Interaction of Cetyl-trimethylammonium Bromide with Swollen and

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Interaction of Cetyl-trimethylammonium Bromide with Swollen and Collapsed Poly(N-isopropylacrylamide) Nanogel Particles Attila Borsos and Tibor Gil
anyi* Laboratory of Interfaces and Nanosize Systems, Institute for Chemistry, E€otv€os Lor
and University, Post Office Box 32, H-1117 Budapest, Hungary ABSTRACT: The interaction of cetyl-trimethylammonium bromide (CTAB) with swollen and collapsed poly(N-isopropylacrylamide) (pNIPAM) monodisperse nanogel particles was investigated by electrophoretic mobility, dynamic light scattering, and potentiometric surfactant activity measurements. The surfactant binds to the nanogel particles as monomers in the whole CTAB concentration range and binds in the form of surfactant aggregates as well above a critical concentration (cac) in both the swollen and collapsed state of the pNIPAM. The swollen particle system is a thermodynamically stable solution. The collapsed nanogel particle system is an electrically stabilized colloid dispersion, which coagulates when the particles are near the electrically neutral state. An analytically undetectably small amount of surfactant binding (5  10
7 mol/g of pNIPAM) leads to a dramatic effect on the stability of the pNIPAM nanogel system. The electrokinetic potential versus surfactant concentration functions unexpectedly strongly depend upon the temperature around the lower critical solution temperature (LCST) of the polymer, which was interpreted by the change of the polymer segment density in the surface layer of the collapsing nanogel particles.

’ INTRODUCTION The stimuli
responsive hydrogels, also called “intelligent” materials, have been widely studied and received widespread interest in the past decade because of their swelling/deswelling properties.1 The hydrogels can swell or shrink discontinuously as a function of different environmental parameters, such as the temperature,2 pH,3,4 ionic strength,4 etc. These materials are used in different fields of industrial technologies (catalysis and separation)5 and therapeutics (controlled drug delivery and biomaterials),6 and they are promising candidates in many other possible applications. The poly(N-isopropylacrylamide) (pNIPAM) hydrogel is one of the most frequently studied temperature-sensitive hydrogels, which has a lower critical solution temperature (LCST) at around 33 °C.2 The swelling properties of the gel can be modified by means of co-polymerization with neutral or electrically charged monomers and by means of ionic surfactant additives.7,8 The interaction of pNIPAM and its different co-polymers with surfactants has been studied in polymer solutions,9
12 macrogels,13
16 and microgel latexes.17
20 Less attention was paid to the interaction of the neutral pNIPAM microgels with surfactants, and the results are contradictory. Abuin et al.19 studied the interaction between the sodium dodecyl sulfate (SDS) and pNIPAM microgel particles by surface tension and dynamic light scattering measurements. The calculated binding isotherm was similar to that of the polymer
surfactant systems. The interaction starts from a critical surfactant concentration (cac, ∼1 mM), and there is no further surfactant r 2011 American Chemical Society

binding above a second critical concentration, as is the general experience in the case of the surfactant
polymer interaction. However, Mears et al.18 did not find a well-defined cac for the SDS binding to the pNIPAM microgel. Following a slight binding, the bound amount sharply increased above 3 mM equilibrium SDS concentration. A further increase was experienced in the binding when the equilibrium surfactant concentration exceeded the critical micelle formation concentration (cmc). Gil
anyi et al.7 also found that the interaction of the pNIPAM nanogels with SDS is a cooperative process. The hydrodynamic diameter of the particles increases with the surfactant in a stepwise manner in correlation with the binding isotherm. The surfactant binds in the nanogel as small aggregates with different aggregation numbers in the two steps, which was interpreted by the inhomogeneous inner structure of the particles. The binding starts in the outer shell of the particles followed by surfactant binding in the particle core. Shirahama et al.21 have investigated the interaction of the cationic surfactant with different types of anionic co-polymers of the pNIPAM polymer gels. They have found that the surfactant binding into anionic polyelectrolytes and polyelectrolyte pNIPAM co-polymer gels is the same process for both the linear polymers and the cross-linked polymer gels. The binding of the

Received: September 17, 2010 Revised: February 23, 2011 Published: March 11, 2011 3461

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Langmuir surfactant starts suddenly at a critical concentration and accelerates in a narrow equilibrium concentration range. Tam et al.22 have studied the interaction of anionic, cationic, and non-ionic surfactants with two different types of pNIPAM microgels with dynamic light scattering and electrophoretic mobility measurements. They found that the ionic surfactants interact with the pNIPAM microgel. The anionic SDS causes the increase of the diameter of the particles and shifts the LCST to higher temperatures. The cationic dodecyl trimethylammonium bromide had no effect on the swelling properties of the microgel particles, but it induced the coagulation of the microgel at the LCST. The investigated non-ionic surfactant had no influence on the properties of the microgel particles at all. The interaction of the ionic surfactants with the nanogel latex has importance in two aspects: the swelling characteristics of the individual nanogel particles and, furthermore, the behavior of the nanogel particles as a system; i.e., the colloid stability of the latex may also be influenced by the interaction. This latter aspect, which is important in practical applications, has not gained attention in the literature. Furthermore, the binding of the surfactant to the collapsed particles (above 33 °C) has not been investigated. In this paper, the interaction of the cetyl-trimethylammonium bromide (CTAB) with monodisperse pNIPAM nanogel particles is studied below and above the collapse temperature of the pNIPAM particles by potentiometric measurements with a surfactant ion-selective electrode, dynamic light scattering, and electrophoretic mobility measurements. The main focus will be on the influence of the surfactant on the colloid stability of the collapsed nanogel particles.

’ EXPERIMENTAL SECTION Preparation of Monodisperse Nanogel Latex. For the preparation of pNIPAM, N-isopropylacrylamide (NIPAM), methylene bisacrylamide (BA), ammonium persulfate (APS), and dodecylbenzenesulfonic acid sodium salt (DBANa) were used. The chemicals were provided by Aldrich and were used for the preparation without further purification. Our procedure was based on a modified method developed by Wu et al.23 for the preparation of monodisperse pNIPAM nanogel particles. A total of 2.85 g of NIPAM monomer, 129 mg of BA, and 46 mg of DBANa were dissolved in 190 mL of distilled water. The temperature of the reactor was kept at 80 °C, and the solution was intensively stirred. To remove oxygen, nitrogen gas was purged through the solution for 30 min. Then, 2 mL of a 2.80 wt % aqueous APS solution (56 mg) was added to the solution, followed by intensive stirring for 4 h. The pNIPAM latex was purified from unreacted monomers and surfactant by dialysis against distilled water for 4 weeks and ultrafiltration with a Vivaflow 200 flip-flop filter. Dynamic Light Scattering Measurements. The dynamic light scattering measurements were performed by means of Brookhaven dynamic light scattering equipment consisting of a BI-200SM goniometer and a BI-9000AT digital correlator. An Omnichrome (model 543) argon-ion laser operating at 488 nm wavelength and emitting vertically polarized light was used as the light source. The signal analyzer was used in the real-time “multi-τ” mode. The time axis is logarithmically spaced over a time interval ranging from 0.1 μs to 0.1 s in this mode, and the correlator used 218 time channels. The pinhole was 100 μm. The measurements were performed at a 90° scattering angle. Prior to the measurements, the nanogel samples were cleaned of dust by filtering through a 0.8 μm pore-size sintered glass filter. The intensity
intensity time-correlation functions were measured (homodyne method) and then converted to the normalized electric field autocorrelation functions

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Figure 1. Binding of CTAB in the pNIPAM nanogel particles at 25 and 36 °C. The concentration of pNIPAM is 0.2%. The solid square symbols represent the data measured at room temperature after separation of the particles from the equilibrium surfactant solution by centrifugation. by means of the Siegert relation. The autocorrelation functions were analyzed by the cumulant expansion and CONTIN methods. Electrophoretic Mobility Measurements. Malvern Zeta NanoZ equipment from Malvern Instruments was used to measure the electrophoretic mobility of the pNIPAM nanogels as a function of the CTAB concentration at different temperatures. The instrument uses a combination of laser Doppler velocimetry and phase analysis light scattering (PALS) in a technique called M3-PALS.24 Prior to the measurements, the instrument was always tested with Malvern ζ potential transfer standard. Potentiometric Measurements. A total of 2 g of polyvinyl chloride (Mw = 1  105) was dissolved in 50 cm3 tetrahydrofuran (THF). A total of 20 cm3 of this solution, 40 cm3 of THF, and 2.97 g of tritolyl phosphate were used as membrane-forming solution. It was poured on a clean, flat glass surface and dried 2 days in a refrigerator. The membrane was conditioned in 1 mM CTAB solution for a day then washed out with distilled water. A piece of membrane was placed in a plastic membrane holder.25 The response time of the membrane was dependent upon its thickness. The optimal thickness of the membrane was found to be 0.3
0.5 mm. The response time of the electrode was within 1
3 min depending upon the measured concentration range. The electromotive force (EMF) values of Ag|AgBr|0.1 M NaBr| membrane|clatex, cCTAB|AgBr|Ag galvanic cell were determined by means of a Radelkis research pH/ion analyzer at 25.0 ( 0.1 and 36.0 ( 0.1 °C. A total of 10 cm3 of clatex nanogel latex solution was placed in the measuring cell and titrated with a CTAB stock solution (of the same latex concentration). The equilibrium EMF values were read at each titration step. The EMF values were converted into equilibrium surfactant concentration (ce) by means of a calibration curve plotting the EMF versus log(c) function without latex (where c = ce below the cmc). The equilibrium surfactant concentration was also determined in the supernatant solution after separating the latex by centrifugation at room temperature. The separation was performed at concentrations ce < cmc, where the thermodynamic condition of the micelle formation is not fulfilled; i.e., free micelles were not in the solutions. The binding isotherm of the surfactant on the nanogel, B(ce), was calculated from the expression7 c ¼ ce þ Bc latex þ cmic 3462

ð1Þ

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Figure 2. Hydrodynamic diameter of the pNIPAM nanogel particles as a function of the CTAB concentration at 25 and 36 °C. In the upper inset, the c(ce) relation is given. In the lower inset, the dh values measured at 15 min after raising the temperature from 25 to 36 °C are shown in the low surfactant concentration range. The concentration of the latex is 0.05%. where c and ce are the total and equilibrium monomer surfactant concentrations, respectively, cmic is the concentration of the micelles in the monomer unit, and clatex is the nanogel latex concentration in g of dry latex/dm3. The c
ce difference gives the sum of the micelle concentration and the bound surfactant concentration. The evaluation of the binding isotherm was restricted to the range ce < cmc when cmic ≈ 0. From the potentiometric measurements, the cmc was found to be 0.92 and 0.98 mM at 25.0 and 36.0 °C, respectively.

’ RESULTS AND DISCUSSION In Figure 1, the bound amount of the surfactant calculated from the potentiometric measurements is plotted as a function of the equilibrium surfactant monomer concentration (ce) below (25 °C) and above (36 °C) the collapse temperature of the pNIPAM nanogel particles. The isotherms reveal a critical interaction concentration at both temperatures. The critical concentration called cac is the thermodynamic criterion of the collective surfactant interaction; i.e., the surfactant molecules interact with the polymer in the form of aggregates.26 The cac value is slightly higher at 36 °C, which means that, with increasing temperature, the polymer-induced aggregation of the surfactant is less favorable. In Figure 2, the hydrodynamic diameter of the pNIPAM particles is shown as a function of the CTAB concentration. The measurements were performed at low latex concentration (0.05%), where the intensity
intensity autocorrelation function of the scattered light was independent from the latex concentration. The cumulant expansion analysis of the autocorrelation functions indicates an extremely narrow size distribution of the gel particles. The second cumulant was found to be p = 0.02 ( 0.01. At 25 °C, the pNIPAM particles are in a swollen state. The hydrodynamic size of the particles is practically constant up to the cac and then starts to increase in correlation with the binding isotherm. The further swelling of the swollen particles above the cac can be interpreted as a consequence of the electrostatic repulsion between the ionic surfactant aggregates formed inside the particles.

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When the temperature is raised to 36 °C, the pNIPAM particles shrinks to almost an order of magnitude smaller volume (the diameter decreases from about 250 to 130 nm). The influence of the CTAB concentration on the particle diameter is similar to that measured at 25 °C, except for a sharp maximum in the low CTAB concentration range. If this maximum is not considered, the particle diameter is constant in the low surfactant concentration range and increases above the cac, as in the case of the swollen particles. The diameter of the particles is smaller than it is at 25 °C in the whole surfactant concentration range; however, the relative volume change caused by the interaction with the surfactant is significantly higher, (dh(c)/dh(c = 0))3 = 2.3 at 25 °C and 10.8 at 36 °C. If we change the temperature of the latex
surfactant system by heating and cooling between 25 and 36 °C, the same diameters can be reproduced at each temperature. Furthermore, the state of the system does not depend upon either pNIPAM being first collapsed by raising the temperature to 36 °C without surfactant and then the surfactant being added to the system or the contrary. The surfactant penetrates into either the swollen or collapsed particles and swells them. In the low surfactant concentration range, where the sharp increase of the particle diameter was observed at 36 °C, the turbidity of the system and the apparent particle size increase in time and, after a sufficiently long period (∼day), precipitation occurs. This clearly indicates that the observed increase of the particle size is related to the aggregation of the collapsed microgel particles and not to the swelling of the individual microgels. As is well-established in the literature, the pNIPAm microgel particles are in a highly swollen state at room temperature in their aqueous solution. The interaction between the swollen nanogel particles is repulsive, which is reflected by the positive value of the second virial coefficient (B2 = 8.5  10
7 mol cm3 g
2).27 The particle stability originates from two sources. On the one hand, polymer chains on the exterior of the microgels provide steric stabilization, and on the other hand, sulfate groups originating from the initiator give electrostatic stability. When the temperature is raised above the LCST (33 °C), the particles collapse in a narrow temperature range, forming compact nanogel particles with a small water content,28 which results in a significant increase of the polymer segment density within the particles. As a consequence, the attractive dispersion force acting between the particles significantly increases (the Hamaker constant characterizing the dispersion forces increases with the square of the polymer segment density in the particles29). Despite the increased attractive interactions, the pNIPAM latex does not coagulate but forms a kinetically stable colloid dispersion as a result of the repulsive forces acting between the particles. Because of the dominant attractive interactions among the polymer segments, the collapsed microgel particles do not bear dangling polymer chains on their surface; thus, only electrostatic forces stabilize the latex above the LCST. According to the classical DLVO theory (named after Derjaguin, Landau, Verwey, and Overbeek), the colloid stability of the electrostatically stabilized dispersions decreases with a decreasing surface charge density and with an increasing ionic strength in the medium. The observed loss of stability of the latex in the low surfactant concentration range implies that the particles lose their electrostatic stabilization (surface charge density) because of the binding of the surfactant to the microgel particles, despite the fact that this interaction cannot be directly detected in the binding isotherm (see Figure 1). To confirm this conclusion, the electrophoretic mobility of the nanogel particles was measured 3463

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Figure 3. (a) Electrokinetic mobility (μ) and the corresponding ζ potential of the pNIPAM nanogel particles as a function of the CTAB concentration at 25 and 36 °C. The concentration of the nanogel latex is 0.05%. (b) Electrokinetic mobility (μ) and the corresponding ζ potential of the pNIPAM nanogel particles as a function of the CTAB concentration below the cac at different temperatures. The open circles show the effect of NaBr without CTAB. The concentration of the nanogel latex is 0.05%.

as a function of the surfactant concentration and plotted in panels a and b of Figure 3. The absolute value of the electrophoretic mobility decreases with an increasing surfactant concentration, and at a certain value, charge reversal occurs. At even higher surfactant concentrations, the electrophoretic mobility further increases to higher positive values. The system coagulates in the range of 0.02
0.05 mM surfactant concentration, and the ζ potential goes through zero at ∼0.03 mM. Coagulation can be expected at both the small finite negative and positive ζ potential values, where the electrostatic repulsion is not high enough to protect the particles from coagulation. It is important to emphasize that charge reversal may occur only if the surfactant specifically binds to the particles. Such specific interaction can be expected from the hydrophobic interaction between the polymer segments and the surfactant alkyl chain. The electrostatic interaction between the hydrated sulfate and surfactant headgroup ions is a contribution to the driving force of surfactant binding but, in itself, may not result in positively charged particles. In Figure 3b, the influence of inert NaBr on the electrophoretic mobility of the microgels is also plotted to compare to the effect of the CTAB. In the case of

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NaBr, there is no specific interaction between the sodium ions and the gel particles, consequently, the charge reversal of the particles cannot be observed. It should be noted that the surfactant concentration range where the particle coagulation was observed coincides with the surfactant concentration range where the charge reversal of the microgel particles occurs, that is, where they lose their (surface) charge. This observation is in good agreement with the theoretical exceptions, as is outlined above. Because neither the direct binding nor the swelling measurements is sensitive enough to indicate the binding of the surfactant in the low surfactant concentration range, only the analysis of the electrophoretic mobility data can provide a deeper insight into the microgel
surfactant interaction below the cac. Unfortunately, currently, the interpretation of the electrophoretic mobility of the microgel particles is rather ambiguous in the literature. Two schools have been developed to interpret the experimental mobility data of the nanogel particles, and despite a number of studies, the dispute about the validity of these approaches has not yet been settled. Ohshima has adopted the free draining model31 of the electrokinetic theory of the polyelectrolytes for the description of the electrophoretic mobility of the microgel particles. This model describes the mobility in terms of the total amount of fixed charges in the gel particles. As an alternative, Penfold et al. used the surface charge model30 developed for the description of the electrokinetic properties of colloid dispersions. The latter model considers the highly cross-linked gel particle as a kinetic unit, in which mobility is determined by its electrokinetic charge, i.e., the amount of charges, which are compensated beyond the slipping plane of the moving particle in the diffuse electric layer. Because macroscopic charge separation cannot take place, the core of the microgel particles has to be electrically neutral (i.e., the charge of the surfactant aggregate is electrically compensated by its counterions inside the particle), the electrokinetic charge of the particles is practically determined by the surface charge density of the microgels. The experimental results are contradicting. Ogawa et al.32 reported that the free draining model is better than the surface charge model for the investigated polyampholytes but only if the counterion shielding parameter is taken as zero. Ohshima et al.33 studied the electrophoretic mobility of polystyrene particles covered by the pNIPAM layer as a function of the ionic strength. They concluded that the free draining model well describes the mobility below and above the LCST temperature of the pNIPAM. However, when pNIPAM is in a collapsed state, it is questionable that the medium can flow through the particle core. The main problem is that, if suitable fitting parameters are chosen, there is not a significant difference in the mobilities calculated by the two models.30 It is noted that further studies are needed to draw a decisive conclusion about the validity of the different electrokinetic models. However, it has to be emphasized that, independent of which model is strictly valid, the change of electrophoretic mobility is qualitatively related to the change of the charge density of the microgel particles; thus, it gives information about the surfactant binding. Finally, it has to be noted that the experimental mobility curves do not reflect the binding of CTAB above the cac at either room temperature (in the case of the swollen microgel particles) or 36 °C (in the case of the collapsed microgel particles) (see Figure 3a). We had the same observation in the case of the interaction of pNIPAM with an anionic surfactant (SDS);7 the electrophoretic mobility did not change during the binding of a 3464

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Figure 4. Change of the surfactant concentration at the electrically neutral state of the pNIPAM particles (μ = 0) with the temperature.

large amount of surfactant in the interior of the gel particle. Because these results imply that the mobility of the microgel particles is primarily determined by their surface charge density, our further analysis is based on the surface charge model, although it has to be emphasized that, as is outlined above, both models would lead to the same qualitative conclusions regarding the microgel/surfactant interaction. With an increasing temperature, the electrophoretic mobility function shifts to a smaller surfactant concentration range and the charge reversal takes place at significantly lower surfactant concentrations. It can be concluded that the adsorption of the surfactant in the surface layer of the particles strongly depends upon the temperature. To interpret the pronounced shift in the electrophoretic mobility (μ) functions, the change of the surfactant concentration with the temperature at the μ = 0 situation will be analyzed. The original negative charge of the particles is independent from the temperature; therefore, it is expected that, at the charge neutralization (when μ = 0), the bound amount of CTAB is the same. In Figure 4, the log ce(μ = 0) values related to the surfactant chemical potential are plotted against the temperature. The surfactant concentration, where the particles are electrically neutral, significantly changes with the temperature around 33 °C, the LCST temperature. The log ce(μ = 0) versus T curve resembles the swelling/deswelling function of pNIPAM,30 and it suggests that the strong dependence of the surfactant binding upon the temperature is associated with the collapse of the particles. The bound surfactant amount, which results in charge reversal, seems to be too small to determine directly by surfactant-binding measurements. Therefore, the binding isotherm of the surfactant in the low surfactant concentration range will be derived from the electrokinetic potential measurements. For small potentials, the relation between the surface charge density σo and the surface potential φo can be approximated as34   1 σ o ¼ εkφo 1 þ ð2Þ ka where ε is the static permittivity of the medium, κ is the Debye
H€uckel inverse screening length, and a is the particle

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Figure 5. Surface charge density of the nanogel particles as a function of the surfactant concentration at 25 and 36 °C. The open circles show the effect of NaBr without CTAB.

radius. If we apply eq 2 for the hydrodynamic particle (a = dh/2, where dh is the hydrodynamic diameter and φo = ζ is the electrokinetic potential calculated by means of the Smoluchowski equation), then the electrokinetic charge or surface charge density of the hydrodynamic particle can be estimated. The surface charge density consists of two contributions, from the original negative charges of the particles (σo(c = 0)) and from the bound positively charged surfactant ions (eNAΓ) σ o ðce Þ ¼ σ o ðce ¼ 0Þ þ eNA Γ

ð3Þ

where e is the elementary charge and Γ is the concentration of the surfactant in the surface layer. Γ is associated only with that fraction of the surfactant that is bound in the surface layer leaving outside its counterion in the diffuse electric layer around the hydrodynamic particle. As discussed above, the binding in the bulk is electrically neutral; the surfactant ions bound in the interior of the gel particles are screened inside the particles and make no direct influence on the electrokinetic behavior of the particles. To make a clear distinction from the total surfactant binding by the particles, B(ce), the binding in the surface layer of the particles as a function of the equilibrium surfactant concentration will be called the adsorption isotherm, Γ(ce). In Figure 5, the surface charge density of the hydrodynamic gel particles calculated by means of eq 2 is plotted against the surfactant concentration at different temperatures. dh was determined by dynamic light scattering measurements as a function of the temperature, and the surfactant concentration, κ, was calculated from the equilibrium surfactant concentration. The temperature dependence of ε was taken from ref 35. The surface charge density of the gel particles starts from negative values, then goes through zero, and further increases with the surfactant concentration. At c = 0, the surface charge density shifts to higher negative values with an increasing temperature. This can be expected because the number of sulfate ions in the surface layer is constant and its surface concentration increases with a decreasing surface area of the particles during the particle collapse. At room temperature, 2.5  10
9 mol/m2 bound surfactant ions compensate the original negative charge of the particles, which corresponds to an extremely small amount of surfactant, 5  10
7 mol/g of pNIPAM. It is understandable 3465

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Figure 6. Adsorption isotherm of CTAB on the pNIPAM particles corrected for the electrostatic term of the driving force at 25 and 36 °C.

Figure 7. Log(KΓo) values as a function of log(dh).

that such a small amount of bound surfactant cannot be detected by means of analytical binding measurements. To proceed in evaluating the adsorption isotherms, a simple adsorption model will be used, in which the specific (chemical) and electrostatic interactions are taken into account. If there are Γo sites available for the surfactant adsorption in the gel surface layer, then Γ ¼ Ke
y ce Γo
Γ

ð4Þ

where K is the binding constant and y = eζ/kT is the dimensionless electric potential in the surface layer. The electrical term promotes the adsorption when ζ is negative and acts against the adsorption after the charge reversal. Because y is known from the experiments, the electrostatic contribution can be eliminated from the adsorption isotherm. In Figure 6, the Γ values are plotted against e
yce. The isotherms are linear, which means that the surfactant ions specifically bind in the surface layer as monomers (otherwise, Γ should be a power function of ce) and the experimentally measurable range of adsorption is far

from the saturation amount (Γ , Γo). Therefore, Γo cannot be determined from the experiment. The slope of Γ versus e
yce function in the linear range directly gives the KΓo product. The temperature dependence of the adsorption can be interpreted by the change of K (i.e., the specific driving force of the adsorption) and/or the change of Γo with the temperature. The number of sites available for adsorption is expected to be proportional to the polymer segment density in the surface layer, because the surfactant can specifically bind only to the polymer. Approximating the gel particles as spheres of homogeneous segment density distribution, Γo, is inversely proportional to the particle volume (πdh3/6). In Figure 7, the log(KΓo) values calculated from the slope of the Γ versus e
yce function at different temperatures are plotted against log(dh). The experimental slope is
3.20 ( 0.25, which supports that the strong temperature dependence of the adsorption is a consequence of the change in Γo. Because of the special character of the gel surface, during the particle collapse, the number density of the adsorption sites in the surface layer may change with the actual particle size, contrary to the surfactant adsorption on free liquid or the solid/liquid interface, where Γo is constant. It can be expected that the surfactant-binding isotherm depends upon the segment density of the particles but is otherwise independent from the initial size of the swollen particles. In summary, it can be concluded that CTAB binds to the pNIPAM particles in two ways. At low surfactant concentrations, the surfactant ions specifically bind to the gel particles as monomers. The size of the nanogel particles is constant, and their electrokinetic potential increases. An extremely small amount of the bound cationic surfactant compensates the original negative surface charge of the particles, resulting in a drastic decrease of the colloid stability if the nanogel particles are in the collapsed state. Below the LCST temperature, the latex system is stable in the whole surfactant concentration range, even when the particles are electrically neutral. Above a cac, the surfactant binds to the particles in the form of aggregates in both the swollen and collapsed states of pNIPAM. The electrokinetic potential of the particle does not reflect the collective surfactant binding, but the size of the individual particles significantly increases.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Hungarian Scientific Research Fund (NKTH-OTKA K68027 and OTKA K68434). We thank Dr. Imre Varga for his valuable discussions. ’ REFERENCES (1) Kratz, K.; Hellweg, T.; Eimer, W. Colloids Surf., A 2000, 170, 137. (2) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1. (3) Varga, I.; Szalai, I.; M
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