Adsorption Properties

The effects of protonation using CF3COOH on adsorption onto the water-air surface or the water-micelle interface had been studied by means of the tensiometries.

Figure 5.26 and 5.27 show respectively the adsorption isotherms at the different protonation degrees and the change in the interfacial tension with X. The isotherms demonstrate synergism definitively: the CMC is reduced by the half protonation of C14DMAO, and moreover the amount of adsorption of the surfactant molecules becomes a maximum at X = 0.5 due to the densely packed molecules. The interaction

0.0 0.2 0.4 0.6 0.8 1.0

Figure 5.25. Changes in the plateau modulus G’ (■) and the characteristic relaxation time τ_R (○) as a function of protonation degree in (NaSCN+HCl)/

C₁₄DMAO system at 25 °C. The ratio of NaSCN/HCl is fixed at 3/7. The surfactant concen-tration is kept at 100 mM.

0.01 0.1 1 10

Figure 5.24. Rheograms for the solutions at different protonation degree X in (NaSCN+HCl)/C₁₄DMAO system at 25 °C. The ratio of NaSCN/HCl is fixed at 3/7. The surfactant concentration is kept at 100 mM. The symbols represent respectively the storage modulus (G’, ●), the loss modulus (G’’, ▲), and the complex viscosity (|η*|, ■).

X = 0.4

X = 0.7

between the molecules are calculated in terms of interaction parameter β, given by surfactants, C1 and C2 the mole concentrations of surfactant 1 and 2 in the bulk phase, C10 and C20 the mole concentrations of surfactant 1 and 2 respectively required at a given surface tension, and f1 the activity coefficient of surfactant 1 at surface. At σS = 35 mN/m, the β value at X = 0.5 is -1.35, and the composition of nonionic component in the surfactants is 0.495 in mole fraction. The two analogues seem to be mixed ideally on the surface. In the present case, Eq. [3.31] can be modified in the following form

( )

Γ is the surface excess amount of the component, and the superscription and the subscription represent the ionic sign and the component, respectively. Eq. [5.7],

0.01 0.1 1 curves for the solutions at different X’s in CF₃COOH/ decane as a function of protonation degree in CF₃COOH/C₁₄DMAO system. The surfactant concentration is kept at 1 mM.

furthermore, is rewritten by C1 = C2 = C,

( )

RT

dσ_S =− Γ₁+2Γ₂ ln [5.8]

The mixing composition X1 from Eq. [5.8] permits to calculate the individual Γ1 and Γ2.

2

And the surface are per molecule in the mixture system can be given by evolution of Eq.

[3.32].

The results are described in Table 5.6.

CMC gives a minimum at X = 0.5, while the average as calculated from Eq. [5.10]

is similar to that at X = 0 and much smaller than that at X = 1.0. Provided that additivity of the head group areas for the composition holds, the as at X = 0.5 should be 0.686 nm². The interacted surface area is indeed smaller than the ideal area. In view of CMC and as, it is conceivable that the non-protonated C14DMAO is the much more hydrophobic than the protonated one as usual, while, in spite of the general synergism, the plateau surface tension decreases uncommonly with increasing X, as well as the interfacial tension σI (Figure 5.27).

Figure 5.28 shows the plots of σI

versus the mixing ratio of two surfactants in the different surfactant systems. The mixture of analogues, C14DMAO/tetradecyl trimethylammonium bromide (C14TMABr), exhibits the upwards deviation from the ideality, while the composite of C14DMAO/sodium dodecyl sulfate (SDS) creates the strong synergistic depression of

0.0 0.2 0.4 0.6 0.8 1.0

0.1 1 10

mol fraction X of the ionic surfactant

C₁₄DMAO/C₁₄TMABr C₁₄DMAO/SDS C₁₄DMAO/HCl

σI [mN/m]

Figure 5.28. Interfacial tension against decane of the surfactant mixtures, C₁₄DMAO/C₁₄TMABr, C₁₄DMAO/SDS, and C₁₄DMAO/

HCl at 25 °C. The surfactant conc-entration is kept at 100 mM.

Table 5.6. CMC and surface area occupied per surfactant molecule (a_s) at different proto-nation degrees in CF₃COOH/C₁₄DMAO and HNO₃/ C₁₄DMAO systems.

σI. The synergistic phenomena, in terms of the surface or interfacial tension, can be explained by the preferable orientation of the neighboring surfactants: the charged amine group of C14DMABr is surrounded by three methyl side chains which would sterically prevent the interaction between its molecules or even with other surfactants molecules. Also the ion-dipole interaction would be centered on the oxygen atom of aminoxide group, and it may be speculated that this location might lead to distortions in the structure of bound water in the neighborhood of the positive charge of the nitrogen atom of aminoxide.¹⁶⁰ The release of bound water molecules would lead to an entropy gain for the aminoxide/cationic surfactant mixtures consistent with the calorimetry data on the dodecylphosphine oxide (C12PO)/dodecyl trimethylammonium bromide (C12TAB) systems.²¹¹ And, on the other, SDS head group is not shielded with any attached chain and the charged sulfate anion interacts directly with the positive portion of the dipole centered on N atom. From the suppression of the cross-sectional area as, the present case probably can be manifested by such ion-dipole interaction, however the monotonic decreases in the surface and interfacial tensions would be necessary to involve the effect of the charge profile normal to the aggregate surface. The protonated surfactant, C14DMAOH⁺CF3COO^-, is more hydrophobic than the non-protonated one, namely, the counter-ion is bounded strongly on the interface, and even may be immersed in the aggregates inside. Certainly, CF3COO^- ions have higher affinity to an oil relative to CH3COO^-.^207,212 Thus, the lyotropic acid causes simultaneously two effects on the surface potential. The complexity may relate to the viscosity curve with protonation degree.

As with, the surface and interfacial tension measurements were carried out in the nitric acid (HNO3) system, in which one can find typical synergism of the zero-shear viscosity η0 against protonation degree. The results are shown in Figure 5.29 and 5.30.

The surface tension curves at different protonation degrees are apparently similar to those in the CF3COOH system, and the CMC and as also show synergism (Table 5.6).

The interaction parameter and the composition on the surface are β = -1.18 and X1 = 0.5121, and the interaction between the two molecules is a little bit weaker relative to the CF3COOH system. The concentration of the non-protonated surfactant on the surface is greater than the protonated one. This indicates that CF3COO^- ion clearly

0.01 0.1 1

28 32 36 40 44 48 52

X = 0 X = 1.0 X = 0.5

σ S [mN/m]

C₁₄DMAO conc. [mM]

Figure 5.29. Surface tension curves for the solutions at different X’s in HNO₃/ C₁₄DMAO system.

enhances C14DMAOH⁺ to adsorb onto the air-water surface. Generally, the higher the hydrophobicity of one component in the mixture, the larger the interaction parameter β is. The tendency can be observed in the present system. The composition difference at the half-protonation may be related to the interaction as well. Although the surface tension decreases with increasing X for HNO3 as well as the CF3COOH system, σI

exhibits a minimum around X = 0.6 for HNO3 system. The behavior is remarkably different with the CF3COOH system, and this is particular evidence that CF3COO -behaves like co-surfactant rather than counter-ion. In Figure 5.30, σI in the HCl system are plotted against X, and protonation with HCl also shows synergistic effect on the interfacial property. The minimum of interfacial tension curve moves to lower protonation degree when the counter-ion changes from NO3- to Cl^-. It is likely that the composition X at the minimum interfacial tension is correlated to X1 and β, in other words, the Hofmeister series.