Temperature and pH Effects on the Coadsorption of Sodium Dodecyl

Rosemary Windsor, David J. Neivandt,† and Paul B. Davies*. Department of Chemistry, University of Cambridge, Lensfield Road,. Cambridge, CB2 1EW, U...
1 downloads 0 Views 58KB Size
Download PDF
Langmuir 2002, 18, 2199-2204

2199

Temperature and pH Effects on the Coadsorption of Sodium Dodecyl Sulfate and Poly(ethylenimine) Rosemary Windsor, David J. Neivandt,† and Paul B. Davies* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K. Received August 15, 2001. In Final Form: December 12, 2001 The polar orientation and alkyl chain conformational ordering of sodium dodecyl sulfate (SDS) at the octadecanethiol/water hydrophobic interface have been investigated using sum frequency vibrational spectroscopy (SFS). The effects of different concentrations of the cationic polymer poly(ethylenimine), PEI, along with the solution pH and temperature have been determined. PEI was found to promote synergistic adsorption of ordered SDS at concentrations as low as 10 ppm at pH 6.0 through cooperative polymer/ surfactant complex formation. At lower pH’s and in the presence of the polymer stronger interfacial ordering of the surfactant was observed, attributed to increased protonation of PEI amino groups in acidic solutions and consequent decreases in the intersurfactant spacings of complexed SDS. SDS was found to be present at the hydrophobic surface at temperatures of up to 60 °C, although thermally induced motion of the surfactant chains resulted in a monotonic decrease in the degree of conformational order with rising temperature. In the presence of PEI the effect of temperature on the ordering of SDS, present predominantly in the form of a polymer/surfactant complex, was qualitatively similar.

Introduction Adsorption from solutions containing polymers and surfactants, both individually and in combination, is widely investigated.1-3 A thorough understanding of the adsorption mechanism allows predictions to be made to achieve desired interfacial effects. Characterization of the effects of variations in parameters such as the nature of the adsorption interface, the concentration of the adsorbates, electrolyte addition, solution pH, and temperature is therefore essential. Recently we investigated adsorption at a model solid/aqueous solution hydrophobic interface from an anionic surfactant/cationic polymer system.4 Specifically the effect of varying the surfactant concentration both in the presence and absence of added electrolyte and polymer was determined for the system of sodium dodecyl sulfate/poly(ethylenimine). This work is an extension of our SDS/PEI investigation in which variations in the concentration of the polymer, the solution pH, and the temperature are considered. The SDS/PEI interaction is strongly electrostatically driven,4 and it might therefore be expected that the ratio of polymeric charge sites to surfactant molecules in solution could significantly effect the adsorption of SDS. Consequently we determine the effect on adsorption of SDS at the hydrophobic d-ODT (deuterated octadeacanethiol)/ solution interface of varying the number of cationic PEI sites in solution in relation to a fixed number of surfactant species. This is achieved in two ways. First, the concentration of PEI is varied at fixed pH to alter the number of polymeric chains in solution while keeping the linear charge density per chain constant. Second, the “tunability” * To whom correspondence should be addressed. E-mail: pbd2@ cam.ac.uk. † Present address: Department of Chemical Engineering, The University of Maine, Orono, ME 04469. (1) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989. (2) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (3) Duffy, D. C.; Davies, P. B.; Creeth, A. M. Langmuir 1995, 11, 2931. (4) Windsor, R.; Neivandt, D. J.; Davies, P. B. Langmuir 2001, 17, 7306.

of the linear charge density of PEI with solution pH at constant polymer concentration is employed. PEI is a cationic polyelectrolyte at most pH values due to protonation of its primary, secondary, and tertiary amino groups. Systematic variation of solution pH provides continuously tunable PEI linear charge density, with the highest charge occurring at the lowest pH value. Approximately 75% of the polymer amino groups are protonated at pH 2 with the degree of protonation dropping approximately linearly to a point of zero charge (pzc) in the region pH 10.8-11.0.5 Temperature changes usually have a pronounced effect on the interfacial adsorption of surfactants. The surface excess of SDS is typically observed to decrease with increasing temperature at both hydrophilic and hydrophobic interfaces. Specifically, Pavan et al.6 observed lower surface excesses of SDS on hydrophilic layered double hydroxide (LDH) at increased temperatures, in common with the trends observed on alumina by Somasundaran and Fuerstenau7 and on quartz by Fridriksberg et al.8 Similarly on hydrophobic polystyrene surfaces Piirma and Chen9 observed decreased amounts of SDS adsorbed on polystyrene latex particles at elevated temperatures, while Singh and co-workers10,11 reported lower SDS surface excesses at the liquid/air interface as the solution temperature was raised. In this study the effect on the adsorption of SDS of varying the temperature over the range 18-60 °C in both the presence and absence of PEI is investigated. The nonlinear optical technique of sum frequency vibrational spectroscopy (SFS) is used in this work to determine the polar orientation and degree of conforma(5) Radeva, T.; Petkanchin, I. J. Colloid Interface Sci. 1997, 196, 87. (6) Pavan, P. C.; Crepaldi, E. L.; Gomes, G. de A.; Valim, J. B. Colloids Surf., A 1999, 154, 399. (7) Somasundaran, P.; Fuersenau, D. W. Soc. Min. Eng. AIME 1972, 252, 275. (8) Fridriksberg, D. A.; Tikhomolova, K. P.; Sidorova, M. P. Croat. Chem. Acta 1979, 52, 125. (9) Piirma, I.; Chen, S.-R. J. Colloid Interface Sci. 1980, 74, 90. (10) Sharma, V. K.; Yadav, O. P.; Singh, J. Colloids and Surf., A 1996, 110, 23. (11) Anand, K.; Yadav, O. P.; Singh, P. P. Colloids Surf. 1991, 55, 345.

10.1021/la011295c CCC: $22.00 © 2002 American Chemical Society Published on Web 02/22/2002

2200

Langmuir, Vol. 18, No. 6, 2002

tional order of SDS at a hydrophobic surface. A visible laser beam of fixed frequency and a tunable frequency infrared beam are spatially and temporally overlapped on an interface, and the intensity of the beam emitted at the sum of the two frequencies is measured. When the infrared frequency coincides with a resonant frequency of the adsorbed species, the intensity of the emitted light changes. Detecting the emitted light as a function of infrared frequency produces a vibrational spectrum of the adsorbate. A spectrum is only produced from molecules at an interface where the centrosymmetry of the bulk phase is broken; further, these interfacial molecules must have a net polar orientation to be sum frequency active. There is no sum frequency emission from molecules arranged in an equal number of opposite orientations or from a completely disordered surface structure. Since surfactants and polymers are isotropic in solution, sum frequency activity only arises if they are adsorbed at an interface. Polar orientation is determined from the relative phase of the SF signal. For the d-ODT/aqueous solution interface investigated here it is well-known that alkyl resonance “dips” in spectra are generated by surfactant molecules with the hydrocarbon chain orientated toward the surface, while resonant “peaks” correspond to molecules orientated with the hydrocarbon chain extending into solution.12 The degree of alkyl chain conformational order is reflected in the relative strength of the symmetric methyl (r+) and methylene (d+) stretching resonances, larger r+/d+ ratios indicating smaller numbers of gauche defects in the alkyl chains and increased conformational order. Experimental Section

Windsor et al.

Figure 1. Sum frequency spectra taken in the PPP beam polarization combination (sum frequency, visible, infrared) of sodium dodecyl sulfate (SDS) adsorbed at the (d-ODT)/solution interface from 0.1 M NaCl, pH 6.0 solution, at 18 °C as a function of surfactant concentration. The solid circles are the individual data points and the solid lines are theoretical fits to the data. Each spectrum is plotted on the same arbitrary intensity scale (which differs from figure to figure). Spectra in this figure and subsequent figures are displaced vertically for clarity. hydrophobic substrates employed were octadecanethiol (ODT) monolayers chemisorbed on gold-coated silicon wafers from methanolic solutions.14 The ODT was fully deuterated to eliminate contributions to sum frequency spectra in the C-H stretching region arising from the substrate. Sodium dodecyl sulfate, kindly supplied by Unilever Research (Port Sunlight Laboratory), was doubly recrystallized from ethanol. Fully deuterated d-SDS was obtained from MSN Isotopes (CDN Isotopes) Canada and was also doubly recrystallized from ethanol prior to use. Poly(ethylenimine), Mr ) 750 000, was obtained from BASF and was used without further purification. Where required, pH adjustments were performed using AR grade hydrochloric acid or aqueous sodium hydroxide. The SDS concentrations given in this paper are expressed as fractions of the SDS critical micelle concentration (cmc) at 25 °C of 8.3 × 10-3 M; PEI concentrations are given in parts per million by weight of the solution (ppm).

Spectra were recorded on a nanosecond sum frequency spectrometer. A visible beam at 532 nm was produced by a frequency-doubled Nd:YAG laser giving 8 ns pulses at a repetition rate of 11.5 Hz. Approximately 90% of the 532 nm beam was used to pump a dye laser giving tunable red light which was converted to tunable infrared light by selecting the third stokes shift of the stimulated Raman-scattered red beam in 34 atm of H2. The residual 10% of the 532 nm beam was temporally delayed in a Herriot cell and overlapped on the sample with the infrared beam in a counterpropagating geometry. The pulse energies of the 532 nm and infrared beams at the sample were approximately 2.5 and 1.8 mJ, respectively. The emitted sum frequency photons were detected by a photomultiplier tube and a gated analogue boxcar. Spectra were collected by scanning the methyl/methylene stretching region (2800-3000 cm-1) at intervals of 2 cm-1, and typically 60 complete scans were averaged for one spectrum. Each spectrum was then normalized by its nonresonant baseline, allowing the ratio of intensities of resonances between spectra to be interpreted. Next, the spectral profiles were modeled by employing a least-squares Levenberg-Marquardt fit to an equation based on a Lorentzian description of the SF resonant line shapes.13 (The theoretical modeling procedure provides the solid lines in the figures.) Although the entire 200 cm-1 spectrum was fitted, only the symmetric methyl/methylene stretching region (2830-2890 cm-1) required for interpretation is presented here. Cleaning procedures for the Teflon and stainless steel sample cells and the experimental protocols used to record in situ spectra at the solid/liquid interface have been described previously.4 For temperature-dependent adsorption studies the stainless steel cell was employed and modified to incorporate two external resistively heated blocks and an internal thermocouple. The sample temperature was assumed to be the same as the thermocouple temperature after a 30 min equilibration time. A temperature stability of (1 °C was typically achieved. Experiments were performed from low to high temperature. The

Sum frequency spectra of SDS adsorbed at the hydrophobic d-ODT solid/solution interface from 0.1 M NaCl solution as a function of increasing surfactant concentration are given in Figure 1. Two resonances assigned to symmetric C-H stretching modes of the surfactant alkyl chains15 are observed which serve to probe the conformational order of the surfactant. The resonance at 2845 cm-1 is due to the d+ symmetric methylene stretching mode, while that at 2873 cm-1 arises from the r+ symmetric stretching mode of the terminal methyl group of the SDS alkyl chains. The fact that the resonances are dips implies that the surfactant molecules are adsorbed at the interface with their hydrophobic tails oriented toward the surface and their polar headgroups extending into solution. At 1/100 cmc only a weak d+ methylene resonance is observed in the SDS spectrum. The absence of a corresponding r+ methyl resonance implies that the alkyl chains of the

(12) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1993, 97, 7141. (13) Lambert, A. G. Ph.D. Thesis, University of Cambridge, 2001.

(14) Duffy, D. C. Ph.D. Thesis, University of Cambridge, 1996. (15) Ward, R. N.; Duffy, D. C.; Davies, P. B. Bain, C. D. J. Phys. Chem. 1994, 98, 8536.

Results and Discussion

Coadsorption of SDS and PEI

Figure 2. Sum frequency spectra (PPP) of SDS adsorbed from 1/100 cmc SDS, 0.1 M NaCl, pH 6.0 solution, at 18 °C as a function of poly(ethylenimine) (PEI) concentration.

adsorbed surfactant have low conformational order, consistent with a small surface excess and hence low packing density of SDS. Raising the solution concentration of surfactant to a value of 1/30 cmc results in the emergence of an r+ resonance, as expected for a rise in the surface excess of SDS and a corresponding increase in the degree of conformational order of the adsorbed surfactant. Increasing the solution concentration of SDS to 1/10 cmc results in a minor rise in the r+/d+ ratio and hence the degree of alkyl chain ordering. No further change in the r+/d+ ratio occurs at surfactant concentrations as high as 3 cmc. This result suggests that a maximum degree of conformational order of the SDS alkyl chains at the interface is achieved at approximately 1/10 cmc, consistent with the attainment of a high packing density and hence full monolayer coverage. Further, the fact that no reduction of SDS conformational order was detected in the region of the cmc confirms that the SDS was not contaminated with its hydrolysis product, dodecanol.16 Varying the Polymer Concentration. The effect of poly(ethylenimine) on the adsorption of SDS is shown in Figure 2 through the addition of increasing concentrations of the polymer to 1/100 cmc SDS solutions containing 0.1 M NaCl at pH 6.0. For experiments performed in the presence of PEI there is a possible contribution to SF spectra from the methylene groups of the polymer. To determine if such a contribution existed, spectra in the C-H stretching region were recorded for a solution containing solely PEI and from mixed deuterated SDS/ protonated PEI solutions (not shown). No resonances were observed in the spectra indicating that the methylene groups of the polymer are SF inactive, most likely due to an absence of macroscopic ordering of PEI at the interface. Consequently all methyl resonances (which are absent in the polymer) and methylene resonances can be confidently assigned to the surfactant. Further, it is noted that the frequencies of the surfactant SF resonances were unchanged in the presence of the polymer. At the lowest PEI concentration investigated, 1 ppm, a strong d+ and a weak r+ resonance are evident, indicative (16) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem. B 1997, 101, 1594.

Langmuir, Vol. 18, No. 6, 2002 2201

of a surfactant layer possessing only a small degree of alkyl chain order. Comparison with the lowest spectrum of Figure 1 suggests that the presence of PEI at 1 ppm may induce a marginal increase in the degree of SDS conformational order over that observed in its absence. Increasing the polymer concentration to 10 ppm however produces a dramatic change in the SF spectrum with a far more dominant r+ resonance observed. Such an increase in the r+/d+ ratio is consistent with a rise in the overall degree of conformational order of the surfactant in the interfacial region. The mode of action of PEI in promoting SDS adsorption is through complexation of the oppositely charged species in solution.4 The hydrophilicity and hence solubility of the complex is lower than the sum of the individual noncomplexed SDS and PEI due to the loss of charge on both species. Further the hydrophobic content of the complex is increased by the inclusion of the alkyl tail of the surfactant. The combined effect of these two factors is synergistic adsorption of the polymer/surfactant complex at the hydrophobic interface. Raising the polymer concentration to a value of 50 ppm and again to 100 ppm has little effect on the r+/d+ ratio. The apparent insensitivity of the r+/d+ ratio to the polymer concentration above 10 ppm is unexpected if the polymer/ surfactant complexation mechanism is driven solely by charge neutralization. On the basis of the assumption that at pH 6.0 30% of the amino groups of PEI are protonated,17 at 10 ppm polymer concentration and 1/100 cmc SDS concentration there is 1.2 times more SDS in solution than there are charged sites of polymer. At 50 ppm PEI, 1/100 cmc SDS, however, there is only sufficient surfactant present to neutralize a maximum of 24% of charged polymer sites, while at 100 ppm PEI concentration this value decreases to 12%. If a statistical distribution of sites of SDS complexation along the polymer is assumed, then the packing density of SDS at the interface would be predicted to decrease with decreasing percentage of SDS to PEI charged sites. Clearly this does not occur, which leads to the conclusion that SDS/PEI complexation in the presence of electrolyte must be cooperative in nature. Cooperative interactions between charged polymers and surfactants derive from the entropic gain that arises from surfactants binding at charged sites neighboring those already occupied by other surfactant molecules, thereby maximizing inter-alkyl chain interactions. The addition of electrolyte favors this interaction, primarily through screening favorable electrostatic interactions between surfactant headgroups and polymer charged sites (which would be predicted to produce a statistical distribution of surfactant along the polymer), thereby increasing the relative strength of the cooperative binding mechanism. We have shown previously that the SDS/PEI interaction is cooperative in the presence of 0.1 M NaCl through studying the effect on the adsorbed conformation of the complex of varying the surfactant concentration.4 The degree of alkyl chain ordering was found to be entirely independent of the SDS concentration, similar to the findings of this study where the extent of SDS chain conformational order is found to be largely insensitive to the polymer concentration once above a threshold PEI concentration. A consistent picture of the polymer/surfactant interaction emerges. Raising the number of polymer chains in solution would be predicted to result in smaller average numbers of surfactant molecules bound/ chain but to have no effect on the average spacing between SDS molecules due to the cooperativity of the polymer/ surfactant interaction. Consequently while the average (17) Product specification from BASF.

2202

Langmuir, Vol. 18, No. 6, 2002

number of SDS molecules/polymer chain at the interface decreases with increasing polymer concentration, the intersurfactant spacing and hence the degree of alkyl chain conformational ordering is unaffected. The extent of SDS conformational order induced by the addition of PEI at 1 ppm is minimal by comparison with that observed at 10 ppm and higher PEI concentrations. This implies that a critical polymer concentration is required before significant SDS/PEI interaction occurs. Such an effect has been observed previously for other polymer/surfactant systems and has been associated with the insensitivity of the critical association concentration to surfactant concentration above a threshold polymer concentration.3,18 Alternatively, the small observable effect may be due to the bulk of the SDS adsorption occurring as noncomplexed surfactant with only a small fraction of SDS adsorbed in a complexed form. This hypothesis is supported by the fact that at 1 ppm PEI and 1/100 cmc SDS concentrations there is 12 times more SDS present in solution than is theoretically required for 1:1 charge neutralization of the protonated polymer. Consequently the adsorption process would be expected to be dominated by free rather than complexed surfactant. At 10 ppm the ratio of SDS molecules in solution to available charge sites on the polymer is far closer (1.2), and hence, the adsorption process occurs primarily as the complexed form with only a minor contribution (at most ≈20%) arising from free SDS. A cooperatively induced high r+/d+ ratio results. Increasing the PEI concentration to 50 ppm gives far more polymeric charge sites than available surfactant molecules in solution, and the adsorbed SDS would be expected to be entirely PEI complexed. It should be noted that the above calculations regarding charge neutralization are only approximately correct since Winnik et al.19 have shown that the degree of protonation is, in addition to being a function of pH, weakly dependent on the polymer solution concentration. Changing the Solution pH. The effect of varying the linear charge density of PEI through modification of the solution pH on the adsorption of SDS is depicted in Figure 3. Specifically, adsorption from 1/100 cmc solution in the presence of 100 ppm PEI and 0.1 M NaCl was investigated over the pH range 4-12. At pH 4.0 similar strength r+ and d+ resonances are observed in the SF spectrum indicating the existence of a comparatively ordered SDS layer at the hydrophobic surface. Raising the solution pH incrementally through 6.0 and 10.0 to a final value of 12.0 results in a monotonic decrease in the r+/d+ ratio, consistent with a gradual decrease in the degree of SDS alkyl chain conformational order. The decrease in the degree of conformational order of SDS adsorbed at the interface in the presence of PEI as a function of rising pH is most likely caused by changes in the linear charge density of the polymer. The percentage of amino group protonation and consequently the linear charge density of PEI are strong functions of solution pH. At pH 4.0 approximately 50% protonation is expected, at pH 6.0 30%, and at pH 10.0 only 10%, while at pH 12.0 no formal charge exists on the polymer. Given that the SDS/PEI interaction has a large electrostatic component, decreasing the degree of protonation gives rise to an increase in the average separation of charges along the polymer and consequently larger inter-surfactant spacings in the polymer/surfactant complex and lower degrees of conformational order. (18) Buckingham, J. H.; Lucassen, J.; Hollway, F. J. Colloid Interface Sci. 1978, 67, 423. (19) Winnik, M. A.; Bystryak, S. M.; Liu, Z.; Siddiqui, J. Macromolecules 1988, 21, 6855.

Windsor et al.

Figure 3. Sum frequency spectra in the PPP polarization combination of SDS adsorbed from 1/100 cmc SDS, 100 ppm PEI, 0.1 M NaCl solution, at 18 °C as a function of pH.

At pH 12.0 the SF spectrum (top of Figure 3) contains a distinct d+ and a weak r+ resonance. Comparison of this spectrum with the lowest spectrum of Figure 1 indicates that the polymer induces a small increase in the conformational order of the surfactant, even though pH 12.0 is above the pzc of the polymer and formal electrostatic interactions between SDS and PEI do not exist. The origin of the weak SDS/PEI interaction at high pH has been investigated in bulk solution by several groups. While it is generally accepted that an attractive interaction does exist, its origin is under debate. Li et al.20 attribute it, at least at SDS concentrations greater than that at which micellar behavior begins, to interactions of uncharged nitrogen atoms with micellar surfactant in an analogous manner to the interaction of SDS with the oxygen atoms of poly(ethylene oxide). An alternative explanation proposed by Winnik et al.21 and others22,23 is that hydrophobic interactions are the dominant attractive driving force. While our present experiments shed no light on the origin of the interaction, they do indirectly support the conclusion that a weak SDS/PEI interaction does exist at pH values as high as 12.0 in the bulk. Effect of Temperature. The spectra presented in Figure 4 demonstrate that temperature has a significant effect on the conformation of SDS adsorbed from 1/30 cmc, 0.1 M NaCl solution at the hydrophobic interface. At 18 °C both strong r+ and d+ modes are observed, consistent with a comparatively ordered surfactant layer. Raising the temperature incrementally through 40 and 50 to 60 °C results in a systematic decrease in the r+/d+ ratio, suggesting a monotonic drop in the conformational order of SDS at the interface. This observation is consistent with other SF studies that have (in the absence of phase transitions24) shown the same behavior.25 The increase in (20) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093. (21) Winnik, M. A.; Bystryak, S. M.; Siddiqui, J. Macromolecules 1999, 32, 624. (22) Yui, T. S. T. I.; Abilov, Zh. A.; Pal’mer, V. G.; Musabekov, K. B. Issled. Ravnovesnykh Sist. 1982, 78. (23) Abilov, Zh. A.; Beisebekov, M. K.; Musabekov, K. B. In Reactions in the Liquid Phase; Kazach State University: Alma-Ata, USSR, 1979; p 134.

Coadsorption of SDS and PEI

Figure 4. Sum frequency (PPP) spectra of SDS adsorbed from 1/30 cmc SDS, 0.1 M NaCl solution, at pH 6.0 as a function of temperature.

the number of gauche defects in the surfactant alkyl chains with temperature is attributed to amplification of the thermal motion of the surfactant and consequent loss of conformational order. No significant change in the strength of the d+ resonance is observed as the temperature is raised (Figure 4), suggesting that the surface excess of the surfactant is relatively insensitive to temperature changes over the range investigated. This result is in contrast to other work (as discussed in the Introduction) in which thermal desorption of SDS at elevated temperatures at both hydrophilic and hydrophobic interfaces has been observed. This apparently anomalous result can be attributed to the presence of added electrolyte. In contrast to the bulk of the work performed in the literature, the current experiment was carried out in the presence of 0.1 M NaCl. Sum Frequency spectra recorded under the same conditions as Figure 4 but in the absence of added electrolyte (not shown) contained steadily decreasing r+ and d+ resonant strengths, with the spectrum at 60 °C almost devoid of resonances. This is consistent with nearly complete thermal desorption of SDS from the hydrophobic interface at 60 °C. An analogous result was obtained by Anand et al.11 at the air/water interface. A 2% decrease was observed in the surface excess of SDS when raising the temperature over a 20 °C range in the presence of 0.1 M NaCl while in the absence of NaCl the surface excess dropped by 12% for a temperature rise of only 10 °C. Consequently it may be concluded that the addition of NaCl acts to stabilize SDS against thermal desorption at hydrophobic interfaces. The thermal stability of the SDS monolayer in the presence of NaCl is attributed to the effect of the electrolyte on the solubility of the surfactant. Surfactant solubility generally rises with temperature due to two complementary mechanisms, one of which is related to the tail group and the other to the headgroup of the surfactant. First, the nature of hydration of the nonpolar alkyl tail is temperature dependent. Both theoretical26,27 and experi(24) Johal, M. S.; Usadi, E. W.; Davies, P. B. J. Chem. Soc., Faraday Trans. 1996, 92, 573. (25) Johal, M. S.; Usadi, E. W.; Davies, P. B. Faraday Discuss. 1996, 104, 231. (26) Mancera, R. L. J. Chem. Soc., Faraday Trans. 1998, 94, 3549. (27) Liu, H.; Ruckenstein, E. J. Phys. Chem. B 1998, 102, 1005.

Langmuir, Vol. 18, No. 6, 2002 2203

Figure 5. Sum frequency (PPP) spectra of SDS adsorbed from 1/100 cmc SDS, 100 ppm PEI, 0.1 M NaCl solution, at pH 6.0 as a function of temperature.

mental28,29 studies of the effect of temperature on nonpolar and amphiphilic solutes suggest that the aqueous solubility decreases slightly with increasing temperature around room temperature (entropically driven), reaches a minimum, and then increases strongly at higher temperatures (enthalpically driven). The structure of the water that hydrates alkyl chains is however not only temperature dependent but also a function of added electrolyte. Addition of NaCl acts to disrupt hydrogen bonding and decreases the solubility of the surfactant (the well-known “salting out effect”). In the presence of added electrolyte the surfactant solubility at a given temperature is therefore lower than in its absence. An additional contribution of the salting out effect to increased surfactant layer stability is the induced rise in the strength of inter-surfactant alkyl chain hydrophobic interactions. Variations in the degree of headgroup counterion association (R) with temperature constitute the second mechanism driving increased surfactant solubility at elevated temperatures. Generally R tends to decrease with rising temperature, thereby increasing surfactant solubility. For SDS R is reported to vary from 0.65 at 25 °C to 0.62 at 45 °C.10 Addition of an electrolyte to solution however increases the degree of counterion association of ionic surfactants and reduces the solubility. At a given temperature then the solubility of SDS is lower in the presence of added NaCl than in its absence. Both the tail group and headgroup related mechanisms that drive increased solubility of surfactants at higher temperatures are therefore damped in the presence of NaCl, and increased thermal stability of the adsorbed SDS layer would be expected. Spectra of SDS adsorbed at the hydrophobic interface from solution containing 100 ppm PEI in addition to 1/100 cmc SDS and 0.1 M NaCl as a function of temperature are given in Figure 5. Little change in the overall strength of the d+ resonance is observed with rising temperature, in common with the spectra of Figure 4. It is concluded therefore that, as for SDS adsorbed in the absence of PEI, (28) Sacco, A.; Holz, M. J. Chem. Soc., Faraday Trans. 1997, 93, 1101. (29) Sacco, A.; De Cillis, F. M.; Holz, M. J. Chem. Soc., Faraday Trans. 1998, 94, 2089.

2204

Langmuir, Vol. 18, No. 6, 2002

thermal desorption of the surfactant does not occur to a significant extent. The thermal stability of the SDS layer is attributed to two effects. First, as described above, the solubility of the surfactant is decreased and inter-alkyl chain hydrophobic interactions are strengthened by the addition of electrolyte resulting in a lower propensity for desorption. The second driving force for thermal stability derives from the fact that adsorption of the surfactant in the presence of the charged polymer occurs as a polymer/ surfactant complex. Since adsorption of the SDS/PEI complex is synergistic, the driving force for adsorption is greater than the sum of the individual polymer and surfactant driving forces; therefore, desorption is energetically more demanding for the complex than for SDS on its own. While the strength of the d+ methylene resonance in Figure 5 is comparatively constant as a function of temperature, the strength of the r+ resonance monotonically decreases with rising temperature as in Figure 4 and is consistent with increasing disorder of the surfactant. A contributing factor to the loss of SDS conformational order is a reduction in the strength of lateral hydrophobic interactions between interfacial surfactant molecules at elevated temperatures. This phenomenon is also reflected in the complexation behavior of the polymer and the surfactant. Both the binding affinity and the degree of cooperativity decrease with rising temperature above a critical temperature that is typically around ambient temperature.30,31 Any exchange processes of complexed surfactant at the interface with free surfactant in solution will therefore result in further induced disorder in the adsorbed layer. A second process that contributes to increased disorder of the SDS interfacial layer with temperature is rising thermal motion of the adsorbed complex. As discussed when considering the effect of temperature on SDS adsorbed from solution containing only the surfactant, raising the temperature increases the thermal motion of adsorbed surfactants and typically produces smaller r+/ d+ ratios in sum frequency spectra. With the addition of PEI, however, the increase in the thermal motion of the surfactant tail is at least partially governed by the effect of temperature on the polymer. The observed decrease in the r+/d+ ratio with rising temperature up to 60 °C is then (30) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; Chapter 5. (31) Goddard, E. D. Colloids Surf. 1986, 19, 301.

Windsor et al.

attributed to both a temperature-induced decrease in lateral alkyl chain hydrophobic interactions and to a rise in the thermal motion of the polymer/surfactant complex. Summary and Conclusions Sum frequency spectroscopy has been employed to show that SDS adsorbed at a hydrophobic surface from aqueous solution is orientated with its alkyl chain toward the surface (and by inference its polar headgroup toward solution) under all conditions investigated here. The degree of conformational order of the hydrocarbon tail does however vary with surfactant and electrolyte concentration, temperature, and the presence or absence of the cationic chargeable polymer PEI. In the presence of even small quantities of the latter (10 ppm), the ordering of SDS is enhanced. This is assumed to be due to incorporation of SDS in a polymer/surfactant complex adsorbed from solution. Strong supporting evidence for this hypothesis comes from the effect on SDS conformation of “tuning” the charge density of the polymer and thereby changing the nature of the interaction between the anionic surfactant and the cationic polymer. Qualitatively similar results have been observed previously in the authors’ laboratory for complementary systems of the cationic surfactant CTAB and several anionic polymers. Temperature is observed to have a significant effect on the conformation of adsorbed SDS both in the presence and absence of PEI. At higher temperatures greater conformational disorder of the surfactant is detected, attributed to increased thermal motion of the alkyl chains. Although less ordered at elevated temperatures, the surface excess of SDS appears to be largely unaffected by temperature changes at least over the 20-60 °C range investigated in this work. Acknowledgment. The authors wish to thank Drs. A. M. Creeth and J. Hines of Unilever Research (Port Sunlight Laboratory) for helpful discussions and the latter for the gift of the recrystallized SDS. Drs. M. S. Johal, E. W. Usadi, and A. M. Briggs are acknowledged for experimental assistance, and Dr. A. G. Lambert is thanked for helpful discussions. R.W. wishes to thank the EPSRC and Unilever Research for a CASE studentship. D.J.N. gratefully acknowledges the Oppenheimer Fund of the University of Cambridge for the award of a Research Fellowship. LA011295C