Bulk phase behaviour and surface properties of oppositely charged - - PDF document
Project work of International Student Summer Programme at ILL student: Marco Valentini marco.valentini1@studenti.unimi.it or marco95.vale@gmail.com Bulk phase behaviour and surface properties of oppositely charged block
Supervisors: Nico Carl (LSS, Universit of Paderborn) and Andrea Tummino (LSS, ELTE University)
Marco Valentini Physics Department, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italia October 30, 2017
Abstract The aim of this short project was to investigate both the bulk and surface properties
ularly, we have been interested in mixtures composed of poly(sodium acryliate)- b-poly(sodium styrene sulfonate)/dodecyltrimethylammonium bromide (PAA-b- PSS/DTAB). The choice of this system is not arbitrary. From the one hand, it is possible to vary the AA-to-SS ratio. On the other the charge density of the AA block depends on the pH of the solution. This two aspects will allow us to investigate thoroughly the effect of the concentration of surfactant at a fixed polymer content, the pH and the chemical composition of the polyelectrolyte onto the bulk and surface properties
The investigation has been carried out by mean of optical density (O.D.) and elec- trophoretic mobility as concern the bulk properties, while surface tensiometry and dynamic ellipsometry measurements were used for the surface characterisation. 2
Project work of International Student Summer Programme at ILL - Marco Valentini
I. Introduction
Surfactants and polymers in aqueous solutions exhibit a tendency to interact with each other and form aggregates. This tendency will acquire more impor- tance if the substances are oppositely charged. This behaviour is used in many applications ranging from large- scale industrial operating to personal care uses and moreover these mixtures are widely present in our body [5]. Therefore, intensive efforts have been made to characterise these interactions and their effects on phase separation, rheological and interfacial properties [6]. The association between surfac- tants and polymers can be caused by both electrostatic and hydrophobic in-
with a weak hydrophobic interaction between the polymer chains and the surfactant head groups or with a strong electrostatic interaction between oppo- sitely charged polyelectrolytes and sur- factant head groups. As regards poly- electrolytes, the interaction tend to be driven almost completely by electro- static forces. Aside from the substances em- ployed, there are many parameters that play a key role in the behaviour of these mixtures, which are [4]:
chain;
trolyte chain.
i. Previous studies
Systems composed by polyelectrolytes and surfactants have already been stud- ied [7] and, momentarily, they have no- ticed common features for these mix-
tration is varied at a fixed polymer con- centration. At very low concentration of sur- factant we do not expect any kind of interaction with polymers, but adding more surfactant, we suppose to achieve surfactant/polyelectrolyte complexes (SPECs)[4]. As the surfactant concen- tration is further increased the solution become turbid and the surfactant be- gin to neutralise the charge of polyelec-
formation of big colloidally unstable aggregates, figure 1. Charge neutrality implies hydrophobicity, thus associa- tive phase separation occurs and the aggregates either precipitate or cream according to their density [2]. The sys- tem is a two-phase equilibrium state. [2]. With time, full phase separation will occur, therefore it is extremely im- portant to analyse the properties for both fresh and aged-settled solutions. Eventually at higher concentration of surfactant, we expect to attain a sta- ble solution. In fact the aggregates will become charged due to the pres- ence of surfactant in excess, thus they will repel each other, becoming an elec- trostatically stabilised colloidal disper- sion, which is still a two-phase system. The aim of this work is to find a relation between the bulk and the sur- face properties of oppositely charged
not yet researched. 3
Project work of International Student Summer Programme at ILL - Marco Valentini
Figure 1: Scheme of the interaction of sodium dodecyl sulfate (SDS) with hyperbranched polyethyleneimine (PEI) [10].
II. Our Systems
We used Dodecyltrimethylammonium bromide (DTAB) as surfactant and two block-copolyelectrolytes, both composed by polyacrylic acid (PAA) and by poly- stirene sulfonate (PSS). The first one, that we will call NC29, is composed by ≈ 90% of PAA and ≈ 10% of PSS, while the second, NC31, is composed by ≈ 50% of PAA and ≈ 50% of PSS. The main difference between PSS and PAA is the value of pKa, and thanks to the Henderson-Hasselbalch equation pH = pKa + log10
[A−] [HA] (1) we expect that they will show a dif- ferent behaviour varying the pH. For
instance at pH 12 both polyelectrolytes are deprotonated, but at pH 2 only PSS is deprotonated.
i. sample preparation
All the stock solutions were prepared either in 10 mM HCl (pH 2) or 10 mM NaOH (pH 12). Two 50 mM DTAB so- lutions were prepared at both pH and, simply by dilution, we obtained 1.0 − 30 mM DTAB solutions. The polymer stocks contained 200 ppm of NC29 or NC31. The mixtures were prepared by fast-adding an aliquot of surfactant into the same volume of the polyelec- trolyte, so that the total final bulk con- centration is halved, under continuous stirring for 20 s. The sample history, i.e. the sam- ple preparation and/or the mixing or- der is fundamental for the final out-
to follow always the same experimen- tal protocol to control the state of the
fect of reversing the mixing order onto the physical state of PEI/SDS solution with the same bulk concentration of PEI and SDS. It is evident that accord- ing to the sample history the system changes dramatically.
III. Bulk properties i. techniques used
First of all it is necessary to understand at which concentrations of surfactant, aggregation starts to occur. The combination of O.D. and elec- trophoretic mobility is a useful tool to have a look on both aggregate forma- tion and their charge.
ii.
If we hit a transparent solution with a wavelength of intensity Iincident, figure 4
Project work of International Student Summer Programme at ILL - Marco Valentini
Figure 2: Order of addition effect. Ex- periment 1: add 5 mL of 0.1% PEI solu- tion into 5 mL of 20 mM SDS solution with continuous stirring. Experiment 2: add 5 mL of 20 mM SDS solution into 5 mL of 0.1% PEI solution very slowly (5 mL/45 min) with continuous stirring [10]. 8a, we would notice that the transmit- ted intensity Itransmitted is ≈ Iincident.
(a) transparent sample. (b) sample absorbing or turbid.
Figure 3: Qualitatively behaviour
through a sample. But if there were aggregates in the sample, the light could be absorbed by them or they could scatter it, figure
decreasing of Itransmitted. In order to quantify this phenomenon we intro- duce the optical density τ τ = ln Iincident Itransmitted (2) A UV-vis spectrophotometer was used to measure the optical density, we opted to use a fixed wavelength, 400 nm. In fact both surfactant and polyelectrolyte absorb below 350 nm, hence an increase
the presence of big aggregates [3].
iii. electrophoretic mobility
Another technique employed to detect the presence of aggregates is the elec- trophoretic mobility. This quantity is the speed of charged particles in a fluid in the presence of an electric field. We expect to record negative values for electrophoretic mobility when the con- centration of surfactant is low. Then, adding more DTAB, we begin to neu- tralise the polyelectrolytes, reaching the zero charge neutrality, that has to correspond to the presence of big ag- gregates. A Malvern Zetasizer NanoZ in- strument was employed for this pur-
velocimetry (DLV) to measure the speed
we sent a monochromatic radiation, λ0, against a moving object, we would re- ceive a radiation with a different wave- length λ1. Figure 4: Principle of laser doppler velocimetry. 5
Project work of International Student Summer Programme at ILL - Marco Valentini
Therefore the particle’s speed is vp = c λ1−λ0
λ1
.
iv. results and discussion
The first substance that we analysed is NC29 at pH 12. As we expected, we do not notice any kind of aggregate for low concentration of DTAB, figure
to observe aggregates, but this aggre- gate are hydrophobic. In fact the op- tical density for aged-settled samples
tend to go to the bottom, because they are heavier than water, creating in this way a transparent solution. As regard higher concentrations we notice, figure 5b, that the solutions seem to be stable in time. This effect could be caused by the excess of positive charge of DTA+ ions adsorbed onto the surface. Another interesting effect was recorded at a concentration of surfactant 4 mM, which is below charge neutrality and before the phase-separation region where the aggregates are negatively charged thanks to excess of polyelec- trolyte segments. At this DTAB con- centration optical density grow in time. This could be provoked by the increase
crease of their number. This is caused by the presence of aggregates, created upon mixing due to instantaneous high concentration
and the solution becomes more turbid. It is worth to notice that the turbidity
fresh-mixed samples. The charge neutrality for NC29/DTAB mixtures at pH 12 was found via elec- trophoretic mobility, ≈ 8.2 mM. This is in good agreement with our O.D. mea- surements, since it coincides with the maximum of turbidity of fresh-mixed systems. Comparing the figures 5a and 5b, we noticed that the value of optical density is strongly linked to the vari- ation of the pH. The optical density for NC29/DTAB mixtures at pH 2 is almost transparent for every concentra-
pHs, and by 90% of PAA that is neutral at pH 2 and negative at pH 12; accord- ingly for the lowest value of pH we have less charge density onto the poly- electrolyte chain, therefore the coulom- bic interaction is strongly diminished. However for this system there is a also little bump at pH 2, figure 6, at around 1 mM, once more correspond- ing to zero charge mobility. The dif- ference of the electrophoretic mobility is explained by the composition of the block copolyelectolyte: we need less surfactant to neutralise all the poly- electrolyte at pH 2. Eventually this little bump recorded for NC29 at pH 2 seems to be stable in time, so it will be interesting to analyse with DLS. As regards NC31, figures 5c and 5d, we noticed a flat trend for low concentrations and a region of rela- tively high optical density increasing the concentration of surfactant. Then the big aggregates precipitate leaving the solutions almost transparent. It is important to notice that, outside the phase separation region, the O.D. re- mains high for several weeks. This is 6
Project work of International Student Summer Programme at ILL - Marco Valentini
100 101 concentration surfactant [mM] 0.2 0.4 0.6 0.8 1
0.5 1 mobility [10 8 m2 V-1 s-1 ] NC29/DTAB pH2 aged-settled fresh mobility
1,3 ¡mM
(a)
100 101 concentration surfactant [mM] 0.2 0.4 0.6 0.8 1
0.5 1 mobility [10 8 m2 V-1 s-1 ] NC29/DTAB pH12 fresh aged-settled mobility
8.2 ¡mM
(b)
100 101 concentration surfactant [mM] 0.2 0.4 0.6 0.8 1
0.5 1 mobility [10 8 m2 V-1 s-1 ] NC31/DTAB pH2 fresh aged-settled mobility 6.1 ¡mM
(c)
100 101 concentration surfactant [mM] 0.2 0.4 0.6 0.8 1
0.5 1 mobility [10 8 m2 V-1 s-1 ] NC31/DTAB pH12 fresh aged-settled mobility 7.9 ¡mM
(d)
Figure 5: Trends of optical density and electrophoretic mobility for NC29 and NC31 at both pHs. The values of optical density are normalised with the highest value recorded. Lines joining the data are only to guide the eye. Aged-settled means a week, hence we will have to take these measurements also after a month, in order to understand better the behaviour of the solutions.
100 101 concentration surfactant [mM] 1 2 3 4 5
10-3 NC29/DTAB pH2 fresh aged-settled
Figure 6: Optical density for NC29 at pH 2 zoomed. related to the production of kinetically trapped aggregates, produced during mixing because of the high concentra- tion gradient. These aggregates are electrostatically stabilised in excess of polyelectrolyte or surfactant (left and right of the phase separation region). For NC31 the difference of the charge neutrality recorded with elec- trophoretic mobility is smaller than for
50% of PSS and by 50% of PAA, hence there is less PAA than in NC29, and PAA is the polyelectrolyte that is af- fected by the pH. 7
Project work of International Student Summer Programme at ILL - Marco Valentini
IV. Surface properties i. techniques used
Once we had some knowledge about the bulk properties, we began to inves- tigate the surface. The first quantity interesting to study is the surface tension γ. In order to measure it, we exploited the Wilhelmy plate, figure 7.
w
L
h
Dynamometer
𝜍" 𝜍# 𝜄
water air
Figure 7: Schematic representation of the Wilhelmy plate. This device is simply composed by a small paper attached to a dynamome-
is given by F = ρp L w t g − ρw h w t g + + 2 (w + t) γ cos θ (3) where g is the gravity, h the height of paper immersed in water, θ the angle formed by surface tension (figure 7), L the height of the paper, w its width and t its thickness and, eventually, ρw and ρp are the density of water and paper respectively. By the way we can lift the pa- per, measure the force and set the dy- namometer to zero, reducing eqn 3 to F = − ρw h w t g + + 2 (w + t) γ cos θ (4) If we only touch the surface, without immersing the paper in the water, we can neglect the contribution of the up-
is also negligible. Therefore we obtain that F = 2 (w + t) γ. (5) Using the equation 5 and the Wilhelmy plate, with the expedients just men- tioned, will be relatively easy to mea- sure the surface tension. Another quantity that it is inter- esting to measure is the surface excess Γ. In general the presence of an in- terface change all the thermodynamic parameters of the system [1]. We imag- ine to divide the systems in three parts, two bulk phases with volume Vα and Vβ and an interface σ. In the Gibbs model we consider Vσ = 0 and all the extensive quantities can be written as a sum of three components. The concentration of the jth ma- terial is, in the bulk phase, cα
i and cβ i
quantity is related to the interface Nσ
i
= Ni − cα
i Vα − cβ i Vβ
(6) where N is the number of molecules. Eventually we define Γi = Nσ
i
A (7) in which A is the area of the interface. In order to measure this quantity we used the ellipsometry, figure 8. In this set-up a beam of known polari- sation is reflected by the sample and 8
Project work of International Student Summer Programme at ILL - Marco Valentini
(a) Picture of the instrument used. (b) Scheme of the ellipsomtery.
Figure 8: Ellipsometry. the polarisation of the reflected beam is measured 8b. The polarisation of the light may be decomposed into an s component and a p component, s is
between the amplitude of the s and p component ρ = rp rs (8) that can be also expressed by the am- plitude component Ψ and the phase shift ∆. In our case the amplitude is negligible, while the phase shift is pro- portional to the total surface excess.
ii. results
We measured the surface properties for
because the measurement revealed to be long. In fact each measurement took at least one hour and we measured at the same time the surface tension and the surface excess. In order to take measurements properly, we fill two dif- ferent low container with the same so- lution, avoiding menisci. After each measurements the solution was com- pletely aspirated using a clean pipet at- tached to a water suction pump, then the container was washed with Milli-
measurement we checked if the value
Milli-Q agreed with the expected. Figure 9: Trend of the surface tension for NC29 at pH 12, varying the concen- tration of surfactant. From surface tension, figure 9, we notice an interesting behaviour. In fact the value of γ does not seem to change with the concentration of surfactant, while the pure surfactant exhibit the same trend (and the same value) only above its critical micelle concentratio (cmc), which is 15 mM for DTAB. It will be fundamental to understand if these values remain constant in time. In fact it will be a great achievement 9
Project work of International Student Summer Programme at ILL - Marco Valentini
to have a low surface tension using a small amount of surfactant. Figure 10: Trend of surface excess for NC29 at pH 12, for different values of concentration of surfactant. Measuring the evolution of sur- face excess during the time, figure 10, we can achieve a clearer idea about the formation of aggregates. First of all, the highest value of surface excess is reached at 8 mM, fig- ure 10, that corresponds to the max- imum of the optical density in fresh solution, figure 5b. The highest con- centrations seem to reach a constant value soon that it agrees with the sta- bility of the solutions in the bulk at 15 and 25 mM. We can notice an irregular trend, especially for 5 and 6 mM. We suppose that this effect is created by aggregates passing through the spot of the beam hitting the sample. These considerations suggest that bulk properties are related to the sur- face. Unfortunately ellipsometry can
and we need the neutron reflectometry to measure the surface excess for each material.
V. Conclusions and outlooks
We begin to investigate a new system that could be interesting for many ap- plications. We showed that we can tune the bulk properties changing the percentage of the polyelectrolytes and varying the pH of the solution. In order to analyse better the bulk, we will use the Dynamic Light Scatter- ing (DLS) for some of the most inter- esting solutions. For instance it would be interesting to analyse with DLS the mixtures at high concentration of DTAB to understand the shape and the size
Then we will exploit Small Angle Neutron Scattering [8] to have a quan- titive measure of the size and shape
will use neutron diffraction [9] to ob- tain information about their internal structure. As regards the surface, we can confirm that the outcomes of the pre- liminary measurement are very inter- esting, hence we will continue to inves- tigate the surface properties, expecting to find a link with the bulk. On the one hand we have to mea- sure the surface tension for NC29 at pH 2 and for NC31, checking also the value after a week and after a month. On the other hand we have to finish to collect the data for ellipsometry for all solutions. Once we performed these exper- iments, we will be ready to do neu- tron reflectometry in order to under- stand the structure and the compo- sition at the interface air-water and Grazing-incidence Small Angle Scat- 10
Project work of International Student Summer Programme at ILL - Marco Valentini
tering (GISAS) to quantify the size and the shape of the aggregates at the sur- face.
Acknowledgments
I would like to thank the organisers of the International Student Summer Pro- gramme, Patrick Bruno (ESRF), Paul Steffens (ILL) and Laurence Tellier (ILL) for the unique opportunity they gave me, my supervisors Nico Carl and An- drea Tummino for their support and their friendliness, Miguel with whom I shared this experience, the LSS group and the PSCM (Partnership for Con- densed Matter) to let us use the instru- ments.
References
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