Gemini surfactant–water mixtures: some physical–chemical properties

Colloids and Surfaces A: Physicochemical and Engineering Aspects 201 (2002) 247– 260 www.elsevier.com/locate/colsurfa

Gemini surfactant –water mixtures: some physical–chemical properties Cesare Oliviero a, Luigi Coppola a, Camillo La Mesa b, Giuseppe A. Ranieri a,*, M. Terenzi a a

Departimento di Chimica, Uni6ersita degli Studi della Calabria, Via P Bucci, Arca6acata di rende, (CS) I-87036, Italy b Departimento di Chimica, Uni6ersita degli Studi di Roma, ‘‘La Sapienza’’, P. le A. Moro 5, 00185 Rome, Italy Received 5 January 2001; accepted 17 August 2001

Abstract The phase diagram of the water-Gemini 16-4-16 system has been investigated and the phase boundaries were determined. DSC and optical microscopy were used to define the region of existence of the different phases. No liquid crystalline phases have been observed, however, a two-phase region and a wide gel phase follow the solution region. The solution region can be highly viscous, depending on composition and temperature. Surface tension and electrical conductance experiments have been performed, to define micelle formation and counter-ion binding to micelles. Interactions and motions over short distances were studied by 1H-NMR relaxation experiments. The drastic decrease of spin–spin relaxation time, T2, with Gemini composition ( : 2 wt.%) was explained in terms of particle growth. Pulsed field gradient spin-echo (PGSE) NMR experiments were used to determine water and surfactant self-diffusion. Some modifications in the micellar structure were inferred on increasing the Gemini content in the mixture. Dynamic rheological experiments were performed for probing the solution microstructure. The observed high solution viscosity and the shear relaxation processes were rationalized in terms of the presence of entangled threadlike aggregates at a moderate concentration ( : 4 wt.%). According to the Bohlin theory of flow as a cooperative phenomenon, the number of the micellar aggregates correlated to each other, and the interaction strength between the micellar units was obtained as a function of Gemini concentration. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Gemini surfactants; Micelles; Phase equilibria; Self-diffusion; Rheology

1. Introduction Efforts have been recently devoted to synthesizing and characterizing second and third generation surface active agents. Among them are * Corresponding author. Tel.: + 39-0984-492047; fax: + 390984-492044. E-mail address: [email protected] (G.A. Ranieri).

promising double chain surfactants, termed Geminis [1–4]. They are amphiphilic molecules possessing, in sequence, a long hydrocarbon chain, a polar head group, a flexible (or rigid) spacer, a second polar group and another hydrocarbon tail. Polar groups can be ionic or not. Geminis are much more efficient than n-alkyl chain surfactants in reducing the surface tension of aqueous solu-

0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 1 0 2 2 - 6

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tions [5] and form micelles at much lower concentrations than conventional surfactants [6,7]. For a Gemini of given alkyl chain the critical micellar concentration, CMC, increases with the spacer length up to a maximum of five methylene units and decreases there from. This behavior is due to a combination of surfactant conformational changes and effects ascribed to the spacer location into aggregates [8,9]. Since Bunton [1], Menger [3] and Rosen [4] several studies have been reported on their synthesis [10– 14] as well as on structural and basic physicochemical characterization of aqueous solutions [6,7,15– 22]. Micelle formation, aggregate size, shape and structure of Geminis surfactants have also received due attention. In particular, information from dynamic light scattering, neutron scattering and from Cryo-TEM studies is available [18,23–26]. In Fig. 1 is reported the chemical formula of the dimeric (Gemini) surfactant investigated here. Its name is N,N,N%,N%-tetramethyl-N,N%dihexadecyl1,4-butan di-ammonium di-bromide (BCTA). The above surfactant molecule, known in the surface chemistry literature as 16-4-16, is made of two long alkyl chains whose head groups are linked by a flexible butyl spacer. In some aspects, it can be formally considered the dimer of the corresponding quaternary hexadecyltrimethyl-ammonium bromide (CTAB) surfactant. In this paper some results for the binary BCTA– H2O system are presented. The phase behavior as a function of temperature and BCTA content was investigated, and special attention was focused on the occurrence of the solution phase. In addition, a preliminary investigation of the whole phase diagram was performed. The experimental methods we used were properly chosen to obtain information on micelle formation and microstructure of this binary system. PGSE and 1H-NMR experiments, surface tension, rheology, differential scanning calorimetry, optical techniques (including polarizing microscopy) and electrical conductivity experiments have been used for that purpose. The present data support former studies and give a more detailed view on their unconventional solution properties. Efforts shall be

made to relate the present findings with those currently available, when comparison is possible. No significant difference between the solution behavior of 16-4-16 surfactant and that of structurally related alkylammonium salts was observed. For this purpose we compared the properties of BCTA with that of CTBA aqueous solutions.

Fig. 1. (a) Chemical formula of the dimeric Gemini 16-4-16 surfactant, hereafter termed as BCTA. The name of the above surfactant is N,N,N%,N%-tetramethyl-N,N%dihexadecyl-1,4-butan di-ammonium di-bromide. (b) Chemical formula of Hexadecyltrimethyl-Ammonium Bromide (CTAB) surfactant, indicated for comparison.

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2. Experimental section

2.1. Materials The raw BCTA salt is a generous gift from Professor G. Cerichelli (University of L’Aquila, Italy). After precipitation in ethanol-acetone mixtures, its purity was found to be : 9%, as inferred by gas chromatography. Heavy water, nominal purity 99.9%, (Aldrich) was used. Conductivity water,  : 1 mS cm − 1 at 25 °C, was used. All samples were prepared by weighing appropriate amounts of the components in standard tubes. Samples used to build up the phase diagram were prepared by weighing the desired amounts of surfactant and water, or D2O, in glass ampoules containing a small magnetic stirring bar. Replacement of water with deuterium oxide was made on a mole fraction basis. The samples were flamesealed, mixed by a vortex 708 (ASAL, Italy), homogenized at 80 °C and left to equilibrate for some days (usually a week) prior to any experiment. As a rule, in fact, the kinetics of phase separation associated with gel formation is completed in 2– 3 days. The phase separation process is perfectly reversible.

2.2. Methods 2.2.1. Surface tension Surface tension measurements, | (mN m − 1), were performed using a Du Nou¨ y ring tensiometer, Nima, whose measuring vessel is thermostated to 9 0.1 °C by circulating water. The maximum uncertainty on | values is 90.2 mN m − 1. The time required to get stable surface tension values is less than 5 min. The shape of the curves in proximity of the critical micellar concentration does not show significant downward inflections and the presence of surface active impurities can be ruled out. The critical micellar concentration, CMC, is determined from the abrupt change in the surface tension versus ln(m) plot. The dependence of | on composition was rationalized by the Gibbs adsorption isotherm according to: d|= − nY2(RT d ln [m])

(1)

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where Y2 is the excess surface concentration, m the surfactant molality, n= 3 (is the constant accounting for the formation of ions and used in previous Gibbs analyses of Geminis with flexible spacers [17]) and other symbols have their usual meaning. With this approach, the area per molecule, A0 (in A, 2), was evaluated from the relation [27,28]: A0 =

1020 NY2

(1a)

where N is Avogadro’s number.

2.2.2. Electrical conductance A Wayne –Kerr bridge, model 6425, operating at 1 KHz, measured the ionic conductivity. The flask containing the solution (volume of about 250 cm3) was immersed in an oil bath, whose temperature was controlled by a Heto thermostatic unit. The temperature was set at 409 0.002 °C and measured by an F25 thermometer, from Automatic System Laboratory. The accuracy on CMC and counter-ion binding constant, i, values is to 9 2%. The former is the concentration at which a departure from the linear (c) trend is observed. Conversely, i is estimated from the ratio of conductivity-versusmolality slopes after and before the CMC [29]. 2.2.3. Rheological techniques A DSR 200 rheometer (Rheometrics, USA) equipped with parallel plates (ƒ= 40 mm) and with a Peltier temperature control system was used. All the samples were tested by stress sweep tests at 1 Hz, to define the presence of a linear sweep-stress range. Different frequency sweep tests were performed between 0.1 and 80 Hz. Measurements were obtained by imposing a sinusoidal deformation k= k0sin …t to samples, where … is the frequency and k0 the maximum strain amplitude. The resultant stress component in phase with the deformation defines the storage or elastic modulus, G%, whereas the stress component out-of-phase with the strain defines the loss or viscous modulus, G¦. The shape and the magnitude of these components is sensitive to microstructures of the system. In this work, G% and

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G¦ modulus have been obtained as a function of frequency. The experimental data were analyzed according to Bohlin’s theory, which considers the flow as a co-operative phenomenon [30]. The theory provides a link between the microstructure of the material and its rheological properties. The most important parameter introduced by Bohlin’s theory is the ‘coordination number’, z, which is the number of unit flow interacting with each other to give the observed flow response of the material. Bohlin demonstrated that: G% =A… 1/z

(2)

A is a proper constant which can be interpreted as the ‘interaction strength’ between the rheological units. Consequently, considering the micellar phase as a network of rheological units, which interact for establishing the system structure, from the G%–… plots were obtained [31,32] the coordination number (z) and the amplitude of such interactions (A).

2.2.4. NMR measurements 2.2.4.1. Self-diffusion. 1H-NMR self-diffusion studies were performed using a Bruker NMR spectrometer, upgraded by Stelar (Milan, Italy), working at the proton resonance frequency of 80 MHz. Self-diffusion measurements were performed by the Fourier Transform of PGSE technique, described by Stilbs [33]. The experimental errors on self-diffusion coefficients are below 9 4%. An airflow regulator, yielding a temperature stability of 9 0.3 °C, controlled the temperature in the measuring chamber. According to Stejskal and Tanner [34] a spinecho signals is induced by a 90° –~ – 180° radio frequency pulse sequence. A magnetic field gradient, of intensity g, is applied during a time l, as a twin pulse separated by a time D, before and after the 180° pulse. The gradient strength (20 G cm − 1) was calibrated with pure water, for which the self-diffusion is known. The signal amplitude, I, for a given chemical species is given by:

I(l)=I(0)exp −

~ exp(− kD) T2

(3)

where ~ is the time over which the spin–spin relaxation (defined by the time constant T2) operates, k=(glg)2(D−l/3), k is the magnetogyric ratio of the proton and D represents the self-diffusion coefficient. In this work, D is kept constant (70 ms) while l is varied between 2 and 5 ms. More details may be found elsewhere [35,36]. The molecular self-diffusion coefficients of surfactant and water were determined by the intensity of their frequency-resolved signals in the spin-echo spectrum, in accordance to Eq. (3). The surfactant self-diffusion was measured by the decay of chain methylene units, giving an intense, sharp, peak on 1H-NMR spectrum. Chemical shifts were taken relative to residual protons in D2O, while the heavy water signal served as an inner lock. In such conditions the spin-echo intensities follow an exponential decay and the observed self-diffusion coefficients do not depend on D. Non linear least-squares fitting procedures were applied and provided the best fit for D. Several points were collected for each frequencyresolved signal over a 10-fold intensity change.

2.2.4.2. Relaxation time measurements. NMR relaxation experiments were performed on the same instrument. The experimental conditions were: 9 ms for the 90° radio-frequency pulses, 1.5 kHz spectral width, 1.9 kHz filter bandwidth, acquisition of 1024 complex data point per transient (in 8 s), accumulation of 128 free induction decays. 1 H-NMR relaxation times (T2) were obtained from BCTA N-methyl protons by means of signal decay on Hahn spin echo sequences [37]. A nonlinear least-squares fitting procedure was used to get T2 values. The temperature was controlled to 9 0.5 °C and calibrated with a copper–constantan thermocouple. Sample rotation was set at 20 Hz. 2.2.5. DSC methods The differential scanning calorimetric unit (Setaram DSC-92) was calibrated by melting indium. 40–50 mg of sample was weighed in open crucibles of 100 ml capacity. DSC runs were performed on four different samples at temperatures

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from 10 to 70 °C. Heating rate was 1 °C min − 1 and an empty pan was used as a reference.

2.2.6. Optical methods Homemade polarizing units were used to check the samples. The investigation was performed in a thermostatic bath. The samples were sealed in glass tubes, equipped with small magnetic stirrers. The temperature was controlled to 90.1 °C. A Heto programmable thermostatic unit controlled heating and cooling modes. Additional experiments were performed by sealing small amounts of samples in thin glass capillaries, which were flame sealed. A Ceti optical polarizing microscope, equipped with a heating stage, performed additional studies. A Leitz microscope, equipped with a rotating stage, operating in polarising mode, or with Bertrand lenses, was also used. Details on the apparatus set-up and measuring procedures are given elsewhere [38].

3. Results and discussion

3.1. Phase diagram In Fig. 2(a) is reported the phase diagram of the BCTA –H2O system. As can be seen there are no lyotropic liquid crystalline phases, and a wide two-phase region follows the solution region. At low surfactant content the solutions have nearly the same viscosity as water, but, when the BCTA concentration exceeds approximately 2 wt.%, the viscosity gradually increases. Some modifications in T2 values as well as in water self-diffusion were observed in that concentration range. The solution region can be arbitrarily divided into two parts, having significantly different transport and rheological properties. For concentrations below approximately 4 wt.%, the solutions are fluid and easily flowing. Above that limit, a sudden and dramatic increase in viscosity was monitored. It is striking that solutions just above the concentration of 4 wt.% show a (low-shear) viscosity of 0.12 Pa s at 40 °C; water viscosity is of approximately 0.65 mPa s at the same tempera-

Fig. 2. (a) Partial phase diagram for the system BCTA/H2O, drawn as temperature versus BCTA concentration. The dashed line dividing the solution region indicates the concentration range above which the solution viscosity suddenly increases (approximately 4 wt.%). The region indicated by the term hydrated solid is a gel phase, in Luzzati’s terminology. (b) DSC thermogram on a sample at 4.9 wt.% BCTA (mass 40.5 mg, heating rate 1 °C min − 1). Pre-transitional effects can be easily inferred.

ture. The above effects can be due to micelle size and shape transition. Cryo-TEM studies [15,18] indicate the occurrence of thread-like micelles in the concentration range mentioned above and support the present hypothesis.

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Thermodynamic information associated with the onset of Krafft boundaries in selected BCTA– H2O mixtures were inferred by DSC. A typical scan on a selected mixture, containing 4.9 wt.% of surfactant, is shown in Fig. 2(b). The plot shows the occurrence of a pre-transition peak, followed by an intense and sharp peak. The latter is ascribed to the melting enthalpy of hydrated BCTA crystals and to the consequent formation of the solution phase. The amplitude of the heat effects is proportional to the amount of BCTA in the mixture. The molal enthalpy of fusion is approximately 0.52 (9 0.06) kJ mol − 1. The Krafft temperature, TK, inferred by electrical conductance experiments, is close to 37 °C. In our experience, solid– liquid phase boundaries from electrical conductance data are more accurate (and self-consistent) than those from other experimental methods [39]. This relative high Krafft temperature observed on BCTA/water mixtures is in a complete disagreement with the Krafft boundaries of C16-trimethyl ammonium derivates [40] (about 27 °C for CTAB). This fact indicates that the linking group (C4) causes an additional stability in packing within the crystal. In addition to what observed we would like to

stress that a variation of the Krafft boundary was described by Fuller et al. [41] for 15-m-15 Gemini surfactants when m increases from 1 to 6. In this case a destruction of crystal packing was obtained by varying the spacer length. At temperatures below TK the micellar solutions are opalescent and slightly bluish. When the solutions are kept below TK for hours, or days, the surfactant precipitates out. The phase separation kinetics depends on the solute concentration and temperature. A significant thermal hysteresis can be observed. Replacement of water with D2O is concomitant to a small but systematic upward temperature shift of the phase boundaries. The width of the two-phase region is moderate, when the occurrence of a gel phase (co-gel in Luzzati’s definition) [42] extends in a wide part of the phase diagram. Platelets of hydrated BCTA crystals can be easily observed. Attempts to get liquid crystalline phases were unsuccessful in the experimental window which was investigated.

3.2. Solution phase and microstructures In Fig. 3 is reported the surface tension, |, versus log(m) plot. As can be seen, BCTA is

Fig. 3. Surface tension, |, in mN m − 1, as a function of BCTA molality, m (mol kg − 1). Both sets of data were measured at 40 °C.

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Fig. 4. The excess electrical conductance, − 0, in mS cm − 1, as a function of Gemini surfactant molality, m (mol kg − 1). The CMC is the intersection point of the two straight lines; the counter-ion binding constant, i, is calculated by the electrical conductance slopes above and below the CMC.

adsorbed at the air/water interface and the surface tension decreases up to the critical micellar concentration. The trend of surface tension versus log(m) is approximately linear up to the critical micellar concentration, CMC. At this point, micelle formation competes with interfacial adsorption and the slope of the plot abruptly decreases. The break, represented by the intersection of two straight lines, defines the CMC. The critical micellar concentration is close to 2×10 − 5 mol kg − 1. Due to the surfactant chain length, the surface tension above the CMC is practically constant and micelle formation of BCTA can be interpreted according to the (charged) phase separation model [43]. Fig. 4 reports the dependence of excess electrical conductance,  −0, on surfactant molality, m, at 40 °C. Data at lower temperatures could not be performed, since the surfactant precipitates out from the solution. From the ratio of electrical conductance slopes above and below the CMC, respectively, the counter-ion binding degree, i, was inferred. It was about 0.67, i.e. nearly the same as the structurally related CTAB [44]. In

other words, there is no significant change in binding, compared with CTAB, when a medium length spacer separates two charged quaternary ammonium groups. Accordingly, the coulombic contributions to the stability of BCTA micelles are not much different from those of Nalkyltrimethylammonium halides. It would be interesting, on this regard, to ascertain how much the distance between adjacent charged groups and counter-ions affect binding to Gemini surfactants. The hypothesis is consistent with SANS studies, which indicate an increasing micelle ionization degree when the spacer length increases [25,26]. This effect may be also related to the occurrence of spherical or thread-like micelles and vesicles on increasing the spacer length. To support the above assumptions simple calculations can be done. We assume, for this purpose, that the volume of C16 chain is 740 A, [3], the area per trimethylammonium group 36 A, [2] and the length of a C16 chain 21.7 A, , [45]. The length of a methylene unit is about 1.3 A, . Proper combination of the above values into the (V/A0l) relation indicates that the packing constraint [46] slightly decreases on in-

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creasing the spacer length. The electrostatic repulsion between head groups operates in the reverse way and allows the effective (V/A0l) parameter to increase. The overall effect is controlled by counter ion binding on alkyltrimethylammonium groups, which decreases on increasing the spacer length and increases, conversely, with concentration. [17] This is nice example of packing constraint modulation exerted by the presence of charges (and by the distance between them). As mentioned above, the structure of BCTA micelles has been previously investigated. At concentrations close to those investigated here the coexistence of entangled threadlike micelles, open membranes and spherical micelles have been observed. [19] To get further information on micelle size and shape preliminary SAXS measurements were performed. According to experimental evidence, average hydrodynamic radii of Gemini micelle increase from about 2– 3 to 25– 30 nm on increasing the concentration from 1.5 to 6 wt.%

BCTA. Thus, micelle growth is evident and in fair good agreement with SANS [25,26] and light scattering experiments [23]. A spectroscopic analysis of the solution properties was performed by a 1H-NMR relaxation study. Spin–spin relaxation times (T2), in fact, are mostly affected by interactions and motions over short distances [47]. For aqueous surfactant solutions the principal relaxation mechanism is due to the time-dependent dipolar interactions, averaged by aggregate tumbling and surfactant lateral diffusion along the micellar surface [48]. That is why T2 values give information on micelle transitions (i.e. growth, elongation of ellipsoidal aggregates, entanglement). Spin–spin relaxation times of Nmethyl protons, at 40 °C, are reported in Fig. 5. A steep linear decrease of T2 values as a function of composition can be observed. At low BCTA content (less than 2 wt.%) T2 values are close to 75 ms. Spin–spin relaxation times of N-methyl protons on CTAB micellar aqueous solutions

Fig. 5. Spin – spin relaxation times (T2/ms) of N-methyl protons as function of BCTA concentration (in weight percent, wt.%), at 40 °C. T2 values are accurate to approximately 9 8. The vertical line indicates the overlap concentration between small and long threadlike micelles. The curves through the points are only a guide of eyes.

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were measured by some of us a few years ago [49]. In a mixture at 15 wt.% BCTA, at 50 °C, the T2 value was of about 62 ms. This mixture is near to the hexagonal mesophase boundary and on consequence the observed relaxation time was interpreted as a typical value of prolate aggregates. With increasing the BCTA concentration, the relaxation times of surfactant decrease dramatically up to a constant value of approximately 8 ms. The change on T2 value observed in Fig. 5 may be due to a decrease of correlation times for aggregate tumbling and surfactant diffusion and can be related to presence of enlarged self-assembled aggregates, i.e. threadlike micelles. At concentration higher than approximately 3 BCTA wt.% the organization into large micellar aggregates is almost complete and transverse relaxation times remain nearly constant. The observed break defines a size and shape transition. Similar lowered T2 values were obtained by Menger and Eliseev [50] in cross linking micelles where 5 mol% of an anionic Gemini surfactant were added to CTAB solution of various concentration. In that case was observed that Gemini molecules promoted a substantial grow in micelle of opposite charge and the T2 values decreased to approximately 5 ms. The dependence of T2 values of BCTA molecules as a function of surfactant concentration supports information inferred from self-diffusion and rheology, as will be discussed now. Fourier transform NMR self-diffusion provides insight into the solution microstructure by determining the long-range mobility of the mixture components. Detection of motion over long distances, compared with typical micelles (some nm), provides a sensitive probe for the aggregate state [51]. Unfortunately, because of the short transverse relaxation times of BCTA protons, surfactant self-diffusion values could be measured with due accuracy only in dilute (non viscous) solution

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range. Here selected self-diffusion coefficients of micellized surfactant as a function of BCTA concentration, at 40 °C, are reported in Fig. 6(a). Typical values are of the order of 10 − 10 m2 s − 1. Extrapolation to infinite dilution gives D omic : (8.99 0.4) 10 − 11 m2 s − 1. An apparent spherical micelle radius, r, of 2.89 0.2 nm may be inferred by the Stokes–Einstein equation: r=

kT 6yD $micpD2O

(4)

where pD2O is the medium viscosity at selected temperature. Apparent hydrodynamic radii, thus, are noticeably different from those of CTAB micelles [49]. Since the maximum length of extended C16 chain never exceeds 2.1 nm, it may be inferred that BCTA micelles already born as small aggregates with a prolate shape. The water self-diffusion in a surfactant solution is influenced by micelle hydration and by obstruction effect due to the micelle [52]. Any growth in aggregate size, as well as in micelle hydration, is concomitant to a decrease in water self-diffusion. In Fig. 6(b) the relative water self-diffusion coefficients are reported as a function of surfactant concentration. The results indicate the occurrence of two different composition ranges: up to 2 wt.% and above that limit, respectively. The observed behavior is perfectly reproducible and implies the occurrence of significant variations in surfactant self-organization. The steep jump in relative water self-diffusion values will be analyzed in the following by assuming the overlapping of hydration with micelle size and shape effects. In accordance with the ‘cell-diffusion model’ the measured water self-diffusion coefficient, Dw, can be interpreted according to [52,53]: DW = f(1− P)Do + PDb

(5)

Fig. 6. (a) The surfactant self-diffusion coefficients, Ds (10 − 10 m2 s − 1), as a function of BCTA concentration (in weight percent, wt.%), at 40 °C. Higher concentrations are not reported, since short T2 values are concomitant to a large uncertainty on self-diffusion values, see the text. (b) Relative water self-diffusion, Dw/D0,as a function of wt.% BCTA, at 40 °C. Dw is the NMR observed self-diffusion coefficient and D0 denotes the self-diffusion of pure water at the experimental temperature. Dw values are accurate to approximately 9 4%. The vertical line indicates the overlap concentration between small and long threadlike micelles. The curves through the points are only a guide of eyes.

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Fig. 6. (Continued)

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where water molecules exist in either ‘free’ or ‘bound’ state, and in fast exchange conditions (compared with the NMR measuring time). In Eq. (5) Do is the water self-diffusion of pure water while Db is the diffusion coefficient of bound water in the mixture. The interaction between water and surfactant head groups is described by a single parameter, P, which gives the fraction of water molecules involved in hydration processes. f, the ‘obstruction factor’, accounts for the hindrance to free diffusion and is dependent on the aggregate shape and on the volume fraction of obstructing particles. f has been calculated for different spheroidal particle shape in [52]. A small obstruction effect ( f : 1) for spherical and prolate shape was observed, and at very low concentrations a small obstruction effect it was also noted for oblate particles with large axial ratios. In on our previous study [49] concerned with C16TAB micellar solutions the obstruction factor was found to be constant to 0.9590.05 within the 5 –20 wt.% surfactant. This range was considered to be the region in which the micellar shape is approximately globular. As a consequence of above findings, the reduction of water self-diffusion coefficients in mixtures at concentration less 2 wt.% BCTA may be influenced only by hydration. Thus, since f is close to unity, the Eq. (5) is simplified to: Dw =(1-P)Do +PDb

(6)

By considering the observed Dw data and assuming that bound water molecules have the same mobility as the aggregates (Db =Dmic), it is possible to obtain P values. The average hydration number of bound water per surfactant molecules, h, can easily be calculated by [51]: h= P

Cw Cs

(7)

where Cw and Cs are water and surfactant mole fractions in the mixture. Accordingly, h values were found to be 129 1. This result corresponds to that obtained in dilute solution of CTAB micellar system. Above the aforementioned limit of approximately 2 wt.%, significant departures from the linear trend are observed on Dw. It is difficult to

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use ‘a priori’ the oversimplified model as in Eqs. (6) and (7), since both f and h terms may change. In this case, as [51] demonstrates, the obstruction factor can be calculated by applying the Eq. (5) to two mixture i and j having a different composition but with the same structure. This is a reasonable approximation in a small concentration range. For mixtures above the 2 wt.% BCTA the obstruction factors were obtained by the following relation: f=

Dw,i − Ki, j Dw, j (1−Ki, j )D0

(8)

where Ki, j is related to the water weight fraction in the mixture i and j. In the concentration range 2–8 wt.% BCTA the obstruction factor calculated by Eq. (8) remains constant, and it is close the one assumed in the very dilute interval, i.e. 0.939 0.03. Consequently, since f is invariant with composition, the average hydration numbers decrease to 3. Such simplified analysis based on water self-diffusion coefficients indicates that an increasing molecular packing, associated to the onset of anisometric aggregates, gives rise to a more compact and less hydrated interface region. Both transverse relaxation times and self-diffusion findings imply significant changes in the micellar structure at concentrations above approximately 2 wt.% BCTA. The observed behavior is in fairly good agreement with CryoTEM experiments [15,18]. Details of rheological properties on concentrated micellar solutions were analyzed by stresssweep experiments in order to define the width of the linear viscoelastic regime [54,55]. In Fig. 7 relevant stress-sweep results are shown; a sinusoidal deformation was imposed to samples while the frequency of oscillation was kept constant at …= 1 rad s − 1. At concentration below approximately 2 wt.% BCTA no elastic behavior was observed and the dynamic elastic modulus, G%, could hardly be measured. The elastic modulus is low for mixtures with a concentration below approximately 4 wt.% BCTA, but markedly increases from there, in proportion to surfactant content. In more concentrated solutions the G%–stress behavior is charac-

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terized by a high and large plateau corresponding to a linear viscoelastic regime. In this regime, the loss modulus G¦ is of the same order of G%. With increasing stress, the material network structure is disrupted and consequently G% and G¦ decrease. The mechanical relaxation times associated to such viscoelastic behavior is tentatively assumed to be in the range of 60– 600 ms, depending on the mixture composition. Finally, frequency-sweep experiments were performed on the micellar phase; G% and G¦, plotted in Fig. 8, were obtained as a function of the frequency with … in the range 0.1– 80 rad s − 1. In this frequency window we find G% \ \G¦ as expected [56]. The G%– … responses are essentially flat in the case of concentrated solutions (\ 4 wt.% BCTA) suggesting the presence of a gel-like network. Note that this behavior is important for

Fig. 8. The storage, G% (Pa), upper figure, and the loss, G¦ (Pa) modulus, lower figure, as a function of the applied frequency, in Hz, for 3.9 () and 6.1 (“) BCTA wt.% samples at 40 °C. The viscoelastic relaxation effects inferred from the above plots are slightly dependent on temperature.

Fig. 7. The storage G% (Pa), upper, and the loss G¦ (Pa) modulus, lower figure, as a function of the applied stress, in Pa, for 1.2 ( + ), 3.1 ( ), 3.9 () and 6.1 (“) BCTA wt.% samples, at 40 °C.

many applications since it reinforces the material consistence. The coordination number associated to the viscous flow, z, can be obtained by fitting G% values, according to Eq. (2). The results are listed in Table 1 and, as can be seen, z values depend on the experimental time scale. For instance, the co-ordination number of a 3.9 BCTA wt.% mixture decreases from 1.7 to 0.7 by tenfold reducing the observation time. At the same time the interaction strength A, related to the experimental elastic modulus by the Eq. (2), increases of one order of magnitude. The above effects are related to variations in the system connectivity, i.e. to the relative amount of large anisometric aggregates and to their average orientation with respect to the applied stress.

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4. Conclusion Information from several techniques indicates that BCTA surfactant forms in water micellar aggregate whose size strongly depends on composition and temperature; as regards the phase equilibria of this binary system, no liquid crystalline mesophase was observed. Considering the high number of experimental findings obtained with a considerable degree of statistical confidence, the main results of this study can be listed as follow. 1. At very low concentration the surfactant selfdiffusion and spin– spin relaxation times indicate the presence of small non-spherical micelles. 2. The transition toward much large aggregates, threadlike micelles, occurs at moderate concentrations, approximately 2 wt.% BCTA. 3. The solution viscosity significantly increases with concentration and entangled thread-like micelles may occur (approximately 4 wt.%). The hypothesis is consistent with NMR transverse relaxation times and water self-diffusion NMR data. Evidence of strong micellar entanglement may be inferred from sweep rate measurements and shear-thinning processes occur. At concentration higher than approximately 4 wt.% BCTA, the rheological behavior exhibit by these mixtures is viscoelastic, similar to that one for entangled polymers or weak gels [31,32]. Accordingly, Gemini may find applications in the preparation of rheological fluids with selected properties. One major drawback, Table 1 Co-ordination numbers, z, and interaction strength, A, for selected BCTA samples Wt.%

Z

3.9

1.7 (1–10 s) 0.7 (0.1–1 s) 4.3 (1–10 s) 2.8 (0.1–1s) 16 (1–10 s) 16 (0.1–1 s)

4.8 6.1

A 5.0 15 67

Values were calculated according toEq. (2). Data in parentheses indicate the observation time window.

259

unfortunately, is the moderate thermal stability of the solution phase. Geminis with shorter spacers may be useful for this purpose. All these considerations are in agreement with analysis from scattering [15–17] and Cryo-TEM experiments [18,19]. From a fundamental point of view, there is no significant differences between the solution behavior of BCTA and that of the structurally related hexadecyltrimethylammonium bromide. Perhaps, compared with most alkylammonium salts, the linear viscoelastic regime of 16-4-16 Gemini solutions occurs at much lower concentrations. A possible explanation for the observed behavior can be found in the packing constraint theory of Gemini molecules. Acknowledgements Professor Giorgio Cerichelli, L’Aquila University is acknowledged with gratitude for giving us significant amounts of BCTA in raw form. We wish to thank Professor Viorel N. Pavel, ‘‘La Sapienza’’ University, for giving us the opportunity to perform preliminary SAXS scattering experiments. Dr Rita Muzzalupo, Calabria University, is acknowledged for help in the product purification. References [1] C.A. Bunton, L. Robinson, J. Schak, M.F. Stam, J. Org. Chem. 36 (1971) 2346. [2] F. Devinsky, I. Lacko, F. Bittererova, K. Tomacekova, J. Coll. Interf. Sci. 114 (1986) 314. [3] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1991) 1451. [4] M.J. Rosen, Chemthech 23 (1993) 30. [5] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 115 (1993) 3840. [6] R. Zana, M. Benrraou, R. Reueff, Langmuir 7 (1991) 1072. [7] E. Alami, H. Levy, R. Zana, Langmuir 9 (1993) 940. [8] M.J. Rosen, Z.H. Zhu, X.Y. Hua, J. Am. Oil Chem. Soc. 69 (1992) 30. [9] H. Hirata, N. Hattori, M. Ishida, M. Okabayashi, M. Frusaka, R. Zana, J. Phys. Chem. 99 (1995) 17778. [10] K. Jennings, I. Marshall, H. Birrell, A. Edwards, N. Haskins, O. Sodermann, A.J. Kirby, P. Camillery, Chem. Commun. (1998) 1951.

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