Importance of head group polarity in controlling aggregation properties of cationic gemini surfactants

Abstract

Cationic gemini surfactants have been extensively studied in the recent past and the effect of chain length, spacer length and nature on aggregation behavior has been examined. But the effect of variation in head group polarity on micellization has not been examined. Hence, the effect of head group polarity of the butane-1,4-bis(dodecyldimethylammonium bromide) surfactants on aggregation properties is studied through conductance, surface tension, viscosity, and small-angle neutron scattering (SANS) measurements. The critical micellar concentration (cmc), average degree of micelle ionization (β_ave), minimum area per molecule of surfactant at air–water interface (A_min), surface excess concentration (Γ_max) and Gibbs free energy change of micellization (ΔG°_mic) of the surfactants were determined from conductance and surface tension data. The aggregation numbers (N), dimension of micelle (b/a), effective fractional charge per monomer (α) were determined from SANS and hydration of micelle (h_m) from viscosity data. The increasing head group polarity of gemini surfactant having spacer chain length of 4 methylene units promotes micellar growth, leading to decrease in cmc, β_ave, ΔG°_mic and increase in N and b/a. This is well supported by the observed increase in hydration (h_m) of micelle with increase in aggregation number (N) and dimension (b/a) of micelle. The Kraft temperature (k_T), foamability and foam stability as a function of head group polarity of gemini surfactants were also examined.

Introduction

Surfactants are among the most versatile materials appearing in diverse products such as motor oils, pharmaceuticals, detergents and petroleum and as flotation agents for beneficiation of ores [1]. The last decade has seen the extension of surfactants applications to high-technology areas such as electronics, printing, magnetic recording, biotechnology and microelectronics. Surfactants are consumed worldwide in large quantities every day. As a result, demands for high-performance surfactants is continuously increasing, since decreasing the quantity of surfactants used can contribute in reducing the load on the natural purification system. Accordingly, novel surfactants have been successfully designed and developed [2], [3], [4], [5]. Although cationic surfactants comprise only a small portion of the surfactant market, their importance in practical applications continues to grow. They are used as antibacterials [6], [7], liquid crystals [8], gene transfection agents [9], in road repair [10], for mineral flotation [11], for protection of metals from corrosion [12], [13], as phase transfer catalysts in regioselective addition reactions [14], [15] and in preparation of crystalline mesoporous materials [16], [17].

Classical surfactant molecules are generally composed of two distinct parts: one polar head and one alkyl chain, incompatible with one another. They are known for their tendency to self-associate and give rise to well developed superamolecular assemblies, called micelles. The micelles formed are of various types, shapes and sizes, such as globular, cylindrical and spherical. The characteristics of these aggregates are governed by the molecular structure of the surfactant molecules as well as by physical parameters such as concentration, temperature and ionic strength [18], [19], [20], [21], [22]. Hence, attempts to design newer molecular structures led to the preparation of new generation surfactants such as geminis. A conventional surfactant has a single hydrophobic tail connected to an ionic or polar head group, whereas a gemini has in sequence a long hydrocarbon chain, an ionic group, a spacer, a second ionic group and another hydrocarbon tail. A schematic representation of a gemini is shown in Fig. 1.

Search for a newer surfactant with greater surface activity gave birth to bis-surfactants, which were later termed as “gemini” surfactants. Geminis are considerably more surface active than conventional monomeric surfactants. All geminis possess at least two hydrophobic chains and two ionic or polar head groups and spacers with different natures [23], such as short or long flexible chains of methylene groups, rigid (stilbene), polar (polyether) and nonpolar (aliphatic, aromatic) groups. The ionic group can be positive (ammonium) or negative (phosphate, sulfate, carboxylate) whereas the polar nonionics may be polyether or sugar. The great majority of geminis have symmetrical structures with two identical polar groups and two identical chains. Some unsymmetrical geminis and geminis with three or more polar groups or tails have recently been reported [24], [25], [26], [27], [28]. The general molecular formula of bis-quaternary surfactants is C_nH_2n+1N⁺(CH₃)₂–(CH₂)_s–N⁺(CH₃)₂C_nH_2n+1, 2Br⁻ and are referred as m–s–m DMA (DMA = dimethyl ammonium bromide) surfactants [29] These surfactants, because of their unique solution properties such as very low cmc, high detergency, high solubilization and high surface wetting capability, possess a wide range of applications in diverse areas such as mining, petroleum, chemical, pharmaceutical industries and biochemical research [30] and as catalysts in several organic and inorganic reactions [31]. They are also used as preservatives [32], anticorrosives [33], [34] and antimicrobial agents [35], [36]. Currently, these gemini surfactants have attracted attention especially as potential gene delivery agents [37], [38] and erythrocyte protectors against hypotonic heamolysis [39]. These gemini surfactants are also used in the synthesis of new mesoporous zeolites MCM-41 and MCM-48 [40], [41], [42].

In the early 1970s, Bunton et al. [30], [43], [44] were the first to report the synthesis of bis-quaternary surfactants and their performance properties in relation to mono-quaternary surfactant as catalysts in several organic reactions. Later, Devinsky et al. reported on the surface activity and micelle formation of some new bis-quaternary ammonium surfactants [45] and Okahara et al. synthesized and examined amphipathic compounds with two sulfate groups and two lipophilic alky chains [46]. In 1991 Menger et al. [47], [48], [49], assigned the term “gemini” to these bis-surfactants having a rigid spacer such as benzene or stilbene. The term was then extended to other bis or double-tailed surfactants, irrespective of the nature of the spacer. Furthermore, Menger et al. [50] examined the effect of heterocyclic head group and acetylenic spacer on the aggregation properties of cationic geminis.

In 1996 Rosen et al. [51], [52] reported on the micellization and premicellization behavior of a series of biscationic surfactants with rigid hydrophobic and flexible hydrophilic spacers. A higher aggregation tendency was reported for a gemini surfactant with flexible hydrophilic spacer than gemini with rigid hydrophobic spacer. Dynamic properties of a gemini surfactant with rigid hydrophobic spacer showed shorter induction time and slower rate of fall in surface tension [53]. The mesomorphic behavior of the gemini surfactants (12-s-12 DMA) has been reported by Alami et al. [54]. The concentration range of lyotropic mesophase was observed to decrease as spacer chain length increased and completely disappeared when spacer length reached 10 to 12 carbon atoms where micelles were present at all compositions up to 90% of surfactant. However, lyotropic mesophases were reported to reappear when spacer length reached 16 carbon atoms.

Zana et al. [55], [56], [57], [58] investigated ion paring and premicellar association of a series of gemini surfactants (m–s–m). Significant ion paring was reported below the cmc for the geminis with short alkyl chain, m ≤ 10, mainly due to the high cmc values of these surfactants. The 12–s–12 DMA surfactants with s ≤ 10 showed neither ion paring nor premicellar association. Premicellar association was observed in solutions of dimeric surfactants with m ≥ 14, having either an octanediyl or p-xylyl spacer. Zana et al. have also reported, influence of change in spacer length, nature (hydrophilic/hydrophobic) and its flexibility on performance properties of gemini surfactants [59], [60], [61].

The influence of the nature of the spacer on the aggregation properties of gemini surfactants in an aqueous solution is also reported by Wang et al. [62]. This study has shown that the nature of the spacer has an important effect on the aggregation properties of gemini surfactants in an aqueous solution. A hydrophilic, flexible spacer promotes micelle formation, which leads to a smaller cmc, smaller average micellar ionization (α), larger aggregation number (N) and more negative ΔG°_m. The micro-viscosity study indicates that a gemini surfactant with a hydrophilic, flexible spacer forms a more closely packed micelle structure than the one with a hydrophobic, rigid spacer.

Recently Wettig et al. [63], [64] have reported a comprehensive study of the aggregation and thermodynamic properties of the 12–4–12 DMA (DMA = dimethylamine) gemini surfactants with one hydroxyl group substituted in one or two of the methylene spacers. It has been shown that the aggregation and thermodynamic properties of gemini surfactants having alkyl tail of fixed carbon atom length depend on the conformation of the spacer group at the micelle/water interface. They have also reported that the aggregation and thermodynamic properties of gemini surfactants (12–EO_x–12 DMA, x = 0–3) having several ethylene oxide moieties within the spacer depend on the hydration and conformation of the spacer groups at the micelle/water interface [65].

Holmberg et al. [66] studied the kinetics of micellization of gemini surfactant by measuring the relaxation time τ₂ for micelles. The micellar life times for the gemini surfactants were found to be an order of magnitude longer than that for the conventional monomeric surfactants. Halder et al. reported the aggregation properties of novel cationic surfactants with multiple pyridinium head groups, through SANS and observed that aggregation number (N) dramatically decreased and fractional charge (α) increased with increase in the number of head groups [67], [68]. However, the effect of variation in the head group polarity on the properties of cationic gemini surfactants, which can offer interesting physicochemical properties, has not been systematically studied so far. Hence, an attempt has been made to study the physicochemical properties of the cationic gemini surfactants with variable head group polarity.

Solutions of surfactants forming exotic structures and mesophases have been examined with electron microscopy at cryogenic temperatures [69]. Though this technique provides absolute information regarding the shape and size of the aggregates formed, no information regarding the aggregation number and immediate micellar environment can be obtained. Small angle neutron scattering (SANS) is a well-established technique for the study of micellar solutions of surfactants [70], [71], [72], [73], [74] which can give information about the micelle size, shape, charge and aggregation number through the use of a proper model [75]. Alkanediyl-α,ω-bis (dimethylhexadecyl/decyl ammonium bromide) types of gemini surfactants, referred to as 16/10–s–16/10 DMA, which contain –N(CH₃)₂ head groups, have been studied in detail [76], [77], [78], [79], [80]. The aggregate structures of these surfactant solutions as studied by SANS have also been reported [81], [82], [83], [84], [85], [86], [87], [88].

Recently we have reported on the effect of spacer and alkyl chain length on aggregation and performance properties of novel gemini surfactants with hydroxylated quaternary heads (12–s–12 MEA, 16–s–16 MEA, MEA = monoethanol amine) and observed that a surfactant with a spacer of four methylene units shows a high aggregation tendency [89], [90], [91], [92]. Hence, a systematic study has been undertaken to examine the effect of varying the head group polarity on physicochemical properties of bis(dodecyldimethylammonium bromide) surfactant with four methylene spacer chain through conductance, surface tension, viscosity and small angle neutron scattering (SANS) measurements. The chemical structures of the surfactants studied are illustrated in Fig. 2.