App Note: Zeta Potential in Pharmaceutical Formulation

APPLICATION NOTE ZETA POTENTIAL IN PHARMACEUTICAL FORMULATION Prepared for Malvern Instruments by Clive Washington Dept. of Pharmacy, University of Nottingham, U.K. Although particle size and its measurement are intuitively familiar to particle technologists, the concept of zeta potential is less widely understood and applied. This is unfortunate since it is at least as fundamentally important as particle size in determining the behaviour of particulate materials, especially those with sizes in the colloidal range below a micrometer. Zeta potential is related to the charge on the surface of the particle, and so influences a wide range of properties of colloidal materials, such as their stability, interaction with electrolytes, and suspension rheology. What is zeta potential? phenomenon. The particles in When a particle is immersed in a their suspending medium are fluid, a range of processes causes the placed in an electric field; if interface to become electrically charged, they will drift in the field, charged. Some of the most positive particles drifting towards commonly found charging the negative electrode, and negative mechanisms include adsorption of particles drifting towards the charged surfactants to the particle positive electrode. However, the surface (for example in an emulsion particles do not drift on their own; stabilised by an ionic surfactant), they carry a thin layer of ions and loss of ions from the solid crystal solvent around them. The surface lattice (silver halide particles used in separating the stationary medium photographic emulsions) and from the moving particle and its ionization of surface groups bound ions and solvent is called the (carboxylate in polymer surface of hydrodynamic shear, and microspheres). These processes lead the zeta potential is the potential at to the production of a surface this surface. Consequently zeta charge density, expressed in potential can be determined by coulombs per square metre, which measuring the drift velocity of the is the fundamental measure of particle in an electrical field of charge at the interface. The charge known strength. Early instruments cannot be measured directly, but for this purpose (the Rank micro only via the electrical field it electrophoresis apparatus) used creates around the particle. Thus manual observation of the particles, the surface charge is normally a procedure that was fraught with characterised in terms of a voltage error and also extremely slow. at the particle surface, the surface Fortunately, we now have a range potential, rather than a charge of instruments that measure the density, although one can usually be velocity using the doppler shift of calculated from the other. The zeta light scattered from the moving potential occurs at a distance from particles – the Zetasizer series. the surface and this will be different Advance signal recovery techniques to the surface potential. In the reliably measure the tiny doppler simplest approximation, the shift due to the particle movement 15 potential decays exponentially with (only a few tens of Hz in 10 Hz) distance from the surface of the and automatically calculate the particle (Fig. 1). As we will see, the distribution of zeta potentials in the rate of decay is dependent on the sample. Normally this value lies + electrolyte content of the fluid. within the range /- 100mV for most systems immersed in aqueous media. How is zeta potential measured? So far, we have not defined zeta potential, and in order to do this we need to understand the basic method for its measurement, which is electrophoresis. To many, this method is familiar because of its use for the separation of macromolecules, and particle electrophoresis is a similar fig 1 complexation with groups on the the attractive Van der Waals’ forces surface. Consequently as their and the electrical repulsion due to concentration is increased, they also the surface charge. If the zeta Low electrolyte concentration High electrolyte concentration screen the zeta potential, but the potential falls below a certain level, additional chemical (as distinct the colloid will aggregate due to from electrostatic) binding on the the attractive forces. Conversely, a surface causes sufficient adsorption high zeta potential maintains a Potential of ions for the original particle stable system. The point at which charge to be neutralised and then electrical and Van der Waals’ forces reversed as the electrolyte exactly balance can be identified Distance from particle surface concentration increases (Figure 3b). with a specific electrolyte In such a system we see a point of concentration, known as the critical zero charge or PZC at a well- flocculation concentration or CFC C defined electrolyte concentration, (Figure 4). Indifferent ions cause counterion prior to charge reversal. the zeta potential to continuously Bulk concentrate Potential-determining ions decline at high concentration, so (PDI) are a special case of we see a single CFC, and the specifically adsorbed ions; this term colloid aggregates at all higher

Ion concentration C electrolyte concentrations. In co-ion is usually reserved for those involved in whatever process is contrast, specifically adsorbed ions Distance from particle surface responsible for the particle charge. cause charge reversal that may be sufficient to re-stabilise the colloid. fig 2 For example, most polymer microspheres are charged because In this case we will see an upper they have carboxylate groups on and lower CFC, with a region of Zeta potential and electrolytes the surface; ionisation of these instability between them. One of the major uses of zeta groups leads to the charge, so H+ is potential is to study colloid- a PDI on this surface. Similarly Diffusion limited electrolyte interactions. Since most Ag+ and I- are PDI’s on silver Aggregation rate colloids, particularly those stabilised iodide particles. The distinction Indifferent ion by ionic surfactants, are charged, it between specifically adsorbed and is not surprising that they interact potential determining ions is often with electrolytes in a complex vague, particularly in those systems Specifically manner. Ions of charge opposite to in which the surface chemistry is Flocculation rate adsorbing ion that of the surface (counterions) are not fully understood. attracted to it, while ions of like The major area of application of Electrolyte concentration charge (co-ions) are repelled from colloid-electrolyte phenomena is to it. Consequently the understand stability and Fig 4 concentrations of ions near the flocculation effects. The simplest surface are not the same as those in model of these phenomena arises Studying zeta potential the bulk of the solution (i.e. at a directly from Figure 3, and is The foregoing discussion shows us long distance from the surface) as known as the DLVO (Deryaguin- that the zeta potential measured in shown in Figure 2. The Landau-Verwey-Overbeek) theory. a particular system is dependent on accumulation of counterions near This simply states that the stability the chemistry of the surface, and the surface causes the particle of the colloid is a balance between charges to be screened, thus also how it interacts with its reducing the zeta potential. Ions surrounding environment. This is a can conveniently be divided into most important point; zeta potential three classes depending on how (a) Indifferent electrolyte must always be studied in well- they interact with the surface: defined environments (specifically Indifferent ions are those pH and ionic strength) or the data which are only attracted to the is valueless. It is quite meaningless surface by virtue of their charge in to talk about "the zeta potential of a purely electrostatic manner, a a surface" unless the conditions are specified. In order to illustrate the process known as non-specific Zeta Potential Electrolyte concentration adsorption. If we measure the zeta planning of a zeta potential study, it potential of a colloid as a function is useful to take a case study on a of concentration of such an ion, we particular system. We have studied find that the screening effect of the (b) Specifically adsorbed electrolyte triglyceride fat emulsions for some ions gradually reduces the zeta years, and these studies provide a Point of useful illustration of the power of potential (not the surface potential), zero charge and this asymptotes to zero at high zeta potential measurement in electrolyte concentrations understanding colloid stability in (Figure 3a). complex systems. Specifically adsorbed ions Zeta Potential Electrolyte concentration interact chemically with the surface, for example by fig 3 Intravenous fat emulsions mixtures contain a large phase Drug targetting and delivery Triglyceride emulsions are medical fraction (1-5%) of the emulsion, systems products; they are sub micron and so are very turbid, and must be Emulsions have also been used as emulsions of vegetable oils in diluted before light scattering drug delivery systems, and in many water, emulsified by phospholipids, measurements can be performed. cases an understanding of the which provide a high zeta Early workers who did not electrophoretic properties is crucial potential, and a correspondingly understand the nature of zeta in formulation design. Although long shelf life (2-3 years). The potential simply diluted the most drugs are water-soluble, an emulsions are used to feed patients mixtures with distilled water. The increasing number are surface- intravenously who cannot be fed resultant zeta potentials bore no active or even hydrophobic, and orally (e.g. due to gastrointestinal resemblance to those of the such materials can provide surgery). Such patients also need emulsion in the original mixture significant problems for other nutrients, including amino since the dominant ions were conventional formulation acids, glucose and electrolytes. For reduced in concentration by some techniques. Consequently some time it has been the common orders of magnitude! In order to hydrophobic drug candidates are practice to mix all of these obtain a relevant zeta potential it is usually sent back to the chemistry materials, in varying proportions, in necessary to maintain the department with a note to prepare a single liquid mixture (a total continuous phase composition on a water-soluble analogue! In some parenteral nutrition or TPN dilution. There are two approaches cases this is not possible, for mixture) and infuse it into a to this problem; if the composition example some natural products or patient, at a rate of about 3 litres a of the continuous phase is known, biotechnology materials, or where day. Naturally, in such a mixture, it can be prepared without any the mode of action is related to the there is a wide scope for interaction emulsion component and used as a lipophilicity, e.g. anaesthetics, between the components, and in diluent. A more common situation hypnotics, and tranquillisers. In many mixtures the fat emulsion is that the continuous phase these cases emulsion delivery is becomes unstable, and coalesces or composition is uncertain; even if increasingly used. Examples are flocculates in a few days. In this you knew what went into it, ICI’s Diprivan, an intravenous condition it is unsuitable for adsorption to the disperse phase anaesthetic, and Kabi’s Diazemuls, a infusion, and so the mixtures are may have depleted some sedative. components. In this case, the usual normally made up just before An example of the problems that trick is to centrifuge the dispersion administration, using sterile can be encountered in this to get a clean sample of the techniques. An understanding of approach is shown in Figure 5, continuous phase for dilution. the stability of the emulsion in which is the zeta potential – pH these systems would be helpful in The second problem with this curve for a drug-containing predicting which mixtures would measurement is the extremely high emulsion that is flocculated at pH be unstable, and even possible in ionic strength (0.2-0.4M) which 7. Data of this type allows a producing stable mixtures with leads to high conductivity and rational selection of formulation long shelf life. consequently rapid sample heating pH and emulsifier to maximise zeta and large cell voltage drops. The potential and hence emulsion early Zetasizer 2 could not cope Formulation protocol stability. particularly well with this problem, Early studies demonstrated that the but the current Zetasizer range has emulsion itself, at a pH of 7 and cell voltage pulsing that keeps the low electrolyte concentration, had a mean current down; and the re- 40 Neutral zeta potential of –40 to –50mV, engineering of the electrophoresis Formulation 30 which is sufficient to provide good cell has resulted in major Flocculates stability and a shelf life of at least 2 improvements in electrical stability. 20 years. This potential was markedly Acid buffered It is now possible to use this 10 formulation reduced by electrolytes, with instrument to routinely measure Zeta potential mV stable pH 0 monovalent cations being zeta potentials in these high 4 5 6 7 8 9 indifferent, while divalent cations conductivity mixtures, and the -10 adsorbed specifically with a PZC of resulting values (± 1-5mV) 3 mM and a significant degree of correlate well with the stability of Fig 5 charge reversal. These ions are all the emulsion in the mixtures. present in TPN mixtures, and this Studies of this type are now accounts for the instability of the allowing us to understand the emulsion in these systems. behaviour of emulsions in complex It should be possible to use the colloidal systems and provide real DLVO theory to correlate the predictive power for formulation stability of the emulsions in a purposes. particular mixture with its zeta potential; unfortunately there are a number of problems involved in making such a measurement. The APPLICATION NOTE c) fi

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80 60 40 20 -20 Zeta potential mV potential Zeta lecithin concentrations above about above concentrations lecithin 10%. our understanding Although in non-aqueous of electrophoresis still primitive,systems is such studies at least an empiricalallow understanding of stability and surfactantadsorption in these systems. Non-aqueous systems Non-aqueous A furtherof example of the use in understandingzeta potential stability occurssuspension in the of drugssuspensions in aerosol of used for delivery propellants drugs inhalation, by for example bronchodilators. The micronised drug the aerosol is suspended in propellant, aerosol so that when the is fired, particles of drug sprayed are out and can be inhaled. It is the particleimportant to control the zeta controlling size by potential, to guarantee a repeatable dose to the patient. The problem is extremely in this case is that it potentials zeta difficult to measure of particles suspended in non- CFC aqueous media like propellants, since the particle small. very mobilities are However, it can be done with appropriate cell, design of the electrophoresis and Malvern Instruments make such a cell for their Zetasizer. of the zeta potential 6 shows Figure lactose (a model solid dispersion) in chloroform(a model non-aqueous medium) as a function of the concentration of lecithin, an ionic surfactant. The lecithin clearly causes major changes to the at small potential even concentrations; the suspension is flocculated in the absence of lecithin, becomes dispersed but at