Quantitative analysis and subsequent effects of partitioning of a mono- and dihydric C4 alcohols into the micelles of cationic surfactants

Colloids and Surfaces A: Physicochem. Eng. Aspects 378 (2011) 79–86

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Quantitative analysis and subsequent effects of partitioning of a mono- and dihydric C4 alcohols into the micelles of cationic surfactants S. Chavda a , K. Singh b , M.G. Perry b , D.G. Marangoni b , V.K. Aswal c , P. Bahadur a,∗ a b c

Department of Chemistry, Veer Narmad South Gujarat University, Surat 395 007, India Department of Chemistry, StFX University, Antigonish, Nova Scotia B2G 2W5, Canada Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India

a r t i c l e

i n f o

Article history: Received 16 December 2010 Received in revised form 28 January 2011 Accepted 3 February 2011 Available online 16 February 2011 Keywords: SANS 2D NMR Alcohols Micellization

a b s t r a c t Surfactant–alcohol systems are not uncommon in colloidal chemistry, what is rare is a specific emphasis on the environmental sensitive behaviour of 1-butanol (C4 OH) in different surfactant solutions. Intermediate solubility of C4 OH (in between totally soluble short chain alcohol and partially miscible long chain alcohol) has prompted us to study its partitioning between bulk and micellar phase for monomeric and dimeric surfactants and consequences on micellar parameters. Micellar aggregation number (Nagg ) and micelle size has been evaluated by using SANS. Partitioning of C4 OH between bulk and micellar phase was evaluated from 2D NMR NOESY and DOSY experiments. The effect of the structure of C4 alcohol was evaluated by taking all the above studies for 1,4-butanediol (1,4-BTD), a non-partitioning alcohol by virtue of its terminal hydroxyl groups. Finally, results from SANS and NMR are supported by the routine study of micellar behaviour from conductivity and viscosity. It is revealed that C4 alcohol plays substantial role in micellization and the two alcohols contribute differently in controlling the overall micellization and micellar parameters. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Surfactants are versatile agents for accomplishing various tasks as simple as washing hands to drug solubilisation and drug delivery. In colloid science, surfactants are best known for the spontaneous aggregation of molecules into micelles above a critical concentration, termed as the critical micelle concentration (CMC). Self-assembly of surface active agents in water has been extensively studied for several decades. The adsorption characteristics and the micellar-phase behaviour of the surfactants are well understood. In the absence of any additive and at concentration just above CMC, surfactant micelles are often spherical in shape. Micelles have two distinct regions – the palisade layer where the polar/hydrophilic compounds may interact and the hydrophobic core where the nonpolar/hydrophobic compounds can be solubilized. Compared to anionic or zwitterionic surfactants, cationic surfactants are not heavily utilized in applications; however they do possess some peculiar properties of adsorption on to negatively charged surfaces that make them useful as cationic softeners, antistatic agents, and antibacterial agents. The most commonly studied

∗ Corresponding author. Tel.: +91 9879132125; fax: +91 2612256012. E-mail addresses: [email protected] (S. Chavda), [email protected] (K. Singh), [email protected] (M.G. Perry), [email protected] (D.G. Marangoni), [email protected] (V.K. Aswal), [email protected] (P. Bahadur). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.02.007

cationic surfactants are the long chain trimethylammonium and pyridinium halides. Cationic gemini surfactants are relatively new class of surfactants that are often described as being like two monomeric surfactants connected at or near the polar head group by different spacers [1,2]. When compared to their single chain, single headed counterparts (i.e., conventional surfactants), gemini surfactants are more efficient in lowering surface or interfacial tension and also have lower CMC values, better wetting/solubilization properties, superior foaming abilities and better cold water solubility. For these reasons, gemini surfactants have been well-studied in both academic and industrial laboratories [3–5]. For the past few decades and as an ongoing research interest, surfactants are deliberately mixed with other surfactants and/or with other micro and macromolecules to optimize the solution properties for various applications. Among these are synthetic/natural polymers (both charged and uncharged), alcohols, polyethers and amines. Specifically, the addition of an alcohol can strongly influence the micellization and micellar properties [6–23]. Water, surfactant and alcohol mixed systems are quite complex and a thorough understanding of their solution behaviour requires a critical examination of a subtle balance of many driving forces. Depending upon the hydrophilic/hydrophobic characters of alcohols, its addition to surfactant solution can strongly affect the micellar properties. Medium and long chain linear alcohols behave differently because of their different partition between bulk and micellar pseudo phase. Generally, it is accepted that short chain

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alcohols (up to three carbons) which are completely miscible with water in all proportions, are located to the hydrophilic shell of micelles. However, water insoluble/partly soluble alcohols (more than four carbon atoms) are solubilised such that their –OH group stays at micelle surface. The location where the alcohol gets solubilized in the micelle plays a substantial role in the evolution of the properties of mixed aggregates as a function of both surfactant and alcohol concentration. Since, the solubility of C4 OH lies in between the short chain and medium chain alcohols; therefore, it can be incorporated into the micelles either way. In literature reports concerning solubilization/partitioning of alcohols in surfactant solution, C4 OH is either grouped with short chain alcohols or long chain alcohols. There are few reports where the molecular details of the partitioning of alcohols and diols in various surfactant solutions have been studied [9,11,16,24–29]. To the best of our knowledge there is no single investigation focussing on the quantitative and qualitative effects of partitioning of C4 OH into cationic surfactant micelles of different types and bulk solution. In the present contribution, we address the question, in which way alcohols get incorporated into the micelles of electrically similar but structurally different surfactants. The partitioning of C4 OH and 1,4-BTD into the micelles of C14 TAB and C12 -4-C12 is evaluated both qualitatively and quantitatively using SANS, NMR, conductivity and viscosity methods. The choice of 1,4-BTD is made to create situation where alcohol remain in bulk phase. 2. Materials and methods 2.1. Materials The cationic surfactant tetradecyltrimethylammonium bromide (C14 TAB) were purchased from Sigma Chemical Company (USA) and used after repeated recrystallization from ethanol–acetone mixture. The gemini surfactant N,N -bis(dimethyldodecyl)-1,4butanediammonium dibromide (C12 -4-C12 ) were synthesized by refluxing the N,N -bis(dimethylbutane)-␣,␻-diamine in dry acetone with 1-bromododecane according to the method reported by Zana and Xia [2]. The solvent was removed from the reaction mixture under vacuum. These were recrystallized at least three times from a hexane–ethyl acetate mixture. The results of elemental analysis and the absence of minimum in surface tension–concentration plot confirmed their high purity [3,30]. The alcohols, C4 OH and 1,4-BTD, were HPLC grade materials from Aldrich. Triply distilled water (specific conductivity approximately 10−3 mS) was used for preparing all solutions. D2 O (99.9% D), used as the solvent in both the SANS and NMR experiments, was purchased from Sigma Chemical Company (USA). 2.2. Small angle neutron scattering (SANS) For the SANS experiments, the sample solutions were kept overnight for equilibration. Immediately prior to the SANS measurements, the solutions were kept at the measurement temperature for at least 1 h to attain thermal equilibrium. Solutions were placed in a quartz cell of 0.5 cm thickness with a tight-fitting teflon stopper. The SANS experiments were performed at the Dhruva reactor, BARC, Mumbai, India [31] which uses a SANS diffractometer with a polycrystalline beryllium oxide (BeO) filter as the monochromator to get an incident neutron beam with a mean wave length () of 5.2 A˚ and a wave length resolution (/) of approximately 15%. The data were recorded in ˚ All the measured SANS distributions the Q range of 0.017–0.35 A. were corrected for background and solvent contributions and were normalized to the unit of cross section using standard procedures [31]. The coherent differential scattering cross section, d˙/d˝ per

unit volume of solution for interacting micelles is given by [32]: d˙ 2 (m − s ){F 2 (Q ) + F(Q )2 [S(Q ) − 1]} + B = nm Vm d˝

(1)

where nm denotes the number density of the micelles of volume Vm , m and s are the scattering length densities of the micelle and solvent, respectively. F(Q) is the single particle (intraparticle) form factor and S(Q) is the interparticle structure factor, and B is a constant term that represents the incoherent scattering background, which is mainly due to hydrogen in the sample. An ellipsoidal shape (a > b) of the micelles is widely used in the analysis of SANS data because it can also account for a range of micelle shapes such as a sphere (a = b), rod-like micelles (a  b) and disc-like aggregates (b  a) where a and b are, respectively, the semimajor and semiminor axes of the micellar aggregate. The interparticle structure factor, S(Q), is decided by the spatial arrangement of micelles in solution. Usually, S(Q) shows a peak at Qm = 2/d, where d is the average distance between micelles and Qm is the value of Q at the peak position. The calculation of S(Q) depends on the spatial arrangement of micelle and on the intermicellar interactions. 2.3. Conductometry Conductometric measurements were done using an ESICO microprocessor-based conductivity bridge, Model 1601, and a diptype cell made of platinum black having unit cell constant. The instrument was calibrated using KCl solutions of known concentrations. Temperature equilibrium was maintained after thorough mixing. In each experiment about 50 experimental values were obtained. The CMCs were calculated from the breaks in the conductivity versus concentration plots. The ˛ values were obtained using the simple ratio of the slopes of the conductivity curves before and after the break points. Data above and below the break points were linearly fitted with correlation coefficients greater than 0.998. 2.4. Viscosity The viscosity measurements were performed by means of Ubbelohde suspended level capillary viscometer which was kept in a thermostat at 30 ◦ C with temperature stability ±0.1 ◦ C. The flow time of the solvent system and the solutions were measured with a calibrated stopwatch were used to determine the relative viscosity (rel ) of the 100 mM surfactant solutions with respect to the particular mixed solvent system. 2.5. 2D NMR Two-dimensional nuclear overhauser enhancement spectroscopy (2D-NOESY) is a noninvasive technique that can be used to determine the locus of solubilization of additives in micelles. This technique does not require an aromatic molecule or added paramagnetic agents to determine the position of solubilizates in surfactant solutions [26,33–37]. The 2D NMR experiments (1 H, gradient NOESY with and without solvent suppression) were performed on a Bruker AVANCE-II 400 MHz spectrometer at StFX University. The mixing times and the delay times for the NOESY experiments were estimated from the spin-lattice relaxation times (T1 values) of the surfactant determined in separate experiments. In all cases, an acquisition delay of 3 × T1 and a mixing time of 1 × T1 were used to obtain the NOESY spectra. For all acquisitions, 256 transients of either 2 or 4 scans over 512 complex data points were acquired. All experiments were done in phase sensitive mode, with and without the saturation of the water resonance at ∼4.70 ppm. The data were zero-filled twice in dimension 1 and multiplied by a squared sine function in both dimensions before 2D

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FT. Obtaining the diffusion coefficients via DOSY experiments was described in detail previously [33].

3. Results and discussion 3.1. Micellar shape/size and aggregation number from SANS Small angle neutron scattering (SANS) experiments were carried out in order for quantitative evaluation of effect of the addition of C4 OH and 1,4-BTD on the size/shape and the aggregation number of C14 TAB and C12 -4-C12 . The SANS distribution of the micellar solution for ionic surfactants generally shows a correlation peak [38] that corresponds to a peak in interparticle structure factor S(Q) and indicates the presence of repulsive interaction between micelles and specifies the correlation between the centres of the micelle. Since S(Q) depends on both the shape and the orientation of the particles, its calculation is quite complicated for any shape other than spheres. Hence, for simplification, prolate ellipsoidal micelles are assumed to be equivalent to spherical micelles. We have calculated S(Q) as derived by Hayter and Penfold [32] using the Ornstein–Zernike equation and the mean spherical approximation. The micelle is assumed to be a rigid equivalent sphere of diameter  = 2(ab2 )1/3 interacting through a screened Coulombic potential. The semimajor axis (a), semiminor axis (b) and the fractional charge (˛) are the parameters required for analyzing the SANS data. From the relation Nagg = 4ab2/3v, where v is the volume of the surfactant, the surfactant aggregation number (Nagg ) was calculated. Throughout the data analysis, corrections were made for instrumental smearing [39]. The parameters in the analysis were optimized by means of a nonlinear least-square fitting program and the errors of the parameters were calculated using standard methods [40]. The micellar properties obtained from the fits are summarized in Table 1. The C14 TAB micelles in 100 mM solutions in D2 O are observed to be prolate with a semi major axis of 38.1 A˚ and a ˚ Nagg at this concentration was 106 which semi minor axis of 18.3 A. is in good agreement with the literature value [41]. The aggregation number of C14 TAB micelles decreased in the presence of both the alcohols. In 10% (v/v) concentration of C4 OH and 1,4-BTD, Nagg are 79 and 33, respectively. Thus it appears that 10% C4 OH is more effective in decreasing Nagg for C14 TAB. For C12 -4-C12 an increase in Nagg was found in presence of 10% C4 OH, while decrease was seen in presence of 10% 1,4-BTD. The SANS results for 100 mM C14 TAB and 100 mM C12 -4-C12 in D2 O, 10% C4 OH and 10% 1,4-BTD at 30 ◦ C are shown in Fig. 1a and b, respectively. Fig. 1a clearly demonstrates that in the presence of 10% alcohol, the scattering intensity of 100 mM C14 TAB decreases and a shift in the correlation peak found towards the high Q region, which indicates a decrease in the micelle size and intermicellar distance (Qm = 2/d, where Qm is the value of Q at the peak position), though the shifting appeared to be somewhat larger in the presence of 10% C4 OH. On the other hand, for 100 mM C12 -4-C12 , in 10% C4 OH, the scattering intensity increased but for 10% 1,4-BTD, a decrease was observed (see Fig. 1b). Further, the correlation peak shifts towards low Q and high Q regions in the presence of C4 OH and 1,4-BTD, respectively. This is consistent with an increase in the micelle size (micellar growth) and a decrease in the intermicellar distance in presence of C4 OH, while a decrease in the micelle size and increase in intermicellar distance occurs in the presence of 1,4-BTD. For both surfactants, in the presence of 1,4-BTD, the micelle size and aggregation number decrease which is not unexpected as this diol is not expected to partition to a great extent inside the micelle [17,18,28]. Hence, addition of 1,4-BTD only leads to solvent structure modification and consequently, to smaller aggregation numbers. However, in the case of C4 OH, the two surfactants

Fig. 1. Plot of normalized neutron scattering cross-section (d˙/d˝) versus the scattering vector Q for 100 mM surfactant in D2 O, 10% C4 OH, 10% 1,4-BTD (a) C14 TAB and (b) C12 -4-C12 .

behave differently, possibly due to the fact that C4 OH, having a chain length of four carbon atoms exhibits behaviour that is intermediate between that of the short chain length alcohols and a medium chain alcohol. Depending on the composition of the system, C4 OH may behave like a short chain alcohol that decreases the Nagg or a medium chain alcohol that increases it. For 100 mM C14 TAB, we observed a decrease in the surfactant Nagg in the presence of C4 OH, likely due to the good solvent power of C4 OH for the nonpolar tails of the surfactant, resulting in a decrease in the both the micelle size and the Nagg [42,43]. An increase in Nagg for C12 -4-C12 in the presence of C4 OH can be attributed to substantial

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Table 1 Parameters obtained from the SANS fits for 100 mM C14 TAB and 100 mM C12 -4-C12 in D2 O, 10% C4 OH, and 10% 1,4-BTD. [Alcohol] % (v/v)

Semimajor axis (a) Å

Semiminor axis (b) Å

Fractional charge (˛)

Aggregation number (Nagg )

Axial ratio (a/b)

C14 TAB 0 10% C4 OH 10% 1,4-BTD

38.1 ± 1.0 32.1 ± 1.2 35.6 ± 1.2

18.3 ± 0.5 14.3 ± 0.5 16.3 ± 0.5

0.20 ± 0.02 0.31 ± 0.03 0.20 ± 0.03

106 ± 5 33 ± 3 79 ± 3

2.1 2.3 2.2

C12 -4-C12 0 10% C4 OH 10% 1,4-BTD

69.5 ± 1.6 115.0 ± 1.7 64.5 ± 1.7

15.8 ± 0.5 17.1 ± 0.5 14.5 ± 0.5

0.20 ± 0.01 0.15 ± 0.02 0.24 ± 0.02

78 ± 7 95 ± 5 61 ± 5

4.3 6.8 4.6

incorporation of C4 OH with the micelles, which means the size (and hence, the Nagg ) of the surfactants will be affected. However, this will leave insufficient amount of alcohol remaining in the solvent to overcome the hydrophobic interactions and may lead to low CMC value. Values of fractional charge (˛) attributes the head group polarity (lower fractional charge attribute less electrostatic repulsion between charged head group) [44]. From Table 1 comparison of fractional charge for 100 mM aqueous surfactant solution with the presence of 10% C4 OH reveal that in case of C14 TAB fractional charge increase where as for C12 -4-C12 decrease. These results support the explanation that at higher concentration C4 OH increases the head group polarity (˛ = 0.31) and enhances the repulsion between C14 TAB monomer. Which, make more numbers of C14 TAB monomer to remain in bulk phase; decreases Nagg for C14 TAB. On the other hand, lower value of fractional charge for C12 -4-C12 (˛ = 0.15) in the presence of 10% C4 OH manifest the decrease in repulsion between head groups that facilitates more number of surfactant molecules to aggregate leading to formation of large micelle with relatively higher aggregation number. Axial ratio (a/b) values listed in Table 1 gives the idea of micellar shape. For 100 mM C14 TAB in water, micelles are prolate (a/b ∼ 2); the presence of alcohol decreases the micelle size but shape remain almost unaffected. However, for C12 -4-C12 micelle size decreases in 10% 1,4-BTD without change in shape but 10% C4 OH leads to increase in micelle size showing micellar growth/transition.

3.2. Micellization in the presence of C4 OH and 1,4-BTD Fig. 2a–d represents changes in the CMC and ˛ values against the amount of alcohols added for both the surfactants. The CMCs and ˛ values for both the surfactants in pure water are in good agreement with literature data [45–47]. As expected both surfactants exhibited changes in the conductometrically determined micellar properties in the presence of alcohols. The CMC values of both the surfactants showed an almost linear increase with 1,4-BTD concentration while a linear decrease was found in that case of C4 OH (up to 4%) (Fig. 2a and c). After 4% (at 5%) of C4 OH increase in CMC was observed as compared to lower concentration for C14 TAB, for C12 -4C12 CMC still decreased. However, for both alcohols, we observed that the degree of counterion dissociation (˛) increased with the progressive addition of alcohols (Fig. 2b and d). For 1,4-BTD additional hydroxyl group tends to decrease number of ordered water molecules around surfactant hydrophobic chain [28] thereby, decreasing the hydrophobic effect, and hence the CMC increases with an increase in the concentration of 1,4BTD. Apart from this, the alkyl chains between two hydroxyl groups may not be of sufficient length for a favourable interaction with the hydrophobic core of a surfactant micelle [48]. On the other hand, the two terminal hydroxyl groups due to their hydrogen bonding capability cause favourable interactions with water (may anchor alkyl tail outside the micelle) leading to an increase in the CMC.

The distribution of C4 OH between bulk phase and pseudo micellear phase has two possible effects on the CMC. First, initially at lower concentration it partitions (solublized) more in micellar phase (behaves like cosurfactant) and intercalation of C4 OH between surfactant molecules results in decrease of electrostatic repulsion leading to decrease CMC. Secondly, at higher concentration of C4 OH, water + C4 OH mixture is proved to be a better solvent for surfactant alkyl tail hence results in increased CMC. We can say that C4 OH behaves as a cosolvent in second case (at higher concentration). The C4 OH at lower concentration is solubilized in such a way that –OH group remain at the surface of micelle and alkyl tail towards the micelle core. This enhances hydrophobic interaction (also reduces electrostatic repulsion) and leads to decrease CMC for both the surfactants. However, at higher concentration (>4%) C4 OH is able to pull surfactant monomer from micelle and shifts micelle–monomer equilibrium towards monomer [43] and leads to increase CMC of C14 TAB. However, in case of C12 -4-C12 due to higher hydrophobic interaction between surfactant (two hydrophobic tails) molecules, micelles are more hydrophobic and allow more partition of C4 OH, (i.e., 5% C4 OH is not enough to overcome hydrophobic interaction and increase CMC). This partition of C4 OH reduces the electrostatic repulsion that facilitates micellization. Hence, in case of dimeric C12 -4-C12 decrease in CMC was found up to 5% C4 OH. CMC of C12 -4-C12 may increase at further higher concentration of C4 OH but solubility of C4 OH limits the conductance measurements. A possible explanation for trends in ˛ values as function of alcohols concentration may be that there is a decrease in the surface charge density of the micelle [48,49] due to interaction of the alcohol with the nearby head groups of the surfactant or the partitions of the alcohol molecules into the micelle [46] leading to a decrease in CMC values but an increase in ˛ values. Further, as manifest from SANS results an increase in the alcohol concentration for C14 TAB causes a decrease in the charge density at micellar surface by lowering the aggregation number of micelle which leads to an increase in the degree of counterion dissociation. Thus in all case ˛ values increase with concentration of alcohol.

3.3. Partitioning of 1-butanol into micelles from NOESY In Fig. 3a and b the NOESY spectra are presented for C14 TAB in C4 OH and 1,4-BTD, respectively. When we examine the NOESY spectrum for the system 10 mM C14 TAB/1.0 wt% 1,4-BTD (Fig. 3a), we observe strong correlations between both sets of equivalent protons of the 1,4-BTD molecules and only the head group protons of the C14 TAB surfactant. This is a strong indication that the 1,4-BTD molecules are only able to interact with the head group protons of the C14 TAB surfactant. In contrast, when we examine the NOESY spectrum of the C14 TAB and the C4 OH (Fig. 3b), a number of other correlations exist between the alcohol peaks and the protons of the C14 TAB, in addition to the strong correlations that exist between the N-CH3 protons and the ␣-CH2 protons on the C4 OH. In partic-

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Fig. 2. Critical micelle concentration (CMC) (a and c) and degree of countion dissociation of micelle (˛) (b and d) in the presence of alcohols for both surfactants at 30 ◦ C.

ular, we can clearly see in the region around 1 ppm that there are cross peaks between the ␻-CH3 peak of the alcohol and the last two sets of chain protons on C14 TAB (on C-13 and C-14). We also see the existence of cross peaks between the C14 TAB methyl protons and the C-3 protons of the C4 OH molecules (∼1.3 ppm). The existence of these particular cross peaks is consistent with the fact that the C14 TAB chains have significant motional freedom inside the micelles, leading to a folding of the carbon chains of the C14 TAB towards the protons on the chains of the C4 OH molecules in the micellar interior. This is in excellent agreement with a wealth of literature on the order parameters and correlation times of surfactant chains in self-assembled form. The NOESY spectra give significant supporting evidence for the SANS results reported above. For the linear n-alcohol, the solubilizate is oriented in the micellar interior such that the head group is mixing with the C14 TAB head groups in the micelle palisade layer, while the hydrocarbon chain is pointed inwards towards the centre of the micelle. Similar results were obtained for C12 -4-C12 .

trations. At lower concentrations, C4 OH partitions between the micellar phase and the bulk phase, and hence, decreases the CMC value, but increases the volume fraction of micelles in the solutions, thereby increasing slightly the viscosity of the solution. However, at higher C4 OH concentrations, the solubility of the nonpolar tail of C14 TAB are increased; hence, the hydrophobic interactions between the surfactant tails are reduced which will affect the amount of surfactant assembling into micelles and the volume fraction of the micelles. This will reduce Nagg as well as lead to a decrease in the micellar size and viscosity. In the presence of 1,4BTD, the relative viscosity values were observed to decrease. The two terminal hydroxyl groups of 1,4-BTD anchor the alcohol mainly in the bulk phase of the system versus the micelle; hence, the 1,4BTD has a greater effect on the water structure which increases the solubility of surfactant in the solvent system and causes a decrease in the micelle size and reduces the viscosity for both surfactant systems.

3.4. Effects of partitioning of alcohols into micelles by viscosity

3.5. Quantitative estimation of partitioning of alcohols into micelles from DOSY

Micellar growth has been investigated by viscosity measurements for 100 mM solutions of each surfactant as a function of the total alcohol concentration in order to evaluate the effect of partitioning of C4 OH into micelles. As a representative case these measurements for C14 TAB C4 OH and 1,4-BTD are shown in Fig. 4. One can clearly see, an initial increase in viscosity occurs up to approximately 5% C4 OH followed by a decrease at higher concen-

The diffusion coefficients for the alcohols in the presence and absence of 100 mM C14 TAB and C12 -4-C12 were determined from the diffusion oriented spectroscopy (DOSY) experiments. From these diffusion coefficients, the partition coefficients (p) of the 2% alcohol systems in C14 TAB and C12 -4-C12 were obtained using the method of Stilbs [50–53]. The partitioning of alcohols between micelles and bulk water is a very important parameter

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Fig. 4. Relative viscosity of 100 mM C14 TAB in presence of increasing concentration C4 OH and 1,4-BTD at 30 ◦ C (in the inset is shown viscosity of 100 mM C12 -4-C12 solution as a function of C4 OH concentration at 30 ◦ C).

sample at a concentration above the CMC. Hence, the binding degree, p, of the alcohol to the micelles can be determined using the following equation: Dobs = pDmic + (1 − p)Daq

(3)

Daq represents the diffusion coefficient of free additive in bulk water (D2 O); this is determined using a solution of the additive in bulk D2 O in the absence of added surfactant. Dmic represents the diffusion coefficient for the micellar bound additive in the micelle; Dmic is conveniently measured as the diffusion coefficient of a small amount of 1-decanol in the 100 mM surfactant solutions in separate experiments. Eq. (3) can be rearranged to give: Dobs − Daq Dmic − Daq

p=

(4)

The mole fraction of the alcohol in the micelle phase is now given by Xmic =

Fig. 3. NOESY spectra for 10 mM C14 TAB in presence of (a) 1% 1,4-BTD and (b) 1% C4 OH.

which helps to determine the physico-chemical properties of the mixed surfactant–alcohol systems. The p-value of the solubilizate between the aqueous and micellar phases is defined as follows p=

Ca, mic Ca, t

(2)

where Ca, mic is the concentration of alcohol in the micellar phase and Ca, t is the total concentration of alcohol. DOSY experiment can be used to obtain the diffusion coefficients with which the partition coefficients of the alcohols can be obtained. If the diffusion of the alcohol between the bulk phase and the micelles is fast on the NMR timescale, the observed diffusion coefficient, Dobs , is the weight averaged diffusion coefficient for the mixed surfactant–alcohol

pCa, t (pCa, t ) + Cs, mic

(5)

where Ca, t is the total alcohol concentration and Cs, mic is the concentration of surfactant in micellar form. The mole fraction of the alcohol in the aqueous phase is Xaq =

(1 − p)Ca, t 49.93

(6)

where 49.93 is the number of moles of D2 O in 1.00 L. The mole fractions are used to determine the partition coefficient, Kx as follows Kx =

Xmic Xaq

(7)

The partition coefficient can be used to determine standard free ◦ energies of transfer of the additives, Gt , from the aqueous phase to the micellar phase ◦

Gt = −RT ln Kx

(8)

T and R are the absolute temperature and gas constant, respectively.

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Table 2 Diffusion coefficients (cm2 s−1 ) and calculated transport properties of additives into the interior of micelles. Alcohol/surfactant

Dobs Dmic Daq p Kx ◦ Gt (kJ mol−1 )

C4 OH/C14 TAB

C4 OH/C12 -4-C12

2.92 × 10−10 7.24 × 10−11 4.57 × 10−10 0.43 ± 0.05 102 ± 20 −11.5 ± 0.5

2.09 × 10−10 7.41 × 10−11 4.57 × 10−10 0.65 ± 0.04 248 ± 37 −13.7 ± 0.4

The diffusion coefficients and the partition coefficients of the mixed systems are given in Table 2. We note from Table 2 that in both cases, the partition constant of the 1,4-BTD is substantially lowered in both cationic surfactant systems versus that of the C4 OH. The Kx values and the Gibbs transfer energies of the alcohol are also given in Table 2. We observe that for both surfactants that the thermodynamic tendency of the 1,4-BTD to transfer to the micellar phase is much lower than that of the C4 OH, in excellent agreement with the literature [17,28], due to the presence of the two hydroxyl groups in the 1,4-BTD and the small size of the alkyl chain connecting them. Hence, the contribution to the formation of mixed micelles from the alkanediol in this case is largely going to be confined to solvent effects and electrostatic effects in the micellar palisade layer region, as the DOSY experiments indicate that the majority of the alcohol remains in the bulk solution and the alcohol that does partition is confined to the headgroup region. Finally, the data in Table 2 also suggest that the cationic gemini amphiphile has a much greater solubilizing power for the alcohols, as evidenced by the larger partition constants for both additives in the dimeric surfactant solutions. 4. Conclusion From the results of SANS, 2D NMR, conductance and viscosity experiments, we conclude that the addition of the C4 monohydric and dihydric alcohols causes a change in the micellar properties of the host cationic surfactants in aqueous solutions. The CMC values indicate that both alcohols contribute to reductions in both electrostatic and hydrophobic interactions, although these alcohols affect each surfactant in a different manner. In examining the SANS results, we also observe variations in the manner in which these alcohols affect the micellization of the cationic gemini surfactant versus the conventional cationic amphiphile. Viscosity measurements indicate that micellar growth occurs as C4 OH is partitioned into both surfactant systems and has stronger tendency for micellar growth in the gemini surfactant system due to enhanced micellar properties and solubilization in the gemini amphiphile. The decrease in viscosity for 100 mM C14 TAB in the presence of [C4 OH] > 5% attributes to decrease in micelle size also supports SANS results for 100 mM C14 TAB/10% C4 OH system. This observation leads to conclude that at higher C4 OH concentration, it is possible that micelle–monomer equilibrium shifts towards monomers hence, micellar size and Nagg decrease. Finally, the partitioning of the alcohols and their locations in the micelles, as deduced from 2D NMR experiments, are consistent with the number and the position of hydroxyl groups on the alcohol chain making a significant contribution to both hydrophobic and electrostatic aspects of the self-assembly process. Acknowledgements SC thanks UGC, New Delhi for Rajiv Gandhi National Fellowship (F.16-1228(SC)/2008(SA-III)). DGM. MGP, and KS thank NSERC (Discovery Grant, GM), StFX University, and the Atlantic Innovation

1,4-BTD/C14 TAB 6.67 × 10−10 7.24 × 10−11 7.76 × 10−10 0.16 ± 0.04 20 ± 12 −7.5 ± 1.3

1,4-BTD/C12 -4-C12 5.62 × 10−10 7.48 × 10−11 7.76 × 10−10 0.30 ± 0.04 48 ± 16 −9.6 ± 0.9

Fund for financial support. PB thanks St. Francis Xavier University, Antigonish, Canada for the grants of a James Chair Visiting Professorship.

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