Association of Siloxane Polymeric Surfactants in Aqueous Solution

Chapter 20

Association of Siloxane Polymeric Surfactants in Aqueous Solution

Downloaded by UNIV OF ROCHESTER on November 16, 2016 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch020

Yining L i n and Paschalis Alexandridis* Department of Chemical Engineering, University of Buffalo, The State University of New York, Buffalo, NY 14260-4200

The micelle formation and structure of a siloxane polymeric surfactant in water and in water mixed with ethanol has been investigated. The formation of micelles (CMC) was established by fluorescent probe molecules that detected hydrophobic domains. The structure of the micelles was determined by small angle neutron scattering (SANS). The siloxane polymeric surfactant, consisting of a hydrophobic polydimethylsiloxane (PDMS) backbone and hydrophilic polyether grafts, forms micelles in water at a concentration of 0.05%. The micelles are spherical at room temperature and consist of a compact siloxane core with a radius of 60 Åand a relatively thin (20 Å) polyether corona. Addition of ethanol suppresses the micelle formation and renders the micelles smaller. The findings presented here are relevant to waterborne coating and ink formulations, where siloxane surfactants are often used in conjunction with cosolvents.

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© 2003 American Chemical Society

Clarson et al.; Synthesis and Properties of Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction The interest in the self-assembly of amphiphiles in water or in mixtures of water with less polar solvents is driven by both fundamental and practical considerations. In waterborne coating and ink formulations, polymeric surfactants are added in order to improve stabilization, solubilization, or provide surface modification. Water is the primary (and desirable) solvent, but cosolvents are needed in order to modulate the formulation performance. In order to improve existing formulations and to design new ones, we need fundamental information on association of polymeric surfactants in solution and on surfaces. In a recent study of Pluronic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) PEO-PPO-PEO block copolymers where water is typically used as a solvent (being selective for PEO) [1-4], we have shown that the addition to water of cosolvents, such as glycerol or ethanol, provides extra degrees of freedom in tailoring the solution properties [4-6]. Cosolvents also cause pronounced effects on the concentration range of stability of the different lyotropic liquid crystals formed by PEO-PPO-PEO block copolymers in water and on the characteristic length scales of the nanostructures [6-8]. Siloxane surfactants have unusually flexible polydimethylsiloxane (PDMS) backbones which may coil in the aggregates in aqueous solution [9]. According to some reports, close to the CMC, siloxane surfactants in aqueous solutions behave much like typical hydrocarbon surfactants [10,11]. However, published information about the association in solution of block and graft siloxane polymeric surfactants, such as the one we examine here, is very limited. Such lack of fundamental knowledge is in contrast with the current trend of increased use of functional polymers and surfactants in aqueous media and adsorbed on particles. Our desire to be able to tune association and adsorption in waterborne coating and ink formulations motivate us in studying interactions between their basic ingredients: polymeric surfactants, cosolvents and particles. In this study, we highlight some of our recent finding on the topic of siloxane surfactant association in water and in water-cosolvent mixtures [12-16]. We first show how to determine the CMC using probe molecules such as DPH (l,6-diphenyl-l,3,5-hexatriene) and methyl orange, which undergo changes in their spectral properties as the environment changes from polar to hydrophobic. We then describe how the structure of micelles can be probed by a combination of SANS (that focuses primarily on the core of the micelle) and DLS (dynamic light scattering, that is sensitive to the solvated micelle). Finally, we discuss how the micelle formation and structure are affected in the addition of ethanol, a solvent fully miscible with, but less polar than water.


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Materials and Methods The siloxane graft copolymer examined here was provided by Goldschrnidt AG, Essen, Germany and has the chemical formula MD oD' M (M: Me Si0 -, D: -Me SiO-, D': Me(R)SiO-, R: PEO-PPO polyether copolymer) with MW=11500. The molecular weight per polyether group in the siloxane copolymer is 1200 and the composition of the polyether is 75% PEO and 25% PPO. l,6-diphenyl-l,3,5-hexatriene (DPH) was obtainedfromMolecular Probes Inc., Eugene, OR. Methyl Orange 99.5% was purchased from Aldrich Co. Ethanol was purchasedfromAcros Co. All solutions were prepared with Milli-Q filtered water (18 ΜΩ.αη) mixed with a cosolvent where appropriate. Deuterated water (D 0) and deuterated ethanol (CD CD OD), used in the neutron scattering experiments, were purchased from Cambridge Isotope Laboratories. 7

5

3

1/2


2

2

3

2

Spectrophotometric measurements DPH is a well-known probe of lipid membrane interiors and can also be used to detect the formation of micelles [17,18]. Thefluorescenceof DPH is minimal in water but is substantially enhanced by association with surfactants. Sample preparation was as follows: a stock solution of 0.4 mM DPH in methanol was prepared; 25pl of the DPH/methanol solution were added to 2.5 ml surfactant solution, so that the final surfactant solution contained 0.004 mM DPH and 250 mM methanol. UV-vis absorption spectra of the surfactant/DPH/water samples were recorded in the 320-400 nm range using a Beckman DU-70 UV-vis spectrophotometer. The main absorption intensity peak, characteristic of DPH, was at 354 nm. Methyl orange has been used to investigate the formation of micelles as a solvatochromic dye molecule [12,13]. The non-covalent binding of methyl orange to the micelles is reflected by a hypsochromic shift of the longwavelength absorption band of methyl orange in the presence of different concentrations of surfactants. The concentration of the dye (2.5xl0" mM) was kept constant for all measurements. 2

Small angle neutron scattering SANS measurements were performed at the National Institute of Standards and Technology (NIST) Center for Neutron Research, beam guide NG3. 5% siloxane surfactant solutions were prepared in D 0 or D 0 mixed with D2

2


225

ethanol, which provides good contrast between the micelle and the solvent. The experimental details are given in [6], The absolute SANS intensity can be expressed as a product of P(q) which is related to the form factor and the structure factor S(q) [5,6,16]:


I(q)=NP(q)S(q) where Ν is the number density of the scattered particles, in our case micelles, which depends on the copolymer concentration and the association number of micelles. The form factor P(q), which takes into account the intramicelle structure, depends on the shape of the colloidal particle. A core-corona form factor has been proposed to describe the scattering generated from the contrast between the micelle core and corona [5,6,16], which have different solvent contents (the core is usually "dry" or has small amounts of solvent, whereas the corona is highly solvated), and the scattering due to the contrast between the micelle corona and the solvent phase: 3

P(qH(4nR /3)(p -p J[3^ (47lR /SXpcorona'PsofyeJfSJjfqRmiceuJ/fqRmicellJJJ core

core

coro

micene

where 5 and ^ceiie are the radii of the micelle core and whole micelle, respectively; p^, p , and Advent ^ scattering length densities (SLD) of the core, corona, and solvent (assuming a homogeneous solvent distribution in each of the domains). J (y) is the first-order spherical Bessel function: core

m

e

Z0XQm

x

2

Ji(yMsin(y)-ycos(y)]/y

In fitting the core-corona model into the scattering data, we view the micelles as consisting of hydrophobic core composed of siloxane plus PPO segments (with little or no solvent present) and a relatively hydrated corona consisting of solvated PEO chains [5,6,16]. The SLD of the core, p , and the corona, p o are a function of the average (over the core radius) volume fraction of siloxane in the core (a ) and of the average volumefractionof PEO in the corona, respectively: core

cor

M5

core

Pcore~ &corePhydrophobie~^(l~Gcor()psolvent Pcorona ~ &coronaPPEO &corona)Psoheni 10

2

10

2

where pydrohobic (=0.1 χ 10 cm") and p (=0.547 χ 10 cm") are the SLD of siloxane plus PPO (where pioxaiu=0.0658 χ 10 cm" and p o =0.325 χ 10 cm" ) and of PEO, respectively, and psoivent is the SLD of the water-cosolvent mixture ( p = 6.33 χ 10 cm" and p thanoi = 5.95 χ 10 cm" ), a^re and h

P

P E 0

10

2

10

si

PE

2

10

D20

2

10

2

De


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can be expressed in terms of the core and micelle radii and the micelle association number, Nas ., i.e., the number of block copolymer molecules which (on the average) participate in one micelle: S0C

&core~~3N Vhydrophobic/(4T&core ) ^corona 3N ^ V /[4 π (R icelle " ^core )] assoc

=

assoc

PE0

m

3

where V is the volume of the siloxane plus PPO block ( = 11904 A ) and VpEo is the volume of the PEO blocks (= 7500 Â ) of one siloxane molecule. In summary, there are three fitting parameters in the core-corona form factor: Rc , R^ceiie, and N ^ . The volumefractionof polymer in the core and corona can be calculated on the basis of these threefittingparameters, according to the above equation. A core-corona form factor which accounts explicitly for the solvent content in the micelle core and corona has been found very useful in the case of PEO-PPO-PEO block copolymer micelles which undergo a progressive solvent loss in both the corona and the core when the temperature increases [5,6,16]. h y d r o p h o b i c


3

0re

Dynamic light scattering Dynamic light scattering measures the time-dependent scattering intensity emanating from the sample which leads to the correlation function G (r) obtained by means of a multichannel digital correlator. a)

(2)

(1)

2

G ( r ) = ^(l + 6 | g ( r ) | ) 1}

where A, b, τ, /"and \£ (τ)\ are the baseline measured by the counter, coherence factor, delay time, decay rate and normalized electric field correlation function, respectively. In our study \£ (τ)\ was analyzed by the EXPSAM (exponential sampling) method, yielding information on the distribution function of Γ from !)

| Λ γ ) | = jG(r)exp(-rr)dT EXPSAM works without any constraints and is based on the eigenvalue decomposition of the Laplace transformation of G(I). G(r) can be used to determine an average apparent (or translational) diffusion coefficient, D =/7q , where ς=(4πη/λ) sin (Θ/2) is the magnitude of the scattering wave vector. The apparent hydrodynamic radius R is related to 2

flpp

h


227

D via the Stokes-Einstein equation, D =kT/6mjR , where k is the Boltzmann constant, Τ is the absolute temperature, and η is the viscosity of the solvent. We used a Lexel model 95 argon ion laser with Brookhaven BI-200SM goniometer to obtain the micelle size distribution from a plot of G(T) versus R by the correlation data with the fitting EXPSAM routine, with G(T{) being proportional to the scattering intensity of particle i having an apparent hydrodynamic radius R^. DLS provides information about the solvation of micelles, the possible presence of larger objects, and the size distribution of the micelles, whereas SANS gives better information about the micelle structure. In this sense, DLS and SANS are complementary. The surfactant concentrations studied at 24 °C were 1% and 5%, which are higher than the CMC (see discussion below). app

app

h


h

Results and Discussion Micelle formation UV-vis spectra of aqueous siloxane solutions containing DPH with siloxane concentrations in the range 0.0001 to 10% w/v were recorded at 24 °C. At low concentrations the siloxane did not associate in aqueous solution and DPH was not solubilized in a hydrophobic environment, therefore, the UV-vis intensities of DPH were very low. At higher concentrations, the siloxanes formed micelles and DPH was solubilized in the hydrophobic micelle interior, giving a characteristic spectrum [17]. The CMC value for siloxane surfactant in aqueous solution was obtained from the first inflection of the absorption intensity at 354 nm vs. siloxane concentration plot for DPH probe (Figure 1). The arrow on the plot indicates the evaluated CMC (0.05%). The spectra of methyl orange at a concentration 2.5x10" M for the siloxane surfactant over the 0.0001-10% (w/v) were used to construct versus siloxane concentration plots (Figure 1). The spectra of methyl orange remain unaltered in the presence of siloxane up to a certain siloxane concentration, but a progressive blue shift in is observed at high concentration also up to a certain value. In Figure 1, the absence of any change in λ , ^ in the dilute region indicates that the copolymer remains fully dissolved in water and does not affect the spectrum of the dye. At higher concentrations, the siloxane forms hydrophobic domains and the dye can be considered to partition from water to these domans resulting in decrease in λ ^ . The CMC value for siloxane surfactant of various concentrations was obtained from the first inflection of the plot (Figure 1). The evaluated CMC value (0.05%), agrees very well with the one we obtained by DPH. 5


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Micelle structure Figure 2 shows the SANS data and model fits. We fit the data from 0.2%, 1% and 5% siloxane solutions using the core-corona form factor. As shown in Figure 2, this modelfitswell the scattering data in the q range 0.01 Â" to 0.1 Â" . The micelle core radius obtainedfromthe fit is 60 Â and the micelle radius is 80 Â. The association number is 76. The micelle core radius, micelle radius and association number do not change as the siloxane concentration increase. The increase in the scattering intensity with increasing concentration is due to the increase in the number density of micelles. For comparison, Pluronic Ρ105 PEOPPO-PEO block copolymer (with molecular weight 6500 and 50% PEO) has core radius 46 Â, micelle radius 80 Â, and association number 78 [6]. Figure 3 shows the size distribution of 1% and 5% siloxane solutions in water obtained by DLS. The size distribution data show two well-distinguished peaks (fitted by Gaussians). The lower size distribution, centered around 130 A, corresponds to the hydrodynamic radius of the siloxane surfactant micelles. The larger micelle radius obtained from DLS, compared to that obtained by SANS, results from the solvation of the siloxane micelles. The surfactant concentration does not show strong effect on the micelle radius, suggesting a closedassociation process [17]. We speculate that the higher size distribution, centered around 500 Â (radius) with a standard deviation of about 80 À, results from hydrolyzed siloxane impurities. The large aggregates contribute about 25 to 30% of the total scattered intensity but their concentration is very low: the number of particles corresponding to the higher peak is

Association of Siloxane Polymeric Surfactants in Aqueous Solution

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