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Interfaces and Nanomaterials Laboratory,

National Institute of Technology, Rour

santanuparia@yahoo.com; sparia@nitrkl.a

Cite this: RSC Adv., 2014, 4, 9182

Received 18th April 2013Accepted 9th July 2013

DOI: 10.1039/c3ra41876f

www.rsc.org/advances

9182 | RSC Adv., 2014, 4, 9182–9188

Interfacial and wetting behavior of natural–synthetic mixed surfactant systems

Nihar Ranjan Biswal and Santanu Paria*

Interfacial and wetting behavior of a double chain cationic synthetic surfactant di-dodecyldimethyl

ammonium bromide (DDAB), a natural surfactant (Shikakai), and their mixtures of five different ratios

have been studied by surface tension and dynamic contact angle measurements. Pure Shikakai has

higher surface tension and contact angle values at critical micelle concentration (CMC) than DDAB,

indicating that it is inferior to DDAB and also commonly used synthetic surfactants. Addition of DDAB to

Shikakai gives a gradual lowering of the CMC, surface tension and contact angle values at the CMC.

When the concentration of DDAB is �50 mol% the final surface tension and contact angle values are

very close to those of pure DDAB. The mixed surfactant solutions show strong non-ideal solution

behaviour because of the interaction between the two surfactants. As the wetting properties of a natural

surfactant are enhanced in the presence of a synthetic surfactant, the use of plant–synthetic mixed

surfactant systems may be useful in wetting and in several other applications. At the same time the

consumption of synthetic surfactant can also be reduced.

1. Introduction

Mixed surfactant systems are generally shown to be superior inperformance in terms of improved interfacial behavior1–6 andlower consumption7,8 in several applications compared to singlesurfactants. The published literature on mixed surfactant systemsalso shows that over at least the past two decades there has been agradually increasing number of papers on mixed surfactantsystems. Published studies on mixed surfactant systems arebroadly classied into three categories, (i) theoretical calculationsto predict different properties,9–13 (ii) experimental studies tosupport the theoretical predictions and explore new mecha-nisms,2,5,14 (iii) applications to show their performance.1–6,8 In mostcases the primary objective in studyingmixed surfactant systems isto discover suitable combinations of surfactants with a synergisticinteraction between the head or tail groups. The majority ofstudies so far reported are on mixed surfactant systems with amixture of ionic–nonionic combinations; but less on two similarionic, non-ionic, and oppositely charged surfactants. Furthermore,it has been also found that mixed surfactant studies are mainly onsynthetic mixtures,1–5,7–14 but rarely on natural–syntheticmixtures.6,15,16

Natural surfactants are a class of surfactants obtaineddirectly from a plant, animal, or microorganism without anyfurther chemical treatment. In recent years there has been agrowing trend of using bio-based or renewable resources as a

Department of Chemical Engineering,

kela, Orissa, 769008, India. E-mail:

c.in; Fax: +91 661 246 2999

“green” product in several applications to reduce environ-mental problems. In the case of surfactant formulation, apartfrom good wetting and surface activity, biodegradability andlow ecotoxicity are also extremely important requirements fromthe environmental view point. The available literature showsthat there have been fewer studies on natural surfactants6,15–23

compared to the enormous number of studies on syntheticsurfactants.1–5,7–14 In practice, also, they are even not able tosubstitute a small fraction of the worldwide consumption oftotally synthetic surfactants.

In general, a huge number of synthetic surfactants are usedin industrial and domestic processes. They are mostly non-biodegradable in nature, and as a result they create environ-mental problems when thrown into the environment withoutany treatment. Hence, researchers are trying to nd biode-gradable or environmentally friendly surfactants for severalapplications. In spite of the fact that there is a huge demand foreco-friendly surfactants, there are only a few studies availableon the wetting behavior of single bio-surfactants,17,18 and noneon only plant or plant synthetic mixed surfactant systems. At thesame time there are few studies available on microorganismbased bio-surfactants, maybe because of their good surfaceactivity. However their major limitations for practical applica-tions are difficulties in large scale production, isolation andpurication from the culture media, and nally the highproduction cost. Natural surfactants from plant sources in turnmay be an alternative option in the future because of thepossibility of production on a large scale to compete withsynthetic surfactants. The plant-based surfactants are saponin.These surfactants are mostly nonionic in nature and of the

This journal is © The Royal Society of Chemistry 2014

Fig. 1 Structure of (a) DDAB and (b) Shikakai.20,23

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triterpenoid glycosides class. The hydrophilic part mainlyconsists of sugars such as D-glucose, D-xylose, L-arabinose, L-rhamnose, and glucuronic acid. Sapindic acid and oleanolicacid are the main constituents of the hydrophobic portion.16–20

The interfacial behavior of different synthetic mixed surfac-tant systems24–26 and their applications for wetting of hydro-phobic surfaces8,27–29 and solubilization6,30–32 was studied bymany researchers. The reported mixed surfactant systemsprimarily emphasized their improved efficiency for the variousapplications compared to that of a single surfactant system.Apart from the surfactant mixture, addition of electrolytes tomixed surfactant systems were also studied by our group, wherewe found that both the CMC and the efficiency of surfactantmixtures can be further signicantly reduced.8

This study focuses on the solution behaviour of mixtures of aplant-based surfactant Shikakai (Acacia concinna) and a double-chain cationic synthetic surfactant DDAB and a comparison ofthe wetting behaviour of mixed surfactant systems with theindividual single surfactants on a PTFE surface. Shikakai waschosen for this study as a natural surfactant because of itssuperior performance compared to other natural surfactants.33,34

DDAB was also chosen as a synthetic surfactant because of itssuperior performance to many other conventionally usedsynthetic single as well as double-chain surfactants.35 Addition ofa cationic surfactant can also enhance the storage life of naturalsurfactant solutions by preventing fungal attack, which is also anadditional advantage. This study shows that the addition of adouble chain cationic surfactant can dramatically enhance theperformance of a plant based surfactant. On the other hand amixed system can reduce the consumption of both surfactantssignicantly in any process by reducing CMC, which is desirablefor any commercial product. The novelty of this work lies in thefact that similar mixed surfactant systems have not been reportedbefore and also the study has some practical importance for thereduction of environmental problems.

2. Experimental section2.1 Materials and methods

A polytetrauoroethylene (PTFE) slide of dimensions25.34 mm � 1.12 mm was cut from a sheet purchased fromJ. Khushal Das and Co., Mumbai, India. DDAB was purchasedfrom Sigma-Aldrich chemicals, Germany (technical grade, catno. 93418) and used without any further purication (structureshown in Fig. 1a). Ultrapure water of 18.20 MU cm resistivity,71.50 mN m�1 surface tension, and 6.5–7.0 pH was used for allexperiments. The plant surfactant Shikakai was extracted fromdried Shikakai pods by the solvent extraction method asreported before.6 The structure of Shikakai with a molecularmass of 1058 is shown in Fig. 1b as reported before.20,23

2.2 Measurement of surface tension and CMC

All the surfactant solutions were prepared freshly just beforemeasurements. The surface tension of the aqueous surfactantssolutions was measured by the Wilhelmy plate method using asurface tensiometer (Data Physics, Germany, DCAT-11 EC) at

This journal is © The Royal Society of Chemistry 2014

25� 0.5 �C. The temperature was maintained using an externalconstant temperature water bath circulator. The motor speedwas 1 mm s�1 and the immersion depth of the platinum platein the surfactant solution was maintained at 3 mm. Aer themeasurement of each concentration the plate was cleanedwith water, acetone and nally burned in an alcohol ame.The CMCs of DDAB and Shikakai were found to be 0.05 and 0.5mM, respectively, by the surface tension measurementtechnique.

2.3 Measurement of contact angle

The dynamic advancing (qA) contact angle was measured by theWilhelmy balance technique using the above mentionedsurface tensiometer. All experiments were carried out at aconstant temperature (25 � 0.5 �C). The motor speed of1 mm s�1 and the immersion depth of 5 mm for the PTFE platewere maintained during the contact angle measurements. Agood quality PTFE plate was chosen and washed rst withacetone and then with ultrapure water to clean it of impurities.The plate was then dried by blowing hot air. The same proce-dure was repeated aer the measurement of each surfactantconcentration.

2.4 Treatment of data

The surface excess at the air–liquid interface (GLG mol m�2) andthe area occupied per surfactant molecule (Amin in A2) werecalculated according to the following equations.

GLG ¼ � 1

2:303nRT

�dgLG

d log c

�(1)

Amin ¼ 1

NAGmax

(2)

RSC Adv., 2014, 4, 9182–9188 | 9183

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where n is a constant equal calculated according to n ¼ n1a1 +n2a2, where n1 and n2 are the number of species of surfactants1 and 2, respectively, whose interfacial concentration changeswith the change in surfactant bulk phase concentration; a1 anda2 are the molar fractions of the surfactants.36

3. Results and discussion3.1 Interfacial behavior of mixed surfactant solutions

The interfacial behavior of the Shikakai and DDAB mixedsurfactant solutions was studied using the following molarmixing ratios: 98 : 2, 95 : 5, 90 : 10, 80 : 20, and 50 : 50, andpresented in Fig. 2. Fig. 2 shows that for pure Shikakai there is agradual decrease in surface tension to a minimum value of38.71 mN m�1 at a concentration of 0.5 mM with a slope of�11.76 and an area of 80.55 A2 occupied per molecule. Abovethat there is no signicant change in surface tension. Incontrast, DDAB shows two transition points in the surfacetension plot. Initially there is a sharp decrease in surfacetension with slope changes at 0.05 mM and 0.8 mM, for theCMC and CVC (critical vesicle concentration), respectively.35,37,38

For DDAB solutions the surface tension values at CMC and CVCare 28.5 and 24.30 mN m�1, respectively, and beyond the CVCthe value of the surface tension remains almost constant. It isalso important to know that Shikakai has a higher minimumsurface tension value (38.79 mN m�1) in contrast to otherconventional single-chain cationic (CTAB ¼ 32.99 mN m�1),anionic (SDBS ¼ 34.22 mN m�1), and nonionic (TX-100 ¼31.01 mN m�1, Igepal CO-630 ¼ 31.02 mN m�1) surfactants.35 Acomparison of the surface tension results between Shikakai andDDAB shows that Shikakai has a higher CMC value as well as ahigher minimum surface tension at CMC than that of DDAB,which indicates that the former is less surface active. Althoughthe nal concentration for the minimum surface tension at CVCis a little higher for DDAB than for Shikakai, the minimumsurface tension is signicantly lower (�24.36 mN m�1). Thegure shows that at the rst mixing ratio (2% of DDAB), theCMC of the mixed solution (0.35 mM) is lower than that of pureShikakai (0.5 mM), with a minimum surface tension value of34.46 mN m�1 at the CMC and a single break point with a slopeof �13.75 below the CMC. However, when DDAB concentration

Fig. 2 The change in surface tension (mNm�1) with the concentration(log c) of different surfactants.

9184 | RSC Adv., 2014, 4, 9182–9188

increases to the ratio of 95 : 5, the CMC value suddenlydecreases to 0.089 mM; subsequently above that mixing ratio,along with CMC the surface tension value at CMC also grad-ually decreases and becomes close to that of pure DDAB at50 : 50 mixing ratio (as shown in Table 2). It is also worthmentioning that, in the presence of 5% or above the DDABconcentration there are two break points in the surfacetension vs. the concentration plot, similar to that of pureDDAB. The change in break points and surface tension valuesare presented in Table 1. These results indicate that when theDDAB composition is above 5 mol%, it dominates in the mixedmicellization process.

When two different (similar or dissimilar types) surfactantsare mixed together, the mixed solutions behave either ideally ornonideally. In general, nonideal behaviour with synergism hasgreat technical and academic interest and is useful for severalapplications. To discover the actual solution behaviour, mixedCMC values were calculated using the ideal solution theory39

from eqn (3) and were compared with the experimental values.

1

CMC12

¼ a1

CMC1

þ ð1� a1ÞCMC2

(3)

where CMC12, CMC1, CMC2, are the critical micelle concentra-tions of the binary mixture, Shikakai, and DDAB, respectively;and a1 is the molar fraction of Shikakai in the binary mixture.

The experimental and calculated CMC values at differentmixing ratios are plotted in Fig. 3. Interestingly, Fig. 3 showsthe experimental CMCs decrease gradually with increasingmole fraction of DDAB and with almost no change above 0.5molar fraction of DDAB (a2). Finally it has been found thatthere are huge differences between the experimental andcalculated CMC values. The experimental CMC values arelower than the calculated one using ideal solution theory,indicating a synergistic interaction between the two surfactantmolecules with negative deviation. To quantify the nonidealbehaviour of the binary surfactant mixture, Rubingh's equa-tion (eqn (4)) of regular solution theory was used to calculatethe micellar molar fraction of Shikakai (x1) by solvingiteratively.36

ðx1Þ2lnða1c12=x1c1Þð1� x1Þ2ln½ð1� a1Þc12=ð1� x1Þc2�

¼ 1 (4)

The interaction parameter (b) can be now be calculated bysubstituting the value of x1 in eqn (5).

Table 2 Values of micellar mole fraction and interaction parameter (b)for different mixing ratios

a1 CMC12 CMCExp x1 b

1 0.5 0.5 1.000 —0.98 0.42 0.35 0.744 �1.0560.95 0.34 0.09 0.542 �5.5610.9 0.26 0.08 0.492 �4.7690.8 0.18 0.07 0.427 �4.2560.5 0.09 0.05 0.288 �3.3050 0.05 0.05 0 —

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Table 1 Surface tension values at CMC and CVC, surface excess atair–water interface, and area occupied per surfactant molecule

a1 CVC (mM)gCMC

(mN m�1)GLG

� 106 (mol m�2) Amin (A2)gCVC

(mN m�1)

1 — 38.71 2.06 80.55 —0.98 — 33.40 1.21 137.73 —0.95 1.1 35.64 1.48 112.55 31.590.9 1.0 34.20 1.39 119.53 30.590.8 0.95 32.68 1.59 104.17 30.090.5 0.8 29.51 1.82 90.78 26.220 0.8 28.04 1.82 91.20 24.31

Fig. 3 The plot of mixed CMC andmicellar molar fraction of DDAB (x2)as a function of DDAB molar fraction, for aqueous Shikakai–DDABmixture.

Table 3 The free energy, enthalpy, and entropy of the mixed micel-lization process

a1 x1 f1DGmix

(kJ mol�1)DHmix

(kJ mol�1)DSmix

(J K�1 mol�1)

0.98 0.744 0.933 �1.903 �0.497 4.7180.95 0.542 0.312 �5.126 �3.418 5.7300.9 0.492 0.296 �4.631 �2.914 5.7590.8 0.427 0.247 �4.268 �2.578 5.6710.5 0.288 0.187 �3.168 �1.680 4.993

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b ¼ lnða1c12=x1c1Þð1� x1Þ2

(5)

Negative, positive, and zero values of b indicate synergism,antagonism, and ideal mixed micelle formations, respectively.The larger the absolute value of b, the stronger the nonideality.The calculated values of b at different mixing ratios are reportedin Table 2. From the table it can be seen that all mixing ratioshave a synergistic behavior; however, that again variesdepending on the composition. The maximum synergisticbehavior was found for a mixing ratio of 95 : 5. This strongsynergistic behavior can be attributed to (i) the van der Waalsinteraction between the tails of DDAB and the hydrophobic partof Shikakai, and (ii) the strong electrostatic interaction betweenthe positively charged head group of DDAB and the carboxylgroup of Shikakai. As there is a strong interaction between thesurfactant molecules, micellization occurs at very low concen-tration in comparison to pure Shikakai. The CMC of pure Shi-kakai is higher than DDAB. As a result Shikakai molecules arepreferably present in the bulk solution than in the mixedmicelle and vice versa for DDAB; additionally the presence ofShikakai reduces the electrostatic repulsive force between thehead group of DDABmolecules inside themicelle, which in turnfavors mixed micelle formation at lower concentrations. Thecalculated micellar mole fractions of DDAB using eqn (4) areplotted in Fig. 3, which shows that there is a sharp increase inmicellar molar fraction of DDAB (x2) with the increase in molarfraction of DDAB (a2) in bulk. The larger distance of separation

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between the diagonal line and the micellar molar fractionplot also indicates that the mixed solutions are highlynonideal in nature, which was essentially concluded before interms of b-parameter.

3.2 Thermodynamic parameters for the micellization ofmixed surfactant solutions

It is evident from the previous section that there are favorableinteractions between the surfactant molecules during the mixedmicelle formation. Now the determination of thermodynamicparameters of the micellization such as free energy (DGmix),enthalpy (DHmix), and entropy (DSmix) play an important role toget an idea about the spontaneity of the process. By consideringthe regular solution theory for these mixed solutions, DSmix ¼DSideal, which indicates excess entropy function SE¼ 0 and GE¼DHmix.10 The thermodynamic parameters are calculated usingthe following equations10,40

DGE ¼ DHmix ¼ RT [x1 ln f1 + x2 ln f2] (6)

DGmix ¼ RT [x1 ln(x1 f1) + x2 ln(x2 f2)]

¼ RT [x1 ln(x1 f1) + (1 � x1 ln{(1 � x1)(1 � f1)})] (7)

DSmix ¼�DHmix � DGmix

T

�(8)

where DGmix, DHmix, DSmix are the free energy (kJ mol�1),enthalpy (kJ mol�1), and entropy (J K�1 mol�1) of mixedmicellization; f1 and f2 are the activity coefficients, and x1 and x2are the micellar molar fractions of Shikakai and DDAB,respectively. The calculated values at 298 K are presented inTable 3.

It can be seen from the table that all mixtures have a negativefree energy of micellization, indicating that the mixed micelli-zation process is favourable. It is also important to note that at a95 : 5 (Shikakai : DDAB) mixing ratio the free energy is morenegative, which is also consistent with a more negative b value.The free energy of mixed micellization is more negative thanpure Shikakai but less than pure DDAB. The negative values ofenthalpy also indicate that the process is exothermic.

3.3 Wettability of mixed surfactant solutions on PTFEsurface

As mixed surfactant solutions show a reduction in CMC andsurface tension compared to pure Shikakai, the mixed solutionswere used to test the wettability of a PTFE surface. The contact

RSC Adv., 2014, 4, 9182–9188 | 9185

Fig. 5 The change in adhesion tension (gLG cos q) versus surfacetension (gLG) of different surfactants.

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angle of aqueous solutions using Shikakai, DDAB, and theirmixtures of different mixing ratios are presented in Fig. 4. Purewater shows a contact angle value of 116.5� on the PTFE surface.With the increasing concentration of pure Shikakai, the contactangle (q) gradually decreases and nally reaches a plateau valueof 98.13� at 0.5 mM concentration with a slope of �6.30 andabove that it remains almost constant. Whereas in the case ofDDAB the contact angle value decreases up to 79�. The contactangle in the presence of DDAB surfactant is 83� at the CMC witha rst slope of �16.85, and above that concentration at CVC thevalue reduces to 79� with a second slope of �3.30. So the totaldecrease in contact angle in the presence of DDAB is more thandouble (�37�) that of Shikakai (�18�) with respect to purewater, indicating that it is a much better wetting agent for thePTFE surface or other hydrophobic surfaces compared to thatShikakai (Fig. 4).

The wettability of Shikakai and DDAB mixed surfactantsolutions on the PTFE surface were studied using differentmolar mixing ratios such as 98 : 2, 95 : 5, 90 : 10, 80 : 20, and50 : 50. In the case of a binary mixture, with increasing DDABconcentration, similar to the surface tension plot, the contactangle at CMC also decreases gradually for all different mixingratios. Upon addition of 2% DDAB to Shikakai, the contactangle value decreases to 92.4� at the CMC with a single breakpoint, which is lower than that of the contact angle value ofShikakai (q ¼ 98.13�) at CMC. Further increasing the DDABconcentrations, the contact angle values at CMC decreasegradually, and at a 50 : 50 molar ratio the contact angle at CMCis close to that of pure DDAB. Similarly to the surface tensionplot, when the concentration of DDAB is 5% or above thecontact angle change because of CVC is prominent, the contactangle values at CMC and CVC for different mixing ratios arepresented in Table 4.

In wetting, detergency, adsorption and many other applica-tions, generally the change in properties reaches a plateau levelabove the CMC. In such cases consumption of the surfactant isoptimal at the CMC, with maximum efficiency. In this study,consumption is simply dened as surfactant concentration atcritical micellization, above which there is no change in contactangle. A comparative analysis of the change in contact angleand surfactant consumption in terms of mol% of different

Fig. 4 The change in contact angle (q) with the concentration (log c)of different surfactants.

9186 | RSC Adv., 2014, 4, 9182–9188

synthetic (nonionic: TX 100, Igepal CO—630; cationic: CTAB,anionic: SDBS) and Shikakai—DDAB mixed surfactants withrespect to Shikakai is presented in Table 5. The negative valuesof Dq and consumption in mol% indicates a lower contact angleand consumption than that of Shikakai. The highest contactangle of Shikakai indicates inferior interfacial properties,however the addition of DDAB enhances the wetting propertiesas well as reduces the consumption of surfactant. Table 5 clearlyshows that at the 98 : 2 mixing ratio the contact angle is �7�

lower than that of pure Shikakai and consumption is 29% lowerthan pure Shikakai; however when themixing ratio is 50 : 50 thecontact angle is 16.3� lower but consumption increases to62.5%. Similarly the contact angle changes and consumption ofother mixing ratios such as 95 : 5, 90 : 10 and 80 : 20 are alsotabulated in Table 5. Although overall consumption iscomparatively high with respect to pure Shikakai, theconsumption of the synthetic surfactant is reduced, which isgood from the environmental view point.

3.4 Adsorption of surfactants at PTFE–water and air–waterinterfaces

The change in contact angle in the presence of aqueoussurfactant solutions is closely related to the adsorption densityas well as to the adsorption pattern on the solid surface. Thecontact angle measurements indirectly give some idea about thesurface excess at the solid–liquid interface compared to that atthe air–water interface, which can be obtained again from thesurface tension results. It has been found before that thesurface excess values at the PTFE–water interface are less than

Table 4 The contact angle (q) values at CMC and CVC for differentmixing ratios

a1 qCMC qCVC

1 98.13 —0.98 91.19 —0.95 90.43 87.690.9 92.86 87.190.8 89.54 86.750.5 86.21 81.750 83.67 79.78

This journal is © The Royal Society of Chemistry 2014

Table 5 A comparison of minimum contact angle and surfactant consumption in mol% with respect to Shikakai for different single and mixedsurfactant systems

Surfactant q� minimum Conc. (mM) Dq Consumption in mol%

Shikakai 98.13 0.5DDAB CMC 83.67 0.05 �14.46 �90

CVC 79.78 0.8 �18.35 +60TX-100 80.42 0.15 �17.71 �70Igepal CO 630 82.79 0.08 �15.34 �84CTAB 84.06 0.9 �14.07 +80SDBS 86.76 1.5 �11.37 +200Shikakai : DDAB 98 : 2 91.19 0.35 �6.94 �29

95 : 5 87.69 0.87 �10.44 +74.1990 : 10 87.19 0.84 �10.94 +68.2780 : 20 86.75 0.82 �11.38 +64.4450 : 50 81.75 0.81 �16.38 +62.56

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that of the air–water interface for the pure surfactants, butthe % reduction depends on the type of surfactant.41,42 Acomparison of surface excess values can be made if the plotof adhesion tension (gLG cos q) vs. surface tension follows alinear relationship (Fig. 5). The adhesion tension is denedas the net force on the liquid at the three-phase boundary.43

In the case of the surfactant mixture, the plot ofadhesion tension vs. surface tension shows a linear rela-tionship. The slopes of linear t using eqn (9) for pure andsurfactant mixtures of different mixing ratios are tabulated inTable 6.

gLG cos q ¼ agLG + b (9)

where a and b are the slope and intercept for the linear t,respectively.

Table 6 shows that pure Shikakai, DDAB, and their mixturesin different ratios have slopes less than 1. This indicates thatthe surface excess of surfactant at the PTFE–water interface islower than that of the air–water interface. It can be seen fromthe table that the slope value is higher than the pure surfac-tants up to a ratio of 80 : 20; when the ratio is 50 : 50 the slopeis close to that of pure DDAB. The increasing adsorptiondensity at the PTFE–water interface is mainly because of (i)strong interaction between the surfactant molecules in thebulk solutions for those mixing ratios, (ii) reduction inrepulsive force between the head groups of DDAB in thepresence of Shikakai.

Table 6 Values of ‘a’ and ‘b’ for the adhesion tension and surfacetension plot for different mixing ratios

a1 a b R2

1 �0.78 24.99 0.990.98 �0.82 27.04 0.990.95 �0.83 27.26 0.990.90 �0.83 27.23 0.990.80 �0.80 26.08 0.990.50 �0.75 23.98 0.990 �0.75 23.59 0.97

This journal is © The Royal Society of Chemistry 2014

4. Conclusions

Surface tension measurements show that Shikakai and DDABhave CMC values of 0.5 and 0.05 mM respectively. The surfacetension values at CMC are 38.79 and 28.5 mN m�1, respec-tively. However, DDAB forms vesicles at 0.8 mM concentrationwith 24.3 mN m�1 surface tension. The mixture of Shikakaiand DDAB shows a strong synergistic interaction between thehead group of DDAB and the carboxyl group of Shikakai. Thesynergistic behavior in terms of interaction parameter usingregular solution theory was calculated and found to be highlynegative (b ¼ �1.056 to �5.561) for entire mixing ratios withmaximum non-ideality (b ¼ �5.561) at the ratio 95 : 5. Themixed solutions also form vesicles when the concentration ofDDAB is 5 mol% or above. Pure Shikakai and DDAB solutionsshow minimum contact angle values of 98.13 and 79.78� onthe PTFE surface. In the case of surfactant mixtures, additionof 2 mol% DDAB to Shikakai shows an �7� lowering in contactangle compared to pure Shikakai, and the consumption is 29%lower than pure Shikakai; however when 50% DDAB is addedthe contact angle is 16.3� lower but consumption increases to62.5%. Here, owing to the fact that the overall consumption iscomparatively high with respect to pure Shikakai, theconsumption of synthetic surfactant reduces signicantly,which is really useful from the environmental view point. Atthe same time, the efficiency of natural surfactants can beenhanced to a better extent than the conventionally usedsynthetic surfactants.

Acknowledgements

N.R.B thanks C.S.I.R., New Delhi, India, for a Senior ResearchFellowship to pursue this work.

References

1 Y. Zhou and L. Zhu, J. Hazard. Mater., 2004, 109, 213–220.2 R. Miller, V. B. Fainerman, M. E. Leser and M. Michel,Colloids Surf., A, 2004, 233, 39–42.

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View Article Online

3 B. Zhao, L. Zhu, W. Li and B. Chen, Chemosphere, 2005, 58,33–40.

4 A. Mohamed and A. M. Mahfoodh, Colloids Surf., A, 2006,287, 44–50.

5 L. M. Bergstrom and T. Bramer, J. Colloid Interface Sci., 2008,322, 589–595.

6 K. J. Rao and S. Paria, J. Phys. Chem. B, 2009, 113, 474–481.7 X. Wang, J. Wang, Y. Wang, J. Ye, H. Yan and R. K. Thomas, J.Colloid Interface Sci., 2005, 286, 739–746.

8 R. Ghosh Chaudhuri, S. Sunayana and S. Paria, So Matter,2012, 8, 5429–5433.

9 A. Shiloach and D. Blankschtein, Langmuir, 1998, 14, 4105–4114.

10 E. Rodenas, M. Valiente and M. S. Villafruela, J. Phys. Chem.B, 1999, 103, 4549–4554.

11 M. Bergstrom and J. C. Eriksson, Langmuir, 2000, 16, 7173–7181.

12 M. Bergstrom, Langmuir, 2001, 17, 993–998.13 A. Goldsipe and D. Blankschtein, Langmuir, 2005, 21, 9850–

9865.14 M. Bergstrom, P. Jonsson, M. Persson and J. C. Eriksson,

Langmuir, 2003, 19, 10719–10725.15 S. Koning, K. Quitzsch, B. Koning, R. Hommel, D. Haferburg

and H. P. Kebler, Colloids Surf., B, 1993, 1, 33–41.16 P. R. Majhi, K. Mukherjee, S. P. Moulik, S. Sen and

N. P. Sahu, Langmuir, 1999, 15, 6624–6630.17 R. Kukhetpitakwong, C. Hahnvajanawong, P. Homchampa,

V. Leelavatcharamas, J. Satra and W. Khunkitti, Int.Immunopharmacol., 2006, 6, 1729–1735.

18 S. Balakrishnan, S. Varughese and A. P. Deshpande, Tenside,Surfactants, Deterg., 2006, 43, 262–268.

19 L. R. Row and C. Rukmini, Indian J. Chem., 1966, 4, 36–38.

20 Y. Tezuka, K. Honda, A. H. Banskota, M. M. Thet andS. Kadota, J. Nat. Prod., 2000, 63, 1658–1664.

21 G. Ozdemira and U. Malayoglu, Colloids Surf., B, 2004, 39, 1–7.

22 Y. Ishigami, Y. Gama, F. Ishii and Y. K. Choi, Langmuir, 1993,9, 1634–1636.

23 M. A. Gafar, T. Obata, F. Kiuchi and Y. Tsuda, Chem. Pharm.Bull., 1997, 45, 620–625.

9188 | RSC Adv., 2014, 4, 9182–9188

24 N. Fatma, W. H. Ansari, M. Panda and Kabir-ud-Din, J.Surfactants Deterg., 2013, 16, 609–620.

25 T. Mehling, L. Kloss, T. Ingram and I. Smirnova, Langmuir,2013, 29, 1035–1044.

26 A. Bera, K. Ojha and A. Mandal, J. Surfactant and Detergent,2013, 16, 621–630.

27 K. Szymczyk, A. Zdziennicka, B. Janczuk and W. Wojcik, J.Colloid Interface Sci., 2006, 293, 172–180.

28 A. Zdziennicka, B. Janczuk andW. Wojcik, J. Colloid InterfaceSci., 2003, 268, 200–2007.

29 Y. G. Bogdanova, V. D. Dolzhikova and B. D. Summ, ColloidJ., 2003, 65, 284–289.

30 W. H. Ansari, N. Fatma, M. Panda and Kabir-Ud-Din, SoMatter, 2013, 9, 1478–1487.

31 J. Wei, G. Huang, L. Zhu, S. Zhao, C. An and Y. Fan,Chemosphere, 2012, 89, 1347–1353.

32 S. Paria and P. K. Yuet, Ind. Eng. Chem. Res., 2006, 45, 3552–3558.

33 N. R. Biswal and S. Paria, Submitted for publication.34 N. R. Biswal, Studies on Adsorption andWetting Phenomena

Associated with Solid Surfaces in Aqueous Synthetic andNatural Surfactant Solutions, Ph. D. Thesis, NIT Rourkela,2013.

35 N. R. Biswal and S. Paria, Ind. Eng. Chem. Res., 2012, 51,10172–10178.

36 Q. Zhou and M. J. Rosen, Langmuir, 2003, 19, 4555–4562.37 A. Fontana, P. D. Maria, G. Siani and B. H. Robinson,

Colloids Surf., B, 2003, 32, 365–374.38 J. F. A. Soltero, F. Bautista, E. Pecina, J. E. Puig, O. Manero,

Z. Proverbio and P. C. Schulz, Colloid Polym. Sci., 2000,278, 37–47.

39 D. N. Rubingh, in Solution Chemistry of Surfactants, ed.K. L. Mittal, Plenum Press, New York, 1979, vol. 1.

40 C. C. Ruiz and J. Aguiar, Colloids Surf., A, 2003, 224, 221–230.41 N. R. Biswal and S. Paria, Ind. Eng. Chem. Res., 2011, 50,

6138–6145.42 R. Ghosh Chaudhuri and S. Paria, J. Colloid Interface Sci.,

2009, 337, 555–562.43 R. E. Johnson Jr and R. H. Dettre, Wetting of low energy

surface, in Wettability, ed. J. C. Berg, Marcel Dekker, Inc.,New York, 1993.

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