Gemini Surfactant Binding onto Hydrophobically Modified Silica

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Gemini Surfactant Binding onto Hydrophobically Modified Silica Nanoparticles Patrizia Andreozzi,†,‡ Ramon Pons,§ Lourdes Pe´rez,§ Maria Rosa Infante,§ Rita Muzzalupo,| Lorenza Suber,⊥ and Camillo La Mesa*,†,‡ Department of Chemistry and SOFT-INFM-CNR Research Center, “La Sapienza” UniVersity, P.le A. Moro 5, Roma I-00185, Italy, Department of Tecnologia Tensioactius, IIQAB-CSIC, C. Jordi Girona 18-26, Barcelona 08034, Spain, Department of Pharmaceutical Sciences, Calabria UniVersity, Via P. Bucci s.n.c., ArcaVacata di Rende, (Cs), I-87030 Italy, and ISM-CNR, Via Salaria, Km 29.300, Monterotondo Stazione, (RM), I-00016, Italy ReceiVed: March 26, 2008; ReVised Manuscript ReceiVed: May 8, 2008

Aqueous systems containing hydrophobically modified, HM, 200 nm silica nanoparticles, strictly monodisperse in size, were added with tiny amounts of a cationic gemini surfactant, 2-Quat-C6 gemini, with two alkyl chains separated by a short, six-carbon, hydrophobic spacer. As observed, the latter surfactant strongly favors the stability of the resulting dispersions. Care was devoted to work in proper concentration ranges, where the adsorption process of gemini surfactant is strongly favored with respect to micelle formation. Binding of the above surfactant onto HM silica was monitored by a combination of different physicochemical methods, spanning from 1H NMR chemical shift to pulsed gradient stimulated echo (PGSE) NMR and from ζ-potential to dynamic light scattering (DLS). Proper combination of results obtained from the above experimental methods allowed us quantifying the extent of gemini binding onto HM silica. Binding depends on the amount of surfactant and, to a less extent, on ionic strength. Estimates of the binding saturation conditions were made by combining results from 1H NMR with electrophoretic mobility ones. The whole binding process has been rationalized in terms of a simplified Langmuir-like adsorption isotherm. Introduction Nanoparticle-based technologies are relevant in many practical applications, among which inks, paints, and coatings are by far the most relevant.1 Nanoparticles also find use in analytical separation,2 in bioremediation-oriented methods,3–5 and in dental surgery, as well.6 These statements are well documented in articles, patents, and reports dealing with nanoparticle coverage by homopolymers,7 polyelectrolytes,8 globular or fibrous proteins,9,10 and DNA.11 Studies along such research lines are progressively orienting toward selected applied aspects, focusing on biocompatibility toward specific tissues12 or dealing with protein adsorption onto cells.13 Nanoparticle dispersants and stabilizers are based on ionic14 or nonionic15,16 surfactants. The binding efficiency toward hydrophobic or hydrophobically modified, HM, nanoparticles depends on the surfactant nature and, particularly, on the relative weight of hydrophobic and electrostatic contributions to binding.17 Use of simple surfactants has many advantages and severe drawbacks. In fact, single-chain surfactants easily adsorb and desorb from nanoparticles, as a result of partitioning between bulk and the HM particle surface,15 unless strong hydrophobic interactions between them occur.18 The same behavior does not hold in double-chain ones, which adsorb significantly and are unlikely to be released. Usually, strong adsorption is an advantage to get stable dispersions. * Corresponding author. Tel.: +39-06-49913707. Fax: +39-06-490631. E-mail: [email protected]. † Department of Chemistry, “La Sapienza” University. ‡ SOFT-INFM-CNR Research Center, “La Sapienza” University. § IIQAB-CSIC. | Calabria University. ⊥ ISM-CNR.

To improve binding, Vincent and co-workers used amphiphilic molecules with two or three alkyl chains facing outward an aromatic residue, onto which a polar poly(oxyethylene glycol) unit was covalently attached.15,16 In that case, the process efficiency was significant. From the above results it is expected that surfactants with large hydrophobic moieties fulfill most requirements for significant binding efficiency. Along this line it is expected that gemini, consisting in two surfactant units joined together by a proper spacer,19,20 may fulfill most requirements for a significant binding onto nanoparticles. The above substances form micelles and supramolecular aggregates in a wide range of composition, thermodynamic conditions, and so forth.21–24 Gemini have a rich polymorphic behavior in water, with formation of micelles, vesicles, and lyotropic mesophases.22 They also show significant surface activity and form micelles at much lower concentrations than the corresponding linear homologues.25,26 They are much more hydrophobic than the corresponding single-chain species and, compared to the latter, efficiently adsorb at oil-water interfaces, and, very presumably, onto hydrophobic solids. Information on this regard, however, is far from being complete. We report and discuss here the results of a systematic investigation on the interactions mechanisms between strictly monodisperse HM-silica nanoparticles and a dicationic surfactant, hereafter referred to as 2-Quat-C6 gemini. Chemically, it consists of two dodecyl chains joined to quaternary ammonium head groups, with six methylene units acting as spacers between them. Preliminary studies, some of which are reported here, have dealt with the physicochemical characterization of gemini, to experience in which conditions it exists in molecular form. They combine surface tension, NMR, potentiometry, and ionic

10.1021/jp8026989 CCC: $40.75  2008 American Chemical Society Published on Web 07/22/2008

Gemini Surfactant Binding onto Si Nanoparticles SCHEME 1: Chemical Formula Pertinent to 2-Quat-C6 Geminia

a

The cmc at 25.0 °C is also reported.

conductivity and univocally indicate what is the lower limit of gemini molecular association in water. Later on, the investigation on binding onto HM silica was performed. Studies on binding are based on the combination of different NMR or light scattering, LS, experimental methods, and are supported by ζ-potential ones. In our opinion, the combination of such methods allows the investigation of several aspects related to gemini surfactant binding onto nanoparticles, in particular, on surface coverage. Data analysis and some thermodynamic hypotheses on the binding efficiency are also dealt with in the forthcoming sections. Experimental Section Materials. The schematic structure of 2-Quat-C6 gemini, a cationic alkanediyl species, [R,ω-bis(N-dodecyl-N,N′-dimethyl ammonium bromide)], with dodecyl groups linked to both ends of an R,ω-N,N’dimethylamine chain, is indicated in Scheme 1. The dodecyl moieties are separated by six methylene units, acting as spacers between the respective polar head groups, located at the end of each alkyl chain. The surfactant was synthesized and purified according to Menger and Littau,20 as indicated elsewhere.23 The product purity was inferred by combining 1H NMR, differential scanning calorimetry (DSC), elemental analysis, and surface tension of its aqueous solutions. Gemini was vacuum-dried before use at 70 °C. Some physicochemical properties, essentially based on thermodynamic methods, of 2-Quat-C6 gemini are reported in previous papers23,27,28 and shall not be discussed here. Those based on self-diffusion are reported in a forthcoming section. In almost all cases reported here on binding efficiency the surfactant content in the mixtures is well below the critical micellar threshold, cmc (which is about 8.0 10-4 mol kg-1).23 Support comes from surface tension, NMR, potentiometry, and ionic conductivity, which indicate the onset of micelle formation above the aforementioned value. HM silica (the particle diameter is very close to 200 nm) was obtained according to the classical Stober’s synthesis, based on tetraethyl orthosilicate hydrolysis in ethanol with controlled amounts of water and ammonia.29 More details on the synthesis, purification, size and polydispersity, hydrophobic modification degree by octadecanol, and so forth are reported elsewhere.30 H2O was doubly distilled in a Pyrex all glass apparatus over KMnO4. The ionic conductivity of freshly prepared doubly distilled water, χ, ranges between 0.7 and 1.0 µS cm-1 at 25.00 °C. NH3, 33.0 vol %, Carlo Erba, was used. DMF (Aldrich) was dehydrated by refluxing over activated MgSO4 and distilled at 80 °C and 60 mmHg. Octadecanol, C18 (Sigma-Aldrich), was

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12143 used as such. Tetraethyl orthosilicate, TEOS (Sigma-Aldrich), and absolute ethanol (Sigma-Aldrich) were vacuum-distilled. Other solvents, Sigma-Aldrich, were of analytical purity. In the preparation of aqueous dispersions containing HM nanoparticles due amounts of NaBr, Sigma-Aldrich, usually between 1.0 and 5.0 mmol kg-1, were used. They modulate the medium ionic strength and avoid flocculation. Methods. Dynamic Light Scattering Analysis. A Malvern light scattering unit, HT Nano ZS series, performed the dynamic light scattering (DLS) measurements in back mode, at 173° and 25.0 ( 0.1 °C. The laser wavelength, λ, is 632.8 nm. The apparatus performances are controlled by measuring the size of 0.10 wt % strictly monodisperse polystyrene latex spheres (Alfa Aesar, 100 nm in size), stabilized by sulfate groups and dispersed in 3.0 mmolal aqueous NaBr. The data analysis facility elaborated the decay-time distribution functions by a CONTIN algorithm. According to DLS experiments, HM particle sizes are constant to within a few percent and the related polydispersity index is low. Data were elaborated combining both intensity and volume statistics,31 to ascertain whether true monodispersity conditions hold. ζ-Potential. A Malvern laser-velocimetry Doppler utility (HT ZS Nano series), determined the electrophoretic mobility, µ (m2 s-1 V-1), of the dispersions, located into polystyrene U-shaped cuvettes, equipped with gold-coated electrodes. A Peltier unit set the temperature to 25.0 ( 0.1 °C. µ values were obtained by scanning the applied potential in the range of -150 mV < V < 150 mV and taken at the maximum of the distribution function. Errors are up to a few percent (usually about 2 mV). The apparatus was controlled by determining the electrophoretic mobility of the aforementioned sulfonated 100 nm polystyrene latex spheres, in 3.0 mmolal NaBr. µ data are transformed into ζ-potential ones according to32

ζ)

[ 4πηµ ε ]

(1)

where ε is the dielectric constant of the dispersing medium and η the solvent viscosity. NaBr content in the HM-silica dispersion is below its critical flocculation concentration, cfc (≈20 mmol kg-1) and stabilizes it. In addition, at 3.0-5.0 mmol kg-1 in NaBr, Hu¨ckel’s approximation is valid and Henry’s law holds.33 Accordingly, µ term in eq 1 is corrected by f(κ, a), where 1/κ is Debye’s screening length and a the particle radius. Thus

ζf(κ, a) )

6πηµ fζ)[ [ 4πηµ ε ] ε ]

(1′)

where, following Hu¨ckel’s approximation, it is assumed that f(κ, a) is ≈2/3. NMR. Chemical Shifts. To determine the free surfactant content in solution, 2-Quat-C6 gemini was dissolved in a D2O/ H2O mixture (with 0.5% H2O). 1H NMR spectra were recorded using a Varian spectrometer at 499.803 (1H) MHz, using the deuterium signal of the solvent as the lock. All measurements were carried out on 1 mL samples in 5.0 mm tubes using a 5 mm indirect broad-band probe. The experimental parameters for 1H NMR spectral acquisitions were the following: pulse width, 8.7 µs (90°), number of scans 128, acquisition time 4 s, relaxation delay (d1) 10 s. All NMR spectra were taken relative to the resonances of the residual solvent signals (HDO in D2O at 4.6 ppm). For consistency, differential measurements, taken relative to the 1H NMR spectra of gemini in the absence of nanoparticles, were also performed. Selected spectra in Figure 1, parts A and B,

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Figure 2. Surfactant self-diffusion, D, in m2 s-1, vs 2-Quat-C6 gemini molality in D2O, at 298 K. The cmc is at the change in slope in the curve.

Figure 1. (A) 1H NMR spectral profile of 2-Quat-C6 gemini in D2O, at 298 K. Its content is 0.30 mg mL-1. Integration of areas is reported as a gray line. (B) Signal intensity of 0.30 mg mL-1 2-Quat-C6 gemini in 6.0 × 10-3 HM-silica Φ (volume fraction) in water.

represent the spectral profiles in the absence or with nanoparticles, respectively. Quantitative 1H NMR measurements are based on the integration of the resonance signals of gemini alkyl chain(s) (integrated for 1.1-1.3 ppm) in the spectra and on comparison with the signal due to the residual amount of H2O present in the sample. In such conditions, the signal area is directly proportional to concentration, within the limits set up by the instrumental sensitivity. Pulsed Gradient Stimulated Echo. Pulsed gradient stimulated echo (PGSE) experiments determined the self-diffusion coefficients of the surfactant at 25.0 ( 0.5 °C. 1H NMR measurements were run on a Varian Inova 500 MHz spectrometer, equipped with a standard 5 mm indirect detection, and a pulse field gradient (PFG) probe.34 The combination provides a z-gradient strength (g) of up to 0.33 T m-1 (33 G cm-1). All NMR signals give rise to single-exponential decays in diffusion experiment, and the diffusion coefficient, D (m2 s-1), is obtained by fitting the data in the equation35 )2(∆-δ/3)D

I ) I ° exp -γGδ (

(2)

There I is the measured signal intensity, I0 is the echo intensity in the absence of field gradient pulses, ∆ is the time between two gradients in the pulse sequence, γ is the magnetogyric ratio of protons, G is the field gradient strength, and δ is the duration of the gradient pulse. No modulation due to the relaxation time of protons is required in eq 2, since both chemical shifts and line widths do not change with concentration. The self-diffusion experiments were carried out by varying the gradient strength G and collecting the FIDs (free induction decays). Typically, a value of 50-250 ms is used for ∆, 5-10 ms for δ, and G is varied from 0.03 to 0.33 T m-1 in 15-20 steps. The combination of G, δ, and ∆ values was generally chosen to obtain 90-95% total signal attenuation throughout the whole experimental procedure. The calibration of the gradient strength was performed using heavy water with trace amounts of light water (DHDO is 18.72 × 10-10 m2 s-1). The

intradiffusion coefficients of 2-Quat-C6 gemini were calculated by the signal intensities of CH2 groups in the alkyl chain. Given the rapid exchange between free and micellized surfactant molecules, which is fast on an NMR time scale, the surfactant intradiffusion coefficient in the micellar range, Dobs, is a mean value between that of free monomers, Dfree, and of associated species, Dmic. This approximation is described by a two-site exchange model as36

Dobs ) PmicDmic + (1 - Pmic)Dmon

(3)

There Pmic ) (c - cmc)/c) is the fraction of micellized surfactant and other symbols are as before. The micellization process was followed by measuring samples at concentrations below and above the cmc, Figure 2. Monitoring of the association process is based on the difference in selfdiffusion coefficient for the surfactant monomers and micelles. The effective translational mobility is considerably reduced when the surfactant diffuses as a whole kinetic entity with micelles, and the association process is, thus, reflected by a marked decrease of self-diffusion coefficients. Results and Discussion This contribution is essentially devoted to understand whether gemini binding to HM silica implies complete surface saturation and how much energetically favored the adsorption process is. Given the strong hydrophobic character of the present surfactant compared to single-chain ones, its binding efficiency should be noticeable. No experimental support to the above hypothesis could be found in the literature, and the present results find their justification in elucidating such aspects. According to considerations on the size and chemical nature of gemini, it is expected that its efficiency as a particle stabilizer is significant. The results reported here strongly support such a hypothesis. It is also worth clarifying if binding onto nanoparticles can be rationalized in terms of a Langmuir adsorption isotherm37 or if extra requirements and different theoretical approaches are necessary.38,39 The aforementioned questions were solved by joining together results obtained from different experimental methods. These are focused on the response peculiar to the surfactant (NMR) or the particles (DLS and ζ-potential), respectively. The combined use of the surfactant-based or the particle-based approach may help clarify some subtleties inferred from experiments. As far as the surfactant-based viewpoint is considered, some NMR methods were used. In low-concentration regimes the more

Gemini Surfactant Binding onto Si Nanoparticles versatile spectroscopic techniques were considered. NMR methods, in fact, offer the way to determine the state and (if properly handled) the relative content of free and particle-bound surfactant. Both self-diffusion and integral area evaluation may be relevant on this regard. Finally, the overall particles behavior and modifications subsequent to surfactant binding were inferred by DLS and, particularly, by ζ-potential methods and interpreted according to the particle-based approach. Micelle Formation. Information on micelle formation and their size was inferred from 1H PGSE NMR, Figure 2. The cmc of gemini, obtained by the self-diffusion plot reported above, is close to that formerly obtained by thermodynamic methods.23 Estimates indicate that micelle size, elaborated in terms of the Stokes-Einstein equation for diffusion, is comparable to that reported in the literature for similar species.34,40 Micelle aggregation numbers were inferred by comparing the StokesEinstein radius in the micellar regime with that pertinent to the molecular species. The diffusivity of the latter is 2.8 10-10 m2 s-1. The self-diffusion pertinent to the associated form, conversely, indicates that micelle radius ranges around 4.3 ( 0.3 nm. This value is of the same order of magnitude as that by DLS (data not shown). According to Tanford, micelles made up of gemini can be slightly anisometric, ellipsoidal particles.41,42 It is questionable, in fact, to assume that 2-Quat-C6 gemini may pack into spherical objects. Presumably, disks or rods are favored over spheres. The latter statement finds correspondence with small-angle neutron scattering (SANS)40,43 and small-angle X-ray scattering (SAXS)34 methods, supporting the hypothesis of nonspherical aggregates. Surfactant Binding. In the following, data relative to the binding of surfactant in molecular form are reported. Given the linearity between concentration and areas underlying the 1H resonance peaks, the free surfactant content was compared to that of molecular gemini solutions (at the same concentration as the samples under test) in the absence of nanoparticles. The contributions ascribed to bound species are missing in the spectra, and only those due to the molecularly disperse form can be detected. For molecules adsorbed onto solids, dipole-dipole and quadrupolar interactions largely modify the NMR spectra, with occurrence of a significant line broadening (unless high-resolution solid-state facilities are used). The effect is particularly relevant in case of nuclei with quantum spin numbers 1/2, such as 1H.44 Data in Figure 3 were obtained by plotting the normalized signal intensity, (I/I°) (the fraction of free gemini in the dispersion), versus surfactant content. The fit indicates the equilibrium concentration of molecular surfactant. This value is inferred by the integral areas underlying the polymethylene signal at 1.2 ppm. As more and more gemini is added, the ratio I/I°, which is proportional to the amount of molecularly dispersed surfactant, increases when there is no more room for its adsorption onto HM silica. This statement is a consequence of significant surfactant partitioning between the bulk and HMsilica surfaces. The lower sensibility limits pertinent to NMR do not allow us to detect precisely the binding onset. Only the salient point of binding and saturation can be estimated. It is inferred by a significant change in slope of the curve. Integral areas are transformed, after proper data manipulation, into excess surface concentration, Γ (mol m-2), versus the total surfactant content in the medium. The adsorbed amount is evaluated by the difference between areas observed with HM silica and those pertinent to free surfactant in the absence of nanoparticles. Subsequently, it was transformed into the number

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Figure 3. Normalized signal intensity (I/I°) vs 2-Quat-C6 gemini concentration (in molality) at 25.0 °C. (I/I°) values were obtained by the signal intensity of the molecular surfactant at the corresponding concentration.

Figure 4. Surfactant adsorption density, Γ (mol m-2), vs 2-Quat-C6 gemini content (in molality) at 25.0 °C. The change in slope at the top of the plot indicates binding saturation conditions.

of moles covering the area of HM silica. The plot obtained by the above procedure is reported in Figure 4. The calculations reported therein were performed elaborating the total surface, as it was obtained from the volume fraction, density, and size of HM-silica nanoparticles, and fitting the adsorbed surfactant excess (in m2 mol-1) in the plot. DLS. The results indicate the occurrence of size monodisperse particles and moderate clustering in the absence of surfactants, Figure 5. The increase in particle size ascribed to adsorbed surfactant is immaterial, as far as DLS methods are concerned. DLS gives significant information on the presence of bare HM nanoparticles or their nucleation. In the absence of gemini, the particles strongly tend to collapse, minimizing the hydrophobic interactions with water, Figure 5. In presence of gemini, conversely, particles are monodisperse and do not coagulate or coalesce. The monolayer coverage operated by the surfactant ensures a uniform surface charge density, and the electrostatic repulsions due to adsorbed 2-Quat-C6 gemini stabilize the nanoparticles. Obviously, changes in the surface state of charge imply modifications in inherent quantities, such as in ζ-potential. ζ-Potential. Data inferred from the latter method indicate significant modifications in the particles surface charge. Addition of progressive amounts of gemini is concomitant to noticeable changes in the particle electrophoretic mobility. The behavior reported in Figure 6 implies a significant increase of surface

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Figure 5. Intensity of scattered light, I, in arbitrary units, vs the particle hydrodynamic diameter, (2Rh) (in nm), for samples containing 6.0 × 10-3 in Φ of HM silica in 5.00 mmolal NaBr solutions, at 25.0 °C. From the top data with 0.30, 0.03, and 0.003 mg/mL 2-Quat-C6 gemini, respectively. Data are rescaled, to avoid the signal overlapping.

density per unit area, σ0.46,47 It is related to the area occupied by a surfactant molecule in an arbitrary reference state. (N.B. It can be considered the area covered by a surfactant molecule at infinite dilution, or at surface saturation.) Different approaches can be used to quantify binding. It can be assumed, in particular, that the area of the surfactant ion adsorbed at the water-HM-silica interface is grossly equivalent to that at the air-water interface. In words, the surface curvature of the nanoparticles is immaterial. This is reasonable, since the radius of HM silica is 2 orders of magnitude higher than that of the gemini alkyl chain. Questions can be addressed as to whether the surfactant located at such interfaces is in liquidexpanded or liquid-compressed state.48 It is safely assumed that at saturation conditions the liquid-compressed form dominates. We may assume that its area is close to that observed at the air-water interface at concentrations close to the cmc.49,50 Incidentally, the area per surfactant molecule in a saturated gemini layer is between 1.2 and 1.3 nm2.51 Equation 5 can be rearranged as

[ ( )]

(ZN)

Figure 6. Particle surface coverage, |θ - θ°|/10, inferred by ζ-potential measurements, vs the concentration of 2-Quat-C6 gemini (in mmolality), at 25.0 °C. The curve was obtained by iterative fitting of data, according to eq 6.

charge density (or surface coverage, as it is obvious) at moderate surfactant content and a progressive saturation of binding sites onto nanoparticles. To quantify the relations between ζ-potential and surface charge density, σ, let us be reminded that the relations between them imply

εζ [ 4πτ ] ) [ ηµτE ]

(4)

[ ( )]( )

(5)

σ)

j is where τ is the double layer thickness around the particles, E the applied electric field, and other symbols are as before. τ depends on the medium ionic strength,45 which is kept constant by added NaBr. In this case, thus, σ or ζ are directly related to the amount of adsorbed surfactant. Given the low concentration of added surfactant (which is always lower than 0.6 mmol kg-1), we assume that modifications in σ values are due to particle coverage. Accordingly, the surface charge density can be expressed as

σ) 1-

Zθs N

e f0

where θS is the fractional surface coverage by the surfactant ion, Z the surfactant ion charge at the given ionic strength, N is the number of binding sites on the particle, each of area f0. The ratio e/f0 is the area covered by a charged entity (bearing two nominal charges) and can be expressed as a surface charge

1-

σ σ0

) 2(N)σred ) θs

(5′)

when Z ) 2. It is worth noticing that σred and θs in eq 5′ are strictly interrelated. Thus, we may relate the observed modifications in surface charge density of the nanoparticles to the amount of adsorbed surfactant and/or with surface concentration. Before proceeding further, let us consider which drawbacks may hinder the present approach. First, it is not known as to whether the adsorbed surfactant is in the fully ionized state. It is reasonably supposed that gemini molecular area depends on the overall surfactant content and on the medium ionic strength, as well. (N.B. That is why the latter is held constant.) In case of partial charge neutralization, σred must be recalculated assuming it is the average value resulting from contributions due to charged and uncharged species, and the term 2 in eq 5′ should be no longer valid. Anyhow, the above equation is a sound basis to quantify surface coverage, in combination with a classical form of the Langmuir adsorption isotherm. Therefore, for systems at equilibrium, and in the presence of finite bulk concentrations, the surface coverage conforms to the following equilibrium conditions

θs )

[ ][

qads bqbulk ) qsat 1 + bqbulk

]

(6)

where qads is the amount of adsorbed surfactant, in g m-2, and the constant b accounts for the equilibrium between adsorption and release. The above equation implies a progressive increase of bound surfactant up to saturation. The slope of the function at concentrations below saturation indicates the system cooperativity, through b. Comparison with data by Vincent and co-workers15,16 indicates that gemini experiences a significant adsorption at HMsilica-water interfaces and behaves as a whole kinetic entity with nanoparticles. This is an univocal indication that binding, essentially driven from hydrophobic interactions between the solid and the surfactant, is significant. No fluctuations or drifts in the particles properties are observed. Binding is fast on the time scale associated to the experimental measurements and imparts to the surface-covered particles a noticeable stability. On thermodynamic grounds, binding processes are related to gemini partitioning between bulk and surface phases. Binding is controlled by the effectively available surface area, i.e., to the particles volume fraction and hydophobicity, by the medium ionic strength and temperature, as well. At present, it is not

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well-known as to whether surfactant adsorption implies formation of hemimicelles or similar aggregates. In the following, we shall consider adsorption in analogy with a progressive binding of individual surfactant entities. At equilibrium, the following equality holds

MSurf + NGem f Surfcov

(7)

where the number of binding sites on the particles, MSurf, is proportional to their surface area, NGem is the amount of surfactant in molecular form, and Surfcov is the surface coverage. The equilibrium constant is expressed as

Keq )

(

aSurf,N a ( Gem)NaM,Surf

)

(7′)

where aSurf,N indicates the activity of the solute on the binding sites, aGem that of the surfactant and aM,Surf that of the binding sites. In the above mass-action approach, there is, thus, direct proportionality between the activity of surface-bound molecules and coverage capacity. The number of available binding sites, Mfree, in the following equation

Mfree )

(

Γ∞ - Γ N

)

(8)

where Γ and Γ∞ refer to the actual adsorption densities at a given concentration, Γ (Γ e Γ∞), and at a reference value, Γ∞ (at saturation), indicates how far the system is from full coverage. Finally, the equilibrium constant is expressed as

((

)

Γ N ) KaGem Γ∞ - Γ)

(9)

In words, the partitioning proceeds up to saturation of binding sites and depends on the amount of available gemini. From the above relation the binding Gibbs energy can be inferred to access the energy, provided studies as a function of temperature are made. Combination of eqs 7′, 8, and 9 with the equilibrium ones, eqs 5 and 6, allows, after some straightforward algebra, the transformation of bulk to surface concentrations. Surface saturation implies that there is no room for more adsorption and that surface potential is constant from the saturation threshold upward. In such a concentration regime, the area covered by a single surfactant molecule at the saturation threshold (1.2-1.3 nm2) is very close to that formerly reported for gemini in a liquid-compressed form.51,52 Hence, gemini adsorbs significantly at the surface of HMsilica nanoparticles and imparts to them a noticeable charge density. From an applied viewpoint, it is argued that the above surfactant is effective in surface adsorption and stabilization. Given the antibacterial activity of ammonium-based surfactants, use of 2-Quat-C6 gemini may avoid surface pollution of the particles by viruses and bacteria and can be used in some technological applications inherent to nanoparticle technology. It is worth noticing, finally, that NMR values in Figure 4 give direct information on the surface coverage density. The ζ-potential values in Figure 6, conversely, are related to changes in surface charge density. There is, obviously, proportionality with values from NMR. However, charge neutralization by counterions and ion exchange to/from the bulk may influence the shape of the resulting charge density plot. Conclusions Heterogeneous mixtures made of HM silica and cationic surfactants of the gemini family were investigated, and adsorp-

tion of the aforementioned species onto HM-silica surfaces was experienced. Combination of experimental methods based on NMR, DLS, and electrophoretic mobility indicates that binding is almost complete and leads to a noticeable surface coverage. It is argued from experiments that coverage leads to the formation of a liquid-compressed film onto HM silica, with subsequent stabilization of the resulting colloidal dispersions. Preliminary information dealt with micelle formation by the above surfactant. The aggregates formed by gemini are small in size and, very presumably, anisometric in shape. Care was taken to avoid the overlapping of micelle formation with surface adsorption, and data relative to the binding of gemini onto HM silica, at concentrations well below the cmc, were elaborated. Binding gives rise to stable dispersions. It has been interpreted in terms of a surface saturation model, which is combined with a Langmuir-like adsorption approach. The results satisfactorily fit in the Langmuir adsorption model. Being much more hydrophobic compared to single-chain surfactants, gemini is extremely efficient in surface coverage. The stability of the resulting particles is retained for several months after preparation and is not influenced by changes in temperature and/or ionic strength. Very presumably, these above effects are related to the high surface activity of gemini compared to single-chain surfactants. Developments along this research lines could be helpful to clarify the role of surfactant-assisted coverage in fundamental aspects related to particles stabilization in biomedicine, paint industries, and so forth. As to ionic strength effects, let us be reminded that NaBr was used to stabilize HM silica and to guarantee a nearly constant ionic strength. In a first approximation, the effect of such moderate ionic strength on the electrical double layer can be neglected. Upon adsorption of gemini there is, surely, exchange of bound ions from the bulk to the particle surface. Even in case of full replacement of sodium with gemini ions onto the surface, this implies small changes in the double layer thickness around the particles. This hypothesis is supported by numerical computations, based on a simplified Poisson-Boltzmann equation for ion distribution around spherical objects.32 Acknowledgment. The stay of P.A. in Barcelona (ES) was financed through a bilateral scientist and young researchers exchange project operating between La Sapienza University, Rome, Italy and CSIC (Consejo Superior de Investigaciones Cientificas), Barcelona, Spain. This work was performed under the auspices of the European Community, by a COST D-35 Action Project on Interfacial Chemistry and Catalysis, 2006-2010. MIUR, the Ministry of University and Research, also supported this work through a PRIN project on polymer-surfactant systems, for the years 2006-2008. Thanks are also due to the Spanish Ministry of Education, CICYT, for supporting us with Grant No. CTQ2006-01582. Special thanks are due to Professor E. F. Marques (Porto University, PT) for giving us information on monolayer surface adsorption studies dealing with gemini surfactants. References and Notes (1) Holme, I. Surf. Coat. Int., Part B 2006, 89, 343. (2) Yoshimatsu, K.; Reimhult, K.; Krozer, A.; Mosbach, K.; Sode, K.; Ye, L. Anal. Chim. Acta 2007, 584, 112. (3) Minc, N.; Slovakova, M.; Dorfman, K. D.; Bokov, P.; Bilkova, Z.; Smadja, C.; Futterer, C.; Taverna, M.; Viovy, J.-L. Spec. Publ.sR. Soc.Chem. 2004, 296, 530. (4) Schwalbe, M.; Pachmann, K.; Hoeffken, K.; Clement, J. H. J. Phys.: Condens. Matter 2006, 18, S2865. (5) Magnani, M.; Galluzzi, L.; Bruce, I. J. J. Nanosci. Nanotechnol. 2006, 6, 2302.

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