Hydrophobic capillaries

The first hydrophilic monoliths based on acrylamide chemistry were based on copolymerization of acrylic acid and (V,/V -methylene bis(acrylamide) in the presence of an aqueous buffer as porogen [66], Shortly after, the first hydrophobic capillary support for hydrophobic interaction chromatography was fabricated by the substitution of acrylic acid by butyl methacrylate, whereas the monomer... 

The capillary inside surface also affects the composite membrane performance. For alcohols with hydrophilic OH groups it is desirable to have a hydrophobic capillary surface. 

Bergero S and Chiari A. Experimental and theoretical analysis of air humidification/dehumidification processes using hydrophobic capillary contactors. Appl. Then Eng. 2001 21 1119-1135. 

In Figure 4 some typical experimental results are presented which show the degree of conversion of the triglycerides as function of time. Only glycerine was present in the permeate. Kicroporous hydrophobic capillary membranes can also be used in this study to distribute small oil droplets in the water phase where the enzyme is dissolved. This emulsion can be controlled in droplets size and formation rate by changing the transmembrane pressure and axial flow rate. 

Capillary Force Valves, Fig. 3 Capillary forces in hydrophilic capillaries result in increased pressure in trapped air bubbles. Capillary forces in hydrophobic capillaries prevent liquid from entering the capillary and can be overcome by larger pressure... 

ARj). Reducing the diameter of a hydrophobic capillary reduces the radius of curvature of the interface and requires larger pressure (AP2) to move the liquid and could also function as a valving mechanism... 

When nonpolar liquids are displaced by water, dynamic wetting films are formed that results in complete wetting of the capillary wall by a retreating meniscus. For viscous dibutyl phthalate = 0. Thinner wetting films form in the case of less viscous tetradecane, which results in 0r = 55°. The amount of nondisplaced oil in hydrophobic capillaries decreases at very low rates of displacement by water when much thinner (equilibrium) wetting films of oils were formed. 

Methylation of quartz surface was used for preparation of hydrophobic capillaries. The methods are based on a reaction of OH groups on quartz surface with trimethylchlorosilane molecules from vapor phase or from a solution in benzene (see Section I). 

Penetration of water into hydrophobic capillaries is retarded or becomes impossible when advancing contact angle exceeds 90°. Wetting of a hydro-phobic surface may be improved by adsorption of surfactant molecules, which are directed with polar groups towards the water phase. Thus, an increase in surfactant concentration leads to gradual decrease of contact angles accelerating spontaneous imbibition. 

A. Acceleration of Impregnation of Hydrophobic Capillaries by Addition of Surfactants... 

Let us consider the spontaneous imbibition of a surfactant solution into a hydrophobic capillary taking into account the possibility of the eventual passing of some surfactant molecules through the meniscus interface to the nonwetted portion of the capillary (Fig. 15). C x,t) and a. x,t) are the local surfactant concentrations (in g/cm ) of the solution inside the capillary and on the capillary surface (in adsorbed state), respectively. A constant surfactant concentration, C(0,t) = Co = constant, is maintained at the capillary inlet. First, the case Co < Cc, where Cc is the CMC, will be considered. 

FIG. 15 Distribution of surfactant solution concentration inside a hydrophobic capillary, C(x), and in adsorbed layer on capillary surface, a(jc), during imbibition of... 

Since the rate of imbibition of surfactant solutions into a hydrophobed capillary is rather slow, the kinetics of adsorption may be ignored and it is possible to include into Eq. (19) an equilibrium adsorption isotherm a = f C). As a first approximation, the linear form of the isotherm will be used here ... 

FIG. 21 Schematic representation of a diffusion mechanism of penetration of a surfactant solution into a hydrophobic capillary with advancing contact angles 0a more than 90°. 

Influence of external pressure on imbibition of surfactant solution has been studied using a horizontally oriented hydrophobed capillary r = 6 pm, 0a = 96°) [1]. The capillary was first washed three times with Cq = 0.1% solution of syntamide-5 up to the length / = 3.7 cm (Fig. 22, point c). Thus, this part of the capillary might be supposed to be in equilibrium with the surfactant solution. 

Spontaneous imbibition of microemulsions into hydrophobed capillaries occurs at much higher rates (K = 0.1-1 cm/s ). Under these conditions complete wetting of the capillary surface is realized [23]. 

The results obtained lead to the conclusion that three mechanisms of penetration of nonionic surfactant solutions into hydrophobic capillaries are possible. The first takes place at a high concentration of surfactant, Q > Q. Spontaneous imbibition advances at a high rate but is limited to some finite length Iq, which depends on Q, r, and Gi. The second mechanism is realized when Co< Q. The rate of penetration in this case is much lower, being controlled by the reduced concentration, Cm = constant, near the meniscus. At still lower concentration of bulk solution Cq, the diffusion mechanism of penetration takes place in thin capillaries. The rate of penetration is determined here by the surface diffusion of surfactant molecules in advance of the meniscus. 

Penetration of surfactant solutions into hydrophobic capillaries under an external pressure is not effective because the surfactant molecules, being adsorbed, cannot approach the advancing meniscus at high Peclet numbers. 

Pretreatment of a hydrophobed capillary with the D-8 solution made the surface hydrophilic, and this secured spontaneous suction of the solution. Nontreated capillary remains hydrophobic, and advancing contact angles differ not mueh from contact angles for water. However, diffusion mechanism of very slow penetration discussed in Section III.B may also take place in this case. 

The capillary counterpressure, which is proportional to the wetting tension 7i2 cos 6r = 10 mN/m, is about three times lower than when silicon oil was displaced by pure water. The value of wetting tension measured in hydrophobed capillaries equals in this case 30 mN/m. Therefore, use of the D-8 trisiloxane surfactant may facilitate displacement of oils from hydrophobic pores. Besides, when water displaces silicon oil a wetting film of the oil remains on the hydrophobed capillary surface. Because the thickness of the remaining dynamic film grows according to Derjaguin [7] with flow rate, the low rates of displacement are preferable in these cases. 

Cai, J., Bo, S., Cheng, R., liang, L., and Yang, Y, Analysis of interfacial phenomena of aqueous solutions of polyethylene oxide and polyethylene glycol flowing in hydrophilic and hydrophobic capillary viscometers, /. Colloid. Interface Sci. 2004, 276(1), 174-181. 

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