Studies on apparent molar volumes and heat capacities of several pluronics (triblock copolymers) in aqueous solutions of sodium dodecyl sulfate at 298.15 and 318.15 K

Density and heat capacity measurements were performed in order to investigate the interactions between the ionic surfactant sodium dodecyl sulfate (SDS) and some triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), at 298.15 and 318.15 K. The pluronics L31, L35, 10R5, L64, F68 and P123, were selected as copolymers because of their convenient hydrophilic-hydrophobic ratio and critical micellar temperature.

Molar volumes and heat capacities of transfer of either SDS or copolymer from water to an aqueous mixed solution were calculated from the density and heat capacity values and analysed as a function of the surfactant concentration.

It was found that the transfer properties depend mainly on the state of the copolymers.

For the unassociated copolymers, the molar volumes of transfer of both SDS and copolymer are positive.

The initial sharp increase seen in the corresponding plots against surfactant molality signals the formation of a surfactant-copolymer complex up to the saturation of the copolymer.

Thereafter, the constant values of transfer volumes of copolymers depend mainly on the length of the poly(propylene oxide) block.

For the associated copolymers, negative volumes of transfer of either SDS or copolymer were observed.

They are ascribed to interactions between SDS and copolymer micelles that give rise to a rapid breakdown of the aggregates.

The profiles of the transfer heat capacity curves show more complicated trends through the critical micellar concentration region, which are interpreted as being due to the large positive contribution of the relaxation terms related to the equilibrium shifts induced by the temperature.


Water-soluble triblock copolymers poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), currently abbreviated Ex–Py–Ex, are commercially available under the trade name “pluronics”.

They are widely used in various industrial applications, especially in pharmaceutical formulations, cosmetics and food industry, because of their low toxicity.

In aqueous solutions, they exhibit interesting structural and phase behaviour which have been investigated in many fundamental studies.1–12

The prominent interest of these copolymers lies in their flexible molecular architecture.

It can be modulated by varying the propylene oxide (PO)/ethylene oxide (EO) ratio and the molecular weight of each block and, therefore, gives rise to various structures in aqueous solution.

From the viewpoint of chemical thermodynamics, their main interest consists in their complex aggregation behaviour in terms of their architecture, concentration and temperature.

The self-assembly occurs above a critical micelle concentration (c.m.c.) and gives rise to micelles whose hydrophobic core is formed by weakly solvated poly(propylene oxide) (PPO) blocks and surrounded by an outer shell of fully hydrated poly(ethylene oxide) (PEO) chains.

The difference in hydration of EO and PO units is dependent on temperature and leads to a thermally reversible aggregation process.

A small increase in temperature may result in a drastic decrease of c.m.c.

This behaviour has led to the concept of critical micellar temperature (c.m.t.) which has been shown to be a very useful micellar parameter.

In many industrial applications of the copolymers, ionic surfactants are widely used in processes of commercial formulations.

They may associate into different microstructures that are likely to affect the functional properties of the systems.

Therefore, it is very important to study the interactions between the copolymers and the ionic surfactants which control the structural behaviour of mixtures.

Recently, extensive studies were carried out using mainly calorimetry, emf- and T-jump measurements and several scattering techniques.13–23

Nevertheless, results are still scarce and available informations are far from being sufficient to understand the complex mechanisms of copolymer–surfactant interactions.

Especially, few quantitative data concerning thermodynamic properties of these systems are presently available.24–28

Most studies were focused on mixtures of a well-known ionic surfactant, sodium dodecyl sulfate (SDS), with pluronics L64(E13P30E13),13,17,28 F68(E75P30E75),13 F88(E100P39E100),15,24 P123(E20P69E20)15,22 and F127(E97P69E97).14–16,18–22

It was shown that the mechanism of interaction between surfactant and copolymer is specific of the systems where different binding modes of SDS have been observed.

These depend mainly on the structural state of the copolymer.18–22

In the first mode, the interactions between monomers and SDS are similar to those between polymers (either PEO or PPO).

Small SDS aggregates are bound onto the copolymer chain up to the saturation of the polymer.

In so far as SDS is added, these bound aggregates are growing in size until the copolymer becomes saturated by normal SDS micelles.

In the second mode, the interactions between SDS and copolymer micelles lead to the formation of mixed micelles followed by their progressive breakdown into smaller mixed aggregates.

The binding process continues until only pluronic monomers are remaining.

Isothermal titration calorimetry and differential scanning calorimetry proved to be very appropriate techniques for the study of polymer–surfactant interactions.18–22

However, a few other thermodynamic properties were investigated.24–28

To the best of our knowledge, the most reliable studies are due to de Lisi et al.25–27

They report measurements of volumes, heat capacities and enthalpies of mixing of aqueous mixtures of L64 and F68 with different surfactants.

Their primary aim consisted in studying the effects of the surfactant headgroups and of the ratio PO/EO of the copolymer on the interactions between monomers with surfactants, at 298.15 K.26

These authors also investigated the behaviour of an associated copolymer in the sodium decanoate/L64 aqueous system.27

However, at 298.15 K, L64 is still partially associated even at high concentrations.

The present contribution aims a better understanding of interactions in systems of ionic surfactants and copolymers showing different hydrophobic properties, as revealed by their thermodynamic properties.

For this purpose, the volumetric properties and the heat capacities of the aqueous systems of ionic surfactant (SDS) + some copolymers were measured.

In order to perform a systematic study of both unassociated and associated copolymers at the same fixed low concentration, measurements were carried out far from the c.m.t. at 298.15 and 318.15 K. Our goal is to describe the effects of the copolymer architecture, namely the ratio PO/EO together with the mass of the PPO block.

Also, it is to determine the influence of the structural state of the copolymer on the intricate nature of the copolymer–surfactant interactions which determine the behaviour of these systems.



Sodium dodecyl sulfate, SDS, was a pure grade reagent (>99%) provided by Merck.

It was used without further purification after drying under vacuum at a temperature below 330 K. Poly(ethylene oxide)x–poly(propylene oxide)y–poly(ethylene oxide)x triblock copolymers where x and y are the numbers of EO and PO units respectively (abbreviated to ExPyEx and PyExPy for the “reverse” structure) are compounds whose trade names are Pluronic or Pluronic-R, respectively.

The selected pluronics, whose properties are summarized in Table 1, namely L31, L35, L64, F68, P123 and 10R5 (“reverse” of L35), were purchased from Aldrich.

Like SDS, the copolymers were used without further purification, since previous analyses have shown that the molar mass distribution has no significant influence on volumes or heat capacities, contrary to some other more sensitive techniques such as light scattering.10,17,29

All solutions were prepared by mass at room temperature, with a precision of 0.1 mg, from copolymer and freshly deionised and degassed water samples.

The copolymer concentration is expressed in terms of molalities using an equation based on the mean molar mass and the PO/EO ratio of the copolymer provided by the manufacturer.

Apparatus and procedures

The density measurements were performed at 298.15 and 318.15 K using a Picker vibrating tube flow densimeter (Model 03-D, Sodev Inc.), whose sensitivity is about 3 ppm.

The temperature was controlled to within ±0.005 K by means of a closed-loop liquid circulation temperature controller (Model CT-L, Sodev Inc.).

The densimeter was calibrated using deionised and doubly distilled water, whose densities were taken from literature30 and vacuum.

The heat capacities by volume unit were measured by means of a Picker flow microcalorimeter (Setaram) at 298.15 K. It operates on the principle of a thermal balance at a constant flow rate of about 0.7 cm3 min–1.

The temperature increment was equal to about 0.5 K, using water as reference liquid.

Temperature was controlled in the same way as for density measurements.

The specific heat capacities were calculated from the density values of solutions.

Both apparatus, calibration and operating procedures have been previously described in detail.31,32

The density and specific heat capacity measurements were assumed to be reproducible to within 3 × 10–6 g cm–3 and 5 × 10–4 J K–1 g–1, respectively.

Thermodynamic properties

The apparent molar volumes, Vφ,S, and heat capacities, Cφ,S, of SDS in aqueous copolymer solutions at a fixed pluronic concentration were calculated from density, ρ, and specific heat capacity, cp, using the following equations:MS and mS are the mean molar mass and the molality of SDS in the binary solvent, respectively.

ρ and cp hold for the density and the specific heat capacity in the ternary systems, while ρo and cp,o are the corresponding properties in the aqueous copolymer solutions, respectively.

The apparent molar properties, Vφ,P and Cφ,P of the pluronic solute in the aqueous surfactant solutions are calculated using also eqns. (1) and (2) from the same density and heat capacity values of the ternary systems.

Here, MP and mP are the molar mass and the molality of the copolymer, respectively.

Molalities of the solutes, SDS and pluronic, need to be recalculated by taking into account the variation of concentration due to the change of the solvent (water + SDS) used as the liquid reference.33

For ternary systems, the molar properties of a solute are generally discussed in terms of the transfer properties (ΔY) from water to binary solutions.

When the solute is transferred at the same concentration of either solution, ΔYP is calculated from the apparent molar properties Vφ,P or Cφ,P using the following relation:ΔYP = Yφ,P(water + SDS) – Yφ,P(water)When the molality of the solute is sufficiently low, the solute–solute interactions can be neglected.

It ensues that the values of transfer quantities are mainly representative of solute–solvent interactions.

They characterize the distribution of the solute between the micellar and aqueous phases.

When their variations are plotted against the surfactant concentration, they clearly show the changes in micellar solution, especially when a transition occurs.

Results and discussion

Apparent molar volumes of SDS, Vφ,S, for the different aqueous pluronic solutions at a fixed composition (1 or 2% by mass) vs. the molality of SDS, mS, have been calculated at 298.15 and 318.15 K, respectively.

Apparent molar heat capacities, Cφ,S, vs.mS in the same solutions, were determined only at 298.15 K. For sake of consistency, the apparent molar volumes and heat capacities of SDS in pure water were calculated from separate runs at the same experimental conditions.

Original experimental density and heat capacity values in the water/pluronic/SDS systems, are reported in the electronic supplementary information (ESI) together with the related apparent molar properties of SDS and molar transfer properties of pluronics.

Depending on the nature of the pluronic or on the temperature, the changes of Vφ,S against mS are shifted towards higher or lower values compared to those in water.

Volumes of transfer of SDS, ΔVS, i.e. the difference between the values of Vφ,S in aqueous pluronic solution and in water, are plotted against mS at 298.15 and 318.15 K in Figs. 1 and 2, respectively.

Similar trends are observed for positive ΔVS values.

A sharp maximum is observed at a molality lower than the c.m.c. of SDS in water (0.0083 mol kg–1).

Thereafter, ΔVS is decreasing slowly and tends to zero, that is Vφ,S merges with its value in water at higher SDS concentrations.

Conversely, the negative ΔVS values appear to be somewhat more dependent on both the nature of the pluronic and the temperature.

Fig. 3 shows the plots of the transfer heat capacities, ΔCS, vs.mS in the aqueous pluronic solutions, at 298.15 K. Large positive values are observed at a concentration far lower than the c.m.c.

The maximum cannot be defined because of the intrinsic accuracy of measurements.

Afterwards, ΔCS decreases steeply towards a flat negative minimum before it tends to zero, like the ΔVS values.

When the pluronic P123 is considered, the ΔCS values are starting from large negative values and tend slowly to zero with increasing SDS concentration.

Typical plots of molar volumes of transfer of some pluronics from water to aqueous SDS solutions, ΔVPvs.mS, are reported in Figs. 4–6 at 298.15 and 318.15 K. Like the transfer volumes of SDS, the changes in ΔVP can be either positive or negative.

Fig. 7 shows the trends in the molar heat capacities of transfer, ΔCP, of some pluronics at 298.15 K. Beyond the c.m.c. the ΔCP values are negative.

For P123, ΔCP is highly negative (see insert in Fig. 7).

In a previous study,11 the interpretation of thermodynamic properties of the same copolymers in water allowed to characterize accurately their structural state in the concentration/temperature phase diagram.

The different regions,2 namely monomer species, monomers–micelles at equilibrium and micelles, were clearly identified.

Thus, at the considered concentrations, L31, L35, 10R5, L64 and F68 are in a monomeric state at 298.15 K, while P123 is partly associated.

At 318.15 K, the mostly hydrophilic pluronics, i.e. L35, 10R5 and F68, whose EO content is large, remain unassociated.

The aggregation of L31 and L64 is thermally induced and their solutions contain both monomers and micelles at equilibrium.

The hydrophobic P123, whose PO content is large, is fully micellized in aqueous solution.

SDS/unassociated pluronic interactions

The plots vs.mS of both transfer properties of SDS and pluronics are similar to those observed when nonionic polymers (PEG or PPO) are bound to SDS.34,35

Thus, a sharp initial increase of ΔVS is observed at the lowest concentrations investigated in this work.

That means that the critical aggregation concentration (c.a.c.) is still lower.

The ΔVS values exhibit rather similar profiles for the various aqueous solutions of unassociated copolymers.

However, some slightly different trends may be observed when scrutinising Figs. 1 and 2.

For example, at 298.15 K, the maximum occurs at a lower SDS concentration when the PPO mass is larger, e.g. at mS ≈ 0.004 mol kg–1 for L64 or F68 (30 PO) and at mS ≈ 0.008 mol kg–1 for L31 or L35 (17 PO).

Also, the width of the peak is quite enlarged with increasing pluronic concentration (1 and 2% L64).

Likewise, increasing the temperature appears to have a minor effect, as evidenced by the plots of ΔVS against mS at 298.15 and 318.15 K for 10R5 and F68.

The magnitude of the positive changes observed on ΔCS curves in Fig. 3 is dependent on the hydrophobic character of the pluronic.

It is also very sensitive to the pluronic concentration (1 and 2% L64).

The area is getting larger and the following minimum is occurring at higher SDS concentrations, 0.02 and 0.04 mol kg–1, respectively.

The shapes of the changes of ΔVPvs.mS are also similar.

ΔVP shows a steep initial increase and then tends to an approximate plateau, whose values are dependent on the mass of the pluronic.

Some values of the transfer quantities of pluronics are reported in Table 2 at two characteristic molalities of SDS, i.e. in the increasing part of ΔVP and on the plateau.

For L64 at 298.15 K (Fig. 5) and L35 at 318.15 K (Fig. 6), ΔVP passes through a shallow maximum at concentrations beyond the c.m.c. of SDS.

Similar results were observed on parent systems (sodium decanoate + L64).27

Fig. 5 shows that this maximum is more smooth and shifted to higher mS with increasing copolymer concentration (1% and 2% L64).

This maximum is probably due to the contribution of relaxation terms, as foreseen by current thermodynamic models,36–40 because the experimental temperature is close to that of the onset of the thermally induced aggregation of the pluronic.

Interestingly, at 298.15 K, the increase of ΔVP is identical for F68 and L64 whose number of PO units is equal.

Indeed, the same values are shown in Table 2, at the molality 0.015 m.

Similarly, the increase of ΔVP is almost identical for F68 at 298.15 and 318.15 K. For the series of pluronics with 17 PO units (Figs. 4 and 6), the positive slope of ΔVP is getting weaker.

In this series, increasing EO content and temperature have also an effect on ΔVP values, as shown in Table 2 by the different ΔVP values at the molality 0.02 m.

Similar features are observed in Fig. 7 about the dependence of molar transfer heat capacities on mS.

ΔCP exhibits a sharp maximum located at very low mS, followed by constant negative values at higher mS.

The more hydrophobic the pluronic, the more pronounced the initial maximum and the steeper the following decrease.

The broadness of the peak is also largely dependent on the concentration of L64 and the decrease spans over a larger SDS concentration range.

From a thermodynamic point of view, the composition dependence of the apparent molar or transfer quantities may be interpreted as the combination of various contributions arising from the equilibrium shifts in solution, due to either the addition of the solute or the temperature changes.

The thermodynamic models that were used in order to explain the behaviour of hydrophobic solutes in the micellization processes of surfactants were related to either the mass action law or the pseudo-phase models.36–40

The large positive ΔVS or ΔVP values in aqueous micellar solutions of pluronics include the terms related to the equilibrium shifts.

They are characteristic of both the dehydration of the surfactant and the copolymer and the changes in the micellar structure arising from the formation of small SDS aggregates bound onto the copolymers.34,35

The changing composition dependence of apparent molar or transfer heat capacities of SDS or pluronics arises from the fact that the heat capacity is a second derivative of the Gibbs free energy.

In this case, additional positive relaxation terms due to the temperature changes are expected.36–38

For the copolymer–SDS systems, these positive contributions occurring at the onset of the SDS binding appear to be quite large.

Upon further addition of SDS, their magnitude decreases steeply until it vanishes.

Afterwards, the decrease of ΔCS or ΔCP arises from the hydrophobic interactions which develop throughout the association process.

Strong attractive interactions between SDS molecules and pluronic monomers have been evidenced by EMF and ITC measurements.18–21

They lead to the formation of a stable copolymer–surfactant complex at a concentration well below the c.m.c. of SDS.

The increasing ΔVS values can be related to the formation of these copolymer/SDS bound aggregates complexes.

Thereafter, upon further addition of SDS, the following decrease of ΔVS is assumed to result from the increase of the aggregation number of the bound SDS micellar aggregates until normal micelles are bound onto copolymer chain.

The apparent molar properties of SDS are nearly equal to those in pure water, or else the transfer quantities tend to zero when the binding process is brought to its close.

The initial increase of ΔVP reveals in the same way the formation of copolymer–surfactant complexes.

It is proceeding until the saturation of the copolymer is reached, i.e. when the solution is supposed to contain only mixed pluronic–SDS aggregates.

For example, the maximum for 1% L64 occurs at mS = 0.018 mol kg–1 (Fig. 5).

The resulting ratio SDS/L64 is close to 5, i.e. it is similar to either the composition of mixed aggregates deduced from fluorescence decay analysis on this system, namely 17–20 SDS and 4–5 L64 monomers,13 or to the ratio found for a complex containing about four surfactant molecules bound on a copolymer chain.16,27

This maximum is shifted to 0.032 mol kg–1 for 2% L64.

The same ratio, i.e. 5, is obtained.

The copolymer saturation occurs at quite similar mS values for F68 monomers.

This means that about the same number of SDS molecules is bound to the pluronic.

A lesser number of SDS molecules may be bound to pluronics having a shorter PPO block (17 PO units).

It ensues that the copolymer gets saturated at larger mS, where a quasi-plateau is reached at mS > 0.05 mol kg–1 (Figs. 4 and 6).

The same effects are prevailing for the molar transfer heat capacities (Fig. 7).

Further addition of SDS gives rise to more or less constant values of the transfer properties.

Two separate groups of ΔVP and ΔCP values are reported in Figs. 4 and 7, respectively.

They correspond to pluronics having about 17 PO and 30 PO repeat units.

Some values are reported in Table 2 at a given SDS molality.

For species with 17 PO units, ΔVP values are close to 20–26 cm3 mol–1 (Fig. 6), while for those containing 30 PO units, such as L64 or F68, they can reach 50 to 60 cm3 mol–1 (Figs. 4 and 5).

Likewise, Fig. 7 shows that the ΔCP values lie in the respective negative ranges 700–1000 J K–1 mol–1 and 2500–3000 J K–1 mol–1 for the above considered species.

These almost steady values let assume that the major contribution to the property is due to the hydrophobic interactions between SDS aggregates and PPO blocks.

A scrutiny of either ΔVP or ΔCP curves or of data reported in Table 2 provides some evidence of the less prominent role of the EO content.

When the PPO mass is equal, the increase of EO percentage is accompanied by an increase of volume, as shown in Fig. 6, and a decrease of ΔCP (Fig. 7).

This may be due to an additive contribution of the interactions between the PEO chains and the SDS aggregates.34

No noticeable influence of the architecture of the copolymer on ΔVP is observed when L35 is compared with 10R5 (Fig. 6).

However, this may result from the small size of the different blocks giving rise to a more expanded conformation of the chain.

Likewise, the ΔVP values are independent on the pluronic concentration (cf. 1 and 2% L64 in Fig. 5).

As long as pluronics are exclusively in a monomeric state, ΔVP is not so dependent on the temperature.

Such a trend is clearly visible in Fig. 4 where the curves for F68 are nearly superimposed at 298.15 and 318.15 K. For the 17 PO series (Fig. 6), although some difference is observed at lower mS at these two temperatures, the ΔVP curves join up all together at higher mS.

SDS/associated pluronic interactions

Negative values of the transfer properties of SDS and pluronics are generally observed when pluronics are existing in an aggregated state.

When they are partly associated (for instance, L64 at 318.15 K or P123 at 298.15 K), ΔVS curves pass through a narrow positive maximum located below the c.m.c. of SDS (Figs. 1 and 2) whose magnitude is dependent on the concentration (1% and 2% for L64).

Thereafter, in their negative part, they intercept at 0.015 mol kg–1, superimpose and level off to zero with increasing SDS concentration.

For P123 at 318.15 K, when the aggregation process is supposed to be completed,11 all of the ΔVS values are negative while the minimum is shifted to a higher concentration, located at 0.02 mol kg–1.

The trends in ΔVP are shown in Fig. 5 (L64 at 318.15 K) and in Fig. 4 (P123 at 298.15 K and 318.15 K).

For micellized P123 at 318.15 K, ΔVP decreases until mS is nearly equal to 0.04 mol kg–1 and then remains constant.

When the pluronic is partly associated, a positive maximum may be observed at very low mS values.

This peak is dependent on the ratio monomer/micelle in solution.

The larger the amount of monomers, the greater the positive peak, as evidenced in Fig. 5 for 1% and 2% L64 at 318.15 K. It is very narrow for P123 at 298.15 K, for which ΔVP is decreasing to still constant, albeit values are less negative than at 318.15 K (see Fig. 4).

Tremendously large negative ΔCp values are observed for P123 (–28 000 J K mol–1, as shown in the insert of Fig. 7).

They should be related to the contribution of the large enthalpy of micellization of P123, due to both the hydrophobic character of P123 (69 PO units) and the high degree of conversion of monomers into micelles.

In this case, this large contribution of the relaxation terms superimposes that one of the hydrophobic interactions (certainly lower than –8000 J K–1 mol–1), if it is compared to that of copolymers having 30 PO units, like L64 or F68.

Previous investigations using several techniques (EMF, ITC, LS, SANS)16,18–20 have shown that SDS is bound to pluronic micelles and gives rise to the formation of mixed micelles.

The hydrophobic interactions between PPO and SDS are stronger than those between the PPO blocks, which involves the removal of bound water molecules from both the micellar surface and the copolymer chain.

The PPO–PPO interactions are weakened to a large extent while the SDS binding leads to a rapid breakdown of pluronic micelles, giving rise to smaller mixed aggregates of varying composition until only monomers are remaining.

Afterwards, the binding process follows the same way than for an unassociated copolymer.

The binding of SDS to copolymer micelles gives rise to lower values of the apparent molar volume of SDS, i.e. to negative transfer volumes, since the neighborhood of the hydrocarbon chain of SDS is comparatively more hydrophilic in pluronic micelles than in SDS aggregates.

Trends in transfer properties of pluronics have been interpreted as the result of simultaneous contributions of the two modes of binding of SDS with monomers and pluronic micelles.

A positive contribution is assumed to be due to the strong attractive hydrophobic monomer–SDS interactions.

As seen in the first part, this contribution is mostly independent on the temperature.

A negative contribution may be ascribed to the interactions between the SDS molecules and the pluronic micelles.

In this case, the PPO blocks are removed from their hydrophobic environment inside the micellar core towards the aqueous phase where their hydrophobic hydration is restored.

Consequently, this contribution appears to be related to the properties of rehydration of monomers and dependent on the equilibrium between monomers and micelles.

Therefore, the decrease of ΔVP may be due to the progressive disruption of pluronic micelles until their complete breakdown which occurs at mS ≈ 0.04 mol kg–1.

When SDS is added beyond mS ≈ 0.04 mol kg–1, only monomers remain present.

Then, small SDS aggregates are bound on the copolymer.

Their aggregation number increases while ΔVP is mainly constant.

In this concentration range, ΔVP values for L64 are close to –32 cm3 mol–1 at 318.15 K, while the contribution for the sole interactions between the L64 monomer and the SDS micelles is nearly equal to +48 cm3 mol–1 at 298.15 K (Fig. 5 or Table 2).

Thus, the contribution arising from the disruption of L64 micelles can be estimated to be close to –80 cm3 mol–1.

This value agrees well with that of the experimental change of the apparent molar volume of aggregation of 1% L64 at 318.15 K, which was previously found equal to +77 cm3 mol–1.11

For P123, ΔVP values are less negative at 298.15 K than at 318.15 K (Fig. 4), because it depends on the ratio monomer/micelle in solution.

For SDS concentrations higher than 0.04 mol kg–1, i.e. when pluronic micelles are assumed to be entirely broken up, the ΔVP values should be decreasing with increasing temperature across the entire range of the thermally induced aggregation process.

It varies from the positive value ascribed to the sole contribution of interactions between pluronic monomers and bound SDS micelles (only monomers are present at T < 293 K)11 to a nearly constant negative value when the micellization of the pluronic is completed at T ≈ 308 K.11

From our previous results dealing with the volumes of aggregation of P123 in water,11 the variations of apparent molar volumes corresponding to the aggregation of 1% P123 were considered to be equal to +210 cm3 mol–1 and +260 cm3 mol–1 at 298.15 and 318.15 K, respectively, if additivity laws are assumed to be valid for the volumes of pluronics in their monomeric state.11

The ΔVP values plotted in Fig. 4 are nearly equal to –100 and –160 cm3 mol–1, respectively.

This means that the contribution to the volume of interactions between the P123 monomers and the bound SDS aggregates may be expected to lie within 100–110 cm3 mol–1.

Since this contribution has been shown to be mainly dependent on the number of PO units in the PPO block and independent on the temperature, an estimate of +110 cm3 mol–1 may be expected for ΔVP of the P123 monomer when compared with those of L35 (+25 cm3 mol–1) and L64 (+48 cm3 mol–1), whose EO percentage is quite similar.

This value agrees well with that obtained from measurements carried out at 288.15 K, where P123 is only in monomeric state41.


The apparent molar volumes and heat capacities of some water/copolymer(pluronic)/surfactant systems have been investigated.

The results obtained for the transfer properties of either SDS or pluronic have clearly shown the prominent role of the aggregation state of pluronics in solution on the interactions between copolymer and surfactant.

When pluronics are in monomeric state, at low concentration range of SDS, the rapid increase in the molar volumes of transfer of SDS and pluronic points to the enhancement of strong interactions between monomers and SDS molecules leading to the formation of copolymer–surfactant complexes up to the saturation of the copolymer.

Thereafter, with increasing SDS concentration, the constant and positive values of the transfer volumes of pluronics reveal that the main contribution stems from hydrophobic interactions between PPO blocks and bound SDS aggregates.

Consequently, the transfer volumes are mainly linked to the PPO mass in the pluronic molecule.

It was also found that they barely change with increasing temperature.

A different behaviour is prevailing for the associated copolymers, for which negative values of transfer volumes are observed.

These should result from the rapid breakdown of pluronic micelles by SDS molecules, which form mixed aggregates until the pluronic micelles are fully disrupted.

The values of the molar transfer volumes of pluronics are interpreted as resulting from two contributions.

A negative one may be due to the rehydration of PPO blocks when monomers are removed from copolymer micelles to the aqueous phase, while a positive one involves the hydrophobic monomer–SDS interactions.

It ensues that their balance is strongly linked to the monomer–micelle equilibrium and to the temperature effect on the aggregation progress in the aqueous pluronic solution.

Analysis of the corresponding molar heat capacities of transfer has revealed a similar behaviour.

However, close to the c.m.c. of SDS, the changes in the molar transfer heat capacities were found to be influenced to a large extent by the relaxation terms arising from temperature effects on association equilibria.