Surface and aggregation properties of heterogemini surfactants containing quaternary ammonium and guanidine moiety

Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 236–242

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Surface and aggregation properties of heterogemini surfactants containing quaternary ammonium and guanidine moiety Yongbo Song a,b , Qiuxiao Li b,∗ , Yunling Li b , Lifei Zhi b a b

College of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, Shanxi Province 030006, PR China China Research Institute of Daily Chemical Industry, 34# Wenyuan Street, Taiyuan, Shanxi Province 030001, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 Heterogemini surfactants containing guanidine group were synthesized.  Higher surface activity than monomeric surfactants and their equimolar mixture.  The adsorption, aggregation and thermodynamic parameters evaluated.

a r t i c l e

i n f o

Article history: Received 10 August 2012 Received in revised form 29 October 2012 Accepted 2 November 2012 Available online 10 November 2012 Keywords: Heterogemini surfactants Guanidine group Surface tension Aggregation behavior

a b s t r a c t A novel gemini surfactant with nonidentical hydrophilic groups containing guanidine group and quaternary salt, N,N-dimethyl-N-[3-(N ,N -dimethyl-N -alkylguanidiumhydrochloride)propyl]-1-alkyl ammonium chloride (diCnGQ, where n represents hydrocarbon chain lengths of 8, 10, and 12) was successfully synthesized. The adsorption and aggregation properties of diCnGQ in aqueous solution have been investigated through surface tension, conductivity, steady-state fluorescence. The critical aggregation concentration (cac) obtained from different techniques showed fairly good agreement. Surface tension measurements have been used to derive surface adsorption properties such as adsorption efficiency and effectiveness (pC20 and cac/C20 ), the maximum surface excess concentration ( max ) and minimum surface area permolecule (Amin ) at the air–water interface. Temperature dependent conductivity measurements have been used to obtain the degree of counterion binding (ˇ), and the thermodynamic parameters such as standard free energy (Gagg ), enthalpy (Hagg ), and entropy (Sagg ) of aggregation. The aggregation number (Nagg ) for diCnGQ has been derived by using the fluorescence quenching technique. As a comparison, the heterogemini surfactants showed a lower cac and higher efficiency in lowering the surface tension than the corresponding monomeric surfactants and their equimolar mixture. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Gemini surfactants contain two hydrophilic head groups and two hydrophobic chains linked by a spacer in a molecule which have been regarded as the next generation surfactants [1,2]. Of the gemini surfactants, cationic types of alkanediyl-␣,␻bis(alkyldimethylammonium)dibromide were first synthesized by

∗ Corresponding author. Tel.: +86 351 4046718; fax: +86 351 4040802. E-mail address: [email protected] (Q. Li). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.11.004

Bunton et al. in 1971, and by far the most investigated surfactants [3]. The quaternary ammonium type gemini surfactants show much lower critical aggregate concentration (cac) values, greater efficiency in lowering the surface tension of water, and interfacial tension at the oil/water interface, and stronger adsorption at the solid/solution interface than the conventional monomeric surfactants [4–8]. Researches have been carried out on gemini surfactants in which the headgroups or hydrocarbon chain lengths are chemically nonidentical which referred to them as “heterogemini” surfactants [9]. Later on several heterogemini surfactants with different head

Y. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 236–242

237

Scheme 1. Compounds prepared and characterized in the present work.

groups, such as anionic–nonionic [10], anionic–cationic [11–13], and nonionic–nonionic [14] have been described. Particularly, Tomokazu Yoshimura group designed and synthesized a series of heterogemini surfactants and investigated their adsorption and aggregation properties [15–18]. Alami group reported some heterogemini surfactants based on natural products as building blocks [10,14,19,20]. Heterogemini surfactants are somewhat more complicated and the literature contains relatively few systematic studies on the physicochemical behavior of such surfactants. Cationic heterogemini surfactants with the same head groups but with different hydrophobic tails have also been reported [21,22]. They showed good surface-active properties, such as low cac and high efficiency in lowering the surface tension. We are attempting to design a new type of cationic heterogemini surfactant with nonidentical headgroups that are still unreported. Alkyl guanidine cationic surfactants were reported exhibiting a smaller cac value and a high efficiency in lowering the surface tension, when compared with conventional cationic quaternary salts which had the same alkyl chain and counterion [23–25]. So we designed a new type of heterogemini surfactant with nonidentical headgroups of guanidine group and quaternary salts. It is expected that the heterogemini surfactants with better properties than those already reported. In this paper, we describe the synthesis of a novel gemini surfactant with nonidentical hydrophilic groups containing guanidine group and quaternary salt, N,N-dimethyl-N-[3-(N ,N -dimethylN -alkylguanidiumhydrochloride)propyl]-1-alkyl ammonium chloride (diCnGQ, where n represents hydrocarbon chain lengths of 8, 10, and 12); we also describe the aggregation behavior and thermodynamics of self-assembling of diCnGQ in the aqueous medium by using tensiometry and conductometry techniques. The micropolarity of the aggregates is usually studied from the steady-state fluorescence spectroscopy using pyrene as polarity probe. Therefore, we recorded the pyrene fluorescence in the aqueous solutions of diCnGQ and used the relative intensity of vibronic bands (I1 /I3 ) to determine the cac of diCnGQ. Fluorescence quenching experiments were performed in order to obtain the aggregation number of diCnGQ aggregates. Scheme 1 shows the synthesis route of novel heterogemini surfactants. DiCnGQ can be regarded as an equimolar combination of the cationic surfactant alkyltrimethylammonium chloride and alkyl guanidine hydrochloride. Therefore, it is interesting to note that the properties of diCnGQ are comparable with those of the corresponding monomeric surfactants and their equimolar mixture.

2. Experimental 2.1. Materials n-Octyl, n-decyl, or n-dodecyl bromide (Acros) had a purity of 99%. 3-(Dimethylamino)-1-propylamine (DMAPA) and N,Ndimethylcyanamide were of industrial grade. Dodecyltrimethylammonium chloride (DTAC) was of analytical grade and was used as received. Di-methyldodecylguanidines hydrochloride (C12MG) was synthesized in our own laboratory and recrystallized triple.

Double distilled water for all analyses and measurements of primary properties. Melting points were taken on a hot plate equipped with a microscope and are uncorrected. FT-IR spectra were recorded in KBr on Avatar 380 FT-IR spectrometer. 1 H NMR (400 MHz) spectra were recorded on a Varian Inova-400 MHz spectrometer. NMR signals are described by use of s for singlet, d for doublet, t for triplet, m for multiplet. Elemental analysis (EA) was also carried out with a PerkinElmer 2400 C H N analyzer. 2.2. Methods 2.2.1. Synthesis of N,N-dimethyl-N-[3-(alkylamino)propyl]-1-alkylammonium chlorides (diCnQ) n-Octyl, n-decyl, or n-dodecyl bromide (20–25 g, 0.1 mol) was added to a stirred solution of N,N-dimethylpropylenediamine (5.1 g, 0.05 mol) in about 100 mL of methanol. The mixture was refluxed for 12 h under an alkaline condition by adding NaOH and NaCl. After the solvent was removed, the residue was dissolved in acetone, and the mixture was filtered to remove the inorganic salt. After acetone was removed, the product was washed with ethyl acetate and then hexane and recrystallized from mixtures of ethyl acetate and ethanol to give N,N-dimethyl-N-[2(alkylamino)propyl]-1-alkylammonium chlorides (refer to diCnQ, n = 8, 10, and 12) as white solids. The yields were 25, 38, and 39% for diC08Q, diC10Q, and diC12Q, respectively. 1 H NMR (400 MHz, CDCl ) [26]: 0.85 (t, 6H, 2CH –CH –), 3 3 2 1.27–1.36 (m, (4n-12)H, 2CH3 –(CH2 )n−3 –CH2 –), 1.45 (m, 2H, –CH2 – CH2 –NH–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –), 1.74 (m, 2H, NH–CH2 – CH2 –CH2 –N+ (CH3 )2 CH2 –CH2 –), 2.58 (t, 2H, –CH2 –CH2 –NH–CH2 – CH2 –CH2 –N+ (CH3 )2 CH2 –), 3.10 (t, 2H, NH–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –CH2 –), 3.44 (s, 6H, –CH2 –N+ (CH3 )2 CH2 –), 3.59 (m, 2H, –NH–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –), and 3.82 ppm (t, 4H, –NH–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –). Elemental analysis: diC08G calcd for C21 H47 N2 Cl: C, 69.52; H, 12.96; N, 7.72. Found: C, 69.75; H, 12.89; N, 7.57. diC10G calcd for C25 H55 N2 Cl: C, 71.68; H, 13.14; N, 6.69. Found: C, 71.99; H, 13.36; N, 7.26. diC12G calcd for C29 H63 N2 Cl: C, 73.34; H, 13.27; N, 5.90. Found: C, 73.72; H, 13.44; N, 5.84. 2.2.2. Synthesis of N,N-dimethyl-N-[3-(N ,N -dimethyl-N -alkylguanidium hydrochloride)propyl]-1-alkylammonium chloride (diCnGQ) Guanidination reactions were conducted with N,Ndimethylcyanamide (4.2 g, 0.06 mol) and diCnQ (18–24 g, 0.05 mol) dissolved in around 100 mL of isopropanol. After pH is set to 3–4 with dilute HCl solution, the mixture was refluxed for 2 h. The crude product was obtained after the evaporation of isopropanol under decompression. The pure product was subsequently obtained after the recrystallization of the crude product in acetone. The yields were 85%, 92%, 93% for diC08GQ, diC10GQ and diC12GQ respectively. 1 H NMR (400 MHz, CDCl ): 0.86 (t, 6H, 2CH –CH –), 3 3 2 1.27–1.36 (m, (4n-12)H, 2CH3 –(CH2 )n−3 –CH2 –), 1.85 (m, 4H, 2.51 –CH2 –CH2 –N(–C–N)–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –CH2 ),

238

Y. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 236–242

(t, 2H, –CH2 –NH–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –), 2.98 (s, 12H, (CH3 )2 N–C–N(CH2 –)–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –), 3.24 (m, 2H, N(–C–N)–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –), 3.39 (t, 2H, N(–C–N)– and 3.52 ppm (t, 2H, CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –), N(–C–N)–CH2 –CH2 –CH2 –N+ (CH3 )2 CH2 –). Elemental analysis: diC08GQ, calcd for C24 H54 N4 Cl2 : C, 61.41; H, 11.51; N, 11.94. Found: C, 61.75; H, 11.89; N, 12.07. diC10GQ calcd for C28 H62 N4 Cl2 : C, 64.00; H, 11.81; N, 10.67. Found: C, 64.99; H, 12.16; N, 11.26. diC12GQ calcd for C32 H70 N4 Cl2 : C, 66.09; H, 12.05; N, 9.64. Found: C, 66.72; H, 12.54; N, 9.59. 2.2.3. Surface tension measurement Surface tension values were obtained by a processor tension ˝ Company, Germany) using the ring technique. meter K12 (Kruss The temperature was 298 K and apparent surface tensions were measured about five times for the sample with a 2 min interval between each reading. Prior to measurement, the surface tension of the doubled distilled water ( 0 ) was confirmed in the range of 72.0 ± 0.3 mN/m. Fig. 1. Plots of equilibrium surface tension versus the concentration at 298 K.

2.2.4. Conductivity measurement Conductivity measurements were performed on a conductivity meter (Model DDS-6700, Shanghai Precision & Scientific Instrument Co. Ltd., accuracy of ±1%) equipped with a conductivity cell having a cell constant of 0.98 cm−1 . The standard deviation for conductivity was estimated around 2% as a maximum over three runs. The conductivity of the doubled distilled water was confirmed in the range of 0.10 ± 0.03 mS at 298 K. 2.2.5. Fluorescence measurement The ratio of the intensities of the first and the third vibronic peaks in the fluorescence spectrum of pyrene (I1 /I3 ) was used to estimate the micropolarity sensed by pyrene in its solubilization site [27,28]. The fluorescence intensities were measured using a Varian Cary-Eclipse spectrofluorimeter. The excitation wavelength was 335 nm, and the emission spectra were scanned over the spectra range of 350–450 nm. The slit widths of excitation and emission were fixed at 1.5 and 3.0 nm, respectively. For the aggregation number of the aggregates, steady state fluorescence quenching measurements were performed using pyrene as a fluorescence probe and benzophenone as a quencher of the fluorescence probe. 3. Results and discussion 3.1. Surface and adsorption properties The surface tension measurement is a classical method of studying the critical aggregation concentration (cac) of surfactants. Fig. 1 shows the variation of surface tension () as a function of concentration for the new heterogemini surfactants diCnGQ at 298 K. The  decreases with the increase in concentration of diCnGQ before reaching a cac, and after that a nearly constant value is obtained. The surface tensions at cac ( cac ) values have a range of 28–30 mN/m, indicating that the surfactants strongly adsorb at the air/water interface. The maximum surface excess concentration ( max ), and the minimum surface area (Amin ) per molecule can be calculated from the Gibbs adsorption isotherm equations (Eqs. (1) and (2)). max = −

Amin =

1 2.303nRT

1016 NA max



∂ ∂ log C



(1) T

(2)

Here  (mN/m) is the equilibrium surface tension at the surfactant concentration, n (the number of ionic species whose concentration

at the interface varies with the surfactant concentration in solution) is assumed to be 3, R is equal to 8.314 J mol−1 K−1 , T is the absolute temperature and (/log C) is the slope of the  versus log C plot, NA is Avogadro’s constant. The adsorption efficiency and effectiveness can be characterized by the value of logarithm of the surfactant concentration C20 at which the surface tension of water is reduced by 20 mN/m (pC20 ) and by the value of cac/C20 , respectively. The pC20 value measures the efficiency of adsorption of surfactant at the air/water interface; the larger the value of pC20 , the greater the tendency of the surfactant to adsorb at the air/water interface, relative to its tendency to self-aggregation, and the more efficiently it reduces the surface tension. The value of cac/C20 ratio is a convenient way of measuring effectiveness which can be correlated with structural factors on the adsorption processes; the larger the values of cac/C20 ratio, the greater the tendency of the surfactant to adsorb at the interface, relative to its tendency to self-aggregation. The surface and adsorption parameters, such as cac,  cac ,  max , Amin , pC20 and cac/C20 are summarized in Table 1. The cac values of the gemini surfactants are remarkably lower than that of the monomeric ones. It is well known that gemini surfactants show lower cac and  cac values than the corresponding monomeric surfactants. One of the reasons for this high activity may be attributed to a decline in the electrostatic repulsion between ionic polar heads. This also suggests that diCnGQ can adsorb efficiently at the air/water interface by a combination of two surfactants connected by a propyl spacer chain. Among these gemini surfactants,  cac increase in the hydrocarbon chain length from 8 to 12 renders them slight less surface active, probably due to a strong cohesion by two long hydrocarbon chains. The pC20 values of diCnGQ increase with increasing hydrocarbon chain length, and they are larger than those for the monomeric surfactants with same chain lengths. This result suggests that the longer hydrocarbon chains of the heterogemini surfactants, the stronger adsorption at the air/water interface. And it is known that the gemini surfactants have large pC20 values in comparison with the conventional monomeric surfactants that are consistent with the results in this study. And the cac/C20 ratios are much larger than monomeric surfactants with same chain lengths. This indicates that the heterogemini surfactants make it easy to adsorb at the air/water interface. On the other hand, cac/C20 values of heterogemini surfactants decrease with an increase in hydrocarbon chain length, probably reflecting the difficulty of packing by two longer hydrocarbon chains in aggregation.

Y. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 236–242

239

Table 1 Surface and adsorption parameters obtained from the surface tension plots of diCnGQ at 298 K. Surfactant

cac(mM)

 cac (mN/m)

 max 10−10 (mol/cm2 )

Amin (Å2 )

pC20

cac/c20

diC8GQ diC10GQ diC12GQ C12MG DTAC

9.02 0.81 0.13 0.67 15.5

28.1 28.5 29.4 32.5 40.5

1.20 1.28 1.63 3.05 2.99

138 130 102 54.4 55.6

3.17 4.19 4.72 3.75 2.64

13.5 12.1 7.24 3.76 5.80

C12MG, di-methyldodecylguanidines hydrochloride; DTAC, dodecyltrimethylammonium chloride.

As shown in Table 1, the cac of diCnGQ decreases with increasing hydrocarbon chain length. The variation of the cac with the number of carbon atoms in the hydrocarbon chain for homologous surfactants can be expressed by the so-called Klevens equation, log cac = A − Bn, where, A and B are constants reflecting the free energy changes involved in transferring the hydrophilic group and a methylene unit of the hydrophobic group from the aqueous phase to the aggregate phase. The relationship between the cac and hydrocarbon chain length for diCnGQ is plotted in Fig. 2. The logarithm of cac decreases linearly with increasing hydrocarbon chain length. The values of the slopes of the straight lines are 0.46, while the value of conventional monomeric cationic surfactant is close to 0.3. This means that the decrease in the cac is very remarkable with an increase of the chain length for heterogemini surfactants studied. Since diCnGQ can be seen as an equimolar combination of a guanidine and a quaternary surfactant built into the same molecule, the result was compared with those equimolar mixtures. These systems are analogous to the heterogemini surfactant. The surface tension curves for DTAC, C12MG, the corresponding equimolar mixture (DTAC/C12MG), and diC12GQ with the surfactant concentration are shown in Fig. 3. It is clear that the heterogemini surfactants show the most active properties as compared with the corresponding monomeric surfactants and their mixture; that is, the cac and  cac for the former are much lower than those for the latter. In addition, cac and  cac for the DTAC/C12MG mixture are close to those for the corresponding monomeric surfactants, indicating that the interaction between both surfactants in the mixture is relatively weak. Therefore, it is suggested that the primary factor contributing to the excellent properties of heterogemini surfactants is the interaction between two hydrocarbon chains connected by a short propyl spacer chain as well as by an electrostatic attraction between ammonium and

Fig. 2. Effect of alkyl chain length on the cac in aqueous solution at 298 K.

guanidine headgroup. This is one of the advantages of the heterogemini surfactants used in this study. 3.2. Conductivity measurement and thermodynamic parameters The temperature dependence of specific conductivity () as a function of concentration for diCnGQ is shown in Fig. 4. The cac values were obtained from  versus concentration plots by the intersection of the lines fitted in the diluted concentrated regions before and after the cac. Besides, the evaluation of the degree of counterion dissociation ˛ can be carried out from the ratio of the slopes obtained above and below the cac while the degree of counterion binding can be obtained by using ˇ = 1 − ˛. The results of the electrical conductivity measurements were analyzed on the basis of the mixed electrolyte model of aggregate solution. The cac and ˇ values determined from Fig. 4 are listed in Table 2 at four different temperatures. Considering the fact that different physical techniques detect different stages of aggregation, the cac derived from the different methods is in fairly good agreement at 298 K. As the temperature increases, the cac values of diCnGQ increase. The values of ˇ also show a trend to increase roughly with the increase in temperature. The effect of temperature on the cac of an ionic surfactant mainly includes two aspects. A higher temperature reduces the degree of hydration of the hydrophilic headgroups, which facilitates aggregate formation. Additionally, the water structures surrounding the hydrophobic chains are destroyed as the temperature increase, which hinders aggregate formation. Because the cac increases with temperature, it is clear that the latter effect plays a crucial role in aggregation in these systems. The specific conductivity of the surfactant aqueous solutions at different temperature was also measured to characterize the thermodynamics of aggregation formation. Applying the phase separation mode to the aggregate equilibrium for amphiphile

Fig. 3. Variation in the surface tension with the surfactant concentration for diC12GQ, DTAC, C12MG, and 1:1 mixture of DTAC/C12MG at 298 K.

240

Y. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 236–242

Fig. 5. Relative intensities of vibronic bands (I1 /I3 ) of pyrene fluorescence in aqueous solutions of diCnGQ at 298 K.

The standard enthalpy change for the aggregation process, Hagg can be determined using the Gibbs–Helmholtz equation for aqueous solutions:



∂(Gagg /T ) ∂(1/T )

Hagg =



= Hagg

(4)

−(1 + ˇ)RT 2 d ln cac dT

(5)

Thus, the standard entropy of aggregation process, Sagg is obtained by the use of the following relation: Sagg =

Hagg − Gagg T

(6)

The thermodynamic parameters of aggregation at different temperatures are listed in Table 2. The aggregation process usually leads to a positive entropy change, which is mainly due to the melting of the “flickering cluster” that arises out of the hydrophobic effect of amphiphilic part of the surfactant molecules. The results show that, as hydrocarbon chain length increases, Hagg and Gagg become more negative. And the more negative of Gagg means the stronger aggregation at the air/water interface. So the changes in Gagg reveal that the aggregation process is more spontaneous for the heterogemini surfactant with hydrocarbon chain length. For the surfactants that have been investigated, it is also seen that the value of −TSagg is much larger than Hagg , which indicates that the aggregation of the diCnGQ series is entropy-driven [30]. 3.3. Fluorescence measurement and aggregation number

Fig. 4. Temperature-dependent conductivity profile of diCnGQ as a function of concentration. The intersection of two straight lines depicts the cac.

molecule [29], the standard Gibbs energy of aggregation can be calculated from the following equation: Gagg = (1 + ˇ)RT ln cac where ˇ is the ionization degree and cac is the critical aggregate concentration expressed as mole fraction (Gagg indicates the free energy difference per mole between molecules in water and in aggregates and also the free energy of transfer 1 mol of surfactants from the aqueous phase to aggregate pseudophase).

Steady-state fluorescence measurements using pyrene as the solvatochromic probe were applied to study the aggregation behavior of diCnGQ in solution. The intensity ratio of the first to the third vibronic peaks of pyrene, i.e. I1 /I3 , was measured as a function of diCnGQ concentrations. The abrupt sigmoidal decrease in I1 /I3 intensity indicates the formation of diCnGQ aggregates and the preferential residence of pyrene molecules in the more hydrophobic environment of the aggregates relative to water. The cac values were taken as the concentration that corresponds to the intersection between the linear extrapolation of the relative stabilized portion corresponding to low diCnGQ concentrations and the abruptly varied portion of the curve. Results are plotted in Fig. 5. The ratio I1 /I3 remains constant initially and then decreases rapidly until cac is reached. After cac, the I1 /I3 becomes almost constant with further increase in the concentration of diCnGQ.

Y. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 236–242

241

Table 2 Thermodynamic parameters of the aggregation process at various temperatures. Temperature (K)

cac (mM)

ˇ

Gagg (kJ/mol)

diC08GQ

298 303 308 313

10.81 11.36 11.50 11.57

0.63 0.63 0.70 0.74

−34.5 −34.3 −35.7 −36.5

−5.29 −5.47 −5.90 −6.24

−29.2 −28.8 −29.8 −30.3

diC10GQ

298 303 308 313

0.91 0.93 1.00 1.04

0.58 0.56 0.55 0.64

−43.1 −45.2 −45.4 −46.4

−11.43 −14.44 −11.98 −13.09

−31.7 −30.8 −31.4 −33.3

diC12GQ

298 303 308 313

0.19 0.21 0.22 0.23

0.57 0.56 0.56 0.63

−48.9 −49.1 −49.7 −52.6

−14.14 −14.53 −15.01 −16.20

−34.8 −34.6 −34.7 −36.4

The cac values from steady-state fluorescence measurements are 5.65 mM, 0.84 mM and 0.18 mM respectively. It can be seen that the values of cac measured from steady-state fluorescence measurements at 298 K are in agreement with those obtained from surface tension and conductivity. The I1 /I3 ratio for all systems is approximately 1.75 for concentrations lower than the cac; this ratio begins decreasing at around the cac. These results indicate that the aggregates are formed in solution at the concentrations around the cac. Interestingly, it shows an abrupter I1 /I3 decrease with increasing the hydrocarbon chain length. At concentrations greater than 10 times the cac, the I1 /I3 ratio reaches approximately 1.2, indicating the formation of aggregates with uniform size distribution. Therefore, it can be safely concluded that the pyrene has been solubilized in the core of aggregates and experiences a highly nonpolar environment. The steady-state fluorescence quenching technique was used to determine the aggregation number (Nagg ) of diCnGQ. The equilibrium of diCnGQ between the aqueous and aggregation phases follows the Poisson distribution. The following equation is applied to fluorescence quenching data: ln

Nagg CQ I0 = I CS − cac

(7)

where CQ , cac, and CS are the concentrations of quencher, aggregate concentration, and total concentration of diCnGQ, respectively, while I and I0 are the fluorescence intensities in the presence and absence of quencher. Fig. 6 shows the plot of ln(I0 /I) against CQ for diCnGQ.

Hagg (kJ/mol)

−TSagg (kJ/mol)

Surfactant

Using the slope of these linear plots and cac, the Nagg was determined by using Eq. (7). The Nagg for diCnGQ varies in the order of 18, 21, and 25. The formation of a large aggregate can be explained by the strong interactions between two long hydrocarbon chains as well as the electrostatic attractions between ammonium and guanidine headgroup. These results are some of the peculiar properties of heterogemini surfactants that possess a structure where two different surfactants are connected by a propyl spacer chain. 4. Conclusions We have synthesized a novel heterogemini cationic surfactant with nonidentical hydrophilic headgroups containing guanidine group and quaternary salt. Aggregation characteristics of diCnGQ have been examined in aqueous media through a variety of physical and spectroscopic techniques. The critical aggregation concentration (cac) of diCnGQ obtained from different techniques showed fairly good agreement. Both the adsorption efficiency (pC20 ) and the effectiveness (cac/C20 ) derived from the surface tension versus concentration profile were rather higher than momomeric cationic surfactants with the same hydrocarbon chain length. Aggregation process in diCnGQ was found to be governed by the entropic parameter. Aggregation number of diCnGQ was formed larger aggregates with lesser aggregation number. Acknowledgments The authors acknowledge the financial supports by the National Science and Technology Support Project of China (No. 2012BAD32B10), Natural Science Foundation of China (No. 21103228), and Shanxi Province graduate innovation project (No. 20113128). References

Fig. 6. Linear plot of ln(I0 /I) for 1 × 10−6 mol/L pyrene in aqueous solutions of diCnGQ.

[1] F.M. Menger, C.A. Littau, Gemini-surfactants: synthesis and properties, J. Am. Chem. Soc. 113 (1991) 1451–1452. [2] F.M. Menger, C.A. Littau, Gemini surfactants: a new class of self-assembling molecules, J. Am. Chem. Soc. 115 (1993) 10083–10090. [3] D. Danino, Y. Talmon, R. Zana, Alkanediyl-alpha,omega-bis(dimethylalkylammonium bromide) surfactants (dimeric surfactants). 5. Aggregation and microstructure in aqueous solutions, Langmuir 11 (1995) 1448–1456. [4] R. Zana, M. Benrraou, R. Rueff, Alkanediyl-alpha,omega-bis(dimethylalkylammonium bromide) surfactants. 1. Effect of the spacer chain length on the critical micelle concentration and micelle ionization degree, Langmuir 7 (1991) 1072–1075. [5] L. Mivehi, R. Bordes, K. Holmberg, Adsorption of cationic gemini surfactants at solid surfaces studied by QCM-D and SPR: effect of the rigidity of the spacer, Langmuir 27 (2011) 7549–7557. [6] C. Keyes-Baig, J. Duhamel, S. Wettig, Characterization of the behavior of a pyrene substituted gemini surfactant in water by fluorescence, Langmuir 27 (2011) 3361–3371. [7] M.N. Maithufi, D.J. Joubert, B. Klumperman, Application of gemini surfactants as diesel fuel wax dispersants, Energy Fuels 25 (2010) 162–171.

242

Y. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 417 (2013) 236–242

[8] Y. You, X. Wu, J. Zhao, Y. Ye, W. Zou, Effect of alkyl tail length of quaternary ammonium gemini surfactants on foaming properties, Colloid Surf. A: Physicochem. Eng. Aspects 384 (2011) 164–171. [9] E.-O. Alami, K. Holmberg, Heterogemini surfactants, advances in colloid and interface science, Adv. Colloid Interface Sci. 100–102 (2003) 13–46. [10] S. Abrahmsén-Alami, E. Alami, J. Eastoe, T. Cosgrove, Interaction between a novel gemini surfactant and cyclodextrin: NMR and surface tension studies, J. Colloid Interface Sci. 246 (2002) 191–202. [11] D.A. Jaeger, B. Li, T. Clark, Cleavable double-chain surfactants with one cationic and one anionic head group that form vesicles, Langmuir 12 (1996) 4314–4316. [12] F.M. Menger, A.V. Peresypkin, A combinatorially-derived structural phase diagram for 42 zwitterionic geminis, J. Am. Chem. Soc. 123 (2001) 5614–5615. [13] A.V. Peresypkin, F.M. Menger, Zwitterionic geminis. Coacervate formation from a single organic compound, Org. Lett. 1 (1999) 1347–1350. [14] E. Alami, K. Holmberg, J. Eastoe, Adsorption properties of novel gemini surfactants with nonidentical head groups, J. Colloid Interface Sci. 247 (2002) 447–455. [15] K. Nyuta, T. Yoshimura, K. Esumi, Surface tension and micellization properties of heterogemini surfactants containing quaternary ammonium salt and sulfobetaine moiety, J. Colloid Interface Sci. 301 (2006) 267–273. [16] T. Yoshimura, K. Nyuta, K. Esumi, Zwitterionic heterogemini surfactants containing ammonium and carboxylate headgroups. 1. Adsorption and micellization, Langmuir 21 (2005) 2682–2688. [17] T. Yoshimura, K. Ishihara, K. Esumi, Sugar-based gemini surfactants with peptide bonds synthesis, adsorption, micellization, and biodegradability, Langmuir 21 (2005) 10409–10415. [18] K. Nyuta, T. Yoshimura, K. Tsuchiya, T. Ohkubo, H. Sakai, M. Abe, K. Esumi, Adsorption and aggregation properties of heterogemini surfactants containing a quaternary ammonium salt and a sugar moiety, Langmuir 22 (2006) 9187–9191. [19] A. Kumar, E. Alami, K. Holmberg, V. Seredyuk, F.M. Menger, Branched zwitterionic gemini surfactants micellization and interaction with ionic surfactants, Colloid Surf. A: Physicochem. Eng. Aspects 228 (2003) 197–207.

[20] E. Alami, K. Holmberg, Heterogemini surfactants based on fatty acid synthesis and interfacial properties, J. Colloid Interface Sci. 239 (2001) 230–240. [21] X. Wang, J. Wang, Y. Wang, J. Ye, H. Yan, R.K. Thomas, Micellization of a series of dissymmetric gemini surfactants in aqueous solution, J. Phys. Chem. B 107 (2003) 11428–11432. [22] Y. Fan, Y. Li, M. Cao, J. Wang, Y. Wangand, R.K. Thomas, Micellization of dissymmetric cationic gemini surfactants and their interaction with dimyristoylphosphatidylcholine vesicles, Langmuir 23 (2007) 11458–11464. [23] Y. Song, Q. Li, Y. Li, Self-aggregation and antimicrobial activity of alkylguanidium salts, Colloid Surf. A: Physicochem. Eng. Aspects 393 (2012) 11–16. [24] M. Miyake, K. Yamadaand, N. Oyama, Self-assembling of guanidine-type surfactant, Langmuir 24 (2008) 8527–8532. [25] M. Miyake, N. Oyama, Effect of amidoalkyl group as spacer on aggregation properties of guanidine-type surfactants, J. Colloid Interface Sci. 330 (2009) 180–185. [26] G. Wang, W. Qu, Z. Du, Q. Caoand, Q. Li, Adsorption and aggregation behavior of tetrasiloxane-tailed surfactants containing oligo(ethylene oxide) methyl ether and a sugar moiety, J. Phys. Chem. B 115 (2011) 3811–3818. [27] E.D. Goddard, N.J. Turro, P.L. Kuo, K.P. Ananthapadmanabhan, Fluorescence probes for critical micelle concentration determination, Langmuir 1 (1985) 352–355. [28] K. Kalyanasundaramand, J.K. Thomas, Environmental effects on vibronic band intensities in pyrene monomer fluorescence and their application in studies of micellar systems, J. Am. Chem. Soc. 99 (1977) 2039–2044. [29] B. Dong, Y.a. Gao, Y. Su, L. Zheng, J. Xu, T. Inoue, Self-aggregation behavior of fluorescent carbazole-tailed imidazolium ionic liquids in aqueous solutions, J. Phys. Chem. B 114 (2009) 340–348. [30] K. Srinivasa Rao, T. Singh, T.J. Trivedi, A. Kumar, Aggregation behavior of amino acid ionic liquid surfactants in aqueous media, J. Phys. Chem. B 115 (2011) 13847–13853.