КОЛЛОИДНЫЙ ЖУРНАЛ, 2008, том 70, № 2, с. 201-206
УДК 541.183.23
ADSORPTION KINETICS INVESTIGATION FOR ALKYLTRIMETHYLAMMONIUM BROMIDES ON ITO-AQUEOUS INTERFACE
© 2008 Liyun Qi*, **, Zhichu Bi**
* School of Chemical and Material Engineering, Southern Yangtze University Wuxi 214122, People's Republic of China **Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry,
Chinese Academy of Sciences Beijing 100080, People's Republic of China E-mail: liyunqi@sytu.edu.cn Поступила в редакцию 11.01.2007 г.
Adsorption of dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide onto indium-tin oxide (ITO) was studied by in-situ monitoring of capacitance and resistance of adsorbed layers for a series of concentrations of aqueous solutions. The experimental results suggested that the adsorption of both surfactants showed a three-region adsorption process, and that the adsorption was driven by electrostatic attraction and hydrophobic interactions in two regions below critical micelle concentration (cmc). Above the cmc, the changes in electrical properties could be explained by the assumption that entire micelles were directly adsorbed onto the ITO surface and then underwent an adjusting process. The variations of water contact angle against surfactant concentration showed two distinct regions. The effect of high electrolyte concentration on the adsorbed layer structure was also discussed.
INTRODUCTION
The wide-ranging application of surfactants at the solid-liquid interface in various industrial processes ranging from ore flotation and detergency to enhanced oil recovery [1] necessitates the complete understanding of surfactant adsorption behavior at the solid-liquid interface. Extensive investigations have focused on static adsorption process in which the adsorption isotherm is widely used [2, 3]. Recently the structure of the adsorbed layers at the solid-liquid interface has been elucidated by innovative experimental techniques, such as atomic force microscopy (AFM) [4-7], neutron reflectivity [810], and fluorescence spectroscopy [11-13]. Most of these studies concern the aggregating behavior of surfactants, and as yet these studies have been limited to the bulk surfactant concentrations close to or above the critical micelle concentration (cmc) [5]. Atkin and his co-workers [2] reviewed exhaustively the results available from traditional methods and relatively new techniques. They commented upon the evolution of the adsorbed layer structure with various concentrations through the equilibrium experiment results. The supposition was that the surfactant adsorption was driven by electrostatic force at low concentrations and as the surface excess increases, lateral hydrophobic interactions led to the formation of surface aggregates (termed hemimicelles), which was followed by the fast increase in surface excess. However, the kinetics of surfactant adsorption at solid/aqueous interface is yet open to question. That is, for a concentration at which aggregates come into being,
is the surface charge first neutralized, followed by hemimicelle formation, then bilayer or discreet aggregate formation, or do these processes occur simultaneously? Additionally, the contribution or otherwise of micelles to the adsorption process is often questioned. Until recently, the kinetics of surfactant adsorption has received comparatively little attention [14—17], despite the importance of adsorption kinetics in actual processes. The scarcity of experimental techniques capable of following the fast kinetic process of the surfactant adsorption is the primary reason for the lack of investigation of adsorption kinetics. Atkin and his coworkers [16, 17] investigated adsorption kinetics using ellipsometry technique and much important information were obtained. However, in their experiments the layer thickness values were determined under artificially controlled conditions. An approach to monitor in situ properties of adsorbed layer is required.
The formation of self-assembled layer on a solid surface can be followed by monitoring its electrical properties [18-22]. And the adsorption of surfactant on charged surface can also be characterized by the appearance of deformed and/or split capacitance peaks according to experimental measurements and theoretical models [2325] based on the principles of thermodynamics. In our studies, the multi-function weak electric analyzer was used to monitoring the average thickness of the membrane by following the changes of the capacitance and resistance. The apparatus allow tiny voltage to be applied and weak electric current (10-3~10-13 A) and thus
small capacitance to be detected. Therefore, the information concerning the properties of the adsorbed layer changing with time can be obtained and the fast kinetic process can be followed conveniently.
This method simplifies the structure of adsorbed films to monolayer or bilayer and cannot give the inplane details of the adsorbed aggregates. However, this technique can follow in-situ changes sensitively and give valuable insight into the nature of the adsorption process. In present paper, we report measurements of surfactant adsorption kinetics onto indium-tin oxide (ITO) surface. These measurements are aimed to follow the structural changes occurring in the adsorbed surfactant layer in real time. Normally, the adsorption of surfactant on the solid substrate can lead to the change of its wettabil-ity depending on the orientation of adsorbed surfactant molecules. Thus, the contact angle measurements for ITO with adsorbed surfactant films were conducted to obtain the information about the surfactant orientation after the adsorbed film formed.
EXPERIMENTAL
Samples of dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB) were of analytical reagent grade from Beijing Chemicals Factory, China. DTAB was purified by repeated recrystal-lization from ethanol/acetone mixtures followed by drying under reduced pressure at 65°C for 12 h. CTAB was recrystallized three times from acetone and freeze-dried before used. The surface tension-concentration curves for the two surfactants showed no minimum point. Other substances of analytical reagent grade were obtained commercially and were used as received. Double-distilled water was used throughout the experimental work.
ITO conducting glass (1 cm x 0.5 cm) was manufactured by Pittsburgh Plate Glass, and its nesa film is n-type semiconductor of resistance of 60 Q per cm-2. ITO wafers were cleaned according to the following procedure: rising in ethanol for 2 min; ultrasonic treatment in acetone for 5 min with subsequent drying; ultrasonic treatment in a solution of 15% (v/v) ammonium hydroxide (30%), 15% hydrogen peroxide (30%), and 70% water for 5 min; heating in the same solution to 60°C for 30 min. Then ITO wafers were rinsed with copious double-distilled water and dried under a stream of nitrogen.
The potentiostat used was JI-100A (Jinke Electronic Co. Ltd., P.R. China) that allowed the average surfactant layer thickness to be calculated throughout adsorption process by monitoring the electrical properties [26]. The used amplitude and scan rate were 50 mV and 10 mV/s, respectively; thus a weak current can be produced so as not to disturb the adsorption process. Adequate supporting electrolyte KCl was added to eliminate the effect of electromigration in surfactant solution. The three-electrode system was used. The ITO was used as working electrode and platinum as counter electrode, calomel electrode as a reference electrode. The electrical behav-
ior was monitored in the assembly cell and the layer thickness was calculated. The measured error of layer thickness was ±0.2 nm.
The cmc values in the presence of KCl were determined by conductance measurements with DOS-12-A conductivity meter. The cell constant of electrode is 1.0 cm-1. Surfactant solutions were thermostated for 15 min before conductivity was measured. Plots of specific conductance versus surfactant concentration exhibited two straight lines intersecting at the cmc. The errors in the conductance measurements were ±0.50%.
The static advancing contact angle measurements were carried out with a 2 pL water drop using an optical contact angle meter (JC2000A POWEREACH, Zhongchen Powereach Company, Shanghai, China). In the experiments, at least three ITO wafers were used to repeat the contact angle measurements for each concentration, and for each treated substrate, the contact angles were measured on at least three different areas of each ITO wafer. The reported values are averages of these measurements. The experimental error was ±2°.
All measurements were performed at 25 ± 1°C.
RESULTS AND DISCUSSION
The adsorption of DTAB and CTAB onto ITO as a function of time for a series of concentrations is investigated. The adsorbed layer thickness is calculated according to the method of references [26] and the results are presented in Fig. 1a for DTAB and Fig. 1b for CTAB. For the convenience of comparison, the surfactant concentration is normalized to its cmc value in present paper. In the initial time a rapid increase in thickness of the adsorbed layer is observed, and within ~100 s, the equilibrium thickness is reached below the surfactant concentration of 0.8cmc. It is found that the rate of change in the adsorbed layer thickness is strongly dependent on the surfactant bulk concentration. The rate of adsorption increases with the increasing of concentration. However, the rate of adsorption decreases gradually after the initial monotonic increase with time, especially at high concentrations.
Noteworthy is that there exists a slow adsorption process in the vicinity of 0.8cmc for both surfactants, which does not be observed for other concentration regions. At 0.8cmc, DTAB and CTAB both exhibit two notable increasing processes in adsorption kinetic curves as shown in Fig. 1. The initially rapid adsorptio
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