13

44

Research ArticleReceived: 10 February 2008 Accepted: 21 May 2008 Published online in Wiley Interscience: 21 July 2008

(www.interscience.com) DOI 10.1002/sia.2900

Simultaneous study of interfacial tension,surface tension, and viscosity of few surfactantsolutions with survismeterMan Singh∗

Surfaces and interfaces are receptive valuable significant property of chemical molecules due to their potential to developseveral phenomena in a self-controlled mechanism. Science of surfaces is vast and is being used industrially since timeimmemorial. Their accurate and simultaneous estimation is necessary; therefore, the survismeter was used for measuringthem along with viscosity. Individually tensiometers, X-ray reflective microscope, and viscometers are used for surface tension,interfacial tension, and viscosity, respectively. These devices are sophisticated, expensive, and individually consume muchtime and resources with poor reproducibility in measurements. Survismeter is an alternative device for similar measurementstogether with higher accuracies and reproducibility. It works on a principle of capillary flow and pressure gradient (PG) insideliquid-holding and air-filled bulbs. Several liquids have been used for study with ±0.01 mN/m, ±0.01 mN/m and ±1 × 10−5 Ns/m2 accuracies in respective data. Copyright c© 2008 John Wiley & Sons, Ltd.

Keywords: interfacial tension; surface tension; hydrophobic; miscibility; hydrophilic

Introduction

Colloidal science and technology has occupied special positionamong sciences like nanotechnology, biotechnology, and micro-biology. Liquids are utilized in all developing and developedfields such as pesticides, medicines, cosmetics, fashion, and lo-tion. ‘Colloidal solutions’ with their physicochemical propertiesand characterization is one of the features of colloid technol-ogy. The major applications of colloid science are quality controlof several industrial products. The colloid science is a studyof systems of small particles of one substance suspended inanother liquid and forms the basis of a wide variety of ap-plications of scientific and technological importance, includingpaints, ceramics, cosmetics, agricultural sprays, detergents, soil,biological cell, many food preparations, and sunscreen lotion.Few molecules like carboxymethylcellulose (CMC) are used widelyas thickener, suspending aid, stabilizer, binder, and film-former.There are no strict limits on the sizes of colloidal particlesbut they tend to vary between 1 nm and 1 µm as seen inFig. 2.

Surfactants lower the surface tension of a liquid, allowing eas-ier spreading and interfacial tension (IFT) between two liquids.The surfactant is a contraction of ‘surface-active agent’ that accu-mulates at the interface between two liquids and modifies theirsurface properties. In a two-phase system, e.g. liquid–liquid orsolid–liquid, a surfactant tends to remain at the interface of twophases to provide continuity between the two different materials.Surfactants are usually organic compounds that are amphipathic,as they contain both hydrophobic groups (tails) that are usuallylong-chain hydrocarbons and hydrophilic groups (heads) whichare often ionic. Cationic surfactant tetramethylammoniumhydrox-ide (TMAH) and anionic dodecylbenzenesulfonicacid (DBSA) wereused in our work. TMAH contains four hydrophobic methyl groupsand hydrophilic –OH groups shown below.

The DBSA contains hydrophobic long alkyl chain as shown inthe following structure.

Surface science fascinates everyone to go deeper and deeperinto the study of surface and frictional forces. Thereby, right,optimistic, acceptable, and approachable exposure of surfacescience and technology perhaps would have encouraging im-plications for young scientists. The survismeter is a simple, fast,multipurpose, green chemistry instrument which fascinates andencourages younger generation for scientific attitude develop-ment. Survismeter synchronized viscosity and surface tension foran equation popularly known as Man Singh equation which ledto a new physicochemical parameter named friccohesity.[1] IFT,surface tension, and viscosity have functional relations[2] and aremeasured simultaneously with survismeter. For example, the vis-cosity is due to friction on the surface, and surface forces too acton the surfaces.[1,3] The surface tension and interfacial tension aremeasured with pendant drop which is formed inside a survismeterbulb. Sessile drop is used for surface properties like contact anglebut there is no reproducible data.

The survismeter is good for accurate determination of suchparameters as certain zwitterionic surfactants like dimethylam-monium carboxylate, dimethylammonium sulfate, and dimethy-lamine oxide. As these are frequently used in cosmetics (Fig. 3)their accurate characterization is necessary.[4]

Interfacial tension of dodecyltrimethylammoniumbromide(DTAB), trimethylsulphoxoniumiodide (TMSOI), methyltrioctylam-moniumchloride (MTOAC), and 3, 5-dihydroxy toluene monohy-

∗ Correspondence to: Man Singh, Chemistry Research Lab., Deshbandhu College,University of Delhi, New Delhi 110 019, India.E-mail: mansingh50@hotmail.com

Chemistry Research Lab., Deshbandhu College, University of Delhi, New Delhi110 019, India

Surf. Interface Anal. 2008, 40, 1344–1349 Copyright c© 2008 John Wiley & Sons, Ltd.

13

45

Interfacial tension with survismeter

(a)

(b)

Figure 1. (a) The structure of TMAH, (b) The structure of DBSA. This figureis available in colour online at www.interscience.wiley.com/journal/sia.

Figure 2. Sizes of colloidal particles.

Dodecyldimethylammoniumbutylcarboxylate

Dodecyldimethylammoniumproplysulfate

Dodecyldimethylammoniumamineoxide

Figure 3. Zwitterionic surfactants.

drate (orcinol) were measured.[3] Electronegativities of O−, I−, andS− of TMSOI, Cl−, and N+ of MTOAC; N+ and Br− of DTAB; OH−

groups and π conjugation of orcinol cause mutual wetting ofbenzene and water.[3 – 6]

Theory for Working

The survismeter consists of three separate pressure circuit links(PCL); PCL1 is 10-9-8-7-2, PCL2 is 10-9-6-5-4, and PCL3 is 10-B1-

B2-19-18 for surface tension, viscosity, and IFT measurementsrespectively. The 10, 9, 8, 7 denote bulb numbers of the PCL1; the10, 9, 6, 5 that of the PCL2, and the 10, B1, B2, 19, 18 that of the PCL3,and the 2, 4, and 19 their sockets, respectively. The bulb numbersare written as b10, b9, b8, b7, b6, and b5. The 10-1 and 18-17-14are two sub-pressure circuit links (SPCL) denoted as SPCL1 andSPCL2 respectively. The SPCL1 monitors hydrostatic pressure andSPCL2 acts as an operator to develop pressure gradient (PG) insidePCL1, 2, and 3. Initially liquid is filled inside b10 at atmosphericpressure and is equilibrated at fairly constant desired temperatureof ±0.05 ◦C with thermostatic control.

Mathematical model

The survismeter working equation (Eqn (1)) is as follows:

P(PCL1) = P(PCL2) = P(SPCL3) (1)

Equation (1) infers equal pressures in P(PCL1), P(PCL2), andP(SPCL3) with PG = 0. The Eqn (1) is bifurcated into Eqns (2)and (3), given below. For surface tension,

Pb10 = Pb9 = Pb8 = Pb7 = 0 (2)

For viscosity,

Pb10 = Pb9 = Pb6 = Pb5 = 0 (3)

These equations infer no flow of liquid via any link since PG = 0,which is a non-operational condition for measurements. Thus thepressure P is useful for working of survismeter. The operationswork on ‘on or off’ principle like electric circuits. The PCL1 isswitched on for operation and then PCL2 and PCL3 are switchedoff. The off denotes defunct or non-operational with the PG = 0or blocked operation.

PCL 1 for surface tension measurement

For its operation, the link (Pb10 = Pb9 = Pb6 = Pb5 = 0) andsocket 3 are blocked with airtight stoppers and the cone number18 of an operator SPCL2 is attached to socket number 2. Theplunger of an operator is moved backward for pressure inside b7,b8, b9, and b10 bulbs. This changes equality in pressure and isdenoted in Eqn (4).

Pb10 > Pb9 > Pb8 > Pb7 = PG �= 0 (4)

The Eqn (4) infers least pressure in b7 to pull liquid from b10 onusing an operator SBPLC2. With F = ma, m is mass, a is constantacceleration, F is force in Newton and is applied on SBPLC2 to pullliquid to b7; liquid V = m/(liquid density) is filled in b7.

Then the pulling of liquid is stopped and the Eqn (4) becomesEqn (5).

Pb10 = Pb9 = Pb8 = Pb7 = PG = 0 (5)

Then the stopper from socket number 3 and SBPLC2 arewithdrawn to permit back flow of liquid from b9. Now b9 attainsatmospheric pressure and there is back flow of liquid from b7-b8-b9-b10. A drop-wise backflow is allowed, a tip of a capillary isextended to b9 to develop a perfect pendant drop. The number ofdetached pendant drops from a liquid volume within two fiducialmarks of b8 was recorded with drop counter. Similarly the PCL2 is

Surf. Interface Anal. 2008, 40, 1344–1349 Copyright c© 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia

13

46

M. Singh

switched on and PCL1 is off for viscosity measurements. The liquidnow flows to Pb10-Pb9-Pb6-Pb5 when pulled up similarly andflows backward when similar operation is executed as in surfacetension. The only difference is that for viscosity there is continuousliquid flow. The flow time is noted with two fiducial marks of b6.The b5 and b7 are for equilibrating the liquid for the respectivemeasurements.

Interfacial tension

Benzene and water together develop an interface but DTAB,TMSOI, MTOAC, and orcinol weaken IFT by mutual miscibilities ofthe phases with wetting.

Experimental: Materials and Method

The 0.45 mm/kg solution of each surfactant was prepared withMillipore water, w/v, DTAB (Sigma, no. D 5047), orcinol (Sigma,no. 1875), TMSOI (Fluka, no. 92763), MTOAC (Fluka, no. 69485), andbenzene (E Merck). The CMC, TMAH, and DBSA Fluka were usedas received. The chemicals were stored in P2O5-filled desiccatoruntil use.

Handling of survismeter

Survismeter was cleaned with standard method and calibrated(calibration no. 06070582/1.01/C-0395, NPL, Govt. of India) andused for viscosity, surface tension, and IFT simultaneously (Fig. 4).It saves time and resources by more than 80% as compared toconventional methods with no escape of poisonous and volatileliquids to environment.[7,8]

Measurements

Initially the liquid sample was filled in b10 and after use wasremoved via tube no. 11 whose inner diameter is 2 mm withoutchanging the position of instrument. The links b4-b5-b6-b7-b9-b10, b2-b7-b8-b9-b10, and 15-16-18-19-B1-B2-1-b10 measureviscosity, surface tension, and IFT respectively (Fig. 4). The two-third volume of b10 is filled with liquid via socket no. 1 when link19-B1-B2 is not fitted in socket no. 1. For viscosity, joint nos. 3 and2 were stopped and piston 14-17-18 is fitted with socket no. 4 tolift the liquid to b5 from b10 via b9 and b6. Stopper from socketno. 3 was withdrawn to evacuate liquid from b9 to b10. The pistonstopper no. 18 of link 17-14 is withdrawn and viscous flow time (tseconds) for liquid within fiducial marks of b6 is noted. Similarlya liquid is lifted to b7 via b9 and b8, the socket no. 4 and 3 arestopped and stopper no. 18 with link 17-14 is fitted in socket no. 2.The liquid from b9 is evacuated like the previous operation andpiston stopper no. 18 is withdrawn and drops falling from capillaryhanging in b9 are counted for a liquid within two fiducial marksof b8.

Interfacial tension

The Millipore water is high-density liquid and is denoted as denserliquid and was taken in bulb no. B1 and B2 of link 13 with openstopper no. 18 for pulling a water sample kept in 25 ml-capacitybeaker for this purpose. When B2 is filled with water, the pistonstopper no. 18 was lifted and airtight stopper was fitted in jointno. 19 to prevent flow of the water. Then the unit 13 was fittedin socket no. 1. The b10 contains benzene (lighter liquid) and

capillary of B1 remains dipped in it. The stopper was removedfrom socket no. 19 and pressure passage link 15-16-18 was fittedin it to control 4–7 drops/min in benzene medium for the volumeof water within two fiducial marks of B1. Similarly the drops werecounted in air in place of benzene.

Result

The surface tension and viscosities were calculated using usualequations and given in Table 1. The interfacial tension wascalculated with Attonoff‘s equation given below.

γIFT =((

nHDL in air

nHDL in LDL

) (ρHDL − ρLDL

ρHDL

))γHDL (6)

The γIFT stands for interfacial tension between low- and high-density liquids, nHDL in air and nHDL in LDL number of drops ofhigh-density liquid in air and low density liquid, the ρHDL andρLDL densities of high and low density liquids and γHDL the surfacetension of high-density liquid.

Our own models

Efforts are on to calculate the γIFT using heights (h) and widths (w)of the (pendant) drops (Fig. 3) in air and liquid media, both withequations given below. Each equation is being used in steps.

γIFT =((

hHDL in air

hHDL in LDL

)(ρHDL − ρLDL

ρHDL

))γHDL (7)

γIFT =((

hHDL in air

hHDL in LDL

)(hHDL − hLDL

hHDL

))γHDL (8)

γIFT =((

wHDL in air

wHDL in LDL

)(ρHDL − ρLDL

ρHDL

))γHDL (9)

γIFT =((

wHDL in air

wHDL in LDL

)(wHDL − wLDL

wHDL

))γHDL (10)

Discussion

Using measurements the t, n, and nIFT data were recorded and usedfor calculation of the η, γ , and γIFT respectively using respectiveequations.[7,8] Each measurement was repeated several times toascertain the reproducibility of the method and statistical analysisof data were made. The data were noted with 95.5% confidencelevel on Gaussian distribution curve. Literature data[9,10] closelyagree to our experimental data of IFT. The 71.1 mN/msurfacetension of water at 31.5 ◦C in air medium was reduced to 30.8mN/m with benzene medium (Fig. 5). This infers a hindrance ofbuoyant force of benzene on drops. Flow times in air medium werelower than those of the benzene due to the influence of buoyancyof benzene medium. The buoyancy resists the normal-drop falls atits interface with a higher decrease in surface tension of water at theinterface of benzene. The IFT values are: benzene > DTAB > TMSOI> water > orcinol > MTOAC with 30, 15.69, 12.86, 10.59, 8.36, and2.97 decreases with respect to the values in air. The MTOAC, orcinol,DTAB, and TMSOI develop stronger hydrophilic and hydrophobicinteractions between water and benzene. MTOAC enhancesthe mutual solubilities as compared to those of DTAB, TMSOI,and orcinol with 2-OH and 1-CH3 developing slightly weakerhydrophilic and hydrophobic interaction (Fig. 5). A comparisonof structure of DTAB (Fig. 5) with alkyl chain of 12 carbon atoms

www.interscience.wiley.com/journal/sia Copyright c© 2008 John Wiley & Sons, Ltd. Surf. Interface Anal. 2008, 40, 1344–1349

13

47

Interfacial tension with survismeter

Figure 4. Survismeter for surface tension, IFT, and viscosity simultaneous measurements.

infers stronger hydrophilic interaction but the MTOAC and TMSOIwith alkyl chain of 12 carbon atoms and alkyl chain with 3 carbonatoms, respectively, develop weaker hydrophobic and strongerhydrophilic interactions due to N+ and Cl− with MTOAC, andthe O atom of TMSOI. The surfactants with a longer alkyl chaindevelop stronger hydrophobic interaction due to higher solubilityin benzene rather than that of the water. Surfactants with a longeralkyl chain develop weaker hydrophilic interactions, and halideions of their functional groups vis-a-vis IFTs show their placementas –Cl− > I− > Br−. Hence the IFTs vis-a-vis structures of TMSOIand MTOAC infer that hydrophobic interactions of 3-CH3 of TMSOIoutweigh the hydrophilic interactions due to O S+ —I−. HenceIFT value is higher for O S+ —I−, water interfaces with O S+ —I−

takes the geometry of O S+ —I− —H2O, titling towards waterphase due to stronger dipolar interactions with H2O. The I−

develops stronger induced polarizability but MTOAC’s Cl− ion

shows slightly stronger hydrophobic interaction than that ofO S+ —I− due to a 10-carbon atom chain that dominates theweaker hydrophilic interactions of N+ —Cl−. The Cl− is a small-sized anion and does not induce any potential. Thus O S+ —I−

contributes greater share to the higher IFTs while N+ —Cl doesnot, but the 12-carbon chains do compensate for the same. DTABincreases the surface tension as its molecules approach the surfaceof water with a higher surface stretching where the N+ and Br−

interact with water, but its dodecyl chain hangs centering towardsbottom, making the surface more stretched. N+ —Br− acts as amild dipole and is unable to disrupt water structure.[11] Thus themildness of N+ is further weakened by 3-CH3 groups attachedto its quaternary nitrogen ion N+ while on the other side thedodecyl chain also weakens it. Due to this reason the N+ and Br−

are not able to break the hydrogen-bonded water surface andinstead make it more stretched. Hence DTAB increases the surface

Surf. Interface Anal. 2008, 40, 1344–1349 Copyright c© 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia

13

48

M. Singh

81 79 93 77 74146

205136

398

975

0

100

200

300

400

500

600

700

800

900

1000

Water DTAB TMSOI MTOAC Orcinol

surfactants

Flow

rim

es, s

ec

Flow times, sec, in air

Flow times, sec, in benzene

27.87

71.2579.55

68.19

28.58

72.70

30

10.5915.69 12.86

2.978.36

0

10

20

30

40

50

60

70

80

90

Benzene Water DTAB TMSOI MTOAC Orcinol

surfactants

Surf

ace

tens

ion,

IFT

Surface tension, dyne/cm, in air IFT, dyne/cm, in benzene

Viscosities, g/cm/s, in air

0.561

0.78400.7664

0.8525

0.76080.7261

0.5000

0.5511

0.6022

0.6533

0.7044

0.7555

0.8066

0.8577

0.9088

Benzene Water DTAB TMSOI MTOAC Orcinol

viscosities, g/cm/s

Surf

acta

nts

Figure 5. The flow time, min, and surface tension 1 × 10−3 N/m in air and benzene medium, viscosities 0.1 N s/m2 in air medium.

tension of water from 71.18 to 79.25 dyne/cm. The TMSOI alsoaccumulates near the surface of water but because of strongerelectronegativity of O− and S+ —I−, does disrupt the hydrogenbonding of surface water which decreases the surface tension ofthe solution by 3 dyne/cm. But MTOAC due to N+ —Cl− dipolestrongly break water structure at the surface, reducing surfacetension to 28.58 dyne/cm.

CMC, TMAH and DBSA systems: viscosities

The viscosities were noted as CMC > TMAH > DBSA (Table 1) withexceptionally higher viscosity for CMC. This is because of strongerhydrogen bonding due to millions of –OH and –CH2COO− groups.The CMC develops stronger intermolecular forces. However theTMAH and DBSA with four –CH3 groups and long alkyl chainrespectively show lower viscosities as compared to CMC. ThusTMAH and DBSA develop stronger hydrophobic interactions that

are responsible for lower viscosity. The DBSA shows lower viscositythan TMAH. It is because DBSA has long hydrophobic chain ascompared to small hydrophobic –CH3 groups of DBSA. Thus theDBSA develop stronger hydrophobic interaction. The viscositiesof the TBSA and DBSA increase with concentration. This reflectsthat in general viscosity increases with concentration at constanttemperature (Table 1).

Surface tension

The surface tensions are TMAH > CMC > DBSA (Table 1) andsurface tension increases with the concentration of each system.The CMC and DBSA show higher surface tension for their di-lute solutions that infer stronger water–CMC interactions thanthose of the DBSA and TMAH with water. The CMC solutionexceptionally produces higher surface tensions for its lower con-centration. But while CMC with higher concentration developed

www.interscience.wiley.com/journal/sia Copyright c© 2008 John Wiley & Sons, Ltd. Surf. Interface Anal. 2008, 40, 1344–1349

13

49

Interfacial tension with survismeter

Table 1. Densities measured with pyknometer for calculation of theviscosities and surface tensions for binary systems with water

c(%) TMAH ρ 103 kg/m3, η Ns/m2 γ mN/m

10 1.46197 1.7811 108.147

8 1.46118 1.6921 108.088

6 1.46063 1.4768 108.047

4 1.46035 1.3966 108.027

2 1.46019 1.3498 108.015

DBSA

10 1.46226 1.3506 44.875

8 1.46173 1.3442 44.998

6 1.46137 1.3391 47.032

4 1.46115 1.3304 47.332

2 1.46096 1.3260 48.925

CMC

2.00 1.46344 32.6152 101.442

1.00 1.46274 16.6682 102.832

0.50 1.46249 7.7345 104.293

0.25 1.46223 3.8119 105.797

ρ = density, η = viscosity and the γ = surface tension

stronger CMC–water–CMC interactions where the surface en-ergy associated with adhesive and cohesive is neutralized theyproduced lower surface tensions with higher concentrations.[13]

Industrially, it is useful in various surface-based surfactants likesol–gel, pharmaceutical formulations, protein-binding, nanocol-loids, nanoparticales, and suspension. TMAH further stresses thewater surface, producing maximum higher surface tensions incomparison to DBSA and CMC. The CMC shows lower surfacetension than TMAH by 4 mN/m but DBSA shows about one-thirdof the surface tension of TMAH and CMC. The sulfate groups at-tached to benzene ring develop stronger hydrophilic interactionand the long alkyl chain develops stronger hydrophobic interac-tion. Such interactions reduce the surface tension of water andhence DBSA is the stronger surfactant used in pharmacy, drugdesigning, pesticides, etc.

Gibbs isotherm equation

This equation was applied to our data and inferred an adsorption ofsurfactant at the interface. The surface tensions were plotted withconcentration of surfactants. Generally sugar causes no effect, ionicsalts increase surface tension, alcohols decrease surface tensionprogressively but surfactants decrease surface tension to thelowest value. Josiah Willard Gibbs correlated the surface tensionand concentration of surfactants through surface concentration(�) parameter. It denotes an excess of solute per unit area ofthe surface over what would be present if the bulk concentration

prevailed all the way to the surface and was measured in mole permeter square. It considers an interface as a bidimensional, whichis not true and hence Guggenheim’s corrected this flaw by givingthe following Gibbs adsorption isotherm equation.

� = − 1

R T

(∂γ

∂ ln c

)(11)

The c is concentration of a solute in the bulk solution, R gasconstant, T temperature, and γ surface tension. The � is negativefor inorganic salts due to stronger solute–solvent interaction butit is positive for surfactants due to interface interaction.[12]

Conclusion

Interfacial tension were listed as Cl− > I− > Br−, and iscorrelated to non-bonding electron transitions due to a greaterelectronegativity of Cl− atom. The orcinol, DTAB, TMSOI, andMTOAC decrease surface tension of water by 63.8, 63.1, 55.2,and 25.3%, respectively. The CMC > TMAH > DBSA and TMAH> CMC > DBSA orders of viscosities and surface tensions findstronger frictional force with CMC while surface force is strongerwith DBSA. This model can be extended to estimate ionic pressureof electrolyte salts like potassium chloride on the interface of theimmiscible-solvent phases. As salts are soluble in water phase,but may have affinity for another phase in contact with the waterphase, they can be useful for the estimation of chemical potentialof interfaces.

Acknowledgements

The author is highly thankful to University Grants Commission,Govt. of India and Dr.A. P. Raste, Principal, DBC, for infrastructuralsupport.

References

[1] M. Singh, J. Ind. Chem. Soc. 2005, 82, 129.[2] M. Singh, Proc. Natl. Acad. Sci. India 2007, 77(A), 1.[3] M. Singh, J. Biochem. Biophys. Methods 2006, 67, 151.[4] M. Singh, J. Instrum. Exp. Techn. 2005, 48, 270.[5] M. Singh, J. Chem. Thermodyn. 2006, 39, 240.[6] M. Singh, J. Appl. Polym. Sci. 2007, 103(3), 1420.[7] M. Singh, J. Mol. Liq. 2007, 135, 188.[8] M. Singh, H. Chand, K. C. Gupta, J. Chem. & Biodiv. Helv. Chim. Acta.

2005, 2, 809.[9] M. Singh, A. Kumar, J. Solution Chem. 2006, 35(4), 567.

[10] M. Singh, Bulg. J. Solution Chem. 2006, 15(6), 426.[11] M. Singh, Surf. Interface Anal. 2007, 40, 15.[12] M. Singh, S. Yadav, S. Ahmad, J. Appl. Polym. Sci. 2003, 91(1), 678.[13] M. Singh, Pak. J. Sci. Ind. Res. 2005, 48(5), 303.

Surf. Interface Anal. 2008, 40, 1344–1349 Copyright c© 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia