DOI: 10.3303/CET25117150 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 3 December 2024; Revised: 30 March 2025; Accepted: 22 April 2025 
Please cite this article as: Giustacori P., Marchini S., Brunazzi E., Hampel U., Schubert M., 2025, Impact of Hydrophobic Surfaces on Mass 
Transfer in Structured Packings, Chemical Engineering Transactions, 117, 895-900  DOI:10.3303/CET25117150 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 117, 2025 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Fabrizio Bezzo, Flavio Manenti, Gabriele Pannocchia, Almerinda di Benedetto 

Copyright © 2025, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-17-5; ISSN 2283-9216 

Impact of Hydrophobic Surfaces on Mass Transfer in 

Structured Packings 

Pietro Giustacoria, Sara Marchinib, Elisabetta Brunazzia, *, Uwe Hampelc, d, Markus 

Schubertb  

aDipartimento di Ingegneria Civile e Industriale, Università di Pisa, Largo Lucio Lazzarino 2, Pisa, 56122, Italy 
bChair of Chemical Process Engineering, Technische Universität Dresden, Dresden, 01062, Germany 
cChair of Imaging Techniques in Energy and Process Engineering, Technische Universität Dresden, Dresden, 01062, 

Germany 
dInstitute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, 01328, Germany 

elisabetta.brunazzi@unipi.it 

This study investigates a potential alternative to traditional film flow in structured packings operating at low liquid 

loads, aiming to improve mass transfer performance by increasing the gas-liquid interfacial area. The potential 

of small droplet flow, which theoretically offers a significantly higher interfacial area compared to film flow, was 

investigated. The water affinity of the structured packing surface was modified by applying self-assembled 

monolayer (SAM) coatings. Contact angles, droplet behavior and coalescence tendencies were investigated on 

smooth, corrugated and embossed metal sheets to identify the best combination of SAM system and coating 

process. The mass transfer performance of SAM-coated packings was compared to standard packings by 

stripping isobutyl acetate with air. These tests showed that the SAM coated packings tested had lower liquid 

holdup and therefore lower mass transfer efficiency compared to uncoated packings. However, the study 

provides a valuable methodology and framework for evaluating the performance of other structured packing 

surface treatments. 

1. Introduction 

Thermal separations are the most widely used processes for separating and purifying mixtures in the chemical 

industry. These operations are usually carried out in columns where the liquid and gas phases are brought into 

contact. Inside the column, various elements promote the contact between the two phases. In recent years there 

has been significant development of structured packings, and many of the newly built columns now use this 

technology (Spiegel and Meier, 2003). In addition, structured packings have been used extensively to revamp 

random packed and tray columns (Shiveler et al., 2007). The purpose of structured packings is to provide a 

large contact area while minimizing pressure drop. For this reason, they are also widely used in vacuum 

operations where pressure drop must be minimized. Typically, structured packings operate in film flow, where 

the liquid, at best, covers the entire surface of the element to utilize all the available geometric area 

(Szulczewska et al., 2003). However, maldistribution within the packing and preferential paths usually prevents 

perfect wetting (Fitz et al., 1999). Furthermore, in the case of film flow, the interfacial area cannot exceed the 

geometric area provided by the packing. 

Flow regimes other than film flow could potentially increase the interfacial area available for mass transfer, an 

even more important aspect for applications that require low liquid loadings where film flow and full wetting is 

also difficult to establish. Droplet flow is one such regime that could achieve this goal. In the extreme case of a 

spherical droplet contacting the packing at a single point, the interfacial area is nearly five times greater than for 

film flow, considering the same liquid volume. 

The possibility of increasing the mass transfer performance of structured packings using hydrophobic coatings 

was investigated in this study. To induce changes in flow regimes, the surface of the packing elements was 

systematically modified to alter their interaction with the liquid phase. Specifically, these modifications were 

895



designed to increase the liquid contact angle on the surface, thereby enhancing the hydrophobic properties of 

the material. By increasing surface hydrophobicity, the modifications promote film instability and the formation 

of discrete droplets on the surface. Self-assembled monolayers (SAMs) were selected for the surface 

modification to implement these flow destabilizations. SAMs offer a simple, yet highly effective method of 

tailoring surface properties, providing robust means of achieving desired contact angles. Their ease of 

application, combined with their ability to produce uniform and reproducible hydrophobic coatings, makes them 

an ideal approach for this study (Harm et al., 2002). 

2.  Materials and methods 

2.1 Surface wettability modification 

Samples were obtained from AISI 316 steel sheets (0.1 mm thick) used in the construction of Montz structured 

packings. The samples were first cleaned with deionized water and degreased with ethanol, dried with 

compressed air and then immersed in a chemical etching bath for half an hour. To enhance the effect of the 

chemical bath and to homogenize the concentration, the etching was performed in an ultrasonic cleaner. After 

the chemical bath, the samples were rinsed with deionized water and dried with compressed air. The samples 

were then immersed in a solution containing 1 mmol of SAM molecules for about 5 minutes at 60 °C. After this 

last immersion, the samples were rinsed with deionized water, dried with compressed air and stored in a closed 

container to avoid contact with dust and to allow self-organization of the SAM molecules. Self-organization 

consists of the reorganization of molecules adsorbed on the surface to maximize interactions with the substrate 

and to align their functional groups to interact with the substrate while their tail groups extend outward, imparting 

hydrophobic properties. A detailed description of the coating process can be found in Harm et al. (2002). 

The wettability characterization of the surfaces was performed by determining the contact angles of ultrapure 

water droplets on the different surfaces analyzed. This analysis was performed using a Krüss DSA100S 

goniometer. The procedure for measuring contact angles using the goniometer sessile drop technique was the 

same as that typically used in the literature (e.g., Kwok et al., 1997). Static contact angle (CA) values were 

extracted using the ADVANCE software associated with the goniometer; while advancing and receding contact 

angles (ACA and RCA) were extracted from the video by the goniometer software and using a custom Python 

script.  

Two different molecules were considered for the coating of steel samples: n-dodecylphosphonic acid (DPA) and 

heptadecafluorodecylphosphonic acid (17FPA). The tested systems have the advantage of forming SAMs at 

low concentrations and do not require complicated coating procedures. In addition, at the used concentrations, 

SAMs solutions are not classified as toxic or carcinogenic. Chemical etching of surfaces can be performed with 

different solutions. In the present study, two different etching temperatures and etching solutions containing 10 

% of nitric acid (HNO3) in water alone or in mixtures with 10 % of ferric chloride (FeCl3) or 10 % hydrogen 

peroxide (H2O2) were considered (all percentages are by weight). The following procedure was chosen as the 

reference case: surface etching with HNO3 solution for 5 minutes at room temperature, followed by coating with 

DPA for 2 minutes at 40 °C. All tests were compared with this system. 

2.2 Fluid dynamic characterization 

The dynamic of the liquid flows on the coated metal surfaces was investigated with a particular focus on droplet 

stability. An inclined plate with variable inclination was constructed to support the samples and to test typical 

contact angles of commercially available structured packings. Flat, embossed, and corrugated embossed sheets 

were used for these experiments. The sheets were placed on the inclined stand and liquid was supplied from 

above through three needles of approximately 1.8 mm OD. The supply system consists of a reservoir, a pump, 

and a valve to control the flow rate. A camera was used to capture the liquid patterns on the sheets for different 

flow rates. To obtain clearer and more distinct images of the liquid flow patterns on the sheets, a dye was used. 

Specifically, an aqueous solution of 0.02 g/L methylene blue was used. The physical properties of the dyed 

solution (i.e., viscosity, surface tension, density, etc.) remained identical to those of pure water. 

2.3 Structured packing characterization 

The performance of coated and uncoated packing elements was compared. A packed column with three Montz 

B1-500 structured packing elements (i.e. consisting of corrugated embossed metal sheets placed side by side) 

was used. The aim of these experiments was to explore the main differences between coated and uncoated 

packings, particularly in terms of holdup and mass transfer performance.  

The experimental setup consists mainly of a DN 100 column with gas and liquid auxiliary circuits. The column 

is composed of several PMMA tube segments screwed together with PMMA flanges. The lower section is a 

bubble column that allows partial pre-saturation of the pressurized dry air entering the system; to prevent liquid 

entrainment, a demister is placed above this section. The upper part of the column is the actual test apparatus, 

896



and this section is equipped with three 100 mm high structured packing elements. The overlapping elements 

are rotated 90° with respect to each other to allow liquid and vapor redistribution at the interfaces between the 

elements. A liquid distributor consisting of 36 1.8 mm OD needles is located at the top of the column. The 

number of irrigation points per square meter is close to the guidelines for ideal packing wetting (Green and 

Perry, 2007). In the system, the liquid is delivered at volumetric flow rates ranging from 0.25 to 2.0 L/min, 

corresponding to liquid loads (𝐵) of 2 to 15 m3/m2h. In this work, a single gas flow rate of 200 SLPM was chosen, 

which corresponds to a gas superficial velocity (𝑢𝑠,𝐺) of 0.45 m/s, ensuring operation below the gas loading 

point. The focus was on understanding how the coating affects the behavior of the liquid phase. Figure 1 shows 

the process diagram of the mini-plant (left) and a picture of the setup (right); a more detailed description of 

experimental procedures follows. 

 

 
Figure 1: Experimental test rig details (left: PFD of the test rig, right: test rig).  
 

The liquid circuit is operated by a centrifugal pump (G) which pumps the liquid from a buffer tank (D1) to the 

liquid distributor. The liquid leaving the packed bed is collected by a chimney tray element and sent to the outlet 

tank (D2), the outlet line is equipped with a sampling point (SP). The gas supply is provided by a compressed 

air line. The gas flow is controlled by a mass flow controller with a range of up to 250 SLPM with an accuracy of 

0.8%. Air is introduced at the bottom of the pre-saturation section and, after bubbling through a batch of water, 

enters directly into the stripping section. After passing through the column, the gas is vented to the atmosphere 

at top of the column. An exhaust hood is placed above the column to collect gaseous emissions. 

For mass transfer experiments, a 1000 ppmv volatile organic compound (VOC) water solution must be prepared. 

To prepare the solution, the pump is used as a mixing device. Two valves allow switching between preparation 

and experimental modes. Pure VOC is injected at the pump inlet via the injection point (IP), and the liquid 

circulates to and from the tank. Liquid exact concentration was checked offline by UV spectroscopy. During the 

experiments, the gas and liquid temperatures above the packed bed and the gas humidity after the bubble 

column were measured at regular intervals. 

Holdup measurement 

For the sake of simplicity, the holdup determination in the present work was made by measuring the amount of 

liquid collected at the bottom of the column when the liquid inlet and outlet were closed, as used by Aferka et 

al. (2011). Liquid level changes were measured as pressure changes using a differential pressure sensor. The 

acquired pressure signals were then converted first to water column heights and then to liquid volumes by 

knowing the area of the liquid collection section. Dividing the collected water by the volume occupied by the 

packing elements, the holdup was derived. First, the packing was pre-wetted; after reaching a steady state at a 

given flow rate, the liquid inlet and outlet valves were closed simultaneously and the liquid was allowed to drain. 

The drain time was ten minutes for each test, a compromise between collecting most of the liquid from the 

packing and keeping the test times short. After the valves were closed, the water supply was immediately shut 

off due to the design of the distributor. At normal operating conditions of the specific experimental setup, the 

gas superficial velocity does not exceed 1 m/s. At this low superficial velocity, the column always operates in 

the preloading regime. In this regime, holdup is essentially independent from the gas velocity but is a strong 

(a) PFD of the test rig (b) Test rig

897



function of liquid flow rate and packing size. Therefore, for simplicity, it was decided to perform holdup 

measurements in the absence of countercurrent gas flow, as the effects were considered negligible. 

Mass transfer tests 

The present work uses air-led stripping of isobutyl acetate from an aqueous solution to measure the mass 

transfer performance of the structured packings. Isobutyl acetate is a colorless liquid with a fruity odor and 

moderate solubility in water. This test system has recently been used to measure liquid-side mass transfer 

coefficients in randomly packed columns by Lamprecht and Burger (2018), and to measure tray and point 

efficiencies in distillation columns by Marchini et al. (2021, 2023). This work represents the first application of 

this system for the determination of mass transfer parameters on structured packings. 

An initial concentration of 1000 ppm isobutyl acetate in the liquid batch was selected based on preliminary tests. 

The physical properties of the resulting batch (i.e., viscosity, surface tension, density, etc.) remained identical 

to those of pure water. An offline UV spectrometer was used for sample analysis and data acquisition. During 

the experiments, solution samples were taken from the sampling point (SP) visible in Figure 2 (left). At first, 

mass transfer resistances were estimated for the considered physical system using the model of Brunazzi et al. 

(1995). It turns out that the resistance in the liquid phase is much higher than in the gas phase, so that the 

experimentally determined mass transfer coefficient corresponds to the liquid-side mass transfer coefficient. 

From the concentration values across the packed bed, the 𝑘𝐿𝑎 values are derived as follows. 

 

𝑁 = 𝐿(𝑥𝐼𝑁 − 𝑥𝑂𝑈𝑇) = 𝐾𝑂𝑋𝑎𝐴𝐶𝐻ΔxLM  (1) 

 

As noted above, the gas phase resistance was considered negligible, so the total liquid-side coefficient is equal 

to the single liquid phase coefficient. 

 

𝑘𝐿𝑎 =  
𝐾𝑂𝑋𝑎

𝜌𝐿/𝑀𝑀
=

1

𝜌𝐿/𝑀𝑀
(

𝐿

𝐴𝐶𝐻

(𝑥𝐼𝑁−𝑥𝑂𝑈𝑇)

Δ𝑥𝐿𝑀
)  (2) 

3. Results and discussion 

3.1 Surface wettability modification 

Depending on the SAM system, the etching solution giving the best results was determined. For DPA, higher 

contact angles were obtained using HNO3 + FeCl3 solution, while for 17FPA greater contact angles were 

obtained using HNO3 + H2O2 solution. The effect of the temperature of the etching bath was also studied. Two 

cases were considered: no heating and heating at 60°C. For the DPA-based system, the combination of HNO3 

+ FeCl3 and heating at 60°C gave the best results, while for 17FPA, heating at 60°C did not have a significant 

effect on the contact angle. The contact angle results are presented in Table 1. The degradation of the contact 

angle by immersion in water was also analyzed. Contact angles were compared before and after immersion in 

water for 72 hours. The degradation of contact angles under these conditions is less than 5% for both systems.  

This feasibility study showed that 17FPA gave the best results in terms of contact angle and hysteresis between 

ACA and RCA. Therefore, it was decided to use the system consisting of 17FPA with a pre-etch treatment at 

room temperature with nitric acid and hydrogen peroxide solution for the subsequent coatings. Figure 2 (left) 

shows the contact angle obtained for the chosen combination of SAMs and etching solution. 

 

Table 1: Static contact angles for different surface treatment combinations (RT: room temperature) 

Surface treatment combination Average CA (°) 

Non coated 80.3 

DPA/HNO3 @RT 93 

DPA/(HNO3 + FeCl3) @ RT 101.5 

DPA/(HNO3 + H2O2) @ RT 95.9 

17FPA/HNO3 @ RT 109 

17FPA/(HNO3 + FeCl3) @ RT 114.7 

17FPA/(HNO3 + H2O2) @ RT 121.9 

DPA/(HNO3 + FeCl3) @ 60°C 106.2 

17FPA/(HNO3 + H2O2) @ 60°C 117.3 

3.2 Fluid dynamic characterization 

The most significant effect of the coating on wettability occurs on smooth surfaces, where the rivulet is unstable 

and tends to break into droplets even at high flow rates. Embossed surfaces are more wettable, but droplets 

can still form at lower flow rates. As the flow rate increases, the embossing provides more stability to the rivulets 

898



that are much narrower and taller on the coated surface compared to the uncoated one. Figure 2 (right) shows 

the effect of coating on the behavior of the liquid flowing over the embossed sheet. It can be seen how the 

coating promotes droplet formation and enhances droplets stability. 

 

 
Figure 2: Feasibility study results (left: static contact angle measurement; right: Liquid behavior on embossed 

metal surface, top uncoated, bottom - coated). 

3.3 Packing performance comparison 

 
Figure 3: Comparison of experimental results for uncoated and coated structured packings (left: holdup; right: 
mass transfer coefficient). 

Holdup results  

Figure 3 (left) shows the holdup results for coated and uncoated packings. The coating significantly reduces the 

amount of liquid retained in the column during operation, this effect is most noticeable at high liquid flow rates. 

The coating reduces the wettability of the surfaces, causing the liquid to flow more rapidly through the packing, 

thereby reducing the amount of liquid instantly present in the packing. Over the range of liquid loads tested, the 

coating results in a macroscopic reduction of the holdup value by a factor of 2 to 3. 

Mass transfer results 

The product of the mass transfer coefficient and the specific interfacial area obtained during the experimental 

campaign is shown in Figure 3 (right). The results show that the mass transfer performance is degraded by the 

surface coating and may be partially due to the reduction in holdup. It is not yet clear whether the degradation 

is due to a reduction in the mass transfer coefficient or the specific interfacial area, or both. Further experiments 

to determine the interfacial area alone would be necessary to distinguish the effects. It should be noted that the 

applied liquid loading values were intentionally limited to study the effect of the hydrophobic coating at low liquid 

flow rates, where the packings operate at the lower limit of the nominal wetting rate.  

4. Conclusions 

The effects of hydrophobic coatings on liquid holdup and mass transfer in structured packings were 

systematically investigated. Self-assembled monolayers (SAMs) were chosen for their non-toxicity at low 

concentrations and ease of deposition on metal substrates. Among the tested systems, n-dodecylphosphonic 

acid (DPA) and heptadecafluorodecylphosphonic acid (17FPA) were evaluated, with 17FPA identified as the 

optimal coating due to its stability, compactness, and superior hydrophobic properties. The pre-treatment of 

metal surfaces using a solution of nitric acid and hydrogen peroxide at ambient temperature further enhanced 

the coating’s effectiveness. The feasibility study demonstrated that the coated surfaces promoted droplet 

formation, potentially increasing the interfacial area within the packing.  

To assess the impact of these coatings, an experimental packed column setup was developed, enabling direct 

performance comparisons between coated and uncoated packing sheets. Holdup measurements revealed that 

hydrophobic coatings significantly reduced liquid retention within the packing, thereby decreasing the available 

(a) Holdup (b) Mass transfer coefficient

899



liquid volume for mass transfer. Mass transfer experiments using air-led stripping of isobutyl acetate confirmed 

that reduced holdup led to shorter liquid residence times, ultimately lowering mass transfer efficiency. 

Despite this reduction in mass transfer performance, the application of hydrophobic coating presents several 

practical implications. In industrial applications where minimizing holdup is advantageous, such as in processes 

requiring rapid phase disengagement, reduced solvent inventory, or enhanced gas flow capacity, hydrophobic 

coatings could be strategically employed. Additionally, in cases where fouling or corrosion due to excessive 

liquid retention is a concern, these coatings may help maintain long-term operational efficiency. However, further 

research is necessary to refine coating techniques and identify conditions where the trade-off between reduced 

holdup and mass transfer efficiency can be optimized for specific process requirements. Future studies should 

also focus on tailoring surface modifications to balance droplet formation with sufficient liquid residence time for 

effective mass transfer. 

Nomenclature

𝑎 – specific interfacial area, m-1 

𝐴𝐶  – column cross-sectional area, m2 

𝐵 – liquid load, m3/m2h 

𝐻 – packed bed height, m 

𝑘𝐿 – liquid-side mass transfer coefficient, m/s 

𝐾𝑂𝑋 – overall mass transfer coefficient, kmol/m2s 

𝐿 – liquid molar flowrate, kmol/s 

𝑀𝑀 – molar mass, kg/kmol 

𝑁 – molar flux, kmol/s 

𝑢𝑠,𝐺 – gas superficial velocity, m/s 

𝑥𝐼𝑁 – liquid inlet molar fraction, - 

𝑥𝑂𝑈𝑇 – liquid outlet molar fraction, - 

Δ𝑥𝐿𝑀 – logarithmic mean driving force, - 

𝜌𝐿 – liquid mass density, kg/m3 

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	Impact of Hydrophobic Surfaces on Mass Transfer in Structured Packings