2
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.
Type: Goal |
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ConceptID: Goa1
3
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.
Type: Result |
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ConceptID: Res1
4
It was found that the transfer properties depend mainly on the state of the copolymers.
Type: Result |
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ConceptID: Res2
5
For the unassociated copolymers, the molar volumes of transfer of both SDS and copolymer are positive.
Type: Observation |
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ConceptID: Obs1
6
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.
Type: Result |
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ConceptID: Res3
7
Thereafter, the constant values of transfer volumes of copolymers depend mainly on the length of the poly(propylene oxide) block.
Type: Result |
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ConceptID: Res4
8
For the associated copolymers, negative volumes of transfer of either SDS or copolymer were observed.
Type: Observation |
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ConceptID: Obs2
9
They are ascribed to interactions between SDS and copolymer micelles that give rise to a rapid breakdown of the aggregates.
Type: Result |
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ConceptID: Res5
10
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.
Type: Result |
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ConceptID: Res6
Introduction
11
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”.
Type: Background |
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ConceptID: Bac1
12
They are widely used in various industrial applications, especially in pharmaceutical formulations, cosmetics and food industry, because of their low toxicity.
Type: Background |
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13
In aqueous solutions, they exhibit interesting structural and phase behaviour which have been investigated in many fundamental studies.1–12
Type: Background |
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ConceptID: Bac1
14
The prominent interest of these copolymers lies in their flexible molecular architecture.
Type: Background |
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ConceptID: Bac2
15
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.
Type: Background |
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ConceptID: Bac2
16
From the viewpoint of chemical thermodynamics, their main interest consists in their complex aggregation behaviour in terms of their architecture, concentration and temperature.
Type: Background |
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ConceptID: Bac3
17
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.
Type: Background |
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ConceptID: Bac3
18
The difference in hydration of EO and PO units is dependent on temperature and leads to a thermally reversible aggregation process.
Type: Background |
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ConceptID: Bac3
19
A small increase in temperature may result in a drastic decrease of c.m.c.
Type: Background |
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ConceptID: Bac3
20
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.
Type: Background |
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ConceptID: Bac3
21
In many industrial applications of the copolymers, ionic surfactants are widely used in processes of commercial formulations.
Type: Background |
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ConceptID: Bac4
22
They may associate into different microstructures that are likely to affect the functional properties of the systems.
Type: Background |
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ConceptID: Bac4
23
Therefore, it is very important to study the interactions between the copolymers and the ionic surfactants which control the structural behaviour of mixtures.
Type: Motivation |
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ConceptID: Mot1
24
Recently, extensive studies were carried out using mainly calorimetry, emf- and T-jump measurements and several scattering techniques.13–23
Type: Method |
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Novelty: Old |
ConceptID: Met1
25
Nevertheless, results are still scarce and available informations are far from being sufficient to understand the complex mechanisms of copolymer–surfactant interactions.
Type: Motivation |
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ConceptID: Mot2
26
Especially, few quantitative data concerning thermodynamic properties of these systems are presently available.24–28
Type: Motivation |
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ConceptID: Mot2
27
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
Type: Background |
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ConceptID: Bac5
28
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.
Type: Background |
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29
These depend mainly on the structural state of the copolymer.18–22
Type: Background |
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ConceptID: Bac5
30
In the first mode, the interactions between monomers and SDS are similar to those between polymers (either PEO or PPO).
Type: Background |
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ConceptID: Bac5
31
Small SDS aggregates are bound onto the copolymer chain up to the saturation of the polymer.
Type: Background |
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ConceptID: Bac5
32
In so far as SDS is added, these bound aggregates are growing in size until the copolymer becomes saturated by normal SDS micelles.
Type: Background |
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ConceptID: Bac5
33
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.
Type: Background |
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ConceptID: Bac5
34
The binding process continues until only pluronic monomers are remaining.
Type: Background |
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ConceptID: Bac5
35
Isothermal titration calorimetry and differential scanning calorimetry proved to be very appropriate techniques for the study of polymer–surfactant interactions.18–22
Type: Method |
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Novelty: Old |
ConceptID: Met2
36
However, a few other thermodynamic properties were investigated.24–28
Type: Background |
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ConceptID: Bac6
37
To the best of our knowledge, the most reliable studies are due to de Lisi et al.25–27
Type: Background |
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ConceptID: Bac6
38
They report measurements of volumes, heat capacities and enthalpies of mixing of aqueous mixtures of L64 and F68 with different surfactants.
Type: Background |
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Novelty: None |
ConceptID: Bac6
39
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
Type: Background |
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Novelty: None |
ConceptID: Bac6
40
These authors also investigated the behaviour of an associated copolymer in the sodium decanoate/L64 aqueous system.27
Type: Background |
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ConceptID: Bac6
41
However, at 298.15 K, L64 is still partially associated even at high concentrations.
Type: Background |
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ConceptID: Bac6
42
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.
Type: Goal |
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ConceptID: Goa1
43
For this purpose, the volumetric properties and the heat capacities of the aqueous systems of ionic surfactant (SDS) + some copolymers were measured.
Type: Method |
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Novelty: New |
ConceptID: Met3
44
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.
Type: Method |
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ConceptID: Met3
45
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.
Type: Goal |
Advantage: None |
Novelty: None |
ConceptID: Goa2
Experimental
Materials
46
Sodium dodecyl sulfate, SDS, was a pure grade reagent (>99%) provided by Merck.
Type: Experiment |
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ConceptID: Exp1
47
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.
Type: Experiment |
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ConceptID: Exp1
48
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.
Type: Experiment |
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ConceptID: Exp1
49
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
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp1
50
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.
Type: Experiment |
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ConceptID: Exp1
51
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.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp1
Apparatus and procedures
52
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.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp2
53
The temperature was controlled to within ±0.005 K by means of a closed-loop liquid circulation temperature controller (Model CT-L, Sodev Inc.).
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp2
54
The densimeter was calibrated using deionised and doubly distilled water, whose densities were taken from literature30 and vacuum.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp2
55
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.
Type: Experiment |
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ConceptID: Exp3
56
The temperature increment was equal to about 0.5 K, using water as reference liquid.
Type: Experiment |
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Novelty: None |
ConceptID: Exp3
57
Temperature was controlled in the same way as for density measurements.
Type: Experiment |
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Novelty: None |
ConceptID: Exp3
58
The specific heat capacities were calculated from the density values of solutions.
Type: Experiment |
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Novelty: None |
ConceptID: Exp3
59
Both apparatus, calibration and operating procedures have been previously described in detail.31,32
Type: Experiment |
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ConceptID: Exp3
60
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.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp3
Thermodynamic properties
61
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.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
62
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.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
63
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.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
64
Here, MP and mP are the molar mass and the molality of the copolymer, respectively.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod1
65
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
Type: Model |
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Novelty: None |
ConceptID: Mod1
66
For ternary systems, the molar properties of a solute are generally discussed in terms of the transfer properties (ΔY) from water to binary solutions.
Type: Model |
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ConceptID: Mod1
67
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.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
68
It ensues that the values of transfer quantities are mainly representative of solute–solvent interactions.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
69
They characterize the distribution of the solute between the micellar and aqueous phases.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
70
When their variations are plotted against the surfactant concentration, they clearly show the changes in micellar solution, especially when a transition occurs.
Type: Model |
Advantage: None |
Novelty: None |
ConceptID: Mod2
Results and discussion
71
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.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
72
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.
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
Type: Experiment |
Advantage: None |
Novelty: None |
ConceptID: Exp4
74
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.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs3
75
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.
Type: Observation |
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Novelty: None |
ConceptID: Obs3
76
Similar trends are observed for positive ΔVS values.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs3
77
A sharp maximum is observed at a molality lower than the c.m.c. of SDS in water (0.0083 mol kg–1).
Type: Observation |
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Novelty: None |
ConceptID: Obs3
78
Thereafter, ΔVS is decreasing slowly and tends to zero, that is Vφ,S merges with its value in water at higher SDS concentrations.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res7
79
Conversely, the negative ΔVS values appear to be somewhat more dependent on both the nature of the pluronic and the temperature.
Type: Result |
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Novelty: None |
ConceptID: Res7
80
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.
Type: Observation |
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Novelty: None |
ConceptID: Obs4
81
The maximum cannot be defined because of the intrinsic accuracy of measurements.
Type: Observation |
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Novelty: None |
ConceptID: Obs4
82
Afterwards, ΔCS decreases steeply towards a flat negative minimum before it tends to zero, like the ΔVS values.
Type: Observation |
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Novelty: None |
ConceptID: Obs4
83
When the pluronic P123 is considered, the ΔCS values are starting from large negative values and tend slowly to zero with increasing SDS concentration.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs5
84
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.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs6
85
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.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs7
86
For P123, ΔCP is highly negative (see insert in Fig. 7).
Type: Observation |
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Novelty: None |
ConceptID: Obs7
87
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.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac7
88
The different regions,2 namely monomer species, monomers–micelles at equilibrium and micelles, were clearly identified.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs8
89
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.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res8
90
At 318.15 K, the mostly hydrophilic pluronics, i.e. L35, 10R5 and F68, whose EO content is large, remain unassociated.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res8
91
The aggregation of L31 and L64 is thermally induced and their solutions contain both monomers and micelles at equilibrium.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res9
92
The hydrophobic P123, whose PO content is large, is fully micellized in aqueous solution.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res9
SDS/unassociated pluronic interactions
93
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
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs9
94
Thus, a sharp initial increase of ΔVS is observed at the lowest concentrations investigated in this work.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs9
95
That means that the critical aggregation concentration (c.a.c.) is still lower.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res10
96
The ΔVS values exhibit rather similar profiles for the various aqueous solutions of unassociated copolymers.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs10
97
However, some slightly different trends may be observed when scrutinising Figs. 1 and 2.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs10
98
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).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs10
99
Also, the width of the peak is quite enlarged with increasing pluronic concentration (1 and 2% L64).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs10
100
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.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs10
101
The magnitude of the positive changes observed on ΔCS curves in Fig. 3 is dependent on the hydrophobic character of the pluronic.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res11
102
It is also very sensitive to the pluronic concentration (1 and 2% L64).
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res11
103
The area is getting larger and the following minimum is occurring at higher SDS concentrations, 0.02 and 0.04 mol kg–1, respectively.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res11
104
The shapes of the changes of ΔVPvs.mS are also similar.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs11
105
ΔVP shows a steep initial increase and then tends to an approximate plateau, whose values are dependent on the mass of the pluronic.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs11
106
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.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs11
107
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.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs11
108
Similar results were observed on parent systems (sodium decanoate + L64).27
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac8
109
Fig. 5 shows that this maximum is more smooth and shifted to higher mS with increasing copolymer concentration (1% and 2% L64).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs12
110
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.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res12
111
Interestingly, at 298.15 K, the increase of ΔVP is identical for F68 and L64 whose number of PO units is equal.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs13
112
Indeed, the same values are shown in Table 2, at the molality 0.015 m.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs13
113
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.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs13
114
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.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res13
115
Similar features are observed in Fig. 7 about the dependence of molar transfer heat capacities on mS.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs14
116
ΔCP exhibits a sharp maximum located at very low mS, followed by constant negative values at higher mS.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs14
117
The more hydrophobic the pluronic, the more pronounced the initial maximum and the steeper the following decrease.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs14
118
The broadness of the peak is also largely dependent on the concentration of L64 and the decrease spans over a larger SDS concentration range.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs14
119
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.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res14
120
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
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res14
121
The large positive ΔVS or ΔVP values in aqueous micellar solutions of pluronics include the terms related to the equilibrium shifts.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res14
122
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
Type: Background |
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ConceptID: Bac9
123
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.
Type: Background |
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ConceptID: Bac9
124
In this case, additional positive relaxation terms due to the temperature changes are expected.36–38
Type: Hypothesis |
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ConceptID: Hyp1
125
For the copolymer–SDS systems, these positive contributions occurring at the onset of the SDS binding appear to be quite large.
Type: Observation |
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ConceptID: Obs15
126
Upon further addition of SDS, their magnitude decreases steeply until it vanishes.
Type: Observation |
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ConceptID: Obs15
127
Afterwards, the decrease of ΔCS or ΔCP arises from the hydrophobic interactions which develop throughout the association process.
Type: Observation |
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ConceptID: Obs15
128
Strong attractive interactions between SDS molecules and pluronic monomers have been evidenced by EMF and ITC measurements.18–21
Type: Background |
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ConceptID: Bac10
129
They lead to the formation of a stable copolymer–surfactant complex at a concentration well below the c.m.c. of SDS.
Type: Background |
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ConceptID: Bac10
130
The increasing ΔVS values can be related to the formation of these copolymer/SDS bound aggregates complexes.
Type: Result |
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ConceptID: Res3
131
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.
Type: Result |
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ConceptID: Res3
132
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.
Type: Result |
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ConceptID: Res15
133
The initial increase of ΔVP reveals in the same way the formation of copolymer–surfactant complexes.
Type: Result |
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ConceptID: Res15
134
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.
Type: Result |
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ConceptID: Res15
135
For example, the maximum for 1% L64 occurs at mS = 0.018 mol kg–1 (Fig. 5).
Type: Observation |
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ConceptID: Obs16
136
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
Type: Result |
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ConceptID: Res16
137
This maximum is shifted to 0.032 mol kg–1 for 2% L64.
Type: Observation |
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ConceptID: Obs17
138
The same ratio, i.e. 5, is obtained.
Type: Observation |
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Novelty: None |
ConceptID: Obs17
139
The copolymer saturation occurs at quite similar mS values for F68 monomers.
Type: Observation |
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ConceptID: Obs17
140
This means that about the same number of SDS molecules is bound to the pluronic.
Type: Result |
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Novelty: None |
ConceptID: Res17
141
A lesser number of SDS molecules may be bound to pluronics having a shorter PPO block (17 PO units).
Type: Result |
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ConceptID: Res17
142
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).
Type: Result |
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ConceptID: Res17
143
The same effects are prevailing for the molar transfer heat capacities (Fig. 7).
Type: Observation |
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Novelty: None |
ConceptID: Obs18
144
Further addition of SDS gives rise to more or less constant values of the transfer properties.
Type: Observation |
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Novelty: None |
ConceptID: Obs18
145
Two separate groups of ΔVP and ΔCP values are reported in Figs. 4 and 7, respectively.
Type: Observation |
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Novelty: None |
ConceptID: Obs18
146
They correspond to pluronics having about 17 PO and 30 PO repeat units.
Type: Observation |
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Novelty: None |
ConceptID: Obs18
147
Some values are reported in Table 2 at a given SDS molality.
Type: Observation |
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ConceptID: Obs18
148
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).
Type: Observation |
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ConceptID: Obs18
149
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.
Type: Observation |
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ConceptID: Obs19
150
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.
Type: Result |
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Novelty: None |
ConceptID: Res18
151
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.
Type: Result |
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ConceptID: Res19
152
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).
Type: Observation |
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ConceptID: Obs20
153
This may be due to an additive contribution of the interactions between the PEO chains and the SDS aggregates.34
Type: Result |
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Novelty: None |
ConceptID: Res20
154
No noticeable influence of the architecture of the copolymer on ΔVP is observed when L35 is compared with 10R5 (Fig. 6).
Type: Observation |
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ConceptID: Obs21
155
However, this may result from the small size of the different blocks giving rise to a more expanded conformation of the chain.
Type: Observation |
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ConceptID: Obs21
156
Likewise, the ΔVP values are independent on the pluronic concentration (cf. 1 and 2% L64 in Fig. 5).
Type: Observation |
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ConceptID: Obs22
157
As long as pluronics are exclusively in a monomeric state, ΔVP is not so dependent on the temperature.
Type: Observation |
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ConceptID: Obs22
158
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.
Type: Observation |
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Novelty: None |
ConceptID: Obs22
SDS/associated pluronic interactions
159
Negative values of the transfer properties of SDS and pluronics are generally observed when pluronics are existing in an aggregated state.
Type: Observation |
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ConceptID: Obs23
160
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).
Type: Observation |
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ConceptID: Obs23
161
Thereafter, in their negative part, they intercept at 0.015 mol kg–1, superimpose and level off to zero with increasing SDS concentration.
Type: Observation |
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ConceptID: Obs23
162
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.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs23
163
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).
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs24
164
For micellized P123 at 318.15 K, ΔVP decreases until mS is nearly equal to 0.04 mol kg–1 and then remains constant.
Type: Observation |
Advantage: None |
Novelty: None |
ConceptID: Obs24
165
When the pluronic is partly associated, a positive maximum may be observed at very low mS values.
Type: Observation |
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ConceptID: Obs24
166
This peak is dependent on the ratio monomer/micelle in solution.
Type: Observation |
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ConceptID: Obs24
167
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).
Type: Observation |
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ConceptID: Obs24
168
Tremendously large negative ΔCp values are observed for P123 (–28 000 J K mol–1, as shown in the insert of Fig. 7).
Type: Observation |
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ConceptID: Obs25
169
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.
Type: Result |
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ConceptID: Res21
170
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.
Type: Result |
Advantage: None |
Novelty: None |
ConceptID: Res20
171
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.
Type: Background |
Advantage: None |
Novelty: None |
ConceptID: Bac11
172
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.
Type: Result |
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Novelty: None |
ConceptID: Res22
173
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.
Type: Result |
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Novelty: None |
ConceptID: Res22
174
Afterwards, the binding process follows the same way than for an unassociated copolymer.
Type: Result |
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ConceptID: Res22
175
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.
Type: Result |
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ConceptID: Res5
176
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.
Type: Result |
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ConceptID: Res23
177
A positive contribution is assumed to be due to the strong attractive hydrophobic monomer–SDS interactions.
Type: Result |
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ConceptID: Res23
178
As seen in the first part, this contribution is mostly independent on the temperature.
Type: Result |
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ConceptID: Res23
179
A negative contribution may be ascribed to the interactions between the SDS molecules and the pluronic micelles.
Type: Result |
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ConceptID: Res23
180
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.
Type: Result |
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ConceptID: Res24
181
Consequently, this contribution appears to be related to the properties of rehydration of monomers and dependent on the equilibrium between monomers and micelles.
Type: Result |
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ConceptID: Res24
182
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.
Type: Result |
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ConceptID: Res5
183
When SDS is added beyond mS ≈ 0.04 mol kg–1, only monomers remain present.
Type: Observation |
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ConceptID: Obs26
184
Then, small SDS aggregates are bound on the copolymer.
Type: Observation |
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ConceptID: Obs26
185
Their aggregation number increases while ΔVP is mainly constant.
Type: Observation |
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ConceptID: Obs26
186
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).
Type: Observation |
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ConceptID: Obs26
187
Thus, the contribution arising from the disruption of L64 micelles can be estimated to be close to –80 cm3 mol–1.
Type: Result |
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ConceptID: Res25
188
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
Type: Result |
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ConceptID: Res25
189
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.
Type: Observation |
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ConceptID: Obs27
190
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.
Type: Hypothesis |
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Novelty: None |
ConceptID: Hyp2
Type: Observation |
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Novelty: None |
ConceptID: Obs28
192
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
Type: Background |
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ConceptID: Bac12
193
The ΔVP values plotted in Fig. 4 are nearly equal to –100 and –160 cm3 mol–1, respectively.
Type: Observation |
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Novelty: None |
ConceptID: Obs29
194
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.
Type: Result |
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Novelty: None |
ConceptID: Res26
195
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.
Type: Result |
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ConceptID: Res26
196
This value agrees well with that obtained from measurements carried out at 288.15 K, where P123 is only in monomeric state41.
Type: Result |
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Novelty: None |
ConceptID: Res26
Conclusion
197
The apparent molar volumes and heat capacities of some water/copolymer(pluronic)/surfactant systems have been investigated.
Type: Object |
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Novelty: New |
ConceptID: Obj1
198
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.
Type: Conclusion |
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ConceptID: Con1
199
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.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con2
200
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.
Type: Conclusion |
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ConceptID: Con2
201
Consequently, the transfer volumes are mainly linked to the PPO mass in the pluronic molecule.
Type: Conclusion |
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ConceptID: Con2
202
It was also found that they barely change with increasing temperature.
Type: Conclusion |
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Novelty: None |
ConceptID: Con3
203
A different behaviour is prevailing for the associated copolymers, for which negative values of transfer volumes are observed.
Type: Observation |
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Novelty: None |
ConceptID: Obs2
204
These should result from the rapid breakdown of pluronic micelles by SDS molecules, which form mixed aggregates until the pluronic micelles are fully disrupted.
Type: Result |
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ConceptID: Res5
205
The values of the molar transfer volumes of pluronics are interpreted as resulting from two contributions.
Type: Conclusion |
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Novelty: None |
ConceptID: Con4
206
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.
Type: Conclusion |
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Novelty: None |
ConceptID: Con4
207
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.
Type: Conclusion |
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ConceptID: Con4
208
Analysis of the corresponding molar heat capacities of transfer has revealed a similar behaviour.
Type: Conclusion |
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ConceptID: Con5
209
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.
Type: Conclusion |
Advantage: None |
Novelty: None |
ConceptID: Con5