Aggregation behavior of anionic sulfonate gemini surfactants with dodecylphenyl tails

Accepted Manuscript Title: Aggregation behavior of anionic sulfonate gemini surfactants with dodecylphenyl tails Author: Zofia Hordyjewicz-Baran Julia Woch Edyta Kuliszewska Jolanta Zimoch Marcin Libera Andrzej Dworak Barbara Trzebicka PII: DOI: Reference:

S0927-7757(15)30156-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.08.012 COLSUA 20103

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

27-4-2015 3-8-2015 6-8-2015

Please cite this article as: Zofia Hordyjewicz-Baran, Julia Woch, Edyta Kuliszewska, Jolanta Zimoch, Marcin Libera, Andrzej Dworak, Barbara Trzebicka, Aggregation behavior of anionic sulfonate gemini surfactants with dodecylphenyl tails, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.08.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Aggregation behavior of anionic sulfonate gemini surfactants with dodecylphenyl tails Zofia Hordyjewicz-Baran a,*[email protected] Julia Woch a Edyta Kuliszewska a Jolanta Zimoch a Marcin Libera b Andrzej Dworak b Barbara Trzebicka b a

Institute of Heavy Organic Synthesis “Blachownia”, Energetykow 9, 47-225 KedzierzynKozle, Poland Institute of Heavy Organic Synthesis “Blachownia”, Energetykow 9, 47-225 KedzierzynKozle, Poland b

Polish Academy of Sciences, Centre of Polymer and Carbon Materials, M. CurieSklodowskiej 34, 41-819 Zabrze, Poland Polish Academy of Sciences, Centre of Polymer and Carbon Materials, M. CurieSklodowskiej 34, 41-819 Zabrze, Poland *Corresponding author. Graphical abstract Highlights    

Anionic sulfonate gemini surfactants with dodecylphenyl tails and oligomethylene spacers of different lengths were obtained. Their physicochemical properties and aggregation behavior in aqueous solution were described. The spontaneous formation of multivesicular structures by gemini surfactants was demonstrated. The properties of the gemini surfactants were compared to those of their single-tail analog.

Abstract A series of anionic sulfonate gemini surfactants with the general structure [(C12H25)(C6H3SO3)O(CH2)mO(C6 H3SO3-)(C12H25)]2Na+, designed as Sm, where m = 2, 4, 6 and 8 denotes the number of carbon atoms in the oligomethylene spacer, was synthesized, and the surfactants’ physicochemical properties and aggregation behavior were characterized. Compared with their single-tail analog, the gemini surfactants were observed to be more effective in decreasing the surface tension and their critical micelle concentrations (CMC) were at least two orders of magnitude lower. The aggregation process of the gemini surfactants in aqueous solution was investigated by dynamic light scattering (DLS). The formation of multivesicular structures consisting of several small vesicles surrounded by vesicular membrane was observed by cryogenic transmission electron microscopy (cryo-TEM). *Corresponding

author E-mail address: [email protected]

Keywords: gemini surfactants, CMC, aggregation, multivesicular structures

Highlights Anionic sulfonate gemini surfactants with dodecylphenyl tails and oligomethylene spacers of different lengths were obtained. Their physicochemical properties and aggregation behavior in aqueous solution were described. The spontaneous formation of multivesicular structures by gemini surfactants was demonstrated. The properties of the gemini surfactants were compared with those of their single-tail analog.

1. Introduction Surface active agents are commonly used products of chemical synthesis. Increases in living standards and industrial development have led to an increase in the demand for surfactants. However, the need to protect the environment has imposed restrictions on the use of additional quantities of chemicals. Therefore, scientists are interested in designing and developing new surfactants that are environmentally friendly, exhibit improved properties and carry reasonable production costs. In recent years, new amphiphilic surface active agents called gemini surfactants have been reported in the literature (Fig. 1). [FIGURE 1, SINGLE] Gemini surfactants consist of two amphiphilic molecules linked covalently by a spacer at the head groups, or very closed to them. Typical gemini surfactants exhibit higher surface activity than their corresponding single-tail analogs with the same head group and tail length [1, 2]. Moreover, the critical micelle concentration (CMC) of gemini surfactants is up to two orders lower than that of their traditional analogs [3, 4]. These properties provide an opportunity to attain surfactants with the same efficiency using much smaller amounts of chemicals. In recent years, many scientific studies have focused on the use of gemini surfactants in many industrial fields, such as those pertaining to detergents, cosmetics, pharmaceuticals, metallurgy, textiles, food processing, petrochemistry and mining [5-7]. The properties of gemini surfactants depends on the type and length of the spacer, the structure of the hydrophobic tail and the hydrophilic head group [8]. Various gemini surfactant structures can be built. The spacer can be hydrophilic or hydrophobic, flexible [9] *Corresponding

author E-mail address: [email protected]

or rigid [10]. The tails of gemini surfactants are typically constructed from a hydrophobic aliphatic or aromatic chain, whereas the polar head group consists of ionic or non-ionic chemical groups, such as tertiary amines [11-13], carboxylates [14], sulfates [15], sulfonates [16], and phosphonates [17]. Information concerning gemini surfactants can be find in the excellent reviews of Rosen [18], Menger and Littau [3], and Zana [19]. Bis quaternary ammonium salts are the most popular and widely studied cationic gemini surfactants [20-25]. Anionic surfactants are much less frequently studied compared with cationic compounds. The most popular anionic gemini surfactants are those that contain sulfonate groups because of their applicability in household chemicals and tertiary oil recovery [16]. Their high solubility in the presence of various ions as well as over a wide range of pH levels enhances their possibility of application. Similarly, the low Krafft point of these surfactants allows them to be used in cold water [16, 26]. In recent years, studies on sulfonate gemini surfactants have been reported. Du et al. [27] reported on the physicochemical properties, surface tension, Krafft temperatures and melting temperatures of dialkyldibenzene disulfonate gemini surfactants with spacer lengths of four or six methylene units and with 6, 8, 10, 12 or 14 hydrocarbon units in the tails. The spacer length was observed to have a stronger effect than the alkyl tail length on the Krafft point and melting temperature. Another study involving a gemini surfactant with a spacer length of four methylene units and 14 hydrocarbon units in the tail [28] showed that the surfactant possessed a high viscosity in solution at a relatively low concentration. This high viscosity was related to the occurrence of a saturated micelles network (multiconnected or threadlike branched), as observed by TEM. The surface properties of sulfonate gemini surfactants with nonylphenol tails and oligomethylene spacers of different lengths were determined by Zhu et al. [29]. A decrease in the CMC and an increase in the aggregation number as a result of spacer lengthening were reported. The studied compounds effectively reduced the surface tension and showed lower CMCs compared with their single-tail analogs. The occurrence of a liquid crystal phase was also demonstrated. Liu et al. [9] synthesized sulfonate gemini surfactants with hydrophobic tails and with one, two, three and four oxyethylene units in the spacer. The CMC decreased with the increase in spacer length. In addition, for gemini surfactants with four oxyethylene units in the spacer, the CMC decreased with an increase in alkyl tail length from 8 to 12 hydrocarbon units. Cao et al. [30] observed that anionic sulfonate gemini surfactants with one, two and three oxyethylene groups as a spacer exhibited a lower CMC and a higher aggregation number than a surfactant with a spacer formed by one ethylene unit. *Corresponding

author E-mail address: [email protected]

Zhu et al. [31] studied the effect of spacer type on the aggregation of sulfonate gemini surfactants; the authors examined an ethylene unit as a flexible hydrophobic spacer; an oxyethylene unit as a flexible hydrophilic spacer; and a vinylene unit with a carbon-carbon double bond as a rigid hydrophobic spacer. The authors showed that the gemini surfactants containing a hydrophilic flexible spacer with an affinity for forming hydrogen bonds and high conformational freedom exhibited a greater capacity for micellization compared with the gemini surfactants with a hydrophobic rigid spacer. Although the physicochemical properties of sulfonate gemini surfactants have been well characterized, the morphology of aggregates has typically not been illustrated. The structures of aggregates have been investigated more systematically for cationic gemini surfactants. Zana et al. [32] observed thread-like micelles for 12-2-12 cationic gemini surfactants, whereas densely packed spheroidal micelles were observed for 12-4-12, 12-8-12, and 12-12-12 surfactants. As reported by Danino et al. [33], an increase in spacer length leads to the formation of vesicles by 12-16-12 and 12-20-12 surfactants. The formation of surfactants’ vesicles has also been reported in mixed cationic-anionic systems [34, 35]. The formation of vesicles by anionic gemini surfactants in aqueous solution has rarely been observed. Zhu et al. [36] observed that sulfonate gemini surfactants with long, fully rigid spacers consisting of two rigid benzene rings connected to two rigid carbonyl groups prefer to form vesicles, whereas those with one or two flexible methylene units placed between the benzene rings aggregate into micelles and vesicles at a low surfactant concentration and mainly into vesicles at a high surfactant concentration. The vesicles formed by gemini surfactants can provide simple models for biological membranes [37], nanoparticle templates [38], micro-reactors [39] and drug carriers [40]. We previously described the properties of sulfonate gemini surfactants with dodecylphenyl tails linked by a hydrophilic spacer consisting of one, two or three ethylene oxide units [41]. An increase in surface tension and CMC as a result of increasing the spacer length was observed. In this work, we synthesized sulfonate gemini surfactants with dodecylphenyl tails and hydrophobic oligomethylene spacers of different lengths. The effect of surfactant spacer length on the aggregation behavior of the surfactants, i.e., CMC and vesicle formation, is herein described. The surfactants’ physicochemical properties, including their density, viscosity and surface tension, are also reported.

*Corresponding

author E-mail address: [email protected]

2. Materials and methods 2.1.

Materials

Dichloromethane (Aldrich) was dried with calcium chloride and distilled prior to use. Dodecylphenol (99.8%) was obtained from the Institute of Heavy Organic Synthesis "Blachownia" (Poland). Sodium dodecylbenzenesulfonate (SDBS) was obtained from PCC Rokita (Poland). Dibromoethane (Aldrich), dibromobutane (Aldrich), dibromohexane (Aldrich), dibromooctane (Aldrich), tetrabutylammonium bromide (Merck KGaA), calcium chloride (POCH), chlorosulfonic acid (Acros Organic) and ethanol (POCH), all of analytical grade, were used without additional purification. 2.2. Synthesis of gemini surfactants The reaction between dibromide 1 Br(CH2)mBr (where m refers to the number of carbon atoms in molecules) and 4-dodecylphenol was carried out in the presence of a phase transfer catalyst (Figure 2). 4-Dodecylphenol (23.0 mmol) and tetrabutylammonium bromide (1.1 mmol) were placed in a three-neck flask; then, 20 mL of aqueous sodium hydroxide solution (15%) was added. The reaction mixture was heated to 90 °C, and then dibromide 1 (13.0 mmol) was added in two portions. The reaction mixture was heated under refluxing for 24 h and then concentrated. The residue was extracted three times with diethyl ether. The organic fraction was dried over anhydrous magnesium sulfate. After filtering off the drying agent, diethyl ether was distilled off under reduced pressure to yield the diether 2, which then was sulfonated to introduce sulfonic functional groups and to obtain an anionic gemini surfactant. Ten mmol of diether 2 was introduced into 15 mL of dichloromethane and sulfonated with chlorosulfonic acid (22.0 mmol) at 0 °C. The reaction was completed after 5 h by neutralizing the substituted sulfonate group with a 5% solution of sodium hydroxide in ethanol. The product was purified by recrystallization from ethanol to yield colorless crystals of gemini surfactant S2. All syntheses of other gemini surfactants were carried out using the same method described above for S2. Gemini surfactants were further denoted as Sm, where m refers to the number of carbon atoms in the oligomethylene spacer. The yields of the reactions are presented in Table 1. [FIGURE 2, 1.5] [TABLE 1] 2.3. Experimental 2.3.1. 1H-NMR *Corresponding

author E-mail address: [email protected]

The chemical structures of the obtained gemini surfactants were confirmed by proton nuclear magnetic resonance spectroscopy (1H-NMR). The spectra were recorded on an Ultrashield NMR 400 MHz spectrometer (Bruker, US). The resonances are presented in ppm with respect to tetramethylsilane (TMS), which was added to the solvent (CDCl3) as a signal standard. The chemical shifts for all obtained surfactants are summarized in Table 2. [TABLE 2] 2.4. Density The densities of 0.5 wt% aqueous solutions of gemini surfactants were measured using a Kruss K100 tensiometer (Krüss, Germany) based on the buoyancy of a solid probe in liquid. The mass of liquid displaced by the measuring probe corresponds precisely to the difference in weight. If the density of the probe is known, then the density of the liquid can be obtained by differential weighing (1):

 L   MK

G MKA  G MKL G MKA

(1)

where L is the density of the liquid [g/cm3], MK is the density of the measuring probe [g/cm3], GMKA is the weight of the measuring probe in air [g], and GMKL is the weight of the measuring probe in liquid [g].

2.5. Dynamic viscosity Viscosity measurements were carried out for 0.5 wt% aqueous solutions of gemini surfactants, at room temperature using a Hoppler viscometer (VEB MLW Prüfgeräte – Werk Medingen, Germany). The viscosity was calculated based on the following formula:

  A   K   C   t (2) where K is the density of balls of the target material [g/cm3 ], C is the density of the investigated liquid [g/cm3], t is the fall time of the balls [s] and A is a viscosimeter constant [mP·s·cm3/g·s].

2.6 Surface tension measurements The surface tension of aqueous solutions of gemini surfactants was determined using a Krüss 100 K tensiometer (Krüss, Germany) equipped with a Wilhelmy plate at 25 C. The system records the force required to break the plate away from the liquid surface. The surface tension was then calculated from the formula

*Corresponding

author E-mail address: [email protected]



F L  cos

(3)

where σ is the surface tension [N/m], F is the maximum force acting on the plate [N], L is the wetted length [m] and θ is the wetting angle. Assuming that the plate is completely wetted, the contact angle θ is equal to 0, which means that the cosθ value is equal to 1. Therefore, the measurement of the surface tension is affected only by the value of the measured force and the wetted length.

2.7 Critical micelle concentration (CMC) The CMC of gemini surfactants in aqueous solution was determined based on tensiometry and conductivity measurements.

2.7.1. Tensiometry The CMC of aqueous solutions of gemini surfactants was determined by a Krüss 100 K tensiometer (Krüss, Germany) equipped with a Wilhelmy plate at 25 C. The measurements began with a highly concentrated aqueous solution, which was then diluted stepwise by the addition of water. The serial dilutions were prepared automatically from an initial stock solution of the surfactant. The CMC was determined from the graph of surface tension versus concentration. The breakpoint in the curve was taken as the CMC value. The surface activity at C20, the surfactant concentration required to reduce the surface tension of water by 20 mNm-1, as well the surface tension at CMC (CMC) were also determined from the same curve.

2.7.1. Conductivity Conductivity measurements were carried out with a Schott CG 853 (Schott, Germany) conductivity meter. Samples of surfactant solutions were prepared by diluting the stock solution. The CMC values of surfactant solutions were determined from the graph of specific conductance versus surfactants concentration at the breakpoint of the curve.

2.8. Dynamic light scattering (DLS) Dynamic light scattering (DLS) measurements were carried out on a Brookhaven BI-200 goniometer with vertically polarized incident light (wavelength =632.8 nm) supplied by a He-Ne laser operating at 35 mW and a Brookhaven BI-9000 AT digital autocorrelator. The autocorrelation functions were analyzed using the constrained regularized algorithm *Corresponding

author E-mail address: [email protected]

CONTIN. The measurements were performed at an angle of 90°. The distribution of particle sizes was given as

2 , where  is the average value of the relaxation rates Γ and μ2 is the 2

second moment. The values were obtained from cumulant analysis. DLS measurements were performed for aqueous solutions of the gemini surfactants at concentrations above the CMC values, as determined by the tensiometry method. For the DLS measurements, three different concentrations were used: 50 CMC, 100 CMC and 200 CMC, which correspond to 50, 100 and 200 times the CMC of the gemini surfactants, respectively, as determined by tensiometry. Before DLS analysis, the gemini surfactant solutions were filtered through membrane filters with a nominal pore size of 0.2 µm (ANATOP 25 PLUS, Whatman).

2.9. Cryogenic transmission electron microscopy (cryo-TEM) Cryogenic transmission electron microscopy (cryo-TEM) images were obtained using a Tecnai F20 TWIN microscope (FEI Company, USA) equipped with a field emission gun operated at an acceleration voltage of 200 kV. The images were recorded on an Eagle 4k HS camera (FEI Company, USA) and processed with TIA software (FEI Company, USA). Specimens were prepared by vitrification of the aqueous solution on grids with holey carbon film (Quantifoil R 2/2; Quantifoil Micro Tools GmbH, Germany). Prior to use, the grids were activated for 15 seconds in oxygen/argon plasma using a Fischione 1020 plasma cleaner (E.A. Fischione Instruments, Inc., USA). The samples were prepared by applying a droplet (2.1 µL) of the solution to the grid, blotting with filter paper and immediately freezing the droplet in liquid ethane using a Vitrobot Mark IV fully automated blotting device (FEI Company, USA). After preparation, the vitrified specimens were stored in liquid nitrogen until they were inserted into a Gatan 626 cryo-TEM holder (Gatan Inc., USA) and analyzed at -178 °C. The gemini surfactant solutions used in cryo-TEM were prepared in the same way as those submitted to DLS measurements.

3. Results and discussion 3.1.

Synthesis

Anionic sulfonate gemini surfactants with dodecylphenyl tails and oligomethylene spacers were synthesized by modifying the method presented by Zhu [31] for nonylphenyl gemini surfactants. The reaction of dibromides with 4-dodecylphenol was carried out much longer (24 h) than in the procedure used by Zhu for gemini surfactants synthesized from nonylphenol (2 h). Dibromides with various numbers of carbon atoms in the molecules (m=2, 4, 6, 8) were *Corresponding

author E-mail address: [email protected]

reacted with 4-dodecylphenol in the first step, resulting in diethers with a hydrophobic oligomethylene spacer. In the second step, diethers were sulfonated with chlorosulfonic acid and then neutralized by NaOH to yield the end products – gemini surfactants Sm. The chemical structures of the gemini surfactants were confirmed by 1 H-NMR spectroscopy, as described in the experimental section.

3.2.

Density, viscosity and surface tension of aqueous solutions of anionic gemini

surfactants The density, viscosity and surface tension of 0.5 wt% aqueous solutions of the gemini surfactants were measured. The results of the measurements are summarized in Table 3 and compared with the corresponding values for sodium dodecylbenzenesulfonate, a single-tail counterpart of the gemini surfactants S m. [TABLE 3] The density and viscosity of the gemini surfactants were lower than those for SDBS only in the case of gemini surfactant S2 containing the shortest spacer. The surface tension of aqueous solutions of all obtained gemini surfactants was lower than that of the SDBS solution. The increase in the hydrophobic spacer length of the gemini surfactants resulted in a decrease in surface tension and an increase in the densities and viscosities of the surfactants’ aqueous solutions.

3.3.

Critical micelle concentration (CMC)

The CMCs were determined using the surface tension and conductivity measurements. The surface tension initially decreased with an increase in the concentration of the gemini surfactants. At the critical concentration, micelles were formed and the surface tension γ reached a plateau (Fig. 3). The CMC values were read out at a concentration corresponding to the breakpoint of the surface tension curves. The surface tension at the CMC (γCMC) and C20 were also determined from the same curve. [FIGURE 3, SINGLE] The relationship between the specific conductance (κ) and the concentration of the gemini surfactants (Fig. 4) was also used to determine the CMC. The addition of an ionic surfactant to the solution by an amount that results in a concentration lower than the CMC causes an increase in the number of charge carriers, leading to an increase in conductivity. Because micelles have a much larger hydrodynamic radius than individual molecules of the surfactant, the diffusion of the molecules through a solution is more sluggish and they are less efficient *Corresponding

author E-mail address: [email protected]

charge carriers. Above the CMC, the conductivity continues to increase with the surfactant concentration; however, the slope is lower above than below the CMC [42]. Consequently, the formation of micelles in a solution is manifested as visible break in the plot of conductivity versus surfactant concentration. [FIGURE 4, SINGLE] The values of the CMC, γCMC and C20 of the gemini surfactants and data corresponding to their conventional analog SDBS [30] are summarized in Table 4. [TABLE 4] The CMCs for the gemini surfactants obtained from tensiometric measurements are similar to those obtained from the conductometric method. The CMCs of the gemini surfactants were at least two orders of magnitude lower than the CMC of SDBS. This result suggests that the studied gemini surfactants possess an excellent ability to form aggregates. The CMC values increased in the series of homologs with increasing spacer length. A different dependence was described by Zhu et al. for sulfonate gemini surfactants with two to eight methylene units in the spacer and with nonylphenyl tails [29]. However, Zana [1] observed that for cationic gemini surfactants with 12 carbon atoms in the tails and a spacer with fewer than six carbon units, the CMC increased with increasing spacer length. The increase in spacer length results in increasing hydrophobicity and flexibility. Long spacers are constrained to lie prone at the air/water interface, and they should inhibit micellization, resulting in the increase in the CMC, as observed for sulfonate gemini surfactants with dodecylphenyl tails. The surface tension of gemini surfactants at the CMC (CMC) was lower compared with that of the single-tail surfactant, indicating that the gemini surfactants were adsorbed strongly at the air/water interface and demonstrated effective surface activities. The efficiency in reducing the surface tension decreased with increasing spacer length, which can be explained by the increase in hydrophobicity for surfactants with longer spacers. The long hydrophobic spacers may bend and move toward the air/water interface, causing an increase in the surface area per molecule. The saturation of surfactants adsorbed at the air/water interface then decreases, which leads to an increase in surface tension at the CMC. The values of C20 indicated that the gemini surfactants were up to three orders of magnitude more efficient at reducing the surface tension of water compared with SDBS. Furthermore, the value CMC/C20, which represents the relationship between surfactant micellization in water and adsorption at the surface, was up to one order of magnitude greater than that of SDBS. The CMC/C20 ratio is indicative of whether surfactants have a better affinity toward micellization or to adsorption at the air/water interface [43]. The higher CMC/C20 ratio of the *Corresponding

author E-mail address: [email protected]

gemini surfactants compared with that of their single-tail analogs SDBS indicates that they adsorb more readily to the air/water interface. The obtained results confirm that the gemini surfactants offer high efficiency in reducing the surface tension of water and show high surface activity. The values of C20 increased according to the series S4 < S6 < S2 < S8, demonstrating that the adsorption of the anionic gemini surfactants at the air/water interface increased with the length of their hydrocarbon chains. However, the steric inhibition of the intramolecular hydrocarbon chains by the short, rigid hydrophobic spacer in the surfactant S2 led to a larger value of C20 compared with the values measured for S4 and S6. The ratio CMC/C20 increased with spacer length from 2 to 6, which indicates an increasing tendency of the surfactant to adsorb at the interface against its tendency to form micelles. However, the gemini surfactant S8, which possessed the longest spacer, showed a relatively small CMC/C20 value, indicating that it favored micelle formation over adsorption at the air/water interface.

3.4.

Aggregation of gemini surfactants in water

The sizes of the structures formed by the gemini surfactants in aqueous solution were obtained by DLS at 90° (

. The 50 CMC, 100 CMC and 200 CMC samples (CMCs

measured by tensiometry) were studied. The experimental results at concentration of 100 CMC are presented in Fig. 5. [FIGURE 5, SINGLE] Two distribution profiles were detected for most of the investigated surfactant solutions. The apparent diameters of both aggregate populations were obtained from the CONTIN analysis performed for individual modes. The average values of five measurements were calculated. The values of

for small (if present) and large aggregates at different concentrations are

summarized in Table 5. [TABLE 5] In all cases, the sizes of the two aggregate populations indicate that the surfactants organized into small and large vesicles. The aggregation morphologies are determined by intermolecular interactions, such as ionic, hydrogen bonding, as well as - interactions, among aromatic groups [44-46]. In the investigated surfactants Sm, the aromatic rings in the heads may be involved in intermolecular - interactions, thereby affecting vesicle formation, as observed by Wang [16].

*Corresponding

author E-mail address: [email protected]

3.5. Cryogenic transmission electron microscopy The morphology and size of the species formed by gemini surfactants in water above the CMC were investigated using the cryo-TEM technique. The types of objects formed by surfactants in aqueous solution were preserved in their natural state by vitrification in the cryopreparation procedure. The thin film of amorphous ice, transparent to electrons, allowed for the observation of aggregated surfactant species. The cryo-TEM micrographs were collected with a small negative defocus to improve phase contrast and at a low electron dose (less than 100e/Å2/s) to prevent crystallization of the ice. Sample micrographs obtained for the same samples used for DLS measurements are presented in Figure 6. [FIGURE 6, SINGLE] The cryo-TEM measurements confirmed the presence of vesicles formed by gemini surfactants in aqueous solution above 100 CMC (Fig. 6). Spherical species of different sizes were observed for all measured surfactants. For all samples (except for S2, Figure 6a) multivesicular structures could also be identified. The largest particles contained one (Fig. 6d, 6f) to many smaller spherical surfactant vesicles (Fig. 6b, 6c) in their interior. It may also be noticed that the membrane of vesicles could be easily deformed when in the vicinity of other particles (Fig. 6b, 6e, 6h). The schematic diagram of vesicles formed is shown in Figure 7. [FIGURE 7, 1.5] The diameters of the vesicles (DTEM) and the thicknesses of the vesicle bilayers (LTEM) were calculated as the average number measured for 20 particles. The results of the cryo-TEM measurements are presented in Table 6. [TABLE 6] The size of the vesicles obtained from the cryo-TEM micrographs is similar to that measured by DLS in all cases. The changes in vesicle size with the gemini surfactants’ concentration in water, as observed in the cryo-TEM results, are analogous to those observed in the light scattering measurements. For all samples, the dimensions of the vesicles formed by the gemini surfactants increased with concentration. Up to the surfactant S6, the particle size for the same concentration also increased with the length of the spacer. The vesicles of the surfactant S 8 reached lower sizes than the S6 vesicles, despite possessing the longest spacer. The size of the vesicles’ bilayer decreased with the spacer length. The thinnest wall was detected for the most concentrated sample of the surfactant S8. The longer spacer was more flexible, which allowed for better packing of the surfactant tails during vesicle formation. *Corresponding

author E-mail address: [email protected]

4. Conclusion A series of gemini surfactants with 4-dodecylphenyl tails, sulfonic head groups and flexible oligomethylene spacer containing two, four, six or eight units was synthesized. The obtained gemini surfactants exhibited lower viscosity and density values and were more effective in decreasing the surface tension compared with SDBS. The CMCs of the gemini surfactants determined from surface tension and conductivity measurements were at least two orders of magnitude lower than the CMC of SDBS. The CMC increased with spacer length. DLS measurements of gemini solutions with concentrations above the CMC in most cases indicated the presence of two populations of aggregates with apparent hydrodynamic diameters of approximately 50 and 200 nm. The cryo-TEM micrographs of surfactants’ aggregates revealed the formation of multivesicular structures consisting of several small vesicles surrounded by vesicular membrane. The membrane of vesicles could be easily deformed when in the vicinity of other particles. The vesicles of gemini surfactants in particular could be used as models for application as nanocontainers or micro-reactors.

Acknowledgments We are grateful for the financial support from the National Science Centre of Poland (Grant project N N209 4469 39). Julia Woch is a recipient of a PhD scholarship under a project founded by the European Social Found.

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TABLES Table 1. The yields of synthesis of gemini surfactants S m. *Corresponding

author E-mail address: [email protected]

Table 2. Chemical shifts of 1H-NMR for gemini surfactants. Table 3. Physicochemical properties of gemini surfactants. Table 4. Values of CMC, CMC and C20 of gemini surfactants and their single-tail analog SDBS. Table 5. Apparent hydrodynamic diameter of gemini surfactants, measured by DLS at different concentrations. Table 6. Diameter and bilayer thickness of vesicles formed by gemini surfactants in aqueous solution above CMC as measured from cryo-TEM micrographs.

FIGURES Fig. 1. Structure of gemini surfactants. Fig. 2. General scheme for the synthesis of gemini surfactants. Fig. 3. Variation in surface tension as a function of surfactant concentration. CMC, γCMC and C20 determined as shown for S8. Fig.

4. Specific conductance (κ) as a function of gemini surfactant concentration in aqueous

solution. Insets: the break in the plots of S2 and S4. Fig. 5. Apparent particle size distribution of gemini surfactants, measured by DLS, at concentration of 100 CMC. Fig. 6. Cryo-TEM micrographs of aqueous solutions of gemini surfactants: (a) S2 100 CMC, (b) S4 100 CMC, (c) S4 200 CMC, (d) S6 100 CMC, (e) S6 200 CMC, (f) S8 50 CMC, (g) S8 100 CMC, and (h) S8 200 CMC; (x, y, z, q, see Fig. 7). Fig. 7. The schematic diagram of vesicles formed.

*Corresponding

author E-mail address: [email protected]