Colloids and Surfaces A: Physicochemical and Engineering Aspects
Self-organization and solubilization properties of gemini hydrotropic compounds in aqueous solution
Graphical abstract
Introduction
Surfactants, as amphiphilic compounds, after reaching critical micelle concentration (CMC), aggregate into micelles in aqueous solutions. Although the definition identifies micelles as aggregates of amphiphilic molecules of colloidal dimensions [1], micelles are considered as organized structures comprising at least several dozens of molecules [2].
Gemini surfactants and their aggregation properties have been a subject of great interest since the late 1990s [3], [4], [5]. A large number of gemini structures inspired by conventional surfactants is currently being synthesized. The specific structure is made up of two amphiphilic units connected by a spacer group and described as m-s-m, where m is the amount of carbon atoms in the tail and s is the amount of carbon atoms in the spacer. Extensive and systematic research on cationic gemini surfactants was presented by Zana [6] and Pisarcik [7].
Geminis exhibit very good surface activity and noteworthy ways of aggregation in aqueous solutions. In general, they easily aggregate into micelles of various shapes and sizes, also at very small concentrations. Besides classical spherical micelles [6], worm-like micelles [8] and vesicles [9] are often detected; rarely, multivesicular structures [10] are formed. The aggregation of gemini surfactants is affected by many factors, of which, besides the general chemical structure, a significant role is played by the length and chemical structure of the spacer [11], [12]. Good surface activity and interesting aggregation properties give gemini surfactants a promising application potential [5], [13].
The group of gemini surfactants comprising cleavable amide bonds in various places within their structure attracted particular attention [14], [15], [16], [17], [18]. The amide bond is not only responsible for biodegradability of surfactants but also affects their aggregation in aqueous solution, although there is no consensus in the literature about their role in aggregation. In some cases its presence propagated large micelle formation [14], [15], but for other gemini surfactants with amide bonds, only small assemblies were detected [17]. Hoque et al. [14], studying the influence of amide groups in the hydrophobic tail and both in the tail and the spacer, observed enhanced micelle formation tendency compared to non-amide analogues and related it to intermolecular hydrogen bonding.
Hydrotropes are amphiphilic compounds, composed of a hydrophilic head and hydrophobic tail, known for their solubilizing activity of hydrophobic compounds in water. Weak hydrophobicity or spatial restrictions are suspected to prevent the formation of micelles by hydrotropes. Instead of micelles, hydrotropes form only small assemblies [19], [20]. Most hydrotropes have a short hydrophobic part, but those with long alkyl tails were also described [21]. As in surfactants, hydrotropes are surface active and capable of solubilization of hydrophobic compounds in water. Above the critical aggregation concentration (CAC), they exhibit surface and solubilization activity in partially aggregated form. CAC is a threshold concentration for solubilization minimum hydrotrope concentration (MHC) [22]. For now, there are no references about hydrotropes comprising two or more amphiphilic units.
Hydrotropes have the ability to increase the solubility of hydrophobic substances in water by several orders of magnitude [23], [24. In general, they are applied as solubilizers in detergents [23] and pharmaceutics [25]. They also exhibit synergistic effects, when mixed with surfactants [26] or amphiphilic block copolymers [27]. The practical use of hydrotropes is limited by relatively high MHC. A typical value is 0.1 M [28], which is substantially higher than the CMC of surfactants.
Although hydrotropes are widely used in industry, they have not aroused great scientific interest. In contrast to surfactants, their solution behaviour still remains not fully understood. Actually, there is no clear agreement in the literature whether hydrotropes are a separate class of compounds distinct from surfactants [28]. Balasubramanian et al. [24] first noticed the surface activity of hydrotropes and indicated the differences between hydrotrope assemblies and classical surfactant micelles, pointing out that hydrotropes do not necessarily aggregate in an on-off mode as surfactants do, but associate stepwise after reaching MHC. Hydrotropic compounds are the subject of reviews summarizing similarities and differences in relation to the behaviour of surfactants [22], [29] and discussing advantages of hydrotropic solubilization [30].
This article describes the surface activity, aggregation and solubilization properties of a group of gemini compounds comprising two hydrophilic quaternary ethylammonium headgroups and two hydrophobic tails with 14 carbon atoms, with an amide bond placed in-between the headgroup and the tail. The headgroups are connected with an alkyl spacer of varying length. Observation, typical for gemini surfactants, of high surface activity and easy formation of aggregates of different shapes and sizes in aqueous solution was expected. However, the measurements revealed behaviour untypical for gemini surfactants. Herein, we reported this behaviour and related it to hydrotropes. The influence of the spacer length on aggregation was described. Data obtained for gemini compounds were compared with those of their single-tail analogue.
Section snippets
Materials
Myristoyl chloride (>98%, Aldrich), 3-diethylamine-1-propylamine (>98%, Merck), bromoethane (>98%, Aldrich), 1,3-dibromopropane (>98%, Aldrich), 1,4-dibromobutane (>98%, Aldrich), 1,6-dibromohexane (>98%, Aldrich), 1,8-dibromooctane (>98%, Aldrich), 1,10-dibromodecane (>98%, Tokyo Chemical Industry) 1,12-dibromododecane (>98%, Tokyo Chemical Industry), acetonitrile (pure p.a., POCH), ethyl acetate (pure p.a., POCH), hexane (pure p.a., POCH), acetone (pure p.a., POCH), pyrene (>98%, Aldrich),
Synthesis
Gemini compounds and their single-tail analogue were synthesized according to Ghumare et al. [16] by quaternization of the amidoamine as presented in Fig. 1. The obtained structures were composed of two amphiphilic units connected by an alkyl spacer: the quaternary ethylammonium headgroups and a long alkyl tail, linked to the headgroups with an amide bond. Obtained compounds were described as 14-s-14, where 14 is the number of carbon atoms in the tail including the carbonyl carbon, and s is the
Conclusions
A series of amphiphilic compounds containing amide bonds close to the hydrophilic headgroup, structurally related to gemini surfactants, were synthesized by quaternization of the corresponding amidoamine. The studied gemini compounds were more effective in lowering the surface tension of water, had significantly lower CAC values and much better solubilization properties than their single-tail analogue. The aggregation behaviour of the investigated gemini compounds differed to that of the
Acknowledgements
Julia Woch was a recipient of a Ph.D. scholarship founded by the European Social Fund. The authors acknowledge Marcin Libera (CMPW PAN, Zabrze, Poland) for cryo-TEM measurements.
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