Dimeric and oligomeric surfactants. Behavior at interfaces and in aqueous solution: a review

https://doi.org/10.1016/S0001-8686(01)00069-0Get rights and content

Abstract

Dimeric and oligomeric surfactants are novel surfactants that are presently attracting considerable interest in the academic and industrial communities working on surfactants. This paper first presents a number of chemical structures that have been reported for ionic, amphoteric and nonionic dimeric and oligomeric surfactants. The following aspects of these surfactants are then successively reviewed the state of dimeric and oligomeric surfactants in aqueous solutions at concentration below the critical micellization concentration (cmc); their behavior at the air/solution and solid/solution interfaces; their solubility in water, cmc and thermodynamics of micellization; the properties of the aqueous micelles of dimeric and oligomeric surfactants (ionization degree, size, shape, micropolarity and microviscosity, solution microstructure, solution rheology, micelle dynamics, micellar solubilization, interaction between dimeric surfactants and water-soluble polymers); the mixed micellization of dimeric surfactants with various conventional surfactants; the phase behavior of dimeric surfactants and the applications of these novel surfactants.

Introduction

A new class of surfactants has recently appeared in the scientific literature. These surfactants are made up of two amphiphilic moieties connected at the level of the head groups or very close to the head groups by a spacer group, as schematically represented in Fig. 1a [1]. Such surfactants have been referred to as dicationic detergents [2] or bis-quaternary ammonium surfactants [3], for those containing two quaternary ammonium moieties, dimeric surfactants [4], gemini surfactants [5], and even siamese surfactants (by analogy with siamese twins) [6]. The current interest in such surfactants arises from three essential properties, which are illustrated by the surface tension vs. surfactant concentration plots in Fig. 2[7], and the viscosity vs. concentration plots in Fig. 3[8]:

(1) Dimeric surfactants are characterized by critical micellization concentration (cmc) that are one to two orders of magnitude lower than for the corresponding conventional (monomeric) surfactants. For instance, Fig. 2 shows that the cmc of the dimeric surfactants dimethylene-1,2-bis(dodecyldimethylammonium bromide) (12-2-12) is approximately 0.055 wt.% whereas that of the corresponding monomeric surfactant dodecyltrimethylammonium bromide (DTAB) is of 0.50 wt.%. Thus the ‘dimerization’ of DTAB results in a surfactant of much lower cmc.

(2) Dimeric surfactants are much more efficient than the corresponding monomeric surfactants at decreasing the surface tension of water. This efficiency is often characterized by the concentration C20, that is the surfactant concentration required for lowering the surface tension of water by 0.02 N/m. Fig. 2 shows that the values of C20 for 12-2-12 and DTAB are 0.0083 and 0.21 wt.%, respectively.

(3) Aqueous solutions of some dimeric surfactants with short spacers can have a very high viscosity at relatively low surfactant concentration whereas the solution of the corresponding monomer remains low viscous. The viscosity vs. concentration plots for 12-2-12 and DTAB in Fig. 3 illustrate this behavior. Solutions of dimeric surfactants can also be viscoelastic: such is the case of 12-2-12 solutions above 1.7 wt.%. They can also display shear-induced viscoelasticity. Thus, 12-2-12 solutions in the concentration range 0.7–1.7 wt.% become viscoelastic by simply shaking the flask in which they are contained [9]. All these properties reflect the ability of dimeric surfactants with short spacers to give rise to worm-like micelles at fairly low surfactant concentration, even in the absence of added salt [10].

The fact that the behavior of dimeric surfactants can differ much from that of conventional surfactants can be qualitatively understood as follows. In solutions of conventional surfactants the head groups are randomly distributed on the surface separating the aqueous phase and the micelle hydrophobic core. The distribution of distances between head groups is a maximum at a thermodynamic equilibrium distance dT (Fig. 4a), determined by the opposite forces at play in micelle formation. The reported values of the surface area per head group at interfaces suggest that dT is of approximately 0.7–0.9 nm. With dimeric surfactants the distribution of distances becomes bimodal. Indeed the head group distance distribution function then exhibits a maximum at the thermodynamic distance dT and another narrow maximum at a distance dS that corresponds to the length of the spacer (Fig. 4b). This length is determined by the number of atoms in the spacer and its conformation. The bimodal distribution of head group distances and the effect of the chemical link between head groups on the packing of surfactant alkyl chains in the micelle core are expected to strongly affect the curvature of surfactant layers, and thus the micelle shape and the properties of the solution. At the outset, it is noteworthy that the distance dS can be adjusted to be smaller, equal or larger than dT by modifying the structure of the spacer. Those different situations are expected to give rise to a rich variety of behaviors.

At this stage it must be stressed that it is essential that the spacer group connects the two amphiphilic moieties of a dimeric surfactant as close as possible to the head groups. Dimeric surfactants where the spacer connects the amphiphilic moieties in the middle of the alkyl chains are in fact bolaform surfactants with a branched alkyl chain (see Fig. 1b). They do not show the properties discussed above.

In addition to those properties dimeric surfactants appear to have better solubilizing, wetting, foaming, and lime-soap dispersing properties than conventional surfactants [11]. These properties are commonly used to evaluate surfactant performances. Besides, the Krafft temperatures of dimeric surfactants with hydrophilic spacers (see below) are generally very low [11], giving to these surfactants the capacity to be used in cold water. Last cationic dimeric surfactants have been shown to possess interesting biological activity [12].

The presence of the spacer group connecting the amphiphilic moieties permits the synthesis of dimeric surfactants with an enormous variety of structures. In principle, it is possible to synthesize a dimeric surfactant by using two of any type of the presently known amphiphiles, identical or different, and connecting them with a spacer group of varied chemical nature, hydrophilic or hydrophobic, rigid or flexible. Obviously this possibility opens the door to infinite variations in the surfactant chemical structure and, thus, of properties that cannot be achieved with pure (in the absence of additives) conventional surfactants. The term dimer is retained throughout this review even when the two amphiphilic moieties are not identical (such surfactants can be referred to as surfactant heterodimers).

Dimeric surfactants have been known in the patent literature since 1935. This literature has been recently reviewed [13] and is not dealt with here. To the best of our knowledge the first report of dimeric surfactants in the scientific literature is due to Bunton et al. in 1971 [2]. These authors synthesized quaternary ammonium bromide dimeric surfactants and studied how the micelles of these surfactants affect the rate of chemical reactions. The work of Bunton et al. was followed by that of Devinsky et al. [14] who synthesized bis-quaternary ammonium dimeric surfactants with a great variety of structures, and that of Zhu et al. [15] who synthesized a large number of anionic dimeric surfactants. These studies and other ones emphasized the low cmc, high efficiency at reducing the surface tension of water, and micelle structural characteristics of dimeric surfactants. The potential applications that these properties suggested led to a renewed interest of the industrial community in these surfactants. Many surfactant-producing companies have now ongoing research on dimeric surfactants. The company Condea (Marl, Germany) is already offering formulations (Ceralution®) based on anionic dimeric surfactants and that can be used as dispersants or emulsifiers, for foam production, etc.

Of course, the discovery of the very interesting and unexpected properties of dimeric surfactants led to the synthesis and investigation of the properties of even longer homologues that are referred to below as oligomeric surfactants [16]. Fig. 1c shows a trimeric surfactant. A limited number of surfactant trimers and one tetramer [17], [18], made of connected quaternary ammonium moieties have been synthesized. Their properties were found to be superior to those of the corresponding dimeric surfactants. Besides these surfactants are intermediate between conventional surfactants and polymeric surfactants (i.e. polymers with a repeat unit that is amphiphilic). Their study helped in understanding some properties of the latter.

Several reviews on dimeric surfactants have been published [1], [11], [13], [19], [20]. However, this topic is developing quite rapidly and it is hoped that this review will help newcomers in the field as well as those already involved in these novel surfactants.

This review is organized as follows: Section 2 presents selected examples of dimeric surfactants that illustrate the enormous variety of structures that have been already synthesized. Examples of oligomeric surfactants are given. Section 3 deals with aqueous surfactant dimer solutions at concentrations below the cmc, and discusses the conflicting views presented in the literature concerning these systems. Section 4 reviews the behavior of dimeric and oligomeric surfactants at interfaces. Section 5 deals with the solubility in water, cmc and thermodynamics of micellization of dimeric and oligomeric surfactants. Section 6 reviews the properties of aqueous micelles of oligomeric surfactant: micelle ionization degree, size, shape, micropolarity, microviscosity and dynamics, solution microstructure and rheology, solubilization and interaction with polymers. Section 7 summarizes the investigations of dimeric surfactant/conventional surfactant mixed micellar systems. Section 8 reviews the mesophases that have been reported to form in water/surfactant dimer mixtures. Section 9 deals with applications of dimeric surfactants.

This review attempts to be as thorough as possible. However, some papers on dimeric and oligomeric surfactants may not have come to my attention. Apologies to their authors.

Section snippets

Chemical structures of dimeric and oligomeric surfactants

Table 1 gives examples of structures of dimeric surfactants (AL) and of quaternary ammonium oligomeric surfactants (M, N) [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48].

The bis-quaternary ammonium halide surfactants (A1A9, I and J) have been the workhorse of the cationic dimeric surfactants because their synthesis and purification are relatively easy. The A1 surfactants,

Aqueous solutions of dimeric surfactants at concentrations below the critical micellization concentration

Many papers discussed the state of ionic dimeric surfactants at concentration below the cmc. Several situations were considered. First, the dimer may be completely disssociated, and give rise to one dimeric ion and two counterions, or not completely dissociated, with partial binding of one counterion to the dimeric ion. The binding equilibrium obeys the mass action law and the binding is expected to increase with the surfactant concentration. Second, depending on the conformation of the spacer

Air–solution interface

A large amount of measurements of surface tension of aqueous solutions of dimeric surfactants were reported [7], [14], [15], [18], [21], [22], [23], [24], [25], [26], [36], [37], [38], [39], [40], [42], [43], [44], [45], [55], [56], [58], [61], [71], [72], [77], [78], [79], [80], [81][82], [82](a), [82](b)[84], [85], [86], [87], [88], [93], [97], [98], [99], [100], [101], [102]. These studies aimed at assessing the efficiency and effectiveness of dimeric surfactants in reducing the surface

Solubility, Krafft temperature and cloud temperature of dimeric surfactants in water

Ionic dimeric surfactants with m≤12 are generally highly soluble in water. Krafft temperatures (TK) below 0°C have been reported for many series of anionic dimeric surfactants with hydrophobic or hydrophilic spacers, as for instance surfactants D and E[15], [36], [37], [38], [42], [78], [79], [80], [81], [82], [82](a), [82](b), [83]. The Krafft temperatures of trimeric surfactants with m<12 are also below 0°C [82], [82](a), [82](b). Such low values of TK permit the use of oligomeric surfactants

Micelle ionization degree

Data concerning the ionization degree α of micelles of ionic dimeric surfactants are scarce. The reported values were obtained from the variation of electrical conductivity, K, with surfactant concentration C, taking α=(dK/dC)C>cmc/(dK/dC)C<cmc[4], [6], [41] or from the analysis of small angle neutron scattering data (SANS) [22], [94], [125], [128], [129], [130], [131], [132].

Both techniques showed a significant increase of the micelle ionization degree upon increasing spacer carbon number for

Mixed micellization between dimeric surfactants and conventional surfactants

Studies of mixed micellization of dimeric surfactants with conventional surfactants were performed with the hope of observing synergism that would make the use of dimeric surfactants in formulations more attractive. Such studies were also performed for the sake of understanding mixed micellization in such systems, measuring micelle sizes and observing the microstructure of the mixed solutions. These various objectives were achieved by using several methods of investigation: surface tension,

Phase behavior

Most studies of phase behavior concerned the m-s-m surfactants [104], [183], [184]. No thermotropism was observed with the pure 12-s-12 surfactants, contrary to the corresponding monomers. This behavior was attributed to geometric constraints on the head group arrangement associated to the presence of the spacer. Thermotropism was observed with the longer 16-m-16 surfactants [184].

The effect of the spacer length was investigated for the 12-s-12/water mixtures. An X-ray study showed that

Applications and uses of dimeric surfactants

Dimeric surfactants m-s-m′ (A9) were used for the synthesis of organized mesoporous silica of cubic [192] and hexagonal symmetry [192], [193].

Gold particles were prepared by reduction of HAuCl4 in solution containing an excess of 12-2-12,2Cl with UV irradiation (see Fig. 23) [194]. The particles were polyhedral or fibrous depending on the HAuCl4 concentration. Another less well-characterized non-ionic dimeric surfactant, the Surfynol 465, was used to prepare colloidal silver [195] and gold

Conclusions

This review focused on the physico-chemistry of a novel class of surfactants, the oligomeric surfactants, made up of two or more amphiphilic moieties that are connected at the level of, or close to, the head groups by a spacer group of varied nature. Several interesting properties have been shown to result from this peculiar structure. In the field of applications the most interesting properties are doubtless the much lower cmc values, the stronger efficiency at decreasing the surface tension

References (206)

  • F Devinsky et al.

    J. Colloid Interface Sci.

    (1986)
  • B Rozycka-Roszak et al.

    J. Colloid Interface Sci.

    (1989)
  • F Marty et al.

    J. Fluor. Chem.

    (1990)
  • Y.-P Zhu et al.

    J. Colloid Interface Sci.

    (1993)
  • P Renouf et al.

    Tetrahedron Lett.

    (1998)
  • T Takemura et al.

    Langmuir

    (1999)
  • K. Jennings, I. Marshall, H. Birrell et al., Chem. Comm. (1998)...
  • M Okahara et al.

    J. Jpn. Oil Chem. Soc.

    (1988)
  • E Alami et al.

    Langmuir

    (1993)
  • R Zana
  • C.A Bunton et al.

    J. Org. Chem.

    (1971)
  • H.C Parreira et al.

    J. Am. Oil Chem. Soc.

    (1979)
  • R Zana et al.

    Langmuir

    (1991)
  • F.M Menger et al.

    J. Am. Chem. Soc.

    (1991)
  • M Frindi et al.

    Langmuir

    (1994)
  • A Espert et al.

    Langmuir

    (1998)
  • F Kern et al.

    Langmuir

    (1994)
  • V Schmitt et al.

    Europhys. Lett.

    (1995)
  • R Zana et al.

    Nature

    (1993)
  • M.J. Rosen, Chem. Technol. (1993)...
  • F Devinsky et al.

    Tensides Detergents

    (1985)
  • M.J Rosen et al.

    J. Surf. Detergents

    (1998)
  • Y Zhu et al.

    J. Am. Oil Chem. Soc.

    (1990)
  • R Zana et al.

    Uzbek J. Phys.

    (1999)
  • R Zana et al.

    Langmuir

    (1995)
  • M In et al.

    Langmuir

    (2000)
  • F.M Menger et al.

    Angew. Chem. Int. Ed.

    (2000)
  • E Fisicaro et al.

    Curr. Top. Colloid Interface Sci.

    (1997)
  • M.J Rosen et al.

    Langmuir

    (1999)
  • M Dreja et al.

    Langmuir

    (1999)
  • F.M Menger et al.

    Langmuir

    (2000)
  • F.M Menger et al.

    J. Am. Chem. Soc.

    (1993)
  • L Song et al.

    Langmuir

    (1996)
  • F Devinsky et al.

    Tensides Detergents

    (1990)
  • B Rozycka-Roszak et al.

    J. Colloid Interface Sci.

    (1996)
  • I. Huc, R. Oda, Chem. Comm. (1999)...
  • M Gaysinski et al.

    J. Fluor. Chem.

    (1995)
  • R. Oda, I. Huc, S.J. Candau, Chem. Comm. (1997)...
  • R Oda et al.

    Langmuir

    (1999)
  • F Duivenvoorde et al.

    Langmuir

    (1997)
  • S Dieng et al.

    Bull. Soc. Chim. Fr.

    (1997)
  • Y.-P Zhu et al.

    J. Jpn. Oil Chem. Soc.

    (1991)
  • Y.-P Zhu et al.

    J. Jpn. Oil Chem. Soc.

    (1993)
  • L Pérez et al.

    Langmuir

    (1996)
  • L Pérez et al.

    Langmuir

    (1998)
  • A Pinazzo et al.

    Langmuir

    (1999)
  • A Masuyama et al.

    J. Jpn. Oil Chem. Soc.

    (1992)
  • J Eastoe et al.

    Langmuir

    (1994)
  • J Eastoe et al.

    Langmuir

    (1996)
  • B.S Gallardo et al.

    Langmuir

    (1997)
  • Cited by (0)

    View full text