Indian Journal of Chemistry Vol. 44A. September 2005. pp . 1793-1799 Micellization behaviour of lithium dodecyl sulphate in aqueous solutions using conductivity, density and adiabatic compressibility measurements Chanchal Das & Dilip K Hazra* Depa rtment of Chemistry. University or North Bengal. Darjeeling 734430. India Email: dkhazra @redilTmail.com Received 22 March 2005; revised 31 May 2005 Temperature effects on micellar behaviour of lithium dodecy l su lphate in aqueous solutio ns have been measured lIsing conductomet ri c, den siometric, and interferrometric techniques. All these methods yield almost identical critica l micelle concentrati on (c mc) ror the surractant system. Conductivity studies, in the range 288.15-318.15 K at 5 K intervals. show a sha ll ow minimum or the cmc values at around 298.42 K. The ionization degree of micelles ( ex) . has been round to increase whereas the standard free energy of micellization decreas es upon increase in temperature . Variation in standard enthalpy upon aggregation has been calculated by using Gibbs-Helmholtz equation. From density measurements carried out at 288 . 15. 298 . 15 and 308.15 K. the changes of the apparent molar vo lume upon micellization of the surfactant have been calculated. The apparent adiabatic compressibilities of th e surfactant so lutions have also been calculated using density and ultrasound veloci ty measurements and pro vide information on the solution behaviour of the surfactant in aqueous solution. IPC Code: Inl. CI 7 GOIN Self-association of hydrophobic molecules generally is of great importance for many physiological molecules, food constituents and drugs. The micellization of surfactants above a critical concentration (called the critical micelle concentrations, cmc) is an important solution property which needs evaluation to know the existence of micelle in solution as well as evaluating the thermodynamic s of the process which is essential for characterization and comparison in the light of spontaneity and stabili ty l.2. Though, many investigations have been done on sodium dodecyl sulphate (S DS )3-IO, but much is not known about micellar behaviour of lithium dodecyl sulphate (LDS) The res ults on conductometric, densiometric, and interferrometric measurements at different temperatures of aqueous so lutions of LDS are report ed here. Among these measurements , densiometric and int erferromet ric measureme nts were never considered in th e past for the evaluation of micellar properties of LDS. From the experimental data, the ionization degree of micelle, a, free energy of micell izati on, 6. G O Ill , standard entropy of mic ellizat io n, 6. HOIll, standard enthalpy of micellization 6.5 0 111 , apparent molar vo lume, and apparent adiabatic compressibility ha ve been calculated . These parameters provide us eful information regarding the solution behaviour of LOS in aqueous solutions. Materials and Methods LDS (E.Merck, Germany) was used as recei ved. Doubly distilled water (s pecific conductance, 2-3 /AS cm· l ) was used in all preparations . Measurements were made at various temperatures in a water-bath maintained within ±O.Ol K. The conductometric measurements were carried out on a Pye-Unicam PW9509 conductivity met er at a frequency of 2000 Hz using a dip-type cell of cell constant 1.14 cm- I and having an accuracy of 0.1 %. Solutions were prepared by mass for the conductance runs, the molarities being converted to molaliti es by the use of densities. Corrections were made for th e specific conductance of water at all temperatures. Densities were meas ured with an Ostwa ld-Sprengel type pycnometer of about 25 mL capacity and an internal diameter of the capillary of about I mm. Uncertainties in th e solute concentration and weighing can produce errors in the va lu es of density, ca. 5x 10- 5 gm cm· 3 , Sound velocities were measured usi ng single- crystal variable-path ultrasonic int erferometer (Mittal Enterprise, New Delhi) working at 4 MHz, which was calibrated with water, methanol and benzene at each
Transcript
Indian Journal of Chemistry Vol. 44A. September 2005. pp. 1793-1799
Micellization behaviour of lithium dodecyl sulphate in aqueous solutions using conductivity, density and adiabatic compressibility measurements
Chanchal Das & Dilip K Hazra* Department of Chemistry. University or North Bengal. Darjeeling 734430. India
Temperature effects on micellar behaviour of lithium dodecyl su lphate in aqueous solutions have been measured lIsing conductometric, densiometric, and interferrometric techniques. All these methods yield almost identical critical micelle concentration (cmc) ror the surractant system. Conductivity studies, in the range 288.15-318.15 K at 5 K intervals. show a shallow minimum or the cmc values at around 298.42 K. The ionization degree of micelles (ex) . has been round to increase whereas the standard free energy of micellization (~G"IlI) decreases upon increase in temperature . Variation in standard enthalpy upon aggregation has been calculated by using Gibbs-Helmholtz equation. From density measurements carried out at 288.15. 298 .15 and 308.15 K. the changes of the apparent molar volume upon micellization of the surfactant have been calculated. The apparent adiabatic compressibilities of the surfactant solutions have also been calculated using density and ultrasound veloci ty measurements and provide information on the solution behaviour of the surfactant in aqueous solution.
IPC Code: Inl. CI 7 GOIN
Self-association of hydrophobic molecules generally is of great importance for many physiological molecules, food constituents and drugs. The micellization of surfactants above a critical concentration (called the critical micelle concentrations, cmc) is an important solution property which needs evaluation to know the existence of micelle in solution as well as evaluating the thermodynamics of the process which is essential for characterization and comparison in the light of spontaneity and stabili ty l.2. Though, many investigations have been done on sodium dodecyl sulphate (SDS)3-IO, but much is not known about micellar behaviour of lithium dodecyl sulphate (LDS)
The results on conductometric, densiometric, and interferrometric measurements at different temperatures of aqueous solutions of LDS are reported here. Among these measurements, densiometric and interferrometric measurements were never considered in the past for the evaluation of micellar properties of LDS. From the experimental data, the ionization degree of micelle, a, free energy of micell ization, 6. GO
Ill, standard entropy of
micellization, 6. HOIll, standard enthalpy of micellization 6.50
111, apparent molar volume, and
apparent adiabatic compressibility have been calculated. These parameters provide useful
information regarding the solution behaviour of LOS in aqueous solutions.
Materials and Methods LDS (E.Merck, Germany) was used as recei ved.
Doubly distilled water (specific conductance, 2-3 /AS cm· l
) was used in all preparations. Measurements were made at various temperatures in a water-bath maintained within ±O.Ol K.
The conductometric measurements were carried out on a Pye-Unicam PW9509 conductivity meter at a frequency of 2000 Hz using a dip-type cell of cell constant 1.14 cm-I and having an accuracy of 0.1 %. Solutions were prepared by mass for the conductance runs, the molarities being converted to molalities by the use of densities . Corrections were made for the specific conductance of water at all temperatures.
Densities were measured with an Ostwald-Sprengel type pycnometer of about 25 mL capacity and an internal diameter of the capillary of about I mm. Uncertainties in the solute concentration and weighing can produce errors in the values of density, ca. 5x 10-5 gm cm·3
,
Sound velocities were measured usi ng singlecrystal variable-path ultrasonic interferometer (Mittal Enterprise, New Delhi ) working at 4 MHz, which was calibrated with water, methanol and benzene at each
1794 INDIAN J CHEM, SEC A, SEPTEMBER 2005
Table I - The cmc, ce, and thermodynamic parameters of LOS at various temperatures'
cmc. mM
Temp,K Cond Dens Camp /1G,,"', kJ mol" I /1/1,,"' . kJ mol" I /1S,,"'. J K·1 mal"I
Fig. 1- Dependence of specific conducti vity of LOS on molality at various temperatures (cmc vs temperature di splayed in the insert).
temperature. The temperature stability was maintained within ±0.01 K by circulating thermostated water around the cell by a circulating pump. The details of these procedures have been described earlier ll
.13
.
Results and Discussion Conductivity studies
The dependence of the specific conductivity on the molality of LOS at various temperatures is shown in Fig.!. An approximate value of the ionization degree of micelles. oc =52/51, was determined from the ratio of the mean gradients of conductivity against concentration plots above (52) and below (51) of the
14·1 8 cmc . According to the charged phase separation model
of micellization, the activity of the monomer remains
constant above cmc, and the standard free energy of micellization per mole of monomer, 1:1 GO
lll, can be
calculated using l9:
.. . ( I)
where Xcrrc is the cmc in mole fraction. The standard enthalpy, l:lHom, and entropy, 1:15o
m, of micellizatlon
were obtained assuming that oc is practically constant (Table 1) and by insertion of Eq.(l ) into a GibbsHelmholtz equation, which gives Eqs (2)_(3)20.21:
I:1Hol11 = -(2 - oc) RT- (0 In Xcrrc/OT )p 1:15o
l11 = (I:1Ho 111 -I:1Go l11)/T ... (2)
. . . (3)
The value of 0 In Xcn·.jOT was determined by fitting the In Xcmc - OT with the polynomial function:
In Xcrrc = a + hT+ cT- ... (4)
The values of the fitting constants were a = 4.33464, b = -.08681, and c = 1.44288x 10-4. The temperature dependent values of cme as well as oc and different thermodynamic parameters are gi ven in Table 1. The cmc value determined conductometrically at 298.15 K for the surfactant LOS is in good agreement with that reported earlier4
.
The temperature dependence of cmc for LOS is given in the insert of Fig. 1 which exhibits a shallow minimum at about 298.42 K.
The degree of ionization, a, of micelles of LOS increases with temperature as found earlier for cationic surfactants 15.22. The higher values of a can be explained qualitatively in terms of larger size of the hydrated Li+ ion which cannot approach the highly charged surface of micelle. However, the value obtained at 298.15 K is much higher than those reported earlier4
•23
. The mean value of l:1aJl:1T estimated in the temperature range 288.15-318.15 K
DAS & HAZRA: MICELLIZATION BEHAVIOUR OF LITHIUM DODECYL SULPHATE 1795
amounts to 0.0014 K· I. It is low compared to that
found earlier for cationic surfactants dodecyldimethylbenzylammonium chloride (0.0030)"4, tetradecyltrimethylammonium bromide (0.0030)"5 and tetradecyldimethyl-phenylammonium bromide (0.0029)26.
Analyses of the thermodynamic parameters of micellization indicate that the aggregation of LOS is driven mainly by the positive tl50
111• Similar changes
from entropic to enthalpic micellization with temperature increase have been noted with many different surfactants2
1.27.29. The standard free energies of micellization, calculated for LOS decrease linearly with temperature. The low rate of decrease of tlGo
l11
with growth of temperature is typical for aqueous solutions of surfactants and results from an entropyenthalpy compensation effect30.34
. Positive values of tlHol11 , such as those noted at the lowest temperatures, are generally attributed to the release of structural water from the hydration layers around the hydrophobic parts of the micelle35. Such hydrophobic interactions become increasingly insignificant with the partial breakdown of the structure of water as the temperature is increased, the negative tlHol11 values suggesting the importance of the London-dispersion interactions as an attractive force contribution for micellizationJ6
. However, it may be seen from Table 1 that at a certain temperature the enthalpy of micellization vanishes and the process is driven only entropically. This is a direct consequence of Eq.(2) only when the temperature dependence of ex: is being neglected .
Density studies
Density (p) measurements of aqueous solutions of LOS are presented in Fig. 2 in the form of plots of P - Po versus molality of LOS, Po being the density of water at the appropriate temperature. It is seen that in both pre- and post-miceller range studied, the densities increase linearly with the increase of molality.
The apparent molar volumes, V¢ of LOS in function of molalities at the temperatures studied were calculated using Eq.(5)34:
M
P ... (5)
where M and m are the molecular mass and the molality of the solute, respectively. The variation of
30
25 ::; 20 '?~ ~~ §
0> 15 , ~/ 0
~
~ 10 0:/ 0}/
5 0 ?" ~/'"
0 0.00 0.02 0.04 0.06 0.08 0.10
m (mol Kg" )
Fig. 2 - Density versus molality plot for LOS at 288. 15(_). 298.15( A), and 308.15 ee) K.
250
245
230
100 200 300 400 500
m" (Kg mOl" )
Fig. 3 -Variation of Ve with the reverse of molality for LOS at various temperatures (Symbols as in Fig. I).
Vii> with the reverse of molality is given in Fig. 3. It can be seen that the apparent molar volumes remain almost constant in the lower concentration range followed first by a sharp increase in the region close to cmc and then by an approach to a saturation value at the highest concentration studied.
In an ionic surfactant system, below cmc , the dependence of density on concentration can be expressed as24
:
.. . (6)
where Mr, Vr, and Me, Ve are the molecular masses and the apparent molar volumes of the surfactant ions
1796 INDIAN J CHEM, SEC A, SEPTEMBER 2005
and counterions. respectively. If these apparent molar volumes were negligibly concentration-dependent, then Eq. (6) predicts a linear relation between density and concentration. Indeed, such a behaviour is seen in Fig. 2. Thus, estimated values for Vr + Vc amount to 229.23±O.26. 235.60±O.24 and 240.44±0.34 cm3 mo)" at 288.15, 298.15 and 308 .1 5 K, respectively. These values can be taken as zero-order approximation of the accurate values as it is well known that the apparent molar volumes are, in general, concentration dependent. The observed increase in V,+ Vc with temperature is related to the relaxation of structured water engaged in the solvation of the hydrocarbon chai n. the head group, and the counterion. A similar behaviour can be observed for the data for decylbenzyldimethyl chloride37 and dodecylbenzyldimethyl chloride24
.
Since it is generally accepted that the solutions of surfactants in the premicellar region behave as singly dispersed system, they may be described b/8
:
... (7)
where Vo" is the value of apparent molar volume at infinite dilution. Av is the Debye Hlickellimiting slope and Bv is an adjustable parameter, which measures the deviations from the limiting law. For 1: I electrolytes at 288.15,298.15 and 308.15 K the values of Av are 1.696, 1.865 and 2.04 cm3kg"2mor3l2, respectively. Experimental values of Vii> in the premicellar region were fitted to Eq.(7) and the values of VeIl° are listed in Table 2. The Vo" values for LDS increase almost linearly with temperature in conformity with observations for other surfactant systems24
.39.
Assuming that the rule of additivity for the apparent molar volumes of the ions in the system is va lid, we can write:
... (8)
where Vm is the apparent molar volume of the micelles in the system and flLDS the total number of moles of the surfactant, whereas 1!r, flc and flm are the number of moles of free monomers, free counterions, and of micelles, respectively. Since LDS is a 1: 1 electrolyte, then, according to the pseudo-phase separation model below cmc:
II!r = mc = m ... (9a)
and above cmc24
mr= cmc mc = cmc + <?C(rn - cmc)
m - C11lC mm=---
N f/Ilx
... (9b)
where Nagg is the aggregation number of the micelle . Dividing Eq. (8) by the mass of the solvent (i n kg) and taking into account relations (9), we obtain the following set of equations24
If we assume that the molar volumes of the ionic species are not concentration-dependent, then, from the last equation a linear decrease of V<I> versus 1/111 plot can be expected in the concentration range above cmc. According to Eq.(lOa), the value of Veil should be constant in the concentration range below cmc. Such a plot is shown in Fig. 3. It can be seen that the system conforms approximately to the expected behaviour. Thus, Eq.(lO) may be considered as an acceptable approximation of the behaviour of the system. We can assign that the intersection point of the I ines above and below cmc as the corresponding cmc. The values, thus obtained, are almost identical with those obtained in conductivity and interferrometric techniques at 288.15,298.15, and 308.15 K, respectively.
Linear fitting of the points in the cmc/Ill ~ 1 region allows us to obtain the value of the apparent molar volume, VeIlcmc
, (intercept + slope) of the surfactant at cmc, and the change in the apparent molar volume upon micellization, t1 vtnc (slope). The results are shown in Table 2. The deviation of about 0.25 to 0.35 cm3 mo)" between the values of Vq,Clnc obtained by applying Eq.( lOb) (for data above cmc) with corresponding values of Vr + Vc obtained via Eq.(6) for data below cmc, can be taken as a measure of the error in the results. The values of V,pcmc found for LDS are positive. A similar observation was reported earlier for other surfactant system37
.40. It is generally
Table 2 - Apparent molar volumes. of the surfactant at infinite dilution, V $0, at erne. V $"''':. and the change upon
mieellization. t::,. V $""'<:. for LOS at 288.15, 298.15 and 308.15 K
T, K V$o, V$"III". em3 mor l t::,. V$"III". em3 mol'l cm3 mol' l
Fig. 5 - Dependence of adiabatic compressibility. ~s, of LOS on molality in aqueous solutions at 288. 15(. ), 298.15(j.), and 308.15 (e) K.
assumed that this effect is mainly due to the release of structured water in the hydration shell of the monomers when the micelles are formed .
According to Eq.(lOb), the slope of V.p versus cmc/m plot, D. vtnc
, shows a positive dependence on the degree of ionization. The last magnitude, as pointed out above, grows with temperature. Thus, an increase in D. V.pco1c values should be expected when the temperature is raised . This effect might be interpreted as being due to growth in the electrostatic repulsion
between the ionic head groups at the surface of the micelles. Table 2 shows, however, a reverse behaviour, i.e.., D. V~c01c decreases with growth of temperature. Again, the structured water surrounding the hydrocarbon chain of the monomers is the origin of this phenomenon. At high temperatures, the water sheath is less structured, and thus the contribution to D. V <jlC IllC due to the release of water molecules is much lower and this effect is mainly responsible to compensate the growth in D. V$c01c due to the increase of the electrostatic repulsion that occurs when the degree of ionization rises24
.
Adiabatic compressibility studies Figure 4 is representative example of the changes
in the ultrasound velocity in aqueous solutions of LOS with the surfactant concentration at temperatures 288.15-308.15 K. Each plot can be divided into two straight-line segments, which correspond to the monomeric and micellar forms of the surfactant in aqueous solution.
The change in adiabatic compressibility, (3s, of the aqueous solutions of LOS with the surfactant concentration at different temperatures is depicted in Fig. 5. The (35 can be calculated from the relation u2 = IIp(3s, where Ll is the ultrasound velocity, expressed in ms- 1
, and p is the density, expressed in Kg m-3 of the aqueous surfactant solution. Again, each plot can be divided into two line segments corresponding to monomeric and micellar forms of the surfactant. Slopes of the plots for the monomeric forms are negative while the sign of the slopes above cmc depends on the temperature and it increases with temperature. A similar increase is observed on the plots of adiabatic compressibility in aqueous solutions of dodecyltrimethylammonium bromide with temperature as shown by Zielinski et at. 40.
For surfactant system of sufficiently low concentration, the dependence of the adiabatic compressibility. ft on molality (below and above cmc), can be expressed by two-approximation equations24 of the exact equations used by Zielinski et al. 40
:
1000 (~ , - ~ 50)
Po (~Sl - ~50) V.pl m m < cmc ... (lla)
1000 (~ '\' - ~so) A A ---"'----'-=- = (I-'sm - I-' so) V$m(m - cmc) m > emc
Po ... (llb)
1798 INDI AN J CHEM, SEC A, SEPTEMB ER 2005
4.202 4.201 4.200 4.199 4.198
~1~
4.203
1 4. 195+-----,-~-_._---,-__,_-~____r-~-.__~ I I I I ~ 3. 900
Fi g. 6 - Molality dependence or the apparent adiabatic compress ibil ity or LOS at 288. 15(. ). 298. 1 5( ~ ) and J08. 15(e) K.
where VOl and VOIll are the apparent molar volumes of the system below and above cmc, Po is the density of pure water (in gm cm-\ 13so, 13s1 and 13sm are the adi abat ic compress ibili ty of pure water and apparent ad iabat ic compress ibilities of surfac tant in the monomeri c and mi cellar states, defined respectively by:
Figure 5 shows that the dependence of 13s on molal ity is we ll represented by two straight lines of di ffe rent slopes in the pre- and post-micell ar concentra ti on range. From the va lue of the slope above cmc, the va lue of 13sm can be es timated by applying Eq.(l l b) . The resul ts are shown in Fig. 6. The va lue of 13sm increases asy mptoti cally at all ex perimental temperature to a limiting value, which compares well with the behaviour fo r other surfactant
140-4' Th I' .. lb ' d f 13 systems· -. e Imltlng va ues 0 taIl1e or Sill are 3.87xJQ-IO, 3.99xlO-10. and 4.20x lO-10 Pa- 1 at 288.15, 298 15, and 308. 15 K, respectively.
Conclusions The present in ves ti gations show that cmc of LDS
in aqueous medium is temperature dependent and exhibits a shall ow minimum at around 298.42 K. The high degree of ioni za ti on fo r the surfac tant system is due to larger size of hydrated Li+ ion. The weak
temperature dependence of the standard Gibbs energy of micellization reflects an enthal py-entropy compensation effect. The apparent molar volume change upon micellization , t. Yom is due to several processes , an important contribution coming probabl y from the dehydrati on of the counteri ons bound to the micell e and also due to the release of structured water in the hydrati on shell of monomers, whic h occ urs upon micellizati on.
Acknowledgement We thank the University Grants Commi ss ion, New
Delhi , fo r financial ass istance th rough DRS Project in Chemistry.
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DAS & HAZRA : MICELLIZATION BEHAVIOUR OF LITHIUM DODECYL SULPHATE 1799
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