1 INTRODUCTION

The formation of charged particles in nonpolar media belongs to a fairly new and poorly studied field in physical chemistry of colloidal systems. The first works devoted to the nature of the origin and the mechanisms of stabilization of charged particles in nonpolar media were published in the 30s of the 20th century [1–5]. In that works, the presence of ions in hydrocarbon media was discovered for the first time. The existence of charged particles is often associated with the phenomenon of electrophoresis, which is one of the methods for their detection and is of importance for practical purposes. Electrophoresis in media with low dielectric constants (of about 2) was first discovered and described in the works by van der Minne and Hermanie [6]. The phenomenon of nonaqueous electrophoresis was already known at that time, but only for media with dielectric constants of about 10–15, for which detection techniques used for “aqueous” systems are applicable. The authors proposed the following criteria indicating that the observed movement of dispersed phase particles under the influence of an external electric field is electrophoresis:

(1) particles of a dispersed phase must move uniformly and rectilinearly;

(2) the velocity of the particles must be independent of their position in the electric field;

(3) the velocity of the particles must be directly proportional to the electric field; when the sign is reversed, the direction of particle movement must be changed to the opposite one.

The authors call the observed phenomenon “real electrophoresis” or “pure electrophoresis.” The term “linear electrophoresis” is more often used for this phenomenon. As the external electric field increases, deviations from the linear character of the particle movement may take place, as was noted by the authors of [6] and was observed in a number of other works [7, 8].

All these criteria are based on the classical Helmholtz–Smoluchowski equation, although, for the case of nonpolar media the Hückel–Onsager equation is usually applicable. Both of these equations are particular cases of the more general Henry equation, which describes the relationship between the velocity of dispersed phase particles and the electrokinetic (zeta) potential:

$$\nu = k\frac{{\zeta \varepsilon {{\varepsilon }_{0}}H}}{{\pi \eta }},$$
(1)

where ν is the velocity of particle movement, k is a numerical coefficient that depends on the shape of the particles and the ratio of their size to the thickness of the electrical double layer (2/3 and 1 in the Hückel–Onsager and the Helmholtz–Smoluchowski approximations, respectively), ζ is the electrokinetic potential, H is the external electric field, ε is the dielectric permittivity of a medium, ε0 is the dielectric constant, and η is the viscosity of the medium.

In addition, the Henry equation shows the main difficulties associated with detecting electrophoresis in hydrocarbon media—the electrophoresis velocity is approximately 40 times lower than that in water, other conditions being equal.

However, it is necessary to study these processes from the practical point of view. The control over the origin of charged particles is important for the petrochemical industry. Electric charges can be accumulated in oil tanks [9], especially under the action of moisture and temperature [10]. In petrochemistry, the formation of charged particles is an extremely undesirable process. The accumulation of charged particles can lead to electrical breakdowns and very disastrous consequences [11]. With the development of electronic device technology, the number of works devoted to nonaqueous electrophoresis has increased markedly: this phenomenon underlies the operating principle of electronic paper. Electronic inks (although it would be more correct to call them electrophoretic inks) are obtained using various modern highly dispersed particles of different materials, such as carbon materials, including nanomaterials [12, 13], silica or titania nanoparticles [14, 15], poly(methyl methacrylate) [16], and others [17, 18]. One thing remains unchanged, namely, a nonpolar dispersion medium.

The difficulties in the development of this field were (and, generally speaking, still are) due to insufficient understanding of how, in principle, charged particles can arise in such an unfavorable environment, and why they remain charged and do not coalesce. The first successful attempt to critically analyze the mechanisms of the origin and stabilization of charged particles in nonpolar media was made by Morrison in the 1990s [19]. In Morrison’s review, attention was paid to the physical processes of charge separation, i.e., the comparison between the energies of thermal and electrostatic interactions. To describe such processes, the authors of many works [20, 21], including reviews [22, 23], often used the concept of the Bjerrum length, which is actually similar to Morrison’s reasoning. The physical meaning of this parameter is quite simple: this is the distance at which the energies of the thermal and electrostatic interactions of two charged particles are equal:

$${{E}_{{{\text{el}}}}} = \frac{{{{e}^{2}}}}{{4\pi h\varepsilon {{\varepsilon }_{0}}}} = {{k}_{{\text{B}}}}T,$$
(2)

from where, it is easy to obtain

$$h = \,\,{{\lambda }_{{\text{B}}}} = \frac{{{{e}^{2}}}}{{4\pi \varepsilon {{\varepsilon }_{0}}{{k}_{{\text{B}}}}T}}.$$
(3)

In these equations, Eel is the energy of the electrostatic interaction; e is the elementary charge; h is the distance between the centers of interacting ions; ε and ε0 are the dielectric permittivity of a medium and the dielectric constant, respectively; kB is the Boltzmann constant; and λB is the Bjerrum length.

In the case of oppositely charged particles, which have an elementary charge and are located at distances shorter than the Bjerrum length, the electrostatic attraction will exceed the thermal interaction [24]. The Bjerrum length is a convenient tool for simulating the processes of the interaction of charged particles with each other [25]. For water under standard conditions, the Bjerrum length is about 0.7 nm [16]. If we consider some simple electrolyte, such as NaCl, the Bjerrum length exceeds the sum of the van der Waals radii of sodium and chlorine by less than two times. Obviously, even a very thin solvation shell will be sufficient to separate the ions to a sufficient distance [19]. In common nonpolar media, such as hydrocarbons, the Bjerrum length under similar conditions will be about 28 nm [22]. Thus, for efficient stabilization of ions in hydrocarbons, the thickness of their “protective” shell must be at least 14 nm [19, 26]. That is, this protective shell should be a kind of a supramolecular structure, such as a micelle. Therewith, the micelle as a whole is considered as a sterically stabilized macroion; i.e., it must not be an electrically neutral system [26]. The uncompensated charge is obviously localized in the polar cavity of the micelle. The term “microemulsion” will be used below for such systems containing charged micelles (of course, reverse micelles are concerned). It is difficult to imagine an ion covered exclusively with a surfactant shell. The ion must be solvated, which implies the presence of a polar phase. It should also be noted that water is almost always present in commercial surfactant samples (purification of surfactants from water is an uneasy procedure and is not always advisable) [27–29]. According to a number of authors, it is polar impurities, mainly water, that are a kind of micellization sites [29, 30]. A significant influence of the amount of impurity water on the critical micelle concentration (CMC) was noted as early as in the 1960–1970s [31]. In other words, if charged micelles are present in a surfactant solution, it is more correct to call it a microemulsion, even if the polar phase has not been intentionally added to it.

The following reasonable questions may arise: how can a micelle be charged in general, and where does the uncompensated charge come from? Since “the environment is not favorable,” isn’t it more advantageous for a micelle to remain electrically neutral? Of course, this is more advantageous. Charged micelles are not formed in amounts sufficient for detection in all microemulsions. Even if they are formed, their fraction is very small. For convenience and simplicity, the types of surfactants, for which the formation of sterically stabilized ions in nonpolar media is observed, will be called “charging” below in this review. They are also referred to as “charge control agents” [32, 33], with this term having a similar meaning. Below, we shall consider the main mechanisms for the formation of charged particles in microemulsions based on various types of surfactants. Anionic surfactants will be considered first, since in their case the stabilization mechanism is simpler and more intuitive. Nonionic surfactants will be considered second. Works devoted to the stabilization of charged particles in nonpolar media using cationic surfactants are currently very scarcer or completely absent.

It is logical to begin the consideration of the mechanisms for the stabilization of charged particles with a brief analysis of more general and broader concepts, such as the electrical double layer (EDL) as a whole and its diffuse part in particular. After considering the diffuse part of the EDL, we shall consider those types of surfactants whose reverse micelles can form it.

2 ELECTRICAL DOUBLE LAYER IN NONPOLAR MEDIA

In accordance with the classical concepts, a surface charge may result from the adsorption of ions on the surface of dispersed phase particles, the dissociation of surface groups, or the orientation of polar molecules on the surface. It is assumed that in nonpolar media the mechanisms of surface charge generation are quite similar, although they have not been completely understood [34]. Significant difference consists in the number of charges on a surface. As noted in [34], this difference can be several orders of magnitude for the same particles.

Differences are also revealed when considering the diffuse part of the EDL. They originate from the slipping plane, which separates the dense and diffuse parts of the EDL. The potential of the slipping plane is usually called electrokinetic, or zeta potential. This is an important parameter, which determines the electrostatic component of the energy of the interparticle pair interaction. Analysis of the balance between different components of the interparticle pair interaction energy makes it possible to predict the stability of dispersed systems, with the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory being used for this purpose. Stability is the key characteristic of any disperse system.

A number of theoretical works are devoted to the development of Derjaguin’s approach, which makes it possible to estimate in a linear approximation the energy of the interaction between two charged particles of a convex shape. An equation for calculating the energy of electrostatic interaction between two spherical particles with different sizes and surface potentials was proposed by Hogg, Healy, and Furstenau [35] in the following form:

$$\begin{gathered} {{E}_{{{\text{edl}}}}} = \frac{{\varepsilon {{\varepsilon }_{0}}{{r}_{1}}{{r}_{2}}\left( {\varphi _{1}^{2} + \varphi _{2}^{2}} \right)}}{{4({{r}_{1}} + {{r}_{2}})}} \\ \times \,\,\left[ {\frac{{2{{\varphi }_{1}}{{\varphi }_{2}}}}{{\left( {\varphi _{1}^{2} + \varphi _{2}^{2}} \right)}}{\text{ln}}\left( {\frac{{\left( {1 + {{{\text{e}}}^{{ - \kappa h}}}} \right)}}{{\left( {1 - {{{\text{e}}}^{{ - \kappa h}}}} \right)}}} \right) + {\text{ln}}\left( {1 - {{{\text{e}}}^{{ - 2\kappa h}}}} \right)} \right], \\ \end{gathered} $$
(4)

where ε is the dielectric permittivity of a medium, ε0 is the dielectric constant, r1 and r2 are the radii of the interacting particles, φ1 and φ2 are their surface potentials, κ is the reciprocal Debye length, and h is the distance between particle surfaces.

The authors have noted that, in the case of the interaction between identical particles (r1 = r2 = r, φ1 = φ2 = φ), Eq. (4) coincides with the Derjaguin equation:

$${{E}_{{{\text{edl}}}}} = \frac{{\varepsilon {{\varepsilon }_{0}}r{{\varphi }^{2}}}}{2}{\text{ln}}\left( {1 + {{{\text{e}}}^{{ - \kappa h}}}} \right).$$
(5)

In addition, the authors have indicated that Eq. (4) is adequate for low potentials of interacting particles (lower than 25 mV), while the thickness of the EDL is small as compared with the particle sizes. For the case of nonpolar dispersion media, the thickness of the EDL is significantly larger than that for polar ones, and this circumstance required additional refinement of the Hogg–Hilly–Furstenau equation. In a number of works [36–38], a modified equation of the following form is applied for the case of κh < 1:

$$\begin{gathered} {{E}_{{{\text{edl}}}}} = \pi \varepsilon {{\varepsilon }_{0}}\frac{{{{r}_{1}}{{r}_{2}}}}{{{{r}_{1}} + {{r}_{2}}}}\left[ {{{{\left( {{{\varphi }_{1}} + {{\varphi }_{2}}} \right)}}^{2}}{\text{ln(}}1 + {{{\text{e}}}^{{ - \kappa h}}}{\text{)}}} \right. \\ + \,\,{{\left( {{{\varphi }_{1}} - {{\varphi }_{2}}} \right)}^{2}}{\text{ln}}\left. {(1 - {{{\text{e}}}^{{ - \kappa h}}})} \right]. \\ \end{gathered} $$
(6)

For the case of the interaction between identical particles, Eq. (6) also takes the more familiar form:

$${{E}_{{{\text{edl}}}}} = 2\pi \varepsilon {{\varepsilon }_{0}}r{{\varphi }^{2}}{{{\text{e}}}^{{ - \kappa h}}}.$$
(7)

Since Eq. (6) has also been obtained under the linear approximation, it is also subject to the limitation on the smallness of the surface potentials of interacting particles. It should be noted that these equations involve the surface potentials rather than the electrokinetic potentials. The surface potential is a static characteristic of a particle and is often determined at the boundary of the stationary part of the EDL. It is assumed that the dense part of the EDL is small in comparison with the diffuse one. This is true for a nonpolar medium; therefore, the equality between the surface and electrokinetic potentials is often postulated for them. In this case, it is the potential at the boundary of the dense EDL layer rather than that at the boundary of the solid phase that is decisive; because, the particle is considered together with the dense EDL layer. Even if there is a charge at the solid phase boundary, the particle may behave as electrically neutral (electrokinetic phenomena will be absent when the external electrical field is turned on), when the charge of the solid phase surface is completely compensated by the stationary EDL layer, as has been noted in [39]. Complete compensation of the solid phase charge by the stationary EDL layer was observed by the authors at low surfactant concentrations and, accordingly, at the impossibility of steric stabilization of charged particles in the diffuse part of the EDL. However, even at a sufficient concentration of a surfactant in a microemulsion, there are much fewer charged particles in nonpolar media than in polar ones. This leads to low electrical conductivity values and a very slow reduction in the surface potential in the diffuse part of the EDL. The structure of EDL in organosols is schematically represented in Fig. 1.

Fig. 1.
figure 1

Scheme of the structure of an electrical double layer in a colloidal system with a nonpolar dispersion medium.

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Electrokinetic potential is a dynamic characteristic, which is determined from the mobility of particles in an external electric field. This is one of the important differences from the surface potential, for which there are no direct methods of experimental determination. However, we can cite work [40] as an example. Using X-ray photoelectron spectroscopy, the authors showed a linear dependence of the binding energy between the 2p orbital of silicon and the cation radius in an electrolyte. An additional confirmation was the fact that the zeta potential of silica particles nearly 9 nm in diameter dispersed in 50 mM solutions of alkali metal chlorides varied from –54 to –40 mV when passing from lithium chloride to cesium chloride. In the approximation of the zero-radius of a cation, the authors managed to determine for the first time the surface potential of silica particles. Moreover, in [41], it has been noted that the potential of the boundary of the stationary layer of the EDL and its thickness can also be determined for particles with a known polydispersity. Therefore, it is quite obvious that, from the point of view of experimental determinations, the zeta potential is a much more convenient parameter than the surface potential. Free-medium electrophoresis is used as the main method for determining electrophoretic mobility and zeta potential with various variants of detecting the movement, mainly visual or optical [34, 42–44], and laser electrophoresis with detecting the particle movement by the phase shift of scattered light or phase analysis light scattering (PALS) [16, 45, 46]. In a number of works it has been noted that this approach allows one to achieve higher sensitivity in comparison with the classical laser Doppler electrophoresis, with this fact being of critical importance for nonpolar media [47, 48]. In addition, it is noteworthy that the authors of [49] have shown that, to determine the zeta potential of particles having a complex geometry (hydrophobic nanotubes), it is sufficient to know their electroosmotic mobility.

Typical electrical conductivity values of water-in-oil (w/o) microemulsions based on charging surfactants range from fS/cm to nS/cm, depending on their concentrations and water contents [26, 50, 51]. Hence, the diffuse layer becomes more merged (the Debye length is much larger in the case of nonpolar media). The size of charged particles also has a significant influence on the increase in the thickness of the diffuse part of the EDL. The typical diameter of reverse micelles without an additionally introduced polar phase ranges from 3 to ~10 nm. The authors of [52] have noted a weak and long-range character of the interaction between surfactant micelles, which changes to a stronger and short-range one with increasing concentration.

3 STABILIZATION OF CHARGED PARTICLES WITH ANIONIC SURFACTANTS

3.1 Aerosol OT as a Classical Charging Anionic Surfactant

When considering the entire variety of commercially available surfactants, one can notice that most of them are water-soluble. Stabilizers for w/o microemulsions are much scarcer. Probably, the most popular oil-soluble surfactant is aerosol OT (AOT, more seldom, NaAOT, sodium bis(2-ethylhexyl) sulfosuccinate). The properties of AOT and its close analogues have been described in detail in a series of four works titled as “What is so special about aerosol-OT?” [53–56]. AOT has rather many special features: it is well soluble in nonpolar organic solvents and worse soluble in water (although in rather large amounts), it is rather popular, and, in many aspects, it is an optimal stabilizer for microemulsion synthesis. Moreover, it can be functionalized with various groups. However, the attention will be primarily focused on its charging ability in this review.

Commercial AOT is a sodium salt. However, using, e.g., cation-exchange extraction, it is possible to replace sodium by another cation with a larger size or charge [57]. In some works, Ca(AOT)2 [58], AgAOT [59], Cu(AOT)2 [60, 61] and other salts [62, 63] have been noted. Therefore, in order to exclude possible misinterpretation, AOT will be understood as the residue of bis(2-ethylhexyl) sulfosuccinic acid, while the sodium salt will be denoted as NaAOT.

Typically, sufficiently high surfactant concentrations are used to impart measurable zeta potentials to nanoparticles. A high surfactant concentration implies the range of concentrations that are typically used for the microemulsion method of producing highly dispersed particles or rather close to them. In other words, it is the concentration range in which the concentration of a surfactant in the molecular form is negligible. The range of surfactant concentrations below or at the level of CMC, in which reverse micelles are either absent or the concentrations of surfactants in the molecular and aggregated forms are rather close, is actually not considered. The range of low concentrations of NaAOT was considered in [64]. A mixture of hexadecane and chloroform (dielectric permittivity of 2–4.8) was used as a medium. In systems with high chloroform contents, electrophoretic mobility values corresponding to zeta potential values of up to 100 mV were found. Diffusion-oriented NMR spectroscopy showed that, at a concentration of 0.25 mM in chloroform, NaAOT was present in the molecular form. In the opinion of the authors, solvated AOT– and (Na2AOT)+ ions were the charge carriers in this system. This hypothesis was confirmed by the concentration dependence of the electrical conductivity of NaAOT solutions, which was linear in a range of 0.1–0.5 mM.

$$2{{\left( {{\text{NaAOT}}} \right)}_{{{\text{solv}}}}} \Leftrightarrow \left( {{\text{N}}{{{\text{a}}}_{{\text{2}}}}{\text{AOT}}} \right)_{{{\text{solv}}}}^{ + } + {\text{ }}\left( {{\text{AOT}}} \right)_{{{\text{solv}}}}^{ - }$$
(8)
$$\left[ {\left( {{\text{N}}{{{\text{a}}}_{{\text{2}}}}{\text{AOT}}} \right)_{{{\text{solv}}}}^{ + }} \right] = \left[ {\left( {{\text{AOT}}} \right)_{{{\text{solv}}}}^{ - }} \right] = {{C}_{{{\text{NaAOT}}}}}\sqrt K $$
(9)

Square brackets indicate the concentration of the corresponding ions and K is the constant of reaction (8).

Chloroform is more polar than saturated hydrocarbons; its Bjerrum length under standard conditions is about 12 nm. Taking into account the size of a NaAOT molecule, the thickness of the solvation layer must be at least 5 nm, which seems to be a quite achievable value.

A more typical NaAOT concentration range used to study electrokinetic phenomena in nonpolar media is several tens or hundreds of millimoles. In the opinion of some researchers, the mechanism for the formation of charged NaAOT micelles is associated with fluctuating exchange processes [65]. Since NaAOT can dissociate, the polar cavity actually contains an electrolyte solution. The reverse micelle itself is a rather labile aggregate capable of intermicellar exchange (this ability manifests itself in the solubilization effect and is actively used in micellar synthesis of nanoparticles). Accordingly, due to fluctuations in the electrolyte composition of the polar cavity, at a certain time moment, some micelles may have unequal numbers of cations and anions. The process is schematically represented in Fig. 2. Measuring the electrical conductivity of a surfactant solution and having known its concentration and the aggregation numbers of micelles, one can easily estimate the number of charged micelles. The corresponding calculations for a NaAOT solution in dodecane have been presented in [52]. In a concentration range of 3–200 mM, the fraction of charged micelles was about 1.2 × 10–5. There was no concentration dependence.

Typically, NaAOT is used as a charging additive for commercial samples of highly dispersed particles of, e.g., silica [45] or poly(methyl methacrylate) [66]. There are some works in which NaAOT is used as a nanoreactor that makes it possible to obtain charged nanoparticles of metals [42, 44] and silica [67]. The obtained electrophoretic mobility values are quite close.

3.2 Zirconyl 2-Ethylhexonate

The second most popular anionic charging surfactant is zirconyl 2-ethylhexonate (usually denoted as ZrO(Oct)2). In terms of popularity, it is significantly inferior not only to NaAOT, but also to all nonionic surfactants, which will be discussed below. Its structure is quite close to NaAOT, and its mechanism of action as a charging agent is similar. In [45], ZrO(Oct)2 was used to charge silica nanoparticles along with NaAOT and nonionic OLOA 11000 (it will be discussed in more detail in Section 4.1). The authors have noted that the characters of the influence of all surfactants on the electrophoretic mobility are quite similar. At low concentrations, the nanoparticles are uncharged due to the lack of micelles for stabilizing counterions.

Fig. 2.
figure 2

Scheme of the formation and stabilization of charged micelles in NaAOT microemulsions.

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4 STABILIZATION OF CHARGED PARTICLES WITH NONIONIC SURFACTANTS

The term “nonionic charging surfactants” is quite capable of causing dissonance in a reader. This really looks like a kind of oxymoron—charging a particle implies the formation of ions, while the nonionic nature of a surfactant implies, on the contrary, their absence. However, works devoted to the control of the electrokinetic potential of nanoparticles in nonpolar media using nonionic surfactants are also present in a comparable amount. The two most common types of nonionic charging surfactants are esters of sorbitol and fatty acids (Span trademark) and poly(isobutylene succinimides) (OLOA trademark). In the literature, one can often find abbreviations PIBS or PIBSI (the PIBSI abbreviation will be used below for poly(isobutylene succinimide), because publications contain not only commercial surfactants under the OLOA trademark, but also those synthesized by the authors [68, 69]). For these surfactants, only the concentration range above the CMC will be considered. In [65], it has been noted that PIBSIs have even higher disproportionality constants in comparison with anionic NaAOT; in the premicellar concentration range, the charged particles are not formed in concentrations sufficient for detecting electrokinetic phenomena, although, in the micellar concentration range, their existence is also not quite obvious.

4.1 Poly(isobutylene succinimides)

The first examples of studying the effect of additives of nonionic charging surfactants on the stability of colloidal systems with nonpolar dispersion media were published in the 1980s. The authors of [70] used a surfactant having a trade name of OLOA 1200 (a solution of PIBSI in mineral oil, 1 : 1 w/w) to stabilize carbon black particles in dodecane. The zeta potential values of the particles were higher than –100 mV. In the same work, a mechanism was proposed for charging particles with nonionic PIBSI. When describing the “charging” of particles with nonionic surfactants in subsequent works [23, 46, 71], namely the mechanism proposed by Fowkes et al. [70, 72, 73] is followed.

The mechanism is based on the acid-base interaction; for PIBSI the term “basic” surfactant is usually used [74]. According to Fowkes’ hypothesis, the formation of charged particles in colloids stabilized with PIBSI consists of the following stages:

(1) adsorption of micelles on dispersed phase particles;

(2) dissociation of functional groups on the surface of dispersed phase particles or desorption of ionizable particles from the surface of dispersed phase particles;

(3) transfer of the charge carrier and its stabilization with surfactant functional groups in the polar cavity of a reverse micelle;

(4) desorption of micelles in the form of sterically stabilized ions.

This mechanism is schematically illustrated in Fig. 3 for the case of a ”basic” charging surfactant of the PIBSI type. The complexity of this mechanism lies in the fact that the efficiency of charging dispersed phase particles is determined not only by the properties of the surfactant itself and the presence of certain functional groups capable of stabilizing ions, but also by the surface chemistry of the dispersed phase particles. Therefore, the most popular objects “for charging” are particles of silica or titania, for which the nature of the appearance of a surface charge has been studied in detail. The interaction of acidic silanol groups on the surface of a silica particle and the basic amino groups of PIBSI provides high values of the zeta potential of the particles [34, 75].

Fig. 3.
figure 3

Schematic representation of the model proposed by Fowkes for stabilizing charged particles in microemulsions of “basic” nonionic surfactants [70]. Process stages:(1) adsorption of reverse micelles on a dispersed phase particle; (2) transfer of a charge carrier (in this case, H+ ion as the simplest and most widespread one) from acidic functional groups A–H localized on the surface of a dispersed phase particle to basic functional groups B of the surfactant molecule; (3) stabilization of the charge carrier by the polar cavity of the reverse micelle; (4) desorption of charged reverse micelles from dispersed phase particles.

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It should also be noted that the efficiency of nonionic surfactants as charging agents is at least as high as that of ionic ones. In [76], it has been noted that, for OLOA 1200, the numerical fraction of charged micelles is very high (about 0.017) and may even exceed that for the case of NaAOT.

4.2 Esters of Sorbitol and Fatty Acids

Probably, surfactants such as Span 80 (sorbitan monooleate) can be found in articles devoted to the stabilization of charged particles in nonaqueous media almost as often as NaAOT. This surfactant is discussed after PIBSI not because of its less popularity, but rather because the works devoted to it were published more recently. As well as NaAOT, Span 80 can be used simultaneously as a charging agent and a stabilizer in the synthesis of nanoparticles [77, 78]. The zeta potential of silver nanoparticles stabilized with Span 80 was about +30 mV. At present, diverse sorbitol esters produced by different manufacturers are commercially available, with this fact explaining their popularity and the large number of works published in the last decades. The following compounds are produced under the Span brand: laurate (Span 20), palmate (Span 40), stearate (Span 60), tristearate (Span 65), trioleate (Span 85) and others. The entire so-called “Span family” (the expression “Span family” can be found, in e.g., [16, 79]) is, to a greater or lesser extent, composed by charging surfactants [80]. In a number of works, these surfactants are called “acidic” ones [71, 81] (in contrast to “basic” PIBSI). Accordingly, the proposed mechanism of action is also acid-base, only the surfactant acts as an acid, while the particles of the dispersed phase act as a base.

The acid-base mechanism can be confirmed by the data obtained by Professor Berg’s group. Work [46] provides a comparison of the zeta potential values of functionalized silica nanoparticles when using Span 80 and OLOA 11000 (dispersion of PIBSI with a molecular weight of about 1.2 kDa in mineral oil [82]) as charging agents. To impart acidic properties to the surface of the nanoparticles, they were functionalized with 3-glycidoxypropyltrimethoxysilane, while, to impart basic properties, they were functionalized with aminopropyltriethoxysilane. Both surfactants charged the particles with both “acidic” and “basic” surfaces. However, Span 80 more efficiently charged basic particles (+93 mV versus +27 mV), while OLOA 11000, on the contrary, more efficiently charged acidic particles (–77 mV versus –19 mV). In later work [74], this group of researchers also noted the importance of considering the surface chemistry of dispersed phase particles. In previous studies, the authors used OLOA 11000 to obtain only negatively charged particles. In this work, the authors studied oxides of silicon, titanium, zinc, aluminum, and magnesium. In the case of magnesia, positively charged particles were also obtained. Magnesia has the most pronounced basic properties, and polyimide OLOA 11000 seems to act as an acid rather than a base in this case. At the same time, the authors have noted that the proposed mechanism is not completely understood and requires clarification.

4.3 Atypical Examples of Stabilization of Charged Particles

The first example of electrokinetic phenomena in organosols stabilized with ethoxylated surfactants has been presented in [83]. Ethoxylates are nonionic surfactants and, accordingly, cannot dissociate. The acid-base properties of ethoxylates have not also been discussed in the literature. Accordingly, the previously described mechanisms for stabilizing charged particles are not applicable to w/o microemulsions stabilized with ethoxylates. The authors assumed that the role of charge carrier in such systems is played by impurity chlorine ions. The presence of a significant amount of chlorine has been shown both qualitatively, by reaction with silver nitrate, and semi-quantitatively, by the method of simultaneous thermal analysis with mass spectrometric detection of evolved gases.

5 CONCLUSIONS

This review has considered the main mechanisms of stabilizing charged particles with surfactants in media having low dielectric permittivities, with these mechanisms being of significance for important areas of science and technology. The main types of popular charging surfactants have been considered and their rather close efficiencies have been noted. For anionic surfactants, the mechanism of appearance, stabilization, and transport of charges described in the scientific literature seems to be logical and understandable for the author of this review mainly due to its simplicity. In the case of nonionic surfactants, acid-base interaction has been suggested in the literature as a possible mechanism. Although a number of works published in authoritative publications have presented experiments that are well consistent with this concept, the author of this work will not undertake to call it unambiguous. The complexity of this model is due to the fact that the electrokinetic phenomena in such systems are determined not only by the chemistry of the surfactants themselves, but also by the chemistry of the surface of dispersed phase particles. However, it is not always simple and understandable. Therefore, the stabilization of charged particles by nonionic surfactants in media with dielectric permittivities of about 2 seems to be an insufficiently studied area of colloid chemistry at the moment from the points of view of both theoretical concepts and their experimental confirmation especially taking into account new results [83] and results that have not been completely understood and require clarification [74, 83].