**3. Addition of polymers or biopolymers**

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, sometimes, controversial fields [23–25].

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, the following areas are observed, from the left:

#### **Figure 1.**

*The surfactant behavior in presence of a nonionic polymer. The black line in the left bottom of the figure indicates the molecular solution region and the dotted one the micellar regime. The turquoise area indicates the molecular regime and is limited by the cac, above which the surfactant starts to interact with the polymer. The red area indicates the interaction regime; the yellow one, the saturation regime, occurs when the polymer is saturated. The line separating the red and yellow regions is indicated as cmc\* line.*

i. a molecular solution region, *I*;

ii. a polymer-surfactant one, *II*; and

iii. a region where free micelles coexist with polymer-surfactant adducts, *III*.

To build up the phase map, surface tension values are measured for a number of polymer *wt%* (**Figure 2**). There splitting of surface tension values in three regimes is evident. *Cac* and *cmc\** are easily determined form these and other experiments, as well [22].

On thermodynamic grounds, the line separating region *I* from *II* indicates the points above which polymer/surfactant interactions start to occur; the line position depends on polymer content and nature. There is an ensemble of critical points, whose location in the phase map depends on the polymer amount. Once the process has occurred, the surfactants located on the polymer backbone act as nucleation sites for the binding of more surface-active species. Thus, entities similar to micelles (*emi-micelles*) aggregate thereon: a sort of "pearl necklace" is formed [26]. Thus, the polymer backbone is decorated by a series of small aggregates, whose number is dictated by its length; the interacting polymer sections, the so termed "polymer

**47**

**Figure 2.**

assume during the flow.

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

binding sites", are a few kdalton long. Surfactant nucleation thereon continues until all possible sites are saturated. In consequence of that, the polymer tends to assume a different conformation with respect to the native one, with subsequent changes in viscosity. This is the reason why polymer/surfactant systems act as "viscosity modulators" [27, 28]. Another important consequence is the fact that they are "kinetic

*Plot indicating how to get the cac, the first minimum, and the cmc\*, at surface saturation, for a given amount of polymer vs. surfactant content. Black points refer to data in presence of polymer, the red ones to the surfactant alone.*

Ancillary effects are concomitant to the mentioned behavior. First, micelles of smaller size compared with free ones are formed; they behave as a whole kinetic entity with the polymer (i.e., the binding energy is significant). This is a feature similar to those occurring in biological systems, as in the binding of molecules to the protruding parts of a receptor. The line separating the two regions is defined as "critical aggregation concentration" or *cac* line. The nucleation of fat droplets on a cotton string is a pertinent example for the formation of polymer-surfactant adducts; their location thereon is energetically more favored than in free form. The *cmc\** one, conversely, is a polymer saturation threshold, above which there is no room for binding. As a consequence, free micelles do form and coexist with polymer-adsorbed ones. Technological applications find place in formulation. The viscoelastic properties that such systems exhibit are used in shampoos, eye-drop fluids, etc. [30, 31]. Viscoelasticity is simply detected by abruptly rotating the fluid-containing vials, with transient formation of ellipsoidal bubbles or, in a more quantitative way, by rheology [27, 28]. An alternative simple procedure requires pressing drops of these formulations between glass slides and looking by a polarizing microscope, to detect the preferred orientation that polymer-surfactant adducts

There is no significant difference when polyelectrolytes replace nonionic polymers. In cases like such, precipitation may also occur; cases are known [32], mostly as to biopolymers are concerned [33–36]. In mixtures containing proteins, precipitates or, eventually, two-phase regions are usually met. As a rule, these are centered around the charge neutralization line, where precipitates or gels may coexist (**Figure 3**). In such systems relevant are the modifications observed in

buffers" as to matter exchange with the bulk is concerned [29].

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

#### **Figure 2.**

*Surfactants and Detergents*

i. a molecular solution region, *I*;

experiments, as well [22].

ii. a polymer-surfactant one, *II*; and

*saturated. The line separating the red and yellow regions is indicated as cmc\* line.*

iii. a region where free micelles coexist with polymer-surfactant adducts, *III*.

*The surfactant behavior in presence of a nonionic polymer. The black line in the left bottom of the figure indicates the molecular solution region and the dotted one the micellar regime. The turquoise area indicates the molecular regime and is limited by the cac, above which the surfactant starts to interact with the polymer. The red area indicates the interaction regime; the yellow one, the saturation regime, occurs when the polymer is* 

To build up the phase map, surface tension values are measured for a number of polymer *wt%* (**Figure 2**). There splitting of surface tension values in three regimes is evident. *Cac* and *cmc\** are easily determined form these and other

On thermodynamic grounds, the line separating region *I* from *II* indicates the points above which polymer/surfactant interactions start to occur; the line position depends on polymer content and nature. There is an ensemble of critical points, whose location in the phase map depends on the polymer amount. Once the process has occurred, the surfactants located on the polymer backbone act as nucleation sites for the binding of more surface-active species. Thus, entities similar to micelles (*emi-micelles*) aggregate thereon: a sort of "pearl necklace" is formed [26]. Thus, the polymer backbone is decorated by a series of small aggregates, whose number is dictated by its length; the interacting polymer sections, the so termed "polymer

**46**

**Figure 1.**

*Plot indicating how to get the cac, the first minimum, and the cmc\*, at surface saturation, for a given amount of polymer vs. surfactant content. Black points refer to data in presence of polymer, the red ones to the surfactant alone.*

binding sites", are a few kdalton long. Surfactant nucleation thereon continues until all possible sites are saturated. In consequence of that, the polymer tends to assume a different conformation with respect to the native one, with subsequent changes in viscosity. This is the reason why polymer/surfactant systems act as "viscosity modulators" [27, 28]. Another important consequence is the fact that they are "kinetic buffers" as to matter exchange with the bulk is concerned [29].

Ancillary effects are concomitant to the mentioned behavior. First, micelles of smaller size compared with free ones are formed; they behave as a whole kinetic entity with the polymer (i.e., the binding energy is significant). This is a feature similar to those occurring in biological systems, as in the binding of molecules to the protruding parts of a receptor. The line separating the two regions is defined as "critical aggregation concentration" or *cac* line. The nucleation of fat droplets on a cotton string is a pertinent example for the formation of polymer-surfactant adducts; their location thereon is energetically more favored than in free form. The *cmc\** one, conversely, is a polymer saturation threshold, above which there is no room for binding. As a consequence, free micelles do form and coexist with polymer-adsorbed ones. Technological applications find place in formulation. The viscoelastic properties that such systems exhibit are used in shampoos, eye-drop fluids, etc. [30, 31]. Viscoelasticity is simply detected by abruptly rotating the fluid-containing vials, with transient formation of ellipsoidal bubbles or, in a more quantitative way, by rheology [27, 28]. An alternative simple procedure requires pressing drops of these formulations between glass slides and looking by a polarizing microscope, to detect the preferred orientation that polymer-surfactant adducts assume during the flow.

There is no significant difference when polyelectrolytes replace nonionic polymers. In cases like such, precipitation may also occur; cases are known [32], mostly as to biopolymers are concerned [33–36]. In mixtures containing proteins, precipitates or, eventually, two-phase regions are usually met. As a rule, these are centered around the charge neutralization line, where precipitates or gels may coexist (**Figure 3**). In such systems relevant are the modifications observed in

**Figure 3.**

*Partial phase diagram for the system water-lysozyme-lithium perfluorononanoate (a stiff, fully fluorinated surfactant), at 25°C. The coexistence of a solution and precipitate occurs in the black area, whereas a pure gel, in dark gray, and one empty of particles, in light gray, are met. The charge neutralization limit is indicated as a blue line. This is the point at which all nominal charges on the protein, at the given pH, are fully neutralized. Partly redrawn from Ref. [26].*

protein conformation. Changes in the relative amounts of alpha-helix, beta-sheet, and random coil conformations are concomitant to protein-surfactant interactions in a wide part of the interaction regime. Such changes are responsible for significant variations in protein activity and three-dimensional structure of the "adducts" that are formed. All these systems are characterized by a not univocally defined stoichiometry, and the definition of "adduct" is more correct with respect to that of "complex." The rationale underlying that behavior finds origin in the fact that alkyl chains are essentially located in the protein hydrophobic tasks. Many possible locations are available in cases like such. The above statements are quite well acquainted from experiments on albumins and, more generally, on protein denaturation strategies [37]. Thus, biopolymer/surfactant systems offer the opportunity to prepare proteins in pure form from extensive dialysis of the corresponding mixtures. For these reasons they find extensive use in biochemically intended procedures.

## **4. Mixtures made of oppositely charged surfactants**

Pioneering studies in the field are due to Wennerstroem [38], who focused on the synthetic analogues of lipids and suggested that stoichiometric mixtures of oppositely charged surfactants could be good substitutes of lipids. The original hypothesis dealt with systems of 1–1 stoichiometry, in terms of charge. There, the electrostatic interactions between polar groups mimic charge separation among entities bound on a glycerol backbone, which is also joining two alkyl chains. The above systems are models of swelling, lamellar domains. The first experimental results were discouraging; in fact, these mixtures often show thermotropic rather than lyotropic behavior [39], due to the high "Krafft point" [40] of alkyl chains in such mixtures. Later work demonstrated that nonstoichiometric *Cat-An* mixtures were more promising. It was noticed there the presence of vesicular entities [41, 42]. Debates occurred on the stability of largely polydispersed in size vesicles. It is actually accepted that they are kinetically stable entities although thermodynamic stability is demonstrated in some cases [43, 44].

The phenomenology of such systems, defined by the acronym "cat-anionic," is extremely appealing from a bio-intended viewpoint. In the phase diagram, in particular, the vesicular areas are located in proximity of micellar ones and are clearly

**49**

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

distinguishable from them. The observation is in favor of a significant modification in the micellar structure induced by the second surfactant. Cat-anionic mixtures, hereafter termed *Cat-An*'s, are characterized by a bluish color and may turn to yellowish or opalescent appearance when vesicle sizes exceed some 100 nms. They are both positively and negatively charged. This fact gives the opportunity to use *Cat-An* vesicles as vehiculating/binding agents of DNA (for positively charged ones) and proteins. In the latter eventuality, both positively and negatively charged

Debates questioned on the possible protein denaturation that could be induced by the surfactants present in *Cat-An* formulations, until it was realized that the surfactant in molecular form is solely responsible for protein denaturation [45]. The amount of such species is orders of magnitude lower than in solutions of the single surfactants. The above behavior is supported by the following thermodynamic considerations. The mutual interactions between polar head groups and alkyl chain packing play a key role in such systems. The observed behavior is different from that expected if ideality of mixing holds. In words, when fluid chains are presumably miscible in all proportions, the effect of surface charges modulates the area on which alkyl chains insist and determines their optimal packing. This results in a strong nonideality of mixing. It is not surprising, therefore, that the *cmc* for an aggregate of given stoichiometry can be orders of magnitude lower than expected from primitive considerations. To quantify such effects, it was assumed the validity of regular solution theory, and it was imposed, accordingly, that "the free monomer has an activity coefficient of unity" [46]. This is an oversimplified viewpoint, since surfactant solutions are strongly nonideal even below the *cmc*. To proceed along, we assume that the concentration above which added surfactant preferentially enters into aggregates (disregarding their size and shape) is the saturation threshold for the molecular species. In this way, the difference in composition between molecular and micellar form is immaterial. In two-component surfactant mixtures, thus, the

vesicles may be used, depending on the demand dictated by protein charge.

*cmc* of the mixed system is defined according to the relation [47].

parameter, *b*, results to be [47].

*b* = ∆*Gexc*,*mixt*[(*X*<sup>2</sup>

cargos for proteins and DNA [49–51].

(1/*cmcmixt*) = [(*X*2/<sup>3</sup> *cmc*3) + (1 − *X*2)/<sup>2</sup> *cmc*2)] (2)

where *γ*2 and *γ*3 are the activity coefficients of the surfactants, having *cmc3* and *cmc2* as the corresponding critical values. *cmcmix*t is the critical concentration of the mixed system. *X*'s are the mole fraction of the given surface-active species. In the limits dictated by the regular solution theory [48], the solute-solute interaction

> <sup>2</sup> + *X*<sup>3</sup> 2 )/(*X*<sup>2</sup> <sup>2</sup>*X*<sup>3</sup> 2

The underlying rationale is as follows. Micelles are in fluid state with freely moving polar head groups. They may change position, adsorb/desorb counterions, and so forth. The constraints acting on alkyl chains are such that polar head groups close each other attract/repel. In consequence of that, mixed systems show strong deviations from the ideal behavior. This tendency is quantified by the mentioned *b* parameter. The effect is substantial (**Figure 4**) and explains why the amount of both surfactants in molecular form is orders of magnitude lower than expected. In words, *Cat-An*'s are in equilibrium with their own counterions and with tiny amounts of free surfactants, as well. This is the basis for using cat-anionic vesicles as

Sizes of *Cat-An* vesicles strongly depend on the formulation stoichiometry. As mentioned above, 1–1 mixtures form indefinitely large smectic crystals; on both sides of this threshold, sizes depend regularly on composition and approach values

)] (3)

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

*Surfactants and Detergents*

**Figure 3.**

*Partly redrawn from Ref. [26].*

protein conformation. Changes in the relative amounts of alpha-helix, beta-sheet, and random coil conformations are concomitant to protein-surfactant interactions in a wide part of the interaction regime. Such changes are responsible for significant variations in protein activity and three-dimensional structure of the "adducts" that are formed. All these systems are characterized by a not univocally defined stoichiometry, and the definition of "adduct" is more correct with respect to that of "complex." The rationale underlying that behavior finds origin in the fact that alkyl chains are essentially located in the protein hydrophobic tasks. Many possible locations are available in cases like such. The above statements are quite well acquainted from experiments on albumins and, more generally, on protein denaturation strategies [37]. Thus, biopolymer/surfactant systems offer the opportunity to prepare proteins in pure form from extensive dialysis of the corresponding mixtures. For these reasons they find extensive use in biochemically intended procedures.

*Partial phase diagram for the system water-lysozyme-lithium perfluorononanoate (a stiff, fully fluorinated surfactant), at 25°C. The coexistence of a solution and precipitate occurs in the black area, whereas a pure gel, in dark gray, and one empty of particles, in light gray, are met. The charge neutralization limit is indicated as a blue line. This is the point at which all nominal charges on the protein, at the given pH, are fully neutralized.* 

Pioneering studies in the field are due to Wennerstroem [38], who focused on the synthetic analogues of lipids and suggested that stoichiometric mixtures of oppositely charged surfactants could be good substitutes of lipids. The original hypothesis dealt with systems of 1–1 stoichiometry, in terms of charge. There, the electrostatic interactions between polar groups mimic charge separation among entities bound on a glycerol backbone, which is also joining two alkyl chains. The above systems are models of swelling, lamellar domains. The first experimental results were discouraging; in fact, these mixtures often show thermotropic rather than lyotropic behavior [39], due to the high "Krafft point" [40] of alkyl chains in such mixtures. Later work demonstrated that nonstoichiometric *Cat-An* mixtures were more promising. It was noticed there the presence of vesicular entities [41, 42]. Debates occurred on the stability of largely polydispersed in size vesicles. It is actually accepted that they are kinetically stable entities although thermodynamic

The phenomenology of such systems, defined by the acronym "cat-anionic," is extremely appealing from a bio-intended viewpoint. In the phase diagram, in particular, the vesicular areas are located in proximity of micellar ones and are clearly

**4. Mixtures made of oppositely charged surfactants**

stability is demonstrated in some cases [43, 44].

**48**

distinguishable from them. The observation is in favor of a significant modification in the micellar structure induced by the second surfactant. Cat-anionic mixtures, hereafter termed *Cat-An*'s, are characterized by a bluish color and may turn to yellowish or opalescent appearance when vesicle sizes exceed some 100 nms. They are both positively and negatively charged. This fact gives the opportunity to use *Cat-An* vesicles as vehiculating/binding agents of DNA (for positively charged ones) and proteins. In the latter eventuality, both positively and negatively charged vesicles may be used, depending on the demand dictated by protein charge.

Debates questioned on the possible protein denaturation that could be induced by the surfactants present in *Cat-An* formulations, until it was realized that the surfactant in molecular form is solely responsible for protein denaturation [45]. The amount of such species is orders of magnitude lower than in solutions of the single surfactants.

The above behavior is supported by the following thermodynamic considerations. The mutual interactions between polar head groups and alkyl chain packing play a key role in such systems. The observed behavior is different from that expected if ideality of mixing holds. In words, when fluid chains are presumably miscible in all proportions, the effect of surface charges modulates the area on which alkyl chains insist and determines their optimal packing. This results in a strong nonideality of mixing. It is not surprising, therefore, that the *cmc* for an aggregate of given stoichiometry can be orders of magnitude lower than expected from primitive considerations. To quantify such effects, it was assumed the validity of regular solution theory, and it was imposed, accordingly, that "the free monomer has an activity coefficient of unity" [46]. This is an oversimplified viewpoint, since surfactant solutions are strongly nonideal even below the *cmc*. To proceed along, we assume that the concentration above which added surfactant preferentially enters into aggregates (disregarding their size and shape) is the saturation threshold for the molecular species. In this way, the difference in composition between molecular and micellar form is immaterial. In two-component surfactant mixtures, thus, the *cmc* of the mixed system is defined according to the relation [47].

$$\left(\mathbf{1}/cmc\_{\rm mix}\right) = \left[\left(X\_2/\gamma\_3cmc\_3\right) + \left(\mathbf{1} - X\_2\right)/\gamma\_2cmc\_2\right] \tag{2}$$

where *γ*2 and *γ*3 are the activity coefficients of the surfactants, having *cmc3* and *cmc2* as the corresponding critical values. *cmcmix*t is the critical concentration of the mixed system. *X*'s are the mole fraction of the given surface-active species. In the limits dictated by the regular solution theory [48], the solute-solute interaction parameter, *b*, results to be [47].

$$b = \Delta G\_{\text{exc}, \text{mix}} \left[ \left( X\_2^{\ 2} + X\_3^{\ 2} \right) / \left( X\_2^{\ 2} X\_3^{\ 2} \right) \right] \tag{3}$$

The underlying rationale is as follows. Micelles are in fluid state with freely moving polar head groups. They may change position, adsorb/desorb counterions, and so forth. The constraints acting on alkyl chains are such that polar head groups close each other attract/repel. In consequence of that, mixed systems show strong deviations from the ideal behavior. This tendency is quantified by the mentioned *b* parameter. The effect is substantial (**Figure 4**) and explains why the amount of both surfactants in molecular form is orders of magnitude lower than expected. In words, *Cat-An*'s are in equilibrium with their own counterions and with tiny amounts of free surfactants, as well. This is the basis for using cat-anionic vesicles as cargos for proteins and DNA [49–51].

Sizes of *Cat-An* vesicles strongly depend on the formulation stoichiometry. As mentioned above, 1–1 mixtures form indefinitely large smectic crystals; on both sides of this threshold, sizes depend regularly on composition and approach values

#### **Figure 4.**

*Dependence of the cmc (in mol kg<sup>−</sup><sup>1</sup> ) on cetyltrimethylammonium bromide, CTAB, mole fraction for SDS-CTAB mixtures, at 25°C. The red line is for visual purposes; the full on the top refers to ideal mixing and the vertical to the nonideality of mixing. The blue area indicates the precipitation regime.*

#### **Figure 5.**

*Vesicle size (in nm) for SDS-CTAB mixtures, at 25°C, vs. the nominal surface charge excess of the vesicular aggregate. The light blue area in the center of the figure refers to the precipitation regime.*

**51**

provided the original work is properly cited.

applications on more conscious grounds.

\* and Gianfranco Risuleo2

1 Department Chemistry, Sapienza University, Rome, Italy

\*Address all correspondence to: camillo.lamesa@uniroma1.it

2 Formerly at Department Biology, Sapienza University, Rome, Italy

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

accordingly retain their size for indefinitely long times.

**5. Conclusions**

**Author details**

Camillo La Mesa1

pertinent to the pure surfactant aggregates. In words, the excess surface charge determines vesicles sizes (**Figure 5**). It is worth to note that similar trends are also observed in mixtures of oppositely charged lipids [52]. The surface charge versatility is reminiscent of statements based on the relations between particles size and surface charge density. The higher the former, the lower the latter. This fact has important consequences on the links between (nominal) surface charge density and sizes. It is a sort of charge-based size tailoring and is quintessential in choosing the proper particles for transfection technologies. Another pertinent possibility along this line arises from thermal cycling procedures, which allow getting stable particles of proper size by raising the temperature above a certain value (which depends on the composition of the *Cat-An* mixture [53]. Thermally quenched vesicles obtained

Sound procedures based on the combination of the above features allow getting vesicles of the desired size and surface charge density. This allows using them for DNA transfection technologies and protein immobilization onto vesicles [54]. An interesting feature is that vesicles of a given composition are destroyed by adding amounts of surfactant required for the complete neutralization of the *Cat-An* mixture. In consequence of that, the biopolymer which is eventually bound onto vesicles is released in its

This contribution focuses on the possibilities offered by surfactants and their mixtures in selected bio-intended applications. The mentioned systems are niche fields, but are becoming of relevant impact in a lot of practical purposes. Think, for instance, that applications in shampoos and similar products almost always include silk proteins as adjuvants of hair state and health. Transfection, conversely, is quite appealing for biochemistry and molecular biology applications. In many aspects, thus, both fields of research are on the same line as those originally intended in the pre-Christian age. It is as if we were moving back to the roots of surfactancy. Luckily, we have much more knowledge in the field, and this allows us to exploit

pristine form [55]. This is a terrific possibility for bio-intended technologies.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

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

pertinent to the pure surfactant aggregates. In words, the excess surface charge determines vesicles sizes (**Figure 5**). It is worth to note that similar trends are also observed in mixtures of oppositely charged lipids [52]. The surface charge versatility is reminiscent of statements based on the relations between particles size and surface charge density. The higher the former, the lower the latter. This fact has important consequences on the links between (nominal) surface charge density and sizes. It is a sort of charge-based size tailoring and is quintessential in choosing the proper particles for transfection technologies. Another pertinent possibility along this line arises from thermal cycling procedures, which allow getting stable particles of proper size by raising the temperature above a certain value (which depends on the composition of the *Cat-An* mixture [53]. Thermally quenched vesicles obtained accordingly retain their size for indefinitely long times.

Sound procedures based on the combination of the above features allow getting vesicles of the desired size and surface charge density. This allows using them for DNA transfection technologies and protein immobilization onto vesicles [54]. An interesting feature is that vesicles of a given composition are destroyed by adding amounts of surfactant required for the complete neutralization of the *Cat-An* mixture. In consequence of that, the biopolymer which is eventually bound onto vesicles is released in its pristine form [55]. This is a terrific possibility for bio-intended technologies.

### **5. Conclusions**

*Surfactants and Detergents*

**50**

**Figure 5.**

**Figure 4.**

*Dependence of the cmc (in mol kg<sup>−</sup><sup>1</sup>*

*Vesicle size (in nm) for SDS-CTAB mixtures, at 25°C, vs. the nominal surface charge excess of the vesicular* 

*) on cetyltrimethylammonium bromide, CTAB, mole fraction for SDS-*

*CTAB mixtures, at 25°C. The red line is for visual purposes; the full on the top refers to ideal mixing and the* 

*vertical to the nonideality of mixing. The blue area indicates the precipitation regime.*

*aggregate. The light blue area in the center of the figure refers to the precipitation regime.*

This contribution focuses on the possibilities offered by surfactants and their mixtures in selected bio-intended applications. The mentioned systems are niche fields, but are becoming of relevant impact in a lot of practical purposes. Think, for instance, that applications in shampoos and similar products almost always include silk proteins as adjuvants of hair state and health. Transfection, conversely, is quite appealing for biochemistry and molecular biology applications. In many aspects, thus, both fields of research are on the same line as those originally intended in the pre-Christian age. It is as if we were moving back to the roots of surfactancy. Luckily, we have much more knowledge in the field, and this allows us to exploit applications on more conscious grounds.

### **Author details**

Camillo La Mesa1 \* and Gianfranco Risuleo2

1 Department Chemistry, Sapienza University, Rome, Italy

2 Formerly at Department Biology, Sapienza University, Rome, Italy

\*Address all correspondence to: camillo.lamesa@uniroma1.it

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
