Adsorption of Alkyldimethylamine and Alkyldimethylphosphine Oxides

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Langmuir 2002, 18, 8436-8446

Adsorption of Alkyldimethylamine and Alkyldimethylphosphine Oxides at Curved Aqueous Solution/Silica Interfaces, Studied Using Microcalorimetry Alf Pettersson* and Jarl B. Rosenholm Department of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FIN-20500 Åbo, Finland Received February 20, 2002. In Final Form: July 31, 2002 We report on microcalorimetric studies of the adsorption of octyldimethylamine oxide (C8DAO), decyldimethylamine oxide (C10DAO), dodecyldimethylamine oxide (C12DAO), and decyldimethylphosphine oxide (C10DPO) on mesoporous, low surface charge density silica gel from aqueous solutions at 298.15 K. Displacement enthalpies combined with adsorption isotherms were measured at natural pH, at which the dilute solutions of the alkyldimethylamine oxides were mixed cationic-nonionic. All surfactants studied adsorb exothermally from dilute solution by forming hydrogen bonding between nonionic headgroup and surface silanol groups, and by electrostatic interactions between ionic adsorbates and oppositely charged surface sites. All the cationic surfactant species and dissociated silanol groups do not lose all their counterions and hydrated water when interacting electrostatically with each other. The surfaces with the exothermally adsorbed constrained species induce endothermic, hydrophobic interactions that increase the adsorption and result in surface self-assembled structures at surfactant contents in solution well below the bulk critical micelle concentration (cmc). The endothermic differential molar enthalpies of displacement of the decyl group substituted surfactants studied show a maximum when the adsorption saturates near the cmc. They form globular surface micelles at the solution/SiO2 interface. Due to interaggregate interactions, the interfaces between neighboring globular surface assemblies are not fully hydrated. C12DAO suddenly forms hemimicelles in the low-affinity adsorption region involving rearrangement of adsorbate species, and probably ellipsoidal micellar aggregates in the high-affinity region at the hydrophilic silica interface. The aggregation number is four C12DAO species per hemimicelle on the average. The adsorption mechanism of C12DAO seems to be different from those of C10DAO and C10DPO.

Introduction Adsorption and the accompanying surface self-assembly of amphiphilic molecules at solid/aqueous solution interfaces are of great importance for processes and applications such as detergency, stabilization of dispersions, spreading of liquids, and surfactant-based separation processes. The surface-active molecules consist of hydrophilic headgroups and hydrophobic chains, and therefore have additional degrees of freedom associated with conformations of the chains. In bulk solution they form curved interfaces on a local scale as a result of the cooperative behavior of the molecules. The phase behavior of binary aqueous aliphatic amine and phosphine oxides systems is defined by stating the number, qualitative nature, and composition of the phases present at equilibrium. This complexity of phase behavior results in part from the existence of a variety of liquid-crystalline states of amphiphile and water and have been thoroughly studied.1-4 Adding silica, with an energetically heterogeneous surface, to the system makes it a completely different one from the bulk solution. The site-bound surfactants at the solid surface are constrained and have a reduced freedom to change conformation. These geometric constraints and the surface charge induce the surfactants to assemble at * To whom correspondence should be addressed. Telephone: +358 2 215 4784. Fax: +358 2 215 4706. E-mail: [email protected]. (1) Ekwall, P. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1975; Vol. 1, p 1. (2) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley-Interscience: New York, 1980. (3) Laughlin, R. G. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1978; Vol. 3, p 41. (4) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994.

the solid/solution interface at a bulk surfactant unimer concentration lower than the critical micelle concentration (cmc). Surfactant adsorption is governed by the interaction of the surfactant with the polar surface and opposed by the hydrophobic chains of the surfactant, giving rise to the hydrophobic effect. The adsorption is determined by the strength of attraction between surfactant headgroup and the hydrophilic silica surface. The latter driving force is closely related to the surfactant structure and hence the solubility characteristics of the surfactants in water. A weak enough headgroup surface attraction will promote the self-assembly that often results in discrete or connected aggregates.5 They are often seen as patchwise formed quasi two-dimensional analogues of the aggregate structures observed in bulk solution; i.e., the half-structured surface aggregates are commonly referred to as hemimicelles.6 The bilayered aggregates are often denoted admicelles, and moreover there may exist randomly orientated spherical aggregates. The structure at the solid/liquid interface is therefore controlled by the boundary conditions and thermodynamics. The surface excess of surfactants at the solid/aqueous solution interface usually features a plateau at the cmc; meanwhile the aggregate layer may take a different structure above the cmc. Some rough guidelines of surface self-assembly of surfactants are known, but many of the structures and geometries of the self-assemblies at solid/liquid interfaces have remained unknown. The alkyldimethylamine oxides and alkyldimethylphosphine oxides are monofunctional amphiphilic molecules (5) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, 1999. (6) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 202, 958.

10.1021/la025642+ CCC: $22.00 © 2002 American Chemical Society Published on Web 10/08/2002

Adsorption of C8DAO, C10DAO, C12DAO, and C12DPO

Langmuir, Vol. 18, No. 22, 2002 8437

Scheme 1. Canonical Structures of the Amphoteric and Zwitterionic Alkyldimethylamine and Alkyldimethylphosphine Oxide Surfactants Studieda

used in various technological fields such as detergent formulations, cosmetics, viscosity builder, wetting agent, emulsifier, foam stabilizer, and antistatic agent. The adsorption of zwitterionic surfactants to powder surfaces has wider interest because of its potential use in processing of ceramics.4,14 The silica gel studied is a very appropriate adsorbent since it represents a silica model surface and has a large enough specific surface area, which is an inevitable requirement for calorimetry and studies of adsorption from solution by measurements of the concentration depletion. The adsorbent silica gel is a noncrystalline solid formed by the sol-gel process. Silica gels have found use as an important component in several biomaterials, as materials for membrane preparation, as a support in heterogeneous catalysis, and as bonding agents between ceramics and metals. Among the various methods available to study adsorption from solution, those using microcalorimetry offer both a direct knowledge of the interactions involved and also a suitability for complex systems involving heterogeneous surfaces, such as those of most adsorbents found in technological applications.15,16 It is well-known that calorimetric effects of adsorption are much more sensitive to the nature of an adsorption system than other experimental adsorption isotherms. As an analyzing technique, calorimetry measures the resulting net heat change of all the different reactions and effects that simultaneously take place in the system. In some cases the studied effects mask complex underlying factors that are not observed. In systems where adsorption occurs from two-component aqueous systems, the adsorbate molecules compete with the solvent molecules for the solid surface sites. When the adsorbate molecules are surfactants which may be involved in complicated phase transitions, and moreover when the solid surface is curved porous material, the complexity increases significantly.

a These structures are dependent on the pH of the aqueous solution. Ignoring water, cationic properties are to the left (at low pH); net neutral properties of the hydrophilic headgroups are in the middle (depicted as nonionic, semipolar compounds) and to the right (depicted as zwitterionics).

classified as amphoteric, nonionic, or zwitterionic surfactants. Amphoterics are by definition surfactants which, depending on the pH, have anionic and/or cationic properties. Amphoterics also have an isoelectric point (or pH range) at which they possess net neutral character. In the net neutral state they may either be seen as having distinct anionic and cationic charges or as dipolar nonionic compounds. They are tertiary compounds with a coordinate covalent bond between the N and the O atoms and between the P and the O atoms, respectively. The hydrophilic headgroup is semipolar; i.e., substantial charge separation exists between the directly bounded O and N, and O and P atoms, respectively. Both N and P are in group V of the elements. The relative hydrophilicities are lower for the PfO group than for the NfO group. The tertiary amine oxide forms three strong hydrogen bonds, while the tertiary phosphine oxide forms two. This is due to differences in both bond moments and basicities. The alkyldimethylamine and alkyldimethylphosphine oxide surfactants studied become, after picking up a proton, a hydroxide salt due to the covalently bonded proton of the cationic or, alternatively, a cationic conjugate acid (see Scheme 1). As previously mentioned, the nonionic structure can also be depicted with the charges separated and referred to as a zwitterionic compound with the charges located on adjacent atoms.7,8 The phase diagrams and thermodynamics of aqueous two-component systems of alkyldimethylamine oxides or alkyldimethylphosphine oxides have been extensively studied.9-13 However, a more systematic study on the selfassembly mechanism of adsorption at aqueous interfaces of amorphous SiO2 with parameters such as variations in hydrophilic structural elements (amine or phoshine oxide), nonionic and cationic characters, and the length of the hydrocarbon chain is still missing from the literature. The long-chain alkyldimethylamine oxides studied have been (7) Lomax, E. G. In Amphoteric Surfactants; Lomax, E. G., Ed.; Surfactant Science Series 59; Marcel Dekker Inc.: New York, 1996. (8) Laughlin, R. G. In Cationic Surfactants; Rubingh, D. N., Holland, P. M., Eds.; Surfactant Science Series 37; Marcel Dekker Inc.: New York, 1991; pp 1-40. (9) Benjamin, L. J. Phys. Chem. 1964, 68, 3575. (10) Herrmann, K. W.; Brushmiller, J. G.; Courchene, W. L. J. Phys. Chem. 1966, 70, 2909. (11) Lutton, E. S. J. Am. Oil Chem. Soc. 1966, 43, 28. (12) Chernic, G. G.; Sokolova, E. P. J. Colloid Interface Sci. 1991, 141, 409. (13) Kresheck, G. C. J. Am. Chem. Soc. 1998, 120, 10964.

Experimental Section Materials. N,N-Dimethyldecylphosphine N-oxide and N,Ndimethyldodecylphosphine N-oxide were kindly supplied by Dr. R. G. Laughlin from The Procter & Gamble Co, Cincinnati, OH. N,N-Dimethyloctylamine N-oxide (purity >99%), N,N-dimethyldecylamine N-oxide (>99%), and N,N-dimethyldodecylamine N-oxide (>98%) used were all supplied by Fluka. They were not purified further, but due to hygroscopicity they were dried in a vacuum over P2O5. The SiO2 used was Davisil Silica gel and supplied by Aldrich. The manufacturer provided the following specifications: grade 644, 100-200 mesh, average pore diameter 150 Å, purity 99+%, specific surface area 300 m2 g-1, pore volume 1.15 cm3 g-1, pH (5% slurry) ) 7.0. The samples were dried in an oven at 150 °C for 4 h before use. Double checking surface area and pore structure by N2 gas adsorption analysis was carried out using an Asap 20100 Sorptometer (by Micromeritics, Norcross, GA), and gave the specific surface area 266 m2 g-1, which is used throughout the calculations. The surface of the granular silica used was found to be a low-charged hydrophilic substrate over the pH range 3-9, and a highly charged hydrophilic substrate outside this range. The point of zero charge (PZC) of the silica used is at pHPZC ) 3.0 and the isoelectric point (IEP) is at pHIEP ) 3.5 (the latter at the ionic strength, I ) 0.001 mol dm-3 KCl). Figure 1 shows a schematic picture compiled from literature data on the pore structure of silica gel, with the most predominant, reactive surface groups. The pores can be featured as open spaces (14) Chavez, P.; Ducker, W.; Israelachvili, J.; Maxwell, K. Langmuir 1996, 12, 4111. (15) Denoyel, R.; Rouquerol, J. J. Colloid Interface Sci. 1991, 143, 555. (16) Lindheimer, M.; Keh., E.; Zaini, S.; Partyka, S. J. Colloid Interface Sci. 1990, 138, 83.

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Langmuir, Vol. 18, No. 22, 2002

Pettersson and Rosenholm

Figure 2. Schematic block diagram of the calorimeter system used.

Figure 1. Schematic illustration of the open pore structure of silica gel, with prevalent surface silanol groups. between the clustered primary sol particles. The exact surface structure of amorphous silica is unknown. The surface of silica gel consists of silanol groups (Si-OH) and generally unreactive siloxane groups (≡Si-O-Si≡). To complete the covalency of 4 of silicon, the surface silanol groups are either singly or doubly Si coordinated, depending on linking to three respectively two ≡Si-O-Si≡ bridges in the bulk.17-19 The total number of hydroxyl groups of a fully hydrolyzed silica gel surface is about 5 per nm2, of which 0.3 are free, isolated OH groups and 4.7 are bridged, hydrogen-bonded silanol groups per nm2.20-22 The water used in all experiments was double distilled and subsequently passed through a Milli-Q water purification system. The minimum resistivity of the purified water was 107 Ω cm and pH ) 5.7. Surface Tension Measurements. The cmc’s of the asreceived used surfactants, with the exception of C8DAO, were determined from surface tension measurements on sets of surfactant solutions using the Du Nou¨y ring method at 298 K and at natural pH, at which all C10DPO and most of the alkyldimethylamine oxide are in the nonionic form (see pK1’s below). Ionizability of the Surfactants Studied. The ionizability below the cmc of the amphoteric surfactants used were calculated from data of potentiometric titrations carried out using a 702 SM Titrino system by Metrohm. Aqueous solution of C ) 0.1 mol dm-3 HNO3 was used as the standard solution. The fraction ionized surfactant unimers (R1) were obtained using the Henderson-Hasselbalch equation (1), in which the equilibrium constant for dissociation of acidic cation unimers (K1) is related to the degree of protonation (R1) at equilibrium. For the alkyldimethylamine oxides studied the relation is

pH ) pK1 - log

[R(CH3)2N+-OH] [R(CH3)2NfO]

w

pK1 ) pH + log

R1 (1) 1 - R1

The equation shows that at pH equal to pK1, half the cationic surfactant molecules are dissociated. The titration procedure has been described in detail elsewhere.23 (17) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Studies in Surface Science and Catalysis 93; Elsevier Science B. V.: Amsterdam, 1995. (18) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; John Wiley & Sons: New York, 1979. (19) El Shafei, G. M. S. In Adsorption on Silica Surfaces; Papier, E., Ed.; Surfactant Science Series 90; Marcel Dekker Inc.: New York, 2000. (20) Van Der Voort, P.; Gillis-D’Hamers, I.; Vansant, F. J. Chem. Soc., Faraday Trans. 1990, 86, 3751. (21) Zhuravlev, L. T. Langmuir 1987, 3, 316. (22) Persello, J. In Adsorption on Silica Surfaces; Papier, E., Ed.; Surfactant Science Series 90; Marcel Dekker Inc.: New York, 2000.

Microcalorimetry of Adsorption. In this work all adsorption isotherms and displacement (abbreviated as “dpl”) enthalpies were determined using liquid flow microcalorimetry (LKB 2107 Sorption microcalorimeter with a Hewlett-Packard 3393A integrator). The adsorption isotherms expressed as the reduced surface excess were measured by the chromatographic method, which has thoroughly been described in the literature.24-26 In this kind of experiment, the adsorbent is placed in the flow cell between two filters, and is successively brought in contact with solvent and solutions of increasing surfactant composition (illustrated in Figure 2). The content of surfactant (weight fraction) of the output from the calorimeter was monitored using a differential refractometer (Knauer, Model 98.00) and recorded using an integrator (HP, Model 3390A). When the surfactant content of the solution exiting the system was equal to the solution entering the calorimeter (constant refractometer output and baseline reached of the calorimeter integrator), the source solution was changed to the next higher surfactant content. Using collected weights of effluents from the system during each step and recordings of the time taken for solvent water to reach the reaction cell from the source bottle and wet the SiO2 (the first step), the time taken for each solution to likewise reach the reaction cell from the source bottle, and the time taken for adsorption to reach equilibrium in the reaction cell during each step, the amount of moles of surfactant adsorbed was determined. The reduced surface excess was calculated using27

Γ(n)2 )

nσ(n) 1 2 ) (n - n0xl2) mas mas 2

(2)

where nσ(n)2 ) the surface excess, n2 is the number of moles of surfactant in the system, n0 ) the total number of moles of solvent and surfactant in the system, xl2 ) the mole fraction of surfactant in bulk liquid at equilibrium with the adsorbent, as ) the specific surface of the adsorbent, and m ) the mass of adsorbent. To obtain the adsorption isotherm, the surface excesses of each step are summed over the concentration range of interest. The molar differential enthalpy of displacement (∆dplhm) is the measured heat change of each step (∆dplh) divided by nσ(n)2 of the step in question. The molar integral (cumulative) enthalpy of displacement (∆dplHm) is the sum of heats of adsorption (∑∆dplh) divided by the sum of moles adsorbed over the concentration range of interest. In comparison with separately measured adsorption and calorimetric enthalpy isotherms, the combined method used in the present study is free from systematic errors. The derived theoretical relationship between curves fitted to experimental calorimetry data was found in the literature as28

∆dplhm )

[

]

∂(∆dplH) ∂(nσ(n)2)

(3)

T,p,as

Tests made prior to the calorimetric measurements showed that (23) Pettersson, A.; Marino, G.; Pursiheimo, A.; Rosenholm, J. B. J. Colloid Interface Sci. 2000, 228, 73. (24) Noll, L. A.; Burchfield, T. E. Silica Gel As a Model Surface for Adsorption Calorimetry of Enhanced Oil Recovery Systems; DOE/BETC/ RI-82/7; U.S. Department of Energy: Bartlesville, 1982. (25) Noll, L. A.; Gall, B. L. Colloids Surf. 1991, 54, 41. (26) Noll, L. A. Colloids Surf. 1987, 26, 43. (27) Kira´ly, Z.; De´ka´ny, I. Colloids Surf. 1988, 266, 663.


Langmuir, Vol. 18, No. 22, 2002 8439 Table 1. Measured cmc, Refractive Index Increments (dn/dm at 950 nm Wavelength) at Natural pH of Surfactant Solutions, pK1 of Alkyldimethylamine Oxide Unimers, and pKa of Alkyldimethylphosphine Oxides Used surfactant

cmc(25 °C) mol dm-3

dn/dm(25 °C) kg mol-1

pK1 (25 °C)

C8DAO C10DAO C12DAO C10DPO

0.19a 1.9 × 10-2 2.0 × 10-3 4.2 × 10-3

0.024 64 0.029 04 0.035 55 0.029 26

4.2 4.8 4.9

a

Figure 3. N2 gas adsorption and desorption isotherms for the silica adsorbent at 0 °C and 1 atm. the response of the refractive index detector was almost linearly proportional to the amount of surfactant in solution, with a slight change of the slope at the cmc’s. Refractive indices of the solutions were obtained by calculating the apparatus constant of the differential refractometer used by calibration with NaBr solutions of known refractive indices. In this study, the adsorption studies were carried out at equilibrium contents of surfactant ranging from diluted solutions to well above the cmc, at natural pH.

Results Surface Area and Pore Structure of Silica Adsorbent by Gas Adsorption Analysis. Figure 3 shows the N2 gas adsorption and desorption isotherms for the silica gel adsorbent used. The hysteresis at quite high relative pressures indicates relatively large pores, while the lack of strong adsorption at low relative pressures indicates only a small proportion of micropores (pores less than about 2 nm in width). The N2 gas adsorption analysis gave an average pore diameter of 18.7 nm (4V/A by BET) and a total pore volume of 1.245 cm3 g-1 (cf. the pore diameter of 15.0 nm and the pore volume of 1.15 cm3 g-1 provided by the supplier). The Harkins and Jura29 thickness plot gave a micropore volume of only 0.0157 cm3 g-1 (less than 1.3% of the total pore volume) and a micropore area of 40.9 m2 g-1 (of the total BET area of 266 m2 g-1). Measured data of characterization are preferred to be used in all calculations throughout this paper rather than the values provided by the supplier. cmc, Refractive Index Increments, and Purity of Surfactants Studied. The cmc values determined decrease with increasing chain length (see Table 1). Due to the smaller hydrophilic repulsion compared with hydrophobic attraction of the alkyldimethylphosphine oxides (accepts only two strong H-bonds from water), they aggregate more easily at low concentrations in comparison with the corresponding amine oxide homologues. Owing to this, the cmc of C10DPO is considerably lower than that of C10DAO. No obvious minima on the surface tension vs concentration curves around the cmc’s were observed. Table 1 also lists the refractive index increments of the aqueous surfactant solutions at 298 K, showing a slight increase with chain length of the alkyldimethylamine oxides studied. The measured cmc’s and refractive index increments were found to be consistent with literature (28) Denoyel, R.; Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1990, 136, 375. (29) Harkins, W. D.; Jura, G. J. Am. Chem. Soc. 1944, 66, 1366.

pKa

-1.5b

Reference 32. b Reference 3.

data, indicating satisfactory purity and accurate contents of prepared solutions.10,30-33 Nonionic-Cationic Equilibrium of the Amphoterics Studied. The measured pK1 values (Table 1) fall well inside the range of values found in the literature.3,34 C10DPO is too weak a base to be titratable in water, but the value (-1.5) was found in the literature expressed in terms of an equivalent pKa using the H0 acidity function.3 The range of pH of the aqueous amine oxide solutions used for the adsorption experiments at 298 K, in the present work, were as follows: pH ) 5.9-7.6 (C8DAO, from dilute solution to solution of the highest molality used), pH ) 5.9-7.5 (C10DAO, from dilute solution to the cmc), and pH ) 5.8-7.0 (C12DAO, from dilute solution to the cmc). Hence, the corresponding ranges of the degree of protonation are R1 ) 2-0.04% of C8DAO, R1 ) 7-0.2% of C10DAO, and R1 ) 11-0.8% of C12DAO, respectively. At the cmc values reported almost all amine oxide is in its nonionic (zwitterionic) form, with the exception of C12DAO of which 0.8% is cationic. The cmc of C12DAO is known to increase with the increase in the ratio of cationic surfactant, due to the electric repulsion among headgroups.35 Surface Ionization of the Silica Studied. The natural pHs of the alkyldimethylamine oxide and alkyldimethylphosphine oxide solutions were within the pH range at which the zeta potential and the relative surface charge density of the silica used are very low. It was found by potentiometric titration that at pH ) 8 (the highest natural pH of the solutions used) the relative surface charge density is only about 2 × 10-20 coulomb nm-2, which gives a maximum of only 0.125 dissociated silanol group nm-2 during the adsorption experiments. The total number of surface hydroxyls is believed to be 5 groups nm-2. Thermodynamics of Adsorption and Surfaceinduced Self-Assembly of Surfactants Studied. Surfactant adsorption on hydrophilic surfaces from aqueous solution is governed by both the interaction of the surfactant with the surface and the hydrophobic effect. The former interactions for the adsorption of nonionic surfactants on hydroxylated silica surface are Londonvan der Waals being universal for any compound and interactions with formation of H-bonds between the hydrophilic groups of surface silanols and surfactant headgroups. In H-bonding the proton acts as a link between two electronegative atoms. H-bonding in many cases brings the main contribution to the adsorption energy. After initial adsorption of surfactants, the hy(30) Herrmann, K. W. J. Phys. Chem. 1962, 66, 295. (31) Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1992, 8, 424. (32) Desnoyers, J. E.; Caron, G.; DeLisi, R.; Roberts, D.; Roux, A.; Perron, G. J. Phys. Chem. 1983, 87, 1397. (33) Corkill, J. M.; Herrmann, K. W. J. Phys. Chem. 1963, 67, 934. (34) Maeda, H.; Tsunoda, M.; Ikeda, S. J. Phys. Chem. 1974, 78, 1086. (35) Imaishi, Y.; Kakehashi, R.; Nezu, T.; Maeda, H. J. Colloid Interface Sci. 1998, 197, 309.

8440


Figure 4. Reduced surface excess and integral enthalpy of displacement of C8DAO at silica gel/water interface vs C8DAO molality at equilibrium in bulk solution at 298 K.

drophobicity of the adsorbates give rise to lateral hydrophobic interactions with adjacent neighbors at the aqueous solution/SiO2 interface resulting in self-assembled structures. Hydrophobic attraction between hydrocarbon chains is entropy driven due to the minimizing of the disruptive effect of the hydrophobe on H-bonding in the water. Hydrophobic interaction is the combined effect of London-van der Waals and H-bonding interactions. Aggregation is opposed by the hydrophilic repulsion between like headgroups, and may be electrical in the case of ionic groups or steric or osmotic in the case of nonionic groups. In this study the solutions of the alkyldimethylamine oxides, from which the adsorption occurs, contain a small amount of the cationic form of the surfactant. The proportion is higher in dilute solution. When present, a protonated alkyldimethylamine oxide surfactant species will adsorb readily to the oppositely charged dissociated silanol groups electrostatically. The counterions present may screen the electrostatic repulsion between the ionic headgroups.5,36 Adsorption of C8DAO. The isotherm of C8DAO adsorbing on silica from aqueous solution in Figure 4 features a plateau starting at molality 0.03 mol kg-1 which is well below the cmc ) 0.19 mol kg-1 (Table 1). The isotherm is therefore incomplete.The packing density of C8DAO at the plateau is only 0.21 molecule nm-2. The integral molar displacement enthalpy of short-chain C8DAO is only exothermic in this molality region, and levels off locally at about -5 kJ mol-1. The cationic alkyldimethylamine oxides adsorb readily onto oppositely charged silica due to long-range Coulombic forces.37 Since the proportion of cationic C8DAO is only 2-0.04% over the range of solution molality studied, the maximum reading of -19.4 kJ mol-1 at initial adsorption in Figure 4 indicates coordination by strong proton transfer reaction (H-bonding) between acidic silanol (strong hydrogen donor, or electron acceptor) and H-bond accepting oxygen atom (electron donor) of nonionic C8DAO.38 The adsorption in Figure 4 is enthalpically driven over the whole molality range studied. Despite the short chain length of C8DAO, (36) Evans, D. F.; Wennerstro¨m, H. The Colloidal DomainsWhere Physics, Chemistry, Biology, and Technology Meet; VCH Publishers: New York, 1994. (37) Somasundaran, P.; Zhang, L. In Adsorption on Silica Surfaces; Papier, E., Ed.; Surfactant Science Series 90; Marcel Dekker Inc.: New York, 2000. (38) Chronister, C. W.; Drago, R. S. J. Am. Chem. Soc. 1993, 115, 4793.


Figure 5. Differential molar enthalpy of displacement of H2O by C8DAO at silica gel/water interface vs C8DAO molality at equilibrium in bulk solution at 298 K.


it is a typical nonionic surfactant.39 Due to surface heterogeneity of silica gel, the high-energy sites are occupied first. This is seen as a decreasing exothermic differential molar enthalpy of displacement vs C8DAO molality in Figure 5. Adsorption of C10DAO and C10DPO. Figure 6 shows the adsorption isotherm for C10DAO on silica together with the integral enthalpy of displacement. The interactions start as exothermic at low packing conditions due to the individual adsorption through H-bonding of headgroups to the energetically strongest surface sites, or electrostatic interaction between cationic headgroups and anionic dissociated silanol groups. The adsorption isotherm is almost linear over this range of surfactant molality in solution. At the point where the integral displacement enthalpy passes to endothermic, the adsorption isotherm features a break point at 0.003 mol kg-1, at which the extent of adsorption becomes high. This critical surface aggregation concentration (CSAC) is obtained as the point of intersection of the two extrapolated lines. (Since for dilute solutions the numerical values of molarity and molality are very close, we use the abbreviation CSAC in the following.) The endothermic (39) Desnoyers, J. E.; Roberts, D.; DeLisi, R.; Perron, G. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. I; p 343.


Figure 7. Differential molar enthalpy of displacement of H2O by C10DAO at silica gel/water interface vs degree of coverage at 298 K.

displacement enthalpies are the result of hydrophobic tail-tail interactions and start to occur at a degree of surface ceverage of less than 10% (Figure 7), suggesting formation of patchwise self-assemblies and influenced by the heterogeneity of the surface. The degree of surface coverage is calculated using experimental data and is given by θ ) Γ/Γmax, where Γ is the reduced surface excess and Γmax is the reduced surface excess at saturation. The adsorption saturates at about bulk cmc and exhibits a plateau of 7.5 × 10-6 mol m-2 or 4.53 adsorbates nm-2. No break point is observed on the adsorption isotherm as an indication of a complete monolayer buildup before formation of a second one; the number of data points are, however, quite few. The integral displacement enthalphy levels off at ∆dplHm,∞ ) 5 kJ mol-1 (∞ stands for the highest molality of solution used in this work). The endothermic differential molar enthalphy of displacement of water by C10DAO in Figure 7 features a maximum of 7 kJ mol-1 at about 80% of full surface coverage. In the most dilute solutions, 7% of C10DAO is protonated, but drops effectively when the content is increased and consequently the natural pH increases. The change from enthalpic to entropic driving force of adsorption interaction occurs for C10DAO at the molality of 0.003 mol kg-1 (Figure 6) and for C10DPO at 0.0017 mol kg-1 (Figure 8). The surface packing densities of the adsorbed surfactants at both these points of CSAC are almost the same i.e., 0.5 ×‚10-6 mol m-2, although they have different cmc’s and conditions for reaching the adsorption plateau (Table 1). Below the CSAC in Figure 8, the adsorption isotherm of nonionic C10DPO seems to be of Langmuir type, and the displacement enthalpy seems to be about constant ∆dplHm ) -7.2 kJ mol-1. The endothermic region of the molar differential enthalphy of displacement of both C10DAO and C10DPO features an extremum at the point where the adsorption approaches saturation (Figures 7 and 9). The same shape of curve has been found for nonionic alkylphenol surfactants adsorbing onto silica gel.16,40 Adsorption of C12DAO. Figure 10 shows the reduced surface excess concentration of C12DAO at the silica gel/ water interface and the molar integral enthalpy of displacement of water by C12DAO at the silica gel/water interface vs the molality of C12DAO in bulk solution at (40) Partyka, S.; Lindheimer, M.; Zaini, S.; Keh, E.; Brun, B. Langmuir 1986, 2, 101.

Langmuir, Vol. 18, No. 22, 2002 8441

Figure 8. Reduced surface excess and integral enthalpy of displacement of nonionic C10DPO at silica gel/water interface vs C10DPO molality at equilibrium in bulk solution at 298 K. Inset is the adsorption isotherm in dilute solution having the axes expanded.

Figure 9. Differential molar enthalpy of displacement of C10DPO at silica gel/water interface vs degree of coverage at 298 K.


equilibrium and 298 K. As in Figures 6 and 8 the adsorption isotherm is of Giles et al.41 type L4, indicating (41) Giles, C. H.; MacEwan, T. H.; Nakhwa, S. N.; Smith, D. J. Chem. Soc. 1960, 3973.

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Table 2. Initial Equilibrium Adsorption Parameters for Displacement of Water with Alkyldimethylamine and Alkyldimethylphosphine Oxide Homologues at the Silica Gel/Water Interface at T ) 298 Ka initial adsorption surfactant

K

∆dplHm (kJ mol-1)

∆dplGm (kJ mol-1)

T∆dplSm (kJ mol-1)

CSAC (mol dm-3)

cmc/CSAC

ΓCSAC (mol m-2)

C8DAO C10DAO C10DPO C12DAO

8.2 31.1 16.0

-19.4 -17.5 -7.2 -8.5

-5.2 -8.5 -6.9

-12.3 1.3 -1.6

0.003 0.0017 0.00032 0.00066

6.3 2.5 6.2 3.0

2.0 × 10-7 2.0 × 10-7 5.0 × 10-8 2.1 × 10-7

a ∆ H ) integral displacement enthalpy; K ) initial equilibrium constant; ∆ G ) integral free energy of displacement; T∆ S dpl m dpl m dpl m ) integral displacement entropy term; CSAC ) critical surface aggregation concentration; ΓCSAC ) reduced surface excess at the CSAC.

to derive the equation are rather stringent, the deviations from the Langmuir equation are often opposite in systems where the assumptions do not strictly apply e.g., when used in many systems with preferred adsorption onto a heterogeneous solid surface. When describing adsorption using the Langmuir equation, interactions between adsorbed molecules must be ignored, and only monolayer formation can be included. In the following we use the Langmuir equation to describe the initial adsorption in the most dilute solutions. The Langmuir equation can at equilibrium be conveniently derived as5,36,42

θ)

Figure 11. Differential molar enthalpy of displacement of C12DAO at silica gel/water interface vs degree of coverage at 298 K. Inset is the expansion of the left part of the x-axis.

multilayer adsorption or porous adsorbent. The adsorption of C12DAO reaches a plateau at solution concentrations slightly above the cmc. With regard to the other surfactants studied, the displacement starts in dilute solution as an exothermic interaction. The proportion of cationic C12DAO is 11% of the surfactant content in dilute solution and 0.8% at the cmc. Attributed to the longer chain length of C12DAO, the hydrophobic interactions onset abruptly at the aqueous solution/SiO2 interface at low surface coverage and show a high maximum of the endothermic molar enthalpy of displacement ∆dplHm ) 10 kJ mol-1 (the integral, see Figure 10) and ∆dplhm ) 16.5 kJ mol-1 (the differential, see Figure 11). Figure 10 features a broad decrease in the endothermic integral displacement enthalpy that levels off at only ∆dplHm,∞ ) 1.1 kJ mol-1, while less lipophilic C10DAO levels off at ∆dplHm,∞ ) 4.9 kJ mol-1 (Figure 6), although the surface packing densities at saturated adsorption are about the same. Due to longer hydrocarbon moiety, the surface self-assembly starts at less than 1% degree of surface coverage (Figure 11), suggesting patchwise surface self-assembly. Unlike C10DAO (and C10DPO), the endothermic differential enthalpy of displacement of C12DAO does not show a local maximum at about cmc when the adsorption saturates, but features a broad plateau of only about ∆dplHm ) 1 kJ mol-1 ranging from the degree of coverage θ ) 0.2 to about 0.8 (Figure 11). Discussion Analysis of the Initial Adsorption in Terms of the Langmuir Equation. An overwhelming majority of the analysis of surfactant adsorption is performed in terms of the Langmuir equation. Although the assumptions used

Ka 1 + Ka

(4)

where θ is the fraction of covered surface (monolayer assumed). a is the activity of surfactant at equilibrium in bulk solution. K is the equilibrium constant and describes the partitioning of the surfactant between the surface phase and the bulk solution phase. In dilute solution the activity becomes equal to the molality. K (without or with the units of reciprocal molality) can be calculated at the limit of infinitely low solution molality when eq 4 is reduced to

K)

(mθ )

mf0

(5)

The displacement free energy is related to the K through ∆dplGm ) -RT ln(K), and the displacement entropy term is given by the relation ∆dplGm ) ∆dplHm - T∆dplSm. The measured and calculated thermodynamic quantities at 25 °C of adsorption processes studied, except for C8DAO (adsorption at cmc was not measured) are listed in Table 2. The initial adsorption of all surfactants studied is enthalpically driven. The high exothermic displacement enthalpy of the amine oxide surfactants compared to nonionic C10DPO may indicate extensive formation of multiple H-bonds between the amine oxide surfactants and surface sites, due to the higher tendency of tertiary amine oxide to form strong H-bonds compared to the tertiary phosphine oxide. The equilibrium constant is high for nonionic C10DPO, indicating strong adsorption. Despite electrostatic adsorption interactions due to 7% cationic species of the C10DAO content in solution, the initial adsorption of C10DPO onto SiO2 at natural pH is thermodynamically more favorable than of C10DAO, stemming from the fact that the PfO group is less strongly hydrophilic than the NfO group. All the cationic surfactant species and dissociated silanol groups seem not to lose all their counterions and hydrated water when interacting electrostatically with each other. This is indicated by the positive entropy term of nonionic C10DPO (42) Hiemenz, P. C. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1986.


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Table 3. Equilibrium Adsorption Parameters for Saturated Displacement of Water with Alkyldimethylamine and Alkyldimethylphosphine Oxide Homologues at the Silica Gel/Water Interface at T ) 298 Ka saturated adsorption surfactant

Γmax (mol m-2)

N2 (nm-2)

m (mol kg-1)

∆dplHm,∞ (kJ mol-1)

degree of pore vol filled (%)

Γmax/ΓCSAC

C10DAO C10DPO C12DAO

7.5 × 10-6 7.4 × 10-6 7.5 × 10-6

4.53 4.49 4.58

0.025 0.0035 0.0025

4.9 5.1 1.1

Adsorption of Alkyldimethylamine and Alkyldimethylphosphine Oxides

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