Micellization and interfacial behavior of 16-E2-16 in presence of inorganic and organic salt counterions

Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

Contents lists available at ScienceDirect

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

Micellization and interfacial behavior of 16-E2-16 in presence of inorganic and organic salt counterions Mohd. Akram b , Sabreena Yousuf b , Tarique Sarwar a , Kabir-ud-Din b,∗ a b

Department of Biochemistry, Aligarh Muslim University, Aligarh 202002, India Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India

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

• Interfacial behavior of ester bonded gemini surfactants on adding salts is studied. • CMC decreases with increasing concentration of salts. • Decrease is more in presence of organic salts. • Synergism was found in the mixed micelles of gemini-hydrotropes in all combinations.

a r t i c l e

i n f o

Article history: Received 1 July 2013 Received in revised form 5 September 2013 Accepted 5 September 2013 Available online 16 September 2013 Keywords: Organic salt Biodegradable gemini surfactant Counterion Critical micelle concentration

a b s t r a c t The effect of salts (inorganic and organic) was investigated and analyzed to probe the micellization behavior of a biodegradable ester-bonded cationic gemini surfactant ethane-1,2-diyl-bis(N,N-dimethylN-hexadecylammoniumacetoxy) dichloride, referred to as 16-E2-16. We investigated the critical micelle concentration (cmc), free energy of micellization (G◦ m ), free energy of adsorption (G◦ ads ) and aggregation number Nagg of the gemini surfactant with different types of salts (KCl, KNO3 , KSCN, NaBen, and NaSal). Indeed, our results clearly indicate that the two kinds of counterions (obtained from the respective salts) affect the aggregation behavior of 16-E2-16 in quite different ways and the order of ability to promote surfactant aggregation is found to be as NaSal > NaBen > KSCN > KNO3 > KCl. The counterions exert strong influence on the cmc, aggregation number and size and shape of aggregates of ionic surfactant systems. The results provide new insight in understanding the effect of ions on the delicate balance of forces controlling aggregate morphology and solution properties of charged amphiphilic molecules. The action mechanism of counterions has been investigated employing surface tension, viscosity, 1 H NMR and fluorescence techniques. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Surfactants are an important category of materials, which have a wide range of applications in many industrial areas (e.g., food, emulsions, cosmetics, detergent formulations, pharmaceuticals, controlled drug delivery, and oil recovery). Characteristically, the surfactant molecules consist of a polar headgroup and one (or two) hydrophobic tail(s). The well known phenomenon of

∗ Corresponding author. Tel.: +91 571 2703515. E-mail address: [email protected] ( Kabir-ud-Din). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.09.007

self-aggregation (or micellization) of surfactants is mainly controlled by the following two tendencies: (i) removal of the non-polar hydrocarbon chains (tails) from the aqueous environment, and (ii) repulsion among the polar headgroups. A delicate balance exists between these two opposing tendencies to make the system stable and any change in the system conditions (e.g., concentration, pH, solvent, temperature, and additives) manifests its due influence. In this context, additives (salts or organics) play a crucial role as they not only affect micellization but control the micellar morphology as well. As such, self-association of surfactants into different microstructures (micelles, vesicles, lamellar phase, etc.) has been achieved with the proper choice of additives.

282

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

Studying the effect of adding salts, in particular, to aqueous surfactant solutions is continuing since long [1]. However, the attempts have mainly been made to examine the effect on micelle formation of ionic surfactants. The oft chosen cetyltrimethylammonium bromide/chloride (CTAB/CTAC) and sodium dodecylsulphate (SDS) have, respectively, been used as the representatives of cationic and anionic surfactants. Owing to their superior properties, due attention has been paid to gemini surfactants too but only with the m-s-m type (where m is the carbon number in alkyl chain length and s is the carbon number in spacer chain length) bearing ammonium headgroups with long alkyl chains (m = 12, 14, 16) and a methylene spacer (s = 2, 4, 6) [2]. However, due to the aquatic toxicity of most of the synthetic surfactants and even of cationic geminis [3–5] coupled with the growing global concern, development of new and novel biodegradable surfactants is gaining much attention. With this aspect in mind, we have investigated the effect of inorganic and organic salts on the aggregation behavior of a cationic gemini surfactant ethane-1,2-diyl-bis(N,N-dimethyl-Nhexadecylammoniumacetoxy) dichloride (16-E2-16) which has ester bonds in the spacer that make it cleavable (biodegradable) and limit its stability [6]. The aim has been to further characterize the recently synthesized biodegradable ester-bonded 16-E2-16 to find its utility in household and other industrial applications. The approach has been to determine micellization parameters at 303.15 K in presence of potassium chloride (KCl), potassium nitrate (KNO3 ), potassium thiocyanate (KSCN), sodium benzoate (NaBen), and sodium salicylate (NaSal) using tensiometric, steady-state fluorescence, viscometric and 1 H NMR measurements. A point worth noting is 16-E2-16’s quite low cmc value (vis-à-vis conventional dicationic ammonium gemini surfactants [7]) which renders it to be called as “green surfactant” because of using a very low amount for any potential application.

(b) Attachment of spacer to polar part. (a) Preparation of spacer. Ethane-1,2-diyl-bis(chloroacetate) was prepared as follows: Chloroacetyl chloride (0.22 mol) was placed in a dried, four-necked, round-bottomed flask equipped with a magnetic stirrer, a thermometer, a condenser (closed with calcium chloride tube) and an additional funnel. Ethylene glycol (0.1 mol) was added dropwise via the additional funnel in a nitrogen atmosphere. The mixture was heated to 50 ◦ C and reaction was continued for 8 h. After completion of the reaction, the HCl gas generated in the reaction was removed under reduced pressure. A small quantity of water was added to the reaction mixture and mixture was transferred to a separating funnel. When organic phase separated from water, it was washed with brine (saturated solution of NaCl) several times until it was neutral. The product was dissolved in ether and dried with MgSO4 and then distilled under reduced pressure. At last, colorless columnar crystals of intermediate ethane-1,2-diyl-bis(chloroacetate) were obtained. (b) Attachment of spacer to polar part. 0.21 mol N,N-dimethylhexadecylamine and 0.1 mol of ethane-1,2-diyl-bis(chloroacetate) were placed in a threenecked, round-bottomed flask equipped with a magnetic stirrer, a thermometer and a condenser. Ethylacetate was added to the flask as a solvent. The solution was heated to reflux and the reaction was carried out for 10 h. The product was recrystallized thrice from ethylacetate–ethanol mixtures (Vethylacetate :Vethanol = 5:1). Gemini 16-E2-16 was finally obtained as a white powder (Scheme 1). The purity of the gemini surfactant was ensured by 1 H NMR and the absence of minimum in surface tension () versus log[gemini] plots [8] (see plots of Fig. 1).

2. Methods and materials

2.2. Methods

2.1. Materials

2.2.1. Surface tension () measurements The tensiometric measurements were performed using a platinum ring by the ring detachment method with a Kruss tensiometer Model K11 MK3 (Germany). Temperature was maintained by circulating water from an Orbit RS10S thermostat (India). All the experiments were performed at 303.15 K. Doubly distilled and deionised water was used throughout. Stock solutions of surfactant were prepared by dissolving the surfactant in aqueous or salt solution. To avoid adsorption kinetic effects, measurements were performed 5–10 min after the addition of surfactant solution. The surface tension values decrease continuously and then become constant along a wide concentration range. The point of break, when the constancy of surface tension begins, was taken as the cmc of the system.

Chloroacetyl chloride, ethylene glycol, ethyl acetate, ethanol and magnesium sulphate were of analytical purity. N,NDimethylhexadecylamine was of chemical purity. KCl (99%, Merck, India), KNO3 (≥99%, Merck, India), KSCN (≥98%, Merck, India), NaBen (≥99.5%, Merck, India), NaSal (≥99.5%, CDH, India), pyrene (99%, Fluka, Switzerland) and CPC (98%, Merck, Germany) were used as received. 2.1.1. Synthesis The synthesis of gemini surfactant involved two steps [6c,6d]. (a) Preparation of spacer, ethane-1,2-diyl-bis(chloroacetate).

CH 2OH

o

+

2 ClCH 2COCl

CH 2OH

50 C, 8h N2

CH 3 CH 2 OOCCH 2 Cl +

CH 2 OOCCH 2Cl

2 NC 16H 33 CH 3

CH 2OOCCH 2 Cl CH 2OOCC H 2 Cl

CH 2 OOCCH 2

+

CH 3

NC 16 H 33.Cl-

ref lux ,10 h

CH 3 CH 3

Ethyl acetate

+

CH 2OOCC H 2

NC 16 H 33.ClCH 3

Scheme 1. Synthetic route to gemini 16-E2-16.

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

(a) 85

16-E2-16 5mM KCl 10mM KCl 20mM KCl 40mM KCl

80 75

(b) 85

10mM KNO3 75

20mM KNO3 40mM KNO3

70

(mN/m)

(mN/m)

16-E2-16 5 mM KNO3

80

70 65 60

65 60

55

55

50

50

45

45 -4.2 -4.0 -3.8 -3.6 -3.4 -3.2 -3.0 -2.8 -2.6

-4.2 -4.0 -3.8 -3.6 -3.4 -3.2 -3.0 -2.8 -2.6

log conc (mM)

log conc (mM) (c)

(d) 90 85

16-E2-16 5mM KSCN 10mM KSCN 20mM KSCN 40mM KSCN

80 75

16-E2-16 5mM NaBen 10mM NaBen 20mM NaBen 40mM NaBen

80

70

70

(mN/m)

(mN/m)

283

65 60

60

55 50

50 45 40

-4.4 -4.2 -4.0 -3.8 -3.6 -3.4 -3.2 -3.0 -2.8 -2.6

-4.4 -4.2 -4.0 -3.8 -3.6 -3.4 -3.2 -3.0 -2.8 -2.6

log conc (mM)

log conc (mM)

(e)

80 16-E2-16 5mM NaSal 10mM NaSal 20mM NaSal 40mM NaSal

76 72

(mN/m)

68 64 60 56 52 48 -4.2 -4.0 -3.8 -3.6 -3.4 -3.2 -3.0 -2.8 -2.6

log conc (mM) Fig. 1. Variation of surface tension () with concentration of 16-E2-16 at different fixed concentrations of (a) KCl, (b) KNO3 , (c) KSCN, (d) NaBen, and (e) NaSal. The scale shown is for pure 16-E2-16. Other curves have been shifted upwards by 5, 10, 15, 20 (mN/m) square units, respectively.

2.2.2. Fluorescence measurements The micelle aggregation numbers of pure and salt systems were determined by steady-state fluorescence quenching technique using Shimadzu spectrofluorometer-5000 (Japan) with slit widths

of 5 nm (both excitation and emission). Pyrene and cetylpyridinium chloride (monohydrate) were used as probe and quencher, respectively. An aliquot of the stock solution of pyrene in ethanol was transferred into a standard volumetric flask and the solvent

284

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

was evaporated. The surfactant solution in salt was added and the pyrene concentration was kept constant at 3 × 10−6 M. The quencher (CPC) concentration was varied from 0 to 0.010 mM. Excitation wavelength was kept at 337 nm and emission spectra were recorded in the range of 350–450 nm. The obtained spectra have five vibronic peaks. 2.2.3. 1 H NMR measurements 1 H NMR spectra of the synthesized geminis were recorded on 300 MHz Bruker Avance NMR spectrometer (Central Drug Research Institute, Lucknow) in D2 O with 1 H chemical shifts relative to internal standard tetramethylsilane (TMS). The stock solution of gemini (in the absence and presence of salts) was prepared in D2 O. About 1 ml of each solution was transferred to a 5 mm NMR tube and chemical shifts were recorded on the ı (ppm) scale (reproducibility within 0.01 ppm) at 303.15 K. The line widths at half heights (lw) were measured from spectra and are accurate to ±0.1 Hz. 2.2.4. Viscosity measurements Viscosity measurements were carried out using an Ubbelohde viscometer, suspended vertically in a thermostat at 303.15 K. The viscometer was cleaned and dried every time before use. In order to check the reproducibility, the time of fall for every measurement was noted at least twice with a calibrated stop watch (reproducibility within ±1%). 3. Results and discussion

the counterion, one with higher hydrophilicity prefers to stay in the bulk of micellar solution and, therefore, is less effective to screen the charge on the micellar surface. The sizes of Cl− and NO3 − ions are only slightly different but, due to their respective positions in the Hofmeister series, NO3 − ion causes a more pronounced cmc decrease. Unlike smaller anions, SCN− ions are large, poorly hydrated, and make stronger ion pairs with cationic surfactants; consequently, their cmc decreasing effect is higher. Such multinuclear large anions are more hydrophobic and have less structured hydration shell, due to which they prefer to stay in the bilayer interior. The cmc reducing capacity of organic salt counterions is even higher than the SCN− ion (Table 1). These counterions contain an aromatic phenyl moiety, and therefore, their influence is dependent upon the extent of their penetration into the micelles. The selfaggregation of the gemini surfactant in presence of organic salts is the resultant of both the electrostatic and hydrophobic interactions. Due to stronger tendency of aromatic counterions to penetrate the head group region, micellization occurs at lower loading vis-à-vis the weakly penetrating inorganic counterions. The anion with more hydrophobic skeleton among organic counterions gives rise to considerably lower cmc. On the basis of this discussion we can say that the properties of aqueous cationic gemini surfactant solutions can be efficiently modified by the addition of salts, especially organic salts and large-sized inorganic salts. A thermodynamic evaluation of the 16-E2-16–salt systems was also made and various parameters were calculated. Values of the ˘ cmc were obtained by the equation: ˘cmc = o − cmc

3.1. Micellization and surface activity The experimental profiles of surface tension () versus log[surfactant] for 16-E2-16 are typical of soluble surfactants that adsorb at the air/liquid interface (Fig. 1). Accumulation of the surfactant molecules at the interface is responsible for the reduction in the surface tension. With increase in bulk concentration of the surfactant, its interfacial concentration also increases until the interface is saturated by the surfactant molecules. Any further increase in the surfactant concentration in the liquid bulk now promotes self-association of the surfactant molecules into the so-called micelles. This equilibrium surfactant interfacial concentration depends on the properties of the bulk (including surfactant characteristics) and the interface. The critical micelle concentration (cmc), at which there is no further decrease in the surface tension with further addition of surfactant, was obtained from the plot of the equilibrium surface tension versus the natural logarithm of surfactant concentration in the liquid bulk (Fig. 1). Thus, the cmc corresponds to the point where a break in the surface tension versus log[surfactant] curve occurs. Under the experimental conditions of the current study, the cmc of 16-E2-16 was found to be 0.00128 mM. It is well established that the presence of electrolytes in the ionic surfactant solutions reduces their cmc’s [9–12] due to the charge screening effect, which leads to a reduction in the Debye length [13,14]. As such (i.e., with decrease in the Debye length), the repulsive interactions between similarly charged head groups decrease and micelle formation is facilitated: this results in increase in surface activity and the consequent lowering of the cmc value. For 16-E2-16 gemini the cmc value decreases with the nature of the added counterion and the magnitude of this decrease follows the order: Sal− > Ben− > SCN− > NO3 − > Cl− The inorganic salts affect through electrostatic interactions. As the structure of the micelle is controlled by the hydrated size of

(1)

where  o and  cmc are the surface tension of the solvent and the mixture at the cmc, respectively. The decreasing and increasing values of  cmc and ˘ cmc with increasing salt concentration indicate that the efficiency of the system increases (Table 1). The values of  max of the gemini surfactant molecules at the air/solution interface were calculated by using Gibbs equation [15]: max =



−

1 2.303nRT

  d  d log C

(2)

where R is the gas constant (8.314 J mol−1 K−1 ) and T the temperature in Kelvin. The factor n is the number of species at the air/aqueous interface which is introduced to allow for simultaneous adsorption of cations and anions. For divalent geminis, n is taken as 3 (the divalent amphiphile and the two counterions) [16,17]. The slope of the tangent at the given concentration of the surface tension () versus log[surfactant] plot was used to calculate  max . We see that the  max increases with an increase in the concentration of salts (Table 1), which means that more gemini surfactant molecules are adsorbed at the interface. This occurs due to the decrease in the repulsion among the head-groups caused by the addition of salts. Using  max values, the minimum area per molecule, Amin , can be evaluated as [18]: Amin =

1020 2

(3)

(NA max )(Å ) where NA is Avagadro’s number. The Amin decreases with increasing the salt concentration (Table 1), which is due to progressive charge shielding and closer packing of the gemini surfactant ions at the interface. To quantify the effect of salts in the mixture on the micellization process, the standard Gibbs energy of micellization, G◦ m , and the standard Gibbs energy of adsorption, (G◦ ads ) [19] were calculated by using Eqs. (4) and (5): G◦ m = RT ln Xcmc

(4)

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

285

Table 1 Various thermodynamic parameters ( cmc , ˘ cmc ,  max , Amin , cmc, G◦ m , G◦ ads ) for 16-E2-16 + salts mixed systems at 303.15 K, evaluated on the basis of surface tension measurements. Conc. (mM) KCl + 16-E2-16 0 5 10 20 40 KNO3 + 16-E2-16 0 5 10 20 40 KSCN + 16-E2-16 0 5 10 20 40 NaBen + 16-E2-16 0 5 10 20 40 NaSal + 16-E2-16 0 5 10 20 40

cmc (mM)

 cmc (mN m−1 )

˘ cmc (mN m−1 )

 max (×107 mol m−2 )

Amin (Å2 )

G◦ m (kJ mol−1 )

G◦ ads (kJ mol−1 )

0.00128 0.00122 0.00107 0.00086 0.00070

49.1 48.1 48.5 46.8 45.9

21.1 21.9 22.0 23.4 25.0

11.0 12.7 13.3 13.9 14.6

150.5 130.7 125.0 119.5 113.7

−22.1 −22.2 −22.3 −22.6 −22.9

−46.2 −46.1 −46.4 −46.9 −47.5

0.00128 0.00098 0.00072 0.00065 0.00047

49.1 47.5 49.5 47.4 47.6

21.1 22.5 22.9 25.3 26.4

11.0 12.6 16.6 13.7 17.8

150.5 131.9 99.9 120.8 93.1

−22.1 −22.4 −22.8 −23.0 −23.4

−46.2 −46.7 −47.1 −47.8 −48.3

0.00128 0.00083 0.00060 0.00057 0.00048

49.1 48.2 46.3 49.0 40.4

21.1 22.1 22.5 25.4 28.7

11.0 19.3 21.9 19.4 21.8

150.5 86.0 75.6 85.5 75.9

−22.1 −22.6 −22.1 −22.1 −23.4

−46.2 −46.5 −47.1 −47.6 −48.0

0.00128 0.00074 0.00050 0.00048 0.00041

49.1 47.5 46.1 44.9 41.4

21.1 23.1 23.9 25.4 26.9

11.0 16.1 15.7 14.5 12.5

150.5 102.8 105.6 113.7 132.6

−22.5 −22.8 −23.3 −23.4 −23.6

−46.2 −47.1 −48.1 −48.5 −49.3

0.00128 0.00067 0.00044 0.00042 0.00034

49.1 48.2 45.4 45.3 43.4

21.1 23.1 23.9 25.4 26.9

11.0 16.0 15.7 13.6 12.2

150.5 103.7 105.6 121.5 135.7

−22.5 −22.8 −23.3 −23.4 −23.6

−46.2 −47.1 −48.1 −48.6 −49.3

G◦ ads = G◦ m −

˘cmc max

(5)

The standard state for the adsorbed surfactant is a hypothetical monolayer at its minimum surface area per molecule, but at zero surface pressure. The values of both G◦ m and G◦ ads are negative (Table 1). It is the hydrophobicity which leads an amphiphile toward the air/water interface and thus is the main cause of adsorption. The work involved in transferring the surfactant molecule from a monolayer at a zero surface pressure to the micelle is represented by the last term in Eq. (5). This is small as compared to G◦ m , indicating the work involved in transferring the surfactant molecules from a monolayer at zero surface pressure to the micelle to be negligible. The spontaneity of adsorption process is obvious by the values of G◦ ads being negative. 3.2. Micellar aggregation number (Nagg ) The steady-state fluorescence probe technique has often been used in a variety of ways to study structural and dynamic aspects of surfactant aggregates in solution. Quantum yield studies are known to provide information regarding micelle size as well as the dynamic properties of both the micelles and of species solubilized therein [20,21]. In the present case the aggregation number (Nagg ) of micelles were determined [22–24] using the relation: ln

I  0

I

=

Nagg [Q] [Ct − cmc]

(6)

where I0 and I are the intensities of fluorescence emission for third vibronic peak in the pyrene emission spectra (at 375 nm), respectively, in the absence and presence of the quencher at [Q], Ct the total surfactant concentration (1 mM) and cmc the critical micelle concentration at the given salt concentration. The plots of ln[I0 /I] versus [Q] in the absence and presence of salts (inorganic/organic)

are shown in Fig. 2. The Nagg values calculated from slopes of linear fits of these plots are given in Table 2. We see that the Nagg varies as a function of nature and concentration of the added salt. Whereas it is slow in case of inorganic salts, the organic salts bring out a faster variation. In case of NaSal and NaBen, the generalization is that such additives do not simply locate at the exterior of the micelle but, due to their hydrophobic nature, insert their hydrophobic part into the interior of micelle. The main support for this behavior of polar organic additives containing a hydrophobic moiety is being provided by NMR studies (see later). The penetration of Sal− and Ben− counterions in the micelle from their hydrocarbon end with the ionic group lying in the outer hydrophilic shell of the micelle produces double effect, i.e., charge neutralization in head group region and simultaneous increase in hydrophobic interactions in the micelle. Both the effects favor micellization and hence result in increase of aggregation number. The inorganic salts (KCl, KNO3 , and KSCN) interact with the micelle only electrostatistically. Thus, the addition of salts leads to an increase in Nagg , the order followed being NaBen > KSCN > KNO3 > KCl. It was not possible to calculate the aggregation number of the surfactants in presence of sodium salicylate because of the absence of characteristic vibrational spectra of pyrene (5 peaks).

Table 2 Aggregation number (Nagg ) of 16-E2-16 in presence of different concentrations of various salts. Aggregation number Salt (mM)

KCl

KNO3

KSCN

NaBen

0 5 10 20 40

28 30 55 76 112

28 44 70 88 115

28 44 72 98 119

28 48 74 100 122

286

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

(a) 1.0

0.8

(b)

16-E2-16 5mM KCl 10mM KCl 20mM KCl 40mM KCl

1.0

0.8

16-E2-16 5mM KNO3 10mM KNO3 20mM KNO3

0.6

ln(Io/I1)

ln(I0/I1)

40mM KNO3 0.6

0.4

0.2

0.2

0.0 0.000

0.4

0.002

0.004

0.006

0.008

0.0 0.000

0.010

0.002

(c) 1.0

0.8

0.010

16-E2-16 5mM NaBen 10mM NaBen 20mM NaBen 40mM NaBen

0.6

ln(I0/I1)

ln(I0/I1)

0.008

(d) 1.0

16-E2-16 5mM KSCN 10mM KSCN 20mM KSCN 40mM KSCN

0.6

0.4

0.2

0.0 0.000

0.006

Q (mM)

Q (mM)

0.8

0.004

0.4

0.2

0.002

0.004

0.006

0.008

0.010

Q (mM)

0.0 0.000

0.002

0.004

0.006

0.008

0.010

Q (mM)

Fig. 2. Variation of ln(I0 /I1 ) versus quencher concentration for 16-E2-16 (1 mM) at different fixed concentrations of (a) KCl, (b) KNO3 , (c) KSCN, and (d) NaBen.

3.3.

1H

NMR studies and viscosity measurements

As we know, at high enough electrolyte concentrations, the charge–charge interaction between surfactant headgroups might be totally diminished, allowing the surfactant molecules to form close-packed micelles (possibly with different morphology). The morphological changes produced by the addition of salts were studied through 1 H NMR and viscosity measurements. A change in the chemical environment of the surfactant molecules provides variation in the chemical shift and line width at half height values (lw), which are taken as direct and foolproof evidences of micellar growth. A line width variation occurs as a consequence of spin lattice/spin–spin relaxation times variation. It surely acknowledges the relative mobility of particular groupings in a range of micellar phases. An increase in lw is observed due to the segmental motion or rotation of rod-like micelles resulting in a decrease in relaxation time. Thus, the morphological transition of micelles from sphere to rod is governed by the peak broadening and increase in lw values [25]. The fact that more pronounced chemical shift and line width changes are observed in case of protons found in the vicinity of cationic head group of the gemini surfactant prompted us to study the effect of varying the surfactant as well as the salt concentrations. 1 H NMR spectrum of pure 16-E2-16 (0.0064 mM) in D2 O is shown in Fig. 3, where various protons attached to carbon atoms are labeled. Table 3 shows the corresponding chemical shift (ı)

Fig. 3. 300 MHz 1 H NMR spectrum of 16-E2-16 (0.0064 mM) in D2 O at 303.15 K.

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

287

Table 3 1 H NMR chemical shifts of gemini 16-E2-16 (0.0064 mM) with various salts. System

Conc. (mM)

Chemical shift, ı (ppm) 1

2

3

4

5

6

7

16-E2-16 KCl KNO3 KSCN

– 20 20 20

0.989 0.980 0.973 0.965

1.408 1.400 1.394 1.389

1.894 1.893 1.873 1.932

3.443 3.442 3.415 3.469

3.745 3.743 3.711 3.772

4.592 4.599 4.559 4.600

4.646 4.641 4.624 4.670

NaBen

5 10 20

1.013 1.029 1.040

1.417 1.429 1.438

1.689 1.869 –

3.342 3.298 3.259

3.861 3.835 3.811

4.463 4.324 4.297

4.537 4.487 4.454

NaSal

5 10 20

2.223 1.043 1.004

2.400 1.445 1.419

– – –

3.283 3.123 3.026

2.917 3.264 3.763

3.283 4.329 4.241

3.478 4.491 4.398

(a)

SCN-

12.5

line width (Hz)

12.0 11.5 11.0

NO3 -

10.5 10.0

Cl-

9.5 3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

polarizibility

(b)

SCN-

12.5 12.0

line width (Hz)

values of various protons of 0.0064 mM 16-E2-16 in pure aqueous media. In order to see the effect of inorganic counter ions on micellar morphological transition, we have added 20 mM KCl, KNO3 or KSCN in 0.0064 mM 16-E2-16 and the 1 H NMR chemical shifts are recorded in Table 3. On addition of salts, the counterions accumulate on the surface, which causes a reduction in the electrostatic potential and the attraction between oppositely charged surface ligands takes place. The line width values of N+ CH3 protons of 16E2-16 at 20 mM inorganic salts are also evaluated where broader signals are observed in case of KSCN as compared to the other two salts (KCl, KNO3 ) (Fig. 4). The end-over-end tumbling motion of rod-shaped micelles [26] is responsible for broadening of proton resonances in the presence of inorganic salts (KCl, KNO3 , and KSCN). Fig. 4 shows the plot of lw values of 0.0064 mM 16-E2-16 + 20 mM inorganic salts as a function of anionic polarizability. Our results strongly support the fact that, being larger, the chaotropic anion SCN− shows more pronounced micellar growth, the reason of which has already been discussed. Hence it can be well stated that the micellar growth of the cationic gemini surfactant is governed by the nature and structure of salt anions following the Hofmeister series as: Cl− < NO3 − < SCN− in the present case. As already reported that organic salts show more pronounced micellar morphological transition [27] we have covered the influence of organic salts (NaBen and NaSal) in detail at their varied concentrations on 0.0064 mM 16-E2-16 by 1 H NMR measurements. In this regard, the role of partitioning site of an aromatic salt counterion on the micellar morphology has been taken into account. The aromatic counterions affect the micellization of gemini surfactants both electrostatistically as well as hydrophobically. Due to their greater tendency of penetrating the head group region of surfactant aggregates, organic counterions are well known growth enhancers at lower loadings [27]. Apart from others (e.g., geometric packing constituents, electronic substituent effects, and solvation effects), electrostatic and hydrophobic effects are of prime importance [28]. In addition to the common carboxylate moiety present in both the benzoate and salicylate ions, the latter contains an additional OH group. This affects the overall hydrophobicity of Sal− . Thus, the adsorption of aromatic counterions on the surfactant aggregates is affected by their refined structure as the hydrophobic interaction between organic counterions and surfactant aggregates is controlled by the position of substituent group in the benzene ring of aromatic counterion, influencing the morphology of surfactant aggregates. We see a significant change in the chemical shift of N+ CH3 protons of the gemini surfactant in presence of NaBen as well as NaSal (Table 3 and Fig. 5a). In this case also, the lw values of N+ CH3 protons are plotted against the added organic salt concentration (Fig. 5b). With the increasing concentration of organic counterion (Ben− , Sal− ), a prominent increase in lw values is observed, which is due to their strong interaction with gemini

11.5 11.0

NO3-

10.5 10.0

Cl-

9.5 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5

lyotropic number (Ln) Fig. 4. Plot of line width at half height of the 1 H NMR signal corresponding to the N+ CH3 group of 16-E2-16 (0.0064 mM) as a function of anion effective polarizibility (a) and lyotropic number (Ln ) (b) in solution.

288

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

(a) 1.10

(a) 3.5

NaBen NaSal

3.4

chemical shift (ppm)

16-E2-16

1.09 1.08 1.07

3.3

r

1.06 1.05

3.2

1.04

3.1

1.03 1.02

3.0 0.002 0.003 0.004 0.005 0.006 0.007 0.008

0

5

10

15

20

[gemini] (mM)

[salt] (mM) (b)

(b)

NaBen NaSal

70

NaBen NaSal

1.16

1.15

60 1.14

r

line width (Hz)

50 1.13

40 30

1.12

20

1.11

10

4

6

8

10

12

14

16

18

20

22

[salt] (mM)

0 0

5

10

15

20

[salt](mM) Fig. 5. (a) Influence of organic salt concentration on variation in line width at half height of the 1 H NMR signal corresponding to the N+ CH3 group of 16-E2-16 (0.0064 mM), and (b) influence of salt concentration on variation in chemical shift of the 1 H NMR signal corresponding to the N+ CH3 group of 16-E2-16 (0.0064 mM).

micelles. It also signifies a reduced mobility and tightly packed components in the gemini–salt mixtures. The increase in lw values is more in case of NaSal than that in the case of NaBen at the same salt concentrations. The relative viscosity (r ) values are also plotted with increasing salt concentration (Fig. 6b). An increase in r values with increasing concentration of both the salts is observed showing micellar transition. Here again, the growth was more pronounced in case of NaSal than that of NaBen confirming stronger interaction of Sal− with the cationic gemini head group than Ben− , as depicted by r values in case of former than the latter. Thus, both the techniques (1 H NMR and viscometry) show correlation in the results obtained in the present study. It has been observed that, in case of NaSal, the ortho position of OH group is more favorable for causing micellar growth [29] (see structures in Figs. 7 and 8). Apart from the possible hydrogen bonding between OH and neighboring COOH, other types of interactions possibly affect the specific orientation at the micellar surface leading to complicated adsorption properties. With an

Fig. 6. Variation in relative viscosities (r ) with the added gemini in water (a) and salt in 0.0064 mM 16-E2-16 (b) at 303.15 K.

increase in concentration of organic salts, the peaks of ethylene protons in alkyl chain show upfield as well as downfield shifts. An increase in ı values is observed for the protons present in the micellar core region and a decrease in ı values is observed for the protons present near the head group region (Table 3). The micellar growth is evidenced by the merging of peaks along with the broadening of peaks with increasing concentration of salts. As a result of the increase in micellar size, there is a restriction in the mobility of salt anions, which causes peak broadening. Therefore, our data support the fact that organic counterions get solubilized in the palisade layer of the micelles present between cationic head groups and outer portion of the micellar core consisting first few carbon atoms of alkyl chains. There is a strong interaction of the cationic geminis with aromatic anions, which is being indicated by the resonance of 4-H and 5-H protons (decrease in ı values) with the increasing salt concentration (Fig. 5a). For better understanding, we have also undertaken a comparative study of NaBen and NaSal into account. The 1 H NMR spectra of ring protons of NaBen and NaSal in D2 O in absence and presence of 0.0064 mM 16-E2-16 are shown in Figs. 7 and 8. In case of NaBen, the 3-, 4-, 5-H protons show upfield shift, whereas, 2-, 6-H protons show downfield shift, which clearly

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

289

Fig. 7. 300 MHz 1 H NMR spectra of NaBen: (a) pure NaBen (20 mM), and (b) at different concentrations of NaBen in presence of 16-E2-16 (0.0064 mM) at 303.15 K. Fig. 8. 300 MHz 1 H NMR spectra of NaSal: (a) pure NaSal (20 mM), and (b) at different concentrations of NaSal in presence of 16-E2-16 (0.0064 mM) at 303.15 K.

indicates that the benzene ring lies in the palisade layer of micelles and the negatively charged carboxylate ion lies in the cationic head group region of gemini micelles. Also, the merging of peaks and peak broadening indicate a strong interaction and micellar growth with increasing salt concentration in the gemini micellar system (Fig. 7). On the other hand, in the spectra of NaSal, the peak of OH proton is not visible separately, as it is labile and has the ability to exchange frequently with deuterium of the solvent. As a result, the OH proton peak merges with the peak of the solvent. The 3-, 4-, and 5-H protons of NaSal are present in nonpolar environment and show upfield shift, whereas 6-H proton is present in polar environment and show downfield shift. It also confirms the intercalation of Sal− in the headgroup region of gemini surfactant. The orientation of Sal− on the micellar surface is such that the negatively charged carboxylic group tends to stay away from the positively charged micellar surface leading to a charge separation to some extent, which thereby increases the energy of the system. On the other hand, the total energy of the system is pacified by the attractive interaction between the carboxylic group of one micelle and the positively charged surface of second micelle. These types of interactions are responsible for the formation of dimers, trimers, tetramers, etc. This indeed supports the fact that NaSal is highly

surface active, which is evidenced by observed peak broadening with increasing concentration of NaSal. 4. Conclusion The physico-chemical properties of cationic gemini surfactants can be finely tuned with counterions (of inorganic and organic salts). Micellar and interfacial properties of gemini surfactant 16-E2-16 containing ester groups in the spacer, in the presence of salts (inorganic and organic) have been studied by tensiometry, flourimetry, viscometry, and 1 H NMR. The results show that the cmc and head group area decrease with increasing salt concentration. Combinations of salts and gemini surfactants exhibit pronounced synergistic effects. Thus, such mixtures are attractive in view of a potential performance enhancement of a given gemini surfactant. The relative importance of the synergistic effects depends on the particular pair of gemini and anions, which must be individually optimized for a given property. The ability to promote surfactant aggregation decreases in the order of NaSal > NaBen > KSCN > KNO3 > KCl. The organic salts are more effective to promote the aggregation because of the hydrophobic nature of the benzene ring of aromatic salt anions. It is observed that both the electrostatic and the hydrophobic interactions

290

Mohd. Akram et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 281–290

exhibited by the salt counterions play an important role in deciding the charge neutralization on the micellar surface and hence the growth of micelles. This work is helpful in understanding the effect of both inorganic and organic salts on the aggregation behavior of gemini surfactants and suggests that applying proper salts can effectively adjust the structure of the surfactant aggregates. The findings reported in this study could form the basis for utilizing biodegradable surfactants in presence of electrolytes in many industrial applications with the ultimate aim of producing surface active agent formulations that are green and sustainable. Acknowledgement KU acknowledges UGC, New Delhi, for award of Emeritus and BSR Fellowships. References [1] (a) S. Ikeda, S. Ozeki, M. Tsunoda, Micelle molecular weight of dodecyldimethylammonium chloride in aqueous solutions, and the transition of micelle shape in concentrated NaCl solution, J. Colloid Interface Sci. 73 (1980) 27–37; (b) S. Ozeki, S. Ikeda, The sphere-rod shaped transition of micelles and two step micellisation of dodecylammonium bromide in aqueous NaBr solutions, J. Colloid Interface Sci. 87 (1982) 424–435; (c) T. Imae, R. Kamiya, S. Ikeda, Formation of spherical and rod shaped micelles of cetyltrimethylammonium bromide in aqueous NaBr solutions, J. Colloid Interface Sci. 108 (1985) 215–255; (d) M.J. Rosen, Surfactants and Interfacial Phenomena, 3rd ed., WileyInterscience, New York, 2004. [2] R. Zana, Dimeric (gemini surfactants): effect of spacer group on the association behavior in aqueous solution, J. Colloid Interface Sci. 248 (2002) 203–220. [3] J. Cross, Environmental aspects of cationic surfactants, in: J. Cross, E.J. Singer (Eds.), Cationic Surfactants, Marcel Dekker, New York, 1994, p. 3. [4] N. Funasaki, M. Ohigashi, S. Hada, S. Neya, Surface tensiometric study of multiple complexation and hemolysis by mixed surfactants and cyclodextrins, Langmuir 16 (2000) 383–388. [5] L. Huber, L. Nitschke, Environmental aspects of surfactants, in: K. Holmberg (Ed.), Handbook of Applied Surface and Colloidal Chemistry, vol. 1, Wiley, Chichester, England, 2002, p. 509. [6] (a) M. Stjerndahl, C.G.V. Ginkel, K. Holmberg, Synthesis and chemical hydrolysis of surface active esters, J. Surfactants Deterg. 6 (2003) 319–324; (b) A.R. Tehrani-Bagha, H. Oskarsson, C.G.V. Ginkel, K. Holmberg, Cationic ester-containing gemini surfactants: chemical hydrolysis and biodegradation, J. Colloid Interface Sci. 312 (2007) 444–452; (c) G. Zhinong, T. Shuxin, Z. Qi, Z. Yu, L. Bo, G. Yushu, H. Li, T. Xiaoyan, Synthesis and surface activity of bisquaternary ammonium salt gemini surfactants with ester bond, Wuhan Univ. J. Nat. Sci. 13 (2008) 227–231; (d) N. Fatma, W.H. Ansari, M. Panda, Kabir-ud-Din, Mixed micellisation behavior of gemini (cationic ester bonded) surfactants with conventional (cationic, anionic and nonionic) surfactants, Z. Phys. Chem. 227 (2013) 133–149. [7] S. De, V.K. Aswal, P.S. Goyal, S. Bhattacharya, Role of spacer chain length in dimeric micellar organisation: small angle neutron scattering and fluorescence studies, J. Phys. Chem. 100 (1996) 11664–11671. [8] R. Zana, J. Xia, Gemini Surfactants: Synthesis, Interfacial and Solution Phase Behavior and Applications, vol. 117, Marcel Dekker, New York, 2004.

[9] S.S. Helvaci, S. Peker, G. Ozdemir, Effect of electrolytes on the surface behavior of rhamnolipids R1 and R2, Colloids Surf. B: Biointerfaces 35 (2004) 225–233. [10] S. Javadian, H. Gharibi, Z. Bromand, B. Sohrabi, Electrolyte effect on mixed micelle and interfacial properties of binary mixtures of cationic and nonionic surfactants, J. Colloid Interface Sci. 318 (2008) 449–456. [11] B. Sarac, M. Bester-Rogac, Temperature and salt-induced micellization of dodecyltrimethylammonium chloride in aqueous solution: a thermodynamic study, J. Colloid Interface Sci. 338 (2009) 216–221. [12] R.M.F. Fernandes, E.F. Marques, B.F.B. Silva, Y. Wang, Micellization behavior of a catanionic surfactant with high solubility mismatch: composition, temperature, and salt effects, J. Mol. Liq. 157 (2010) 113–118. [13] J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1991. [14] S. Chiruvolu, J.N. Israelachvili, E. Naranjo, Z. Xu, J.A. Zasadzinski, E.W. Kaler, K.L. Herrington, Measurement of forces between spontaneous vesicle-forming bilayers, Langmuir 11 (1995) 4256–4266. [15] D.K. Chattoraj, K.S. Birdi, Adsorption and the Gibbs Surface Excess, Plenum Press, New York, 1984. [16] L. Perez, J.L. Torres, A. Manresa, C. Solans, M.R. Infante, Synthesis, aggregation, and biological properties of a new class of gemini cationic amphiphilic compounds from arginine, bis(Args), Langmuir 12 (1996) 5296–5301. [17] K. Tsubone, Y. Arakawa, M.J. Rosen, Structural effects on surface and micellar properties of alkanediyl-␣, ␻-bis(sodium N-acyl-␤-alaninate) gemini surfactants, J. Colloid Interface Sci. 262 (2003) 516–524. [18] K. Anand, O.P. Yadav, P.P. Singh, Studies on surface and thermodynamic properties of some surfactants in aqueous and water + 1,4-dioxane solutions, Colloids Surf. 55 (1991) 345–358. [19] M.J. Rosen, A.W. Cohen, M. Dahanayake, X. Hua, Relationship of structure to properties in surfactants. 10. Surface and thermodynamic properties of 2dodecyloxypoly(ethenoxyethanol)s, C12 H25 (OC2 H4 )x OH, in aqueous solution, J. Phys. Chem. 86 (1982) 541–545. [20] K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, New York, 1987. [21] S. Reekmans, D. Bernik, M. Gehlen, J. van Stam, M. Van Der Auweraer, F.C. De Schryver, Change in the micellar aggregation number or in the size distribution? A dynamic fluorescence quenching study of aqueous cetyltrimethylammonium chloride, Langmuir 9 (1993) 2289–2296. [22] J. van Stam, S. Depaemelaere, F.C. De Schryver, Micellar aggregation numbers: a fluorescence study, J. Chem. Edu. 75 (1998) 93–98. [23] N.J. Turro, A. Yekta, Luminescent probes for detergent solutions. A simple procedure for determination of the mean aggregation number of micelles, J. Am. Chem. Soc. 100 (1978) 5951–5952. [24] R.G. Alargova, J.I. Kochijashky, M.L. Sierra, R. Zana, Micelle aggregation numbers of surfactants in aqueous solutions: a comparison between the results from steady-state and time-resolved fluorescence quenching, Langmuir 14 (1998) 5412–5418. [25] F.A.L. Anet, Novel spin–spin splitting and relaxation effects in the proton NMR spectra of sodium salicylate in viscoelastic micelles, J. Am. Chem. Soc. 108 (1986) 7102–7103. [26] U. Olsson, O. Soderman, P. Guering, Characterization of micellar aggregates in viscoelastic surfactant solutions. A nuclear magnetic resonance and light scattering study, J. Phys. Chem. 90 (1986) 5223–5232. [27] U.S. Siddiqui, F. Khan, I.A. Khan, Kabir-ud-Din, Role of added counterions in the micellar growth of bisquaternary ammonium halide surfactant (14-s-14): 1 H NMR and viscometric studies, J. Colloid Interface Sci. 355 (2011) 131–139. [28] D. Ohlendorf, W. Interthal, H. Hoffmann, Surfactant systems for drag reduction: physico-chemical properties and rheological behavior, Rheol. Acta 25 (1986) 468–486. [29] C. Manohar, U.R.K. Rao, B.S. Valaulikar, R.M. lyer, On the origin of viscoelasticity in micellar solutions of cetyltrimethylammonium bromide and sodium salicylate, J. Chem. Soc., Chem. Commun. (1986) 379–381.