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 copolyelectrolyte/surfactant mixtures Supervisors: Nico Carl (LSS, Universit of Paderborn) and Andrea Tummino (LSS, ELTE University)
Bulk phase behaviour and surface properties of oppositely charged block copolyelectrolyte/surfactant mixtures 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 of oppositely charged diblock copolyelectrolyte/surfactant (P/S) mixtures. Partic- 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 of their mixture. 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. I ntroduction i. Previous studies Systems composed by polyelectrolytes Surfactants and polymers in aqueous and surfactants have already been stud- solutions exhibit a tendency to interact ied [7] and, momentarily, they have no- with each other and form aggregates. ticed common features for these mix- This tendency will acquire more impor- tures. Typically, the surfactant concen- tance if the substances are oppositely tration is varied at a fixed polymer con- charged. This behaviour is used in centration. many applications ranging from large- At very low concentration of sur- scale industrial operating to personal factant we do not expect any kind of care uses and moreover these mixtures interaction with polymers, but adding are widely present in our body [5]. more surfactant, we suppose to achieve Therefore, intensive efforts have been surfactant/polyelectrolyte complexes made to characterise these interactions (SPECs)[4]. As the surfactant concen- and their effects on phase separation, tration is further increased the solution rheological and interfacial properties become turbid and the surfactant be- [6]. The association between surfac- gin to neutralise the charge of polyelec- tants and polymers can be caused by trolytes. This turbidity is related to the both electrostatic and hydrophobic in- formation of big colloidally unstable teractions. In general, we could deal aggregates, figure 1. Charge neutrality with a weak hydrophobic interaction implies hydrophobicity, thus associa- between the polymer chains and the tive phase separation occurs and the surfactant head groups or with a strong aggregates either precipitate or cream electrostatic interaction between oppo- according to their density [2]. The sys- sitely charged polyelectrolytes and sur- tem is a two-phase equilibrium state. factant head groups. As regards poly- [2]. With time, full phase separation electrolytes, the interaction tend to be will occur, therefore it is extremely im- driven almost completely by electro- portant to analyse the properties for static forces. both fresh and aged-settled solutions. Aside from the substances em- Eventually at higher concentration of ployed, there are many parameters that surfactant, we expect to attain a sta- play a key role in the behaviour of ble solution. In fact the aggregates these mixtures, which are [4]: will become charged due to the pres- • the ratio Z = [poly]/[surfactant]; ence of surfactant in excess, thus they will repel each other, becoming an elec- • the total concentration; trostatically stabilised colloidal disper- sion, which is still a two-phase system. • the weight of the polyelectrolyte; The aim of this work is to find a • the stifness of the polyelectrolyte relation between the bulk and the sur- chain; face properties of oppositely charged of a new copolyelectrolyte/surfactant • charge density in the polyelec- not yet researched. trolyte chain. 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. O ur S ystems into the same volume of the polyelec- trolyte, so that the total final bulk con- We used Dodecyltrimethylammonium centration is halved, under continuous bromide (DTAB) as surfactant and two stirring for 20 s. block-copolyelectrolytes, both composed The sample history, i.e. the sam- by polyacrylic acid (PAA) and by poly- ple preparation and/or the mixing or- stirene sulfonate (PSS). The first one, der is fundamental for the final out- that we will call NC29, is composed come. Therefore it is very important by ≈ 90% of PAA and ≈ 10% of PSS, to follow always the same experimen- while the second, NC31, is composed tal protocol to control the state of the by ≈ 50% of PAA and ≈ 50% of PSS. system. In fact figure 2 shows the ef- The main difference between PSS and fect of reversing the mixing order onto PAA is the value of pK a , and thanks to the physical state of PEI/SDS solution the Henderson-Hasselbalch equation with the same bulk concentration of [ A − ] PEI and SDS. It is evident that accord- pH = pK a + log 10 (1) [ HA ] ing to the sample history the system changes dramatically. we expect that they will show a dif- ferent behaviour varying the pH. For instance at pH 12 both polyelectrolytes III. B ulk properties are deprotonated, but at pH 2 only PSS i. techniques used is deprotonated. First of all it is necessary to understand i. sample preparation at which concentrations of surfactant, aggregation starts to occur. All the stock solutions were prepared The combination of O.D. and elec- either in 10 mM HCl (pH 2) or 10 mM NaOH (pH 12). Two 50 mM DTAB so- trophoretic mobility is a useful tool to lutions were prepared at both pH and, have a look on both aggregate forma- simply by dilution, we obtained 1.0 − tion and their charge. 30 mM DTAB solutions. The polymer stocks contained 200 ppm of NC29 or ii. optical density NC31. If we hit a transparent solution with a The mixtures were prepared by wavelength of intensity I incident , figure fast-adding an aliquot of surfactant 4
Project work of International Student Summer Programme at ILL - Marco Valentini duce the optical density τ I incident τ = ln (2) I transmitted 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 of the optical density will be related to the presence of big aggregates [3]. Figure 2: Order of addition effect. Ex- iii. electrophoretic mobility periment 1: add 5 mL of 0.1% PEI solu- tion into 5 mL of 20 mM SDS solution Another technique employed to detect with continuous stirring. Experiment the presence of aggregates is the elec- 2: add 5 mL of 20 mM SDS solution trophoretic mobility. This quantity is into 5 mL of 0.1% PEI solution very the speed of charged particles in a fluid slowly (5 mL/ 45 min ) with continuous in the presence of an electric field. We stirring [10]. expect to record negative values for electrophoretic mobility when the con- centration of surfactant is low. Then, 8a, we would notice that the transmit- adding more DTAB, we begin to neu- ted intensity I transmitted is ≈ I incident . tralise the polyelectrolytes, reaching the zero charge neutrality, that has to correspond to the presence of big ag- gregates. (a) transparent sample . A Malvern Zetasizer NanoZ in- strument was employed for this pur- pose. This device uses the laser doppler velocimetry (DLV) to measure the speed (b) sample absorbing or turbid . of the particles in the solution. In fact if we sent a monochromatic radiation, λ 0 , Figure 3: Qualitatively behaviour against a moving object, we would re- of the intensity of a beam passing ceive a radiation with a different wave- through a sample. length λ 1 . But if there were aggregates in the sample, the light could be absorbed by them or they could scatter it, figure 3b. These possible effects provoke the decreasing of I transmitted . In order to Figure 4: Principle of laser doppler quantify this phenomenon we intro- velocimetry. 5
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