Surfactant Mixtures: Performances vs. Aggregation States

*Camillo La Mesa and Gianfranco Risuleo*

## **Abstract**

The focus of this chapter is on bio-intended procedures based on mixing surfactants with polymers and biopolymers, or surfactants among them (provided they are oppositely charged). In the first case, polymer-surfactant and protein-surfactant systems are dealt with. Both are characterized by the splitting of the solution phase into, at least, three regions having peculiar properties. At first, surfactant nucleation onto polymers takes place; this implies large modifications in properties with respect to the starting materials. The formation of gels is possible in some instances. As to mixtures of oppositely charged surfactants, it is indicated how they form cat-anionic vesicles if mixed in nonstoichiometric amounts. Vesicle sizes are modulated by the charge ratio. These systems are excellent vectors for biomedical purposes.

**Keywords:** ionic surfactant mixtures, size and shape, surface charge density of micelles and vesicles, polymer-surfactant systems, protein-surfactant systems

#### **1. Introduction**

The certified history of surfactants and detergents goes back to the Mesopotamian and Egyptian ages. In the Roman period, authors contemporary of Julius Caesar described the procedures in use from Gauls and Belges to produce soaps from the alkaline hydrolysis of beef fat [1]. They were horribly shocked for the excessive use of soaps that Gauls consumed in hair cleaning. Such procedures are still in use in the preparation of niche products as Marseille soap. In much more recent times, new procedures largely improved the preparation of surface-active products, synthetizing alkyl sulfates. These studies date back to the 1930s of the last century [2]. Later on, nonionic surfactants of the alkyl-polyoxyethylene family, as Triton TX-100, or zwitterionic ones were worked out and synthetized [3]. This induced chemists to prepare new classes of solid or liquid formulations, with better performances in terms of surface activity and solvent capacity. These efforts allowed preparing chemicals capable to operate in all working conditions, irrespective of pH, the presence of calcium, and ionic strength of the dispersant [4–6].

Nowadays, focus is on surfactant mixtures, improving the intrinsic quality of formulations and allowing applications to much more cases than those originally intended for. Applications of surfactant-based systems are much more versatile with respect to canonical laundry and personal body care formulations that were exploited until now. Current research lines focus on unexpected fields, as applications in biomedicine and in the feminine personal hygiene formulations. We do not consider, in this review, the adjuvant action played by cosurfactants, as long-chain

#### *Surfactants and Detergents*

alkanols, glycerol, sterols, perfumes, softeners, bleaching adjuvants, and so forth. We mainly focus on the addition of species increasing the surface activity and solvency of existing surface-active/cleaning formulations and in applications thereof. In particular, the synergistic properties that are observed in surfactant mixtures [7, 8] are discussed.

Cases of interest span from mixtures of ionic species of the same charge, to ionic/nonionic ones, and to mixtures of species having oppositely charged polar head groups. Relevant are also the cases where polymers, enzymes, and proteins are added. We discuss separately all the above fields taking into account the reasons underlying such research lines. In turn, focus is on the following aspects:


The above items are more strictly interconnected than one could think at a first glance. In both the organizing role played, surfactants are crucial both on small or medium size scale (for polymer/surfactant systems) and on a much larger size scale, in case of surfactant mixtures. Both classes of formulations are biomimetic, and the efficiency is related to biopolymer modifications induced by surfactants and to surfactant-driven vesicle formation, respectively.

As a starting point, we report the essential details on the physical meaning of surface activity and solvent capacity; both requisites are necessary to understand biomimicry, surfactancy, and detergency on solid grounds. For more details, the interested reader is referred to pivotal books and reviews that have dealt with that field [11–14]. In many aspects, we follow the "main street" that is suggested in a seminal book, which allowed scientists to unify in a whole field the formation of both micelles, vesicles, and biological membranes [15].

### **2. Solvent capacity and surface activity**

The term surface active, or surfactant, refers to substances capable to lower significantly and permanently the surface tension of water, i.e., to decrease the work required increasing the surface area of a liquid. In terms of the classical Gibbs surface adsorption equation valid for aqueous binary mixtures, we define as surface active all species fulfilling the equation [16]:

$$\mathbf{d}\sigma = -\mathbf{G}\_2 \mathbf{d}\mathbf{R} \mathbf{T} \text{d}\ln a\_2 \tag{1}$$

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*Surfactant Mixtures: Performances vs. Aggregation States DOI: http://dx.doi.org/10.5772/intechopen.85437*

addition of fats and oils.

assumed to be in a "liquid-like" form.

hoc" information will be necessary.

**3. Addition of polymers or biopolymers**

sometimes, controversial fields [23–25].

the following areas are observed, from the left:

nonpolar (i.e., hydrophobic) molecules. The polar groups facing outward the bulk guarantee thermodynamic stability to the aggregates so formed. In words, the solubilizing capacity toward oils and fats starts to occur only when micelles do form. For this reason, micelles are swelling units which grow in size upon

From a thermodynamic viewpoint, micelle formation is mainly entropydriven. This is a rather counterintuitive behavior, if we consider that several molecules associate in a given entity. The reason underlying the entropy-based statement is that water molecules hydrophobically interacting with alkyl chains are released during micelle formation [15]. This substantially increases the number of degrees of freedom for H2O and those of the chains, as well. It is also worth noticing that an increase in temperature increases the number of rotational degrees of freedom of geometrically constrained surfactant alkyl chains, which are free to move into micelles. This is the main reason why micelle interior is

To unify the above features, that is, surface activity and solvent capacity, in a whole definition, we assume that the point at which surface activity ends and micelles begin to form is a "pseudo" phase separation threshold, indicated as critical micellar concentration or *cmc* [18, 19]. The definition "critical" indicates the steep discontinuity in many thermodynamic quantities (molar volumes, dilution enthalpies, activity coefficients, and so forth) observed in close proximity of the *cmc*. For a given class of surfactants, such as alkali metal alkylsulfates, alkyltrimethylammonium halides, polyoxyethylene glycol alkyl ethers, etc., the two features jointly depend on the length of alkyl chains. The longer the latter are, the lower is the *cmc*, the steeper is the decrease in surface tension, and the more efficient is solvent capacity. We do not enter in more details about micelle sizes, shape, and polydispersity and assume, in a first approximation, that such aggregates are spheroidal colloids. For these reasons, they scatter light, have much lower diffusion coefficients than molecules from which they are made of, and their solutions can be moderately or significantly viscous. At high concentrations, they form ordered phases known as lyotropic liquid crystals [20, 21]. More aspects, such as the role of salts and cosolvents in micelle formation, shall be introduced when the need of "ad

Studies on additives as salts and cosolvents have been widely investigated in the past and will not be reported, unless this is strictly necessary. Conversely, studies on systems containing synthetic polymers or biopolymers are still a matter of debate and investigation and will be discussed in this section. The first efforts along this line go back to the 1950s and were essentially dealing with protein separation from biological membrane lipids. These efforts were led to convergence in a classical textbook of the early 1990s [22]. This induced many scientists to focus on new and,

The underlying phenomenology can be understood by looking at **Figure 1**. In the plot the behavior of a ternary system containing water, surfactant, and polymer is reported. If the relative *wt%* of the latter substances is much lower than water, the ternary phase diagram can be simplified in a pseudo-binary one. As can be seen in **Figure 1**, a pseudo-phase behavior occurs in absence of polymer; the *cmc* is the point separating the micellar from the molecular regime. Added polymer induces the splitting of the solution phase into three regions. For finite amounts of polymer,

where *σ* is the surface tension and *a2* is the solute activity. *G2*, the surface excess concentration, indicates as to whether the surface tension will decrease, or increase, upon addition of a given solute. *G2* is defined with respect to the concentration of the given chemical in the bulk and depends on its modulus. That is the rationale underlying the meaning of the term "surface active." When d*σ* = 0, there is no more room for adsorption, and the surface is saturated. In addition, if dln*a*2 is zero, the solute activity is constant and a new phase is being formed. This is the basis for the so-called phase separation approach to micelle formation [17], discussed later on.

The solvent capacity arises from a more subtle behavior and is univocally related to micelles onset. The organization of surfactant molecules arises from the "schizophrenia" that such molecules suffer from. They associate in micellar entities whose interior, mostly composed of alkyl groups, is capable to dissolve

*Surfactants and Detergents*

systems [10].

surfactant-driven vesicle formation, respectively.

both micelles, vesicles, and biological membranes [15].

**2. Solvent capacity and surface activity**

active all species fulfilling the equation [16]:

are discussed.

alkanols, glycerol, sterols, perfumes, softeners, bleaching adjuvants, and so forth. We mainly focus on the addition of species increasing the surface activity and solvency of existing surface-active/cleaning formulations and in applications thereof. In particular, the synergistic properties that are observed in surfactant mixtures [7, 8]

Cases of interest span from mixtures of ionic species of the same charge, to ionic/nonionic ones, and to mixtures of species having oppositely charged polar head groups. Relevant are also the cases where polymers, enzymes, and proteins are added. We discuss separately all the above fields taking into account the reasons

ii.use of mixtures made of oppositely charged surfactants, defined as *Cat-An*

The above items are more strictly interconnected than one could think at a first glance. In both the organizing role played, surfactants are crucial both on small or medium size scale (for polymer/surfactant systems) and on a much larger size scale, in case of surfactant mixtures. Both classes of formulations are biomimetic, and the efficiency is related to biopolymer modifications induced by surfactants and to

As a starting point, we report the essential details on the physical meaning of surface activity and solvent capacity; both requisites are necessary to understand biomimicry, surfactancy, and detergency on solid grounds. For more details, the interested reader is referred to pivotal books and reviews that have dealt with that field [11–14]. In many aspects, we follow the "main street" that is suggested in a seminal book, which allowed scientists to unify in a whole field the formation of

The term surface active, or surfactant, refers to substances capable to lower significantly and permanently the surface tension of water, i.e., to decrease the work required increasing the surface area of a liquid. In terms of the classical Gibbs surface adsorption equation valid for aqueous binary mixtures, we define as surface

d = −*G*2dRTdln*a*<sup>2</sup> (1)

where *σ* is the surface tension and *a2* is the solute activity. *G2*, the surface excess concentration, indicates as to whether the surface tension will decrease, or increase, upon addition of a given solute. *G2* is defined with respect to the concentration of the given chemical in the bulk and depends on its modulus. That is the rationale underlying the meaning of the term "surface active." When d*σ* = 0, there is no more room for adsorption, and the surface is saturated. In addition, if dln*a*2 is zero, the solute activity is constant and a new phase is being formed. This is the basis for the so-called phase separation approach to micelle formation [17], discussed later on. The solvent capacity arises from a more subtle behavior and is univocally related to micelles onset. The organization of surfactant molecules arises from the "schizophrenia" that such molecules suffer from. They associate in micellar entities whose interior, mostly composed of alkyl groups, is capable to dissolve

underlying such research lines. In turn, focus is on the following aspects:

i. addition of polymers/biopolymers, referred to as *PSS*s [9]; and

**44**

nonpolar (i.e., hydrophobic) molecules. The polar groups facing outward the bulk guarantee thermodynamic stability to the aggregates so formed. In words, the solubilizing capacity toward oils and fats starts to occur only when micelles do form. For this reason, micelles are swelling units which grow in size upon addition of fats and oils.

From a thermodynamic viewpoint, micelle formation is mainly entropydriven. This is a rather counterintuitive behavior, if we consider that several molecules associate in a given entity. The reason underlying the entropy-based statement is that water molecules hydrophobically interacting with alkyl chains are released during micelle formation [15]. This substantially increases the number of degrees of freedom for H2O and those of the chains, as well. It is also worth noticing that an increase in temperature increases the number of rotational degrees of freedom of geometrically constrained surfactant alkyl chains, which are free to move into micelles. This is the main reason why micelle interior is assumed to be in a "liquid-like" form.

To unify the above features, that is, surface activity and solvent capacity, in a whole definition, we assume that the point at which surface activity ends and micelles begin to form is a "pseudo" phase separation threshold, indicated as critical micellar concentration or *cmc* [18, 19]. The definition "critical" indicates the steep discontinuity in many thermodynamic quantities (molar volumes, dilution enthalpies, activity coefficients, and so forth) observed in close proximity of the *cmc*.

For a given class of surfactants, such as alkali metal alkylsulfates, alkyltrimethylammonium halides, polyoxyethylene glycol alkyl ethers, etc., the two features jointly depend on the length of alkyl chains. The longer the latter are, the lower is the *cmc*, the steeper is the decrease in surface tension, and the more efficient is solvent capacity. We do not enter in more details about micelle sizes, shape, and polydispersity and assume, in a first approximation, that such aggregates are spheroidal colloids. For these reasons, they scatter light, have much lower diffusion coefficients than molecules from which they are made of, and their solutions can be moderately or significantly viscous. At high concentrations, they form ordered phases known as lyotropic liquid crystals [20, 21]. More aspects, such as the role of salts and cosolvents in micelle formation, shall be introduced when the need of "ad hoc" information will be necessary.
