Effect of Alkane Chain Length and Counterion on the Freezing

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Effect of Alkane Chain Length and Counterion on the Freezing Transition of Cationic Surfactant Adsorbed Film at Alkane Mixture − Water Interfaces Yuhei Tokiwa,† Hiroyasu Sakamoto,‡ Takanori Takiue,§ Makoto Aratono,† and Hiroki Matsubara*,† †

Department of Chemistry, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan Department of Visual Communication Design, Faculty of Design, Kyushu University, Fukuoka 815-8540, Japan § Faculty of Arts and Sciences, Kyushu University, Fukuoka 819-0395, Japan ‡

ABSTRACT: Penetration of alkane molecules into the adsorbed film gives rise to a surface freezing transition of cationic surfactant at the alkane−water interface. To examine the effect of the alkane chain length and counterion on the surface freezing, we employed interfacial tensiometry and ellipsometry to study the interface of cetyltrimethylammonium bromide and cetyltrimethylammonium chloride aqueous solutions against dodecane, tetradecane, hexadecane, and their mixtures. Applying theoretical equations to the experimental results obtained, we found that the alkane molecules that have the same chain length as the surfactant adsorb preferentially into the surface freezing film. Furthermore, we demonstrated that the freezing transition temperature of cationic surfactant adsorbed film was independent of the kind of counterion.

1. INTRODUCTION The adsorbed film of amphiphiles have three different physical states called gaseous, expanded, and condensed states,1−9 which respectively correspond to two-dimensional gas, liquid, and solid phases. Phase transitions between these surface phases are thought to have great potential in industrial applications because the abrupt change in interfacial properties accompanied by surface phase transition may lead to a switching of the stability of colloidal systems composed of gas−liquid and liquid−liquid interfaces. However, in reality, such a study has not been implemented. One of the disadvantages of this idea is that the condensed film formation has been observed in most cases for nonionic, and in particular, sparingly water-soluble amphiphiles.1,2,4−9 This limitation is obviously crucial for the application of the surface phase transition to the stabilization of foam and emulsion systems in which higher water-solubility is generally required for surfactant molecules. For general surfactants, the electrically charged or hydration capable head groups give rise to their high water solubility, however, they simultaneously generate in-plane electric or hydration repulsions that hinder the formation of condensed film. A noteworthy report on a novel type of surface phase transition was proposed by the Bain group in this regard where they found that the adsorbed film of cetyltrimethylammonium bromide (CTAB) underwent a first-order freezing transition at the tetradecane−water interface upon cooling from interfacial tensiometry and ellipsometry.10 With the aid of the Gibbs adsorption equation, they found from their interfacial tension data that the interfacial density of CTAB did not change © 2015 American Chemical Society

significantly at the surface freezing transition. This suggests that this surface-freezing transition is induced by the van der Waals interaction between surfactant tail groups, and that alkane molecules penetrated into the CTAB adsorbed film. The generality of this surface-freezing transition was later confirmed for CTAB and stearyltrimethylammonium bromide (STAB) with alkanes of various chain length by using interfacial tension and X-ray reflectivity (XR) measurements.11,12 With increasing alkane chain length, the surface-freezing transition appeared first with tridecane and finally with hexadecane for CTAB where the surface-freezing temperature versus alkane chain length curve intersects with the bulk freezing temperature curve of alkanes. For STAB, the end point of surface-freezing was extended to octadecane, however, its starting point was also raised to tetradecane. Similar freezing transitions have also been observed for mixed CTAB-alkane monolayers at the solution− vapor interface.13−16 These findings suggest that only alkanes having similar chain length with cationic surfactant can induce the surface-freezing transition at temperatures above their melting points. The subject of the present study is to propose two additional insights about the surface-freezing transition of cationic surfactant at the oil−water interface. We have previously showed in our study of the interface between water and cyclohexane−benzene mixture that the adsorbed amount of benzene at the interface can be calculated relative to that of Received: March 13, 2015 Revised: May 1, 2015 Published: May 1, 2015 6235

DOI: 10.1021/acs.jpcb.5b02448 J. Phys. Chem. B 2015, 119, 6235−6241

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The Journal of Physical Chemistry B

Figure 1. Interfacial tension versus temperature curves for CTAB aqueous solutions against (a) C12−C14, (b) C12−C16, and (c) C14−C16 mixtures at m = 0.600 mmol kg−1. x2 = (1, red) 0, (2, yellow) 0.200, (3, yellow green) 0.400, (4, green) 0.600, (5, blue) 0.800, and (6, purple) 1. The inset shows the surface freezing temperature Teq versus x2 curve. In the panel (c), the melting point of bulk alkane mixture is shown by a thin solid curve.

2-2. Interfacial Tensiometry. Interfacial tension, γ, was determined by the analysis of the shape of a pendant drop of surfactant solution hanging on a glass capillary in the oil phase20 at varying temperatures, T, at a constant surfactant molality in an aqueous solution, m, and with mole fraction of alkanes in the oil phase, x2. The experimental error of the interfacial tension was within 0.1 mN m−1, which is smaller than the size of the symbols used for interfacial tension versus temperature curves shown in Figure 1. 2-3. Ellipsometry. Ellipsometric measurements were performed by a Picometer Ellipsometer (Beaglehole Instruments Wellington, NZ) equipped with a HeNe laser at 632.8 nm. The surfactant solution of about 20 mL was contained in a 6.5 cm diameter glass dish and the same amount of oil was poured onto it. The dish was placed in a brazen jacket then thermostated by circulating temperature-controlled water. In order to attain temperature equilibrium, the temperature of the circulating water was kept constant at least 30 min before each measurement. The coefficient of ellipticity, ρ̅, defined as the imaginary part of rp/rs at the Brewster angle was recorded (rp and rs are the complex Fresnel reflection coefficients for p- and s-polarized light). A laser beam was applied to the air−alkane interface at ∼75°, and its refracted light hit the alkane−water interface at an incident angle of ∼43°, which is the Brewster angle of the alkane−water interface. The experimental error of the ρ̅ was again very small compared with the size of the symbols in Figure 4.

cyclohexane, from the variation of interfacial tension against the bulk composition of the oil phase.17 By applying the same experimental procedure to the mixture of n-dodecane (C12), ntetradecane (C14), and n-hexadecane (C16) in the presence of CTAB, we will later discuss the van der Waals interaction between CTAB and alkanes from the viewpoint of the preferential adsorption of alkanes at the oil−water interface both in their surface frozen and surface liquid states. Another interesting issue that remains unsolved is the counterion effect on the surface phase transition of cationic surfactant adsorbed film. It is well-known that the physical behavior of ionic surfactant systems are greatly influenced by their counterions. For example, the critical micelle concentration (cmc) of CTAB (0.94 mmol kg−1 at 30 °C)18 increases about 40% by exchanging its counterion to Cl − , and the cmc of cetyltrimethylammonium chloride (CTAC) is ∼1.30 mmol kg−1.18 Similarly, the Krafft temperature of CTAB (25 °C)18 decreases to 2 °C for CTAC.19 In the latter half of this paper, we will examine the counterion effects on surface freezing by comparing the surface freezing temperatures of the CTAB and CTAC systems. At the end of this paper, we would like to consider a possible experimental extension of surface freezing transition to control emulsion stability based on the results of this study.

2. EXPERIMENTAL SECTION 2-1. Materials. CTAB was purchased from Nacalai tesque Co Ltd. (>99%) and recrystallized eight times from a mixture of acetone and ethanol (4:1 v/v ratio). Oil-soluble impurities were removed by extraction six times from hexane. CTAC was purchased from Tokyo Chemical Industry Co Ltd. (95%) and recrystallized six times from a mixture of acetone and ethanol (9:1 v/v ratio). These purities were confirmed by the absence of a minimum on the surface tension vs molality curves around their critical micelle concentration (cmc). All surfactant solutions used in this study were prepared with Milli-Q water. C12 (>99%), C14 (>99%) and C16 (>98%) were also purchased from Nacalai tesque Co Ltd. and purified by fractional distillation under reduced pressure. Their purities were checked by measuring the oil−water interfacial tensions.

3. RESULTS AND DISCUSSIONS Figure 1 shows the interfacial tension measured against temperature at fixed m and x2 for (a) C12−C14, (b) C12− C16, and (c) C14−C16 systems. In the C12−C14 system, all curves have a break point, and in the C12−C16 and C14−C16 systems, it was observed up to x2 = 0.4 and x2 = 0.6, respectively. The interfacial tension decreased gradually with decreasing T above these break points and rather rapidly below them. The slope of γ versus T curves gives the entropy change associated with the adsorption of CTAB, ΔsH, calculated by the following equation:17,21 6236

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Figure 2. (a) Interfacial density versus molality curves for the C6−water interface with (1) icosanol and (2) 2-perfuluorooctylethanol (FC10OH) and for the C14−water interface with CTAB at (3) surface liquid and (4) surface frozen states. (b) Entropy change associated with adsorption versus temperature curves for the C6−water interface with (1) octadecanol and (2) FC10OH, and (3) for the C14−water interface with CTAB.

⎛ ∂γ ⎞ Δs H = −⎜ ⎟ = s H − (Γ oHso + Γ sHss) ⎝ ∂T ⎠ p , m , x 2

surface freezing at the C16−water interface was previously denied10 and confirmed;12 however, judging from the systematical variation in Teq with x2 observed in the present study, the observation of surface freezing at the C16−water interface only seems possible in its supercooling state. Figure 2a shows the interfacial density of CTAB, ΓHs , versus m curves calculated for the C14−water interface at 18 and 8 °C. These temperatures respectively correspond to the surface liquid and surface frozen states of the system. The interfacial density of alkanol and fluorinated alcohol at the hexane−water interface are shown together by way of comparison.23 In Figure 2b, the temperature dependence of ΔsH is also compared in the same systems.1,24 It is now very clear that the surface frozen state observed here is as an expanded state from the viewpoint of the interfacial density of CTAB, but is highly ordered on the same level as the condensed film of fluorinated alcohol. The absolute value of ΔsH in the condensed state strongly depends on the nature of molecular interaction between adsorbed molecules. For alkanols, the absolute values of ΔsH are about twice those obtained for fluorinated alkanols and the surface frozen state of the CTAB system. This is because the hydrogen bonding between alkanols leads to an additional entropy decrease, as does the ordering of hydrocarbon chains by the lateral van der Waals interactions. This hydrogen bonding contribution becomes much weaker for fluoroalkanols because their large cross sectional area (0.3 nm2) is about 1.5 times of that of alkanols (0.2 nm2). Therefore, the similarity in ΔsH between the surface frozen state of CTAB and the condensed film of fluoroalkanols implies that the origin of negative ΔsH in the surface frozen state mainly comes from the ordering of hydrocarbon chains of CTAB and C14 in the adsorbed film due to the lateral van der Waals interaction. To elucidate the lateral orientation of molecules at the interface, the grazing incidence X-ray diffraction (GIXD) is most useful; however, as pointed out by Tamam et al., the application of GIXD to the oil−water interface is technically difficult because of the highly diffused background from the liquid bulk.12 Due to the similarity of ΔsH values, Bain et al. suggested that the structure of surface frozen film at the oil− water interface is similar to that at the alkane−air interface in which the chains are hexagonally packed in an all-trans

(1)

where sH is the interfacial excess entropy and si and ΓHi are the partial molar entropy and interfacial densities of oil (i = o) and surfactant (i = s), respectively. We found that ΔsH is ≈−0.1 mJ K−1 m−2 and ≈−0.8 mJ K−1 m−2, respectively, above and below the break point in these systems. Considering that ΔsH indicates the difference in partial molar entropy of components in the interface and the bulk phase, it is likely that the ordering of molecules is enhanced more in the adsorbed film than in the bulk phase when the ΔsH has a negative value. Therefore, the near zero ΔsH value in the high T region means that the extent of the ordering of molecules in the adsorbed film is similar to that in the bulk phase. On the contrary, the large negative ΔsH value in the low T region indicates that the adsorbed film is highly ordered compared with the bulk phase. The ΔsH value obtained in the low T region is almost identical with that obtained for CTAB adsorbed film at the water−C14 interface (−0.76 mJ K−1 m−2) reported by the Bain group10 and therefore, it is possible that we observed the same surface freezing transition in our systems. Two new results obtained here were as follows. First, we demonstrated that the surface freezing transition occurred at the C12−water interface at 2.7 °C as shown in Figure 1a,b. This was not examined in Tamam’s paper mentioned above;12 however, the difference between the surface freezing transition temperature, Teq, and the melting point of the water phase was very small and the explicit determination of the ΔsH value was rather difficult. Second, we found that the surface freezing transition temperature rose by increasing the composition of longer alkanes in the bulk oil phases (see Figure 1 insets). This suggests that the lateral van der Waals interaction becomes stronger when the difference in chain length between CTAB and alkane molecules becomes smaller due to the mixing of longer chain alkanes in the adsorbed film. In Figure 1c, the melting point of bulk alkane mixture is shown as a thin solid line.22 It is clear from the relationship between the two curves that the surface freezing transition of the adsorbed film of alkane mixtures, including C16, is interrupted by the freezing of the bulk alkane mixture at around x2 = 0.8. The existence of 6237

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Figure 3. Relative amount of longer alkane component in the surface liquid (filled circles) and surface frozen (open circles) films as a function of x2: (a) C12−C14, (b) C12−C16, and (c) C14−C16 mixtures.

Figure 4. Ellipticity versus temperature curves for CTAB aqueous solutions against (a) C12−C14, (b) C12−C16, and (c) C14−C16 mixtures at m = 0.600 mmol kg−1. x2 = (1, red) 0, (2, yellow) 0.200, (3, yellow green) 0.400, (4, green) 0.600, (5, blue) 0.800, and (6, purple) 1.

component.17,25 Therefore, it should be noted that ΓH2 does not express the real interfacial density of the alkanes. However, ΓH2 is still useful as an intuitive indicator to show which one of two solvent components is included preferentially in the adsorbed film. Figure 3 shows ΓH2 values obtained in the present study. The fact that ΓH2 values of the three systems are almost zero in the surface liquid state, suggests that there is no preferential adsorption of alkane molecules in these cases. That is, the composition of the two alkane species in the adsorbed film is almost the same as that in the bulk phase. Furthermore, from the fact that ΓH2 has a positive value in the surface frozen state in all systems, we can deduce that longer alkane species are adsorbed preferentially at the alkane mixture−water interface. As mentioned above, CTAB does not form surface frozen film at the pure C16−water interface because the bulk phase is frozen prior to the surface freezing transition. However, this thermodynamic analysis shows that the C16 molecules penetrate into the surface frozen film preferentially compared to other alkanes in alkane mixed systems. This indicates that the lateral van der Waals interaction is maximized when the alkane chain length is the same as that of the

conformation same as the rotator II phase of bulk alkane.10 Furthermore, Tamam et al. suggested from the interfacial tension and XR measurements that the surface frozen film of STAB at the C17−water interface first adopts the rotator phase, then undergoes a second transition to the full crystalline state. The change in the slope of γ versus T curve for this second transition was only 0.3 mJ K−1 m−2 and was not apparent in our experiments.12 More detailed measurements are probably necessary to detect such a small break on the γ versus T curves. Next let us consider the preferential adsorption of alkane at the interface by applying17,25 Γ 2H = −

x1x 2 ⎛ ∂γ ⎞ ⎟ ⎜ RT ⎝ ∂x 2 ⎠T , p , m

(2)

to the γ versus x2 curves, obtained from Figure 1. Here, ΓH2 is defined with respect to the two dividing planes chosen to make the excess numbers of moles of water, and the summation of the two kinds of alkanes are simultaneously zero. This shows the interfacial density of one of the alkanes in the mixture relative to the interfacial density of the other alkane 6238

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and εo = 2.12 used for our calculation were taken from Bain et al.32 The obtained ρ̅c values roughly coincide with Δρ̅ of curve No.1 (−3.7 × 10−3) and 6 (−4.73 × 10−3) in panel a, and No. 4 (−5.22 × 10−3) in panel c. The coincidence between these values also gives evidence of the formation of surface frozen film in the present system. In this important finding, it should be noted that, especially in the C12−C14 and C12−C16 systems, the Δρ̅ decreases considerably in the alkane mixture when the x2 changes from 0 to 0.2 but it decreases only slightly for the further change in x2. In the C12−C14 system, the values of Δρ̅ obtained at x2 > 0.2 are almost the same as that obtained for the pure C14 interface. Considering that the interfacial density of CTAB is not increased significantly by the freezing transition, we can conclude that the observed decrement in Δρ̅ is attributable to the increment in the composition of longer alkane components in the adsorbed film. Moreover, the independence of Δρ̅ above x2 > 0.2 suggests that the composition of surface frozen film hardly changes in this region. Taking account of all these findings, the surface frozen film consists almost only of CTAB and longer alkane components in the C12−C14 and C12−C16 systems even when the bulk oil phase contains only 20% of the longer alkane component. This trend becomes weaker in the C14−C16 system probably because the difference in the lateral van der Waals interaction with CTAB molecules is rather small for these alkanes. This confirms the finding that the extent of preferential adsorption of alkane is smallest in the C14−C16 system. Finally, we will discuss the effect of interfacial density on the surface freezing transition temperature for the C14−water interface in the presence of CTAB and CTAC (Figure 5). The

surfactant. This idea is also confirmed from the finding that the magnitude of preferential adsorption becomes larger as the chain length gap between the two alkanes increases from the C12−C14 to the C12−C16 systems. Due to the stronger lateral van der Waals interaction between CTAB and C16 molecules, we think that C12 and C14 molecules are expelled from the interface at the surface frozen transition. In order to observe this issue more closely, we also performed ellipsometry at the alkane mixture−water interface. Figure 4 shows the coefficient of ellipticity as a function of T at m = 0.600 mmol kg−1. There are discontinuous changes corresponding to the surface freezing transition. In the surface liquid state at high T region, the ρ̅ is almost independent of x2. On the other hand, in the C12−C14 and C12−C16 systems, the ρ̅ decreases in the alkane mixture in the surface frozen state at low T region, while the x2 changes considerably from 0 to 0.2, but ρ̅ decreases only slightly for any further change in x2. In the C12−C14 system, the values of ρ̅ obtained at x2 > 0.2 are almost the same as those obtained for the pure C14 interface. A detailed analysis of the ellipsometry data was carried out by Bain et al.10,26 Here, we would like to discuss the qualitative features that can be deduced from our ellipsometric results. The ρ̅ is expressed by the Drude equation10 ρ̅ =

π ε1 + ε2 η λ (ε1 − ε2)

(3)

where λ is the wavelength of light and ε1 and ε2 are the dielectric constants of alkane and water and η is the ellipsometric parameter. For a thin isotropic monolayer of thickness d and permittivity ε, η is calculated as follows:26−31 η=

(ε − ε1)(ε − ε2) d ε

(4)

The hydrocarbon chain in the surface liquid state, the trimethylammonium headgroup, and the counterion layers are regarded as isotropic layers and have individual ε and d. For the optically uniaxial layer, the hydrocarbon chain layer in the surface frozen state, the η is rewritten as10,31 ⎡ (ε − ε1)(εe − ε2) ⎤ + (εo − εe)⎥d η=⎢ e εe ⎣ ⎦

(5)

in which the dielectric constants for perpendicular (εe) and parallel (εo) to the interface are taken into account. In addition, there is a negative contribution to ρ̅ arising from the capillary wave roughness of the interface; however, if we consider the difference in ρ̅ (Δρ̅) at the surface freezing temperature, this contribution disappears between the surface liquid and frozen states, because the amplitude of the capillary wave is determined by the square root of interfacial tension and temperature at the experimental point considered.32 The contribution from the headgroup and the counterion layers is also identical between the two interfaces because the interfacial density only slightly changes at the freezing transition (see Figure 2a). Furthermore, the hydrocarbon chain contribution from the surface liquid state is negligible because the ε is almost equal to that of the bulk alkane (ε1). As a result, the Δρ̅ is determined almost solely by the contribution from the optically uniaxial hydrocarbon chain layer in the surface frozen state, ρ̅c. By using eqs 3 and 5, we obtained ρ̅c = −4.15 × 10−3, −4.69 × 10−3, and −5.24 × 10−3, respectively, for the frozen monolayer of C12, C14, and C16 molecules. The εe = 2.23

Figure 5. Surface freezing temperature versus interfacial density curves for the C14−water interface with (1) CTAB and (2) CTAC.

Teq rises linearly with increasing ΓHs both in the CTAB and CTAC systems. If one extrapolates these data to the cmc, the highest temperature for the occurrence of the surface freezing transition would be 11.5 and 9.5 °C for the CTAB and CTAC systems, respectively. The difference between the two curves is almost constant and is only about 1−2 °C in the whole ΓHs range. The observed small surface freezing temperature difference is in striking contrast to the large difference between the Krafft temperatures of these surfactants (about 25 °C). This result is also related to the physical mechanism of the surface 6239

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The Journal of Physical Chemistry B freezing transition. In the surface frozen films, the distance between cationic surfactants is still as large as that in the expanded films; therefore, the electric repulsion between head groups is not very influential in the surface frozen films compared to the solid hydrates of the same cationic surfactants. In this paper, we showed that the surface freezing transition could be realized at the interfaces of C12−C14, C12−C16, and C14−C16 mixtures against CTAB aqueous solutions. The most unique features of the surface freezing transition is that the interfacial density of surfactants is kept almost constant at the surface freezing transition. This is because the hydrocarbon chain density required for the surface frozen film formation is attained by the penetration of alkane molecules into the adsorbed film. In the alkane mixture systems, the alkanes that have chain length similar to that of CTAB adsorb preferentially to the oil−water interface accompanied by the surface freezing transition; nevertheless, there is no detectable difference in the adsorbed amount of different alkanes in the surface liquid state. The ellipsometric measurements at the oil−water interfaces suggested that the preferential adsorption of alkanes becomes very significant for the C12−C14 and C12−C16 mixtures. In these cases, it was expected that the surface frozen film was almost completely composed of CTAB, and longer chain alkanes and shorter alkanes were expelled from the interface to the bulk oil phases. We believe that this finding gives us a new experimental approach to stabilize oil-in-water emulsions because there is a reasonable prospect that the surface freezing transition is likely to occur widely for systems containing only a small amount of alkane components of proper chain length in the bulk. Similarly, such a preferential adsorption is also expected in mixed surfactant systems with a given alkane. Furthermore, it was also revealed that the effect of counterions on the surface freezing temperature was suppressed considerably by the mixing of alkanes in the adsorbed film. Considering that the Krafft temperature is largely decreased by exchanging surfactant counterion from Br− to Cl−, the temperature range of surface frozen film may increase significantly with the use of CTAC. Relevant experiments are now ongoing.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Cosmetology research grant (2013-2014) and by the Grant-in-Aid for Scientific Research (C) of the Japan Society for the Promotion of Science (No. 00372748). We thank Nicholas Shillingford and the Front Researcher Program for help editing this paper.

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REFERENCES

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DOI: 10.1021/acs.jpcb.5b02448 J. Phys. Chem. B 2015, 119, 6235−6241

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DOI: 10.1021/acs.jpcb.5b02448 J. Phys. Chem. B 2015, 119, 6235−6241