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Adsorption of Acetylenic Diol-Based Nonionic Surfactants on Lampblack and Phthalocyanine Blue Pigment
Journal of Colloid and Interface Science 256, 91–99 (2002) doi:10.1006/jcis.2001.7987
Adsorption of Acetylenic Diol-Based Nonionic Surfactants on Lampblack and Phthalocyanine Blue Pigment S. W. Musselman and S. Chander1 Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, 123 Hosler Building, University Park, Pennsylvania 16802 Received June 4, 2001; accepted September 20, 2001; published online November 9, 2001
out blending. The adsorption of surfactants at the solid/liquid interface is strongly influenced by a number of factors including the nature of the structural groups on the solid surface, the molecular structure of the surfactant, and the environment of the aqueous phase (pH, temperature, electrolyte content, etc.). Together these factors determine the mechanism by which adsorption occurs and the efficiency and effectiveness of adsorption (2). Three surfactant types, namely, octylphenol-, nonylphenol-, and acetylenic diol-based surfactants were used in this investigation to delineate the influence of molecular structure on surfactant adsorption by hydrophobic surfaces.
The adsorption of acetylenic diol based surfactants on lampblack and phthalocyanine blue has been studied to determine the mechanisms of surfactant adsorption and the orientation of these molecules at hydrophobic surfaces. The TMDD series of reagents, commonly known as acetylenic diol-based surfactants, are derivatives of 2,4,7,9-tetramethyl-5-decyne-4,7-diol. While studies of conventional nonionic surfactants have included alkyl ethoxylates and ethoxylated alkyl phenol surfactants, limited information is available for acetylenic diol-based surfactants. The effect of the degree of ethoxylation of these surfactants was determined. Although the Langmuir model could be used to fit adsorption data in many cases, further analysis of the data showed that the free-energy per molecule was a function of adsorption density. The change in free energy with adsorption density could be interpreted by considering a change in the orientation of the adsorbed molecules. Acetylenic diol-based surfactants were found to have an area per molecule of two to three times that of similarly ethoxylated alkyl phenols. The area per molecule was found to increase linearly with increased ethoxylation within each family of surfactants. C 2002 Elsevier Science (USA) Key Words: adsorption; pigment; lampblack; surfactant; phthalocyanine blue; hydrophobic.
MATERIALS AND METHODS
The nonionic surfactants used in this study were obtained from various sources: TMDD series of reagents from Air Products and Chemicals, Inc., Triton X-100 from Rohm and Haas, and Tergitol NP-9 and NP-15 from Union Carbide. The TMDD series of reagents, commonly known as acetylenic diol-based surfactants, are derivatives of 2,4,7,9-tetramethyl-5-decyne-4,7diol. Ethoxylated versions of this molecule containing 0, 3.5, 10, 20, and 30 mol of ethylene oxide per molecule were used. Some of these surfactants are commercially available as wetting and defoaming agents under the Surfynol series manufactured by Air Products and Chemicals. Triton X-100 is a branched chain octylphenol with 9.5 mol of ethylene oxide per molecule. The Tergitol NP-9 and Tergitol NP-15 are nonylphenols with 9 and 15 mol of ethylene oxide per molecule, respectively. Although nonylphenol is commonly depicted with a linear alkyl chain, NMR spectroscopy studies show it to be a branched chain. Selected properties of these nonionic surfactants are given in Table 1. Structural formulae of selected reagents are given in Fig. 1. Lampblack 101, manufactured by Degussa, was used as a model hydrophobic surface. Carbon blacks contain carbon atoms arranged in graphitic layers, which are continuous from one particle to the next (3). Therefore, these carbon structures exist as aggregates in solution. Phthalocyanine blue 15 : 3 pigment with a molecular structure of C32 H16 CuN8 , illustrated in Fig. 2 (4), obtained from Sun Chemical Co., was used as a model of a complex pigment that might consist of hydrophobic and hydrophilic sites. It has been
INTRODUCTION
The adsorption of nonionic surfactants on solid surfaces from aqueous solutions is important for controlling many properties including wetting, dispersion, and rheology. The ability of surfactants to modify interfacial properties even at low bulk concentrations has led to their widespread use in numerous applications in the mineral, environmental, pharmaceutical, biological, agricultural, cosmetic, textile, and coatings industries. The growth of the nonionic surfactant industry in recent decades was aided by an increased environmental concern over the nonbiodegradable nature of most anionic surfactants (1). Nonionic surfactants are widely used for technical reasons such as the ease of varying surfactant properties. Simply changing the length of the polyoxyethylene group can modify the properties of many nonionic surfactants. These surfactants can also be mixed with other reagents to enhance properties that are not easily attained with1 To whom correspondence should be addressed. Fax: (814) 865-3248. E-mail: [email protected].
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0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
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TABLE 1 Properties of Surfactants Used in This Study
Reagent
Surfynol 104 Surfynol 440 Surfynol 465 Surfynol 480 Surfynol 485 Dynol 604 Triton X-100 Tergitol NP-9 Tergitol NP-15
Formula
Molecular CMC weight (mmol/L)
Acetylenic diols C14 H24 (OH)2 226 380 C14 H24 (OH)2 EO3.5 C14 H24 (OH)2 EO10 666 1106 C14 H24 (OH)2 EO20 1546 C14 H24 (OH)2 EO30 NA NA Alkylphenols C8 H17 EO9.5 624 C9 H19 EO9 616 880 C9 H19 EO15
N/A N/A 10.5 N/A 11.2 NA 0.22 0.078 0.093
Surface tension (dynes/cm) 0.1% @ 25◦ C
33.1 33.2 41.9 51.1 NA 31 30 36
Note. NA, not available.
shown that phthalocyanine pigments contain crystals as aggregates, which are grown together along crystallographic lattice directions (4). Total organic carbon (TOC) analysis showed that there were very small amounts of a carbon-based substance (<1–2 ppm) leaching from the phthalocyanine blue surface when dispersed in water. According to the manufacturer, the phthalocyanine blue sample has approximately 3–5% impurities in the form of short chain glycols, inorganic salts, and other
FIG. 2.
Chemical structure of phthalocyanine blue 15 : 3 (C32 H16 CuN8 ).
compounds produced in the ring closure process for manufacturing phthalocyanine blue. Adsorption studies were conducted by dispersing 200 mg of the lampblack in 20 mL of known concentrations of the nonionic surfactants in aqueous solutions at 25◦ C. The particles were dispersed using a Branson ultrasonic bath for 3 min. The solutions were filtered and analyzed using a Shimadzu TOC-5000A carbon analyzer. The adsorption density of the surfactants on the solid was calculated by using the following expression: = ((C0 − Cf )V )/(m SA ),
[1]
where C0 and Cf are the initial and final (equilibrium) concentrations of the surfactant in solution (micromol/L), V is the solution volume (L), m is the mass of adsorbent (g), and SA is the BET surface area of the sorbent (m2 /g). RESULTS AND DISCUSSION
Adsorption on Lampblack The adsorption isotherms for the acetylenic diol based and alkylphenol surfactants are presented in Figs. 3 and 4, respectively. The adsorption experiments show a clear dependence of adsorption density on the degree of ethoxylation. A decrease in the plateau region with an increase in the ethylene oxide content for each family of surfactants is evident. The data was fitted to the Langmuir model, written in its linear form as C/ = 1/(m K ) + C/ m ,
FIG. 1.
Chemical structure of selected surfactants used in this investigation.
[2]
where C is the concentration, is the adsorption density, m is the adsorption density at monolayer coverage, and K is an equilibrium constant. A plot of C/ versus C thus allows the calculation of m and K . The Langmuir plots for TMDD series of surfactants are presented in Fig. 5. Two important assumptions that are made in the derivation of this equation which are not met when considering surfactant adsorption from aqueous adsorption are that the solute and solvent molecules occupy the same surface area and that there is no interaction between the
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ADSORPTION OF ACETYLENIC DIOL-BASED SURFACTANTS
FIG. 3. Adsorption of acetylenic diol-based surfactants on lampblack. The solid lines are Langmuir fits to the data.
solute and solvent on the surface. It has been shown that the invalidity of these assumptions causes deviations from the Langmuir equation that are in opposite directions to one another and therefore the equation is obeyed due to a compensation of the two errors (5). The adsorption density at monolayer coverage and area per molecule for each surfactant is presented in Table 2. The calculated areas per molecule for these surfactants at the solid/liquid
FIG. 5. Langmuir model for the adsorption of acetylenic diol-based surfactants on lampblack.
interface are similar to the values at the liquid/vapor interface. This indicates that the adsorption of nonionic surfactants from solution onto hydrophobic surfaces results in a molecular orientation much like the liquid/vapor interface, which is characterized by a vertical configuration of the molecules. The hydrophobic part of the surfactant chain is adsorbed vertically onto the hydrophobic surface of the lampblack particles, leaving the bulky, hydrated, hydrophilic part of the molecule extending into the solution. Calculated areas per molecule for an octylphenol with 9 and 10 mol of ethylene oxide per molecule adsorbing in a vertical orientation are 0.56 and 0.62 nm2 (6), respectively. These numbers agree quite well with the results of the Triton X-100 (0.58 nm2 ). The calculated values for the area per molecule of a nonylphenol with 9 and 10 mol of ethylene TABLE 2 Adsorption Density and Area per Molecule for Acetylenic Diol and Alkylphenol-Based Surfactants Reagent
Surfynol 104 Surfynol 440 Surfynol 465 Surfynol 480 Surfynol 485 Dynol 604
FIG. 4. Adsorption isotherms of alkylphenol surfactants on lampblack. The solid lines are Langmuir fits to the data.
Triton X-100 Tergitol NP-9 Tergitol NP-15
Formula Acetylenic diols C14 H24 (OH)2 C14 H24 (OH)2 EO3.5 C14 H24 (OH)2 EO10 C14 H24 (OH)2 EO20 C14 H24 (OH)2 EO30 Proprietary Alkylphenols C8 H17 EO9.5 C9 H19 EO9 C9 H19 EO15
m (µmol/m2 )
Am (nm2 )
2.65 2.10 1.40 1.28 0.89 2.21
0.63 0.79 1.19 1.30 1.87 0.75
2.88 3.18 1.97
0.58 0.52 0.84
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FIG. 6. groups.
MUSSELMAN AND CHANDER
Area per molecule as a function of the number of ethylene oxide
oxide per molecule adsorbing in a horizontal plane parallel to the particle surface are 2.44 and 2.70 nm2 (6), respectively. The nonylphenols used in this study (Tergitol NP-9 and Tergitol NP15) offer further proof that the adsorption of these nonionic surfactants occurs in a vertical orientation, as their areas per molecule are approximately equal to that for a vertically oriented layer. The area per molecule as a function of the number of ethylene oxide groups is presented in Fig. 6. The data at 0 mol of ethylene oxide per molecule for the Tergitol series has been taken as 0.22 nm2 from a close packed layer of hydrocarbons and verified by the extrapolation of the data of Kronberg et al. (5). Although there is scatter evident in the data, it is reasonable to suggest that the straight line fits for the Tergitol and Surfynol series of surfactants are parallel, thereby indicating a systematic increase in area per molecule at hydrophobic surfaces with increased ethoxylation. The results show that the Surfynol reagents occupy a significantly greater area at the solid/liquid interface when compared to similarly ethoxylated octyl and nonylphenol surfactants. The bulky shape of the hydrophobic part of the acetylenic diol based molecule is most likely the reason that Surfynol molecules occupy approximately two to three times the area at the solid/liquid interface when compared to similarly ethoxylated alkyl phenols. In contrast, the hydrophobic part in alkylphenols is relatively linear which allows these reagents to pack closer together. The area per molecule for the nonylphenol was slightly smaller than the corresponding octylphenol. These results show that the shape of the hydrophobic portion of the molecule and the size of the hydrocarbon chain determines adsorption density at monolayer coverage. Although comparably ethoxylated alkylphenol reagents show a higher adsorption density than acetylenic diol-based surfactants at monolayer cover-
age, in some applications it may be advantageous to have a low adsorbing reagent. These advantages are discussed in a later section of this paper. The dependence of area per molecule on the moles of ethylene oxide per molecule for nonylphenols adsorbing on various surfaces in other studies is shown in Fig. 7. The area per molecule results from this study appears to agree quite well with results published by other investigators (5, 7). The studies by Abe and Kuno (8) and Narkis and Ben-David (9) show a significant deviation from the other results. A possible explanation for this discrepancy could be due to the large surface area of furnace black and powdered activated carbon that were used in these studies. Surface areas determined by nitrogen adsorption can lead to errors when calculating surfactant adsorption densities if pores are fine. Surfactants cannot access all pore spaces that are available to a nitrogen molecule due to physical size constraints. This results in a much smaller surface area available for surfactant adsorption and causes errors when determining adsorption densities. The results from this work and other studies (7, 10) suggest that there is a linear dependence of area per molecule on ethylene oxide content. At low ethylene oxide contents, the area per molecule at the solid/liquid interface is similar to the liquid/ vapor interface. As ethylene oxide content is increased, the area per molecule at the solid/liquid interface becomes significantly
FIG. 7.
Area per molecule for nonylphenols adsorbing on various surfaces.
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greater. This suggests that the packing of ethylene oxide at these two interfaces is different. A possible reason for these differences can be explained by considering the forces that exist on the surfactant molecules at the interfaces. The steric repulsive force that exists between ethylene oxide chains and the attractive forces between hydrocarbon chains govern the packing of ethoxylated surfactants. At the liquid/vapor interface, the hydrocarbon chains are expelled from the solution and are separated by air. This leads to a large attractive force between the hydrocarbon chains, and they pack close together in monolayer formation with the ethylene oxide groups extended into solution. At the solid/liquid interface, water is not completely expelled from the adsorbed surfactant layer. The presence of water reduces the attractive force between the hydrocarbon chains by reducing the effective Hamaker constant. Under these circumstances, the surfactant molecules are not packed as tightly. The ethylene oxide groups are still extended into solution, but are now free to coil, thus preventing the close packing that is observed at the liquid/vapor interface. Adsorption on Phthalocyanine Blue The Freundlich plots for adsorption of acetylenic diol and alkylphenol on phthalocyanine blue are presented in Figs. 8 and 9, respectively. Two phthalocyanine blue samples were used. The second sample gave slightly higher amount of organics leached into water (approximately 1–2 ppm of carbon) but it seemed to have no effect on adsorption. Experiments were conducted on both powder samples with Surfynol 465. No significant differences in the adsorption data were observed. All studies were made using the second sample of phthalocyanine blue with the exception of Surfynol 104.
FIG. 8. blue.
Adsorption of acetylenic diol-based surfactants on phthalocyanine
FIG. 9.
Adsorption of alkylphenol surfactants on phthalocyanine blue.
The Freundlich equation is used to describe adsorption onto heterogeneous surfaces and is expressed in the following form: θ=
1 = kC n , m
[3]
where θ is fractional coverage, is adsorption density, m is adsorption density at monolayer coverage, C is concentration, and k and n are constants. The adsorption data for Dynol 604 was also fitted to the Freundlich equation. For each surfactant there is a break in the straight line fit of this equation. Other studies have shown that this type of break in the data can be due to a change in the diffusion mechanism responsible for the rate-controlling step in the adsorption process (11). Adsorption on phthalocyanine blue is less at low concentrations when compared to lampblack. This is likely due to structural differences between the two surfaces and the presence of surface-active impurities on the phthalocyanine blue. Because of these differences, a Langmuir equation better fits adsorption on lampblack while Freundlich equation is better for phthalocyanine blue. A decrease in the plateau region with increasing ethylene oxide content was observed in a manner similar to the results with lampblack. The adsorption densities at monolayer coverage are compared in Table 3 for phthalocyanine blue and lampblack. Similar results were obtained for both surfaces with the exception of Surfynol 480 and 485. Although a definitive reason for these results is not easy to obtain, several possibilities exist. The larger, less surface-active molecules may not be able to displace the impurities present on the surface. It is also possible that these larger surfactant molecules are unable to enter some of the pore space in phthalocyanine blue. Phthalocyanine blue has three times the surface area of the lampblack powder and
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TABLE 3 Adsorption Densities at Monolayer Coverage for Phthalocyanine Blue and Lampblack Reagent
Surfynol 104 Surfynol 440 Surfynol 465 Surfynol 480 Surfynol 485 Triton X-100 Tergitol NP-9 Tergitol NP-15
M for Phthalocyanine blue (µmol/m2 ) Acetylenic diols 2.90 2.10 1.75 0.83 0.52 Alkylphenols 2.80 3.10 1.78
m for Lampblack (µmol/m2 )
2.65 2.10 1.40 1.28 0.89 2.88 3.18 1.97
likely contains micropores that are too small to be accessed by Surfynol 480 and Surfynol 485, thereby causing a reduction in apparent adsorption density. Thermodynamics of Adsorption The free energy for the adsorption of nonionic surfactants on hydrophobic surfaces is likely to include contributions from the hydrophobic interaction between the surfactant molecule and the solid surface, the chain interactions between the adsorbed surfactant molecules, the interaction between the solid surface and water, and the interaction between the surfactant and water molecules. These interactions involve van der Waals dispersion forces and hydrogen bonds. The free energy of adsorption may be calculated from the equation
G ◦ = −RT ln K ,
[4]
where G ◦ is the free energy of adsorption of 1 mol of surfactant, R is the ideal gas constant, T is the temperature, and K is the equilibrium constant typically calculated from the Langmuir equation. This simple calculation sometimes leads to confusing results as the literature often suggests that the affinity of the surfactant for the hydrophobic surface decreases with increasing ethylene oxide content, yet the free energy calculation can show no difference in the affinity for the surface. The free energy of adsorption calculated on a molar basis is given in column two of Table 4 for various surfactants used in this study. The free energy of adsorption calculated on a molar basis appears to be a constant independent of the degree of ethoxylation or surfactant type. A value of −33 ± 0.7 kJ/mol may be taken as the mean value. Free energy per unit surface area. Although the free energy of adsorption per mole gives information regarding the affinity of 1 mol of surfactant for the surface, it may be more useful for wetting studies to calculate the free energy of adsorption per unit surface area. This calculation is more relevant to a system in which the purpose is to cover the surface with adsorbed surfactant molecules so as to promote wetting. Therefore, the
free energy of adsorption per unit area was calculated and the values are given in the last column in Table 4. The free energy decreases with an increase in ethylene oxide content of the molecule. These results are similar to those calculated by Romero-Cano et al. (12) for octylphenols and Kronberg et al. (5) for nonylphenols. Their value of −33.7 kJ/mol obtained for the octylphenol with 9.5 mol of ethylene oxide is very similar to the value of −33.2 kJ/mol obtained in the current study. When the free energy of adsorption is calculated on a per unit surface area basis, the energy shows large decreases as the ethylene oxide content of the molecule is increased. The data of other investigators studying ethoxylated alkyl chain surfactants without a phenol group show similar trends concerning the free energy of adsorption (13, 14). From the current study, it is obvious that the free energy of adsorption on a unit surface area basis is strongly dependent on the relative size of the nonylphenol chain to the length of the ethylene oxide chain. Similar results are evident in the data of Romero-Cano et al. (12) for octylphenols. These results demonstrate that the most hydrophobic portion of the molecule controls the adsorption process. Kronberg’s data also shows that attaching an additional alkyl chain to the phenol group (DNP-PO-EO) decreases the free energy of adsorption when calculated on a surface area basis possibly due to steric effects in which the second alkyl group hinders close packing. This result indicates that a branched alkyl chain decreases the free energy of adsorption. Free energy as a function of adsorption density. Another approach to understanding the importance of the free energy of adsorption is to estimate the free energy change as a function of adsorption density of the surfactants as shown in Figs. 10 through 12. This approach gives more insight into adsorption phenomena. From these results it is possible to get some information about the change in the configuration of the surfactant molecule with adsorption density. For this purpose, the free energy was calculated using the equation
G = −RT ln((/(C(m − ))),
[5]
TABLE 4 Free Energy of Adsorption on Lampblack on a per Mole and Unit Surface Area Basis Surfactant
Surfynol 104 Surfynol 440 Surfynol 465 Surfynol 480 Surfynol 485 Triton X-100 Tergitol NP-9 Tergitol NP-15
G (kJ/mol) Acetylenic diols −33.6 −33.7 −33.9 −32.2 −32.3 Alkylphenols −33.2 −31.6 −33.5
−m G (J/m2 ) × 103
89.0 70.8 47.5 41.0 28.7 95.6 100.5 66.0
ADSORPTION OF ACETYLENIC DIOL-BASED SURFACTANTS
FIG. 10. Free energy for Tergitol NP-9 as a function of adsorption density on lampblack.
where G is the free energy of adsorption, R is the ideal gas constant, T is the temperature (approximately 298 K in this study), is the adsorption density, C is the equilibrium mole fraction of surfactant, and m is the adsorption density at monolayer coverage. The free energy of adsorption of Tergitol NP-9 on lampblack is shown in Fig. 10. The maximum in the absolute value of free energy is obtained in the region between an adsorption density that corresponds to a horizontal orientation of the surfactant molecules and it was below the critical micelle concentration (CMC). At low surfactant concentrations, the contact angle of a drop of liquid increased, indicating an increase in hydrophobicity. Similar results were obtained for Tergitol NP-15 and Triton X-100. With a further increase in concentration, the surfactant molecules pack closer together in a horizontal orientation so that the hydrophobic chains can interact, thus resulting in a more negative value for free energy. As shown in Fig. 10, the maximum absolute value of free energy occurs at approximately this point. Further increase in surfactant concentration leads to the reorientation of the adsorbed surfactant molecules from a horizontal to a vertical position in such a way that the hydrophobic group of the surfactant molecule replaces the ethylene oxide groups that had been adsorbed parallel to the surface. In this region, there is a sharp decrease in free energy until an approximate constant value of −30 ± 1 kJ/mol is reached. The transition to a vertical configuration appears to be a fairly rapid transformation and has been found to occur around the CMC of the alkyl phenol surfactants studied. The surface became hydrophilic at high surfactant concentrations. The ethylene oxide groups are therefore considered to be extended into solution and control the packing of the surfactant molecules at the interface through steric effects.
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The free energy of adsorption of the acetylenic diol based surfactants on lampblack is shown in Fig. 11. An initial increase in the magnitude of the free energy of adsorption is followed by a sharp decrease until a final average of −30 ± 1 kJ/mol is attained. This pattern is observed consistently with all the reagent and adsorbent combinations tested so far. At relatively low adsorption densities (which correspond to a low coverage of the solid surface), the absolute value of the free energy of adsorption is greatest, indicating a strong interaction between the hydrophobic surface and the surfactant molecule. As the number of moles of ethylene oxide in the acetylenic diol molecules is increased, the constant value of free energy is reached at a lower adsorption density. The hydrated hydrophilic ethylene oxide group prevents close packing of the surfactant molecules, thus inhibiting the interaction of hydrophobic chains and leading to a decrease in the absolute value of the free energy of adsorption. Somewhat similar results were observed for adsorption of these surfactants on phthalocyanine blue, as can be seen from the results in Fig. 12. At low adsorption densities, the free energy per mole increased with increase in degree of ethoxylation. These results support the hypothesis presented in a previous section that ethoxylated reagents adsorb both through the hydrophobic and the polyethylene oxide groups at low coverage. This behavior was different for lampblack for which the free energy per mole was greatest at an intermediate degree of ethoxylation corresponding to Surfynol 465. At high adsorption densities, the free energy per mole was a function of adsorption density. This trend was different for the same series of surfactants on lampblack. For lampblack the free energy per molecule became constant at high adsorption densities and it was independent of degree of ethoxylation.
FIG. 11. Free energy for acetylenic diol-based surfactants as a function of adsorption density on lampblack.
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MUSSELMAN AND CHANDER
FIG. 12.
Normalized free energy of adsorption for acetylenic diol-based surfactants adsorbing on phthalocyanine blue.
Wetting Studies Contact angle measurements were made on a crystal of pyrolytic graphite as model solid for lampblack and the results for selected surfactants are presented in Fig. 13. At low surfactant concentrations the contact angle increased to about 90◦ before decreasing in value at higher surfactant concentrations. The results were similar for other surfactants and pigments tested so far. The increase at low concentrations is considered to be due to the molecule lying flat at the surface. This hypothesis is consistent with a more negative free energy of adsorption at low to
moderate adsorption densities. At high concentrations the contact angle became small, promoting wetting.
SUMMARY
The adsorption of TMDD (2,4,7,9-tetramethyl-5-decyne-4,7diol) series of surfactants on lampblack and phthacyanine blue is compared with that of alkylphenols. At low concentrations, the surfactants adsorb parallel to the hydrophobic surface, resulting in a high free energy of adsorption. Such an adsorption behavior results in an increase in contact angle. As the reagent concentration was increased further, the adsorbed surfactant molecules reoriented to form a monolayer in which the hydrophobic portion of the surfactant adsorbed in a vertical configuration leaving the polyethylene oxide chains extended into solution. This orientation promoted wetting. Acetylenic diol based surfactants were found to have an area per molecule of two to three times that of similarly ethoxylated alkyl phenols due to their bulky shape. Acetylenic diol based surfactants with ethylene oxide contents between 3.5 and 10 mol per molecule and alkyl phenol based surfactants with approximately 9 mol of ethylene oxide per molecule were found to provide superior wetting.
ACKNOWLEDGMENTS
FIG. 13.
Contact angle of graphite in the presence of selected surfactants.
The authors acknowledge the financial and technical support of Air Products and Chemicals, Inc., under the Strategic Alliance Program and for supplying acetylenic diol-based surfactants. The authors also thank Dr. Kevin R. Lassila for supplying TMDD-20, an experimental reagent prepared for this study. The authors also acknowledge Degussa, Union Carbide, and Rohm and Haas for supplying lampblack, Tergitol NP-9 and Tergitol NP-15, and Triton X-100, respectively.
ADSORPTION OF ACETYLENIC DIOL-BASED SURFACTANTS
REFERENCES 1. Srivastava, S. K., Gupta, V. K., Anupam, and Mohan, D., Removal of some anionic and cationic detergents using an inorganic gel adsorbent. Indian J. Chem. 34, 342–350 (1995). 2. Rosen, M. J., “Surfactants and Interfacial Phenomena.” Wiley, New York, 1978. 3. Medalia, A. I., and Rivin, D., “Carbon Blacks, Characterization of Powder Surfaces” (G. D. Parfitt and K. S. W. Sing, Eds.). Academic Press, New York, 1976. 4. Sappok, R., and Honigmann, B., “Organic Pigments, Characterization of Powder Surfaces” (G. D. Parfitt and K. S. W. Sing, Eds.). Academic Press, New York, 1976. 5. Kronberg, B., Stenius, P., and Thorsell, Y., Adsorption of nonylphenol– poly(propylene oxide)–poly(ethylene oxide) nonionic surfactants on polystyrene latex. Colloids Surf. 12, 113–123 (1984). 6. Mohal, B., “Enhancement of the Wettability of Coal Powders Using Surfactants.” Ph.D. thesis, The Pennsylvania State University, 1988. 7. Furlong, D. N., and Aston, J. R., Adsorption of polyoxyethylated nonyl phenols at silica/aqueous solution interfaces. Colloids Surf. 4, 121–129 (1982).
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8. Abe, R., and Kuno, H., The adsorption of polyoxyethylated nonylphenol on carbon black in aqueous solution. Kolloid-Zeitschrift 181, 70 (1962). 9. Narkis, N., and Ben-David, B., Adsorption of nonionic surfactants on activated carbon and mineral clay. Water Res. 19, 815–824 (1985). 10. Scales, P. J., Greiser, F., Furlong, D. N., and Healy, T. W., Contact angle changes for hydrophobic and hydrophilic surfaces induced by nonionic surfactants. Colloids Surf. 21, 55–68 (1986). 11. Mohan, Dinesh, Gupta, V. K., Srivastava, S. K., and Chander, S., Kinetics of mercury adsorption from wastewater using activated carbon derived from fertilizer waste. Colloids Surf., in press. 12. Romero-Cano, M. S., Martin-Rodriguez, A., Chauveteau, G., and de las Nieves, F. J., Colloidal Stabilization of Polystyrene Particles by Adsorption of Nonionic Surfactants, J. Colloid Interface Sci. 198, 266–272 (1998). 13. Klimenko, N. A., Tryasorukova, A. A., and Permilovskaya, A. A., Effect of the association of nonpolar groups on the adsorption of nonionic surfactants from aqueous solutions on a carbon sorbent. Kolloidn. Zh. 36, 678–681 (1974). 14. Klimenko, N. A., Permilovskaya, A. A., and Koganovskii, A. M., Structure of the adsorbed layer with the adsorption of unassociated molecules of nonionogenic surfactants on carbon black. Kolloidn. Zh. 36, 788–792 (1974).
Adsorption of Acetylenic Diol-Based Nonionic Surfactants on Lampblack and Phthalocyanine Blue Pigment S. W. Musselman and S. Chander1 Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, 123 Hosler Building, University Park, Pennsylvania 16802 Received June 4, 2001; accepted September 20, 2001; published online November 9, 2001
out blending. The adsorption of surfactants at the solid/liquid interface is strongly influenced by a number of factors including the nature of the structural groups on the solid surface, the molecular structure of the surfactant, and the environment of the aqueous phase (pH, temperature, electrolyte content, etc.). Together these factors determine the mechanism by which adsorption occurs and the efficiency and effectiveness of adsorption (2). Three surfactant types, namely, octylphenol-, nonylphenol-, and acetylenic diol-based surfactants were used in this investigation to delineate the influence of molecular structure on surfactant adsorption by hydrophobic surfaces.
The adsorption of acetylenic diol based surfactants on lampblack and phthalocyanine blue has been studied to determine the mechanisms of surfactant adsorption and the orientation of these molecules at hydrophobic surfaces. The TMDD series of reagents, commonly known as acetylenic diol-based surfactants, are derivatives of 2,4,7,9-tetramethyl-5-decyne-4,7-diol. While studies of conventional nonionic surfactants have included alkyl ethoxylates and ethoxylated alkyl phenol surfactants, limited information is available for acetylenic diol-based surfactants. The effect of the degree of ethoxylation of these surfactants was determined. Although the Langmuir model could be used to fit adsorption data in many cases, further analysis of the data showed that the free-energy per molecule was a function of adsorption density. The change in free energy with adsorption density could be interpreted by considering a change in the orientation of the adsorbed molecules. Acetylenic diol-based surfactants were found to have an area per molecule of two to three times that of similarly ethoxylated alkyl phenols. The area per molecule was found to increase linearly with increased ethoxylation within each family of surfactants. C 2002 Elsevier Science (USA) Key Words: adsorption; pigment; lampblack; surfactant; phthalocyanine blue; hydrophobic.
MATERIALS AND METHODS
The nonionic surfactants used in this study were obtained from various sources: TMDD series of reagents from Air Products and Chemicals, Inc., Triton X-100 from Rohm and Haas, and Tergitol NP-9 and NP-15 from Union Carbide. The TMDD series of reagents, commonly known as acetylenic diol-based surfactants, are derivatives of 2,4,7,9-tetramethyl-5-decyne-4,7diol. Ethoxylated versions of this molecule containing 0, 3.5, 10, 20, and 30 mol of ethylene oxide per molecule were used. Some of these surfactants are commercially available as wetting and defoaming agents under the Surfynol series manufactured by Air Products and Chemicals. Triton X-100 is a branched chain octylphenol with 9.5 mol of ethylene oxide per molecule. The Tergitol NP-9 and Tergitol NP-15 are nonylphenols with 9 and 15 mol of ethylene oxide per molecule, respectively. Although nonylphenol is commonly depicted with a linear alkyl chain, NMR spectroscopy studies show it to be a branched chain. Selected properties of these nonionic surfactants are given in Table 1. Structural formulae of selected reagents are given in Fig. 1. Lampblack 101, manufactured by Degussa, was used as a model hydrophobic surface. Carbon blacks contain carbon atoms arranged in graphitic layers, which are continuous from one particle to the next (3). Therefore, these carbon structures exist as aggregates in solution. Phthalocyanine blue 15 : 3 pigment with a molecular structure of C32 H16 CuN8 , illustrated in Fig. 2 (4), obtained from Sun Chemical Co., was used as a model of a complex pigment that might consist of hydrophobic and hydrophilic sites. It has been
INTRODUCTION
The adsorption of nonionic surfactants on solid surfaces from aqueous solutions is important for controlling many properties including wetting, dispersion, and rheology. The ability of surfactants to modify interfacial properties even at low bulk concentrations has led to their widespread use in numerous applications in the mineral, environmental, pharmaceutical, biological, agricultural, cosmetic, textile, and coatings industries. The growth of the nonionic surfactant industry in recent decades was aided by an increased environmental concern over the nonbiodegradable nature of most anionic surfactants (1). Nonionic surfactants are widely used for technical reasons such as the ease of varying surfactant properties. Simply changing the length of the polyoxyethylene group can modify the properties of many nonionic surfactants. These surfactants can also be mixed with other reagents to enhance properties that are not easily attained with1 To whom correspondence should be addressed. Fax: (814) 865-3248. E-mail: [email protected].
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0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
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TABLE 1 Properties of Surfactants Used in This Study
Reagent
Surfynol 104 Surfynol 440 Surfynol 465 Surfynol 480 Surfynol 485 Dynol 604 Triton X-100 Tergitol NP-9 Tergitol NP-15
Formula
Molecular CMC weight (mmol/L)
Acetylenic diols C14 H24 (OH)2 226 380 C14 H24 (OH)2 EO3.5 C14 H24 (OH)2 EO10 666 1106 C14 H24 (OH)2 EO20 1546 C14 H24 (OH)2 EO30 NA NA Alkylphenols C8 H17 EO9.5 624 C9 H19 EO9 616 880 C9 H19 EO15
N/A N/A 10.5 N/A 11.2 NA 0.22 0.078 0.093
Surface tension (dynes/cm) 0.1% @ 25◦ C
33.1 33.2 41.9 51.1 NA 31 30 36
Note. NA, not available.
shown that phthalocyanine pigments contain crystals as aggregates, which are grown together along crystallographic lattice directions (4). Total organic carbon (TOC) analysis showed that there were very small amounts of a carbon-based substance (<1–2 ppm) leaching from the phthalocyanine blue surface when dispersed in water. According to the manufacturer, the phthalocyanine blue sample has approximately 3–5% impurities in the form of short chain glycols, inorganic salts, and other
FIG. 2.
Chemical structure of phthalocyanine blue 15 : 3 (C32 H16 CuN8 ).
compounds produced in the ring closure process for manufacturing phthalocyanine blue. Adsorption studies were conducted by dispersing 200 mg of the lampblack in 20 mL of known concentrations of the nonionic surfactants in aqueous solutions at 25◦ C. The particles were dispersed using a Branson ultrasonic bath for 3 min. The solutions were filtered and analyzed using a Shimadzu TOC-5000A carbon analyzer. The adsorption density of the surfactants on the solid was calculated by using the following expression: = ((C0 − Cf )V )/(m SA ),
[1]
where C0 and Cf are the initial and final (equilibrium) concentrations of the surfactant in solution (micromol/L), V is the solution volume (L), m is the mass of adsorbent (g), and SA is the BET surface area of the sorbent (m2 /g). RESULTS AND DISCUSSION
Adsorption on Lampblack The adsorption isotherms for the acetylenic diol based and alkylphenol surfactants are presented in Figs. 3 and 4, respectively. The adsorption experiments show a clear dependence of adsorption density on the degree of ethoxylation. A decrease in the plateau region with an increase in the ethylene oxide content for each family of surfactants is evident. The data was fitted to the Langmuir model, written in its linear form as C/ = 1/(m K ) + C/ m ,
FIG. 1.
Chemical structure of selected surfactants used in this investigation.
[2]
where C is the concentration, is the adsorption density, m is the adsorption density at monolayer coverage, and K is an equilibrium constant. A plot of C/ versus C thus allows the calculation of m and K . The Langmuir plots for TMDD series of surfactants are presented in Fig. 5. Two important assumptions that are made in the derivation of this equation which are not met when considering surfactant adsorption from aqueous adsorption are that the solute and solvent molecules occupy the same surface area and that there is no interaction between the
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ADSORPTION OF ACETYLENIC DIOL-BASED SURFACTANTS
FIG. 3. Adsorption of acetylenic diol-based surfactants on lampblack. The solid lines are Langmuir fits to the data.
solute and solvent on the surface. It has been shown that the invalidity of these assumptions causes deviations from the Langmuir equation that are in opposite directions to one another and therefore the equation is obeyed due to a compensation of the two errors (5). The adsorption density at monolayer coverage and area per molecule for each surfactant is presented in Table 2. The calculated areas per molecule for these surfactants at the solid/liquid
FIG. 5. Langmuir model for the adsorption of acetylenic diol-based surfactants on lampblack.
interface are similar to the values at the liquid/vapor interface. This indicates that the adsorption of nonionic surfactants from solution onto hydrophobic surfaces results in a molecular orientation much like the liquid/vapor interface, which is characterized by a vertical configuration of the molecules. The hydrophobic part of the surfactant chain is adsorbed vertically onto the hydrophobic surface of the lampblack particles, leaving the bulky, hydrated, hydrophilic part of the molecule extending into the solution. Calculated areas per molecule for an octylphenol with 9 and 10 mol of ethylene oxide per molecule adsorbing in a vertical orientation are 0.56 and 0.62 nm2 (6), respectively. These numbers agree quite well with the results of the Triton X-100 (0.58 nm2 ). The calculated values for the area per molecule of a nonylphenol with 9 and 10 mol of ethylene TABLE 2 Adsorption Density and Area per Molecule for Acetylenic Diol and Alkylphenol-Based Surfactants Reagent
Surfynol 104 Surfynol 440 Surfynol 465 Surfynol 480 Surfynol 485 Dynol 604
FIG. 4. Adsorption isotherms of alkylphenol surfactants on lampblack. The solid lines are Langmuir fits to the data.
Triton X-100 Tergitol NP-9 Tergitol NP-15
Formula Acetylenic diols C14 H24 (OH)2 C14 H24 (OH)2 EO3.5 C14 H24 (OH)2 EO10 C14 H24 (OH)2 EO20 C14 H24 (OH)2 EO30 Proprietary Alkylphenols C8 H17 EO9.5 C9 H19 EO9 C9 H19 EO15
m (µmol/m2 )
Am (nm2 )
2.65 2.10 1.40 1.28 0.89 2.21
0.63 0.79 1.19 1.30 1.87 0.75
2.88 3.18 1.97
0.58 0.52 0.84
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FIG. 6. groups.
MUSSELMAN AND CHANDER
Area per molecule as a function of the number of ethylene oxide
oxide per molecule adsorbing in a horizontal plane parallel to the particle surface are 2.44 and 2.70 nm2 (6), respectively. The nonylphenols used in this study (Tergitol NP-9 and Tergitol NP15) offer further proof that the adsorption of these nonionic surfactants occurs in a vertical orientation, as their areas per molecule are approximately equal to that for a vertically oriented layer. The area per molecule as a function of the number of ethylene oxide groups is presented in Fig. 6. The data at 0 mol of ethylene oxide per molecule for the Tergitol series has been taken as 0.22 nm2 from a close packed layer of hydrocarbons and verified by the extrapolation of the data of Kronberg et al. (5). Although there is scatter evident in the data, it is reasonable to suggest that the straight line fits for the Tergitol and Surfynol series of surfactants are parallel, thereby indicating a systematic increase in area per molecule at hydrophobic surfaces with increased ethoxylation. The results show that the Surfynol reagents occupy a significantly greater area at the solid/liquid interface when compared to similarly ethoxylated octyl and nonylphenol surfactants. The bulky shape of the hydrophobic part of the acetylenic diol based molecule is most likely the reason that Surfynol molecules occupy approximately two to three times the area at the solid/liquid interface when compared to similarly ethoxylated alkyl phenols. In contrast, the hydrophobic part in alkylphenols is relatively linear which allows these reagents to pack closer together. The area per molecule for the nonylphenol was slightly smaller than the corresponding octylphenol. These results show that the shape of the hydrophobic portion of the molecule and the size of the hydrocarbon chain determines adsorption density at monolayer coverage. Although comparably ethoxylated alkylphenol reagents show a higher adsorption density than acetylenic diol-based surfactants at monolayer cover-
age, in some applications it may be advantageous to have a low adsorbing reagent. These advantages are discussed in a later section of this paper. The dependence of area per molecule on the moles of ethylene oxide per molecule for nonylphenols adsorbing on various surfaces in other studies is shown in Fig. 7. The area per molecule results from this study appears to agree quite well with results published by other investigators (5, 7). The studies by Abe and Kuno (8) and Narkis and Ben-David (9) show a significant deviation from the other results. A possible explanation for this discrepancy could be due to the large surface area of furnace black and powdered activated carbon that were used in these studies. Surface areas determined by nitrogen adsorption can lead to errors when calculating surfactant adsorption densities if pores are fine. Surfactants cannot access all pore spaces that are available to a nitrogen molecule due to physical size constraints. This results in a much smaller surface area available for surfactant adsorption and causes errors when determining adsorption densities. The results from this work and other studies (7, 10) suggest that there is a linear dependence of area per molecule on ethylene oxide content. At low ethylene oxide contents, the area per molecule at the solid/liquid interface is similar to the liquid/ vapor interface. As ethylene oxide content is increased, the area per molecule at the solid/liquid interface becomes significantly
FIG. 7.
Area per molecule for nonylphenols adsorbing on various surfaces.
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greater. This suggests that the packing of ethylene oxide at these two interfaces is different. A possible reason for these differences can be explained by considering the forces that exist on the surfactant molecules at the interfaces. The steric repulsive force that exists between ethylene oxide chains and the attractive forces between hydrocarbon chains govern the packing of ethoxylated surfactants. At the liquid/vapor interface, the hydrocarbon chains are expelled from the solution and are separated by air. This leads to a large attractive force between the hydrocarbon chains, and they pack close together in monolayer formation with the ethylene oxide groups extended into solution. At the solid/liquid interface, water is not completely expelled from the adsorbed surfactant layer. The presence of water reduces the attractive force between the hydrocarbon chains by reducing the effective Hamaker constant. Under these circumstances, the surfactant molecules are not packed as tightly. The ethylene oxide groups are still extended into solution, but are now free to coil, thus preventing the close packing that is observed at the liquid/vapor interface. Adsorption on Phthalocyanine Blue The Freundlich plots for adsorption of acetylenic diol and alkylphenol on phthalocyanine blue are presented in Figs. 8 and 9, respectively. Two phthalocyanine blue samples were used. The second sample gave slightly higher amount of organics leached into water (approximately 1–2 ppm of carbon) but it seemed to have no effect on adsorption. Experiments were conducted on both powder samples with Surfynol 465. No significant differences in the adsorption data were observed. All studies were made using the second sample of phthalocyanine blue with the exception of Surfynol 104.
FIG. 8. blue.
Adsorption of acetylenic diol-based surfactants on phthalocyanine
FIG. 9.
Adsorption of alkylphenol surfactants on phthalocyanine blue.
The Freundlich equation is used to describe adsorption onto heterogeneous surfaces and is expressed in the following form: θ=
1 = kC n , m
[3]
where θ is fractional coverage, is adsorption density, m is adsorption density at monolayer coverage, C is concentration, and k and n are constants. The adsorption data for Dynol 604 was also fitted to the Freundlich equation. For each surfactant there is a break in the straight line fit of this equation. Other studies have shown that this type of break in the data can be due to a change in the diffusion mechanism responsible for the rate-controlling step in the adsorption process (11). Adsorption on phthalocyanine blue is less at low concentrations when compared to lampblack. This is likely due to structural differences between the two surfaces and the presence of surface-active impurities on the phthalocyanine blue. Because of these differences, a Langmuir equation better fits adsorption on lampblack while Freundlich equation is better for phthalocyanine blue. A decrease in the plateau region with increasing ethylene oxide content was observed in a manner similar to the results with lampblack. The adsorption densities at monolayer coverage are compared in Table 3 for phthalocyanine blue and lampblack. Similar results were obtained for both surfaces with the exception of Surfynol 480 and 485. Although a definitive reason for these results is not easy to obtain, several possibilities exist. The larger, less surface-active molecules may not be able to displace the impurities present on the surface. It is also possible that these larger surfactant molecules are unable to enter some of the pore space in phthalocyanine blue. Phthalocyanine blue has three times the surface area of the lampblack powder and
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TABLE 3 Adsorption Densities at Monolayer Coverage for Phthalocyanine Blue and Lampblack Reagent
Surfynol 104 Surfynol 440 Surfynol 465 Surfynol 480 Surfynol 485 Triton X-100 Tergitol NP-9 Tergitol NP-15
M for Phthalocyanine blue (µmol/m2 ) Acetylenic diols 2.90 2.10 1.75 0.83 0.52 Alkylphenols 2.80 3.10 1.78
m for Lampblack (µmol/m2 )
2.65 2.10 1.40 1.28 0.89 2.88 3.18 1.97
likely contains micropores that are too small to be accessed by Surfynol 480 and Surfynol 485, thereby causing a reduction in apparent adsorption density. Thermodynamics of Adsorption The free energy for the adsorption of nonionic surfactants on hydrophobic surfaces is likely to include contributions from the hydrophobic interaction between the surfactant molecule and the solid surface, the chain interactions between the adsorbed surfactant molecules, the interaction between the solid surface and water, and the interaction between the surfactant and water molecules. These interactions involve van der Waals dispersion forces and hydrogen bonds. The free energy of adsorption may be calculated from the equation
G ◦ = −RT ln K ,
[4]
where G ◦ is the free energy of adsorption of 1 mol of surfactant, R is the ideal gas constant, T is the temperature, and K is the equilibrium constant typically calculated from the Langmuir equation. This simple calculation sometimes leads to confusing results as the literature often suggests that the affinity of the surfactant for the hydrophobic surface decreases with increasing ethylene oxide content, yet the free energy calculation can show no difference in the affinity for the surface. The free energy of adsorption calculated on a molar basis is given in column two of Table 4 for various surfactants used in this study. The free energy of adsorption calculated on a molar basis appears to be a constant independent of the degree of ethoxylation or surfactant type. A value of −33 ± 0.7 kJ/mol may be taken as the mean value. Free energy per unit surface area. Although the free energy of adsorption per mole gives information regarding the affinity of 1 mol of surfactant for the surface, it may be more useful for wetting studies to calculate the free energy of adsorption per unit surface area. This calculation is more relevant to a system in which the purpose is to cover the surface with adsorbed surfactant molecules so as to promote wetting. Therefore, the
free energy of adsorption per unit area was calculated and the values are given in the last column in Table 4. The free energy decreases with an increase in ethylene oxide content of the molecule. These results are similar to those calculated by Romero-Cano et al. (12) for octylphenols and Kronberg et al. (5) for nonylphenols. Their value of −33.7 kJ/mol obtained for the octylphenol with 9.5 mol of ethylene oxide is very similar to the value of −33.2 kJ/mol obtained in the current study. When the free energy of adsorption is calculated on a per unit surface area basis, the energy shows large decreases as the ethylene oxide content of the molecule is increased. The data of other investigators studying ethoxylated alkyl chain surfactants without a phenol group show similar trends concerning the free energy of adsorption (13, 14). From the current study, it is obvious that the free energy of adsorption on a unit surface area basis is strongly dependent on the relative size of the nonylphenol chain to the length of the ethylene oxide chain. Similar results are evident in the data of Romero-Cano et al. (12) for octylphenols. These results demonstrate that the most hydrophobic portion of the molecule controls the adsorption process. Kronberg’s data also shows that attaching an additional alkyl chain to the phenol group (DNP-PO-EO) decreases the free energy of adsorption when calculated on a surface area basis possibly due to steric effects in which the second alkyl group hinders close packing. This result indicates that a branched alkyl chain decreases the free energy of adsorption. Free energy as a function of adsorption density. Another approach to understanding the importance of the free energy of adsorption is to estimate the free energy change as a function of adsorption density of the surfactants as shown in Figs. 10 through 12. This approach gives more insight into adsorption phenomena. From these results it is possible to get some information about the change in the configuration of the surfactant molecule with adsorption density. For this purpose, the free energy was calculated using the equation
G = −RT ln((/(C(m − ))),
[5]
TABLE 4 Free Energy of Adsorption on Lampblack on a per Mole and Unit Surface Area Basis Surfactant
Surfynol 104 Surfynol 440 Surfynol 465 Surfynol 480 Surfynol 485 Triton X-100 Tergitol NP-9 Tergitol NP-15
G (kJ/mol) Acetylenic diols −33.6 −33.7 −33.9 −32.2 −32.3 Alkylphenols −33.2 −31.6 −33.5
−m G (J/m2 ) × 103
89.0 70.8 47.5 41.0 28.7 95.6 100.5 66.0
ADSORPTION OF ACETYLENIC DIOL-BASED SURFACTANTS
FIG. 10. Free energy for Tergitol NP-9 as a function of adsorption density on lampblack.
where G is the free energy of adsorption, R is the ideal gas constant, T is the temperature (approximately 298 K in this study), is the adsorption density, C is the equilibrium mole fraction of surfactant, and m is the adsorption density at monolayer coverage. The free energy of adsorption of Tergitol NP-9 on lampblack is shown in Fig. 10. The maximum in the absolute value of free energy is obtained in the region between an adsorption density that corresponds to a horizontal orientation of the surfactant molecules and it was below the critical micelle concentration (CMC). At low surfactant concentrations, the contact angle of a drop of liquid increased, indicating an increase in hydrophobicity. Similar results were obtained for Tergitol NP-15 and Triton X-100. With a further increase in concentration, the surfactant molecules pack closer together in a horizontal orientation so that the hydrophobic chains can interact, thus resulting in a more negative value for free energy. As shown in Fig. 10, the maximum absolute value of free energy occurs at approximately this point. Further increase in surfactant concentration leads to the reorientation of the adsorbed surfactant molecules from a horizontal to a vertical position in such a way that the hydrophobic group of the surfactant molecule replaces the ethylene oxide groups that had been adsorbed parallel to the surface. In this region, there is a sharp decrease in free energy until an approximate constant value of −30 ± 1 kJ/mol is reached. The transition to a vertical configuration appears to be a fairly rapid transformation and has been found to occur around the CMC of the alkyl phenol surfactants studied. The surface became hydrophilic at high surfactant concentrations. The ethylene oxide groups are therefore considered to be extended into solution and control the packing of the surfactant molecules at the interface through steric effects.
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The free energy of adsorption of the acetylenic diol based surfactants on lampblack is shown in Fig. 11. An initial increase in the magnitude of the free energy of adsorption is followed by a sharp decrease until a final average of −30 ± 1 kJ/mol is attained. This pattern is observed consistently with all the reagent and adsorbent combinations tested so far. At relatively low adsorption densities (which correspond to a low coverage of the solid surface), the absolute value of the free energy of adsorption is greatest, indicating a strong interaction between the hydrophobic surface and the surfactant molecule. As the number of moles of ethylene oxide in the acetylenic diol molecules is increased, the constant value of free energy is reached at a lower adsorption density. The hydrated hydrophilic ethylene oxide group prevents close packing of the surfactant molecules, thus inhibiting the interaction of hydrophobic chains and leading to a decrease in the absolute value of the free energy of adsorption. Somewhat similar results were observed for adsorption of these surfactants on phthalocyanine blue, as can be seen from the results in Fig. 12. At low adsorption densities, the free energy per mole increased with increase in degree of ethoxylation. These results support the hypothesis presented in a previous section that ethoxylated reagents adsorb both through the hydrophobic and the polyethylene oxide groups at low coverage. This behavior was different for lampblack for which the free energy per mole was greatest at an intermediate degree of ethoxylation corresponding to Surfynol 465. At high adsorption densities, the free energy per mole was a function of adsorption density. This trend was different for the same series of surfactants on lampblack. For lampblack the free energy per molecule became constant at high adsorption densities and it was independent of degree of ethoxylation.
FIG. 11. Free energy for acetylenic diol-based surfactants as a function of adsorption density on lampblack.
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FIG. 12.
Normalized free energy of adsorption for acetylenic diol-based surfactants adsorbing on phthalocyanine blue.
Wetting Studies Contact angle measurements were made on a crystal of pyrolytic graphite as model solid for lampblack and the results for selected surfactants are presented in Fig. 13. At low surfactant concentrations the contact angle increased to about 90◦ before decreasing in value at higher surfactant concentrations. The results were similar for other surfactants and pigments tested so far. The increase at low concentrations is considered to be due to the molecule lying flat at the surface. This hypothesis is consistent with a more negative free energy of adsorption at low to
moderate adsorption densities. At high concentrations the contact angle became small, promoting wetting.
SUMMARY
The adsorption of TMDD (2,4,7,9-tetramethyl-5-decyne-4,7diol) series of surfactants on lampblack and phthacyanine blue is compared with that of alkylphenols. At low concentrations, the surfactants adsorb parallel to the hydrophobic surface, resulting in a high free energy of adsorption. Such an adsorption behavior results in an increase in contact angle. As the reagent concentration was increased further, the adsorbed surfactant molecules reoriented to form a monolayer in which the hydrophobic portion of the surfactant adsorbed in a vertical configuration leaving the polyethylene oxide chains extended into solution. This orientation promoted wetting. Acetylenic diol based surfactants were found to have an area per molecule of two to three times that of similarly ethoxylated alkyl phenols due to their bulky shape. Acetylenic diol based surfactants with ethylene oxide contents between 3.5 and 10 mol per molecule and alkyl phenol based surfactants with approximately 9 mol of ethylene oxide per molecule were found to provide superior wetting.
ACKNOWLEDGMENTS
FIG. 13.
Contact angle of graphite in the presence of selected surfactants.
The authors acknowledge the financial and technical support of Air Products and Chemicals, Inc., under the Strategic Alliance Program and for supplying acetylenic diol-based surfactants. The authors also thank Dr. Kevin R. Lassila for supplying TMDD-20, an experimental reagent prepared for this study. The authors also acknowledge Degussa, Union Carbide, and Rohm and Haas for supplying lampblack, Tergitol NP-9 and Tergitol NP-15, and Triton X-100, respectively.
ADSORPTION OF ACETYLENIC DIOL-BASED SURFACTANTS
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