Mixed Micellar Aggregates of Nonionic and Anionic Surfactants with Short Hydrophobic Tails: A Microcalorimetric Study

Mixed Micellar Aggregates of Nonionic and Anionic Surfactants with Short Hydrophobic Tails: A Microcalorimetric Study

Journal of Colloid and Interface Science 249, 481–488 (2002) doi:10.1006/jcis.2002.8280, available online at http://www.idealibrary.com on Mixed Mice...

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Journal of Colloid and Interface Science 249, 481–488 (2002) doi:10.1006/jcis.2002.8280, available online at http://www.idealibrary.com on

Mixed Micellar Aggregates of Nonionic and Anionic Surfactants with Short Hydrophobic Tails: A Microcalorimetric Study Ornella Ortona,1 Gerardino D’Errico, Vincenzo Vitagliano, and Lucia Costantino Chemistry Department of Naples University, Federico II, Complesso di Montesantangelo, via Cinzia, Napoles, Italy Received November 2, 2000; accepted February 7, 2002; published online April 10, 2002

Apparent molar relative enthalpies were measured for the nonionic ethoxylated surfactant CH3 –(CH2 )5 –(OCH2 –CH2 )5 OH (C6 E5 ) in aqueous solution at constant molality of the ionic sur◦ + factant CH3 –(CH2 )5 –SO− 3 Na (C6 SNa) at 25 C. The experimental data obtained by a stepwise dilution process allowed evaluation of the C6 E5 first interaction parameter at several constant molalities of C6 SNa. The C6 E5 critical micelle composition as a function of the C6 SNa molality was also estimated. The experimental calorimetric data, together with the mixed micelles composition computed in the past by some of us [Ciccarelli et al., Langmuir 14, 7130 (1998)], allowed computation of the hMic of micellization. The experimental data are compared to those predicted by the ideal solution model and regular solution model of mixed micellization. From a calorimetric study performed on the water–hexanol– C6 SNa and water–penthaethylene glycol–C6 SNa model systems, it can be argued that the interactions among the hydrophilic heads in the C6 E5 –C6 SNa mixed micelles prevail on the contribution of the hydrophobic tails in ruling the enthalpic properties of the system. C 2002 Elsevier Science (USA) Key Words: calorimetry; ionic surfactant; nonionic surfactant; mixed micelles.

I. INTRODUCTION

The aggregation of surfactants in aqueous solution is controlled by the competition between the tendency of the hydrophilic head of the surfactant to deeply interact with the solvent and the tendency of the hydrophobic tail to avoid this kind of interaction. The compromise is reached organizing the molecules in aggregates, the micelles, where both these tendencies balance. The micellization process is not a first-order transition; in fact, it appears through a short composition range. However, it is often convenient to describe it as a phase transition process, assuming that the micelles appear at a single composition, defined as the critical micelle composition, cmc (1, 2). This assumption will be taken in this paper.

1 To whom correspondence should be addressed. E-mail: [email protected] unina.it.

481

In an aqueous solution of two or more surfactants, the aggregation process is controlled by the interactions of each solute with the solvent and by the interactions among the solutes themselves. The importance of surfactant mixtures is related to their wide application in industrial and detergent processes (3). The thermodynamic and structural characterization of these systems is difficult because almost all the experimental techniques do not distinguish between the contribution of each surfactant to the observed property; furthermore, theories and models present in the literature (4) are not completely satisfactory. In the past some of us studied the ternary system water– pentaethylene glycol monohexylether (C6 E5 )–sodium hexyl sulfonate (C6 SNa) measuring mutual and intradiffusion coefficients through the interferometric Gouy technique (5) and the NMRPGSE method (6). We focused our attention on short tail surfactants having cmc values at concentrations high enough to allow experimental investigation also in the premicellar composition range. At the same time C6 E5 and C6 SNa show all the characteristics of a typical surfactant (7, 8). The investigations on the water–C6 E5 –C6 SNa system showed the formation of mixed micellar aggregates, to which both surfactants participate. The intradiffusion coefficients allowed computation of the partitioning of the two surfactants between the aqueous environment and the micelles (6); the data were interpreted in terms of the regular solution theory proposed by Holland and Rubingh (9) that assumes an ideal mixing entropy of the surfactants into the micelles. In this work we present a calorimetric study on the same system. In the premicellar composition range the experimental data were interpreted in terms of the interaction of the surfactants as monomers, while in the micellar composition range the enthalpy h Mic of micellization was computed (in this paper small letters have been used to define molar quantities and capital letters to define extensive quantities). The results were compared to those predicted by the ideal and regular solution models. To our knowledge, all the calorimetric investigations on mixed micellar systems present in the literature (10–13) dealt with surfactants having hydrophobic tails longer than those studied in this work. In all cases a not negligible difference between the 0021-9797/02 $35.00

 C 2002 Elsevier Science (USA)

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experimental results and those predicted through the Rubingh approach was found. II. EXPERIMENTAL

A. Materials. C6 E5 (Bachem, declared purity >99.8%), C6 SNa (Sigma, declared purity ∼98%), 1-hexanol (C6 E0 ) (Aldrich, declared purity ∼98%), and pentaethylene glycol (C0 E5 ) (Aldrich, declared purity ∼99%) were used as received. Water was twice distilled. B. Apparatus and methods. The calorimetric measurements were performed on a TAM device (Thermal Analysis Monitor) (Thermometric) adding by a motor-driven syringe precise volumes of a titrating solution to a weighed amount of the solution to be titrated. All experiments were performed at 25.00 ± 0.01◦ C with a thermal stability of 0.0001◦ C. For the binary system water–C6 SNa, water was added to a C6 SNa aqueous solution well above the surfactant cmc. For the binary system water–C6 E5 , a set of dilutions was performed starting from the pure liquid surfactant. For the ternary systems, precise volumes of a water–C6 SNa solution were added to a weighed amount of a water–C6 E5 – C6 SNa solution at the same molality of the ionic surfactant. In this way any problem connected with the C6 SNa dilution was avoided. Ten C6 SNa molalities were considered. The last one (0.587 mol kg−1 ) is slightly above the C6 SNa cmc; in this case micelles are present through the whole titration run. Two test runs were also performed for the ternary systems water–C6 E0 –C6 SNa and Water–C0 E5 –C6 SNa at total constant C6 SNa molalities above its cmc. III. RESULTS AND DISCUSSION

Binary Systems Dilution runs have been carried out on the C6 SNa aqueous solutions as a preliminary collection of data. The apparent molar relative enthalpies, ( )S , computed as discussed in a previous paper (8), are shown in Fig. 1.2 The ( )S data, in the premicellar composition range (m s < 0.58 mol kg−1 ), fit the following equation: ( )S /J mol−1 = As m s .

[1]

The value of the empirical fitting parameter As was found to be 3770 (±80) J kg mol−2 . Actually, from a theoretical point of view, the ( )S data should extrapolate to infinite dilution according to the limiting Debye-Huckel expression ( )S /J mol

−1

= 1973



mS + · · · .

[2]

2 The  experimental data as a function of surfactant composition are avail able by request to the authors.

FIG. 1.

Apparent molar relative enthalpy of C6 SNa in aqueous solution.

However, the composition range in which the limiting DebyeHuckel theory is adequate to reproduce experimental data is far more dilute than that in which calorimetric measurements are reliable, so that in the following Eq. [1] will be used. The cmc and the molar enthalpy of micellization of C6 SNa, h S , shown in Table 1, have been evaluated as reported previously (8). For the nonionic surfactant C6 E5 , a linear trend of the apparent molar relative enthalpy, ( )E , with molality, m E , in the premicellar composition range (m E < 0.104 mol kg−1 ) was found by some of us in the past (8). The data fit the following equation: ( )E /J mol−1 = AE m E = 22600(±100)m E .

[3]

The cmc and the molar enthalpy of micellization of C6 E5 , h E , are shown in Table 1. In this work we extended the measurements to the entire C6 E5 –water composition range. Figure 2 is a graph of the apparent molar relative enthalpy of C6 E5 , ( )E , drawn as a function of its mole fraction. At least three possible conformational transitions were put in evidence as maxima of the function ∂( )E /∂ xE (Fig. 3). The very sharp transition, shown in the inset of Fig. 3, corresponds to the micellization process. Two transitions clearly appear in the range of concentrated C6 E5 solutions (xE ∼ = 0.1 and xE ∼ = 0.2). They can be attributed to different hydration structures of C6 E5 present in solution. Similar results were found for the C8 E5 solutions (8). It is interesting to note that surfactants with short hydrophobic tails, such as C6 E5, that usually form small aggregates with loose structure (14, 15), also give rise to phase transitions in the concentrated composition range. Ternary System: Experimental Approach Dilution experiments in mixed surfactant solution were performed to measure the apparent molar relative enthalpy of the ethoxylated surfactant and the enthalpy variation due to the micellization process in aqueous solution containing a constant molality of the ionic surfactant, m S .

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AGGREGATION OF NONIONIC/ANIONIC SURFACTANTS

TABLE 1 Thermodynamic Properties for Water–C6 SNa, Water–C6 E5 , and Water–C6 SNa–C6 E5 Mixturesa

m cmc S (mol kg−1 ) C6 SNa C6 E5

0.58 ± 0.01

b

m cmc E (mol kg−1 )

0.104 ± 0.005

mS (mol kg−1 )

ASE (J kg mol−2 )

0.044 0.046 0.074 0.105 0.199 0.300 0.429 0.495 0.557 0.587

22,600 ± 200 (17,000 ± 200) 23,200 ± 200 24,000 ± 200 27,300 ± 200 32,600 ± 500 42,000 ± 9000 48,000 ± 10000

Binary systems AS (J kg mol−2 ) 3770 ± 50

Ternary systems m cmc E (mol kg−1 ) 0.077 ± 0.010 0.087 ± 0.010 0.077 ± 0.010 0.065 ± 0.020 0.060 ± 0.015 0.035 ± 0.010 0.015 ± 0.008 0.005 ± 0.002 d 5 × 10−4

AE (J kg mol−2 )

22600 ± 100

YE 0.78 0.76 0.74 0.71 0.52 0.54 0.46 0.42

h S (J mol−1 ) 3700 ± 50

h E b (J mol−1 )

18800 ± 200

h Mic c (J mol−1 ) 15,800 14,800 15,000 14,400 11,300 10,100 7,300 5,600 (3,700)

19000

a m cmc , m cmc molality of surfactants at the cmc. A , coefficient of Eq. [1]. AS , coefficients of Eq. [10]. h , h molar enthalpies of micellization of surfactants. s S E S E E YE , mole fraction of C6 E5 in the micelles, Eq. [17]. h Mic , molar enthalpy of micellization in the ternary mixtures, Eq. [16]. b Recomputed data from Ref. (8). c Estimated error on h −1 Mic ∼ 1000 J mol . d Interpolated data.

We made a stepwise dilution of a concentrated solution of C6 E5 in the presence of C6 SNa with a solution of C6 SNa at the same molality. An experimental run can be summarized as a series of steps: 

  n 01 H2 O + n 0E E + n 0S S + (i − 1)(n 1 H2 O + n S S) + (n 1 H2 O + n S S)  0   = n 1 H2 O + n 0E E + n 0S S + i(n 1 H2 O + n S S) .

[4]

FIG. 2. Apparent molar relative enthalpy of C6 E5 in aqueous solution. , experimental data; , data interpolated with a polynomial equation.

The various terms have the following meaning: E, ethoxylated surfactant; S, sulfonate surfactant; n 0E , moles of E to be titrated; n 01 and n 0S , initial moles of water and S; n 1 and n S moles of water and S added at each step i. In Eq. [4] n 01 H2 O + n 0E E + n 0S S is the original amount of solution in the titration vessel, (i − 1) (n 1 H2 O + n S S) accounts for the surfactant solution added up to the step i − 1, and n 1 H2 O + n S S for the surfactant solution added at the step i. The condition of constant molality of the sulfonate surfactant corresponds to n 0S /n 01 = n S /n 1 .

FIG. 3. First derivative of the apparent molar relative enthalpy of C6 E5 in aqueous solution as a function of C6 E5 mole fraction.

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Ternary System: Premicellar Composition Range

FIG. 4. Apparent molar relative enthalpy of C6 E5 for various molalities of C6 SNa: 1, m S = 0.105; 2, m S = 0.199; 3, m S = 0.300; 4, m S = 0.429; 5, m S = 0.495; 6, m S = 0.587.

The experimental enthalpy of dilution after N steps is h N = −

N 

 (qi − qi, f ) n 0E ,

In the premicellar composition range, the solution formed by water and the two surfactants can be conceived in two ways: (i) a ternary system of the C6 E5 and C6 SNa solutes in water; (ii) a pseudo-binary system of the solute C6 E5 and the solvent water–C6 SNa at constant composition. The former approach agrees with the treatment of solution properties proposed by McMillan and Mayer (16) and specifically applied to the properties of aqueous solutions of nonelectrolytes by Kozak et al. (17) and Friedman and Krishnan (18); the latter, followed in the past by Castronuovo et al. (19), highlights how the mixed solvent affects the solute–solute interaction. Actually, the latter approach applies the McMillan and Mayer theory to pseudo-binary systems as it was developed for binaries. Our experimental data, having been obtained at constant C6 SNa molality, are more properly interpreted in terms of the latter approach. In the ternary water–C6 E5 –C6 SNa system, in the premicellar composition range, ( )E was found to be almost a linear function of C6 E5 molality, m E , as found for the surfactant in the absence of C6 SNa, see Eq. [3]:

[5]

i=1

( )E = ASE m E .

where qi is the experimental heat measured at each addition; qi, f is the heat of friction due to the input of a titrating drop of volume Vi into the vessel. By a preliminary assay, we obtained qi, f /JµL−1 = 5.0 × 10−6 Vi .

[6]

At any step of the titration run the experimental h N , referred to 1 mol of C6 E5 , is related to the apparent molar relative enthalpy of C6 E5 , ( )E , by the equation 

h N = ( )E − ( ) E ,

[7]

where ( ) E refers to the enthalpy of the initial solution put into the calorimetric vessel. In agreement with the definition of apparent molar relative enthalpy,

[9]

ASE is the C6 E5 –C6 E5 first interaction parameter. It accounts for the C6 E5 –C6 E5 interaction in an aqueous solution of C6 SNa at constant molality. Its value depends on the C6 SNa molality. Some deviations of the experimental ( )E data from linearity below the cmc may be imputed to the formation of premicellar aggregates. In Table 1 the ASE values for C6 E5 are collected as a function of C6 SNa composition. These values are also shown in Fig. 5. It can be seen that ASE increases smoothly up to a C6 SNa

50000 45000 40000

lim ( )E = 0,

m E→0

[8]

so that it is possible to obtain ( )E values from Eq. [7]. It must be pointed out that Eq. [8] refers to C6 E5 infinite dilution in each C6 SNa solution at its fixed m S molality, and that also in the ternary system the molalities of both surfactants are always computed as number of moles per 1000 g of neat water. Figure 4 collects the apparent molar relative enthalpies of C6 E5 at various C6 SNa compositions. The curves obtained with a C6 SNa molality lower than 0.58 mol kg−1 show a large slope change in the composition range where micelles appear.

S

AE J kg mol-2 30000 25000 20000 15000 0.00

0.10

0.20

0.30 m S / mol kg-1

0.40

0.50

0.60

FIG. 5. C6 E5 –C6 E5 first interaction parameter as a function of C6 SNa molality: , below C6 SNa cmc; , above C6 SNa cmc. The datum at m S = 0.046 has been rejected.

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AGGREGATION OF NONIONIC/ANIONIC SURFACTANTS

equal to the values obtained from the intercept of the straight line of Eq. [9] with the tangent to the inflection of the ( )E trend. Figure 6 is a graph of the cmc data given in the table. As it can be seen, the correlation between the C6 E5 molality and that of C6 SNa at a given cmc is almost linear. The cmc’s computed from calorimetric data are in very good agreement with those evaluated using the NMR-PGSE method (6).

0.12

0.10

0.08 cmc

m

E

mol kg-1 0.04

Ternary System: Micellar Composition Range

0.02

0.00 0.00

0.10

0.20

m S / mol kg

-1

0.40

0.50

0.60

FIG. 6. C6 E5 molality at the cmc, m cmc E , as a function of C6 SNa molality, m S , in the ternary systems: , from calorimetric data (this work); , from intradiffusion measurements (6).

molality ∼0.58:  ASE J kg mol−2 = 22600(±150) + 6980(±1500)m S + 88900(±3000)(m S )2 .

[10]

Our experimental method does not explicitly prove the C6 SNa contribution to the premicellar surfactants interactions. However, this contribution is implicit in the dependence of ASE on the C6 SNa molality. For m S < 0.58 mol kg−1 the micelles are absent up to a C6 E5 concentration corresponding to the cmc value given in Table 1. The ASE dependence on m S can be interpreted as due to a smooth increase of the interactions between the C6 E5 monomers in the presence of C6 SNa. This is in agreement with the results of intradiffusion measurements made on the same system by some of us (6). At a C6 SNa molality larger than ∼0.58 mol kg−1 , Eq. [10] cannot apply anymore. Under this condition ionic micelles are already present also in the absence of C6 E5 so that the added nonionic surfactant enters the preexisting C6 SNa micelles. As a consequence, at m S = 0.587 mol kg−1 , the ( )E slope at the origin drops drastically, it accounts for the interactions between C6 E5 molecules solubilized inside the C6 SNa micelles (see Table 1 and Fig. 5). The ( )E trends allow evaluation of the C6 E5 molalities corresponding to the appearance of micelles, m cmc E . Various thermodynamic criteria can be chosen to define the cmc (20, 21), depending on both the experimental technique and the micellization model used to treat the data. Nevertheless, they all lead to similar results. In connection with our calorimetric measurements, we tested various methods, finding that the most reliable and reproducible is to assume as cmc the composition at which a maximum appears on the graph of the second derivative of the apparent molar relative enthalpies, ∂ 2 ( )E /∂m 2E , drawn as a function of the solution composition. The evaluated cmc values are collected in Table 1. They are almost

In a ternary system, in the micellar composition range, both surfactants are present as bound species into the micelles and as free monomers in the aqueous phase. Their partitioning between the two pseudo-phases varies changing the C6 E5 total molality at F M each C6 SNa constant molality. The molalities, m FE , m M E , mS, mS of the four species C6 E5 (free), C6 E5 (micellized), C6 SNa(free), and C6 SNa(micellized), were obtained in the past from intradiffusion data for several constant C6 SNa molalities (6). From now forward, the apices F and M will refer to the free and micellized species. Figure 7 shows the results obtained varying the C6 E5 composition at a constant C6 SNa molality, namely 0.3 mol kg−1 . Above the cmc, mixed micelles form. Since a fraction of total C6 SNa enters the micelles, the molality of monomeric C6 SNa decreases, i.e., m FS < m S . Our calorimetric measurements allow evaluation of the enthalpy of micellization, HMic , through the following procedure. The extensive apparent relative enthalpy of the water– C6 SNa binary mixture at m S molality, computed assuming the C6 SNa infinite dilution as reference state, is given by L bin = m S ( )S = m S (AS m S )

[11]

(remember that in this paper capital letters refer to extensive properties).

0.30

0.25

0.20 mS mol kg-1 0.15

2 1

cmc

0.10 4

0.05

0.00 0.00

3

0.10

0.20

mE / mol kg-1

0.40

0.50

0.60

FIG. 7. Composition of mixed surfactant solutions for the total C6 SNa molality 0.3, obtained from intradiffusion data (6). 1, Free C6 SNa; 2, micellized C6 SNa; 3, free C6 E5 ; 4, micellized C6 E5 .

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The slopes of these plots are the molar enthalpies of micellization in the mixed surfactants solutions: ∂HMic . ∂m M

h Mic =

FIG. 8. Enthalpy of micellization, HMic , see Eq. [14], as a function of M the total molality of micellized surfactants, m M = m M S + m E , at various C6 SNa molalities: 1, m S = 0; 2, m S = 0.105; 3, m S = 0.199; 4, m S = 0.300; 5, m S = 0.429.

The extensive apparent relative enthalpy of the water–C6 E5 – C6 SNa ternary mixtures, computed assuming the infinite dilution of both surfactants as reference state, is given by L tern = m E ( )FE + m S ( )FS + HMic     = m E ASE m FE + m S AS m FS + HMic ,

[12]

where HMic is the enthalpic effect connected with the process M in which m M E moles of C6 E5 and m S moles of C6 SNa form mixed F micelles in equilibrium with m E and m FS moles of the monomeric surfactants dissolved in 1000 g of water. ( )FE and ( )FS are the molal apparent relative enthalpies of the free surfactants. In this work the apparent relative enthalpy in the ternary mixtures, m E ( )E , was measured assuming as reference state the C6 SNa aqueous solution at molality m S (see Eq. [8]). As a consequence

[16]

The plots of Eq. [14] were found almost linear (see Fig. 8). Accounting for all the approximations inherent in our treatment, we assumed as h Mic the value computed from a linear interpolation of the HMic data obtained, for each calorimetric run, at compositions reasonably far from the cmc. In fact, the experimental error on the estimated molalities of free and bound surfactants largely increases near the cmc, leading to unreliable m M and h Mic values. The results, plotted as a function of the average mole fraction of C6 E5 into the micelles, YE = m M E



 M mM S + mE ,

[17]

are shown in Fig. 9, the corresponding data are given in Table 1. It is interesting to note that the experimental h Mic ’s run quite well between those of the two binary systems. Particularly, our data show a linear trend for 0.5 < YE ≤ 1. This is in agreement with the ideal solution approach to the mixed micelles proposed by Clint (22). In the past some of us showed that the C6 E5 –C6 SNa mixed micellar aggregates behave as a regular solution. This evidence appears particularly clear by plotting the surfactant composition in the bulk vs the surfactant composition in the micellar aggregates. These compositions were obtained by nmr PGSE technique (see Fig. 7 of Ref. 6). In the case of regular solution, h Mic = YE h E + (1 − YE )h S + β RT YE (1 − YE ),

[18]

m E ( )E = L tern − L bin

     = HMic + m E ASE m FE + m S AS m FS − m S . [13]

Equation [13] allows the computation of HMic from the experimental ( )E provided that m FS and m FE are estimated from intradiffusion data (6). The coefficients ASE and AS are taken from Eqs. [10] and [1], respectively. The HMic values were plotted, for each calorimetric run, as a function of the total molality of micellized surfactants, HMic = f (m M ),

[14]

where M F F mM = mM S + mE = mS − mS + mE − mE.

[15]

FIG. 9. Molar enthalpy of micellization, h Mic = ∂HMic /∂m M , see Eq. [16], as a function of the mole fraction of C6 E5 present into the micelles. Solid line: prediction of the ideal solution approximation. Dotted line: prediction of the regular solutions approximation, see Eq. [18]. The datum at m S = 0.557 has been rejected.

AGGREGATION OF NONIONIC/ANIONIC SURFACTANTS

where h E and h S are the enthalpies of micellization of the surfactants in their binary aqueous solutions. The third term on the right-hand side of Eq. [18] accounts for the deviation from the ideal behavior and β is a measure of such deviation. For the system studied here, intradiffusion data allowed evaluation of β = −0.8 (6). In Fig. 9 the h Mic experimental data, together with the trends for the ideal and regular solution behavior, are shown. Although calorimetric data do not allow choosing unequivocally between these two models, however, they confirm, at least in the 0.5 < YE ≤ 1 micellar composition range, the low value of the β interaction parameter if the regular solution approach is chosen. The regular solution model has been used in the past to predict the composition of mixed micelles (23–25). However, in none of the cited papers, the authors compared the experimental h Mic ’s with those calculated through this theory. Holland (26) measured, in the past, the heat of mixing of sodium dodecyl sulfate with ethylene glycol monodecyl ether, with the result that the experimental h Mic values are largely different from those predicted through the regular solution model. This deviation was attributed by the author to the decrease of both the electrostatic repulsion among the ionic heads and the steric repulsion among the ethoxylic chains in mixed aggregates. To our knowledge, the system we explored is the first one for which the experimental h Mic values show that surfactants in the micelles form effectively a regular or almost ideal solution, in a quite wide micellar composition range. In the case of the mixed micelles studied in this paper, the steric repulsions among C6 E5 polar heads are low, as it was shown by some of us in the past (6). Indeed, if the hydrophilic head of the nonionic surfactant is small, the introduction of an ionic surfactant molecule into the nonionic micelles never improves their stability. This is consistent for the almost ideal behavior shown in this paper for 0.5 < YE ≤ 1, where the C6 E5 – C6 E5 interaction should prevail. In contrast the effect of steric repulsions becomes a main problem when mixed micelles are formed by an ethoxylated surfactant with long and bulky hydrophilic head (27, 28). In mixed micelles rich in ionic surfactant (YE < 0.5), the C6 SNa–C6 SNa electrostatic interactions are important. In this composition range our experimental data show a deviation from both ideal and regular solution behavior (see Fig. 9). This suggests that the insertion of an ethoxylated surfactant molecule into an ionic micelle is favored, reducing the interaction among the charged heads with the consequent lowering of the counterion concentration (29).

IV. COMPARISON WITH MODEL SYSTEMS

The apparent molar relative enthalpy of C6 E5 in a water– C6 E5 –C6 SNa ternary system, at a constant molality of the ionic

487

FIG. 10. Comparison between the apparent molar relative enthalpies of 1hexanol, C6 E0 , and pentaethylene glycol, C0 E5 , with that of C6 E5 in ∼0.6 molal C6 SNa solution.

surfactant above its cmc, is the enthalpic effect of the solubilization of C6 E5 molecules into the ionic micelles. As it can be seen from the experimental run we performed at m S = 0.587 mol kg−1 this effect has positive values (see Fig. 4, curve 6). When C6 E5 molecules enter an ionic micelle, their tails interact with the micellar hydrophobic core, while their heads are constrained on the micellar surface together with the ionic heads, forming the external layer of the aggregates. This layer is similar to a very concentrated solution of ethoxylic chains and sulfonic groups in water. With the aim to put in evidence the different contribution of the hydrophobic and hydrophilic interactions to the experimental ( )E ’s, two calorimetric runs were performed on the ternary water–C6 SNa–C6 E0 (1-hexanol) and water–C6 SNa–C0 E5 (pentaethyleneglycol) systems at a constant molality of the ionic surfactant above its cmc (see Fig. 10). C6 E0 mimes the hydrophobic tail of C6 E5 . Since the solubility of C6 E0 in water is very low, the high solubility of C6 E0 in the presence of micellized C6 SNa accounts for its solubilization into the aggregates. The apparent molar relative enthalpy assumes negative values. C0 E5 mimes the polar head of C6 E5 and, being a very hydrophilic molecule, it dissolves in the aqueous pseudo-phase, where it can have some hydrophilic interactions with the C6 SNa heads (30, 31). The apparent molar relative enthalpy assumes positive values and increases almost linearly with the molality, so that it probably assumes very high values in concentrated solution. The presence of C6 E0 and C0 E5 may affect both the C6 SNa aggregation number and the cmc value, thus allowing only a qualitative interpretation of the experimental data. However, since in the case of the water–C6 SNa–C6 E5 system, ( )E assume positive values, it can be qualitatively deduced that the interactions among the hydrophilic heads prevail on the contribution of the hydrophobic tails in ruling the enthalpic properties of the system.

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V. CONCLUSION

A wide thermodynamic characterization of the ternary system water–C6 E5 –C6 SNa has been carried out. The microcalorimetric technique allowed computation of the enthalpic interaction parameter among monomers, the cmc, and the h Mic for the mixed micellar aggregates at different surfactant compositions. In the micellar composition range in which the nonionic surfactant is prevailing, the experimental h Mic values are in quite good agreement with those predicted through the ideal and regular solution approach. In the latter case a very low value of the interaction parameter, β, is compatible with the experimental data. Such behavior can be attributed to the weak steric interactions among the ethoxylic heads on the surface of mixed aggregates formed by these surfactants. In contrast, in the micellar composition range in which the ionic surfactant is predominant, deviation from ideal and/or regular behavior is observed, probably due to the efficiency of the electrostatic interactions among the sulfonic heads. ACKNOWLEDGMENT This research was carried out with the financial support of Italian Cofin. MURST 97 CFSIB and Italian CNR.

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