπe as a basic material with a grain size below 10 microns are mixed with a small quantity of water in a frictional mortar, the mixture thus obtained is diluted with 50 cm of distilled water and allowed to stand for three days. For measuring a measuring pipe with an internal diameter of 4.0 mm with elec¬ trodes having been arranged therein with a spacing of 10 cm, a 5 cm synthetic sprayer are used. In a microscope with 10 X magnification the flow time on the 1000 micron path in secoπd(s), as well as voltage πeedes for stopping the particle are deter¬ mined. Thereafter measuring is performed for a 318 microns path with 30 X magnification. ZP-values calculated from measuring data are as follows: 1000 microns pa ,th s V ZP 11.0 230 -59.3 12.0 222 -56.3 12.0 222 -56.3 318 microns path 3.80 220 -57.1 3.73 220 -58.1 3.73 219 -58.4 3.93 205 . -59.2 3.87 208 -59.3 Average value: -58.0 coefficient of varia¬ tion + 1.2 (- * - 2.07 %). These test results demonstrate well that the measuring system and the testing method can be used for determining the ZP-value with an accuracy which corresponds to the experiences of technical literature, the more, when compared to some publications, even a higher accuracy can be achieved. In technical literature ZP-value used to be given generally as an "apparent" value, from which conclusions can be drawn on the inaccuracies resulting from the difficulties of determination. Taking into consideration that measuring technique based on a flow system eliminates numerous disturbing factors, the ZP-value having been determined in the said way may be considered as .a virtual value. In order to be able to demonstrate reproducibility of the data of technical literature, comparative tests were also carried out. Example 2 (Comparative example) In the earlier described manner, colloidal iron(III)-hydroxyde was tested in freshly boiled and cooled distilled water at 22 °C after having precipitated ferri-chloride with sodium hydroxide and washed to chlorine-free. Measuring results: 318 microns path: s V ZP mV 4.58 235 -44.3 4.80 220 -45.1 4.76 222 -45.1 4.90 215 -45.3 5.00 231 -41.3 5.57 203 -42.2 6.43 181 -41.0 7.12 162 -41.3 Average value: -43, .23 variation coefficient: + 1.78 (+ 4.11 %) The value given in technical literature is 45 mV, the difference is -3.93 % (Martin, A.N. et al. Physical Phyrmacy (Lea and Febiger, Philadelphia * 1969) page 458). * As according to experiences ZP-value is considerably influenced by traces of carbonic acid impurities in v/ater, or chloride ion impurities of the sample, the difference in comparison to the value of technical literature can be considered as a good congruency. Example 3 (Comparative example) The second comparative test having been per- formed for determining ZP-values in alumiπa/10 -4 M potassium-nitrate solution in dependence of PH was carried out as previously described (Wiese, R.G. et al.: Colloid Interface Sci. 5_1_, 427) (1975)); the results obtained are summarized in Table 1. From the figure it becomes obvious that the ZP-values measured in the flowing system show a good congruency with the data of technical literature. Difference of larger extent, as observed in high pH-raπges may result from the differences in the structure of the solid material tested in addition • to the error sources as mentioned before, to this phenomenon we shall revert in the examples. Example 4 Comparative test for comparing the ZP- -values of polymorphous crystal modifica- tioπs It is a well known fact that some elements have allotropic structure, furtheron, mainly organic compounds can be produced in several crystal modifica¬ tions. As such an organic compound we may refer to the pesticidal pharmaceutical basic material Mebendazol, the three polymorphous crystal modifica- cations (A, B and C) of which are known (Lancos, Kriszti- πa: Ph. D thesis, Medical University Semmelweis, Pharma¬ ceutical Institute, Budapest, 1985). In respect to chemical composition the single Mebendazol poly orphes are quite identical, however, the structure of their solid body is reversibily different. Three Mebendazol polymorphous crystal modifications were comminuted to a grain size finer, than 30 microns and 2 mg thereof were suspended in distilled water. ZP-value was measured in a flowing system at 20 °C temperature, as specified earlier. The average value of six measurements and variation coefficient of the measuring series, were, as follows: sample + ZP mV Mebendazol A 106.3 2.1 Mebendazol B 34.9 0.9 Mebendazol C 104.0 3.0 Example 5 Stabilization of the suspension by controlling the ZP-value by using additives According to technical literature stability of a suspension is considered as maximally satisfactory, if the value of ZP is around - 100 mV. The stability of " the suspension does not meet the requirements if the ZP-value thereof is lower, under -40 agglomeration and under -10 precipitation will occurre . By means of auxiliary materials absorbed onto the surface of the solid body, properties of the so-called electric double-layer and thus the ZP- -value can be controlled. For the sake of illustration the possibility of ZP-value regulation is illustrated by the example of the aqueous suspension of 17o<l-hydroxy- -21-acetoxy-cαrticosteroπe, as specified hereinabove. If ZP-value is measured in distilled water, at a temperature of 20 °C in the presence of different auxiliary materials, as described earlier,the following results are obtained: g.cm " auxiliary materials ZP mV _+ 0 (in pure distilled water) -58 . 0 1 . 2 0.5 sodium stearate -74 . 0 2 . 0 0.5 Tween-80 -81 . 0 1 . 4 0.5 + 0.5 sodium stearate + Tweeπ-80 -140.2 4.1

Claims: What we claim is 1. Measuring method for determining the zeta-poteπtial of a material suspended in a liquid medium which comprises streaming the suspension in a laminar flow, than subjecting the system to the effect of D.C. voltage and determining the electropho¬ retic velocity from the such way induced flow velocitie's change of a selected particle and calculating zeta- -potential in a manner known per se. 2. The method as claimed in claim 1 which comprises streaming the suspension in a laminar flow, then subjecting the suspension to a D.C. voltage by which a selected particle stops to move, thus obtain- ing the flow velocity of the particle without voltaga loading as electrophoretic ' velocity, and calculating the zeta-potential in a manner known per se. 3. Measuring system for determining the zeta-poteπtial of a material suspended in_ liquid medium comprising a sample feeder, a sample holder provided with two electrodes, a supply unit provided with a D. C. voltage regulator and with a pole changer, a microscope and a detector and/or signal converter unit.

Inventors: DAVID AGOSTON, DAVID NE KENEZ MARIA, RACZ ISTVAN, DAVID NE KENEZ, MARIA, DAVID, AGOSTON, RACZ, ISTVAN
Applicants: MTA KUTATASI ESZKOZOKET KIVITE, MTA KUTATASI ESZKOEZOEKET KIVITELEZOE VALLALAT
Application year: 1987
Main classification: G01N13
IPC codes: G01N27