phase and rheological behavior of cetyldimethylbenzylammonium salicylate (cdbas) and water.pdf

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Journal of Surfactants andDetergents ISSN 1097-3958Volume 14Number 2 J Surfact Deterg (2011)14:269-279DOI 10.1007/s11743-010-1223-6

Phase and Rheological Behavior ofCetyldimethylbenzylammonium Salicylate(CDBAS) and Water

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ORIGINAL ARTICLE

Phase and Rheological Behaviorof Cetyldimethylbenzylammonium Salicylate (CDBAS) and Water

Francisco Carvajal-Ramos • Alejandro Gonzalez-Alvarez • J. Roger Vega-Acosta •

Donato Valdez-Perez • Vıctor Vladimir Amilcar Fernandez Escamilla •

Emma Rebeca Macıas Balleza • J. Felix Armando Soltero Martınez

Received: 27 May 2010 / Accepted: 6 July 2010 / Published online: 4 August 2010

� AOCS 2010

Abstract The temperature–composition phase diagram in

the diluted region of the cationic surfactant cetyldi-

methylbenzylammonium salicylate/water system was stud-

ied with a battery of techniques. The Krafft temperature

(Tk = 33 ± 1 �C) was measured by differential scanning

calorimetry, polarizing microscopy, conductimetry, viscos-

imetry, and rheometry. The critical vesicle concentration

(cvc, *0.002 wt%) and a vesicle–micellar transition

(cvm, *0.005 wt%) was detected at a temperature of 35 �C.

Below Tk and concentrations B2 wt%, a transparent solution

is formed (I). Above 2–8.5 wt%, a lamellar (L1) phase forms.

At higher concentrations and up to 12 wt%, a second

lamellar phase (L2) is detected. From 12.4 to 15.5 wt%, an

emulsion phase (E) is formed. Rheological dynamic mea-

surements for the I phase indicate that the system exhibits a

predominantly viscous behavior (G0\ G00) for concentra-

tions lower than the overlap or entanglement concentration

(Ce, *0.75 wt%). At higher concentrations, wormlike

micelles form and the elastic behavior predominates

(G0[ G00). The elastic (G0) modulus collapses in a concen-

tration–time master curve in the whole reduced frequencies

range xsc examined, whereas the viscous modulus (G00)collapses only at reduced frequencies lower than 0.1.

Reduced stress plotted as a function of the reduced

shear rate yields a good superposition of the curves at the

different concentrations up to the onset of the non-linear

behavior.

Keywords Phase behavior � Vesicle � Wormlike �Hydrotropic counterion � Rheology � Shear banding flow

Introduction

Surfactants are amphiphile substances that self-organize in

solvents, forming spontaneously a large variety of struc-

tures (micelles, wormlike micelles, liquid crystals, vesicle,

etc.) [1]. These structures are of fundamental interest in

science research and in many technological applications,

such as drug delivery, in medical diagnostics, cosmetic

formulations, and in the food industry [2–11].

The structure transitions in surfactants solution systems

can be induced by using a great variety of parameters, e.g.,

changes in temperature, surfactant concentration, pressure,

ionic strength, pH, additives like alcohol, salts, and hy-

drotropes [12, 13], and by external fields such as shear

deformations, magnetic, and electric fields [14]. Self-

organization behavior of surfactants in solutions is a

spontaneous process that is thermodynamically driven.

This phenomenon can be described based on the packing

parameter, P ¼ vt

ahlc;t

� �; where vt is the volume of the

hydrocarbon tail of the surfactant in the core, ah is the

optimal head group area, and lc,t is the critical chain length

of the tail [14–29]. This parameter is used to predict the

most presumable shapes that a surfactant can form, i.e.,

F. Carvajal-Ramos � A. Gonzalez-Alvarez �E. R. Macıas Balleza � J. Felix Armando Soltero Martınez (&)

Departamento de Ingenierıa Quımica, Universidad de

Guadalajara, Boul. M. Garcıa Barragan #1451, Guadalajara,

Jalisco 44430, Mexico

e-mail: [email protected]; [email protected]

J. Roger Vega-Acosta � D. Valdez-Perez

Instituto de Fısica, Universidad Autonoma de San Luis Potosı,

Alvaro Obregon #64, San Luis Potosı, SLP 78000, Mexico

V. V. A. Fernandez Escamilla

Departamento de Ciencias Tecnologicas,

Universidad de Guadalajara, Av. Universidad #1115,

Ocotlan, Jalisco, Mexico

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J Surfact Deterg (2011) 14:269–279

DOI 10.1007/s11743-010-1223-6

Author's personal copy

P \ 1/3 for spherical micelles, P = 1/3 - 1/2 for rodlike

micelles, P = 1/2 - 1 for flexible bilayer, vesicles [30,

31].

Many studies have been performed in the past two

decades into understanding the effect of hydrotropes in the

phase behavior of surfactants/water systems [32–34]. Hy-

drotropes are molecules that were described first by Neu-

berg [32]. They have an amphiphilic structure and the

ability of increasing the solubility of scarcely soluble

organic molecules in water [32]. Cationic surfactants are

very sensitive to changes in the molecular structure of

aromatic hydrotropes [35–45], since even small changes in

the organic counterion structure can dramatically modify

the phase and rheological behavior [46]. Hydrotropes cause

profound decreases in critical micelle concentration (cmc)

and induce the formation of viscoelastic wormlike micelle

solutions at lower surfactant concentrations (i.e., salicylate,

tosilate, p-vinylbenzoate) [46–48] compared to that of the

respective halide counterions [36]. Buwalda et al. [49]

studied the influence of aromatic hydrotropes on single-

chain amphiphiles aqueous solutions, observing that hy-

drotropes modify the packing parameter by interacting with

the amphiphiles head, thereby, changing the structure.

The spontaneous formation of vesicles and their appli-

cation in drug delivery and DNA compacting have been

reported in numerous articles [50, 51]. The most interesting

property of vesicles is their capacity to incorporate

hydrophilic and lipophilic substances [14]. Over the last

two decades, mostly natural and synthetic double-chain

amphiphiles have been used for forming vesicles [14].

Great effort has been placed into understanding and

controlling the stability of this self-assembled structure

[12]. Kaler et al. [16] reported the spontaneous formation

of vesicles from single-chain mixed cationic and anionic

surfactants. These catanionic surfactants are a cheaper

option compared to lipids for spontaneous vesicles

formation.

Cetyldimethylbenzylammonium chloride (CDBAC) is a

cationic surfactant that forms in water spherical micelles

with low aggregation numbers at low concentrations [52–

54]. The benzyl group can act as a second hydrophobic

chain, which can interact with the hydrocarbon chains and

with the counterions that are around the micelles [54].

In this work, we studied the effect of the hydrotrope

counterion salicylate on the phase and rheological behavior

of the cetyldimethylbenzylammonium ion (CDBA?) in

water from diluted concentrations of up to 15 wt% sur-

factant. Salicylate is a highly hydrophobic counterion that

is able to reduce the effective head-group area (ah) by

shielding the positive charge of the surfactant head.

Besides interacting with the benzyl group as a second

hydrophobic tail, this produces an increase in the packing

parameter value and the possibility of forming lamellar or

vesicular structures. Studies were performed by using a

battery of analytical techniques, such as: differential

scanning calorimetry (DSC), conductimetry, viscosimetry,

polarizing microscopy, transmission electron microscopy

(TEM), and rheometry.

Experimental

Materials

Cetyldimethylbenzylammonium chloride (CDBAC) with a

purity of 97.0 wt% and sodium salicylate (NaSal) with a

purity of 99.5 wt% were purchased from Fluka and used as

received. Two aqueous solutions (100 mL each) of NaSal

(20.2 9 10-3 M) and CDBAC (20.0 9 10-3 M) were

prepared at 40 �C and sonicated for 1 h. The cetyldi-

methylbenzylammonium salicylate (CDBAS) was prepared

by mixing the solutions and standing at 40 �C for 48 h,

when a white precipitate would be observed. The precipi-

tate was filtrated and recrystallized three times from a

water–acetone mixture (10:90 by volume) and the surfac-

tant was dried using phosphorous pentoxide for 1 week.

Methods

The surfactant structure was confirmed by NMR spec-

troscopy (Advance DMX500 Bruker spectrometer). HPLC-

grade water was used.

For phase and rheological studies, samples were prepared

by weighing appropriate amounts of CDBAS and water in

glass vials. For the critical vesicular concentration (cvc), the

vesicular-to-sphere, and sphere-to-rod concentrations tran-

sitions, samples in a range from 5 9 10-4 to 5 9 10-2 wt%

were prepared by dilution from a 0.1 wt% stock solution.

Samples were placed in a water batch at 60 �C for a week,

where they were frequently shaken. Then, the samples were

allowed to reach equilibrium at the examination or mea-

surement temperature. All samples were centrifuged to

remove suspended air bubbles before being tested. Critical

concentrations were determinated by viscosimetry and

conductimetry at 35 �C. Electrical conductivities were

measured with a conductimeter (Oakton Model 510) and an

immersion cell (YSI) with cell constant = 1 cm-1. The cell

was calibrated with standard KCl solutions of known con-

ductivity. Viscosity measurements were performed in an

Anton Paar AMVn microviscometer with a repeatability of

\0.1% for viscosity and 0.01 �C for temperature.

Phase behavior was determinated by visual observa-

tions, polarized light microscopy, differential scanning

calorimetry, and rheometry. In order to discriminate

between isotropic and liquid crystalline phases, samples

were viewed through crossed polarizers.

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To identify the liquid crystalline phases, samples were

examined with an Olympus BX51 optical microscope

equipped with polarizers and a Linkam CSS450 heating-

and-cooling stage, with a digital temperature controller

(±0.1 �C).

Thermograms were obtained with a TA Instruments

Q2000 differential scanning calorimeter that was previ-

ously calibrated with indium, water, and n-octane stan-

dards. All scans were made with heating and cooling rates

of 0.1 �C/min. In order to minimize any loss due to

evaporation, aluminum pans for volatile samples (TA

Instruments) were used. Samples in the sealed pans were

weighed before and after each test. Results from the sam-

ples that lost weight were discarded. Transition tempera-

tures were determined to within 0.5 �C upon heating from

the point where the base line changed slope significantly.

Steady and oscillatory simple-shear experiments were

performed in a strain-controlled TA Instruments Ares-22

rheometer with a cone-and-plate geometry of 0.1 radian

and a diameter of 50 mm. An environmental control unit

was placed around the cone-and-plate fixture to prevent

water evaporation. The temperature was controlled to

within 0.1 �C during the measurements.

Transmission electron microscopy (TEM) was per-

formed following the negative staining technique with

uranyl acetate using a HRTEM FEI TECNAI F30 STWIN

G2. Details of the procedure have been described else-

where [55].

Results and Discussion

Phase Behavior

Figure 1 shows plots of the conductivity and viscosity as a

function of CDBAS concentration measured at 35 �C. At

low surfactant concentrations below the cmc, the viscosity

decreases slightly with increasing concentration and it is

smaller than the viscosity of pure water. There are several

reports about the reduction of water viscosity due to the

addition of surfactants in the premicellar range [56–58].

This behavior was explained in terms of the theory

developed by Nemethy and Scheraga [59, 60]. At around

0.0024 wt% (arrow a), the viscosity begins to increase up

to 0.0050 wt%, where a maximum appears (arrow b). This

slope change is due to the formation of structures larger

than the monomeric ions. From 0.0050 to 0.0085 wt%, the

viscosity decreases as the surfactant concentration aug-

ments and a minimum is observed (arrow c). This decrease

in viscosity indicates a transition to a smaller structure.

After this minimum, the viscosity increases sharply, indi-

cating a third structural transition. The conductivity, on the

other hand, increases linearly with surfactant concentration

up to 0.0024 wt%. At this concentration, the conductivity

decreases steeply (arrow a) and then increases linearly

again up to 0.0050 wt%, where the conductivity increases

steeply (arrow b). At higher surfactant concentrations, the

conductivity increases linearly.

The formation of structures larger than monomer ions is

indicated by the viscosity increase and the conductivity

drop detected at 0.0024 wt% (arrow a). The second tran-

sition observed from 0.0050 to 0.0085 wt%, where the

viscosity decreases and the conductivity increases with

surfactant concentration, suggests that there is a transition

from larger to smaller structures. We believe that the first

transition is due to the formation of vesicles and the second

one is a vesicles-to-micelle transition. On the other hand,

the third slope change in conductivity, detected at con-

centrations higher than 0.0085 wt%, is probably due to the

micelle-to-rod transition.

Measurements of TEM were performed in order to

discriminate the structures that are formed in these transi-

tions. Figure 2 shows the negative-staining TEM photo-

graph for the sample with a concentration of 0.005 wt%.

This micrograph affirmably proved the existence of vesi-

cles. In this picture, it is evident that vesicles coexist with

spherical micelles.

CDBAS is quite soluble in water above 33 ± 1 �C. At

temperatures lower than 33 �C, CDBAS/water samples

exhibit a white, granular appearance due to the presence of

CDBAS crystals dispersed in an isotropic saturated sur-

factant solution. At 35 �C and for surfactant concentrations

lower than 0.75 wt%, the solutions are isotropic and

transparent, with viscosities similar to that of water. At

higher concentrations and up to around 2.3 wt%, the vis-

cosity increases as the concentration augments. Samples

are transparent and isotropic when viewed through crossed

polarizers, but they show streaming birefringence that

increases in intensity upon increasing concentration and

decreases as temperature increases. This behavior is similar

to that exhibited by wormlike micellar solutions of single-Fig. 1 Electrical conductivity (filled squares) and viscosity (opensquares) as a function of CDBAS measured at 35 �C

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tail surfactants with aromatic hydrotropes, e.g., cetyltri-

methylammonium tosilate (CTAT)/water system [47], but

the streaming birefringence intensity and viscosity values

for solutions with the same concentration are much lower

for CDBAS/water than those of the CTAT/water system

[47]. At concentrations higher than 2.3 wt% and up to

12 wt%, samples are birefringent, with textures typical of

lamellar liquid crystals. To the naked eye, samples with

concentrations from 2.3 to 8.5 wt% are milky and opales-

cent; at higher concentrations, the samples turned bluish

and turbid. Figure 3a depicts photograph of a 7 wt%

CDBAS sample taken through crossed polarizers in the

polarizing microscope at 35 �C. In this picture, a cloudy

texture, positive, and negative spherulites are detected;

these textures are consistent with the lamellar phase [61].

For a 9 wt% CDBAS sample (Fig. 3b), the micrographs

show changes in textures, oily streaks are observed, neg-

ative spherulites are predominant, and only a few positive

spherulites are observed. With increasing temperature, the

lamellar phase texture disappears slowly in the polarizing

microscopy at around 80 �C for a 6 wt% CDBAS sample,

indicating a transition into an isotropic phase. At concen-

trations higher than 12 wt% and up to 15 wt%, the samples

are completely milky and isotropic. When samples are

observed in the polarizing microscopy with parallel

polarizers, a suspension of small and isotropic droplets

dispersed in an isotropic solution is detected. We noted this

phase as emulsion (E). When diluted samples

(C \ 12 wt%) are cooled at temperatures lower than Tk,

they do not crystallize for months, up to around 23 �C;

these solutions are named pseudo-solutions by Ravin et al.

[62]. Recently, Benedini et al. [63] reported this behavior

in the amiodarone/water system. For pure surfactant, three

transitions were detected in the polarizing microscopy with

crossed polarizers. At temperatures lower than 35 �C,

hydrated crystals of surfactant (Ch) are observed. From 35

and up to 45 �C, waxy crystals (Cw) are detected, and for

higher temperatures, an isotropic solution is formed.

Figure 4 shows DSC thermograms at different CDBAS

concentrations. For concentrations lower than 2 wt% (see

thermogram for 0.5 wt%), only two peaks are detected, the

large peak (peak a) that appears at 0 �C is due to the

melting of free water, the peak area of which diminishes

with surfactant concentration, as expected. The second

smaller peak (peak b) begins at around 33 �C and increases

in intensity with surfactant concentration; this temperature

coincides with that observed visually when the surfactant

begins to be soluble in water. This confirms that this

temperature corresponds to the Krafft temperature

(Tk = 33 �C), which is bigger than that observed in sur-

factants with the same length tail and with different hy-

drotropes, such as tosilate [47] and p-vinylbenzoate [48].

At concentrations higher than 2 wt% and up to 12 wt%

(see thermogram for 4 wt% sample), a third peak (peak c)

is detected at 73 �C, which coincides with the transition L1

to I, observed by temperature sweeps in the polarizing

Fig. 2 Negative-staining TEM photograph for 0.005 wt% CDBAS

concentration

Fig. 3 Photograph of CDBAS/water samples taken at 35 �C through

crossed polarizers in a polarizing microscope: a 7 wt%, b 9 wt%

272 J Surfact Deterg (2011) 14:269–279

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microscopy. For more concentrated solutions (see ther-

mogram for 15 wt% sample), only both the melting (a) and

Tk (b) peaks are observed. When the pure surfactant is

heated, two peaks are detected. The first one (peak d)

appears at 35 �C and is due to the surfactant hydrocarbons

chain melting when waxy crystals are formed, which was

observed by polarizing light microscopy, and a second

sharp peak (peak e) is seen at ca. 43 �C, which is associated

to the waxy crystal-to-isotropic transition seen at 45 �C in

the polarizing microscopy.

Rheological Behavior

Figure 5 depicts G0 and G00 as a function of temperature for

selected CDBAS concentrations, 1 (c), 3 (b), and 6 (c) wt%

samples. Measurements were performed at a frequency of

10 rad/s and a deformation of 20% for the (a) and (b)

samples and 0.1% for the (c) sample, which are within the

linear viscoelastic regime (LVR). The 1 wt% sample

(Fig. 5a) exhibits a slightly predominant viscous behavior,

that is, G00[ G0 for temperatures lower than 15 �C; in this

temperature range, both moduli diminish gradually with

increasing temperature. Above 15 �C, G0 and G00 cross over

(arrow a) and a predominantly elastic response is observed

(G0[ G00); both moduli increase with temperature up to

around 23 �C, where a maximum is observed (arrow b).

Above 23 �C and up to 34 �C, the moduli decrease by more

than one magnitude order. The slope change detected at

15 �C is due to the transition from hydrated crystals (Ch) to

a mixture of waxy crystal (Cw) and isotropic phase (I), as

confirmed by polarizing light microscopy. The reduction

exhibited for the moduli from 23 to 34 �C is due to the

melting of waxy crystals into a transparent, isotropic phase

that exhibits streaming birefringence; this transition was

seen by the naked eye. At temperatures higher than 34 �C

(arrow c), both G0 and G00 increase sharply by around two

magnitude orders. This sharp increase in moduli with

temperature is a sign of the Krafft temperature. This

increase in the elastic modulus is due to the formation,

growth, and entanglement of wormlike micelles. Above Tk

and up to the temperature of the G0 and G00 crossover (Ts),

the sample response is dominantly elastic. For temperatures

higher than Ts, the sample becomes viscous again. This

response is observed for concentrations up to 2 wt%

CDBAS. The sample with 3 wt% concentration (Fig. 5b)

and for temperatures lower than Tk (arrow c) exhibits a

similar behavior, and two transitions were detected (arrows

a and b). For temperatures above Tk, the rheological

behavior is predominantly viscous, where both moduli

decrease with temperature and around 70 �C, a slope

change is detected. At this temperature, a transition from a

milky and opalescent solution to an isotropic non-bire-

fringent solution was visually detected. For concentrations

in the range where the lamellar phase was detected, the

rheological behavior at temperatures lower than Tk is

similar to that shown for 1 and 3 wt% samples. The

moduli–temperature dependence for a 6 wt% sample is

depicted in Fig. 5c. At temperatures higher than Tk, both G0

Fig. 4 DSC thermograms of CDBAS/water samples as a function of

CDBAS concentration. The heating rate was 0.1 �C/min

Fig. 5 Storage G0 (filled squares) and loss G00 (open squares) moduli

at 10 rad/s within the linear viscoelastic region as a function of

temperature for CDBAS concentrations of: a 1, b 3, and c 6 wt%

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and G00 decrease by almost two magnitude orders and a

predominantly elastic response is observed. It is evident

that, from 34 to 70 �C, the moduli are temperature-inde-

pendent; above 70 �C, both G0 and G00 diminish up to

80 �C, where the moduli cross over (Ts) and a predomi-

nantly viscous behavior is observed. Figure 6 depicts

moduli temperature dependence performed in the heating

and cooling modes for the sample with 3 wt% in surfactant.

It is evident in this figure that, for temperatures higher than

Tk, the data for both experiments have the same tendency.

For temperatures lower than Tk, in the cooling mode data,

the Krafft temperature is not observed and it is possible to

measure moduli data even up to 15 �C lower than Tk

(18 �C, arrow a) without the sample crystallizing. In the

temperature range from 18 to 33 �C, G0 and G00 crossover is

observed at Ts = 28 �C.

With the data collected visually, by DSC, rheometry,

and optical microscopy, the temperature–concentration

phase diagram was generated (Fig. 7). At temperatures

below 0 �C, ice and surfactant crystals coexist. Between 0

and ca. 15 �C, an aqueous solution (I) and hydrated sur-

factant crystals are present. From 15 up to 23 �C, waxy

crystals in an aqueous solution are formed. For

concentrations between the cvc and ca. 15 wt% (shaded

area), a pseudo-solution is formed. For temperatures higher

than the Krafft temperature (ca. 33–34 �C) and concen-

trations between the cvc and up to around 2 wt%, a

micellar solution (I) forms; this phase is transparent,

streaming-birefringent, and viscoelastic. Two lamellar

phases, L1 and L2, span from 2 to 12 wt%; the boundary

between phase the L1 and L2 phases is around 8.5 wt%.

The phase behavior of the CDBAS/water system is very

different from that reported for surfactants with the same

length tail, with different head (–(CH3)3N? instead of –

(CH3)2–CH2C5H6N?) and aromatic counterions, e.g., tos-

ilate [47] and p-vinylbenzoate [48]. The phase diagram of

the surfactant with the cetyltrimethylammonium tosilate

(CTAT) in water [47] exhibits a larger micellar phase (I)

that extends to 25 wt% at 25 �C and to higher concentra-

tions at higher temperatures; in fact, at 80 �C, the isotropic

micellar phase extends to 90 wt% CTAT. At room tem-

perature and in a concentration range between 27 and

47 wt% CTAT, a hexagonal phase forms, but the extent of

this region diminishes with increasing temperature. At even

higher concentration, the hexagonal phase (H-phase)

coexists with a viscous–isotropic phase (V1) [47]. Inter-

estingly, no lamellar phase was reported for the CTAT/

water system [47], in contrast to the system reported here.

However, the lamellar phase was reported in CTAT/water

by adding some electrolytes [64]. Hassan et al. have

observed the formation of vesicles and lamellar phase by

substituting the counterion in CTAT by 3-hydroxy-naph-

thalene 2-carboxylate (HNC). They suggested that the

hydrophobicity of the counterion plays an important role in

controlling the curvature of the interface and, thus, in set-

ting the structure of the supramolecular assemblies [65,

66]. In the case of cetyltrimethylammonium p-vinyl-

benzoate (CTAVB), a micellar solution (I) is formed for

temperatures higher than the Krafft temperature (ca.

17–18 �C) and concentrations between the cmc and ca.

23 wt%, a H-phase is detected from 30 to 70 wt%, and a

lamellar phase is detected at high concentrations from 80 to

near 100 wt% [48]. The lamellar phase formed by the

CDBAS in water at relatively lower concentrations is due

to the benzyl group that acts as a second hydrophobic

chain. Besides, the salicylate counterion that is highly

hydrophobic reduces the effective head-group area (ah), by

shielding the positive charge of the surfactant head and by

interacting with the benzyl group that produces an increase

in the packing parameter value, inducing the forming of

lamellar or vesicular structures.

Figure 8 shows the elastic (G0) and viscous (G00) moduli

measured at 35 �C as a function of CDBAS concentration.

The frequency sweeps are made at oscillatory strain

deformation within the linear viscoelastic region. All

samples except the 0.5 wt% sample exhibit a crossover of

Fig. 6 Temperature sweeps made at 10 rad/s within the linear

viscoelastic (20%) region for 3 wt% CDBAS concentration: (filledsquares, open squares) heating mode; (filled circles, open circles)

cooling mode

Fig. 7 Temperature–composition phase diagram of binary mixtures

of CDBAS and water. I isotropic micellar solution, L1 lamellar phase,

L2 lamellar phase, E emulsion, Cw waxy crystals, Ch hydrated crystals

274 J Surfact Deterg (2011) 14:269–279

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G0 and G00 at a characteristic frequency (xc), the reciprocal

of which corresponds to the main relaxation time (or dis-

entanglement time) of the system sc. For concentrations up

to 1.5 wt%, xc shifts to lower values; however, above this

concentration, xc shifts to higher values. It is evident that,

for CCDBAS B 0.5 wt%, the rheological behavior is pre-

dominantly viscous for all frequencies. For higher con-

centrations, the wormlike micelles begin to overlap and

entangle, and, hence, a predominantly elastic behavior is

observed. This characteristic concentration is the critical

entanglement concentration (Ce) [67]. A value of

Ce & 0.7 wt% is determined [on the basis of the observed

onset of the elastic plateau (G0)]. The predominantly elastic

behavior region increases with concentration. From

CCDBAS = 1 wt% to the highest concentration studied

(2 wt%), the elastic modulus (G0) increases with frequency,

but its slope decreases with concentration. However, the

plateau modulus (G0) is not detected in the frequencies

range studied.

The G0 and G00 frequency dependence performed in the

linear viscoelastic zone (c = 0.1%) for 6 and 10 wt%

samples is shown in Fig. 9. The moduli for both samples

exhibit a predominantly elastic response and are frequency

independent; this gel-like behavior is characteristic of

lamellar phase structures [68].

Figure 10 shows the concentration dependencies of G0

(estimated as G0 ¼ scg�0), the main relaxation time (sc), the

zero shear viscosity (g0, obtained under steady shear rate

experiments), and the complex zero shear viscosity (g0*) at

35 �C. G0 follows a power law concentration dependence

of the form: G0 / C2:5surf (Fig. 10a) from 0.75 up to 2.3 wt%.

This concentration dependence of G0 has been reported

for a variety of micellar and polymer solutions with a

similar exponent [40, 41, 69, 70]. Around 2.3 wt%, G0

exhibits a slope change, which coincides with that of the

isotropic-to-lamellar transition (see Fig. 7). Figure 10b

shows the main relaxation time as a function of concen-

tration. It is clear that sc increases with concentration and at

around 1.4 wt%, a maximum is exhibited and it then

decreases up to a concentration of 2.3 wt%, where a

departure from the tendency is observed. The same

behavior is depicted by g0 and g0* (Fig. 10c); the departing

from the tendency is due to the I-to-L1 transition.

Figure 11 depicts the time–concentration master curves

of G0/G0 and G00/G0 for CDBAS concentrations from 0.75

to 2.25 wt% at 35 �C. G0 collapses in all of the dimen-

sionless frequencies (scx) that were studied. On the other

hand, G00 collapses only at scx B 1 values, where the effect

of the entanglements is not important (terminal zone). At

higher scx, where the effects of the entanglements become

important, G00 data departs from the master curve.

Fig. 8 Storage G0 (filled symbols) and loss G00 (open symbols) moduli

versus frequency measured at 35 �C for various CDBAS aqueous

solutions (wt%): (filled squares) 0.50; (filled circles) 0.75; (filledtriangles) 1; (filled inverted triangles) 1.25; (filled left pointingtriangles) 2

Fig. 9 Storage G0 (filled symbols) and loss G00 (open symbols) moduli

versus frequency measured at 35 �C for CDBAS aqueous solutions of

(wt%): (filled circles) 6; (filled squares) 10

Fig. 10 Dependence on CDBAS concentration with a plateau elastic

modulus (G0), b relaxation time (sc = 1/xc), and c zero-frequency

complex viscosity (filled squares) and zero-shear viscosity (opensquares) measured at 35 �C

J Surfact Deterg (2011) 14:269–279 275

123


Figure 12 shows the steady-shear master dynamic

r=G0 vs: _cscð Þ diagram proposed by Berret et al. [71] for

samples with concentrations from 0.75 to 2.25 wt% at

35 �C. At _csc\0:3 where the behavior is Newtonian, all

the data collapse in a single line. For _csc� 0:3; where a

shear thinning behavior develops, the data depart from the

master diagram. However, two different patterns can be

discerned; at lower concentrations, the stress continuously

increases with the shear rate, whereas at higher concen-

trations, a stress plateau or a sigmoid behavior in the stress

versus shear rate is observed. The inflexion in the shear

stress–shear rate curve has been observed in several sur-

factant systems, which has been associated with the

appearance of an unstable region in a critical shear rate

value [72–74]. Reduced logarithmic shear stress increases

again linearly at higher logarithmic reduced shear rates

values, indicating a second Newtonian behavior (g?). The

stress plateau has been associated with the appearance of

the shear banding flow, in which two phases coexist, sup-

porting different shear rates. The reduced stress plateau

occurs at r=G0 � 0:62; which is slightly lower than that

predicted by the Cates and Fielding’s model [75].

In conclusion, the Krafft temperature for the cetyldi-

methylbenzylammonium salicylate (CDBAS) in water is

33 ± 1 �C. This value is higher than that measured for

surfactants with the same tail length, trimethylammonium

head, and different aromatic counterions, e.g., tosilate

(Tk = 23 �C) and p-vinylbenzoate (Tk = 18 �C). The

CDBAS/water system forms vesicles at 0.002 wt% and a

vesicle–micellar transition at 0.005 wt% is detected at a

temperature of 35 �C. The formation of vesicles is proba-

bly due to the salicylate counterion, which is highly

hydrophobic. This reduces the effective head-group area

(ah) by shielding the positive charge of the surfactant head

and by interacting with the benzyl group, producing an

increase in the packing parameter value. This kind of

surfactant is a cheap alternative for spontaneous vesicle

formation instead of catanionic surfactants, double-tail

surfactants, or lipids.

Acknowledgments This work was supported by a project of the

National Council of Science and Technology of Mexico (CONACYT

grant no. 25463). FC-R acknowledges the scholarship from the

CONACYT.

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Author Biographies

Francisco Carvajal-Ramos received his B.Sc. and M.Sc. and is

studying for his Ph.D. in Chemical Engineering at the University of

Guadalajara, Mexico.

Alejandro Gonzalez-Alvarez received his B.Sc. and M.Sc. in

Chemical Engineering from the University of Guadalajara, Mexico,

and his Ph.D. in Genie des Procedes from the INPG, Grenoble,

France. He is currently Head Professor of the Chemical Engineering

Department of the University of Guadalajara, Mexico.

J. Roger Vega-Acosta received his Ph.D. at the Institute of Physics

of the Universidad Autonoma de San Luis Potosı, Mexico. His

research interests include the application of viruses to nanotechnol-

ogy. He uses viral protein as nanocontainers and studies different

forms of assemblies and techniques of the characterization of

materials.

Donato Valdez-Perez is an invited professor of research at the

Institute of Physics of the Universidad Autonoma de San Luis Potosı,

S.L.P., Mexico. His research interests include the fundamental studies

focused on scanning probe techniques in the interdisciplinary areas of

bio/nanotribology, bio/nanomechanics and bio/nanomaterials charac-

terization, and applications to bio/nanotechnology.

Vıctor Vladimir Amilcar Fernandez Escamilla is a research

professor at Guadalajara University. He received his M.Sc. and

Ph.D. in Chemical Engineering from Guadalajara University, Mexico.

He undertook a stay at the Laboratory of Rheology in Grenoble,

France, in 2006.

Emma Rebeca Macıas Balleza received her B.Sc., M.Sc., and Ph.D.

from the Universidad de Guadalajara in Chemical Engineering. She

also received her Ph.D. in Physics at the Universite Joseph Fourier at

278 J Surfact Deterg (2011) 14:269–279

123


Grenoble, France. She is a Professor at the University of Guadalajara

and works in the field of the rheology of complex fluids.

J. Felix Armando Soltero Martınez is chief of the Laboratory of

Rheology in the Chemical Engineering Department of the University

of Guadalajara. He received his B.Sc., M.Sc., and Ph.D. from the

University of Guadalajara, Mexico. He is a research professor in the

Chemical Engineering Department. His interests include ionic and

non-ionic surfactant properties for consumer product formulation in

the food and pharmaceutical industries.

J Surfact Deterg (2011) 14:269–279 279

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