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Colloids and Surfaces A: Physicochem. Eng. Aspects 450 (2014) 106–114
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical andEngineering Aspects
j ourna l h om epa ge: www.elsev ier .com/ locate /co lsur fa
riton X-100 micelles modulated by solubilized cinnamic acidnalogues: The pH dependant micellar growth
ijay Patela,∗, Debes Rayb, Vinod K. Aswalb, Pratap Bahadura
Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, IndiaSolid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India
i g h l i g h t s
Antioxidants from cinnamic acidfamily were solubilized in TX-100micelles.Marked micellar growth/transitionwas seen in the presence of cinnamicacid.Micellar shape/size depends on pHand interaction of cinnamic acid withmicelles.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 30 November 2013eceived in revised form 28 February 2014ccepted 3 March 2014vailable online 12 March 2014
eywords:
a b s t r a c t
Aqueous micellar solutions of a nonionic surfactant p-tert-octylphenoxy polyethylene (9.5) ether, TritonX-100 (TX-100) in the presence of cinnamic acid and its hydroxyl derivatives viz. p-coumaric acid, caffeicacid, ferulic acid and sinapic acid were examined by cloud point (CP), viscosity, dynamic light scatter-ing (DLS), nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS) measurements.These solubilizates influence the CP, viscosity and micelle size of TX-100 depending on their hydropho-bicity. Cinnamic acid, a biomedically useful compound displayed maximum growth in the micelles and
riton X-100innamic acidicelles
cattering
was therefore studied in detail. We found that an increase in the cinnamic acid concentration, a decreasein pH and the presence of sodium chloride all favour the micellar growth at low temperature. The locationof cinnamic acid in micelles was elucidated from 1H NMR. SANS study showed that ellipsoidal micellestransform into rod-like on solubilization of cinnamic acid which otherwise change to spherical ones inalkaline medium.
© 2014 Elsevier B.V. All rights reserved.
. Introduction
Micellar growth and sphere-to-rod shape transitions in sur-
actant solutions are observed in the presence of inorganic saltsnd organic additives [1,2]. Hydrophobic and weakly polar organicolutes have also been examined for their effect on micelles [3].hile highly polar substances like short chain alcohols [4,5] and
∗ Corresponding author. Tel.: +91 9979509888.E-mail addresses: [email protected] (V. Patel), [email protected]
D. Ray), [email protected] (V.K. Aswal), [email protected] (P. Bahadur).
ttp://dx.doi.org/10.1016/j.colsurfa.2014.03.015927-7757/© 2014 Elsevier B.V. All rights reserved.
amides [6,7] disintegrate micelles; weakly polar and sparingly solu-ble substances such as medium chain alcohols [8,9], amines [10,11]and phenols [12–14] favour micellar growth.
There are few reports on the effect of aromatic acids on thebehaviour of surfactant solutions [15–17]. Acids with low aque-ous solubility act as solubilizates and favour micellar growth buttheir salts behave like hydrotropes [17]. The study of the pHeffect on the surfactant systems containing solubilized anthranilicacid in Triton X-100 (TX-100) micelles is interesting in this
context [18]. Hydrophobic organic anions interact with cationicsurfactant which leads to the micellar growth developing high vis-cosity/viscoelasticity in the dilute solutions. The most extensivelystudied system has been CTAB-sodium salicylate [19,20].
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V. Patel et al. / Colloids and Surfaces A: P
TX-100 is a polyoxyethylene based alkyl aryl type nonionic sur-actant, most extensively studied for its micellar behaviour [21–26].ts critical micellar concentration (CMC) and CP are ∼0.27 mM and67 ◦C, respectively [12]. TX-100 is commonly used for solubilizingembrane proteins [27] and in detergent formulations from heavy
uty industrial products to gentle detergents [28]. The effects ofromatic solutes like p-toluidine [11], phenols [12] and anthraniliccid [18] on TX-100 micelles have been documented. p-Toluidinend various phenolic compounds markedly alter the micellar char-cteristics strongly dependant on the pH which determines theocation of these solubilizates in the micelles.
Cinnamic acid (CA) occurs naturally in many spices (cinnamonnd cloves), cranberries and prunes and acts against pathogenicrganisms [29]. It is an FDA approved flavouring agent used inerfumes, pharmaceuticals and food industries. Cinnamic acid andelated polyphenols show antimicrobial and antifungal activity30]. Hydroxycinnamic acids viz. p-coumaric acid, caffeic acid, fer-lic acid and sinapic acid are well known antioxidants which onolubilization in surfactant solutions, can alter micelle size or shapend rheological behaviour. The solubility of CA and related polyphe-ols in water [31] and supercritical CO2 [32] has been reported.innamic acid possesses an aqueous solubility of 0.002 M withissociation constant (pKa) 4.55 [31]. Cinnamic acid is proton-ted at the pH conditions below the pKa value while in alkalineedium it becomes more soluble in water. The enhanced solubilityould make CA very applicable for industrial use viz. pharma-
eutical and perfume industry. Studies on the solubilization andnteraction of some phenolic antioxidants viz. p-hydroxybenzoiccid, syringic acid, sinapic acid and quercetin [33] and parabens34] in Pluronic® micelles have been reported. In the presence ofrans-ortho-methoxycinnamic acid, CTAB forms long and entangledormlike micelles resulting in highly viscous solutions [35].
To the best of our knowledge, there are no reports of solubili-ation and interaction of CA and its analogues in TX-100 micelles.n view of this, here we report the effect of solubilization of CAnd its hydroxyl derivatives viz. p-coumaric acid, caffeic acid, fer-lic acid and sinapic acid on the phase behaviour of TX-100 inqueous solution. Cinnamic acid displays the strongest interactionmong other solubilizates studied with TX-100 micelles. As a con-equence, we have studied the effect of CA on TX-100 micellesn detail at different temperature, pH and salt concentration con-itions using physical and scattering techniques (DLS and SANS).he location of cinnamic acid in micelles is revealed from 1HMR.
. Experimental
.1. Materials
Triton X-100 (TX-100), cinnamic acid (CA), p-coumaric acidPCA), caffeic acid (CFA), ferulic acid (FA) and sinapic acid (SA) werebtained from Sigma-Aldrich and used as received. Triply distilledater was used to prepare aqueous solutions. Deionized water from
Millipore Milli-Q system was used to prepare samples for DLS. D2O99%) was used for sample preparation for SANS and NMR.
.2. Methods
.2.1. ViscosityThe viscosity measurements were performed in a temperature
ontrolled water bath with stability of ±0.1 ◦C. Calibrated Cannon
bbelohde viscometers were used with size 25 and 150 havingonstants 0.001869 and 0.03462 cSt s−1, respectively [36]. The vari-tion in flow time was found to be ±5 s. The absolute viscosities ofhe solutions obtained are multiplied by viscometer constant to getchem. Eng. Aspects 450 (2014) 106–114 107
kinematic viscosity in centi-stokes and then multiplied by densityof solvent (water) to give solution viscosities in centipoise. Theseviscosities of solutions were divided by viscosity of water to obtainthe relative viscosity [8].
2.2.2. Cloud point (CP)The CPs of the solutions were determined by slowly heating the
sample solutions in thin 20 mL test tube immersed in a beaker hav-ing water. The solution temperature was increased slowly (heatingrate ∼1 ◦C min−1) with constant stirring using magnetic stirrerequipped with a heater. Temperature of first appearance of tur-bidity was taken as cloud point. The CP results were reproducibleup to ±0.5 ◦C.
2.2.3. Solubility experimentsUV–Vis double beam spectrophotometer (Thermo Fisher) was
used for solubilization experiments. It contains a matched pair ofstoppered quartz cells with optical path length of 1 cm. For analysis,wavelength used for CA, PCA, CFA, FA and SA are 272, 291, 321,320 and 319 nm, respectively, which correspond to their maximumabsorption. Dilute solutions of acids with concentrations rangingfrom 0.01 to 0.1 mM in methanol were used to prepare calibrationplot. It follows Beer–Lambert law (R2 = 0.9982). Sample solutionssaturated with acids were prepared by dissolving excess amount ofacids in water and 5% TX-100 and then stirred at 30 ◦C at 200 rpm for48 h. Excess amount of acid was removed by filtering the solutionswith millipore of size 0.45 �m. Filtered solution was diluted 10–100times with methanol. The very low amount of water permits thedirect use of calibration plot. The solubilities of acids in water andin 5% TX-100 were determined using the standard analytical shake-flask method.
2.2.4. Dynamic light scattering (DLS)The apparent hydrodynamic size of the micelles was obtained
from Zeta sizer Nano-ZS 4800 (Malvern Instruments, UK). The scat-tering angle was kept 90◦ fixed and the He–Ne laser was operatedat 633 nm. The Stokes–Einstein relationship was used to get thehydrodynamic size. Results are reproducible up to ±0.3 nm.
2.2.5. Small angle neutron scattering (SANS)SANS measurements were performed at the SANS diffractome-
ter at Guide Tube Laboratory, Dhruva reactor, BARC, Mumbai, India.The scattering data for a mean incident wavelength of 5.2 A with��/� = 15% were measured at 30 ◦C in the wave vector transfer(Q = 4�sin(�/2)/�, where � is scattering angle) range of 0.017 to0.35 A−1. The measured SANS data were corrected for the back-ground, the empty cell contributions and the transmission andnormalized to absolute cross-sectional unit using standard proto-cols.
2.2.5.1. SANS analysis. The differential scattering cross-section(d�/d�) per unit volume as a function of wave vector transfer Qand for a system of monodisperse particles in a medium can beexpressed as [37,38]
(d˙
d˝
)(Q ) = nV2(�p − �s)
2P(Q )S(Q ) + B, (1)
where n denotes the number density of particles, �p and �s are,respectively, the scattering length densities of particle and solvent
and V is the volume of the particle. P(Q) is the intraparticle structurefactor and S(Q) is the interparticle structure factor. B is a constantterm representing incoherent background, which is mainly due tothe hydrogen present in the sample.
108 V. Patel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 450 (2014) 106–114
on X-1
af
P
F
tfS
(
p
wa
f
w(b
datlps
2
4Stpt
acts as a kosmotrope which reduces the solubility of surfactant inwater [46,47]. In 1.0 M NaCl, the CP of a TX-100 solution with solu-bilized CA decreases more sharply compared to only CA (Fig. 2(c)).
Table 1Solubility data of cinnamic acid and its analogues.
Organic acids Solubility (g/L) at 30 ◦C Po/w
Fig. 1. Chemical structures of Trit
The intraparticle structure factor P(Q) is decided by the shapend size of the particle and is the square of single-particle formactor F(Q) as determined by
(Q ) =⟨|F(Q )|2
⟩(2)
For a spherical particle of radius R, F(Q) is given by
(Q ) = 3
[sin(QR) − QR cos(QR)
(QR)3
]. (3)
S(Q) describes the interaction between the particles present inhe system and it is the Fourier transform of the pair correlationunction for the mass centres of the particles. For dilute systems,(Q) ∼1 and Eq. (1) subsequently reduces to
d˙
d˝
)(Q ) = nV2 ×
(�p − �s
)2 × P(Q ) + B. (4)
The polydispersity in size distribution of the particles is incor-orated using the following integration [39]:
d˙
d˝(Q ) =
∫d˙
d˝(Q, R) × f (R)dR + B, (5)
here f(R) is the particle size distribution and usually accounted by log–normal distribution as given by
(R) = 1√2�R
exp
[− 1
22
(ln
R
Rmed
)2]
, (6)
here Rmed is the median value and is the standard deviationpolydispersity) of the distribution. The mean radius (Rm) is giveny Rm = Rmed exp(2/2).
The data have been analyzed by comparing the scattering fromifferent models to the experimental data. Throughout the datanalysis, corrections were made for instrumental smearing, wherehe calculated scattering profiles smeared by the appropriate reso-ution function to compare with the measured data [40]. The fittedarameters in the analysis were optimized using nonlinear least-quare fitting program to the model scattering [41].
.2.6. Nuclear magnetic resonance (NMR)1H NMR experiments were performed on a Bruker AVANCE-II
00 MHz spectrometer. All measurements were performed at 30 ◦C.amples were prepared in D2O. The spectrum was calibrated by set-ing the HDO peak at a chemical shift of 4.65 ppm at 298 K. The HDOeak due to residual water was eliminated by solvent suppressionechniques.
00 and cinnamic acid analogues.
3. Results and discussion
3.1. Solubility of cinnamic acid analogues in TX-100 micelles
The CA analogues find numerous industrial as well as biologicalapplications/activity viz. antioxidant, antitubercular, antidiabetic,antimicrobial, antiviral and anticancer agents [30,42]. These acidshave limited aqueous solubility and get solubilized in TritonX-100 micelles. The surfactant solution behaviour depends onhydrophobicity, quantity solubilized and location of these acids inmicelles. Depending upon the octanol–water partition coefficients(Po/w), these solubilizates partition in micelles and accordinglyalter the micellar characteristics. All the studied solubilizatespossess a common aromatic ring, –COOH group, double bondand vary in functional groups like –OH, –OCH3 which play akey role in determining solubility and location in the micelles(Fig. 1).
The solubility data are presented for cinnamic acid analogues(Table 1). It clearly shows that CFA has the maximum aqueoussolubility due to two –OH group while CA possesses maximum sol-ubility in 5% TX-100 compared to other solubilizates studied due tothe highest Po/w. SA has the lowest aqueous and micellar solubilitydue to the presence of two –OCH3 groups and lower Po/w. With thehighest Po/w, CA penetrates in the TX-100 micelles and displays thestrongest interaction. The solubilization of CA in TX-100 micellesleads to an increase in solution viscosity, indicating micellar growthwith a corresponding decrease in CPs (Fig. 2). TX-100 forms rodlikemicelles with progressive solubilization of CA (discussed later inSANS part of MS). Here, CA acts as a water structure maker andtherefore, dehydration of TX-100 occurs which results in loweringits CP (Fig. 2(c)). Addition of inorganic salt like sodium chloride alsoleads to dehydration of micelles [44,45]. It is well known that NaCl
Water (Exp.) Water* (L) 5% TX-100
CA 0.29 0.31 7.75 2.41PCA 0.97 1.02 4.86 1.87CFA 1.25 1.23 3.16 1.42FA 0.88 0.92 4.28 1.64SA 0.19 0.22 2.65 0.99
* Data are taken from Ref. [31].Po/w Data are taken from Ref. [43] (RSC website).
V. Patel et al. / Colloids and Surfaces A: Physico
30
40
50
60
70
0
2
4
6
8
10
0 10 20 30 40 500
20
40
60
80
CP,
°Cc
a
b
rel
Dh,
nm
[Cinnamic aci d], mM
Fig. 2. (a) Micelle hydrodynamic diameter (Dh), (b) relative viscosity and (c) cloudpoint of 5% TX-100 with CA in the (�) presence and (©) absence of 1.0 M NaCl at3
Hiid(
hiilhohDgsih
h1Tuotis
0 ◦C.
ere, the lowering of the cloud point is dominated by dehydrat-ng effects of CA and NaCl which favours the micellar growth andt is further supported by an increase in apparent hydrodynamiciameter (Dh) of TX-100 micelles acquired from DLS measurementsFig. 2(a)).
The presence of –OH groups in PCA and CFA makes them moreydrophilic and hence it is difficult for their molecules to penetrate
nto the micelle. This is probably the reason for their lower solubil-ty in TX-100 micelles (Table 1). Consequently, PCA and CFA have aower solubility compared to CA in TX-100. Now, FA has a slightlyigher solubility than CFA due to the presence of –OCH3 in placef second –OH group. p-Coumaric acid, CFA and FA due to theirydrophilic nature do not have a significant effect on CP, rel andh of TX-100 (figure not shown). Sinapic acid has one extra –OCH3roup compared to FA. Hence, it becomes more hydrophobic andhould have a higher solubility in the micelle but an opposite effects observed (Table 1). This may be due to its large structure i.e. stericindrance and low Po/w.
Incorporation of solubilizates to TX-100 micelles inducesydrophobicity in the system. The size distribution plots for 5% TX-00 in the presence of cinnamic acid analogues are shown in Fig. 3.he Dh of 5% TX-100 micelle is 10.6 nm which increases with sol-bilization of all acids. The order of Dh of micelles in the presence
f acid is as follows: CA > PCA > FA > CFA > SA. This clearly reflectshat solubilization of CA (highest Po/w) has a maximum modulat-ng effect on the apparent micelle size (73.7 nm) indicating thetrongest interaction with TX-100 micelles [12].chem. Eng. Aspects 450 (2014) 106–114 109
3.2. Effect of temperature
Dehydration of nonionic micelles can be brought up by the addi-tion of salt, increasing temperature or solubilizing the hydrophobicsolutes [8,12,38,25,48]. Nonionic surfactant solutions often showa micellar transition near the phase separation temperature (CP).Solubilization of CA lowers it to room temperature. Fig. 4 showsthe effect of temperature on an aqueous solution of TX-100 in theabsence and presence of CA. This indicates that the cloud point isdecreased to 53, 39 and 32 ◦C (Fig. 2) by the addition of 20, 40 and50 mM CA, respectively. The increase in the hydrodynamic size andviscosity confirms the micellar growth.
3.3. Effect of pH
3.3.1. Viscosity, cloud point and DLSCinnamic acid shows enhanced antimicrobial/antioxidant activ-
ity in acidic media [49]. Hence, microstructure changes in TX-100micelles with solubilized CA are of prime importance and arestudied in detail. Fig. 5 shows the CP, rel and micelle Dh of 5%TX-100 solutions in the presence of 40 mM CA (fixed) at varyingpH at 30 ◦C. Being a nonionic surfactant, the micellar behaviourof TX-100 is unaffected by pH [12]. The presence of pH respon-sive –COOH group makes CA sensitive to pH. Our SANS data showthat originally (5% TX-100 + 40 mM CA) micelles are rodlike. Beinghydrophobic, CA molecules penetrate into the TX-100 micelles byhydrophobic interaction and accordingly, induce micellar growthwhich is supported by increase in both the viscosity as well asthe apparent hydrodynamic size (Fig. 3). Originally, pH of the 5%TX-100 solution having 40 mM solubilized CA is 3.6. Now, theionization of CA is opposed at the pH <3.6 and protonation ofCA molecules induces more hydrophobicity and eventually micel-lar growth is observed due to its penetration within the micelle.Protonation of CA induces the highest hydrophobicity and con-sequently maximum micelle growth is observed. The solutionviscosity attains the maximum with a reverse trend for CP. Furtherlowering of pH does not involve any change in micelle param-eters and a plateau is observed. The pKa of CA is 4.55 [31]. AtpH > 4.55, negative charge is developed on CA and hence its solubil-ity in water is increased and hydrophobic interaction is decreasedand thus CA is pushed out towards the micelle interface. As aresult, the viscosity of the solution is decreased with a simultane-ous increase in the CP. Above pH > 6.0 almost all the CA moleculesare converted in to cinnamate ions and as a result, no markedchange in the viscosity and corresponding CP has been observed.Here, the change in pH towards alkaline media pulls CA towardsthe micellar interface and rod-like micelles are transformed intospherical-like geometry. This has been discussed in the later partof the manuscript. Furthermore, the size distribution profiles ofmicelles obtained from DLS measurement can give a better ideaabout the pH-induced change in hydrodynamic size of micelles(Fig. 6).
3.3.2. SANSThe progressive solubilization of cinnamic acid leads to morpho-
logical changes in TX-100 micelles. At higher level of solubilization,the large increase in viscosity and Dh suggests the formation ofbigger size micelles. SANS measurements have been performed tomonitor CA-induced morphological changes in TX-100 micelles andare shown in Fig. 7. SANS data show the effect of CA varied over aconcentration range from 0 to 40 mM with fixed concentration (5%)of TX-100. There is a strong build up of scattering intensity in the
low-Q region with the increase in the CA concentration whereasthey overlap at higher Q region irrespective of the CA concentra-tion. This is an indication of the morphological changes in micelles.The analysis shows that originally TX-100 (5%) micelles are
110 V. Patel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 450 (2014) 106–114
Fig. 3. Size distribution curves for 5% TX-100 in the presence of CA, PCA, CFA, FA and SA at 30 ◦C.
30 35 40 45 500
20
40
60
80
100
0
2
4
6
8
10
Dh,
nm
Temperatur e, °C
a
0 mM 20 mM 40 mM
0 mM 20 mM 40 mM
b
rel
Fig. 4. Effect of temperature on (a) micelle hydrodynamic diameter (Dh) and (b)viscosity of 5% TX-100 solutions in the absence and presence of CA.
0 2 4 6 8 10 12 140
10
20
30
401
2
3
4
5
6
730
45
60
75
90 c
a
Dh,
nm
pH
b
rel
CP
°C
Fig. 5. Effect of pH on (a) hydrodynamic size, (b) relative viscosity and (c) cloudpoint of 5% TX-100 micelles in the presence of 40 mM CA at 30 ◦C.
V. Patel et al. / Colloids and Surfaces A: Physico
Fig. 6. Size distribution curves for 5% TX-100 + 40 mM CA at varying pH at 30 ◦C.
Fig. 7. Effect of concentration and pH on SANS pattern of 5% TX-100 in the presenceof CA at 30 ◦C. Inset shows the plots in a linear scale. The solid lines represent thefitted data.
chem. Eng. Aspects 450 (2014) 106–114 111
ellipsoidal with axial ratio of ∼3 [50]. Mahajan et al. [51] showedthat Nagg of 3% TX-100 is 378 and approximately same semi-minoraxis (20 A). However, a slight change was observed in the semi-major axis which may be due to the compact packing of TX-100micelles. In the presence of a small amount of CA (10 mM) the axialratio slightly increases and becomes 3.8, presenting the ellipsoidalshaped micelles (Table 2). We conclude that the micelle shape isnot much affected by lower [CA]. With increase in [CA], the axialratio increases and at 30 mM CA, the axial ratio is ∼4.7 depictingstill ellipsoidal micelles. Also a marked increase in the aggrega-tion number is observed as a result of the concentration gradient ofCA. At 40 mM CA, uniaxial micellar growth was observed; micelleshave an axial ratio ∼10 which presents a rod-like structure with aradius 20.3 A and length 206.8 A. SANS results for 40 mM at pH = 3.6show typical slope of about (−1) on log–log scale in low-Q regionwhich corresponds to the presence of rod-like micelles [52,53].These fitted parameters have been obtained by the model fittingas well as by the normalization of scattering data in the absoluteunit.
A change in the pH of the solution has a striking effect on themicellar transition and aggregation characteristics [11,54]. Lower-ing of the pH leads CA into its nonionized form (hydrophobic) whichgets solubilized in the micelles and consequently induces micellargrowth. Now, a change in the pH of the system towards the alkalinemedium has a prominent effect on micelle shape, size and aggre-gation characteristics [54]. Fig. 7 shows that at pH = 9.0, the overallscattered intensity in the low Q-region decreased drastically (lowerthan pure TX-100) which indicates decrease in micellar size. This isdue to the development of negative charge on CA which increasesits solubility in water and consequently CA is expelled out of themicelle. The absence of slope (−1) and a flat scattering pattern sug-gest the transformation from rod-like to spherical micelles at pH9. Spherical micelles have a radius of 23.1 A and an aggregationnumber of 142. Our SANS results are also supported by viscosityand DLS results. Micellar parameters obtained from SANS data aresummarized in Table 2.
3.3.3. NMRChanges in the microenvironment of the surfactant solution can
easily be monitored by the analysis of 1H NMR spectrum. Cinnamicacid-induced microstructural changes can be quantified from theobserved chemical shifts of TX-100 protons (Fig. 8). The 1H NMRspectra and chemical shifts of TX-100 and CA protons are shownin Fig. 8(A). TX-100 protons are highly sensitive to the change inthe polarity of their microenvironment. In the presence of 20 mMcinnamic acid, all TX-100 protons show an upfield shift indicat-ing that these protons experience more hydrophobic environment(Fig. 8(B)). The decrease in the intensity of the NMR peaks of cin-namic acid protons clearly indicates that they are shielded whereasthe broadening of NMR peaks indicates the micellar growth [55,56].At a higher level of solubilization (40 mM), NMR peaks of the aro-matic protons of cinnamic acid (C4 and C5) completely merged withT8 proton peak while the other protons show upfield shifts. Herethe T3, T2 and T1 protons of the alkyl chain of TX-100 form the coreof the micelle and are located at 1.59, 1.22 and 0.65 ppm which in40 mM CA shifts to 1.25, 0.87 and 0.31 ppm, respectively. At thesame time, the vinyl protons of CA also experience large upfieldshifts. The shell protons of TX-100 are affected more compared tothe core and aromatic protons by the presence of cinnamic acidwhich indicates that majority of CA molecules may reside at thecore–shell interface at higher level of solubilization.
Now, a change in the pH of the solution has a significant effect
on the NMR spectrum also. Fig. 8(B) shows the 1H NMR spectrum of5% TX-100 in the presence of 40 mM cinnamic acid at pH ∼ 9. All theprotons exhibit a downfield shift and the intensity of the aromaticprotons of CA increases suggesting that they are deshielded and
112 V. Patel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 450 (2014) 106–114
Table 2Micellar parameters for 5% TX-100 in the presence of CA at 30 ◦C.
[CA] (mM) Semi-major axis a (Å) Semi-minor axis b (Å) Axial ratio Nagg Shape
0 63.5 19.8 3.2 287 Ellipsoidal10 76.0 20.0 3.8 351 Ellipsoidal30 95.6 20.3 4.7 454 Ellipsoidal40 (pH = 3.6) 206.8 20.3 10.2 737 Rod-like
40 (pH = 9.0) 23.1 23.1 1.0 142 Spherical
F fect of
eCmt
ig. 8. (A) Structures and 1H NMR spectra of (a) TX-100 and (b) cinnamic acid (B) Ef
xperience a relatively more hydrophilic environment. As a resultA molecules are pushed out from core–shell interface towardsicelle interface and hence rod-like micelles are transformed in
o spherical micelles.
concentration and pH on TX-100 protons in the presence of cinnamic acid at 30 ◦C.
4. Conclusion
The influence of the solubilization of phenolic antioxidantsfrom cinnamic acid family viz. CA, PCA, CFA, FA and SA on the
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V. Patel et al. / Colloids and Surfaces A: P
ggregation behaviour of TX-100 micelles in aqueous medium haseen investigated by viscosity, DLS, SANS and NMR studies. The sol-bility data for CA and its analogues in TX-100 micelles follow therder CA > PCA > FA > CFA > SA, a trend expected from their chemicaltructures.
Cinnamic acid alters the micellar characteristics of TX-100hich is strongly dependant on pH as shown from DLS and SANS
esults and supported by the CP and viscosity data. The behaviourf CA as a hydrophobic solubilizate in acidic pH and a hydrotrope inlkaline pH dictates its location in micelles and as a consequence,hanges the micellar size and shape. SANS data shows that theicellar aggregation number increases with the concentration of
A. The formation of rodlike micelles on progressive solubilizationf cinnamic acid and their transformation into spherical micelles inlkaline medium has been confirmed from SANS. NMR experimentsndicate that majority of CA molecules may reside at core–shellnterface of TX-100 micelles at higher level of solubilization. As cin-amic acid and related compounds are biologically important, thistudy might be useful for their surfactant based formulations inharmaceutical and food industry.
cknowledgement
The authors thank UGC New Delhi for the financial assistanceProject No: 37-527/(2009) SR).
eferences
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