Interfacial properties of zwitterionic surfactants: N-dodecylbetaine

171

Thermochimica Acta, 122 (1987) 171-179 Elsevier Science PublishersB.V., Amsterdam- Printed in The Netherlands

INTERFACIAL PROPERTIESOF ZWITTERIONIC SURFACTANTS: N-DCDECYLEETAINE A. AMIN-ALAMI,N. KAMENKA, and S. PAR’IYKA Laboratoire Universitk

de Physico-Chimie des Sciences

des Systemes Polypha&

et Technique du Languedoc

Place Eugene Bataillon,

34060 Montpellier

(France)

The interfacial properties and adsorption of a zwitterionic surfactant, n-dodecylbetaine, were investigated in interfacial tension and calorimetric studies.The enthalpy of micellization was directly obtained from calorimetric curve of surfactant dilution measurements and changed as expectedAdsorption onto silica gel showed mechanisms dependent on amphiphile concentration and temperature. INTRODUCTION In spite increasing aspects

of the wide applicability conrnercial

have received

use,it

of zwitterionic

is clear

far less

surfactants

from literature,that

attention

their

and their theoretical

than those of nonionic

or ionic

sur-

factants. Recent studies

have concerned phase diagrams (ref.l),micellization

the upper consolute microemulsion

(ref.3),the

surface

surfactants

where R,R’ and R”

have the following

represent

aliphatic

general

enthalpies

groups,R’

of micellization

from experimental surfactant

Three alkyl purity

and R”

micelle

those of betaines

calorimetric

being methyl or ethyl, ...

have been calculated

of the temperature dependence of the critical purpose of our work is to obtain ionic

(refs.4,5,6),and

formula:RR’R’~N+(CHZ)mA-,

group A- may be sulfate,sulfonate,carboxylate

Generally,the

tly

properties

systems (refs.7,8).

Zwitterionic the anionic

temperature

(ref.Z),

from studies

concentration

(cmc).The

in aqueous solutions

measurements,as has already

direc-

been done for

systems (ref .9) . betaines

h(CHZ),N’(CH3) ZCHZCOO-with n=l 2 ,14 and 16 of high

have been synthetized

and their

micellization

of n-dodecylbetaine,its

of adsorption

onto silica

surface

properties

isotherms

and its

studied.The differential

gel were investigated.

EXPERIMENTAL Surfactants mo methods were employed to obtain 0040-6031/87/$03.50

the following

01987 Elsevier Science PublishersB.V.

betaines:

enthalpy molar heat

172 a) n-dodecylbetaine was preparedusing&e methodof Tori and Nakagawa(ref.10) as describedelsewhere(ref.Z).Several recrystallizations from dry acetonewere performedto eliminatesodiumchloride,a by-productof the reaction,followed by treatmentof the aqueoussolutionswith an ion exchangeresin.Theaqueouss&utionswere subsequently liophilized. b) A singlestep reactio~~!was??p~~~sed startingfrom commercially available N-N d~~l~y~inehydr~oride*: Na2C03 (CH3)2-N-GW-2-COCHj HCl + RX

+ ,R-N+(CH3)2-CH2-CC0

The reactionwaszuarried out by refluxingthe reactantsin methanolfor 48 H. Aftenzool&g;the reactionmixturewas ifiltered and evaporatedunder vacuumand then dissolvedin acetone;Thesalt.was'Zi&tered~off and the resulting&k&betaineswere obtainedafter severalrecrystallisations in acetone. The structureof n-alkylbetaines was confirmedby direct inspectionof their 'H and '3C UiR spectraand the purityof the compoundsobtainedfrom microanalyses. Adsorbent The silicagel&da&bent-was,ra:,s~cimen SPHEROSILXOB,.O1;5 ,manufactured by IBF-Rhmne*Poulenc France.Themeasurements of the surfacearea of this adsorbent were carriedout by using the BET method and Harkins-Jura method (ref,lS),giving 2 -1 24,.6 and 25 m g respectively.The lattervalue has been,msedin our calculatio~.~e~~so~tion

isothermwas determinedby measuringthe surfactantcon-

centaration before.:&drafter adsorption.The'bulk solutionconcentration was measured'bymeans of the differential refractometer WatersR 403 . Measuremets The surfacetensionmeasurements of the betainesolutionswere made by applyingthe WIFE

method and using a PROmO

tensiometer.

The,calorimetric curvesof the differential molar enthalpyof dilution AdilH were obtainedby means of the calorimeterdevelopedby Partykaet al. (ref.l3).The water and the surfactantstock solution,with an initialconcentrationaofabout 28 timesi~higher than"thatcorresponding to unc, were placed into the calorimetric cellWhen the thermalequilibrium was reached,the experiment was carriedout in the followingway: 0.2 g of the surfactantstock solutioncontaininga definitequantityof surfactant(monomers+micelles), was injectedby the pump into the water.The quantityinjectedwas chosenso that the equilibriumconcentration at the bottom of the calerimetric cell be smallerthan cmc.Theacc~~ying

thermaleffectwas

recorded. * I.RICO,E.R.A. 264, Universit&Paul Sabatier,Toulouse,France .

173 The equilibriumstate of the calorimeteris characterizeti1by the return to theinitial base l,ine3Re1~, the next injectionis made;andso on.Eachnew injecfion-allowsusto attain a higher equilibriumconcentration of surfactantand is followedby a consistentthermaleffect.After many injectionsof the stock solu-, fion;intothewater, the water soltionconcentrationreachesthe ant value. l'fhe next injectionsof the stock solutionintothat micellarsolutionare accompaniedby smallerand.smaliler -thermal effects,which are due to the&iUtion of the stock solution(20 times a&).. Initially,the surfactantin solutionexists in monomericform.Afterreaching the cmc, the concentration of monomersbecomesconstantas determinedpreviously in diffusionstudies (ref.2). The AmicH

of micellization at differenttemperatures was calculated'from

the calorimetric curvesof dilutioncof$-2&.e-teine by extrapolation to the cmc region.Thesamecalorimetricmethod was used (ref.13) for the measurementsof the differential molar heat of adsorption(AadsH).Thistime,however,silica gel particleswere placed at the‘bottomof the calorimetric cell.Afterinjectinga given quantityof surfactantsolutioninto the water suspensionof particles,a part of the injectedsurfactantmoleculesis adsorbedand anotherone remains in water.Then,the accompanyingthermaleffect.isrecorded&&

new injection

leads to a higher surfacecoverageof the silicaparticles. RESULTSAND DISCUSSION

Fig.1 Surfacetension8 versus concentrationlog of n-alkylbetaines in aqueoussolution,n=12,14 and 16 .T=ZS'C

174

.4

-5

Fig.2

Surface

-3

tension

8 versus

n-alkylbetaines Figures

1 and 2

of the surfactant

concentration

in aqueous solution,

show the surface

concentrations

tension

of ionic

and nonionic

rements are restricted

solution

of zwitterionic

surfactants.On

to surfactants

surfactants,

as a function

it can be applied

we have shown, that the aggregates

case cannot be obtained

from conductivity

determined by us is this way was also confirmed

In Fig.1

,the plot

of 5

data (ref.11) as a function

In all surfactants

cases,

interface

of dodecylbe-

measurements.The cmc value in density

and refractive

index

2.0*10m3 mole kg -1 , in

of 1ogC ,(C is the molar concenration) studied.For

n-dodecyl

betaine,

a

was found.

the shape of the 5

studied

The values /gas

mole kg-’

measuthe case

.

shows a net break at the cmc of the betaines value of 1.9*10-3

head.In

at pH=6,7 .Thus, the cmc value

measurements.At T=25oC ,the cmc value was found to be agreement with the literature

for a

conductivity

a charged polar

(ref.2)

of the log

at two temperatures.

the contrary,

carrying

taine have a net charge equal to zero. in this

log of

)I plotted

in the final

log c

n=12, 14 and l6.T=35 C .

This method is widely used to determine cmc, since variety

*

-2

curves around cmc shows that the three

were pure.

of cmc,and the areas occupied are listed

by the polar

in Table I .‘Ihe area occupied

heads at the liquid/ by one polar

head was

175 calculated

from the slope

of the

8 = f(logC)

curves,

using the following

relationship, 1

r = 2.303

is the surface

where I’ molecule

RT

excess

A = 12

and

(-6y) &log

C T

concentration

Nl-

-2 in mol cm , A is the area per

in nm’, and N is Avogadro number. ,F======================================================= II cmc/ (mole kg-‘) II BETAINE T/ (OC) 6/ (8) 2 jl II II II II II j/ It // II

cl2

%4

‘16

57

25

1.9.10-3

35

1 .8.1o-3

59

25

l.1.10-4

44

35

1 .05.10-4

46

25

1 .9.1o-5

55

II 35 1 .9.lo-5 57 II n:=============================P=========================== TABLE1. The experimentally cross-sectional

estimated

cmc values,and

areas at the liquid/air

the

interface

and Cl6 betaines have cmc and the surface area occupied cl2 head in agreement with the literature data reported (ref.l2).On the

cmc value for Cl4 betaine

basis

of the evolution

not studied

estimated

of betaine

yet calorimetrically

by us is smaller

series

by the polar the contrary,

than expected

ant with chain length.So

the C,4 and C,6 betaines,

on the

far,we

due to their

have low

cmc values. kiicellization

is an interfacial

composed of two main contributions:(l) betaine

from the bulk solution

transfer

of the polar

The effect interfacial

effect.The

to the hydrophobic

of the aliphatic

group from the bulk solution

interactions.The

sum of the enthalpies

of the heat of dilution

Figure 3 shows the curves corresponding

to the micellar formation

the

environment.

results

of these processes

from the is given

of the betaines.

to the heats of dilution

C,2 per mole at 25 and 35’C respectively.

is part of

core of the micelle,(2)

of temperature on the enthalpy of micelle

by the measured values betaine

enthalpy of micellization

the transfer

of

176

Zs”C

__._F “:

1

:

3

~

Co. mol.kg-‘.103

i cmc

Fig.3

N-dodecylbetaine

enyhalpy of dilution

at T=2S°C and T=35’C as a function

Adil~

of the equilibrium

concentration. At the lower temperature is constant,but value.At

in the cmc region

below cmc, the enthalpy of dilution

it decreases

3S°C, the enthalpy of dilution

From the last for

(2S°C),and

dramatically

of C,2jbetaine

to almost a zero

is very close

experiment we were not able to evaluate

to zero.

the very small value

A micH . The decrease

the variation

in

AmicH

when temperature increases

of the iceberg

structure

of water around the alkyl

free monomers.During micellization,that ‘l&y
structure

decreases

part of molecules

with increasing

very small values the literature Our present effect betaines

contribution

temperature.Ihus,one

of AmicH

data provide

accompanying the micelle with various

polar

should not be surprised for C, 2 betaine

by the indirect

(4.7 and 5.85 W mole-’ calorimetric

is due to the hydrausually by the

.

data (ref.l4),obtained

in our laboratory

chain of the

and the flexibi-

(-N+(CH2)2CH2-COO-) .That hydration

The comparison between the measured A micH cant differences

is disrupted

by

chain increases.

The second,much more important enthalpic tion of the polar

could be explained

at 25’C and

method, shows signifi-

respectively). us only with the total

formation.Experiments

groups and different

enthalpy

on homologous series

chain lengths

of

are in progress

now.We hope that these experiments tiwilb :aUow us to separate

the energetic aliphatic

contributions

chains

Fig.4

to the enthalpy of micellization

and polar

Experimental

adsorption

on Spherosil

at 25 and 35’C .

Figure 4 shows the isotherms

tion

constitues

centration tinues

brings

parts

about structuring

above cmc ,and the plateau

faces

not typical

few surface

the increase

increase

sites

direct

adsorption

of the isotherms,

for a nonionic

(@=r/r*

step&

adsorp-

shown by the

in equilibrium

of the adsorptionAdsorption formation,far

surfactant

concon-

concentration

enough from the cmc

adsorbed onto hydrophilic

sur-

(ref.16).

The cross-sectional adsorption

isotherm

-water solution

area of one admolecule

temperature on adsorption Direct d ads H

calorimetric

calculated

from the plateau

is equal to 33 W2 .‘Ihe same value calculated

interface

yelds

58 w2 per molecule.(Ref.6).The

of

for the airinfluence

of

is negligible.

measurements of differential

seem to provide

a further

,

are opened

“gaseous”

is reached at an equilibrium

.Such a plateau

gel

8 < 0.05

adsorbed amount increases

molecules.This

the background for a further

of about 3.1~1 0W3 mole. kg-’ region,is

increasing.A

anchorage of the surfactant

shape of the steeper

coverages

at plateau),the

with the bulk concentration

of n-dodecylbetaine

of C,2 onto silica

the low surface

where r* is the adsorbed quantity to direct

isotherms

of adsorption

measured at two temparatures.At slowly

coming from

heads.

molar heats of adsorption

support for the above interpretation.

178

-A-

T:25°C

*_

T: 3 5 ‘C

Fig.5 Differential

molar enthalpies

of n-dodecylbetaine Figure 5 shows that,at one observes

a strong

sites

of silica

The

A adsH values

becomes constant physical

adsorption.It betaine

structured

by the silica

seems,that

molecule

of betaine,

molecules

and isolated are negligible.

large;(Fig.S).

exceedes

and very low.(About

individual

concentrations

between adsorbed molecules

are relatively coverage @

of adsorption

at 25 and 35’C .

between individual

surface.Interactions

When the surface

,versus@

small bulk equlibrium

interaction

(AadsH)

a value of about 0.05

3.5 kJ per mole).This

the betaine

already

molecules

, AadsH

would indicate

a

are adsorbed around

anchored on the water molecules

,well

gel surface.

CONCLUSIONS Isotherm and calorimetric adsorbed layer At the first

data suggest

at the saturation stage,

the molecules

head (-N+(CH3)2CH2COO-) to the sites mic heats of adsorption adsorption

the following

structure

for the

plateau. are attached of silica

seem to be a convincing

individually

gel

(O<@

experimental

by the polar

(O.OS).The proof

exoter-

for that

mechanism.

Adsorption

then progresses

due to hydrophobic

interactions

between aliphatic

179

parts of betaineon the silicagel surfacehomogenizedby stronglybondedwater molecules.This kind of ice-waterstructurecan give a relativelyhomogeneous solid surface. Beginningfrom 8 =O.OSup to 1, the valuesof Aaadsfi are very small. (3.5kJ mol-').Calorimetric experimentsgive the sum of the enthalpicexchange betweenthe exit of monomerfrom bulk solutionand the adsorptionof this monomer in the interfacialsurfactantstructure.It would be desirableto estimate separatelythe variousenthalpiccontributions due to the differentparts of surfactantmolecules.Perhaps it might be feasibleby studyingthe adsorption of the same kind of surfactantmoleculeswith variousaliphaticparts. The experimental cross-sectional area of one moleculecalculatedfrom the saturationplateau,d,was33 w'.The same quantitycalculatedfor the solution-air interfacewas found to be about 58 w2.The theoreticalsurfaceoccupiedby polar head of betaine (-N+(CH3)2CH2CC0-) in one of differentpossibleconfigurationsis 55.9 R2.Thatcalculation,and the measuredcalorimetric data suggest that the adsorbedmoleculeshave a bilayerstructure. REFERENCES 1

J.C.Lang,in: V.Degiorgionand M.Corti (Eds.),Physics of Amphilities,North-HollandPhysicsPublishing,Elsevier,Amsterdam,l985,pp.336-371 2 N.Kamenka,G.Haouche,B.Faucompre,B.Brun and B.Lindman,J.Colloid and Interface Sci.,lO8(1985) 451-456 3 P.G.Nilsson,B.Lindman and R.G.Laughlin,J.Phys.Chem.,88(1984) 6357-6362 4 M.Dahanayaheand M.J.Rosen,in:M.J,Rosen (Ed,),Structure PerformanceRelationshipsin Surfactants,A.C.S.,Washington DC, 1984,pp.49-59 5 M.J.Rosenand Bu Yao Zhu,in:M.J.Rosen(Ed.),Structure PerformanceRelationships in Surfactants,A.C.S.,Washington lX,l984,pp.61-71 Bu Yao Zhu and M.J.Rosen,J.Colloid and InterfaceSci.,lO8(1985),423-429 N.Kamenka,G.Haouche,B.Brun and B.Lindman,Colloids and Surfaces,/in press/ B.Brun,G.Haouche and N.Kamenka,Tenside Detergents,21(1984)307-311 M.Lindheimer,S.Partyka,J.Rouv~ere,N.Cygankiewicz and B.Brun,Cat.Anal.Therm., 14 (1983)274-279 10 K.Toriand T.Nakagawa,Kolloid Z.Polym.188 (1962)47-52 11 J.Swarbrick and J.Daruwala,J.Phys.Chem. 74 (1970)1293-1296 12 R.Ernstand E.J.Miller,in:B.R.Buerstein andC.L.Hilton (Eds.),Amphoteric Surfactants,Marcel Dekker In C,l982,pp.71-161 S.Partyka,M.Lindheimer,S.Zaini,E.Keh and B.Brun,Langmuir,2 (1986)101-105 ii P.Molyneux,C.T.Rhodes and J.Swarbrick,Trans.Faraday Soc.,61(1965)1043-1052 15 S.Partyka,F.Rouquerol,J.Rouquerol,J.Colloid and INterfaceSci.,68 (1979) 21-31 16 S.Zaini,Thesis,Montpellier,l986